Ind. Eng. Chem. Res. 1997, 36, 2641-2645
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Preparation and Characterization of Fine Ceria Powders by Hydrolysis of Cerium(III) Carboxylate Dissolved in Organic Solvent Yasuhiro Konishi,* Tetuya Murai, and Satoru Asai Department of Chemical Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 593, Japan
Tertiary carboxylate solutions of cerium(III) were hydrolyzed and precipitated micrometer, crystalline ceria (CeO2) particles, when contacted with both water and atmospheric oxygen at 130-170 °C (0.27-0.78 MPa) for 30-180 min. The rate of ceria precipitation in a batch autoclave was markedly dependent on processing parameters such as temperature, liquid-phase stirring speed, and initial organic-phase concentrations of cerium(III) carboxylate and free carboxylic acid. The particle size distributions of the ceria particles were considerably influenced by these processing parameters. In addition, the nature of aliphatic and aromatic hydrocarbon diluents had an appreciable effect on the size distribution of ceria. Introduction Wet-chemical processing techniques, such as the hydrolysis of metal alkoxides in alcohols (sol-gel method) and the hydrothermal reaction of aqueous metal-salt solutions, have been investigated for producing a wide range of advanced oxide ceramics. An alternative technique involves precipitation of metal oxides by hydrolysis of metal carboxylates dissolved in an organic solvent using pure water at temperatures around 200 °C (Doyle-Garner and Monhemius, 1985; Doyle, 1992; Konishi et al., 1993, 1994, 1996). The direct hydrolysis route of metal carboxylates exhibits certain interesting features which extend the possible applications of solvent extraction to the processing of oxide ceramics. Because the carboxylic acids are widely used as a commercial solvent extractant in hydrometallurgical processes, the metal carboxylate solutions are readily prepared in solvent extraction systems and are much cheaper starting materials than the metal alkoxides used in the sol-gel method. The hydrolysis of metal carboxylate in the organic phase enables the elimination of anion contamination of the oxide product in aqueous media because the organic solvent having a low dielectric constant is free from ionic species. In addition to these particular features, an integration of solvent extraction with ceramic processing techniques appears to offer much savings on both capital and running costs. Using the carboxylate hydrolysis technique, powders of R-Fe2O3, CuO + Cu2O, Mg(OH)2, γ-Mn2O3, ZrO2, and several ferrites MFe2O4 (M: Ni, Co, Zn, Mn) were precipitated from nonaqueous carboxylate solutions using water at 140-210 °C (Thorsen and Monhemius, 1981; Doyle-Garner and Monhemius, 1985; Doyle and Ye, 1987, Doyle and Monhemius, 1994). However, the properties of the resulting precipitates have been poorly characterized, and little attention has been directed toward the influence of process conditions on powder characteristics. In our recent work (Konishi et al., 1993, 1994, 1996), submicron, crystalline powders of R-Fe2O3, Fe3O4, and NiFe2O4 have been prepared from carboxylate solutions at 140-245 °C with or without water, and the characteristics of the iron oxide powders have been discussed in terms of process conditions. This paper describes the preparation and characterization of ceria (CeO2) powders by direct hydrolysis of * To whom all correspondence should be addressed. Telephone: 81-722-52-1161. Fax: 81-722-59-3340. Email:
[email protected]. S0888-5885(97)00047-X CCC: $14.00
cerium(III) carboxylate dissolved in an organic solvent with water at temperatures up to 170 °C and total pressures up to 0.78 MPa. The effects of process conditions on the precipitation rate and the particle size distribution are considered. Experimental Section Materials. The carboxylic acid used in this work was commercially available Versatic 10, a synthetic tertiary aliphatic monocarboxylic acid (Shell Chemical Co.). The synthetic acid contains at least 98% C9H19COOH and has an acid value of 320 mg of KOH/g. The Versatic 10 was diluted to desired concentration levels using commercial Exxsol D80, an aliphatic hydrocarbon diluent (Exxon Chemical Co.). In some runs, commercial Solvesso 150, an aromatic hydrocarbon diluent (Exxon Chemical Co.), was employed to dilute the Versatic 10. These organic materials were used without further purification. An aqueous solution of cerium(III) was prepared by dissolving pure cerium carbonate (Santoku Metal Industry Co., Ltd., 99.9% Ce2(CO3)3‚5H2O) in hydrochloric acid. The excess hydrochloric acid in the aqueous solution was removed by boiling. Cerium(III)-loaded Versatic 10 solutions were prepared by solvent extraction from the aqueous cerium(III) solutions into the dilute Versatic 10 solutions. During the extraction operation, the aqueous phase pH was adjusted around 6.5 by the addition of a dilute NaOH solution. The cerium-loaded organic solution was washed with distilled water to remove residual anions and then passed through glass fiber paper and phaseseparating paper to remove physically entrained water. The initial concentrations of cerium in the organic solution were 0.0491 and 0.150 kmol/m3, and the initial concentrations of free Versatic 10 were 0.394, 0.738, and 1.61 kmol/m3. Apparatus and Procedure. A stainless steel autoclave lined with glass was used to prepare ceria powders at elevated temperatures and pressures. The autoclave was 7-cm i.d. and 16-cm height, and a sixblade turbine impeller of 4.5-cm diameter was placed 3 cm above the bottom of the autoclave. A 100-cm3 volume of the metal carboxylate solution was charged into the autoclave with an equal volume of distilled water. The organic and aqueous solutions were mixed by the impeller at room temperature, and air was continuously bubbled into the solution for 30 min before heating was started. In some precipitation tests, an © 1997 American Chemical Society
2642 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 Table 1. Hydrothermal Treatment of Cerium(III) Carboxylate Solutions at 150 °C and 0.43 MPa for 180 min initial Ce(III) conc. in organic solution (kmol/m3)
water
atmosphere
total pressure in autoclave before heating (MPa)
percentage precipitation (%)
0.0491 0.0491 0.0491 0.150 0.150 0.150
absence presence presence presence presence presence
air nitrogen air air air air
0.1 0.1 0.1 0.1 0.2 0.3
0 0 100 43 65 96
oxygen-free condition in the autoclave was achieved by sparging continuously with nitrogen gas at 600 cm3/min for 30 min. After continuous gassing with either air or nitrogen was performed at room temperature, the organic and aqueous phases were heated in the autoclave and maintained at a synthesis temperature for 60-180 min. The experimental temperature was varied from 130 to 170 °C ( from 0.27 to 0.78 MPa). Because of the low volatility of the organic solutions, the total pressure in the autoclave was approximately equal to the saturated steam pressure at the operating temperature. The time required to reach the reaction temperatures of 130-170 °C was from 5 to 10 min. The starting solution was mixed at three different stirring speeds of 100, 300, and 500 rpm. A solution sample of 5 cm3 was periodically withdrawn from the autoclave and centrifuged for analysis. To determine the organicphase cerium concentration, the organic samples were mixed with a 6 kmol/m3 hydrochloric acid solution to strip the metal species, and the aqueous extracts were analyzed for cerium by EDTA titration. The resulting precipitates were filtered, washed with distilled water and acetone, and dried for 10 h at either 50 or 200 °C. The precipitates were characterized by X-ray diffraction (XRD) analysis. Thermogravimetry (TG) and differential thermal analysis (DTA) was conducted at a heating rate of 10 °C/min. The particle morphology was observed by scanning electron microscopy (SEM). The particle size distributions of the precipitates were measured with a Leeds and Northrup Microtrac analyzer. Results and Discussion Conditions for Precipitation of Ceria. Some experiments were performed to establish conditions for the precipitation of cerium oxide from nonaqueous cerium(III) carboxylate solutions. Table 1 gives the results of precipitation tests in which the cerium(III) carboxylate solutions were treated at 150 °C in the absence or presence of distilled water and atmospheric oxygen. The percentages of precipitation were determined from the concentration of cerium in the organic solution at any time, divided by the initial organic phase concentration. The cerium carboxylate gave no precipitation in the absence of water under an air atmosphere and in the presence of water under an oxygen-free nitrogen atmosphere. When contacted with water at the elevated temperature under an air atmosphere, the cerium carboxylate solutions were hydrolyzed and precipitated cerium compound. The percentage precipitation decreased from 100 to 43% as the initial cerium concentration in the organic solution increased from 0.0491 to 0.150 kmol/m3. This result indicates that the amount of oxygen needed for the precipitation tends to increase with an increase in the initial cerium concentration in the organic solution. To increase the amount of atmospheric oxygen in the autoclave before heating the starting solution, air from a compressor was bubbled through a 0.150 kmol/m3 cerium carboxylate solution at room temperature and
Figure 1. X-ray diffraction pattern of precipitate from cerium(III) carboxylate solution in the presence of water and atmospheric oxygen. Conditions: 160 °C, 0.61 MPa, 0.0491 kmol/m3 cerium(III) carboxylate, 0.394 kmol/m3 free carboxylic acid, aliphatic hydrocarbon diluent, 500 rpm, and 60 min.
a total pressure of either 0.2 or 0.3 MPa. Table 1 indicated that the percentage of precipitation at 150 °C increased with the air pressure at room temperature. This demonstrates that atmospheric oxygen, which was dissolved in the organic and aqueous phases and existed in the gas phase over the solution in the autoclave, plays an important role in the precipitation of cerium compound from the cerium(III) carboxylate solution. The cerium(III) carboxylate solution gave precipitation when contacted with both water and atmospheric oxygen at temperatures between 130 and 170 °C (0.27 and 0.78 MPa). The resulting precipitates were identified by XRD analysis. As shown in Figure 1, the observed XRD patterns were characteristic of ceria (CeO2) (JCPDS 34-0394), and the sharpness of the peaks indicated the highly crystalline nature of the oxide product. There were no peaks due to unidentified crystalline phases. Because the cerium(IV) oxide was precipitated directly from the cerium(III) carboxylate solution in the presence of atmospheric oxygen, the cerium(III) in the starting solution is likely to be oxidized with oxygen at elevated temperatures and pressures. The overall reaction for the precipitation of ceria can be represented as
4Ce(RCOO)3 + 6H2O + O2 ) 4CeO2 + 12RCOOH (1) where overbars denote species present in the organic phase and RCOOH represents the Versatic 10 (free carboxylic acid). In view of the stoichiometry based on eq 1, the amount of oxygen in the autoclave was found to suffice for the full conversion of 0.0491 kmol/m3 cerium(III) carboxylate in the organic solution to the oxide product. Because the ceria powders were prepared in the presence of the organic phase, there was a fear of unfavorable contamination of the oxide product with the organic starting material, the Versatic 10-hydrocarbon diluent. The organic contamination of the ceria was examined by using thermal analysis. Figure 2 shows the TG-DTA curves of the ceria powders which were airdried in an oven at two different temperatures before the thermal analysis. The ceria particles dried at 50 °C exhibited a prominent exothermic peak at around
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Figure 3. Rate data for precipitation of ceria at different temperatures and pressures: (2) 160 °C and 0.61 MPa; (9) 150 °C and 0.43 MPa; ([) 140 °C and 0.36 MPa; (b) 130 °C and 0.27 MPa. Conditions: 0.0491 kmol/m3 cerium(III) carboxylate, 0.394 kmol/m3 free carboxylic acid, aliphatic hydrocarbon diluent, and 500 rpm.
Figure 2. Thermal analysis of ceria precipitated at 160 °C, 0.61 MPa, 0.0491 kmol/m3 cerium(III) carboxylate, 0.394 kmol/m3 free carboxylic acid, aliphatic hydrocarbon diluent, 500 rpm, and 60 min: (a) ceria particles dried at 50 °C for 10 h; (b) ceria particles dried at 200 °C for 10 h.
270 °C on the DTA curve and a weight loss on the TG curve, which result from combustion of the organic material. The weight loss of the ceria dried at 50 °C took place with a loss equivalent to 10% of the initial sample weight. However, the ceria dried at 200 °C did not exhibit an exothermic peak and a weight loss on the TG-DTA curve, indicating that the ceria particles are uncontaminated by the organic material. These TGDTA data demonstrated that the removal of the organic material can be achieved by drying the oxide products at 200 °C for 10 h. Since the organic starting solution has a boiling point of about 220 °C, the drying of precipitates at around the boiling point is a useful means for eliminating completely the organic contamination of the oxide products. Rate of Precipitation. The rates of ceria precipitation were followed by measuring the organic-phase concentrations of cerium as a function of time, in order to examine the effect of various processing parameters, such as the hydrolysis temperature, the liquid-phase stirring speed, and the organic-phase composition. Rate data collected at different operating conditions are shown in Figures 3-6, where the precipitation percentages are plotted against time. Figure 3 shows rate data for the precipitation of ceria at different hydrothermal conditions between 130 and 160 °C (0.27 and 0.61 MPa). The precipitation rate markedly increased with increasing hydrolysis temperature. At 160 °C, the cerium was completely stripped and precipitated from the carboxylate solution within 30 min. An Arrhenius plot of the initial precipitation rates at 130-160 °C gave an apparent activation energy of 143 kJ/mol. This high activation energy suggests that the precipitation process is controlled by either hydrolysis reaction or nucleation, the resistance to mass transfer being insignificant. Figure 4 shows rate data for the ceria precipitation at different liquid-phase stirring speeds at 150 °C (0.43 MPa). The precipitation rate of ceria was markedly enhanced as the stirring speed was increased from 100 to 500 rpm. Since previous work on crystallization has indicated that the formation rate of nuclei in the solution is strongly dependent on the liquid-phase stirring intensity (Garside, 1985), the change in the
Figure 4. Rate data for precipitation of ceria at different liquidphase stirring speeds: (9) 500 rpm; ([) 300 rpm; (b) 100 rpm. Conditions: 150 °C, 0.43 MPa, 0.0491 kmol/m3 cerium(III) carboxylate, 0.394 kmol/m3 free carboxylic acid, and aliphatic hydrocarbon diluent.
Figure 5. Rate data for precipitation of ceria at different initial organic-phase concentrations of cerium(III) carboxylate and free carboxylic acid: (2) 0.0967 kmol/m3 cerium(III) carboxylate and 0.394 kmol/m3 free carboxylic acid; (9) 0.0491 kmol/m3 cerium(III) carboxylate and 0.394 kmol/m3 free carboxylic acid; ([) 0.0491 kmol/m3 cerium(III) carboxylate and 0.738 kmol/m3 free carboxylic acid; (b) 0.0491 kmol/m3 cerium(III) carboxylate and 1.61 kmol/ m3 free carboxylic acid. Conditions: 150 °C, 0.43 MPa, aliphatic hydrocarbon diluent, and 500 rpm.
nucleation rate is considered to be responsible for the precipitation rate of ceria. A similar dependence on the liquid-phase stirring speed was observed in the precipitation of nickel ferrite from an iron-nickel carboxylate solution (Konishi et al., 1996). Figure 5 shows that the precipitation rates of ceria were influenced by the initial concentrations of cerium carboxylate and free carboxylic acid in the organic
2644 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997
Figure 6. Rate data for precipitation of ceria using aliphatic and aromatic hydrocarbon diluents: (2) aromatic diluent, 170 °C and 0.78 MPa; (b) aliphatic diluent, 170 °C and 0.78 MPa; ([) aromatic diluent, 140 °C and 0.36 MPa; (9) aliphatic diluent, 140 °C and 0.36 MPa. Conditions: 0.0491 kmol/m3 cerium(III) carboxylate, 0.394 kmol/m3 free carboxylic acid, and 500 rpm.
Figure 8. Particle size distributions of ceria particles prepared at different temperatures and pressures: (- - -) 170 °C and 0.78 MPa; (s) 150 °C and 0.43 MPa; (- ‚ -) 130 °C and 0.27 MPa. Conditions: 0.0491 kmol/m3 cerium(III) carboxylate, 0.394 kmol/ m3 free carboxylic acid, aliphatic hydrocarbon diluent, 500 rpm, and 120 min.
Figure 9. Particle size distributions of ceria particles prepared at different liquid-phase stirring speeds: (s) 500 rpm; (- ‚ -) 300 rpm; (- - -) 100 rpm. Conditions: 150 °C, 0.43 MPa, 0.0491 kmol/ m3 cerium(III) carboxylate, 0.394 kmol/m3 free carboxylic acid, aliphatic hydrocarbon diluent, and 120 min.
Figure 7. Scanning electron micrographs of ceria prepared at 160 °C, 0.61 MPa, 0.0491 kmol/m3 cerium(III) carboxylate, 0.394 kmol/ m3 free carboxylic acid, aliphatic hydrocarbon diluent, 500 rpm, and 60 min.
phase. The precipitation rate increased with an increase in the initial concentration of cerium carboxylate in the organic phase. Moreover, a marked increase in the precipitation rate occurred when the initial concentration of free carboxylic acid was decreased from 1.61 to 0.394 kmol/m3. An increase in the cerium carboxylate concentration and a decrease in the free carboxylic acid concentration shift the position of equilibrium in eq 1 to the right and appear to promote the precipitation of ceria. Because the overall precipitation process depletes the cerium carboxylate in the organic phase and produces the free carboxylic acid, the system appears to be approaching equilibrium as the ceria precipitation proceeds. Therefore, the precipitation of ceria was initially rapid and then gradually decreased the rate, as shown by Figures 2-5. Figure 6 compares the precipitation rates of ceria in two different diluents. The precipitation rates were similar, regardless of whether the aliphatic hydrocarbon (Exxsol D80) or the aromatic hydrocarbon (Solvesso 150) was used as a diluent for the preparation of the cerium carboxylate solution. Morphology and Particle Size of Ceria. As shown in Figure 7, the ceria formed as slightly elongated particles, and there was a slight agglomeration. Moreover, microscopic examination revealed that the particle
morphology was not changed appreciably by the operating variables such as temperature, liquid-phase stirring speed, and organic-phase composition. However, the particle size of ceria was highly sensitive to the processing parameters. Figure 8 shows cumulative undersize distributions of ceria particles precipitated at three different temperatures. The particle size distribution was wide at the lower temperatures. The average size of ceria particles increased from 1.3 to 2.0 µm as the temperature decreased from 170 to 130 °C (from 0.78 to 0.27 MPa). The decrease in temperature also gave lower precipitation rate (Figure 3), which suggests the slow formation of nuclei over a wide time interval. Such a slower nucleation during the precipitation process is likely to produce larger particles and a wide particle size distribution of precipitates. Figure 9 shows the particle size distributions of ceria particles precipitated at different liquid-phase stirring speeds. An increase in the stirring speed of starting solution gave a pronounced decrease in particle size, although fairly broad size distributions were observed. The average particle size of ceria prepared at 100 rpm was 3.4 µm and was found to decrease by a factor of 2 as the stirring speed increased from 100 to 500 rpm. Since the precipitation of ceria was promoted by increasing the liquid-phase stirring speed (Figure 4), the decrease in particle size is likely to be due to a rapid formation of many nuclei. Figure 10 shows the size distributions of the ceria particles prepared at different initial concentrations of cerium carboxylate and free carboxylic acid in the
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processing parameter for determining the particle size distribution. Conclusions
Figure 10. Particle size distributions of ceria particles prepared at different initial organic-phase concentrations of cerium(III) carboxylate and free carboxylic acid: (- - -) 0.0491 kmol/m3 cerium(III) carboxylate and 1.61 kmol/m3 free carboxylic acid; (- ‚ -) 0.0967 kmol/m3 cerium(III) carboxylate and 0.394 kmol/m3 free carboxylic acid; (s) 0.0491 kmol/m3 cerium(III) carboxylate and 0.394 kmol/m3 free carboxylic acid. Conditions: 150 °C, 0.43 MPa, aliphatic hydrocarbon diluent, 500 rpm, and 120 min.
Upon treatment with both water and atmospheric oxygen at 130-170 °C (0.27-0.78 MPa) at reaction times between 30 and 180 min, the cerium(III) carboxylate solutions were hydrolyzed and directly precipitated crystalline ceria with average particle sizes on the order of 1 µm. The organic contamination of the resulting precipitates by the organic solution was largely eliminated by drying the oxide particles at 200 °C for 10 h. The precipitation rate of ceria markedly increased with increasing temperature, liquid-phase stirring speed, and cerium carboxylate concentration and with decreasing free carboxylic acid concentration. Moreover, these processing parameters considerably influenced the particle size distributions of ceria powders. The nature of the diluent had also a significant effect on the size distribution of ceria. Acknowledgment We thank Messrs. Takashi Goto, Yoshimasa Katayama, Michiya Ohashi, and Hirobumi Takemori, Santoku Metal Industry Co., Ltd., Kobe, Japan, for their assistance in characterizing the precipitated powders. Abbreviations
Figure 11. Particle size distributions of ceria particles prepared using aliphatic and aromatic hydrocarbon diluents: (- - -) aromatic diluent; (s) aliphatic diluent. Conditions: 170 °C, 0.78 MPa, 0.0491 kmol/m3 cerium(III) carboxylate, 0.394 kmol/m3 free carboxylic acid, and 500 rpm.
organic solution. A 2-fold increase in the initial cerium concentration produced a decrease in the particle size, and an increase in precipitation rate was also observed (Figure 5). These results suggest that higher precipitation rates produced finer precipitates. Moreover, the particle size decreased by 40% as the initial concentration of free carboxylic acid increased from 0.394 to 1.61 kmol/m3. Despite slower ceria precipitation at the higher free carboxylic acid concentration [RCOOH] (Figure 5), the average particle size decreased from 1.7 µm at [RCOOH] ) 0.394 kmol/m3 to 0.9 µm at [RCOOH] ) 1.61 kmol/m3. This is likely to be that the free carboxylic acid adsorbs on the oxide particles and suppresses agglomeration by steric repulsion during the precipitation of ceria. Figure 11 compares the size distributions of the ceria particles prepared using the aliphatic and aromatic hydrocarbon diluents. The ceria particles precipitated in the aromatic hydrocarbon diluent had the average size of 0.7 µm and were smaller than the aliphatic counterpart. The rates of ceria precipitation in the different diluents were similar, although a slight difference was apparent (Figure 6). Again, the diluent clearly had an appreciable effect on the particle size distribution. This effect is probably due to the difference in agglomeration behavior during precipitation in the aliphatic and aromatic media. Thus, it can be concluded that the nature of the diluent, along with the liquidphase stirring speed, temperature, and initial cerium and carboxylic acid concentrations, is an important
DTA ) differential thermal analysis JCPDS ) joint committee on powder diffraction standards TG ) thermogravimetry XRD ) X-ray diffraction
Literature Cited Doyle, F. M. Integrating Solvent Extraction with the Processing of Advanced Ceramic Materials. Hydrometallurgy 1992, 29, 527-545. Doyle, F. M.; Monhemius, A. J. Kinetics and Mechanisms of Precipitation of Nickel Ferrite by Hydrolytic Stripping of Iron(III)-Nickel Carboxylate Solutions. Hydrometallurgy 1994, 35, 251-265. Doyle, F. M.; Ye, W. ZrO2 Powders from Zirconium(IV) Carboxylates. J. Met. 1987, 39 (7), 34-37. Doyle-Garner, F. M.; Monhemius, A. J. Hydrolytic Stripping of Single and Mixed Metal-Versatic Solutions. Metall. Trans. B 1985, 16B, 671-677. Garside, J. Industrial Crystallization from Solution. Chem. Eng. Sci. 1985, 40, 3-26. Konishi, Y.; Kawamura, T.; Asai, S. Preparation and Characterization of Fine Magnetite Particles from Iron(III) Carboxylate Dissolved in Organic Solvent. Ind. Eng. Chem. Res. 1993, 32, 2888-2891. Konishi, Y.; Kawamura, T.; Asai, S. Preparation and Properties of Fine Hematite Powders by Hydrolysis of Iron Carboxylate Solutions. Metall. Mater. Trans. B 1994, 25B, 165-170. Konishi, Y.; Kawamura, T.; Asai, S. Preparation and Characterization of Ultrafine Nickel Ferrite Powders by Hydrolysis of Iron(III)-Nickel Carboxylate Dissolved in Organic Solvent. Ind. Eng. Chem. Res. 1996, 35, 320-325. Thorsen, G.; Monhemius, A. J. U.S. Patent 4,282,189, 1981.
Received for review January 17, 1997 Revised manuscript received April 9, 1997 Accepted April 15, 1997X IE9700472
X Abstract published in Advance ACS Abstracts, June 1, 1997.
