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SBA-15 nanomaterials
Accepted Manuscript Preparation and photocatalytic application of magnetic Fe2O3/ SBA-15 nanomaterials
Junhong Wang, Xianzhao Shao, Qiang Zhang, Jianqi Ma, Hongguang Ge PII: DOI: Reference:
S0167-7322(18)30588-9 doi:10.1016/j.molliq.2018.03.109 MOLLIQ 8883
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
2 February 2018 22 March 2018 26 March 2018
Please cite this article as: Junhong Wang, Xianzhao Shao, Qiang Zhang, Jianqi Ma, Hongguang Ge , Preparation and photocatalytic application of magnetic Fe2O3/SBA-15 nanomaterials. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:10.1016/j.molliq.2018.03.109
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ACCEPTED MANUSCRIPT Preparation and photocatalytic application of magnetic Fe2O3/SBA-15 nanomaterials
Authors:
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The affiliations and addresses of the authors:
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Junhong Wang*, Xianzhao Shao, Qiang Zhang, Jianqi Ma, Hongguang Ge
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Shaanxi Key Laboratory of Catalysis, College of Chemical and Environment Science, Shaanxi University of Technology, Hanzhong 723000, PR China
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Corresponding authors:
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Junhong Wang
Shaanxi Key Laboratory of Catalysis, College of Chemical and Environment
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Tel: +86 0916- 2641660
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Science, Shaanxi University of Technology, Hanzhong 723000, PR China
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Email: [email protected]; [email protected]
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ACCEPTED MANUSCRIPT Abstract: Magnetic Fe2O3/SBA-15 mesoporous nanomaterials were synthesized by means of a facile impregnation and calcination method. As-prepared magnetic nanomaterials were characterized using XRD, N2 adsorption-desorption isotherms, SEM, TEM, VSM and XPS techniques. And the photocatalytic activities of magnetic Fe2O3/SBA-15 on the degradation of rhodamine B were also evaluated. Results
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indicate that the order structure of as-synthesized nanomaterials still remain, but its specific surface area, pore volume and pore size decrease. The saturation
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magnetization of the samples increases and its particle size decreases gradually with
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increasing the calcination temperature. And the existence of SBA-15 mesoporous molecular sieves increases largely the stability of γ-Fe2O3. Moreover, as-synthesized
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magnetic Fe2O3/SBA-15 shows high photocatalytic activities in the course of the degradation of rhodamine B by photo-Fenton-like reaction at natural pH about 6.3 and
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visible light irradiation. The structure and amount of catalyst, the concentration of H2O2 and rhodamine B are main influence factors on the degradation of rhodamine B, and these influence results have been depicted quantitatively from dynamics.
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Rhodamine B in aqueous solution could almost be removed completely after a period
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of illumination by visible light and overnight in the dark. Meanwhile, as-prepared magnetic Fe2O3/SBA-15 has excellent reusability and stability, which its
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photocatalytic activity has only a slightly decrease after five consecutive runs. Therefore, as-synthesized magnetic Fe2O3/SBA-15 has a broad prospect of practical
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application.
Keywords: magnetization, photocatalyst, Fe2O3/SBA-15, Fenton-like reaction, degradation
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ACCEPTED MANUSCRIPT 1. Introduction Environmental protection problem is an important subject that human society is facing today. With the rapid development of human society and industrialization, more and more waste is discharged into the environment, causing serious pollution of water quality, air and soil [1]. How to clean up these pollutants and restore a good
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living environment for humanity has become a serious challenge for scientists and technicians.
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To remove heavy metal, organic dyes, halogenated pollutants, polycyclic
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aromatic hydrocarbon and other pollutants in water, many methods have been used such as adsorption, chemical precipitation, catalytic and photocatalytic reaction [2-6].
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Among of the present methods, photocatalytic degradation of pollutants under visible light irradiation is an efficient and clean technology, which can make full use of the
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solar energy and save fossil resources. Due to its low band-gap 2.1 eV [7], high efficiency and low-cost, FexOy has been extensively used in the degradation of many pollutants as catalyst or photocatalyst [8-10]. It is usually known that Fenton reaction
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as an advanced oxidation process has been widely used in degradation of many toxic
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and non-biodegradable organic pollutants [11-14]. The combination of Fenton reaction and photocatalytic reaction, namely, photo-Fenton reaction, is an important
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reaction process and will have a broad application prospect in the field of environmental wastewater remediation [11, 15].
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Metal oxide nanoparticles have a widely application in adsorption, catalysis, drug delivery and so on, but they are easy to aggregation and leaching during reaction so that prevents them from fully functioning. Therefore, metal oxides are usually loaded onto a carrier in order to increasing their specific surface areas, dispersivity and catalytic performance [16, 17]. Therein, SBA-15 mesoporous molecular sieve is an excellent carrier, which has big specific surface areas, big pore diameters, ordered channel structure and higher thermal and hydrothermal stability [18-20]. Yan et al [19] synthesized Fe/SBA-15 to use for the ozonation of oxalic acid and got a high removal efficiency of 86.6%. Barros et al [21] prepared ZnO/SBA-15 and MgO/SBA-15 and 3
ACCEPTED MANUSCRIPT used to catalytic esterification reaction of lauric acid with 1-butanol under atmosphere pressure and reflux conditions, giving to 81% yield of butyl ester. Wang et al [22] synthesized SBA-15 supported iron oxide materials to catalytic removal of hydrogen sulfide and gained a remarkable effect. Karthikeyan et al [13] synthesized the nanomaterials of Cu and Fe oxide dispersed on SBA-15. This kind of material
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revealed a high catalytic activity for the oxidative degradation of N, N-diethyl-pphenyl diamine (DPD). Our research group [23] prepared Cu/SBA-15 for catalytic
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reduction of p-nitrophenol to p-aminophenol and obtained over 99% conversion ratio. Gaudin et al [24] synthesized CuO/SBA-15 by the solid state grinding method and
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showed a better adsorption efficiency to removal of SOx.
To improve the reusability and practicability of catalysts, magnetic Fe2O3/
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SBA-15 mesoporous nanomaterials were prepared by simple incipient wetness impregnation and burning method at different calcination temperatures. Then, the
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photocatalytic activities of as-synthesized Fe2O3/SBA-15 nanoparticles as photocatalyst were tested by photo-Fenton reaction to degrade organic dye
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2. Experimental
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Rhodamine B under visible light irradiation and original solution pH condition.
2.1. Chemicals and materials
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Tetraethoxysilane (TEOS), Pluronic P123 (poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol)) and rhodamine B (RhB) were bought
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from Sigma-Aldrich; poly (vinyl alcohol) (PVA) was purchased from Beijing chemical reagent three factory; ferric ammonium citrate was purchased from Sinopharm Beijing Co., Ltd. (Fe content 20.5-22.5%); hydrochloric acid was obtained from Tianjin Guangfu fine chemical institute. All of these reagents were used as received without further purification. 2.2. Synthesis of SBA-15 SBA-15 mesoporous molecular sieves were synthesized according to our previous literature [18], but made a simple modification. In a typical procedure, 4 g of P123 was added into a 200 mL of beaker with 120 mL of 0.75 M HCl and 30 mL of 4
ACCEPTED MANUSCRIPT deionized H2O. After P123 dissolved completely, 2 g of PVA was added into the above solution with stirring, and added 8.6 g of TEOS after 4 h. Then, the above viscous liquid was stirred continuously for 20 h at 35 ºC and hydrothermal crystallized for another 24 h at 100 ºC static condition. At last, SBA-15 was gained by filtered, washed, dried and calcined at 550 ºC for 5.5 h.
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2.3. Preparation of magnetic Fe2O3/SBA-15 Magnetic Fe2O3/SBA-15 mesoporous material was prepared by a simple dipping
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and calcination method. Typically, 1.0 g of as-synthesized SBA-15 was immersed in 20 mL of deionized H2O with 6.0 g of ferric ammonium citrate overnight. Then the
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loads were filtered and washed with deionized water and ethanol three times, dried at 60 ºC, calcined in static air for 6 h at different temperature (300, 400 and 550 ºC) ,
and S3, respectively.
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2.4. Characteristics of Fe2O3/SBA-15
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obtained magnetic Fe2O3/SBA-15 mesoporous nanomaterials and marked as S1, S2
X-ray diffraction (XRD) measure was completed with a Netherlands X'Pert PRO
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power diffractometer using Cu Kα radiation. The data were collected by scanned the
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samples in the 2θ ranges from 10 to 90 with a resolution of 0.02º. N2 adsorptiondesorption isotherms were measured on a BelSorp Max volumetric adsorption analyzer at -196 ºC. The samples were degassed for 6 h at 180 ºC under vacuum
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before measurement. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area and the Barrett-Joyner-Halenda (BJH) model was
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utilized to measure the pore size distributions by the adsorption branch of the isotherm. Scanning electron microscopy (SEM) images were obtained by a Hitachi S4800 field emissions scanning electron microscopy. Transmission electron microscopy (TEM) images were recorded on a FEI Tecnai G2 F20 S-Twin microscope operated at 200 kV. The static magnetic properties of samples were measured through an American Lake Shore VSM 7307 Vibrating sample magnetometer (VSM) with a sensitivity of 5×10-6 emu. X-ray photoelectron spectra (XPS) analysis was performed on a PHI-5400 instrument with Mg Kα as the X-ray source under a pressure of 1.33 × 10−7 Pa and a step of 0.05 eV. The C (1s) level 5
ACCEPTED MANUSCRIPT (285.0 eV) was taken as the reference binding energy. 2.5. Photocatalytic activity evaluation The photocatalytic activity of as-synthesized samples was evaluated by photoFenton-like reaction for degradation of rhodamine B in aqueous solution. 20 mg of Fe2O3/SBA-15 was added into 30 mL 30 mg/L of rhodamine B aqueous solution and
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ultrasound for 10 min. Then stood still for 120 min in the dark in order to ensure the establishment of an adsorption-desorption equilibrium. After that, added 2 mL 30% of
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H2O2 and ultrasound for 10 min, the photocatalytic reaction was carried out by using a 300 W xenon lamp (CEL-HXP 300, Jinyuan Technology Co., Ltd., Beijing) with a
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420 nm cutoff filter as visible light source under stirring occasionally. Ordinary glass beaker is used as the reaction vessel. The reaction solution is always kept about 12 cm
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from the light source and the reaction temperature is about 25 ºC. About 4 mL aliquots were took out at frequent interval of 30 min, magnetic and centrifuged separation,
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measured its absorbance at the certain wavelength of 553 nm using a 722 G spectrophotometer (INESA Analytical Instrument Co., Ltd., China).
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The residual degradation efficiency of rhodamine B (RhB) is calculated
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according to following formula:
Ct A 100% t 100% C0 A0
(1)
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Where, C0 is the initial concentration of RhB, mg/L; Ct is the concentration of RhB at time t, min; A0 and At are the maximum absorbance at time 0 and time t, min,
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respectively.
In general, the photocatalytic degradation reaction of organic pollutants is a pseudo first order reaction and obeys Langmuir-Hinshelwood kinetic [25], thus the apparent rate constant k could be calculated from the following equation:
ln
C0 kt Ct
(2)
Where, k is the apparent rate constant, min-1. C0 and Ct is the same as above. By the slope of fitting curve ln(C0/Ct) ~ t, the rate constant k could be calculated. 6
ACCEPTED MANUSCRIPT 3. Results and discussions 3.1. Structure, morphology and magnetic properties of as-synthesized Fe2O3/SBA-15
A
1000
S3
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600 400
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Intensity (a.u.)
800
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200 0 1
2
3
4
5
6
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2()
S3 20
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S2
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S1
B
40
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Relative intensity (a.u.)
60
80
2 ()
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Fig. 1. Low angle XRD pattern (A) of the sample S3 and conventional XRD patterns (B) of as-synthesized sample S1, S2 and S3. Fig. 1 is x-ray diffraction patterns of the samples, therein, Fig. 1A is the low angle pattern of the sample S3 and Fig. 1B is the conventional XRD patterns of the samples. As shown in Fig. 1A, the peak of 2θ at 1.1 º is (100) reflection of SBA-15 which associated with the two dimensional hexagonal structure of mesoporous channels [18], indicating that the structure orderliness of the sample Fe2O3/SBA-15 still exists. This is in good agreement with the result of TEM image of the samples S3 (Fig. 3D). From Fig. 1B, it can be seen that the sample S1 has only a few of wide angle x-ray 7
ACCEPTED MANUSCRIPT diffraction patterns of γ-Fe2O3, the peaks intensity of XRD patterns gradually increase as increase of the calcination temperature of the samples. Fig. 1B S3 shows the narrow peaks at 2θ values of 30.4, 35.7, 43.7, 54.1, 57.6 and 63.2 °, which can be assigned to (220), (311), (400), (422), (511) and (420) of γ-Fe2O3 (JCPDS NO. 24-0081), indicating that the sample Fe2O3/SBA-15 has individual γ-Fe2O3 crystalline
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structure [26]. And particle sizes of the samples increase gently with increase of the calcination temperature. This well agrees with the SEM images of the samples S1, S2
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and S3 in the Fig. 3A, 3B and 3C. In addition, according to our previous research [27], it can be known that the sample is α-Fe2O3 when ferric ammonium citrate was
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calcined at 550 ºC under air atmosphere. This clearly indicates that SBA-15 can
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increase the stability of γ-Fe2O3. 400
adsorption desorption
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300
S1
S2
100
0 0.0
0.2
D
200
S3
0.6
0.8
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3
Adsorbed volume (cm /g)
A
0.4
1.0
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B
0.15
0.20
3
0.15
dV(d) (cm /nm/g)
3
dV(d) (cm /nm/g)
0.25
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Relatively pressure (p/p0)
0.10
S1 0.05
S3
0.10
0.05
0.00 5
0.00
S2 5
10
10
15
20
Pore diameter (nm) 15
20
25
Pore diameter (nm) 8
30
ACCEPTED MANUSCRIPT Fig. 2. N2 adsorption-desorption isotherms (A) and pore diameter distribution curves (B) of the sample S1, S2 and S3. Table 1 Structure parameters of the sample SBA-15, S1, S2 and S3. BET surface area (m2/g)
Total pore volume (cm3/g)
Micropore volume (cm3/g)
Micropore area (m2/g)
SBA-15
639.3
0.964
0.894
596.4
S1
258.0
0.434
0.222
162.0
S2
213.3
0.456
0.155
98.0
S3
164.1
0.371
0.129
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Pore Diameter (nm)
22.2
4.1
97.3
2.8
115.1
3.1
82.8
2.8
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84.8
External surface area (m2/g)
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Sample
N2 adsorption-desorption isothermals and pore diameter distribution curves of the
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sample S1, S2 and S3 are displayed in Fig. 2, and the corresponding structure parameters are given in Table 1. From Fig. 2A, it can be seen that N2 adsorptiondesorption isotherms of the samples are the Ⅳ-type curves with H1 hysteresis loop
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corresponding to the filling of mesoporous, indicating that the addition of Fe2O3 does
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not destroy the ordered structure of SBA-15, but its orderliness reduces. As shown in Fig. 2B, all of the three samples have narrow pore size distributions, which their pore diameters concentrate at about 2.8, 3.1 and 2.8 nm, respectively, showing also the
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existence of ordered mesoporous. In addition, from Table 1, it can be seen that BET specific surface area, micropore area and micropore volume decrease with increase of
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the calcination temperature, whereas total pore volume, external surface area and pore diameter change irregularly. Compared to parent SBA-15, the decrease of BET surface area, total pore volume, micro pore volume, micropore surface area and pore diameter of as-synthesized Fe2O3/SBA-15, which is due to the larger Fe3+ instead of the smaller Si4+ [28], indicates Fe2O3 has been loaded successfully onto SBA-15.
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10
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Fig. 3. SEM images of the sample S1 (A), S2 (B), S3 (C) and TEM image of the
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sample S3 (D).
To research intuitively the surface morphologies and structure properties of
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as-synthesized Fe2O3/SBA-15, the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images are displayed in Fig. 3. From Fig.
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3A to 3B to 3C, the particle size of the samples gradually decrease, from bigger
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irregular particles become gently smaller particles and at last become very small spherical particles with regular structure. This indicates that particle size of the samples is closely related to the calcining temperature. The higher calcination
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temperature, the smaller the sample particle size is. This is consistent with the result of XRD in Fig. 1B. Fig. 3D is the TEM image of the sample S3. It shows the channel
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structure of the sample still retain, Fe2O3 particles are filled in the channel or located on the outer surface of SBA-15, and their mean particle sizes is about 20 nm. This also fully demonstrates that the addition of Fe2O3 particles did not change the ordered structure of SBA-15, and that the good confinement effect of SBA-15 avoids excessive enlarge of the sample size.
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S1 S2 S3
10
0
-20 -10000
-5000
0
5000
10000
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-10
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Moment (emu/g)
20
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Field (Oe)
Fig. 4. Magnetization curves of the as-synthesized Fe2O3/SBA-15 at 300 K.
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Table 2
Magnetism parameters of as-synthesized Fe2O3/SBA-15 samples.
S1
7.09
S2
14.95
S3
19.15
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Remanent magnetization (Mr)/ emu/g
Coercive field (Hc)/ Oe
0
0
0.19
4.76
0.73
19.34
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Saturation magnetization (Ms)/ emu/g
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Sample
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Fig. 4 is the magnetization curves of the as-synthesized Fe2O3/SBA-15 sample S1, S2 and S3 at 300 K and corresponding magnetism parameters are presented in Table 2.
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As shown in Fig. 4, it can be seen that the magnetization curves of the samples hardly any hysteresis loop exist, suggesting the as-synthesized samples have superparamagnetic properties. This can also be proved from their very small of remanent magnetization (Mr) and coercive field (Oe) (Table 2). And the superparamagnetic behavior of the samples becomes weak as increase of the calcination temperature. Furthermore, the saturation magnetizations (Ms) of the samples gradually enlarge when the calcination temperature of the samples increases. The higher saturation magnetization of the samples can be sure they will be well recycled under the influence of the external magnetic field. 12
ACCEPTED MANUSCRIPT 3.2. Catalytic activities of as-synthesized materials in photo-Fenton reaction In order to evaluate the catalytic activities of as-synthesized Fe2O3/SBA-15, the heterogeneous photo-Fenton reaction of rhodamine B aqueous solution were performed at the natural pH (or so 6.3) and room temperature, and some influencing factors such as the amount and calcination temperature of catalyst, the amount of
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hydrogen peroxide and the initial concentration of reaction solution were also investigated. The primary experiments demonstrate that the adsorption capacity and
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degradation efficiency of RhB on all samples were negligible when RhB solutions
1.0
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were not illuminated by light.
A
0.8
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S1 S2 S3
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C/C0
0.6
0.4
0
60
120
180
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0.0
D
0.2
240
300
Time (min)
Equation
y = a + b*x
Adj. R-Squ 0.97221 0.9534
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3
Value
-- Intercep -0.363 300 degree Slope
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Ln(C0/C)
2
1
0.15814
0.0134 9.87427E-4
-- Intercep -0.257
550 degree Slope
0.116
0.0128 7.24283E-4
-- Intercep -0.482 400 degree Slope
B
0.95766
Standard Er
0.09104
0.0077 5.12971E-4
0 0
60
120
180
240
300
Time (min) Fig. 5. Effects of the different catalyst on the degradation efficiency of RhB (A) and the kinetic fitting curves (B). Reaction conditions: RhB concentration 30 mg/L, H2O2 13
ACCEPTED MANUSCRIPT 2 mL. Fig. 5A is the variation curves of the residual degradation ratio of RhB in different catalysts, and that Fig. 5B is the corresponding kinetic fitting curves. As shown in Fig. 5A, the residual degradation efficiencies of RhB decrease gradually with the process of heterogenous photo-Fenton reaction. But at the very start, the rates
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of degradation are very slow. Then the reaction becomes quickly until the end of the reaction after a period of time. This indicates that the photo-Fenton-like reaction has a
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period of induction due to production of •OH free radicals [29], and the length of induction period is closely related to some parameters such as the structure of catalyst
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(Fig. 5), amount of catalyst (Fig. 6) and hydrogen peroxide (Fig. 7), the initial concentration of organic pollutants (Fig. 8), and so on. Fig. 5A is the degradation
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curves of RhB using different structure catalyst under photocatalytic conditions. It can be seen that the catalytic activity of the catalyst is closely related to its structure
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properties. Compare the data in Table 1, we know that the total pore volume, external surface areas and pore diameters of the catalysts are consistent with their catalytic
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activities. On the other hand, From Fig. 1, it can be known that catalytic activities of
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catalyst also relates to their particle size and crystallinity. Therefore, the sample S2, namely, calcined at 400 ºC, has the highest catalytic activity, next is S1, and that the activity of S3 is the lowest, but their activity difference is little. This can also be seen
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from Fig. 5B. The apparent rate constant of the three catalysts is 0.0128, 0.0134 and 0.0077 min-1, respectively. The final degradation percentage of RhB to the three
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catalysts is S1 89.26% at 300 min, S2 96.43% and S3 96.82% at 270 min, respectively. Compare with the literature, we can found that using Fe2O3/SBA-15 as the catalyst, the degradation percentage of RhB under photo-Fenton reaction is higher than that without light condition [30], but slightly lower than that of photo-Fenton reaction using Kaolin as the catalyst carrier [31].
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1.0
A 0.8
10 mg 20 mg 30 mg
C/C0
0.6
0.4
0
60
120
180
240
300
3
Value
0.96725 Standard E
-- Intercep -0.517
20 mg
2
Slope
0.18513
0.0103
0.00104
-- Intercep -0.363
0.116
Slope
0.0128 7.24283E-
-- Intercep -0.348 30 mg
Slope
0.11322
0.0115 7.06944E-
1
B
0 0
60
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D
Ln(C0/C)
10 mg
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y = a + b*x
Adj. R-Squ 0.9061 0.9722
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Equation
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Time (min)
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0.0
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0.2
120
180
240
300
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Time (min)
Fig. 6. Effects of the amount of catalyst on the degradation efficiency of RhB (A) and
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the kinetic fitting curves (B). The amount of catalyst has obvious influence on its catalytic activity. These results were depicted in Fig 6. It can be seen that the photo-Fenton-like reaction proceeded gradually as time went on, but the rate of reactions is different. As shown in Fig. 6B, the apparent rate constant of the three reactions is 0.0103, 0.0128 and 0.0115 min-1, respectively. This indicates that the rate of photocatalytic reaction first increases and then reduces as further increasing the amount of catalyst. This is because the active sites of catalyst increase as the increase of the amount of catalyst, and thus lead to increase of the amount of •OH free radicals by the reaction of Fe2+ 15
ACCEPTED MANUSCRIPT and H2O2 (Eq. (6), (7) and (9)). On the other hand, the excess increase of catalyst active sites can accelerate the decomposition of H2O2 to a number of O2 and H2O (Eq. (10)), or excess catalyst can produce excess •OH radicals accelerating the reaction between •OH radicals (Eq. (11)) or •OH radicals and H2O2 (Eq. (12)), thus decrease the amount of •OH free radicals [32, 33] and at last result in a slower reaction rate.
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This is consistent with the experimental phenomena we have observed. Therefore, 20 mg of Fe2O3/SBA-15 is a suitable amount of catalyst in the current photocatalytic
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degradation reaction of RhB. And the reaction equations of the whole process could be summed up as follows:
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Fe3+ +H2O2→ Fe–OOH2+ +H+ Fe–OOH2+→ Fe2+ +HOO•
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Fe3+ +HOO• → Fe2+ +O2 +H+
Fe2+ +H2O2→ Fe3+ +•OH + OH−
H2O2 + hv → 2HO•
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Fe2O3 + hv → e- + h+
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H2O2 +•OH → H2O + HOO•
(12)
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(11)
A 1 mL 2 mL 3 mL
0.2
0
(7)
(10)
0.4
0.0
(6)
HO• + HO• → H2O2
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C0/C
0.6
(5)
(9)
2H2O2 → 2H2O+ O2
0.8
(4)
(8)
H2O2 + e- → OH- + HO•
1.0
(3)
60
120
180
240
Time (min)
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Equation
y = a + b*x
Adj. R-Squar 0.96525
0.9722 Value
0.97918 Standard Err
-- Intercept -0.3699
3
1 mL H2O2
Slope
0.0112
0.11334 7.07677E-4
-- Intercept -0.3636 Slope
0.0128
0.116 7.24283E-4
-- Intercept -0.3614
2
3 mL H2O2
Slope
0.0137
0.10657 6.65389E-4
B
1
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Ln(C/C0)
2 mL H2O2
0 60
120
180
240
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0
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Time (min)
Fig. 7. Effects and kinetic fitting curves of the amount of H2O2 on the degradation
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efficiency of RhB under photo-Fenton process. Reaction conditions: catalyst S2, RhB solution concentration 30 mg/L.
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It is well known hydrogen peroxide is the important oxidizing agent of Fenton reaction. It is the •OH free radicals from H2O2 decomposition that contribute to the rapid progress of Fenton reaction. Accordingly, the amount of hydrogen peroxide in
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the reaction solution will greatly affect the speed of Fenton reaction. Fig. 7 is the
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degradation efficiency and kinetic fitting curves of RhB at different concentration of hydrogen peroxide in heterogenous photo-Fenton process. As can be seen from Fig.
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7A, the reaction rate increases with increase of the amount of H2O2 [34], however, the rate increases is limited when the amount of H2O2 increases to 3 mL. This is due to
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the production of a large number of •OH free radicals by more H2O2 (Eq. (8) and (9)), thus facilitate rapid progress of photo-Fenton reaction. But, as the excess addition of hydrogen peroxide the rate of reaction slows down, this may be due to increasing the level of •OH radicals scavenging by H2O2 itself [34, 35], just as shown from the reaction Eq. (11) and (12). From Fig. 7B, the apparent rate constant of photocatalytic reaction under the different concentration of H2O2 is 0.01121, 0.01287 and 0.01371 min-1, respectively. This shows the apparent rate constant increases as increasing the amount of H2O2. But from Fig. 7A, we can know that the reaction rate at 3 mL H2O2 is higher than that at 2 mL H2O2 just at the beginning of the reaction. The final 17
ACCEPTED MANUSCRIPT degradation percentage of RhB in the different concentration of H2O2 is 93.88%, 96.43% and 97.03% at 270 min, respectively. Therefore, for the sake of economic consideration, 2 mL of hydrogen peroxide is a suitable dosage in the current heterogenous photo-Fenton reaction of RhB solution degradation. 1.0
0.4
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Fig. 8. Effect of the initial concentration of RhB solution on the degradation efficiency of RhB. Reaction conditions: catalyst S2, 30% H2O2 2 mL, RhB solution
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30 mL.
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The concentration of organism in wastewater has an important influence on its degradation. For this reason, the degradation experiments of different initial
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concentration of RhB solution under visible light irradiation were performed and the results were given in Fig. 8. From Fig. 8, the degradation curve of RhB almost linear
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decreases, and there is hardly any obvious induction period when the concentration of RhB solution is 20 mg/L. Then the induction period appears and gets longer gradually as increase of the initial concentration of RhB solution from 30 to 40 mg/L. And the degradation efficiency of RhB gradually decreases as increasing of the initial concentration of RhB solution from 20 to 30 to 40 mg/L. This is because the active sites are certain to a certain amount of catalyst, namely, the catalytic capacity of the catalyst is certain, as increase of RhB concentration, the excess RhB molecules will cover on the active sites of the catalyst, thus leading to the decrease of RhB degradation efficiency. In addition, it can also be seen that the final degradation 18
ACCEPTED MANUSCRIPT percentage of RhB is 95.94% at 240 min, 96.43% at 270 min and 86.70% at 300 min, respectively, when the initial concentration of RhB solution is 20, 30 and 40 mg/L, respectively. 1.0
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Fig. 9. Reusability of the catalyst and kinetic fitting curves. Reaction conditions: catalyst S2, 30% H2O2 2 mL, RhB concentration 30 mg/L. Table 3 The kinetic parameters of RhB degradation under the condition of reused catalyst. Reuse times Rate constant/ min-1
1
2
3
4
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0.01287
0.01128
0.01006
0.00863
0.00614
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0.9722
0.9415
0.9470
0.9441
0.9331
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ACCEPTED MANUSCRIPT Reusability of catalyst is one of the important factors to measure the activity and stability of solid catalysts. To this end, recovered catalyst S2 was taken as an example to exam its catalytic activity of degradation RhB. Used catalyst S2 was recovered by an external magnet, washed with deionized water and ethanol, and reused after dried. The catalytic degradation efficiencies of RhB under reused for five times of catalyst
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S2 were shown in Fig. 9. And the kinetic fitting curves and parameters were given in Fig. 9B and Table 3, respectively. From Fig. 9A, it can be seen that the degradation
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percentage of RhB at 270 min is 96.43%, 94.95%, 92.60%, 87.93% and 76.70%, respectively, when the catalyst S2 was reused from first time to fifth time. This
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indicates that as-synthesized catalyst Fe2O3/SBA-15 has excellent reusability and stability. According to our experiment, it can be found that RhB solution became clear
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and transparent when the solution was degraded by photocatalysis and after overnight in the dark (Fig. 10), indicating RhB has been degraded completely. Therefore,
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as-synthesized magnetic Fe2O3/SBA-15 nanomaterials have higher photocatalytic degradation activity and stability, and as well as a certain application prospect.
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Furthermore, as shown in Table 3, the apparent rate constant for 5 times degradation
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of RhB is 0.01287, 0.01128, 0.01006, 0.00863 and 0.00614 min-1, respectively. This shows that the degradation speed of RhB slows gradually with increase of the catalyst
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repetition numbers.
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Fig. 10. Photograph of RhB solution after photocatalytic reaction. (A) Primary
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catalyst reaction solution, (B) Fifth time recovery catalyst reaction solution. To investigate the elemental composition and chemical states of the photocatalyst
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S2 after photocatalytic reaction, the XPS analysis was performed and the results were presented in Fig. 11. The survey XPS spectrum of Fe2O3/SBA-15 (S2) in Fig. 11A confirmed the existents of C, Si, O and Fe elements and the content of each element is
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O 57.4%, Fe 13.43%, C 18.27% and Si 10.9%. The higher carbon content on the
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surface of the photocatalyst indicates that the degradation products of RhB may be deposited on the surface of the catalyst. As shown in the high resolution XPS
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spectrum Fig. 11B, C 1s spectrum is divided into three peaks at 284.7, 286.4 and 288.8 eV, which is respectively contributed to C-C, C-O and C=C (C=O) bands [36,
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37]. This also shows there is the degradation produce of RhB deposited on the surface of the photocatalyst. From Fig. 11C, the peak at 103.3 eV is corresponding to Si 2p spectrum, which is the characteristic of Si-O-Si band in SiO2 of SBA-15 [38, 39]. The O 1s spectrum shown in Fig. 11D was fitted to two peaks respectively at 530.4 and 532.4 eV, which is assigned to the lattice oxygen, chemisorbed oxygen and the band of O-C [37, 40]. The Fe 2p spectrum displayed in Fig. 11E contains two peaks at binding energy of 711.3 and 724.9 eV, which is attributed to Fe 2p3/2 and Fe 2p1/2. The split of 13.6 eV between the two peaks is identical to the reported result [37, 41], indicating that the chemical state of Fe2O3 has hardly any change after photocatalytic 21
ACCEPTED MANUSCRIPT reaction. Furthermore, it can be also discovered the specific surface area, pore volume and pore size of catalyst have not obvious decrease by BET analysis of photocatalyst (S2) after used fifth times, manifesting as-prepared photocatalyst has a high stability. This may be the reason that as-synthesized catalysts have higher photocatalytic
Fe2p1/2
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103.3 eV
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532.4 eV O 1s scan B
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724.9 eV Fe 2p1/2
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Binding energy (eV) Fig. 11. XPS spectra of survey (A), C 1s (B), Si 2p (C), O 1s (D) and Fe 2p (E) of catalyst S2 (Fe2O3/SBA-15) used once. 23
ACCEPTED MANUSCRIPT 4. Conclusions In summary, magnetic Fe2O3/SBA-15 nanomaterials were facile synthesized by environmentally friendly impregnation and calcination method. As addition of Fe2O3, BET areas, whole pore volume and pore size of nanomaterials decrease, but their order structure still remains. SBA-15 mesoporous molecular sieves have obvious
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confinement effect, which can prevent the excessive growth of iron oxide nanoparticles. Prepared Fe2O3/SBA-15 has higher saturation magnetizations so that it
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can be expediently recycled and reused as heterogenous catalyst. As-synthesized
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Fe2O3/SBA-15 shows a higher photocatalytic activity and excellent reusability and stability on the degradation of rhodamine B under visible light irradiation and at about
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6.3 of solution natural pH. Experimental results show the structure and amount of catalyst, the concentration of H2O2 and rhodamain B are important influence factors
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on the degradation of rhodamine B. And these influence process are also discussed from kinetics. Rhodamine B can almost be degraded completely when its solution is irradiated for a certain time and after rest overnight in the dark. Therefore, we
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synthesized magnetic Fe2O3/SBA-15 nanomaterials have a good prospect of industrial
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application.
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ACCEPTED MANUSCRIPT Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (Project No. 21503125, 21502109), the Natural Science Foundation of Shaanxi Province (No. 2017JQ2017) and Key Scientific Research Program Funded by Shaanxi Provincial Department of Education (No.
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17JS025).
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Highlights
1. Magnetic Fe2O3/SBA-15 was prepared by a facile immersion
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and calcination method.
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2. Saturation magnetization of as-prepared samples increases as
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heating temperature.
3. As-prepared Fe2O3/SBA-15 has a higher photocatalytic activity
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under visible light.
consecutive runs.
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4. Photocatalytic activity of samples slightly decreases after five
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Junhong Wang, Xianzhao Shao, Qiang Zhang, Jianqi Ma, Hongguang Ge PII: DOI: Reference:
S0167-7322(18)30588-9 doi:10.1016/j.molliq.2018.03.109 MOLLIQ 8883
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
2 February 2018 22 March 2018 26 March 2018
Please cite this article as: Junhong Wang, Xianzhao Shao, Qiang Zhang, Jianqi Ma, Hongguang Ge , Preparation and photocatalytic application of magnetic Fe2O3/SBA-15 nanomaterials. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:10.1016/j.molliq.2018.03.109
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ACCEPTED MANUSCRIPT Preparation and photocatalytic application of magnetic Fe2O3/SBA-15 nanomaterials
Authors:
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The affiliations and addresses of the authors:
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Junhong Wang*, Xianzhao Shao, Qiang Zhang, Jianqi Ma, Hongguang Ge
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Shaanxi Key Laboratory of Catalysis, College of Chemical and Environment Science, Shaanxi University of Technology, Hanzhong 723000, PR China
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Corresponding authors:
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Junhong Wang
Shaanxi Key Laboratory of Catalysis, College of Chemical and Environment
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Tel: +86 0916- 2641660
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Science, Shaanxi University of Technology, Hanzhong 723000, PR China
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Email: [email protected]; [email protected]
1
ACCEPTED MANUSCRIPT Abstract: Magnetic Fe2O3/SBA-15 mesoporous nanomaterials were synthesized by means of a facile impregnation and calcination method. As-prepared magnetic nanomaterials were characterized using XRD, N2 adsorption-desorption isotherms, SEM, TEM, VSM and XPS techniques. And the photocatalytic activities of magnetic Fe2O3/SBA-15 on the degradation of rhodamine B were also evaluated. Results
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indicate that the order structure of as-synthesized nanomaterials still remain, but its specific surface area, pore volume and pore size decrease. The saturation
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magnetization of the samples increases and its particle size decreases gradually with
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increasing the calcination temperature. And the existence of SBA-15 mesoporous molecular sieves increases largely the stability of γ-Fe2O3. Moreover, as-synthesized
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magnetic Fe2O3/SBA-15 shows high photocatalytic activities in the course of the degradation of rhodamine B by photo-Fenton-like reaction at natural pH about 6.3 and
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visible light irradiation. The structure and amount of catalyst, the concentration of H2O2 and rhodamine B are main influence factors on the degradation of rhodamine B, and these influence results have been depicted quantitatively from dynamics.
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Rhodamine B in aqueous solution could almost be removed completely after a period
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of illumination by visible light and overnight in the dark. Meanwhile, as-prepared magnetic Fe2O3/SBA-15 has excellent reusability and stability, which its
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photocatalytic activity has only a slightly decrease after five consecutive runs. Therefore, as-synthesized magnetic Fe2O3/SBA-15 has a broad prospect of practical
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application.
Keywords: magnetization, photocatalyst, Fe2O3/SBA-15, Fenton-like reaction, degradation
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ACCEPTED MANUSCRIPT 1. Introduction Environmental protection problem is an important subject that human society is facing today. With the rapid development of human society and industrialization, more and more waste is discharged into the environment, causing serious pollution of water quality, air and soil [1]. How to clean up these pollutants and restore a good
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living environment for humanity has become a serious challenge for scientists and technicians.
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To remove heavy metal, organic dyes, halogenated pollutants, polycyclic
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aromatic hydrocarbon and other pollutants in water, many methods have been used such as adsorption, chemical precipitation, catalytic and photocatalytic reaction [2-6].
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Among of the present methods, photocatalytic degradation of pollutants under visible light irradiation is an efficient and clean technology, which can make full use of the
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solar energy and save fossil resources. Due to its low band-gap 2.1 eV [7], high efficiency and low-cost, FexOy has been extensively used in the degradation of many pollutants as catalyst or photocatalyst [8-10]. It is usually known that Fenton reaction
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as an advanced oxidation process has been widely used in degradation of many toxic
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and non-biodegradable organic pollutants [11-14]. The combination of Fenton reaction and photocatalytic reaction, namely, photo-Fenton reaction, is an important
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reaction process and will have a broad application prospect in the field of environmental wastewater remediation [11, 15].
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Metal oxide nanoparticles have a widely application in adsorption, catalysis, drug delivery and so on, but they are easy to aggregation and leaching during reaction so that prevents them from fully functioning. Therefore, metal oxides are usually loaded onto a carrier in order to increasing their specific surface areas, dispersivity and catalytic performance [16, 17]. Therein, SBA-15 mesoporous molecular sieve is an excellent carrier, which has big specific surface areas, big pore diameters, ordered channel structure and higher thermal and hydrothermal stability [18-20]. Yan et al [19] synthesized Fe/SBA-15 to use for the ozonation of oxalic acid and got a high removal efficiency of 86.6%. Barros et al [21] prepared ZnO/SBA-15 and MgO/SBA-15 and 3
ACCEPTED MANUSCRIPT used to catalytic esterification reaction of lauric acid with 1-butanol under atmosphere pressure and reflux conditions, giving to 81% yield of butyl ester. Wang et al [22] synthesized SBA-15 supported iron oxide materials to catalytic removal of hydrogen sulfide and gained a remarkable effect. Karthikeyan et al [13] synthesized the nanomaterials of Cu and Fe oxide dispersed on SBA-15. This kind of material
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revealed a high catalytic activity for the oxidative degradation of N, N-diethyl-pphenyl diamine (DPD). Our research group [23] prepared Cu/SBA-15 for catalytic
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reduction of p-nitrophenol to p-aminophenol and obtained over 99% conversion ratio. Gaudin et al [24] synthesized CuO/SBA-15 by the solid state grinding method and
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showed a better adsorption efficiency to removal of SOx.
To improve the reusability and practicability of catalysts, magnetic Fe2O3/
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SBA-15 mesoporous nanomaterials were prepared by simple incipient wetness impregnation and burning method at different calcination temperatures. Then, the
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photocatalytic activities of as-synthesized Fe2O3/SBA-15 nanoparticles as photocatalyst were tested by photo-Fenton reaction to degrade organic dye
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2. Experimental
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Rhodamine B under visible light irradiation and original solution pH condition.
2.1. Chemicals and materials
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Tetraethoxysilane (TEOS), Pluronic P123 (poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol)) and rhodamine B (RhB) were bought
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from Sigma-Aldrich; poly (vinyl alcohol) (PVA) was purchased from Beijing chemical reagent three factory; ferric ammonium citrate was purchased from Sinopharm Beijing Co., Ltd. (Fe content 20.5-22.5%); hydrochloric acid was obtained from Tianjin Guangfu fine chemical institute. All of these reagents were used as received without further purification. 2.2. Synthesis of SBA-15 SBA-15 mesoporous molecular sieves were synthesized according to our previous literature [18], but made a simple modification. In a typical procedure, 4 g of P123 was added into a 200 mL of beaker with 120 mL of 0.75 M HCl and 30 mL of 4
ACCEPTED MANUSCRIPT deionized H2O. After P123 dissolved completely, 2 g of PVA was added into the above solution with stirring, and added 8.6 g of TEOS after 4 h. Then, the above viscous liquid was stirred continuously for 20 h at 35 ºC and hydrothermal crystallized for another 24 h at 100 ºC static condition. At last, SBA-15 was gained by filtered, washed, dried and calcined at 550 ºC for 5.5 h.
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2.3. Preparation of magnetic Fe2O3/SBA-15 Magnetic Fe2O3/SBA-15 mesoporous material was prepared by a simple dipping
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and calcination method. Typically, 1.0 g of as-synthesized SBA-15 was immersed in 20 mL of deionized H2O with 6.0 g of ferric ammonium citrate overnight. Then the
SC
loads were filtered and washed with deionized water and ethanol three times, dried at 60 ºC, calcined in static air for 6 h at different temperature (300, 400 and 550 ºC) ,
and S3, respectively.
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2.4. Characteristics of Fe2O3/SBA-15
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obtained magnetic Fe2O3/SBA-15 mesoporous nanomaterials and marked as S1, S2
X-ray diffraction (XRD) measure was completed with a Netherlands X'Pert PRO
D
power diffractometer using Cu Kα radiation. The data were collected by scanned the
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samples in the 2θ ranges from 10 to 90 with a resolution of 0.02º. N2 adsorptiondesorption isotherms were measured on a BelSorp Max volumetric adsorption analyzer at -196 ºC. The samples were degassed for 6 h at 180 ºC under vacuum
CE
before measurement. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area and the Barrett-Joyner-Halenda (BJH) model was
AC
utilized to measure the pore size distributions by the adsorption branch of the isotherm. Scanning electron microscopy (SEM) images were obtained by a Hitachi S4800 field emissions scanning electron microscopy. Transmission electron microscopy (TEM) images were recorded on a FEI Tecnai G2 F20 S-Twin microscope operated at 200 kV. The static magnetic properties of samples were measured through an American Lake Shore VSM 7307 Vibrating sample magnetometer (VSM) with a sensitivity of 5×10-6 emu. X-ray photoelectron spectra (XPS) analysis was performed on a PHI-5400 instrument with Mg Kα as the X-ray source under a pressure of 1.33 × 10−7 Pa and a step of 0.05 eV. The C (1s) level 5
ACCEPTED MANUSCRIPT (285.0 eV) was taken as the reference binding energy. 2.5. Photocatalytic activity evaluation The photocatalytic activity of as-synthesized samples was evaluated by photoFenton-like reaction for degradation of rhodamine B in aqueous solution. 20 mg of Fe2O3/SBA-15 was added into 30 mL 30 mg/L of rhodamine B aqueous solution and
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ultrasound for 10 min. Then stood still for 120 min in the dark in order to ensure the establishment of an adsorption-desorption equilibrium. After that, added 2 mL 30% of
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H2O2 and ultrasound for 10 min, the photocatalytic reaction was carried out by using a 300 W xenon lamp (CEL-HXP 300, Jinyuan Technology Co., Ltd., Beijing) with a
SC
420 nm cutoff filter as visible light source under stirring occasionally. Ordinary glass beaker is used as the reaction vessel. The reaction solution is always kept about 12 cm
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from the light source and the reaction temperature is about 25 ºC. About 4 mL aliquots were took out at frequent interval of 30 min, magnetic and centrifuged separation,
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measured its absorbance at the certain wavelength of 553 nm using a 722 G spectrophotometer (INESA Analytical Instrument Co., Ltd., China).
D
The residual degradation efficiency of rhodamine B (RhB) is calculated
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according to following formula:
Ct A 100% t 100% C0 A0
(1)
CE
Where, C0 is the initial concentration of RhB, mg/L; Ct is the concentration of RhB at time t, min; A0 and At are the maximum absorbance at time 0 and time t, min,
AC
respectively.
In general, the photocatalytic degradation reaction of organic pollutants is a pseudo first order reaction and obeys Langmuir-Hinshelwood kinetic [25], thus the apparent rate constant k could be calculated from the following equation:
ln
C0 kt Ct
(2)
Where, k is the apparent rate constant, min-1. C0 and Ct is the same as above. By the slope of fitting curve ln(C0/Ct) ~ t, the rate constant k could be calculated. 6
ACCEPTED MANUSCRIPT 3. Results and discussions 3.1. Structure, morphology and magnetic properties of as-synthesized Fe2O3/SBA-15
A
1000
S3
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600 400
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Intensity (a.u.)
800
SC
200 0 1
2
3
4
5
6
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2()
S3 20
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S2
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D
S1
B
40
CE
Relative intensity (a.u.)
60
80
2 ()
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Fig. 1. Low angle XRD pattern (A) of the sample S3 and conventional XRD patterns (B) of as-synthesized sample S1, S2 and S3. Fig. 1 is x-ray diffraction patterns of the samples, therein, Fig. 1A is the low angle pattern of the sample S3 and Fig. 1B is the conventional XRD patterns of the samples. As shown in Fig. 1A, the peak of 2θ at 1.1 º is (100) reflection of SBA-15 which associated with the two dimensional hexagonal structure of mesoporous channels [18], indicating that the structure orderliness of the sample Fe2O3/SBA-15 still exists. This is in good agreement with the result of TEM image of the samples S3 (Fig. 3D). From Fig. 1B, it can be seen that the sample S1 has only a few of wide angle x-ray 7
ACCEPTED MANUSCRIPT diffraction patterns of γ-Fe2O3, the peaks intensity of XRD patterns gradually increase as increase of the calcination temperature of the samples. Fig. 1B S3 shows the narrow peaks at 2θ values of 30.4, 35.7, 43.7, 54.1, 57.6 and 63.2 °, which can be assigned to (220), (311), (400), (422), (511) and (420) of γ-Fe2O3 (JCPDS NO. 24-0081), indicating that the sample Fe2O3/SBA-15 has individual γ-Fe2O3 crystalline
PT
structure [26]. And particle sizes of the samples increase gently with increase of the calcination temperature. This well agrees with the SEM images of the samples S1, S2
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and S3 in the Fig. 3A, 3B and 3C. In addition, according to our previous research [27], it can be known that the sample is α-Fe2O3 when ferric ammonium citrate was
SC
calcined at 550 ºC under air atmosphere. This clearly indicates that SBA-15 can
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increase the stability of γ-Fe2O3. 400
adsorption desorption
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300
S1
S2
100
0 0.0
0.2
D
200
S3
0.6
0.8
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3
Adsorbed volume (cm /g)
A
0.4
1.0
AC
B
0.15
0.20
3
0.15
dV(d) (cm /nm/g)
3
dV(d) (cm /nm/g)
0.25
CE
Relatively pressure (p/p0)
0.10
S1 0.05
S3
0.10
0.05
0.00 5
0.00
S2 5
10
10
15
20
Pore diameter (nm) 15
20
25
Pore diameter (nm) 8
30
ACCEPTED MANUSCRIPT Fig. 2. N2 adsorption-desorption isotherms (A) and pore diameter distribution curves (B) of the sample S1, S2 and S3. Table 1 Structure parameters of the sample SBA-15, S1, S2 and S3. BET surface area (m2/g)
Total pore volume (cm3/g)
Micropore volume (cm3/g)
Micropore area (m2/g)
SBA-15
639.3
0.964
0.894
596.4
S1
258.0
0.434
0.222
162.0
S2
213.3
0.456
0.155
98.0
S3
164.1
0.371
0.129
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Pore Diameter (nm)
22.2
4.1
97.3
2.8
115.1
3.1
82.8
2.8
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84.8
External surface area (m2/g)
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Sample
N2 adsorption-desorption isothermals and pore diameter distribution curves of the
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sample S1, S2 and S3 are displayed in Fig. 2, and the corresponding structure parameters are given in Table 1. From Fig. 2A, it can be seen that N2 adsorptiondesorption isotherms of the samples are the Ⅳ-type curves with H1 hysteresis loop
D
corresponding to the filling of mesoporous, indicating that the addition of Fe2O3 does
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not destroy the ordered structure of SBA-15, but its orderliness reduces. As shown in Fig. 2B, all of the three samples have narrow pore size distributions, which their pore diameters concentrate at about 2.8, 3.1 and 2.8 nm, respectively, showing also the
CE
existence of ordered mesoporous. In addition, from Table 1, it can be seen that BET specific surface area, micropore area and micropore volume decrease with increase of
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the calcination temperature, whereas total pore volume, external surface area and pore diameter change irregularly. Compared to parent SBA-15, the decrease of BET surface area, total pore volume, micro pore volume, micropore surface area and pore diameter of as-synthesized Fe2O3/SBA-15, which is due to the larger Fe3+ instead of the smaller Si4+ [28], indicates Fe2O3 has been loaded successfully onto SBA-15.
9
AC
CE
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D
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SC
RI
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ACCEPTED MANUSCRIPT
10
SC
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Fig. 3. SEM images of the sample S1 (A), S2 (B), S3 (C) and TEM image of the
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sample S3 (D).
To research intuitively the surface morphologies and structure properties of
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as-synthesized Fe2O3/SBA-15, the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images are displayed in Fig. 3. From Fig.
D
3A to 3B to 3C, the particle size of the samples gradually decrease, from bigger
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irregular particles become gently smaller particles and at last become very small spherical particles with regular structure. This indicates that particle size of the samples is closely related to the calcining temperature. The higher calcination
CE
temperature, the smaller the sample particle size is. This is consistent with the result of XRD in Fig. 1B. Fig. 3D is the TEM image of the sample S3. It shows the channel
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structure of the sample still retain, Fe2O3 particles are filled in the channel or located on the outer surface of SBA-15, and their mean particle sizes is about 20 nm. This also fully demonstrates that the addition of Fe2O3 particles did not change the ordered structure of SBA-15, and that the good confinement effect of SBA-15 avoids excessive enlarge of the sample size.
11
ACCEPTED MANUSCRIPT
S1 S2 S3
10
0
-20 -10000
-5000
0
5000
10000
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-10
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Moment (emu/g)
20
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Field (Oe)
Fig. 4. Magnetization curves of the as-synthesized Fe2O3/SBA-15 at 300 K.
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Table 2
Magnetism parameters of as-synthesized Fe2O3/SBA-15 samples.
S1
7.09
S2
14.95
S3
19.15
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Remanent magnetization (Mr)/ emu/g
Coercive field (Hc)/ Oe
0
0
0.19
4.76
0.73
19.34
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Saturation magnetization (Ms)/ emu/g
D
Sample
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Fig. 4 is the magnetization curves of the as-synthesized Fe2O3/SBA-15 sample S1, S2 and S3 at 300 K and corresponding magnetism parameters are presented in Table 2.
AC
As shown in Fig. 4, it can be seen that the magnetization curves of the samples hardly any hysteresis loop exist, suggesting the as-synthesized samples have superparamagnetic properties. This can also be proved from their very small of remanent magnetization (Mr) and coercive field (Oe) (Table 2). And the superparamagnetic behavior of the samples becomes weak as increase of the calcination temperature. Furthermore, the saturation magnetizations (Ms) of the samples gradually enlarge when the calcination temperature of the samples increases. The higher saturation magnetization of the samples can be sure they will be well recycled under the influence of the external magnetic field. 12
ACCEPTED MANUSCRIPT 3.2. Catalytic activities of as-synthesized materials in photo-Fenton reaction In order to evaluate the catalytic activities of as-synthesized Fe2O3/SBA-15, the heterogeneous photo-Fenton reaction of rhodamine B aqueous solution were performed at the natural pH (or so 6.3) and room temperature, and some influencing factors such as the amount and calcination temperature of catalyst, the amount of
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hydrogen peroxide and the initial concentration of reaction solution were also investigated. The primary experiments demonstrate that the adsorption capacity and
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degradation efficiency of RhB on all samples were negligible when RhB solutions
1.0
SC
were not illuminated by light.
A
0.8
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S1 S2 S3
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C/C0
0.6
0.4
0
60
120
180
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0.0
D
0.2
240
300
Time (min)
Equation
y = a + b*x
Adj. R-Squ 0.97221 0.9534
CE
3
Value
-- Intercep -0.363 300 degree Slope
AC
Ln(C0/C)
2
1
0.15814
0.0134 9.87427E-4
-- Intercep -0.257
550 degree Slope
0.116
0.0128 7.24283E-4
-- Intercep -0.482 400 degree Slope
B
0.95766
Standard Er
0.09104
0.0077 5.12971E-4
0 0
60
120
180
240
300
Time (min) Fig. 5. Effects of the different catalyst on the degradation efficiency of RhB (A) and the kinetic fitting curves (B). Reaction conditions: RhB concentration 30 mg/L, H2O2 13
ACCEPTED MANUSCRIPT 2 mL. Fig. 5A is the variation curves of the residual degradation ratio of RhB in different catalysts, and that Fig. 5B is the corresponding kinetic fitting curves. As shown in Fig. 5A, the residual degradation efficiencies of RhB decrease gradually with the process of heterogenous photo-Fenton reaction. But at the very start, the rates
PT
of degradation are very slow. Then the reaction becomes quickly until the end of the reaction after a period of time. This indicates that the photo-Fenton-like reaction has a
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period of induction due to production of •OH free radicals [29], and the length of induction period is closely related to some parameters such as the structure of catalyst
SC
(Fig. 5), amount of catalyst (Fig. 6) and hydrogen peroxide (Fig. 7), the initial concentration of organic pollutants (Fig. 8), and so on. Fig. 5A is the degradation
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curves of RhB using different structure catalyst under photocatalytic conditions. It can be seen that the catalytic activity of the catalyst is closely related to its structure
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properties. Compare the data in Table 1, we know that the total pore volume, external surface areas and pore diameters of the catalysts are consistent with their catalytic
D
activities. On the other hand, From Fig. 1, it can be known that catalytic activities of
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catalyst also relates to their particle size and crystallinity. Therefore, the sample S2, namely, calcined at 400 ºC, has the highest catalytic activity, next is S1, and that the activity of S3 is the lowest, but their activity difference is little. This can also be seen
CE
from Fig. 5B. The apparent rate constant of the three catalysts is 0.0128, 0.0134 and 0.0077 min-1, respectively. The final degradation percentage of RhB to the three
AC
catalysts is S1 89.26% at 300 min, S2 96.43% and S3 96.82% at 270 min, respectively. Compare with the literature, we can found that using Fe2O3/SBA-15 as the catalyst, the degradation percentage of RhB under photo-Fenton reaction is higher than that without light condition [30], but slightly lower than that of photo-Fenton reaction using Kaolin as the catalyst carrier [31].
14
ACCEPTED MANUSCRIPT
1.0
A 0.8
10 mg 20 mg 30 mg
C/C0
0.6
0.4
0
60
120
180
240
300
3
Value
0.96725 Standard E
-- Intercep -0.517
20 mg
2
Slope
0.18513
0.0103
0.00104
-- Intercep -0.363
0.116
Slope
0.0128 7.24283E-
-- Intercep -0.348 30 mg
Slope
0.11322
0.0115 7.06944E-
1
B
0 0
60
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D
Ln(C0/C)
10 mg
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y = a + b*x
Adj. R-Squ 0.9061 0.9722
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Equation
SC
Time (min)
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0.0
PT
0.2
120
180
240
300
CE
Time (min)
Fig. 6. Effects of the amount of catalyst on the degradation efficiency of RhB (A) and
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the kinetic fitting curves (B). The amount of catalyst has obvious influence on its catalytic activity. These results were depicted in Fig 6. It can be seen that the photo-Fenton-like reaction proceeded gradually as time went on, but the rate of reactions is different. As shown in Fig. 6B, the apparent rate constant of the three reactions is 0.0103, 0.0128 and 0.0115 min-1, respectively. This indicates that the rate of photocatalytic reaction first increases and then reduces as further increasing the amount of catalyst. This is because the active sites of catalyst increase as the increase of the amount of catalyst, and thus lead to increase of the amount of •OH free radicals by the reaction of Fe2+ 15
ACCEPTED MANUSCRIPT and H2O2 (Eq. (6), (7) and (9)). On the other hand, the excess increase of catalyst active sites can accelerate the decomposition of H2O2 to a number of O2 and H2O (Eq. (10)), or excess catalyst can produce excess •OH radicals accelerating the reaction between •OH radicals (Eq. (11)) or •OH radicals and H2O2 (Eq. (12)), thus decrease the amount of •OH free radicals [32, 33] and at last result in a slower reaction rate.
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This is consistent with the experimental phenomena we have observed. Therefore, 20 mg of Fe2O3/SBA-15 is a suitable amount of catalyst in the current photocatalytic
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degradation reaction of RhB. And the reaction equations of the whole process could be summed up as follows:
SC
Fe3+ +H2O2→ Fe–OOH2+ +H+ Fe–OOH2+→ Fe2+ +HOO•
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Fe3+ +HOO• → Fe2+ +O2 +H+
Fe2+ +H2O2→ Fe3+ +•OH + OH−
H2O2 + hv → 2HO•
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Fe2O3 + hv → e- + h+
D
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H2O2 +•OH → H2O + HOO•
(12)
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(11)
A 1 mL 2 mL 3 mL
0.2
0
(7)
(10)
0.4
0.0
(6)
HO• + HO• → H2O2
AC
C0/C
0.6
(5)
(9)
2H2O2 → 2H2O+ O2
0.8
(4)
(8)
H2O2 + e- → OH- + HO•
1.0
(3)
60
120
180
240
Time (min)
16
ACCEPTED MANUSCRIPT 4
Equation
y = a + b*x
Adj. R-Squar 0.96525
0.9722 Value
0.97918 Standard Err
-- Intercept -0.3699
3
1 mL H2O2
Slope
0.0112
0.11334 7.07677E-4
-- Intercept -0.3636 Slope
0.0128
0.116 7.24283E-4
-- Intercept -0.3614
2
3 mL H2O2
Slope
0.0137
0.10657 6.65389E-4
B
1
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Ln(C/C0)
2 mL H2O2
0 60
120
180
240
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0
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Time (min)
Fig. 7. Effects and kinetic fitting curves of the amount of H2O2 on the degradation
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efficiency of RhB under photo-Fenton process. Reaction conditions: catalyst S2, RhB solution concentration 30 mg/L.
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It is well known hydrogen peroxide is the important oxidizing agent of Fenton reaction. It is the •OH free radicals from H2O2 decomposition that contribute to the rapid progress of Fenton reaction. Accordingly, the amount of hydrogen peroxide in
D
the reaction solution will greatly affect the speed of Fenton reaction. Fig. 7 is the
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degradation efficiency and kinetic fitting curves of RhB at different concentration of hydrogen peroxide in heterogenous photo-Fenton process. As can be seen from Fig.
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7A, the reaction rate increases with increase of the amount of H2O2 [34], however, the rate increases is limited when the amount of H2O2 increases to 3 mL. This is due to
AC
the production of a large number of •OH free radicals by more H2O2 (Eq. (8) and (9)), thus facilitate rapid progress of photo-Fenton reaction. But, as the excess addition of hydrogen peroxide the rate of reaction slows down, this may be due to increasing the level of •OH radicals scavenging by H2O2 itself [34, 35], just as shown from the reaction Eq. (11) and (12). From Fig. 7B, the apparent rate constant of photocatalytic reaction under the different concentration of H2O2 is 0.01121, 0.01287 and 0.01371 min-1, respectively. This shows the apparent rate constant increases as increasing the amount of H2O2. But from Fig. 7A, we can know that the reaction rate at 3 mL H2O2 is higher than that at 2 mL H2O2 just at the beginning of the reaction. The final 17
ACCEPTED MANUSCRIPT degradation percentage of RhB in the different concentration of H2O2 is 93.88%, 96.43% and 97.03% at 270 min, respectively. Therefore, for the sake of economic consideration, 2 mL of hydrogen peroxide is a suitable dosage in the current heterogenous photo-Fenton reaction of RhB solution degradation. 1.0
0.4
SC
C/C0
0.6
0
60
120
180
240
300
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Time (min)
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0.2
0.0
RI
20 mg/L 30 mg/L 40 mg/L
PT
0.8
Fig. 8. Effect of the initial concentration of RhB solution on the degradation efficiency of RhB. Reaction conditions: catalyst S2, 30% H2O2 2 mL, RhB solution
D
30 mL.
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The concentration of organism in wastewater has an important influence on its degradation. For this reason, the degradation experiments of different initial
CE
concentration of RhB solution under visible light irradiation were performed and the results were given in Fig. 8. From Fig. 8, the degradation curve of RhB almost linear
AC
decreases, and there is hardly any obvious induction period when the concentration of RhB solution is 20 mg/L. Then the induction period appears and gets longer gradually as increase of the initial concentration of RhB solution from 30 to 40 mg/L. And the degradation efficiency of RhB gradually decreases as increasing of the initial concentration of RhB solution from 20 to 30 to 40 mg/L. This is because the active sites are certain to a certain amount of catalyst, namely, the catalytic capacity of the catalyst is certain, as increase of RhB concentration, the excess RhB molecules will cover on the active sites of the catalyst, thus leading to the decrease of RhB degradation efficiency. In addition, it can also be seen that the final degradation 18
ACCEPTED MANUSCRIPT percentage of RhB is 95.94% at 240 min, 96.43% at 270 min and 86.70% at 300 min, respectively, when the initial concentration of RhB solution is 20, 30 and 40 mg/L, respectively. 1.0
A
0.2
0.0
0
60
120
180
240
Time (min)
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B
3
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Ln(C0/C)
2
2nd 4th
D
1st 3rd 5th
SC
0.4
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1st 2nd 3rd 4th 5th
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C0/C
0.6
PT
0.8
1
CE
0 0
60
120
180
240
AC
Time (min)
Fig. 9. Reusability of the catalyst and kinetic fitting curves. Reaction conditions: catalyst S2, 30% H2O2 2 mL, RhB concentration 30 mg/L. Table 3 The kinetic parameters of RhB degradation under the condition of reused catalyst. Reuse times Rate constant/ min-1
1
2
3
4
5
0.01287
0.01128
0.01006
0.00863
0.00614
R2
0.9722
0.9415
0.9470
0.9441
0.9331
19
ACCEPTED MANUSCRIPT Reusability of catalyst is one of the important factors to measure the activity and stability of solid catalysts. To this end, recovered catalyst S2 was taken as an example to exam its catalytic activity of degradation RhB. Used catalyst S2 was recovered by an external magnet, washed with deionized water and ethanol, and reused after dried. The catalytic degradation efficiencies of RhB under reused for five times of catalyst
PT
S2 were shown in Fig. 9. And the kinetic fitting curves and parameters were given in Fig. 9B and Table 3, respectively. From Fig. 9A, it can be seen that the degradation
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percentage of RhB at 270 min is 96.43%, 94.95%, 92.60%, 87.93% and 76.70%, respectively, when the catalyst S2 was reused from first time to fifth time. This
SC
indicates that as-synthesized catalyst Fe2O3/SBA-15 has excellent reusability and stability. According to our experiment, it can be found that RhB solution became clear
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and transparent when the solution was degraded by photocatalysis and after overnight in the dark (Fig. 10), indicating RhB has been degraded completely. Therefore,
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as-synthesized magnetic Fe2O3/SBA-15 nanomaterials have higher photocatalytic degradation activity and stability, and as well as a certain application prospect.
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Furthermore, as shown in Table 3, the apparent rate constant for 5 times degradation
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of RhB is 0.01287, 0.01128, 0.01006, 0.00863 and 0.00614 min-1, respectively. This shows that the degradation speed of RhB slows gradually with increase of the catalyst
AC
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repetition numbers.
20
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ACCEPTED MANUSCRIPT
Fig. 10. Photograph of RhB solution after photocatalytic reaction. (A) Primary
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catalyst reaction solution, (B) Fifth time recovery catalyst reaction solution. To investigate the elemental composition and chemical states of the photocatalyst
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S2 after photocatalytic reaction, the XPS analysis was performed and the results were presented in Fig. 11. The survey XPS spectrum of Fe2O3/SBA-15 (S2) in Fig. 11A confirmed the existents of C, Si, O and Fe elements and the content of each element is
D
O 57.4%, Fe 13.43%, C 18.27% and Si 10.9%. The higher carbon content on the
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surface of the photocatalyst indicates that the degradation products of RhB may be deposited on the surface of the catalyst. As shown in the high resolution XPS
CE
spectrum Fig. 11B, C 1s spectrum is divided into three peaks at 284.7, 286.4 and 288.8 eV, which is respectively contributed to C-C, C-O and C=C (C=O) bands [36,
AC
37]. This also shows there is the degradation produce of RhB deposited on the surface of the photocatalyst. From Fig. 11C, the peak at 103.3 eV is corresponding to Si 2p spectrum, which is the characteristic of Si-O-Si band in SiO2 of SBA-15 [38, 39]. The O 1s spectrum shown in Fig. 11D was fitted to two peaks respectively at 530.4 and 532.4 eV, which is assigned to the lattice oxygen, chemisorbed oxygen and the band of O-C [37, 40]. The Fe 2p spectrum displayed in Fig. 11E contains two peaks at binding energy of 711.3 and 724.9 eV, which is attributed to Fe 2p3/2 and Fe 2p1/2. The split of 13.6 eV between the two peaks is identical to the reported result [37, 41], indicating that the chemical state of Fe2O3 has hardly any change after photocatalytic 21
ACCEPTED MANUSCRIPT reaction. Furthermore, it can be also discovered the specific surface area, pore volume and pore size of catalyst have not obvious decrease by BET analysis of photocatalyst (S2) after used fifth times, manifesting as-prepared photocatalyst has a high stability. This may be the reason that as-synthesized catalysts have higher photocatalytic
Fe2p1/2
1.5
0.5
Si2p Fe3p
SC
Fe2p3/2
1.0
C1s
0.0 0
200
400
600
800
1000
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Binding energy (eV)
284.7 eV
Intensity (a.u.)
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D
C 1s
AC
CE
286.4 eV
280
285
PT
O1s
A
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cps 10
5
2.0
O KLL
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Survey
2.5
Fe LMM a Fe LMM b Fe LMM c
activities.
288.8 eV
290
295
Binding energy (eV)
22
1200
B
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103.3 eV
C
98
100
102
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110
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Binding energy (eV)
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96
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Intensity (a.u.)
Si 2p
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O 1s
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Intensity (a.u.)
530.4 eV O 1s scan A
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532.4 eV O 1s scan B
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535
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Binding energy (eV)
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711.3 eV Fe 2p3/2
724.9 eV Fe 2p1/2
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Intensity (a.u.)
Fe 2p
700
710
720
730
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Binding energy (eV) Fig. 11. XPS spectra of survey (A), C 1s (B), Si 2p (C), O 1s (D) and Fe 2p (E) of catalyst S2 (Fe2O3/SBA-15) used once. 23
ACCEPTED MANUSCRIPT 4. Conclusions In summary, magnetic Fe2O3/SBA-15 nanomaterials were facile synthesized by environmentally friendly impregnation and calcination method. As addition of Fe2O3, BET areas, whole pore volume and pore size of nanomaterials decrease, but their order structure still remains. SBA-15 mesoporous molecular sieves have obvious
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confinement effect, which can prevent the excessive growth of iron oxide nanoparticles. Prepared Fe2O3/SBA-15 has higher saturation magnetizations so that it
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can be expediently recycled and reused as heterogenous catalyst. As-synthesized
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Fe2O3/SBA-15 shows a higher photocatalytic activity and excellent reusability and stability on the degradation of rhodamine B under visible light irradiation and at about
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6.3 of solution natural pH. Experimental results show the structure and amount of catalyst, the concentration of H2O2 and rhodamain B are important influence factors
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on the degradation of rhodamine B. And these influence process are also discussed from kinetics. Rhodamine B can almost be degraded completely when its solution is irradiated for a certain time and after rest overnight in the dark. Therefore, we
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synthesized magnetic Fe2O3/SBA-15 nanomaterials have a good prospect of industrial
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application.
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ACCEPTED MANUSCRIPT Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (Project No. 21503125, 21502109), the Natural Science Foundation of Shaanxi Province (No. 2017JQ2017) and Key Scientific Research Program Funded by Shaanxi Provincial Department of Education (No.
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17JS025).
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1.5
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Si2p
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Fe2p1/2
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cps 10
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Fe LMM a Fe LMM b Fe LMM c
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Graphical abstract
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Highlights
1. Magnetic Fe2O3/SBA-15 was prepared by a facile immersion
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and calcination method.
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2. Saturation magnetization of as-prepared samples increases as
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heating temperature.
3. As-prepared Fe2O3/SBA-15 has a higher photocatalytic activity
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under visible light.
consecutive runs.
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4. Photocatalytic activity of samples slightly decreases after five
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5. As-synthesized photocatalyst has an excellent prospect for
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industrial application.
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