Effect of templates on catalytic activity of ordered mesoporous ceria for CO oxidation

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1 Advanced Materials Research Submitted: ISSN: , Vols , pp Accepted: doi: / Online: Trans Tech Publications, Switzerland Effect of templates on catalytic activity of ordered mesoporous ceria for CO oxidation Peng Yang a, Shiyu Zhou b and Jiaheng Lei * c Department of Chemistry, Wuhan University of Technology, Wuhan, Hubei, China a yp_03@163.com, b zsywhut@163.com, c yhx2000@263.com Keywords: Mesoporous ceria; CO oxidation; Hard template method. Abstract. Mesoporous ceria catalysts were prepared by nanocasting of a mesoporous carbon and KIT-6. The prepared mesoporous ceria catalysts were used to catalyze CO oxidation. The various characterization techniques were employed to study the relationship between catalyst nature and catalytic properties. The results indicate that ceria prepared by using mesoporous carbon as template possess an ordered mesoporous architecture and exhibit much better catalytic activity compared with those prepared by using KIT-6 as template. The residual SiO 2 from KIT-6 was demonstrated to be the main reason for the much worse catalytic activities by the characterization results of X-ray Fluorescence Spectrometer and FTIR spectra. Introduction Ceria-based catalysts have been extensively investigated because of their major role in three-way catalysis (TWC) for automotive exhaust-gas purification, fuel cell technology for H 2 production and purification, combustion of hydrocarbons and CO oxidation and so on [1-4]. The catalytic properties of ceria-based catalysts have been found to be highly dependent on the specific surface areas and size of ceria [1]. Mesoporous CeO 2 has high specific surface areas and porous small crystallite size, therefore mesoporous CeO 2 can improve the catalytic performance. The hard template method has advantages for the preparation of mesoporous materials with stable and controllable structure. The ordered mesoporous ceria, which is replicated using silica hard templates, shows high surface area and pore volume, and the framework of this materials is composed of crystal ceria and ceria-zirconia phase [5, 6]. Mesoporous carbon has also been applied as a hard template to prepare mesoporous materials, and carbon template was from target products by calcination. Here, we report in this paper the synthesis of ordered mesoporous ceria using KIT-6 and mesoporous carbon as a template, and the structural of mesoporous ceria were investigated. The effects of template of mesoporous ceria upon the CO oxidation activity were investigated. Experimental Preparation of the mesoporous silica (KIT-6) and mesoporous carbon (MC), have been reported previously [7, 8]. Mesoporous ceria were prepared by a conventional impregnation method. 1.0g cerium nitrate (Ce(NO 3 ) 3 6H 2 O, Aladdin) was dissolved in 10mL of ethanol (Aladdin), followed by addition of 0.5mL of KIT-6 or MC. The sample was dried at 60 o C, calcined to 600 o C in N 2 atmosphere for 3 h with a heating rate of 1 o C/min. To remove the silica template, CeO 2 /MS was treated with a hot aqueous solution of 2M NaOH. The resultant product was centrifuged and washed with distilled water, dried at 100 o C to obtain the product. The prepared sample was denoted as MS-CeO 2. To remove the carbon template, CeO 2 /MC was calcined to 500 o C in air atmosphere. The prepared sample was denoted as MC-CeO 2. Non-mesoporous structure ceria was prepared by a precipitation method. Proper amounts of Ce(NO 3 ) 3 6H 2 O was dissolved in distilled water. Under stirring, 30wt.% ammonia solution was added drop wise until ph of the mixture remained at approximately 11. The precipitate was filtered and washed with distilled water, the drying at 100 o C All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-11/05/16,06:38:49)

2 96 Advances in Chemical Engineering and Advanced Materials IV overnight. The resultant product was calcined at 600 o C in air atmosphere for 3 h. The prepared sample was denoted as P-CeO 2. The Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with a CuΚα radiation (λ= Å) operating at 40 kv, 50 ma. Transmission electron microscopy (TEM) images were taken with a JEM 2100F electron microscope operating at 200 kv. Nitrogen adsorption-desorption data were measured with a Quantachrome Autosorb-1 analyzer at 77K. Prior to the measurement, the samples were first degassed at 200 o C for at least 6 h. The surface areas were calculated by the Brunauer-Emmett-Teller (BET) method. The pore size distributions were derived from the adsorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) model. X-ray Fluorescence Spectrometer (XRF) data were used for SiO 2 content with a Axios Advanced. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet AVATAR 370 spectrometer in the transmission mode using KBr to make the sample wafers. The CO oxidation activities of the catalysts were carried out a fixed bed continuous-flow reactor (8 mm O.D.) in the constant temperature (±1K) zone of an electric furnace. A sample 100 mg of catalyst held between two quartz wool plugs. The reaction feed consisted of 1.0 vol. % CO, 2.5 vol. % O 2 and N 2 balance and the flow rated is 50ml min 1 (GHSV=30000ml h 1 g 1 ). Samples were pretreated in N 2 flow at 150 o C for 30 min. The CO and CO 2 signals were detected by the FID detector. Results and Discussion Small angle X-ray diffraction (SAXRD) patterns of KIT-6, MC, MS-CeO 2 and MC-CeO 2 are shown in Fig. 1. The well-resolved peaks in the pattern of KIT-6 and MC can be indexed reflections of the 3D cubic Ia3d, consistent with reported in literatures [7, 8]. The diffraction peaks of MS-CeO 2 and MC-CeO 2 samples are observed only one at 2θ=~1.0 o (Fig. 1(b)), indicating that the prepared samples are the ordered mesoporous structure to be widespread. Both MS-CeO 2 and MC-CeO 2 were examined by transmission electron microscopy (TEM). By examining many different particles, it has been demonstrated that this highly ordered mesoporous structure is preserved throughout each material (Fig. 2). From the analysis results of SAXRD and TEM, MS-CeO 2 and MC-CeO 2 are an ordered mesoporous materials, while MC as template gives rise to less ordered mesoporous. Fig. 1. Small angle XRD patterns for (a) MC Fig. 2. TEM images of (a) MS-CeO 2 ; (b) MC-CeO 2 and KIT-6; (b) MC-CeO 2 and MS-CeO 2 Table 1 Structural parameters and catalytic performance of ceria samples Sample BET surface area Pore size Crystallite size SiO 2 (wt.%) T 50 ( o C) (m 2 g 1 ) (nm) a (nm) MS-CeO MC-CeO P-CeO a The crystallite sizes were calculated from the Scherrer Equation from the FWHM of (111) reflection. The pore structure of sample was further confirmed by nitrogen adsorption-desorption measurements. Nitrogen adsorption-desorption isotherms and BJH pore size distribution curves of the MS-CeO 2 and MC-CeO 2 samples are shown in Fig. 3, and the corresponding pore structural

3 Advanced Materials Research Vols parameters including BET surface areas and pore sizes are summarized in Table 1. A type IV isotherm observed for all samples, indication that these samples are the ordered mesoporous structure, consistent with the results of SAXRD and TEM. Similar H 3 -type hysteresis loop are observed for these composites, implying that part pore channels may be caused by aggregates of particles giving rise to interparticle pores. The BET surface areas for MS-CeO 2 and MC-CeO 2 are m 2 g 1 and m 2 g 1 (Table 1). Pore size distributions and average pore size, calculated from the adsorption branches data, exhibit a narrow distribution for the samples centred at around 3.4 nm and 3.0nm. Fig. 3. N 2 adsorption-desorption isotherms and BJH Fig. 4. XRD of MC-CeO 2, MS-CeO 2 and size pore distributions of MS-CeO 2 and MC-CeO 2. P-CeO 2 The wide-angle XRD patterns of MS-CeO 2, MC-CeO 2 and P-CeO 2 are shown in Fig. 4, and the corresponding crystallite size are summarized in Table 1. The wide-angle XRD patterns confirm that all samples present the oxide phase with the cubic, fluorite-type structure typical of CeO 2. All of the diffractograms contain the six main reflections typical of a fluorite-structured material with an fcc unit cell, corresponding to corresponding to (111), (200), (220), (311), and (222) respectively [5]. Average crystallite sizes of MC-CeO 2, MS-CeO 2 and P-CeO 2 is 9.1nm, 8.2nm, 11.8nm, respectively. It show that KIT-6 and MC templates prevent crystallite of CeO 2 growth. Fig. 5. Catalytic activity of all catalyst for CO Fig. 6. FTIR spectra of MS-CeO 2 and MC-CeO 2. oxidation. The catalytic activities of catalysts for CO oxidation are shown in Fig. 5, and the temperatures of T 50 at which the CO conversion reached to 50% were summarized in Table 1. It can be seen that the catalytic activity of MC-CeO 2 and MS-CeO 2 are significantly higher than P-CeO 2, because both of them have higher surface area and smaller crystallite size than P-CeO 2 (Table 1), which is favorable to catalytic reaction [2]. Compared with MS-CeO 2, MC-CeO 2 demonstrates much better catalytic activity for CO oxidation. In consideration of the same preparation procedure for the two mesoporous

4 98 Advances in Chemical Engineering and Advanced Materials IV CeO 2 except for the type of template, we can suppose the residual SiO 2 from KIT-6 might be responsible for the inactivity of MS-CeO 2. To identify the assumptions, the silica content of MS-CeO 2 and MC-CeO 2 were analyzed by XRF. The silica content of MS-CeO 2 and MC-CeO 2 are shown in Table 1. Samples were removed the template, Residual silicon content of MS-CeO 2 and MC-CeO 2 is 3.3wt.% and 0, respectively. The XRD patterns results show that the peaks ascribable to SiO 2 species were not observed, implying that the SiO 2 is highly dispersed in the sample or belong to amorphous. Fig. 6 presents the FTIR spectra of MS-CeO 2 and MC-CeO 2. In the region from 1300 cm -1 to 400 cm -1, the vibration bands 1090 cm -1 is assigned to Si-O-Si asymmetric stretching; ~970 cm -1 to Si-OH and Si-O-Ce stretching; and the ~500 cm -1 strong peaks corresponding to fluorite type CeO 2. We can clearly see that the peak of MS-CeO 2 for symmetric stretching vibration of Si-O-Si bond, Si-OH bond and Si-O-Ce bond [9, 10]. Combining the catalytic properties and the characterization results, it can be deduced that the residual SiO 2 in catalyst is the main factor accounts for the inactivity of MS-CeO 2. The main reason is due to the fact that highly dispersed SiO 2 in MS-CeO 2 sample hinder to contact CO with an oxygen atom from a surface (-Ce IV -O- linkage) of ceria. Conclusions Ordered mesoporous ceria with cubic fluorite-structure were successfully prepared by using KIT-6 and MC templates. The CeO 2 possess order mesoporous architecture show promising catalytic performance for CO oxidation. However, the mesoporous CeO 2 prepared by hard-template method using the KIT-6 owns much worse activity for CO oxidation. The negative effect of KIT-6 as template can be mainly corroborated to the residual SiO 2, which hinder to contact CO with active sites of ceria, resulting in very low activity for CO oxidation. References [1] Zhao Z, Jin R, Bao T, et al. Applied Catalysis B: Environmental, 2011(110): [2] Di Monte R, Kašpar J. Journal of Materials Chemistry, 2005,15(6): [3] Aneggi E, Divins N J, de Leitenburg C, et al. Journal of Catalysis, 2014,312: [4] Yuan Q, Liu Q, Song W G, et al. Journal of the American Chemical Society, 2007,129(21): [5] Abdollahzadeh-Ghom S, Zamani C, Andreu T, et al. Applied Catalysis B: Environmental, 2011( ): [6] Ying F, Wang S, Au C T, et al. Microporous and Mesoporous Materials, 2011,142(1): [7] Laha S C, Ryoo R. Chemical Communications, 2003(17): [8] Liu D, Lei J H, Guo L P, et al. Carbon, 2012,50(2): [9] Liu B, Baker R T. J. Mater. Chem., 2008,18(43): [10] Dai Q, Wang X, Chen G, et al. Microporous and mesoporous materials, 2007,100(1):

5 Advances in Chemical Engineering and Advanced Materials IV / Effect of Templates on Catalytic Activity of Ordered Mesoporous Ceria for CO Oxidation /