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Solvothermal Preparation of Cuprous Oxide Fine Particles by Hydrolysis of Copper(II) Carboxylate in Two-Phase Liquid-Liquid System Yasuhiro Konishi,* Toshiyuki Nomura, and Dai Satoh Department of Chemical Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan
Nonaqueous copper(II)-loaded carboxylate solutions were hydrolyzed directly by water at elevated temperatures to precipitate copper oxides fine particles. Under solvothermal conditions, 170 °C and 0.8 MPa to 210 °C and 2.1 MPa for 120 min, a mixture of cupric oxide (CuO) and cuprous oxide (Cu2O) was prepared from the organic starting solution of 0.15 kmol/m3 copper(II) carboxylate and 0.36 kmol/m3 free carboxylic acid. When the initial carboxylic acid concentration in the organic starting solution was increased to 1.0 kmol/m3, thermal treatment of the copper(II) carboxylate solutions at 180-210 °C resulted in the formation of cuprous oxide alone. The average particle size of the resulting cuprous oxide significantly decreased from 16 to 5.5 µm as the stirring speed of the starting solution was changed from 300 to 100 rpm. The precipitation rate of cuprous oxide in a batch autoclave was markedly dependent on the operating temperature, and the copper(II) in the carboxylate solution was completely precipitated as cuprous oxide within 60 min at 210 °C. Introduction Fine particles of metal oxides with high purity are of importance as industrial materials in the fields of metallurgy and electronics. Among the metal oxides, cuprous oxide (Cu2O) fine particles are widely used in catalysts, pigments of glass and enamel, antifouling paints, and so on. There are several wet-chemical methods for producing cuprous oxide fine particles in aqueous environments: (i) hydrothermal production of aqueous copper-salt solutions;1 (ii) reduction of cupric salts,2-4 and cupric oxide (CuO)5 in aqueous solutions; and (iii) γ-irradiation for aqueous solution of cupric sulfate in the presence of acetic acid and sodium acetate.6 One of the main disadvantages of these aqueous systems is that the oxide fine particles are precipitated in aqueous media containing foreign anions, which tend to be a major source of anion contamination of the oxide products. Alternatively, a precipitation technique in the twophase aqueous-organic system is a promising route for producing oxide materials, which involves precipitation of metal oxides by direct hydrolysis of nonaqueous metal-loaded carboxylate using water at elevated temperatures.7,8 The metal carboxylates are a cheap starting material because the carboxylic acids are widely used as a commercial solvent extractant in hydrometallurgical processes. The hydrolysis of concentrated metal carboxylate has an attractive ability to produce oxides in reasonably high productivity. This synthesis route has another advantage that the foreign anion contamination of oxide products is largely eliminated because ionic species are absent in the organic solvent having a low dielectric constant (low polarity). Using this technique, crystalline powders of various metal * To whom correspondence should be addressed. Tel.: +8172-254-9297. Fax: +81-72-254-9911. E-mail: yasuhiro@ chemeng.osakafu-u.ac.jp.
oxides have been prepared such as hematite (R-Fe2O3),8,9 magnetite (Fe3O4),10 nickel ferrites (NiFe2O4),8,11,12 zirconia (ZrO2),13 and ceria (CeO2).14,15 Previous work8 on the direct hydrolysis of copper(II)-loaded carboxylate has shown that a mixture of cuprous oxide (Cu2O) and cupric oxide (CuO) precipitate from copper(II)-loaded carboxylate solution at 200 °C. However, the properties of the resulting precipitates have been poorly characterized. Moreover, no reports have appeared concerning the preparation of cuprous oxide alone from copper(II) carboxylate solution using water. The purpose of this paper is to find suitable conditions for the preparation of crystalline cuprous oxide fine particles from nonaqueous copper(II) carboxylate solutions under mild solvothermal conditions and to examine the effects of process conditions on the precipitation rate and the particle size of cuprous oxide. Experimental Section Materials. The tertiary monocarboxylic acid used in this work was commercially available Versatic 10, a synthetic tertiary aliphatic monocarboxylic acid (Shell Chemical Co., Tokyo, Japan). The synthetic carboxylic acid contained at least 98% C9H19COOH and had 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., Tokyo, Japan). These organic materials were used without further purification. Organic solutions of copper(II) carboxylate were prepared by solvent extraction from an aqueous copper sulfate solution. During the extraction operation, the aqueous solution pH was adjusted at pH 4.2 by the addition of dilute sodium hydroxide solution. The copper(II)-loaded organic solutions were washed with distilled water to remove residual anions and then passed through glass fiber paper and phase-separating paper to remove physically entrained water. The initial con-
10.1021/ie0305115 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/30/2004
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centrations of copper(II) in the organic solution were 0.05, 0.15, and 0.30 kmol/m3, and the initial concentrations of free carboxylic acid were from 0.36 to 1.0 kmol/ m3. Apparatus and Procedure. A stainless steel autoclave of 400 cm3 was used to prepare copper oxide particles at elevated temperatures and pressures. The autoclave was 6 cm i.d. and 16 cm high, and a six-blade turbine impeller of 4.5 cm diameter was placed 3 cm above the bottom of the vessel. A 100 cm3 volume of the copper(II)-loaded organic solution was charged into the autoclave with an equal volume of distilled water. The organic and aqueous solutions were sufficiently mixed at room temperature by the stirrer, and nitrogen gas was continuously bubbled into the solution for 20 min. After that, the organic and aqueous phases were heated in the autoclave and maintained at a reaction temperature for 120 min. The experimental temperature was varied from 170 to 210 °C. The time required to reach the reaction temperatures was from 10 to 20 min. 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, that is, 0.8 MPa at 170 °C and 2.1 MPa at 210 °C. The starting solutions were mixed at two different stirring speeds of 100 and 300 rpm. A solution sample of 5 cm3 was withdrawn from the autoclave and centrifuged for analysis. The organic samples were mixed with 6 kmol/m3 hydrochloric acid solution to strip completely the copper species in the organic phase, and the aqueous solutions were analyzed for copper by ion chromatography. The resulting precipitates were filtered, washed with distilled water and acetone, and dried for 5 h at 50 °C. The precipitates were characterized by X-ray diffraction (XRD) analysis. The particle size and morphology were observed with scanning electron microscopy (SEM).
Figure 1. X-ray diffraction patterns of precipitates obtained from copper(II) carboxylate solutions with water for 120 min. Effect of operating temperature: (a) 170 °C and 0.8 MPa; (b) 180 °C and 1.0 MPa; (c) 210 °C and 2.1 MPa. Conditions: 0.15 kmol/m3 copper(II) carboxylate and 0.36 kmol/m3 free carboxylic acid. (O) Cupric oxide (JCPDS 41-254). (b) Cuprous oxide (JCPDS 5-667).
Results and Discussion Conditions for Preparation of Cuprous Oxide. The copper(II)-loaded carboxylate solutions were hydrolyzed and gave precipitation when contacted with distilled water at 170 °C and 0.8 MPa to 210 °C and 2.1 MPa for 120 min. Figure 1 shows the XRD patterns of the resulting precipitates, which were obtained from the organic solution of 0.15 kmol/m3 copper(II) carboxylate and 0.36 kmol/m3 free carboxylic acid. The observed XRD peaks were patterns of mixtures of cupric oxide (JCPDS 41-254) and cuprous oxide (JCPDS 5-667), and the sharpness of peaks indicated the highly crystalline nature of the oxide products. There were no peaks due to unidentified crystalline phases. At a lower temperature of 170 °C, cupric oxide was the predominant product from the copper(II) carboxylate solution. As the operating temperature was raised from 170 to 210 °C, the amount of the resulting cuprous oxide increased relative to the amount of cupric oxide. The presence of cuprous oxide was also evidenced by the color change of the precipitated particles: black particles were obtained at 170 °C, whereas dark reddish particles were prepared at 210 °C. A suitable condition for preparing cuprous oxide alone was examined by changing the free carboxylic acid concentration in the starting solutions at a lower temperature of 180 °C. Figure 2 shows the XRD patterns of precipitates obtained from 0.15 kmol/m3 copper(II) carboxylate solutions at different initial carboxylic
Figure 2. X-ray diffraction patterns of precipitates obtained from 0.15 kmol/m3 copper(II) carboxylate solutions with water at 180 °C and 1.0 MPa for 120 min. Effect of initial carboxylic acid concentration: (a) 0.36 kmol/m3 free carboxylic acid; (b) 0.5 kmol/ m3 free carboxylic acid; (c) 1.0 kmol/m3 free carboxylic acid. (O) Cupric oxide (JCPDS 41-254). (b) Cuprous oxide (JCPDS 5-667).
acid concentrations. A mixture of cupric oxide and cuprous oxide was prepared at initial carboxylic acid concentrations of 0.36 and 0.50 kmol/m3. When the initial carboxylic acid concentration was increased from 0.50 to 1.0 kmol/m3, the observed XRD patterns were characteristic of pure cuprous oxide (JCPDS 5-667), and the sharpness of peaks indicated the high crystalline nature of the oxide product. Therefore, an increase in the free carboxylic acid concentration is likely to promote the precipitation of cuprous oxide. Figure 3 shows the XRD patterns of precipitates obtained by changing the organic composition in the starting solution at higher temperature of 210 °C. In the presence of 1.0 kmol/m3 free carboxylic acid, pure
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Figure 4. Scanning electron micrographs for (a) cupric oxide (predominant product) precipitated from 0.15 kmol/m3 copper(II) carboxylate and 0.36 kmol/m3 free carboxylic acid; (b) cuprous oxide precipitated from 0.15 kmol/m3 copper(II) carboxylate and 1.0 kmol/m3 free carboxylic acid. Conditions: 180 °C, 0.8 MPa, 120 min, and 300 rpm.
Figure 3. X-ray diffraction patterns of precipitates obtained from copper(II) carboxylate solutions with water at 210 °C and 2.1 MPa for 120 min. Effect of organic phase concentrations: (a) 0.15 kmol/ m3 copper(II) carboxylate and 1.0 kmol/m3 free carboxylic acid; (b) 0.30 kmol/m3 copper(II) carboxylate and 1.0 kmol/m3 free carboxylic acid; (c) 0.05 kmol/m3 copper(II) carboxylate and 1.0 kmol/m3 free carboxylic acid; (d) 0.05 kmol/m3 copper(II) carboxylate and 0.36 kmol/m3 free carboxylic acid. (b) Cuprous oxide (JCPDS 5-667).
cuprous oxide was prepared from the starting solutions ranging in copper(II) concentration from 0.05 to 0.30 kmol/m3. Even when the initial carboxylic acid concentration was decreased from 1.0 to 0.36 kmol/m3, pure cuprous oxide was prepared at a lower copper(II) carboxylate concentration of 0.05 kmol/m3. Thus, it can be concluded that the copper(II) carboxylate solution gave only cuprous oxide when the free carboxylic acid concentration was sufficiently high as compared to the copper(II) carboxylate concentration. For the preparation of cuprous oxide from the copper(II) carboxylate solution, it is necessary to reduce some of the copper(II) in the organic starting solution into copper(I). In this connection, previous work7,8 has reported that a mixture of cuprous oxide and cupric oxide was precipitated from copper(II) carboxylate solution using water at 200 °C. Similar behavior was observed when magnetite (Fe2O3‚FeO) particles were prepared from iron(III) carboxylate solutions at 245 °C.10 Doyle-Garner and Monhemius7 have found that the reduction of Cu(II) in carboxylate/carboxylic acid solution takes place with the oxidation of carboxylic acid to CO2, which suggests that the carboxylic acid was the reducing agent. These investigators have suggested that the reduction of copper(II) carboxylate complex is probably associated with the change in coordination or electronic structure caused by replacing the organic ligands (carboxylic acid and carboxylate ions) by hydroxide ions and water. In the present work, the organic solutions were analyzed for the free carboxylic acid by alkalimetry before and after the formation of cuprous oxide from the
Figure 5. Scanning electron micrographs for (a) cuprous oxide precipitated at 100 rpm and (b) cuprous oxide precipitated at 300 rpm. Conditions: 200 °C, 1.7 MPa, 120 min, 0.15 kmol/m3 copper(II) carboxylate, and 1.0 kmol/m3 free carboxylic acid.
organic solution of 0.30 kmol/m3 copper(II) carboxylate and 1.0 kmo/m3 carboxylic acid. The quantitative analysis revealed that the formation of cuprous oxide resulted in a significant decrease in the carboxylic acid concentration and that the molar amount of carboxylic acid consumed is about one-third that of copper precipitated as the oxide product. This indicates that the carboxylic acid serves as a reducing agent and is partially oxidized during the preparation of cuprous oxide. Morphology and Particle Size. Figure 4a shows the SEM images of copper oxides prepared at 180 °C and an initial carboxylic acid concentration of 0.36 kmol/ m3 for 120 min. The predominant product particles, cupric oxide particles, were micrometer agglomerates of fine crystallites. Figure 4b shows the cuprous oxide particles prepared at 180 °C and an initial carboxylic acid concentration of 1.00 kmol/m3 for 120 min. The particle size of cuprous oxide was from 10 to 20 µm, and the individual particles are cuboctahedral morphology with a well-defined habit. Comparing Figure 4a with Figure 4b, it is clear that the particle size and morphology were changed appreciably by the nature of copper oxides. Figure 5 shows the SEM images of cuprous oxide prepared at different stirring speeds of the starting solution. As the liquid-phase stirring speed was decreased from 300 to 100 rpm, the average particle size of cuprous oxide drastically decreased from 16 to 5.5 µm. This result demonstrates that the liquid-phase stirring speed is one of the important processing parameters for changing the particle size of cuprous oxide. It is likely that, at vigorous stirring of the copper(II) carboxylate
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Figure 6. Rate data for precipitation of different oxide products: (1) cupric oxide (predominant product) from organic solution of 0.15 kmol/m3 copper(II) carboxylate and 0.36 kmol/m3 free carboxylic acid; (4) cuprous oxide from organic solution of 0.15 kmol/ m3 copper(II) carboxylate and 1.0 kmol/m3 free carboxylic acid. Conditions: 180 °C, 1.0 MPa, and 100 rpm.
Figure 7. Rate data for precipitation of cuprous oxide at different liquid-phase stirring speeds: (0) 300 rpm; (b) 100 rpm. Conditions: 200 °C, 1.7 MPa, 0.15 kmol/m3 copper(II) carboxylate, and 1.0 kmol/m3 free carboxylic acid.
solution, stable nuclei tend to collide and coalesce to form large particles. In addition, microscopic examination revealed that the morphology and the average particle size of cuprous oxide are little affected by other operating variables such as temperature and organic phase concentration of copper(II) carboxylate. Precipitation Rate of Copper Oxides. The precipitation rates of copper oxides were followed by measuring the organic phase copper concentrations as a function of time. Rate data collected at different operating conditions are shown in Figures 6-8, where the precipitation percentages are plotted against time. Because the aqueous solution was free from copper ions during the precipitation process, the percentages of oxide precipitation were determined from the concentration of copper in the organic solution at any time divided by the initial organic-phase copper concentration. Figure 6 shows rate data for the precipitation of copper oxides at 180 °C and 1.0 MPa. The precipitation rate was strongly dependent on the nature of copper oxide precipitates. When cupric oxide was a predomi-
Figure 8. Rate data for precipitation of cuprous oxide at different hydrothermal conditions: (0) 210 °C and 2.1 MPa; (b) 200 °C and 1.7 MPa; (4) 190 °C and 1.3 MPa; (O) 180 °C and 1.0 MPa. Conditions: 0.15 kmol/m3 copper(II) carboxylate, 1.0 kmol/m3 free carboxylic acid, and 100 rpm.
nant product at an initial carboxylic acid concentration of 0.36 kmol/m3, the copper in the organic solution was completely stripped and precipitated for 120 min. On the other hand, the precipitation of cuprous oxide was slower at an initial carboxylic acid concentration of 1.0 kmol/m3, and the copper(II) in the organic solution was precipitated beyond 60% in 120 min at 180 °C. Figure 7 compares the precipitation rates of cuprous oxide at two different stirring speeds of starting solution at 200 °C and 1.7 MPa. The precipitation rate of cuprous oxide was not appreciably enhanced when the stirring speed was changed from 100 to 300 rpm. This result suggests that the resistance to mass transfer is insignificant in the precipitation of cuprous oxide. Figure 8 shows rate data for the precipitation of cuprous oxide alone under different thermal conditions. The precipitation rate increased sharply as the operating temperature was increased from 180 °C and 1.0 MPa to 210 °C and 2.1 MPa. The precipitation of cuprous oxide was completed within 60 min at 210 °C and 2.1 MPa. An Arrhenius plot of the initial precipitation rates at 180-210 °C gave the apparent activation energy of 105 kJ/mol. This high activation energy suggests that the precipitation process is controlled by hydrolysis reaction, the resistance to mass transfer being insignificant. Conclusions Upon treatment with water at 170 °C and 0.8 MPa to 210 °C and 2.1 MPa for 120 min, the nonaqueous copper(II) carboxylate solutions were hydrolyzed and gave precipitates. Characterization of the resulting particles demonstrated that a mixture of cupric oxide and cuprous oxide was prepared from the organic starting solution of 0.15 kmol/m3 copper(II) carboxylate and 0.36 kmol/m3 free carboxylic acid. When the initial carboxylic acid concentration in the starting solution was increased to 1.0 kmol/m3, the copper(II) carboxylate solutions gave the crystalline particles of cuprous oxide alone at 180-210 °C. The particle size of cuprous oxide decreased from 16 to 5.5 µm as the liquid-phase stirring speed was changed from 300 to 100 rpm. The precipitation rate of cuprous oxide increased sharply with
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increasing temperature, and the precipitation of cuprous oxide was completed within 60 min at 210 °C. Acknowledgment This research was supported in part by a Grant-inAid for Scientific Research (B) (13555281) from the Ministry of Education, Science, Sports and Culture, Japan. We wish to thank Mr. Hiroshi Doi (Japan New Metals Co., Ltd., Osaka, Japan) for his assistance in characterizing the precipitates. Literature Cited (1) Ermakova, L. L.; Puzakov, V. V.; Lanskikh, A. D. Improving the Manufacture of Powdered Copper(I) Oxide. Tsvetn. Metall. 1986, 7, 35. (2) McFadyen, P.; Matijevic, E. Copper Hydrous Oxide Sols of Uniform Particle Shape and Size. J. Colloid Interface Sci. 1973, 44, 95. (3) Shah, I. D.; Ruzzi, P. D.; Schluter, R. B. Decomposition of Cupric Oxide Using a Reducing Scavenger. U.S. Patent 806116, 1977. (4) Dong, Y. J.; Li, Y. D.; Wang, C.; Cui, A. L.; Deng, Z. X. Preparation of Cuprous Oxide Particles of Different Crystallinity. J. Colloid Interface Sci. 2001, 243, 85. (5) Muramatsu, A.; Sugimoto, T. Synthesis of Uniform Spherical Cu2O Particles from Condensed CuO Suspensions. J. Colloid Interface Sci. 1997, 189, 167. (6) Zhu, Y. J.; Qian, Y. T.; Zhang, M. W.; Chen, Z. Y.; Xu, D. F.; Yang, L.; Zhou, G. Preparation and Characterization of Nanocrystalline Powders of Cuprous Oxide by Using γ-Radiation. Mater. Res. Bull. 1994, 29, 377.
(7) Doyle-Garner, F. M.; Monhemius, A. J. Hydrolytic Stripping of Single and Mixed Metal-Versatic Solutions. Metall. Trans. B 1985, 16B, 671. (8) Doyle, F. M. Integrating Solvent Extraction with the Processing of Advanced Ceramic Materials. Hydrometallugy 1992, 29, 527. (9) 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. (10) 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. (11) 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. (12) 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. (13) Doyle, F. M.; Ye, W. ZrO2 Powders from Zirconium(IV) Carboxylates. J. Met. 1987, 39, 34. (14) Konishi, Y.; Murai, T.; Asai, S. Preparation and Characterization of Fine Ceria Powders by Hydrolysis of Cerium(III) Carboxylate Dissolved in Organic Solvent. Ind. Eng. Chem. Res. 1997, 36, 2641. (15) Konishi, Y.; Asai, S.; Murai, T.; Takemori, H. Preparation of Fine Ceria Powders by Hydrolysis of Cerium(IV) Carboxylate Solutions. Metall. Mater. Trans. B 1997, 28B, 959.
Received for review June 17, 2003 Revised manuscript received January 14, 2004 Accepted February 18, 2004 IE0305115
