规则中孔氧化铈的纳米铸型合成及作为高 CO 氧化活性金催化载体的应用

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1 第 30 卷第 11 期催化学报 2009 年 11 月 Vol. 30 No. 11 Chinese Journal of Catalysis November 2009 文章编号 : (2009) 研究论文 : 1085~1090 规则中孔氧化铈的纳米铸型合成及作为高 CO 氧化活性金催化载体的应用 张慧丽, 阎晓洁, 李文翠 大连理工大学化工学院精细化工国家重点实验室, 辽宁大连 摘要 : 以有序的中孔炭材料 CMK-3 为模板, 以硝酸铈为前体, 利用纳米铸型法合成了具有规则结构的中孔氧化铈, 考察了模板脱除温度对中孔氧化铈微结构的影响. 热重 元素分析 X 射线衍射 透射电镜和氮气物理吸附结果表明, 炭模板的脱除温度可低至 350 o C, 所得氧化铈具有二维六方规则结构, 比表面积高达 167 m 2 /g, 孔径分布集中在 3~5 nm. 采用胶体沉积法将 2~5 nm 的金溶胶粒子沉积到所得氧化铈表面制得了 Au/CeO 2 催化剂, 考察了 Au/CeO 2 在 CO 氧化反应中的活性. 结果表明, 该催化剂的活性较常规氧化铈制备的金催化剂有明显提高, 这可能与载体的规则结构有关. 关键词 : 氧化铈 ; 炭模板 ; 纳米铸型 ; 金 ; 一氧化碳 ; 氧化中图分类号 :O643 文献标识码 :A Nanocast Ordered Mesoporous CeO 2 as Support for Highly Active Gold Catalyst in CO Oxidation ZHANG Huili, YAN Xiaojie, LI Wencui * State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian , Liaoning, China Abstract: Ordered mesoporous ceria oxides (CeO 2 ) were synthesized by nanocasting using ordered mesoporous carbon (CMK-3) as the template and cerium nitrate as the CeO 2 precursor. The as-prepared materials were characterized by thermogravimetry, elemental analysis, powder X-ray diffraction, transmission electron microscopy, and N 2 adsorption. The obtained ceria exhibits a 2-D hexagonal ordered structure with a high surface area of up to 167 m 2 /g and pore size in the range of 3 5 nm. The carbon template can be completely removed at a temperature as low as 350 o C. Such ceria was then employed as the support for the preparation of gold catalysts by the colloid deposition method. The catalytic activity of the Au/CeO 2 catalyst was evaluated in CO oxidation. An enhanced catalytic activity was observed over such a catalyst possibly due to the nano-sized particle and narrow pore size of the ordered mesoporous ceria. Key words: ceria; carbon template; nanocasting; gold; carbon monoxide; oxidation Cerium oxide (CeO 2 ) has inspired much research interest due to many applications such as high temperature ceramics [1], catalysts [2,3], solid oxide fuel cells [4], oxygen sensors [5], and capacitor devices. Especially, ceria is a suitable catalyst or catalyst support [6,7] for a number of reactions such as carbon monoxide oxidation [8,9], low-temperature water gas-shift reaction [10], catalytic combustion of volatile organic compounds [11], decomposition of nitrous oxide, etc. The high catalytic activity of ceria in the redox reaction is associated with the many oxygen vacancies in the ceria matrix and the low redox potentials between Ce 3+ and Ce 4+. For instance, gold supported on ceria has been reported as an effective catalyst for CO oxidation [12]. Excluding the function of the active gold component, Carrettin et al. [13] have demonstrated that the rate of CO oxidation on gold deposited on nanocrystalline particles of cerium dioxide was a hundred times higher than that of Au on a regular CeO 2 support. For selective aerobic oxidation of Received date: 5 June *Corresponding author. Tel/Fax: ; wencuili@dlut.edu.cn Foundation item: Supported by the Key Project of the Chinese Ministry of Education (108138) and the Basic Funding of Dalian University of Technology (DUT). English edition available online at ScienceDirect (

2 1086 催化学报第 30 卷 aldehydes, they have obtained similar results [14]. Obviously, nano-structured ceria possesses certain specific physical properties compared to the conventional one. As accepted by most people, the particle size reduction down to the nanoscale will create a large number of more reactive sides or edges that would improve the reaction activity [15,16]. Therefore, the synthesis of nanostructed ceria, especially with ordered mesoporous structure, has been widely explored. Ordered mesoporous ceria has been synthesized by different methods [17,18]. For example, using organic amphiphiles as structure-directing units, ordered mesoporous cerium (IV) oxide has been obtained in aqueous media under mild conditions [19,20]. However, a following thermal treatment that allows the materials to crystallize is often required to satisfy desired applications. Nevertheless, it would unavoidably collapse the pore structure. Extraction/ion exchange at room temperature can be employed to remove the organic amphiphiles; however, crystallization of the pore walls would not happen in this case. In contrast, the material prepared by nanocasting pathway shows a big advantage, which can endure much higher reaction temperatures without the loss of nanostructure of metal oxide than the one prepared by the aforementioned method [21]. The ordered mesoporous ceria replicated from mesoporous silica or carbon has high surface area and pore volume, and the pore wall of this material is well crystallized. Mesoporous CeO 2 has been synthesized by the nanocasting pathway using the mesoporous SBA-15 or KIT-6 silica as structure templates [22,23]. As an alternative and more advantageous template, mesoporous carbon has also been used to nanocast mesoporous ceria. In this case, the carbon template can be easily removed through combustion instead of a treatment with HF or concentrated NaOH solution in case of silica template removal. The catalytic tests reveal that the activity of the mesoporous ceria in methanol decomposition is substantially higher than that for a non-porous sample [24]. Hereby, we report the synthesis of ordered mesoporous CeO 2 materials by nanocasting using ordered mesoporous carbon (CMK-3) with 2-D hexagonal periodic arrangement as the template. The temperature of carbon template removal by combustion was explored in detail to maintain the ordered structure of ceria wall. The obtained material was used as support for deposition of Au nanoparticles, and the catalytic activity of the Au/CeO 2 obtained was evaluated in the CO oxidation reaction. In order to compare the effect of ordered mesoporous ceria as support, disordered ceria was also prepared by nanocasting using disordered porous carbon aerogel as template. 1 Experimental 1.1 Preparation of the mesoporous CeO 2 The template of CMK-3 was synthesized through a nanocasting pathway as previously reported by us [25]. For the preparation of ceria, CMK-3 (0.5 g) was impregnated with a saturated solution (890 μl) of Ce(NO 3 ) 3 (Tianjin Guangfu Fine Chemicals). After drying at 50 o C, the sample was thermally treated at 200 o C under nitrogen to decompose the Ce(NO 3 ) 3. In order to solidify the ordered structure of ceria, this process was repeated once. Finally, the sample was subject to combustion in air at different temperature (350, 400, and 500 o C) for 4 h to remove the carbon template, and the powder products were collected and denoted CeO 2-350, CeO 2-400, and CeO 2-500, respectively. For comparison, ceria was also templated with disordered carbon aerogel with large mesopores (A BET = 623 m 2 /g, V total = 1.33 cm 3 /g), which was obtained by polymerization of resorcin with formaldehyde. This sample was named CeO 2 - C400, implying a template removal temperature of 400 o C. 1.2 Characterization of the mesoporous CeO 2 Thermogravimetry (TG) was performed under nitrogen atmosphere flow with a Mettler-Toledo TGA/SDTA851 e thermobalance. The residual carbon content was determined in a Vario EL III analyzer (Elementar, German). The powder X-ray diffraction (XRD) of the product was recorded on a Philips X pert PRO X-ray diffractometer (Cu K α radiation, λ = nm). Transmission electron microscopy (TEM) analyses were carried out with a Tecnai G220S-Twin instrument operated at 200 kv. The samples for TEM analysis were prepared by dipping carbon-coated copper grids into the ethanol solutions of ceria nanoparticles and drying at room temperature. N 2 adsorption measurement was performed at 196 o C using a micromeritics ASAP 2020 device. Before measurement, samples were degassed at 80 o C under vacuum for 4 h to remove all adsorbed species. The surface area was determined by the Brunauer-Emmett- Teller (BET) method. 1.3 Preparation of the Au/CeO 2 catalyst and activity test The ceria-supported gold catalyst was prepared by the colloidal deposition method as described elsewhere [26]. The gold colloidal solutions were prepared using poly vinyl alcohol (PVA, M W = 10000) as the protecting agent. The support was then added to the colloidal gold solution under stirring and kept in contact until total adsorption (1 wt% of gold on the support) had occurred, which was indicated by decoloration of the solution. The solids were collected by centrifugation followed by washing with distilled water and dring under a vacuum of 1 Pa in a desiccator. The activity of

3 第 11 期张慧丽等 : 规则中孔氧化铈的纳米铸型合成及作为高 CO 氧化活性金催化载体的应用 1087 the catalysts for CO oxidation was measured using 50 mg of sieved catalysts ( μm) in a gas mixture of 2% CO-1% O 2-97% N 2 at a flow rate of 67 ml/min (corresponding to a space velocity of 80 L/(g h)). The concentrations of CO 2 and CO were analyzed at the outlet of the reactor by nondispersive IR spectroscopy using two URAS 3E instruments (Hartmann and Braun). The system was calibrated with a gas mixture of 1% CO 2 /N 2. CO conversion (X(CO)) was calculated as X(CO) = ([CO] in [CO] out )/ [CO] in. 2 Results and discussion 2.1 Textural properties of the as-synthesized CeO 2 Generally, the carbon template can be completely removed by combustion at high temperature of at least 550 o C in the nanocasting process [27]. A high-temperature treatment always leads to a decrease in specific surface area of the carbon replica. Ceria performs catalytically very active in oxidation reactions; it thus may catalyze the combustion of carbon during the template removal. Therefore, the thermal analysis of the composite was performed and the results of TG and its derivative (DTG) curves are shown in Fig 1. The mass loss of the composite occurs mainly in three steps. The first one below 100 o C may be ascribed to the physisorbed water, the second step at o C could mainly correspond to the conversion of Ce(NO 3 ) 3 to CeO 2, and the last one ( o C) is attributed to the thermal combustion of the mesoporous carbon matrix. Obviously, the catalytic activity of ceria lowers the temperature of carbon combustion. Although there is a very weak step at o C, a prolonged combustion time will ensure a complete removal of carbon. Therefore, in the experiment, the composite was treated at 350 o C for 6 h under air to completely remove carbon and meanwhile to maintain the ordered structure maximally well. The elemental analysis yielded a Mass (%) TG Temperaure ( o C) DTG (dm/dt)/(%/min) Fig. 1. Thermal analysis of mesoporous CMK-3 carbon impregnated with Ce(NO 3 ) 3. residual carbon content of 0.587% in CeO In order to investigate the influence of the temperature of carbon removal on the structure of ceria, the composite was also treated at 400 and 500 o C. Figure 2(a) shows the wide angle XRD patterns of ceria. All the XRD peaks can be assigned, in sequence, to well crystallized ceria. The particle size of ceria calculated by the Scherrer s equation is nm for CeO 2-350, CeO 2-400, and CeO 2-500, which is quite similar to each other. The sample CeO 2 -C400 also can be attributed to crystallized ceria; however, the particle size is determined as 11.5 nm by the Scherrer s equation, which basically corresponds to the pore size of the carbon aerogel template. The low-angle XRD patterns of ceria as well as CMK-3 template are shown in Fig. 2(b). The CMK-3 template shows clear (100), (110), and (200) reflections, indicating a well ordered structure with a 2-D hexagonal symmetry. The patterns of CeO 2-350, CeO 2-400, and CeO occur at the same angle, which verifies the maintenance of ordered structure in the nanocasting process. A slightly less resolved XRD reflection was observed in CeO High temperature may cause a certain loss of ordered structure in ceria. TEM measurements further confirmed the ordered structure of ceria. As shown in Fig. 3, the obtained CeO and CeO exhibit well-distinguished 2-D hexagonal ordered Intensity Intensity CeO CeO 2-C θ/( o ) (100) (110) (200) CMK-3 CeO θ/( o ) Fig. 2. Wide-angle (a) and small-angle (b) XRD patterns of CeO 2 prepared at different template-removing temperature (TRT). (a) (b)

4 1088 催化学报第 30 卷 Fig. 3. TEM images of CeO 2 prepared at different TRT. (a) and (b) CeO 2-350; (c) and (d) CeO arrays in broad view, which demonstrated a well-performed nanocasting process in the experiments. Figure 4(a) shows the N 2 sorption isotherms of CeO 2-350, CeO 2-400, CeO 2-500, and CeO 2 -C400. Essentially, all the samples gave the type IV isotherms with a visible hysteresis loop, indicating mesoporous characteristics. With decreasing temperature of carbon combustion, the amount of adsorption volume gradually increases, corresponding to a higher surface area and pore volume. The calculated textural parameters of ceria are compiled in Table 1. A high BET surface area of up to 167 m 2 /g was obtained for CeO Pore size distribution of ceria (Fig. 4(b)) shows that all the samples from CMK-3 exhibit a narrow pore size distribution in the range of 3 5 nm, while the sample CeO 2 -C400 has a wide pore size distribution in the range of nm. According to the XRD, TEM, N 2 adsorption, and thermal analysis, ordered mesoporous ceria can be prepared by nanocasting with the CMK-3 template. Due to the catalytic combustion assisted by ceria, the carbon template can be completely removed at the low temperature of 350 o C, which is responsible for a high surface area and well-arranged ordered structure of the ceria replica. 2.2 Catalytic activity of Au/CeO 2 V/(cm 3 /g, STP) (dv/dd)/(cm 3 /(g nm) (a) 100 CeO CeO 2-C p/p 0 (b) 0.01 CeO CeO 2-C400 Supported Au/CeO 2 catalysts show very high low-temperature activity for CO oxidation [28]. The nature of the support material as well as the physical state is found to significantly influence the catalytic activity of the resulting D/nm Fig. 4. Nitrogen isotherms (a) and pore size distribution (b) of CeO 2 prepared at different TRT.

5 第 11 期张慧丽等 : 规则中孔氧化铈的纳米铸型合成及作为高 CO 氧化活性金催化载体的应用 1089 Table 1 Texture parameters of CeO 2 prepared with different TRT Sample Combustion temperature ( o C) A BET /(m 2 /g) V total /(cm 3 /g) V mic /(cm 3 /g) D * peak /nm CeO CeO CeO CeO 2 -C * * Average pore size calculated from formula of 4V total /A BET. gold catalysts [29,30]. Therefore, the ordered mesoporous CeO 2 was selected as support to prepare a gold catalyst. The colloidal deposition method was used because it allowed the isolation of support effect on the catalytic activity and excluded the influence of gold particle formation from the deposition on the support. The HRTEM image (Fig. 5) of the gold colloid shows that most of the gold particles are in the range of 2 5 nm. From our previous work [26], we know that the gold particles would not grow bigger below 350 o C, which is much higher than the current application range, so gold particles deposited on the CeO 2 should have the same size before and after reaction. Every sample was run three cycles in the CO oxidation. In the first run, Au/CeO 2 shows a very low activity, which is due to the decomposition of the protecting agent (PVA) of the gold colloids. The activities were almost identical in the second and third runs. Therefore, the temperature of 50% CO conversion (T 50% ) was evaluated from the second run. Figure 6 displays the activity results of the catalysts prepared from CeO 2-350, CeO 2-500, and CeO 2 -C400. Obviously, gold supported on ordered ceria shows a relatively high activity towards CO oxidation. The T 50% for Au/CeO and Au/CeO were 21 and 31 o C, respectively, whereas T 50% for CeO 2 -C400 reached 71 o C. This may be related to the differences of surface area and pore structure among the supports. CeO was subject to a treatment at 350 o C to remove the carbon template. At such low temperature the ceria can well maintain the ordered structure of mesoporous carbon over a wide range, as indicated by the TEM image (Fig. 3). Nevertheless, the treatments at 500 o C will unavoidably more or less lead to a loss of ordered structure and decrease of surface area. The sample CeO 2 -C400 shows a disordered structure with the lowest surface area and big particle size, which may be responsible for the relatively low activity. Chen et al. [31] reported that nano-sized ceria with a particle size of 14.6 nm was used as the gold support, and the Au/CeO 2 catalysts (Au mass content = 0.88% 1.54%) showed enhanced catalytic activities. T 100% was o C for the CO oxidation in 2.8% CO-7.2% O 2-90% N 2 at a space velocity of 51.6 L/(g h). Obviously, the catalytic activity of nanocast ceria (from CMK-3)-supported gold in our experiment is comparable to the reported results. This implies that the smaller the particle size of ceria is, the higher the activity of Au/CeO 2 will be in CO oxidation. The use of the ordered mesoporous carbon template limits the growth of ceria particles during the thermal treatment and leads to the formation of ordered mesoporous ceria with high surface area and small particle size. Such ceria as gold support exhibits a significantly improved activity for the CO oxidation. However, Au/CeO 2 -C400 nanocast from carbon aerogel shows an apparent decrease of activity due to a disordered structure. In general, in order to improve the activity of the supported catalysts, nanocasting can be used to prepare nano-sized oxide supports with ordered structure and small particle size by choosing proper templates. CO conversion (%) Au/CeO Au/ Au/CeO 2-C Temperature ( o C) Fig. 5. HRTEM image of the gold colloid. Fig. 6. CO oxidation results for the 1% Au/CeO 2 nanoparticles.

6 1090 催化学报第 30 卷 3 Conclusions Ordered mesoporous ceria has been prepared by nanocasting using mesoporous carbon as the template. The influence of the template removal temperature on the final structure of CeO 2 was investigated. A temperature as low as 350 o C is sufficient to completely remove the carbon template to ensure the formation of ordered mesoporous ceria with a high surface area and small particle size. Gold colloid particles deposited on such ceria show a better catalytic activity for CO oxidation. Acknowledgements The authors are grateful to Prof. Ferdi Schüth for useful discussion. References 1 Koichi E. J Alloys Compd, 1997, 250: Park B E, Sakai I, Tokumitsu E, Ishiwara H. Appl Surf Sci, 1997, : 赵波, 王秋艳, 葛昌华, 李光凤, 周仁贤. 催化学报 (Zhao B, Wang Q Y, Ge C H, Li G F, Zhou R X. Chin J Catal), 2009, 30: Zhen Y D, Tok A I Y, Jiang S P, Boey F Y C. J Power Sources, 2008, 178: 69 5 Jasinski P, Suzuki Z, Anderson H U. Sens Actuators B, 2003, 95: 73 6 邵建军, 张平, 唐幸福, 张保才, 宋巍, 徐奕德, 申文杰. 催化学报 (Shao J J, Zhang P, Tang X F, Zhang B C, Song W, Xu Y D, Shen W J. Chin J Catal), 2007, 28: 郑修成, 张晓丽, 王淑荣, 于丽华, 王向宇, 吴世华. 催化学报 (Zheng X Ch, Zhang X L, Wang Sh R, Yu L H, Wang X L, Wu Sh H. Chin J Catal), 2005, 26: Shen W H, Dong X P, Zhu Y F, Chen H G, Shi J L. Microporous Mesoporous Mater, 2005, 85: Pillai U R, Deevi S. Appl Catal A, 2006, 299: Yuan Z Y, Idakiev V, Vantomme A, Tabakova T, Ren T Z, Su B L. Catal Today, 2008, 131: Scire S, Minico S, Crisafulli C, Satriano C, Pistone A. Appl Catal B, 2003, 40: Wang X Y, Wang S P, Wang S R, Zhao Y Q, Huang J, Zhang S M, Huang W P, Wu S H. Catal Lett, 2006, 112: Carrettin S, Concepcion P, Corma A, Nieto J M L, Puntes V F. Angew Chem, Int Ed, 2004, 43: Corma A, Domine M E. Chem Commun, 2005: Yin L X, Wang Y Q, Pang G S, Koltypin Y, Gedanken A. J Colloid Interface Sci, 2002, 246: Xie X W, Li Y, Liu Z Q, Haruta M, Shen W J. Nature, 2009, 458: Brezesinski T, Erpen C, Iimura K, Smarsly B. Chem Mater, 2005, 17: Lyons D M, McGrath J P, Morris M A. J Phys Chem B, 2003, 107: Lundberg M, Skårman B, Cesar F, Wallenberg L R. Microporous Mesoporous Mater, 2002, 54: Terribile D, Trovarelli A, de Leitenburg C, Dolcetti G, Llorca J. Chem Mater, 1997, 9: Lu A H, Schüth F. Adv Mater, 2006, 18: Rossinyol E, Arbiol J, Peir o F, Cornet A, Morante J R, Tian B, Bo T, Zhao D. Sens Actuators B, 2005, 109: Laha S C, Ryoo R. Chem Commun, 2003: Roggenbuck J, Schäfer H, Tsoncheva T, Minchev C, Hanss J, Tiemann M. Microporous Mesoporous Mater, 2007, 101: Lu A H, Li W C, Schmidt W, Kiefer W, Schüth F. Carbon, 2004, 42: Comotti M, Li W C, Spliethoff B, Schüth F. J Am Chem Soc, 2006, 128: Lu A H, Schmidt W, Taguchi A, Spliethoff B, Tesche B, Schüth F. Angew Chem, Int Ed, 2002, 41: Sun C W, Li H, Chen L Q. J Phys Chem Solids, 2007, 68: Kozlow A I, Kozlova A P, Liu H C, Iwasawa Y. Appl Catal A, 1999, 182: 9 30 Si R, Flytzani-Stephanopoulos M. Angew Chem, Int Ed, 2008, 47: Chen Z, Gao Q. Appl Catal B, 2008, 84: 790