Natural kaolin derived stable SBA-15 as a support for Fe/BiOCl: A novel and efficient Fenton-like catalyst for the degradation of 2-nitrophenol

Article abstract of DOI:10.1039/c5ra01804h

The ordered mesoporous material SBA-15 (FCSBA-15) with an enhanced hydrothermal stability was successfully synthesized from natural kaolin in the presence of a fluorocarbon surfactant. FCSBA-15 was further used as the support for Fe/BiOCl with the aim of exploring its Fenton-like catalytic performance toward the degradation of 2-nitrophenol. Based on characterization techniques including X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), no phases referring to the Fe species were observed, suggesting that the Fe³⁺ ions may be present on the surface of flower-like BiOCl. It is noted that the Fe³⁺ ions could lead to a morphological reconstruction from BiOCl nanosheets to BiOCl flowers. The obtained Fe/BiOCl-FCSBA-15 exhibited an excellent degradation efficiency for 2-nitrophenol, which reached nearly 100% within 40 min by optimizing parameters such as the H₂O₂ dosage, pH value, temperature, Fe/Bi molar ratio and Bi/Si molar ratio. The finding reported here is important and may help to develop novel mesoporous matrix based systems for advanced catalysts.

Full text of DOI:10.1039/c5ra01804h

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Natural kaolin derived stable SBA-15 as a support
for Fe/BiOCl: a novel and efficient Fenton-like
catalyst for the degradation of 2-nitrophenol†
Cite this: RSC Adv., 2015, 5, 36948
*
*
Qihang Zhao, Xiaoyan Liu, Menglin Sun, Chunfang Du and Zhiliang Liu
The ordered mesoporous material SBA-15 (FCSBA-15) with an enhanced hydrothermal stability was
successfully synthesized from natural kaolin in the presence of a fluorocarbon surfactant. FCSBA-15 was
further used as the support for Fe/BiOCl with the aim of exploring its Fenton-like catalytic performance
toward the degradation of 2-nitrophenol. Based on characterization techniques including X-ray
diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), energy
dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), no phases referring to the Fe
species were observed, suggesting that the Fe3+ ions may be present on the surface of flower-like
BiOCl. It is noted that the Fe3+ ions could lead to a morphological reconstruction from BiOCl
nanosheets to BiOCl flowers. The obtained Fe/BiOCl–FCSBA-15 exhibited an excellent degradation
efficiency for 2-nitrophenol, which reached nearly 100% within 40 min by optimizing parameters such as
the H2O2 dosage, pH value, temperature, Fe/Bi molar ratio and Bi/Si molar ratio. The finding reported
here is important and may help to develop novel mesoporous matrix based systems for advanced catalysts.
Received 29th January 2015
Accepted 27th March 2015
DOI: 10.1039/c5ra01804h
www.rsc.org/advances
Fe-involved homogeneous catalysis (Fenton reaction)13,14 and
heterogeneous catalysis (Fenton-like reaction)15–17 have been
1. Introduction
extensively reported. Compared with homogeneous Fenton
catalysis, the heterogeneous solid ion-based reaction is a highly
promising alternative to expand the reaction conditions and
prevent the precipitation of soluble iron and the accumulated
iron sludge.18,19
Nitrophenols, widely used in the chemical industry for
manufacturing, synthetic dyes, wood preservatives and pesti-
cides, are common pollutants of wastewater and the environ-
ment and most of them have been included in the US
Environmental Protection Agency (USEPA) list of priority pollut-
ants.1–3 2-Nitrophenol (2-NP) is one of the most bio-refractory,
persistent, and highly toxic nitrophenols, the concentration of
Fenton-like catalysis refers to the catalyst aggregation and
difficult recycling from the solution. Therefore, catalysts are
always loaded onto various supports, taking mesoporous
matrices,20,21 clay minerals22 and zeolites23,24 as examples, for the
purpose of retaining the nanostructure of the catalysts and
easier recycling. SBA-15, an attractive mesoporous material due
to its huge surface area, high pore volume, uniform pore size
distribution, regular structure and higher thermal and hydro-
thermal stability than M41S materials,25,26 is oen employed as
the host matrix to participate in the catalysis process.27–29 The
reagents used for synthesizing SBA-15 are mainly concentrated
on tetramethylorthosilicate (TMOS),30 tetraethylorthosilicate
(TEOS)31 and water glass (Jo et al.),32 which are relatively
expensive and hazardous to the environment. Available natural
minerals, such as layered silicates rich with SiO2 or Al2O3, would
be more favorable raw starting materials for the preparation of
mesoporous materials.33,34 Although many studies referring to
the synthesis of ordered mesoporous materials, such as MCM-41
from kaolin, talc, bentonite, etc. have been reported,35–37 the
relative work about SBA-15 being synthesized from natural
minerals is limited.38
which is restricted by USEPA to less than 4.8 mg Lꢀ1 4,5 Thus, the
.
efficient control and degradation of 2-nitrophenol through a
green and economic route is a most urgent task worldwide.
Many techniques have been proposed to treat hazardous
2-nitrophenol, including conventional biological oxidation,6,7
solvent extraction,8 thermal treatment and adsorption,9,10 etc. By
comparison, catalytic wet peroxide oxidation (CWPO), an
advanced oxidation process, is expectedly attractive due to its
mild conditions, simple equipment and being environmentally
friendly.11 This process mostly involves some suitable catalyst,
in particular iron based catalysts in the presence of hydrogen
peroxide (H2O2), to improve the degradation of 2-nitrophenol
through the formation of active hydroxyl radicals (cOH).12
College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot,
Inner Mongolia 010021, P. R. China. E-mail: cedchf@imu.edu.cn; cezlliu@imu.edu.
cn; Fax: +86-471-4995414; Tel: +86-471-4995414
† Electronic supplementary information (ESI) available: Details of TEM images,
EDS spectra, adsorption and degradation data of various samples. See DOI:
10.1039/c5ra01804h
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In the present work, natural kaolin was chosen as a raw FCSBA-15 was added into the solution and the mixture was then
material to synthesize SBA-15 with an enhanced hydrothermal transferred into a 100 mL Teon-lined stainless steel autoclave,
ꢁ
stability in the presence of a uorocarbon surfactant (FSO-100). heated up to 110 C and kept at this temperature for 8 h. The
Furthermore, Fe3+ modied BiOCl particles loaded onto the resulting precipitate was collected by centrifugation, washed
SBA-15 was used as a heterogeneous green catalyst to degrade with ethanol and deionized water several times, then dried at
2-nitrophenol in the presence of H2O2. Various reaction param- 60 ꢁC under vacuum and treated at 400 ꢁC for 3 h with a heating
eters, including H2O2 dosage, pH value, reaction temperature, rate of 3 C minꢀ1. The nal composite was marked as (y)Fe/
ꢁ
Fe/Bi molar ratio and Bi/Si molar ratio were investigated.
BiOCl–FCSBA-15(x), where x and y indicate the molar ratio of
BiOCl to FCSBA-15 and Fe to BiOCl, respectively.
2. Experimental
2.1 Materials
2.4 Characterization
Detailed crystallographic information and evidence of the
porous structures of the synthesized samples were obtained
with an X-ray diffractometer (Empyrean Panalytical) with Cu Ka
radiation (l ¼ 0.15406 nm) over a scanning range of 2q ¼ 1–10ꢁ
for small angle XRD (SAXRD) and 2q ¼ 10–80ꢁ for wide angle
XRD (WAXRD). The detailed morphologies and structures of the
samples were recorded by transmission electron microscopy
(TEM) and high resolution TEM (HRTEM) on a JEM-2010
apparatus with an acceleration voltage of 200 kV. The surface
states and chemical compositions of the samples were analyzed
by X-ray photoelectron spectroscopy (XPS), which was carried
out on a Thermo Escalab 250Xi with monochromatic Al Ka
radiation (hv ¼ 1486.6 eV). The specic surface areas (SBET) of
the samples were obtained from N2 adsorption–desorption
isotherms at 77 K (ASAP 2020). Prior to the sorption experiment,
the materials wꢁere dehydrated by evacuation under specic
conditions (200 C, 10 h).
Natural kaolin from Maoming, China was used as a raw mate-
rial to synthesize the mesoporous material FCSBA-15. The main
chemical composition of the kaolin were as follows (wt%): SiO2,
46.90; Al2O3, 37.80; MgO, 0.22; Fe2O3, 0.41; TiO2, 0.25; CaO,
0.15; K2O, 0.12; Na2O, 0.18; and the loss on ignition was 14.00.
Analytical grade chemicals of Bi(NO3)3$5H2O, Fe(NO3)3$9H2O,
KCl, glycerol, NaOH, HCl were purchased from the Shanghai
Chemical Industrial Co., Pluronic P123 was purchased from
Sigma-Aldrich Co. and the uorocarbon surfactant (FSO-100,
F(CF2CF2)mCH2CH2O(CH2CH2O)nH, m ¼ 1–7, n ¼ 0–15) was
purchased from DuPont Co., all of which were used as the
starting materials without further purication.
2.2 Fluorocarbon surfactant assisted preparation of FCSBA-
15 from kaolin
Kaolin was rst treated at 850 ꢁC to transform it into metakaolin,
this was then leached with 6 M HCl at 80 ꢁC for 4 h. The collected
solid was washed with deionized water several times until the pH
value of the washed water became about 7.0. The obtained solid
2.5 Degradation of 2-nitrophenol over Fe/BiOCl–FCSBA-15
The typical degradation procedure was as follows: 0.1 g of the
prepared sample was added into 50 mL of 1 ꢂ 10ꢀ4 M 2-nitro-
phenol solution at 40–80 ꢁC with a pH value of 2.0–6.0 adjusted
by 0.1 M HCl. Then the homogeneous suspension was stirred
for 20 min to ensure adsorption–desorption equilibrium. A
certain amount of H2O2 was then added. At a given time
interval, 3 mL of the suspension was taken out and then
centrifuged for the subsequent concentration measurement.
The 2-nitrophenol concentration was checked using a Hitachi
U-3900 spectrophotometer.
ꢁ
was dried overnight and then calcined at 950 C for 2 h again.
This was then mixed with 5 M NaOH and hydrothermally treated
in the conditions of 160 ꢁC and 0.5 MPa for 3 h (solution A). The
pH value of the obtained solution A was adjusted to 12.0 with 3 M
HCl. The uorocarbon surfactant FSO-100 and P123, with a
molar ratio of FSO-100/P123 ¼ 0.18, were mixed and dissolved in
a solution of 25 mL HCl and 15 mL H2O at 40 ꢁC (solution B) for
30 h. Aer that, solution A was dropped into solution B and the
mixture was transferred into a 100 mL Teon-lined stainless
steel autoclave and kept at 160 ꢁC for 48 h. The resultant white
product was ltered, washed several times with deionized water
and ethanol, dried at 100 ꢁC overnight, and then treated at
550 ꢁC in air for 5 h. The nal product was denoted as FCSBA-15.
For comparison, the counterpart SBA-15 without the uoro-
3. Results and discussion
3.1 Characterization of FCSBA-15 and its hydrothermal
stability
ꢁ
carbon surfactant was also prepared at 100 C, the procedure
It is known that uorocarbon surfactants are stable and can
endure high temperatures (>200 ꢁC), but they are not suitable as
templates for the formation of ordered mesostructures due to
the rigidity and strong hydrophobicity of the uorocarbon
chains.39,40 Expectably, the combination of a uorocarbon
surfactant and a hydrocarbon surfactant can induce various
used was the same as that of FCSBA-15 except for the addition of
the uorocarbon surfactant and hydrothermal temperature.
2.3 Synthesis of Fe/BiOCl loaded onto FCSBA-15 (Fe/BiOCl–
FCSBA-15)
In a typical procedure, 0.776 g Bi(NO3)3$5H2O and 0.129 g ordered mesoporous structures41,42 and improve the hydro-
Fe(NO3)3$9H2O were dissolved in 76 mL glycerol, denoted as thermal stability of the mesoporous materials,43 which is help-
solution I. Then, 0.12 g KCl was dissolved in 4 mL deionized ful in Fenton-like reaction processes.
water (solution II), which was subsequently poured into solu-
Fig. 1 shows the SAXRD patterns of FCSBA-15, SBA-15 and
tion I and stirred. Aer stirring for 15 min, a certain amount of their counterparts aer hydrothermal treatment. The
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Fig. 1 SAXRD patterns of FCSBA-15 and SBA-15 before and after
hydrothermal treatment.
hydrothermal treatment (HT) waꢁs done in a 100 mL Teon-lined
stainless steel autoclave at 100 C for 12 h. As can be seen in
Fig. 1, there is a distinct peak corresponding to the (100) plane
of SBA-15 for the prepared products, with or without the uo-
rocarbon surfactant, which indicates that SBA-15 with an
ordered mesostructure could be successfully synthesized from
natural kaolin, even in the presence of the uorocarbon
surfactant FSO-100. Aer hydrothermal treatment, the peak
intensity of the crystalline (100) plane for FCSBA-15 was main-
tained but it decreased for SBA-15, which implies that the
prepared FCSBA-15 in the presence of the uorocarbon surfac-
tant displayed better hydrothermal stability than SBA-15 in the
absence of the uorocarbon surfactant.
To further investigate the porous properties of the synthe-
sized samples, N2 adsorption–desorption measurements were
used to characterize the samples. The N2 adsorption–desorp-
tion isotherms and pore size distributions of FCSBA-15 and
SBA-15, as well as the counterparts aer hydrothermal treat-
ment, are shown in Fig. 2. As can be seen in Fig. 2, the shapes of
the N2 adsorption–desorption isotherms of FCSBA-15 and SBA-
15 are typical IV type isotherms, corresponding to the standard
mesoporous materials. The maximum pore size distributions
were 6.2 nm and 3.9 nm for FCSBA-15 and SBA-15, respectively.
Aer hydrothermal treatment, there was an obvious decrease in
the N2 adsorption amount for SBA-15 in comparison with
FCSBA-15. Furthermore, the peak broadening in the pore size
distribution curve conrms the collapse of the mesoporous
structure of SBA-15 aer hydrothermal treatment. The relative
parameters of the porous properties for FCSBA-15 and SBA-15
before and aer hydrothermal treatment are listed in Table 1.
The maintained specic surface area and pore volume, as well
as the average pore diameter, further conrm the improved
hydrothermal stability of FCSBA-15 in contrast to SBA-15.
Fig. 3 presents TEM images of FCSBA-15 and SBA-15 before
and aer hydrothermal treatment. As shown in Fig. 3(a)–(c),
Fig. 2 N2 adsorption–desorption isotherms and pore size distribu-
tions of FCSBA-15 and SBA-15 before and after hydrothermal
treatment.
Table 1 The relative parameters of the porous properties of FCSBA-15
and SBA-15 before and after hydrothermal treatmenta
Average pore
diameter (nm)
Pore volume
SBET (m2 gꢀ1
)
(cm3 gꢀ1
)
Samples
HTb
HTa
HTb
HTa
HTb
HTa
FCSBA-15
SBA-15
281.3
787.3
272.4
276.0
8.7
4.1
8.5
6.9
0.69
0.62
0.68
0.44
a
HTb values are before hydrothermal treatment and HTa values are
aer hydrothermal treatment.
hydrothermal stability than SBA-15, which is expected to be
benecial for an application in a Fenton-like reaction process.
3.2 Characterization of catalyst Fe/BiOCl–FCSBA-15
both FCSBA-15 and SBA-15 exhibited well ordered meso- Fig. 4 shows the XRD patterns of the Fe/BiOCl–FCSBA-15 cata-
structures. Aer hydrothermal treatment for 12 h, the uniformly lysts with different Bi/Si and Fe/Bi molar ratios. All the diffrac-
ordered mesostructure of FCSBA-15 remained (Fig. 3(b)) but it tion peaks were in good agreement with those of the BiOCl
collapsed for SBA-15 (Fig. 3(d)). All of the information standard (JCPDS 06-0249). The narrow sharp peaks suggest that
mentioned above indicates that FCSBA-15 exhibits higher the as-prepared catalysts are highly crystalline. The diffraction
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peaks of BiOCl in Fig. 4(a) become stronger with the increasing
Bi/Si molar ratio. In Fig. 4(b), no diffraction peaks referring to
iron species are observed, even when the Fe/Bi molar ratio
reaches 1.0, which is in agreement with results of Cu-modied
BiOCl.44 In Cu-modied BiOCl, no typical patterns of copper
species were observed even when the Cu/Bi molar ratio was
10 : 1, which was ascribed to the Cu ions being present on the
surface of the BiOCl. Furthermore, no obvious peak shi is
observed when increasing the Fe/Bi molar ratio in Fig. 4(b),
implying that the Fe3+ ions were not doped into the lattice or
substituted for Bi3+ ions in BiOCl.45 Thus, it can be concluded
that Fe3+ ions were present on the surface of the BiOCl.
The morphology and crystal structure of the catalyst 0.5Fe/
BiOCl–FCSBA-15 (0.6) were obtained using the TEM technique.
As shown in Fig. 5(a), Fe3+ ion modied BiOCl with a ower-like
and nanosheet morphology was loaded onto mesoporous
FCSBA-15. The EDS spectrum (Fig. 5(b)) reveals the co-presence
of Fe, Bi, O, Cl and Si elements in the catalyst 0.5Fe/BiOCl–
FCSBA-15 (0.6). The HRTEM image (Fig. 5(c)) is a magnied
micrograph of part 1 (ower-like morphology) in Fig. 5(a), which
clearly presents the lattice fringes of BiOCl with a d spacing of
0.276 nm, matching well with the (110) crystalline plane of
BiOCl.
Fig. 3 TEM images of FCSBA-15 (a) and SBA-15 (b) and counterparts
after hydrothermal treatment (c and d).
Fig. 5(d) shows the magnied micrograph of part 2 (nano-
sheet morphology) in Fig. 5(a), which also displays the distinct
lattice fringes corresponding to the (110) crystalline plane of
BiOCl. The selected area electron diffraction (SAED) pattern
(inset in Fig. 5(d)) displays a spot pattern, suggesting a single-
crystalline structure and it presents different crystalline
planes of BiOCl of (110), (200) and (310). In comparison with
pure BiOCl, which was composed of nanoplates displaying
approximate round edges with a diameter of 50 nm and width of
10 nm, the Fe/BiOCl sample was mainly composed of hierar-
chical micro-owers with a small number of square nanosheets
(Fig. S1†). It has been reported that the introduction of Fe3+ ions
Fig. 4 XRD patterns of 0.5Fe/BiOCl–FCSBA-15 with different Bi/Si
molar ratios (a) and Fe/BiOCl–SBA-15 (0.6) with different Fe/Bi molar
ratios (b).
Fig. 5 TEM image (a), EDS spectrum (b) of catalyst 0.5Fe/BiOCl–
FCSBA-15 (0.6) and magnified micrographs of part 1 (c) and part 2 (d) in
(a). The inset in (d) is the SEAD pattern of part 2.
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can induce a morphology transformation in bismuth oxy- which is characteristic of the Bi3+ in the catalyst 0.5Fe/BiOCl–
halides.46,47 The results from the TEM images in our work FCSBA-15 (0.6). In Fig. 6(c), there are two obvious peaks located
indicate that the Fe3+ ions can induce the self-assembly of BiOCl at 198.1 eV and 199.8 eV, which are assigned to Cl 2p3/2 and Cl
nanosheets to form hierarchical micro-owers. For Fe doped 2p1/2, respectively. The high-resolution spectrum (Fig. 6(d))
BiOBr, the morphology transformation was ascribed to the shows that the binding energies of 532.8 eV and 530.2 eV
oriented attachment and Ostwald ripening.46 The EDS spectrum correspond to O 1s, with the former assigned to the defect-oxide
indicates that there were only Bi, O, Cl and Si elements in the and hydroxyl-like groups48 and the latter assigned to the lattice
nanosheet Fe/BiOCl–FCSBA-15, but Fe, Bi, O, Cl and Si elements oxygen in Bi–O and Si–O bonds. As depicted in Fig. 6(e), the
were present in the ower-like samples (Fig. S2†), which further binding energies of Fe 2p were observed at 724.2 eV and 710.7 eV,
suggests that the Fe modied BiOCl nanosheets were more which are assigned to Fe 2p1/2 and Fe 2p3/2 for Fe3+, respectively.
favorable to assembling into ower-like structure.
The contribution at around 720.0 eV belongs to the surface iron
To determine the surface composition and the chemical species and satellite signal of Fe3+ species.49 The XPS technique
states of the elements, as well as to better understand the roles was also used to determine the chemical composition. From the
of the iron ions in Fenton-like catalytic oxidation, the catalyst XPS data, the molar ratios of Fe/Bi and Bi/Si were estimated to
0.5Fe/BiOCl–FCSBA-15 (0.6) was studied by XPS (Fig. 6). The be 0.36 and 0.57, respectively, which were close to the original
survey XPS spectrum indicates the co-presence of Bi, O, Cl, Fe, Si molar ratios of Fe/Bi and Bi/Si.
and C elements. The C peak was from the adventitious carbon
Fig. 7 presents the N2 adsorption–desorption isotherms of
on the surface of the sample. As shown in the high-resolution FCSBA-15 and 0.5Fe/BiOCl–FCSBA-15 with different amounts of
spectrum (Fig. 6(b)), the two strong peaks located at 164.7 eV BiOCl. As displayed in Fig. 7, no obvious shape change in the N2
and 159.4 eV are assigned to Bi 4f5/2 and Bi 4f7/2, respectively, adsorption–desorption isotherm is observed when increasing
the Bi/Si molar ratio. However, the N2 adsorption amount, as
well as SBET, decreased when increasing the Bi/Si molar ratio.
The SBET value decreased from 247.9 m2 gꢀ1 for FCSBA-15 to
115.6 m2 gꢀ1 for 0.5Fe/BiOCl–FCSBA-15 with a Bi/Si molar ratio
of 0.8. The decreased N2 adsorption amount and SBET value
could be ascribed to the big particle size of the ower-like
Fe/BiOCl, which greatly occupies the mesostructure of FCSBA-
15 and thus inhibits the N2 molecules from entering the mes-
ochannels of the mesoporous material.
3.3 Evaluation of the degradation efficiency
The inuence of various parameters, such as H2O2 dosage, pH
value, degradation temperature, Fe/Bi molar ratio and Bi/Si
molar ratio, on the degradation efficiency toward 2-nitro-
phenol are displayed in Fig. 8. Before the degradation process,
the catalysts were immersed in the 2-nitrophenol solution for
20 min to ensure an adsorption–desorption equilibrium. It is
Fig. 6 XPS spectra of the catalyst 0.5Fe/BiOCl–FCSBA-15 (0.6): survey Fig. 7 N2 adsorption–desorption isotherms of 0.5Fe/BiOCl–FCSBA-15
scan (a), Bi 4f (b), Cl 2p (c), O 1s (d), Fe 2p (e).
with different Bi/Si molar ratios.
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lower degradation rate.52 Thus, a reasonable concentration of
H2O2 is indispensable for Fenton and Fenton-like reactions.
The degradation efficiency for 2-nitrophenol can reach nearly
100% within 40 min over the catalyst 0.5Fe/BiOCl–FCSBA-15
(0.6) with the concentration of H2O2 being 800 mg Lꢀ1. For
comparison, the degradation reaction with H2O2 in the absence
of the catalyst was also conducted (Fig. S4†), the result of which
showed that the 2-nitrophenol solution is relatively stable in the
presence of H2O2 without the catalyst Fe/BiOCl–SBA-15 and the
degradation efficiency only reaches about 30%. Thus, the co-
presence of H2O2 and the catalyst enhances the degradation
efficiency for 2-nitrophenol.
Fig. 8(b) displays the effect of the pH value on the degra-
dation efficiency toward 2-nitrophenol over the catalyst Fe/
BiOCl–FCSBA-15. It has been reported that an acid environ-
ment is favorable for generating the maximum amount of cOH,
which is helpful for the Fenton-like reaction.53 Thus, the
adopted range of pH values for the degradation of 2-nitro-
phenol was between 2 and 6. As shown in Fig. 8(b), the
degradation rate of 2-nitrophenol is optimal at a pH value of 4.
Furthermore, the degradation efficiency can reach 81% even in
weak acid conditions (pH ¼ 6). It also can be observed that the
strong acid environment does not facilitate the degradation
reaction. This phenomenon is attributed to the easy decom-
position of H2O2, which is due to the scavenging of cOH by H+
ions to form O2 and H2O51 or the scavenger effect of Clꢀ on
cOH.54
Fig. 8 Effects of various variables on 2-nitrophenol degradation by the
catalyst Fe/BiOCl–FCSBA-15; (a) effect of H2O2 dosage (T ¼ 60 ꢁC,
pH ¼ 4, Fe/Bi molar ratio ¼ 0.5, Bi/Si molar ratio ¼ 0.6); (b) effect of pH
value (T ¼ 60 ꢁC, H2O2 dosage ¼ 800 mg Lꢀ1, Fe/Bi molar ratio ¼ 0.5,
Bi/Si molar ratio ¼ 0.6); (c) effect of degradation temperature (pH ¼ 4,
H2O2 dosage ¼ 800 mg Lꢀ1, Fe/Bi molar ratio ¼ 0.5, Bi/Si molar ratio ¼
0.6); (d) effect of Fe/Bi molar ratio (T ¼ 60 ꢁC, pH ¼ 4, H2O2 dosage ¼
800 mg Lꢀ1, Bi/Si molar ratio ¼ 0.6); (e) effect of Bi/Si molar ratio (T ¼
60 ꢁC, pH ¼ 4, H2O2 dosage ¼ 800 mg Lꢀ1, Fe/Bi molar ratio ¼ 0.5); (f)
determination of reaction rate constants in the degradation of 2-
nitrophenol over the catalyst Fe/BiOCl–FCSBA-15 with different Fe/Bi
molar ratios (T ¼ 60 ꢁC, pH ¼ 4, H2O2 dosage ¼ 800 mg Lꢀ1, Bi/Si
molar ratio ¼ 0.6).
The degradation of 2-nitrophenol over the catalyst Fe/BiOCl–
FCSBA-15 was also studied by varying the degradation temper-
ature from 40 ꢁC to 80 ꢁC, which is plotted in Fig. 8(c). As
depicted in Fig. 8(c), the degradation rate increased with raising
the temperature. At 40 ꢁC, a y percent degradation efficiency
ꢁ
was observed and at 60 C, nearly 100% degradation efficiency
was obtained within 40 min. It has been reported that an
elevation of temperature can accelerate the generation of cOH,
which indicates that there would be less H2O2 available for
scavenging the cOH. At a temperature of 40 ꢁC, the lower
degradation efficiency was ascribed to the slow generation of
cOH,51 and the higher degradation efficiency at 60 ꢁC may be
due to the faster production of cOH. Aer the temperature
reaches 80 ꢁC, the degradation efficiency was not enhanced
dramatically, but the degradation rate was improved dramati-
cally, which was in accordance with the results of
Inchaurrondo.54
The optimization study for the Fe dosage amount was carried
out by varying the Fe/Bi molar ratio from 0.2 to 1.0, which is
presented in Fig. 8(d). It can be observed that the maximum 2-
nitrophenol degradation efficiency was achieved with a Fe/Bi
molar ratio of 0.5, and the degradation efficiency decreased
with both an increase and a decrease from the Fe/Bi molar ratio
of 0.5. The increased availability of active sites for the genera-
tion of radicals is responsible for the enhanced degradation
efficiency, and the decomposition of H2O2, caused by the high
catalytic activity of the extra catalyst, may be the reason for the
decreased degradation efficiency. Furthermore, Zhan55
proposed that in the conditions of a high catalyst dosage, the
cOH may react immediately with the intermediate compounds
seen that the Fe/BiOCl–FCSBA-15 catalyst showed no apparent
adsorption toward 2-nitrophenol (Fig. S3†). It is well known that
H2O2 plays a vital role in the degradation of dyes with Fenton
and Fenton-like reactions.50 Thus, the inuence of the H2O2
dosage on the degradation of 2-nitrophenol was investigated. As
shown in Fig. 8(a), the degradation efficiency was improved
when increasing the amount of H2O2, whereas the degradation
efficiency obviously decreased with an excessive H2O2 dosage of
1200 mg Lꢀ1. At a lower concentration of H2O2, there are only a
small number of cOH radicals produced. So the limited cOH
cannot initiate and nish the degradation reaction rapidly and
completely. When increasing the amount of H2O2 to more than
800 mg Lꢀ1, the extra H2O2 would be a powerful scavenger
which reacts with cOH to generate HO2c, O2 or H2O:50,51
ꢀH2O
þcOH
H O þ OH
HO
O þ H O
ƒƒƒƒƒ!
ƒƒƒƒƒ!
2
2
2
2
2
c
c
The generated HO2c are much less reactive and have no
contribution to the degradation reaction, which causes the
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produced from dye degradation, which can result in the ions are present on the surface of BiOCl, just like Cu-modied
reduced utilization of H2O2 for the further degradation of dyes. BiOCl,44 which indicates that the Fe3+ ions do not exist in a
The inuence of the BiOCl dosage on the 2-nitrophenol crystalline state, therefore they are very unstable and favorable
degradation efficiency was evaluated by changing the Bi/Si molar to be leached into the solution.
ratio over the range of 0.2 to 0.8. As indicated in Fig. 8(e), the
degradation rate accelerated rst and then slowed down when
increasing the BiOCl amount, reaching a maximum efficiency
with the Bi/Si molar ratio at 0.6. For comparison, the catalyst
Fe/FCSBA-15 without BiOCl was also synthesized and the cata-
lytic properties for 2-nitrophenol degradation were also studied
(Fig. S5†). As shown in Fig. S5,† the degradation efficiency of the
catalyst Fe/BiOCl–FCSBA-15 was higher than that of the catalyst
Fe/FCSBA-15. Thus, it is concluded that the degradation effi-
ciency of 2-nitrophenol is greatly improved by BiOCl. To further
conrm the role of BiOCl in the 2-nitrophenol degradation
process, both pure BiOCl and Bi2O3 were synthesized and their
degradation behavior toward 2-nitrophenol were also checked
(Fig. S6†). It can be seen that both BiOCl and Bi2O3 show a
degradation efficiency toward 2-nitrophenol, which may be
ascribed to Bi ions. In addition, the pure Fe/BiOCl catalyst
without the matrix FCSBA-15 was also prepared and its catalytic
behaviour for 2-nitrophenol degradation is displayed in Fig. S7.†
The degradation results suggestthat the catalyst Fe/BiOCl loaded
onto FCSBA-15 shows a better catalytic performance than pure
Fe/BiOCl, indicating that the porous matrix has an important
role in the Fenton-like reaction. Considering these results, it can
be concluded that the catalyst Fe/BiOCl–FCSBA-15 with a Fe/Bi
molar ratio of 0.5 and a Bi/Si molar ratio of 0.6, with degrada-
tion conditions of H2O2 dosage ¼ 800 mg Lꢀ1, pH value ¼ 4.0 and
T ¼ 60 ꢁC, displays the best degradation efficiency toward
2-nitrophenol, which reached nearly 100% within 40 min.
The 2-nitrophenol concentration versus the reaction time
was further analysed using the equation for pseudo-rst-order
kinetics ln(C/C0) ¼ ꢀkt, where C0 and C are the 2-nitrophenol
concentration in the solution at time t ¼ 0 and t, respectively,
and k is the pseudo-rst-order rate constant. The degradation
rates of 2-nitrophenol over the Fe/BiOCl–FCSBA-15 (0.6) cata-
lysts with different Fe/Bi molar ratios are displayed in Fig. 8(f).
The degradation rates as inuenced by the other parameters,
including H2O2 dosage, pH value, degradation temperature and
Bi/Si molar ratio are shown in Fig. S8.† The related calculated
degradation constants k are listed in Table S1.† As shown in
Fig. 8(f), S8 and Table S1,† the catalyst 0.5Fe/BiOCl–FCSBA-15
3.4 Possible mechanism for the degradation of 2-
nitrophenol
It has been proposed that for heterogeneous Fenton-like reac-
tions, the Fe ions form complexes with peroxide species and
this is then followed by steps of dissociation reactions to
generate cOH, which plays a vital role in the Fenton-like
oxidation process.52 The possible mechanism for hydroxyl
radical generation by Fe/BiOCl–FCSBA-15 is given in Fig. 9.
Generally, under acidic or neutral conditions, a complex
between Fe3+ and H2O2 is formed through H2O2 activation by
the catalysts.56 The complex subsequently converts to Fe2+ and
HO2c, and the generated HO2c further reacts with Fe3+ to
produce Fe2+. Then, all the produced Fe2+ reacts with H2O2 to
generate the cOH, which is prepared for participating in the
degradation process of 2-nitrophenol. The possible reactions
involved can be summarized as eqn (1)–(3).
Fe3+ + H2O2 / Fe2+ + HO2c + H+
Fe2+ + H2O2 / Fe3+ + cOH + OHꢀ
(1)
(2)
2-Nitrophenol+2cOH/12CO2+HNO2+HNO3+31H2O (3)
Moreover, the presence of Bi ions was also favorable for the
degradation efficiency of 2-nitrophenol, and thus the combined
effect of the Fe ions and Bi ions is responsible for the excellent
degradation efficiency for the catalyst Fe/BiOCl–FCSBA-15 on
2-nitrophenol.
To further evaluate the important role of cOH, various scav-
engers including isopropanol (IPA) and methanol (MeOH) were
used (Fig. 10). As shown in Fig. 10, large decreases are seen in
the degradation efficiency for 2-nitrophenol, from nearly 100 to
20 and 30% aer adding IPA and MeOH, respectively. The
results strongly suggest that a large number of cOH radicals are
generated in the oxidation process, which have a prominent
role in the degradation of 2-nitrophenol.
(0.6) in oxidation conditions of H2O2 dosage ¼ 800 mg Lꢀ1
,
ꢁ
pH ¼ 4 and T ¼ 60 C (consideration of energy consumption)
showed the fastest degradation rate of 9.85 ꢂ 10ꢀ2 minꢀ1
.
To check the structural stability of the synthesized catalysts,
the XRD pattern of the catalyst 0.5Fe/BiOCl–FCSBA-15 (0.6) aer
the degradation of 2-nitrophenol was measured, and is shown
in Fig. S9.† It can be observed that there are no obvious peak
changes between the XRD patterns of the catalyst before and
aer the degradation process, which indicates the structural
stability of the synthesized catalysts. Furthermore, the reus-
ability of the catalyst 0.5Fe/BiOCl–FCSBA-15 (0.6) was also
investigated. However, the result was not ideal and needs to be
further improved (Fig. S10†), which may be related to the Fe3+
ions leaching into the solution. As described in Fig. 4, the Fe3+ Fig. 9 Hydroxyl radical generation mechanism.
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ꢀ
ꢀ
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ordered mesoporous material SBA-15 (FCSBA-15) the assistance
of uorocarbon surfactant. The material showed a specic
surface area (SBET) of 247.9 m2 gꢀ1, a pore volume of 0.69 m3 gꢀ1
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC, no. 51102129 and 51462025) and
the Inner Mongolia Provincial Natural Science Foundation of
China (no. 2013MS0204).
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