Cerium dioxide with large particle size prepared by continuous precipitation

JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009, p. 991

Cerium dioxide with large particle size prepared by continuous precipitation LI Mei (ᴢṙ), WANG Mitang (⥟㾙ූ), LIU Zhaogang (᷇ী߮), HU Yanhong (㚵㡇ᅣ), WU Jinxiu (ਈ䫺㒷) (School of Material and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014010, China) Received 28 August 2008; revised 27 April 2009

Abstract: Cerium dioxide (CeO2) has attracted much attention and has wide applications such as automotive exhaust catalysts, polishing materials for optical glasses and additives for advanced glasses, as well as cosmetic materials. The particle size and its distribution are key factors to the performance of the materials in the functional applications. However, control of particle size is still a challenge in materials synthesis. Therefore, continuous precipitation of cerium oxalate (precursor of ceria) was carried out at different operational conditions such as reactant concentration, agitation speed, feeding rate and reaction temperature, and their effect on the particle size distribution, distribution width and morphology of cerium oxalate were investigated. The optimum conditions for preparing cerium oxalate with large particle size were determined based on orthogonal test as follows: [Ce(NO3)]=0.02 mol/L, agitation speed: 200 r/min, feeding rate of solutions of cerium nitrate and oxalic acid: 10 ml/min, reaction temperature: 80 °C. The results showed that the shape of cerium oxalate was sheet and the phase structure was amorphous. The median particle size of the final product was 27.60 ȝm, the particle size distribution width was very narrow and the micrograph was still sheet-like. Some attempts were made to explain the experimental phenomena in terms of agglomeration, disrupt, and precipitation kinetics. Keywords: cerium oxalate; continuous precipitation; large particle; particle size distribution; morphology; rare earths

During the past decades, rare earth metals and their compounds (mostly oxides) have been found to be versatile as key materials for high temperature application, catalysts, fluorescence, laser applications, and electronic applications such as memory chips and superconductors[1–3]. Cerium is the most abundant rare earth element in both bastnasite and monazite. Cerium dioxide (CeO2) has important and wide applications such as automotive exhaust catalysts, polishing materials for optical glasses and additives for advanced glasses[4,5] , as well as cosmetic materials. With the development of functional materials, great interest has been focused on the synthesis and characterization of ceria powder with some distinct properties. Therefore, a large number of studies have been worked out to relate properties of precursor to that of the final product and prepare precursor under those conditions, which can lead to the production of oxides of desirable characteristics[6–16]. The need and interest in relating properties of the precursor to the final product is important because properties of the final product are inherited from that of the precursor. Monitoring and control at the early stage and suitable modification of the characteristics of the precursor itself will be of great importance for processing technology. So in order to improve

the efficiency during sintering process, which also significantly affects the final material's properties, precursor of desirable properties is required. Therefore, various methods have been proposed to prepare metal oxide particles of different properties. These methods may largely be classified into three categories, such as solid, liquid and vapor methods, depending on the phase of materials involved. Solidphase methods include thermal dissociation[6–8] and mechanical milling[9,10]. Liquid-phase methods are based on direct chemical precipitation, homogeneous precipitation[11,12], hydrolysis, and sol-gel techniques[13–15]. Lastly, vapor condensation, vapor-vapor, vapor-liquid and vapor-solid reaction are all typical examples of vapor-phase methods[16]. Among these methods, it is generally agreed that liquid-phase methods are suitable to precisely control chemical composition and industrialize easily because of simple process and devices as well as cheap raw materials. Research on nano-materials prepared by batch precipitation has been active in the past few years[17,18], and decomposition process and mechanism of rare earth oxalates have been reported. However, control of particle size and distribution of powder prepared by continuous precipitation is still a challenge in material synthesis. In fact, particle size and distribu-

Foundation item: Project supported by the National Natural Science Foundation of China (2056601, 50662002) Corresponding author: LI Mei (E-mail: [email protected]; Tel.: +86-472-5934390) DOI: 10.1016/S1002-0721(08)60376-2

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tion are key factors to the performance of materials in functional applications. CeO2 as fining and decolorizing agent of glass, for example, its particle size is very important for uniformity of glass batch whose particle size is larger, so fining and decolorizing effect on the glass is close to the particle size of CeO2.

1 Experimental 1.1 Reagents Cerium carbonate (Ce2(CO3)3) was purchased from Inner Mongolia Baotou Steel Rare-Earth Hi-Tech Co., Ltd. (CeO2/ ™REO•99.9%, ™REO•45%). Ce(NO3)3 standard solution was prepared as follows. First, any trace of impurities was removed, then, it was dissolved in nitric acid (A.R.), and the remaining acid was removed by prolonged heating above the solvent boiling point. Ce(NO3)3 thus obtained was further diluted using distilled water, and the solid concentration was adjusted to 0.518 mol/L (pH=2.5–3.5), and was used as standard solution. The standard aqueous oxalate solution was prepared, and its concentration was adjusted according to concentration of Ce(NO3)3. All other reagents were of analytical grade. 1.2 Equipment LS-230 Laser particle sizer (Coulter, USA); WH-S stirring (Tianjin Weihua Instruments Co., Ltd.); SHB-III vacuum pump (Zhengzhou Dufu Instruments Co., Ltd.); BT01-YZ1515 peristaltic pump (Tianjin Xieda Instruments Co., Ltd.); DZKW-D-4 Constant temperature water bath (Beijing Yongguangming Inc.); 101C-2 Oven (Shanghai Experiment Instruments Co., Ltd.). Scanning electron microscopy S3400 (Japan). 1.3 Preparation of cerium oxalate precursor and ceria A stirred tank reactor with a working volume of 1.2 L is used as continuous crystallizer (self made) for preparing product. It is equipped with a draft tube and a flat bottom. A plate propeller is used as stirrer. An industrial reactor with a working volume of 100 L is used as ageing crystallizer (to keep recovery). It is equipped with a shaped bottom and a two blade propeller. The heating device is necessary to maintain the desirable temperature for experiment. The experimental sketch is shown in Fig. 1. The prepared Ce(NO3)3 and H2C2O4·2H2O solution were added into the continuous crystallizer simultaneously at the same feeding rate with a peristaltic pump. After ageing, the precipitate was filtrated and washed with distilled water, after filtration, the precipitate was dried in the oven at 100 °C for 24 h. Then the final product of ceria was obtained by calcining the precipitates at

Fig. 1 Experimental setup A: Continuous crystallizer; B: Growth tank; C: Two blade propeller

800 °C for 2 h.

2 Results and discussion 2.1 Concentration of reactant Ce(NO3)3 (0.518 mol/L) obtained was further diluted using distilled water to adjust the solid concentration to 0.02, 0.06, 0.10, and 0.14 mol/L. The aqueous oxalate solution was prepared, its concentration (0.03, 0.09, 0.15, and 0.21 mol/L) was adjusted stoichiometrically according to Ce(NO3)3 in order to make the experiment work simultaneously. The particle size distribution and width of cerium oxalate were collected in Table 1 under different concentrations of reactant. A series of particle size distribution of samples shown in Fig. 2 presents the effect of reactant concentration on particle size, its distribution and width of cerium oxalate. All other experimental conditions, such as agitation speed, reaction temperature and feeding rate, were all fixed. As anticipated, the effect of concentration of reactant on particle size and size distribution is very significant, the median size (D50) of samples varies from 59.63 to 53.05 ȝm with the concentration of cerium nitrate ranging between 0.02 and 0.14 mol/L. As shown in Fig. 2, as the concentration of cerium nitrate is lower than 0.14 mol/L, the median particle size decreases from 59.63 to 39.70 ȝm; when the concentration of cerium nitrate is 0.14 mol/L, the median particle size increases to 53.05 ȝm. These are the result from nucleation and growth kinetics. When the reactant concentration is low (0.02 mol/L), the surpersaturation is also low, so there are few nuclei in the precipitator, but the remaining ionic (Ce3+ and C2O42–) improves the growth of nuclei, therefore, the particle size is larger than those of 0.06 and 0.10 mol/L. Severe agglomeration, however, at higher concentration such as 0.14 mol/L, takes place among particles, thereby, the particle size also

LI Mei et al., Cerium dioxide with large particle size prepared by continuous precipitation

becomes larger. From Table 1 and Fig. 2, it also can be seen that the distribution width is the smallest in all samples at cerium nitrate concentration of 0.14 mol/L, although its median particle size is not the smallest. 2.2 Agitation speed Table 2 shows particle size distribution and distribution width of the samples as a function of agitation speed. From Table 2, one important rule can be observed. The effect of agitation speed on particle size distribution and distribution width of sample is very important. The median size (D50) decreases from 59.63 to 27.82 ȝm, then increases to 49.87 ȝm with the agitation speed ranges between 200 and 500 r/min. It is known that agitation speed has two effects on precipitation process, one of which is the enhancement of the diffusion ability of ionic, the other is shear force which makes the unstable particles break. During the precipitation process, when the strength of the aggregate formed is not sufficient to resist against the intensity of shear force, the aggregate will be broken into two or several smaller particles. Thereby, these two effects (positive and negative) make the smallest media particle size (27.82 ȝm) occur at 400 r/min. This result is in agreement with the phenomena reported in Refs. [19, 20]. In view of distribution width of sample, agitation speed of 300 r/min is the best condition (that is near-monodisperse) as evidenced in Fig. 3. Table 1 Particle size distribution of cerium oxalate vs. concentration of reactant

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2.3 Feeding rate Both solutions of cerium nitrate and oxalic acid were added into the continuous precipitator with peristaltic pump. Feeding rate is a very important factor in continuous precipitation. The effect of feeding rate of raw materials on residence time of the newly formed particles and supersaturation of solution is significant. Table 3 and Fig. 4 are the effect of feeding rate of raw materials on particle size distribution and distribution width of samples. As shown in Table 3 and Fig. 4, the median size of the particles decreases from 53.90 to 35.54 ȝm with the increase of feeding rate from 10 to 40 ml/min. However, the dependence of distribution width of sample on feeding rate exhibits opposite trend. These phenomena are the result of short residence time caused by much higher feeding rate. Although the surpersaturation becomes larger and larger with the increase of feeding rate, the particles do not have enough time to grow and aggregate, so the size becomes smaller and smaller. Table 2 Particle size distribution of cerium oxalate vs. agitation speed Agitation speed/

Particle size distribution

(r/min)

D10/ȝm

D50/ȝm

D90/ȝm

Distribution width/R

200

9.257

59.63

112.1

1.725

300

30.76

53.90

78.81

0.8915

400

13.89

27.82

42.84

1.041

500

9.739

49.87

85.06

1.510

Table 3 Particle size distribution of cerium oxalate vs. feeding rate

Cerium nitrate/

Oxalic acid/

Particle size distribution

Distribution

(mol/L)

(mol/L)

D10/ȝm

D50/ȝm

D90/ȝm

width/R

Feeding rate/

Particle size distribution

0.02

0.03

9.257

59.63

112.1

1.725

(ml/min)

D10/ȝm

D50/ȝm

D90/ȝm

width/R

0.06

0.09

14.81

46.00

84.31

1.511

10

30.76

53.90

78.81

0.8915

0.10

0.15

13.67

39.79

66.25

1.321

20

12.42

42.04

68.85

1.342

0.14

0.21

21.19

53.05

80.45

1.117

30

8.966

41.62

80.47

1.718

40

8.599

35.54

68.15

1.676

Note:

R

D90  D10 D



Distribution

50

Fig. 2 Particle size distribution of samples vs. reactant concentration

Fig. 3 Particle size distribution of sample vs. agitation speed

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Fig. 4 Particle size distribution of sample vs. feeding rate

With the longer residence time (at a feeding rate of 10 ml/min), the smaller particles aggregate, therefore, the distribution width becomes wider. 2.4 Reaction temperature Similar to other phase transition behaviors, such as crystallization, nucleation and growth which are involved, the rate of cerium oxalate precipitation will critically depend on reaction temperature. In order to study this effect, reaction temperature was varied between 20 and 80 °C. Other experimental conditions were kept constant ([Ce(NO3)3]: 0.02 mol/L; [H2C2O4]: 0.03 mol/L; agitation speed: 200 r/min; feeding rate: 10 ml/min). SEM micrographs in Fig. 5 show the morphology of cerium oxalate formed under different reaction temperatures. The variation of size and size distri-

bution of samples as a function of reaction temperature is presented in Table 4. As shown in Fig. 5, within the studied temperature range, precipitates maintain sheet shape, and the median size and size distribution vary significantly. From Table 4, median size of particles shows severe increase with reaction temperature. The effect of reaction temperature on size distribution and morphology of precipitates can be ascribed to the effect of reaction temperature on precipitation kinetics including nucleation and growth rate. Both nucleation and growth rate are slow at low temperature, but nucleation rate is bigger than growth rate, so it facilitate to form smaller particles. However, growth rate is bigger than nucleation rate at high temperature, thereby the particle becomes larger. This result is in agreement with the phenomena reported in Ref. [21]. 2.5 Preparation and characterization of cerium oxalate Base on the above experiments, orthogonal test was carried out to find the optimum conditions for preparing cerium oxalate with large particle size. The optimum conditions are Table 4 Particle size distribution of cerium oxalate vs. reaction temperature Reaction

Particle size distribution

temperature/°C

D10/ȝm

D50/ȝm

D90/ȝm

width/R

20

9.257

59.63

112.1

1.725

40

12.58

63.43

130.1

1.853

60

15.66

70.07

136.3

1.722

80

19.10

78.14

151.9

1.700

Fig. 5 SEM micrographs of cerium oxalate vs. reaction temperature (a) 25 °C; (b) 40 °C; (c) 60 °C; (d) 80 °C

Distribution

LI Mei et al., Cerium dioxide with large particle size prepared by continuous precipitation

as follows: [Ce(NO3)]=0.02 mol/L, agitation speed is 200 r/min, feeding rate of solutions of cerium nitrate and oxalic acid is 10 ml/min, and reaction temperature is 80 °C. Cerium oxalate powder prepared under the optimum conditions was characterized by scanning electron microscopy and X-ray diffraction. SEM micrograph and XRD pattern of the morphology and phase structure of cerium oxalate formed under optimum conditions are shown in Fig. 6. SEM micrograph shows that its shape is sheet. However, the phase structure of the sample is amorphous, this results is not in agreement with the phenomena reported in Ref. [21]. This result might be caused by different crystallization kinetics. 2.6 Characterization of ceria The cerium oxalate precursor prepared under optimum conditions was calcined at 800 °C for 2 h and the final product cerium dioxide was obtained. The ceria was characterized by XRD, SEM and laser particle size. SEM micrograph, XRD pattern and PSD of ceria were shown in Fig. 7. As shown in Fig. 7(a), the powder is cerium dioxide according to standard card JCPDS(81-0792), which suggests that the amorphous precursor has no influence on the phase structure of cerium dioxide. From Fig. 7(b), it can be seen that the median size of ceria particle is very large (D50= 27.60 ȝm), however, its size distribution width is very nar-

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row (D10=13.56 ȝm, D90=40.22 ȝm), these are in accordance with SEM micrographs. In addition, SEM micrograph shown in Fig. 7(c) indicates that ceria is still sheet-like, which is in agreement with Fig. 6(a). This suggests that the properties of the final product are inherited from that of the precursor.

3 Conclusions Continuous precipitation of cerium oxalate (precursor of ceria) was carried out at different operational conditions such as reactant concentration, agitation speed, feeding rate and reaction temperature, and their effects on particle size distribution, distribution width and morphology of cerium oxalate were investigated in this paper. It was found that reactant concentration, varying from 0.02 to 0.14 mol/L, had profound impact on particle size and its distribution width, which decreased from 59.63 to 53.05 ȝm and 1.725 to 1.117 ȝm respectively. The agitation speed and feeding rate played a very important role in the precipitation of cerium oxalate. It has been found that the effect of reaction temperature on the particle size distribution and morphology was also very significant. On the basis of orthogonal test, the optimum conditions for preparing cerium oxalate with large particle size were as follows: [Ce(NO3)]=0.02 mol/L, agitation speed

Fig. 6 SEM micrograph (a) and XRD pattern (b) of cerium oxalate formed under optimum conditions

Fig. 7 XRD pattern (a), PSD (b) and SEM micrograph (c) of cerium dioxide

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 was 200 r/min, feeding rate of solutions of cerium nitrate and oxalic acid was 10 ml/min, reaction temperature was 80 °C. SEM micrograph shows that the shape of cerium oxalate was sheet. The phase structure of sample was found firstly to be amorphous. The median size of the final product cerium dioxide was 27.60 ȝm, the particle size distribution width was very narrow and its shape was still sheet-like. Acknowledgements: The authors would like to express their sincere thanks for the financial support by the National Natural Science Foundation of China (2056601) and the National Natural Science Foundation of China (50662002).

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