Effects of the synthesis methods on the physicochemical properties of cerium dioxide powder

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1 Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) Effects of the synthesis methods on the physicochemical properties of cerium dioxide powder Mei Li a,b, Zhaogang Liu b, Yanhong Hu b, Zhenxue Shi b, Hangquan Li a, a College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing , China b Material and Metallurgy School, Inner Mongolia University of Science and Technology, Baotou , China Received 1 October 2006; received in revised form 3 December 2006; accepted 12 December 2006 Available online 16 December 2006 Abstract The CeO 2 powders were synthesized using several different methods and characterized by X-ray diffraction (XRD), laser diffraction particle size analyzers, BET surface area analysis and transmission electron microscopy (TEM). The results show that CeO 2 crystals prepared by different methods possess all the same fluorite structure. On the other hand, the physicochemical properties including particle size, cubic specific surface area, fluidity and density of cerium dioxide powder depend on the different synthesis procedures. This means that the properties requested for industrial applications can be tuned by generative methods. The calcined temperature is another very important parameter for the preparation of monodisperse superfine powder Elsevier B.V. All rights reserved. Keywords: Cerium dioxide; Specific surface area; Particle size; Crystal structure; Fluidity 1. Introduction The rare earth composites were praised as the treasury of materials due to their special optical, electric and magnetic properties that were given by the specific f electron of rare earth elements [1]. Studies on rare earth materials attracted an extensive interest with the development of new technologies. Various rare earth materials possessing different physicochemical properties have been produced due to the demand of different applications. For example, a powder having different particle size, density, specific surface area and morphology can be used in different industry areas. CeO 2 powder is widely used in ceramic, catalyst, polishing, luminescence and solid electrolyte materials [2]. However, its performance in different application areas is directly determined by its physicochemical properties. For example, super fine CeO 2 can decrease the calcination temperature and increase the density of ceramic; various particle size of CeO 2 can meet the different demands of polishing, in general, the polishing of semiconductor will need CeO 2 powder with a fine particle size [3]; in catalytic area, using CeO 2 with a higher specific sur- Corresponding author. Tel.: ; fax: address: lihq@mail.buct.edu.cn (H. Li). face area as carriers can enhance catalytic activity of catalyst [4]; the glass additive need a higher loose density of CeO 2 [5]. The physicochemical properties of CeO 2 are so important for its application that the synthesis of CeO 2 powder with different physicochemical properties attracts widely investigate interests. The preparation methods of CeO 2 powder are mainly precipitation [6], hydrothermal [7,8], sol gel [9], alkoxide method [10], microemulsion [11], template synthesize [12], gas reaction [13,14], etc. In this study, several improved precipitation and sol gel methods were used to prepare CeO 2 powders. The physicochemical properties of CeO 2 powders were characterized. The crystal structure of cerium dioxide was determined by XRD and TEM measurements. The effect of calcination temperature on crystal properties was also investigated. This work provides much useful information for the industry and fundamental research of rare earth materials. 2. Materials and methods 2.1. Sample preparation Cerium carbonate [Ce 2 (CO 3 ) 3 ] was provided by Inner Mongolia Baotou Steel Rare-Earth Hi-Tech Co., Ltd /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.colsurfa 转载

2 154 M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) (CeO 2 / REO 99.9%, REO 45%). Cerium carbonate was purified and dissolved by nitric acid in order to get 1 M Ce(NO 3 ) 3 solution. A 2% dispersant was then added to the Ce(NO 3 ) 3 solution and were stored for the following experiments. All other reagents were of analytical grade. Cerium dioxide powders were synthesized by the following methods: Precipitation method: A given concentration ( M, depending on the different precipitation methods) precipitator was added to the Ce(NO 3 ) 3 solution at a given temperature varying between 30 and 80 C and stirring intensity (500 r/min). Keep stirring for 10 min after deposition is entirely finished. The deposition was dried at 100 C after filtrating and washing with water for several times. Sol gel method: Citric acid was added to Ce(NO 3 ) 3 solutions, the final ratio of Ce 3+ and citric acid is 1:1.2, followed by slowly evaporating to a transparent sol status under stirring at C, it will become a gel by the slowly and continuous evaporating, finally, the gel was dried at 100 C. The CeO 2 powders were gained by calcinated the precursors at C Characterizations The particle size distribution of sample was done with Coulter LS230 laser diffraction particle size analyzer. The specific surface area was tested with Coulter SA3100 specific area testing equipment. The morphology, dispersing character and phase conformation were analyzed by TEM (JEM-200CX). The crystal structure and phase conformation were analyzed with XRD (Philips PW170). The loose density and fluidity was tested with BT-1000 synthesis performance testing instrument. 3. Results and discussion 3.1. Preparation methods Several improved precipitation and sol gel methods were used to prepare CeO 2 powder in order to fulfill different demands for industry materials. Using the oxalic acid as precipitator to deposit Ce 3+ is a traditional industry preparation method for CeO 2 powder. [15] The particle size of powder obtained is about 3 6 m. However, the superfine powder is difficult to obtain by this method. Improving precipitation methods or controlling the condition of precipitation can generate the powder with nano size particles. EDTA oxalic acid complex and ammonia oxalic acid complex were used as precipitators to improve the traditional oxalic acid method and carried out as the following: first, adding EDTA into the solution of Ce 3+, EDTA combines with Ce 3+ to form a very stable complex, then a certain mount of oxalic acid is added. At this point, no precipitation formed. The deposition begins to form when HNO 3 is dropped into this solution because it destroys the complex of EDTA and Ce 3+. In this way, the speed of precipitation is well controlled to avoid the non-uniformity. On the other hand, the super saturation of system could keep proper range that makes the particle uniform. Ammonia oxalic acid deposition method is adding ammonia into the solution to form a hydroxide deposition. Then the hydroxide deposition is transformed into oxalate deposition by adding oxalic acid. This method improved the problem caused by the fine particle size of hydroxide deposition that is very difficult for afterward disposal. However, the fine CeO 2 powder can be generated by this method because the oxalate deposition of cerium was translated from very fine hydroxide deposition. Using NH 4 HCO 3 as precipitator has several advantages including the low cost, having less pollution and the easy and controllable technique. However, the phenomena of local precipitation have been observed by this method. We improved this method by gradually releasing the precipitator into the solution instead of adding the precipitator at once. The result shows no local precipitation formed. Hydrolyzing carbamide to produce NH 4 OH as precipitator is a traditional homogenous precipitation method. This method can decrease the precipitation speed of metal ions to obtain the homogenous deposition. However, quite an amount of carbamide is required to produce NH 4 OH in order to adjust the ph value since Ce 3+ only can completely precipitate at the high ph value. We used carbamide ammonia and carbamide NH 4 HCO 3 as co-precipitator and were added into Ce 3+ solution to get CeO 2 powder. It is obvious that the dosage of carbamide can dramatically decrease by this method. Using formic acid and acetic acid as precipitator is another precipitation method to synthesize cerium dioxide powder [16]. Adding hydrazine in this process of deposition yields the mixture deposition of formic cerium, acetic cerium, formic hydrazine Table 1 The physical properties of CeO 2 powder prepared by different method Methods Particle size, D 50 ( m) Specific surface area, S (m 2 g 1 ) Loose density, ρ (g cm 3 ) Fluidic index Oxalic acid precipitation EDTA oxalic acid precipitation Ammonia oxalic acid precipitation NH 4 HCO 3 precipitation Carbamide NH 4 HCO 3 precipitation Carbamide ammonia precipitation Formic acid hydrazine precipitation Acetic acid hydrazine precipitation Sol gel method

3 M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) Fig. 1. The particle size distribution of CeO 2 prepared by carbamide NH 4 HCO 3 method. Fig. 4. The XRD patterns of CeO 2 calcined at different temperature The physicochemical properties of CeO 2 powder Fig. 2. The XRD patterns of CeO 2 powder prepared by (a) common oxalate, (b) sol gel, and (c) ammonia-oxalic acid method. and acetic hydrazine. The cerium dioxide particles do not aggregate and have amount of pores after calcinations because the formic hydrazine and acetic hydrazine released a lot of heat energy and gas during the calcination process. The sol gel method is adding citric acid into the cerium nitrate solution and concentrate it to form gel, then the heating treatment is performed to generate the powder material. The process includes the following phases: molecule polymer sol gel crystal (or non-crystal). This method is used to synthesize the powder material with special structure and aggregation states [9]. The physicochemical properties of CeO 2 powders including particle size, specific surface area, loose density and the fluidity index are characterized. The results are listed in Table 1. The particle size of CeO 2 decreased with both improved methods (EDTA oxalic acid, m and ammonia oxalic acid, m) compared to the oxalate precipitation method (0.896 m). At the same time, the specific surface area increased slightly with both improved method. The loose density and the fluidity index are quite similar. This kind of CeO 2 powder is good to be used as polishing agent and pottery materials. Fig. 1 shows the particle size distribution of CeO 2 prepared by carbamide NH 4 HCO 3 method. The particle size in the middle of the cumulative volume (D 50 ) is m, and the width of particle size distribution is 0.8 calculated by R =(D 90 D 10 )/D 50. It is obvious that the particle size distribution is very narrow. The volume-particle size distribution shows a normal distribution that means CeO 2 particle obtained by this method are homogenous. Comparison with the other methods, the particle size and specific area of CeO 2 prepared by NH 4 HCO 3, carbamide NH 4 HCO 3, carbamide ammonia methods is in the medium range. This kind of CeO 2 powder can be used as pottery and catalyst materials. NH 4 HCO 3 precipitation method is a better Fig. 3. (a) The electron diffraction pattern of CeO 2 obtained by NH 4 HCO 3 method. (b) The analysis result of (a).

4 156 M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) Fig. 5. TEM of CeO 2 calcined at different temperature. (a) 500 C and (b) 900 C. industrial method to prepared CeO 2 catalyst material considering the cost. Using hydrazine improved formic acid and acetic acid precipitation method, the super fine cerium dioxide powders with a larger specific surface are obtained (103.3 m 2 /g, Table 1). This kind of cerium dioxide powder is an ideal catalyst material. The size of the particles prepared by sol gel method is small and its distribution is uniform. The specific surface area is bigger comparing with other methods (Table 1). So the powder prepared by this method is also an ideal catalyst and pottery materials candidate. The loose density of powders prepared by different methods are all relatively small except those prepared by three oxalate methods. The fluidity index of CeO 2 shows a little difference between all different methods (Table 1). Above all, we found that using different improved methods can control the physicochemical properties of cerium dioxide powder The crystal structure of CeO 2 powder Fig. 2 shows X-ray diffraction patterns of CeO 2 powders prepared by common oxalate, sol gel, and ammonia-acid methods. One notes that specific diffraction peaks of three samples are consistent with the peaks of the cubic structure of CeO 2.It obviously demonstrates that the crystal structures of these samples are cubic. On the other hand, comparing with the typical CeO 2, The specific peak of CeO 2 with a size of nanometer is relatively lower and broader which is typical character of nanomaterials [7]. This means the particle size of CeO 2 crystal is very small and the crystallization is not integrity. On the other words, the crystal structure of nano CeO 2 particles possesses some defects. Fig. 3a shows the electron diffraction pattern of CeO 2 that was prepared with NH 4 HCO 3 method. The analysis of this pattern (Fig. 3b) shows that the CeO 2 has a face-centered cubic (FCC) structure, which is the same structure as the CeO 2 prepared by common oxalate, sol gel, and ammonia acid methods The effect of calcinated temperature Fig. 4 shows X-ray diffraction patterns of CeO 2 prepared by NH 4 HCO 3 method and calcined at different temperatures. We can observe that the powder calcined at 100 C has an amorphous structure. The specific diffraction peak of CeO 2 appeared when the powder was calcined at 300 C. It means that the powder basically decomposed into CeO 2 after it was calcined at 300 C. In the calcinations reaction of all rare earth salts, the decomposed temperature of the Ce 3+ salt was usually lower than for other rare earth salts. For example, the decomposed temperature of Ce 2 (C 2 O 4 ) 3 is 500 C, but the decomposed temperature of Nd 2 (C 2 O 4 ) 3 is 800 C [17]. This is because that cerium possesses a different isotope that Ce 4+ is more stable than Ce 3+ in presence of O 2, so normally cerium is present as CeO 2 instead of Ce 2 O 3 in nature. From Fig. 4, one also notes that the product shows an obvious cubic crystal structure. The diffraction peak of CeO 2 becomes sharp with increase of the calcined temperature. It means that the degree of crystallization is enhanced and the particle size increases when powder are calcined at higher temperature. Fig. 5 shows the TEM images of CeO 2 that were obtained by calcined at 500 and 900 C, respectively. The result shows that CeO 2 powder consisted of fine particles and aggregated after calcined at 500 C. However, the particle size increased as a result of aggregation of small particle when the powder is calcined at 900 C. This is consistent with the result of XRD. It means that the calcination is also very important condition to prepare monodisperse superfine powder. 4. Conclusion CeO 2 powder having different physicochemical properties can be obtained with different methods and precipitators. All the CeO 2 obtained by different methods possess a same cubic crystal structure. Their fluidities are all similar. The fine CeO 2 powder with medium specific surface area can be obtained by EDTA oxalic acid and ammonia oxalic acid precipitation methods. This kind of CeO 2 powder is an ideal candidate for polishing and pottery material. The particle

5 M. Li et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) size and specific surface area of CeO 2 obtained by NH 4 HCO 3, carbamide NH 4 HCO 3 and carbamide ammonia methods is in the medium range. However, the particle size and specific surface area of CeO 2 obtained by formic acid hydrazine and acetic acid hydrazine methods are big which has advantages in applying as catalyst materials. The CeO 2 obtained by sol gel method has small particle size and big specific surface area, which is good for catalyst and pottery application. The calcined temperature also can control the particle size of CeO 2 powder. Indeed, the calcined temperature is also very important for the preparation of the monodisperse superfine CeO 2 powder. Acknowledgements We greatly acknowledge the financial support by the National Basic Research Foundation of China (Grant No. 2004CCA03900) and the National Natural Science Foundation of China (Grant No ). We also thank Dr. Andersen for helping us in revising the English. References [1] Q. Su, Rare Earth Chemistry, Henan Sci. & Technol. Publishing Com., Zhengzhou, 1993, p [2] X.T. Dong, X.G. Qu, G.Y. Hong, et al., Chin. Sci. Bull. 41 (1996) [3] Y.X. Li, X.Z. Zhou, Y. Wang, et al., Mater. Lett. 58 (2003) 245. [4] B. Jacques, L. Oloviero, B. Renard, et al., Catal. Today 75 (2002) 29. [5] Z.Z. Xu, Y.X. Yang, S.L. Yuan, et al., J. Rare Earth 20 (2002) 119. [6] B. Djuriclic, S. Pickering, J. Eur. Ceram. Soc. 19 (1999) [7] T. Masui, H. Hirai, N. Imanaka, et al., J. Mater. Sci. Lett. 21 (2002) 489. [8] X.T. Dong, J.H. Yan, Y. Wei, Rare Metal Mater. Eng. 1 (2002) 312. [9] W.H. Hou, L. Xu, J.H. Qiu, et al., J. Nanjing Univ. 7 (1997) 487. [10] X.T. Dong, G.X. Liu, J. Sun, et al., Rare Metal Mater. Eng. 31 (2002) 229. [11] S. Shi, Y.H. Lu, H.Q. Wang, Chemistry 12 (1998) 51. [12] L.D. Zhang, W.P. Cai, C.M. Mo, Prog. Natural Sci. 9 (1999) 103. [13] N. Guillou, L.C. Nistor, H. Fuess, et al., Nanostructured Mater. 8 (1997) 545. [14] R.X. Valenzuela, G. Bueno, A. Solbes, et al., Topics Catal. 15 (2001) 181. [15] G.X. Xu, Rare Earth, Metallurgical Industry Press, Beijing, 1995, p. 68. [16] H.G. Brittain, P.S. Grade, J. Less Common Met. 94 (1983) 277. [17] Y.W. Zhang, Z.G. Yan, C.S. Liao, et al., J. Rare Earth 19 (2001) 378.