ferrite composites

ferrite composites

Available online at www.sciencedirect.com Chinese Chemical Letters 21 (2010) 122–126 www.elsevier.com/locate/cclet Preparation and characterization ...

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Available online at www.sciencedirect.com

Chinese Chemical Letters 21 (2010) 122–126 www.elsevier.com/locate/cclet

Preparation and characterization of rice husk/ferrite composites Xue Gang Chen a, Shuang Shuang Lv b, Ying Ye a,*, Ji Peng Cheng c, Su Hang Yin a a

Department of Ocean Science and Engineering, Zhejiang University, Hangzhou 310027, China b Zhejiang Institute of Geology & Mineral Resources, Hangzhou 310007, China c Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 20 April 2009

Abstract A novel ferrite composite using rice husk as substrate has been prepared via high temperature treatment under nitrogen atmosphere. The rice husk substrate consists of porous activated carbon and silica, where spinel ferrite particles with average diameter of 59 nm are distributed. The surface area of the composite is greater than 170 m2 g 1 and the bulk density is less than 0.6 g cm 3. Inert atmosphere is indispensable for the synthesis of pure ferrite composites, while different preparation temperatures of above 600 8C result in composites with similar structures and morphologies. Due to the presence of ferrite particles, this novel composite shows enhanced adsorption ability for acid orange II. # 2009 Ying Ye. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Spinel ferrite; Rice husk; Composite materials; Catalysts; Adsorbent

Spinel ferrite, with general formula of AFe2O4 (A = Mn, Zn, Ni, Co, Fe, etc.), presents excellent structural stability and catalytic ability because of its variable valence of iron and resumable structure [1,2]. Ferrite catalysts have been employed in various reactions such as organic dehydrogenation, catalytic oxidation, fenton reaction, and CO2 reduction [3–5]. When combined with activated carbon, the obtained activated carbon/ferrite composites are expected to have both adsorption and catalytic abilities. Many activated carbon-supported ferrite composites including LiFe2O4 [6], ZnFe2O4 [7], and CuFe2O4 [8] have been prepared and showed effective adsorption and catalytic ability for air and wastewater purification. However, the production cost of activated carbon is usually high and thus restricted itself to be substrates of ferrite composites. Rice husk is the milling byproduct of rice and is a major waste material of agricultural industry. Dry rice hulk contains 70–85% of organic matter and the remainder consists of silica [9,10]. The abundance and low cost make rice husk as good precursor for activated carbon. Activated carbon with high performance can be prepared from rice husk by pyrolyzing under inert atmosphere [11,12], acidification, or alkali treatment [13–15]. The production cost of activated carbon is lower using rice husk than commonly used pecan shell, wood, coal, etc. In this study, we prepared a novel ferrite composite using pyrolyzed rice husk as substrates. In a typical procedure, 50 mmol Mn(NO3)2 or Co(NO3)26H2O and 100 mmol Fe(NO3)39H2O (40.4 g) were dissolved in 200 mL anhydrous ethanol under continuous stirring. The as-obtained solutions were poured onto 50 g rice husk that had been washed

* Corresponding author. E-mail address: [email protected] (Y. Ye). 1001-8417/$ – see front matter # 2009 Ying Ye. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2009.08.003

X.G. Chen et al. / Chinese Chemical Letters 21 (2010) 122–126

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Table 1 Characteristics of the samples. Samples

RHA RHM RHC

Bulk density (g/cm3)

Average pore size (nm)

Surface area (m2/g)

Chemical compositions C (%)

SiO2 (%)

Mn (%)

Co (%)

Fe (%)

0.19  0.01 0.39  0.01 0.57  0.03

1.958 2.047 2.109

433.90  9.17 214.54  5.20 175.15  6.37

45.1  4.4 19.5  5.1 21.7  3.5

52.7 27.6 24.8

0 11.2 0

0 0 13.5

0 23.5 25.5

thoroughly. After stirred for 15 min, the products were dried at 70 8C and calcined at 950 8C for 2 h with a heating rate of 30 8C min 1 under a nitrogen atmosphere. After cooled to room temperature naturally, rice husk/MnFe2O4 composite (RHM) and rice husk/CoFe2O4 composite (RHC) were separately prepared. For comparison, rice husk was directly calcined at 950 8C for 2 h with a heating rate of 30 8C min 1 under a nitrogen atmosphere. The product was named rice husk ash (RHA). Table 1 shows the characteristics of the obtained samples. The carbon content and surface area of RHA are almost twice than that of RHM and RHC, while the bulk density of RHA is half of RHM and one third of RHC. It is suggested that the organics of rice husk has been carbonized to activated carbon, which determines the surface area and bulk density of samples. All the samples show a similar average pore size of around 2 nm. In addition, the chemical compositions of RHA, RHM and RHC are consistent with the molar ratio of MnFe2O4 and CoFe2O4 and confirm that the ferrite composites consist of activated carbon, SiO2 and ferrite. The XRD patterns of samples are shown in Fig. 1. All three samples show an identical set of diffraction peaks that marked with ‘‘&’’, which can be indexed to SiO2 (JCPDF#82-0512). No other phases including carbon can be found in the XRD pattern, indicating that the activated carbon in all samples is amorphous. In addition to the diffraction peaks of SiO2, RHM presents another set of peaks that marked with ‘‘’’, which can be easily indexed as MnFe2O4 (JCPDF# 74-2403). Correspondingly, a set of sharply peaks that marked with ‘‘*’’ can be identified in the XRD pattern of RHC, which can be steadily ascribed to CoFe2O4 (JCPDF# 03-0864). Calculated from the ferrite (3 1 1) peak corresponding to 2u = 358, the Scherrer analysis [16] indicates that the average crystalline sizes of MnFe2O4 and CoFe2O4 are 22 nm and 31 nm, respectively. Fig. 2 shows the SEM patterns of the samples. Numerous pores that formed by the decomposition of organics are distributed on the surface and inner part of RHA (Fig. 2A). These pores are varying from 0.3 mm to 30 mm in diameter and slit-like to ellipsoidal in shapes. The substrates of RHM and RHC, however, are relatively smooth and even no pores can be found (Fig. 2B and C), indicating that the initial generated pores have been covered by the afterwardsynthesized ferrites. Covered on the substrate, the MnFe2O4 of RHM are monodisperse nanoparticles with diameters of 20–900 nm (average = 59 nm) and 73% particles of less than 100 nm (Fig. 2D). In contrast, the CoFe2O4 of RHC is

Fig. 1. XRD patterns of as-obtained samples: (A) RHA, (B) RHM, and (C) RHC.

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Fig. 2. SEM patterns of as-prepared samples: (A) RHA, (B) RHM, and (C) RHC. (D) is the size distribution of MnFe2O4 particles in RHM.

blocked on the substrate in micro-clusters with diameter of greater than 1 mm. The SEM observations prove that rice husk/ferrite composites have been successfully prepared. In addition, we adjusted the preparation conditions of RHM to investigate the effects of calcination temperature and atmosphere on the structure and morphology of ferrite composites. As shown in Fig. 3A–C, the crystal structure and surface morphology of RHM are almost identical at different preparation temperatures of above 600 8C. The sole change is the intensity of the XRD peaks and the grain size of ferrite particles increase slightly with the temperatures.

Fig. 3. XRD and SEM patterns of RHM that prepared under 600–800 8C (A–C), and air atmosphere (D and E).

X.G. Chen et al. / Chinese Chemical Letters 21 (2010) 122–126

Fig. 4. Adsorption of AO7 on RHA, RHM and RHC (100 mL AO7 solution with initial concentration of 2000 mg L

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1

and 2 g adsorbent).

When under air atmosphere, however, the obtained ferrite composite presents an extra Fe2O3 phase in the XRD pattern (Fig. 3D). The corresponding SEM image merely exhibits particles of 70–500 nm in diameter and no substrate can be found (Fig. 3E). The carbon content of this ferrite composite is only 4.7  1.2 wt.%, and the crystalline size of MnFe2O4 that calculated from its XRD pattern is 29.9 nm. These changes can be attributed to the presence of oxygen in air atmosphere, which oxidized the elemental Fe to Fe2O3 and carbon to CO2. Therefore, inert atmosphere is indispensable for the synthesis of pure rice husk/ferrite composites. To compare the adsorption abilities of these rice husk/ferrite composites, we investigated the removal of acid orange II (AO7) by RHC, RHM, and RHA as a function of adsorption times. As shown in Fig. 4, although the surface area of RHA is almost twice than that of RHM and RHC, the AO7 removal of RHM and RHC is higher than that of RHA at all stages. At the first 3 h, the adsorption rates of AO7 on RHM and RHC are higher than that RHA, either. The adsorption capacities of RHM, RHC and RHA are 69.4 mg g 1, 65.0 mg g 1 and 59.1 mg g 1, respectively. These results are contributed to the presence of ferrite, which enhanced the catalytic ability and retained the adsorption activity of activated carbon [8]. Note that because the surface area of RHC is lower and the particle size of ferrite in RHC is higher than that of RHM, the AO7 removal of RHC is lower than RHM. In conclusion, a novel ferrite composite using pyrolyzed rice husk as substrate has been successfully synthesized. It consists of porous activated carbon, SiO2 and ferrite particles, showing surface area of greater than 170 m2 g 1 and bulk density of less than 0.6 g cm 3. The optimum condition to prepare this composite is 600–900 8C under inert atmosphere. Due to the presence of ferrite particles, the adsorption of AO7 on rice husk/ferrite composites is higher than that on rice husk ash. This ferrite composite will extend the applications of rice husk, and may gain promising applications in air and wastewater purifications. Acknowledgment This work was financial supported by National High Technology Research and Development (863) Program of China (No. 2007AA06Z128). References [1] [2] [3] [4] [5] [6] [7] [8]

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