Continuous Hydrothermal Synthesis of Fe2O3 Nanoparticles Using a

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Ind. Eng. Chem. Res. 2010, 49, 8841–8846

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Continuous Hydrothermal Synthesis of Fe2O3 Nanoparticles Using a Central Collision-Type Micromixer for Rapid and Homogeneous Nucleation at 673 K and 30 MPa Kiwamu Sue,*,† Toshiyuki Sato,‡ Shin-ichiro Kawasaki,§ Yoshihiro Takebayashi,† Satoshi Yoda,† Takeshi Furuya,† and Toshihiko Hiaki‡ Nanosystem Research Institute, National Institute of AdVanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, Department of Applied Molecular Chemistry, College of Industrial Technology, Nihon UniVersity, Izumi-cho 1-2-1, Narashino, Chiba 275-8575, Japan, and Research Center for Compact Chemical System, National Institute of AdVanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan

Continuous hydrothermal synthesis of Fe2O3 nanoparticles was carried out at 673 K and 30 MPa. Two types of mixers, a conventional T-type micromixer and a central collision-type micromixer (CCM), were used for the synthesis. CCM was newly fabricated on the basis of the concept for preventing heterogeneous nucleation induced on the inner wall of the mixer. Residence time and Fe(NO3)3 molality of the starting solution varied from 0.1 to 1.0 s and 0.05 to 0.50 mol/kg, respectively. Effects of the mixer structure, the residence time, and the Fe(NO3)3 molality on crystal structure, Fe3+ conversion, average particle size, and size distribution were discussed. CCM effectively worked for producing smaller Fe2O3 nanoparticles with narrow and monomodal distribution. 1. Introduction Nanoparticles of metal oxides have been attracting considerable attention in the field of several functional materials. Desirable characteristics of these nanoparticles are narrow size distribution and high crystallinity. Several techniques have been proposed to prepare these nanoparticles such as sol-gel,1 spray pyrolysis,2 hydrothermal,3 and solvothermal4 methods. However, these techniques often require multistep treatment,1 long reaction time,1 high temperature,2 concentrated base,3 and organic solvents.4 A continuous hydrothermal synthesis method using supercritical water (SCW, TC ) 647 K, PC ) 22.1 MPa) as a reaction medium has been developed for preparing highly crystalline functional nanoparticles of metal oxides.5-7 This method uses some features of SCW such as low solubility of metal oxides and high hydrothermal reaction rate and is recognized as an environmentally benign technique these days. In this method, an aqueous solution of metal salts is fed by a pump, mixed in a T-type mixer (TM) with preheated water fed from another line as shown in Figure 1a,b and rapidly heated to reaction temperature above TC.5 The rapid heating promises production of smaller particles compared with an ordinary batch-type hydrothermal method. Development of the mixer for more rapid heating and homogeneous nucleation is a key task to obtain smaller particles having a narrow size distribution and to control the size of the particles. Over the past 5 years, some types of mixers with different structures have been proposed such as a mixer with a movable needle,8 a nozzle-type mixer,9 and a swirl-type mixer,10 as shown in Figure 1c-g, respectively. These mixers showed a * To whom correspondence should be addressed. Telephone: +8129-861-4866. E-mail: [email protected]. † Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology. ‡ Nihon University. § Research Center for Compact Chemical System, National Institute of Advanced Industrial Science and Technology.

significant advantage for decreasing the average size and the size distribution of the nanoparticles. On the other hand, a miniaturization of flow channels in the mixers from macroscale down to microscale is effective for promoting the mixing. For example, an average particle diameter of AlOOH nanoparticles using TM having an inner size of 0.3 mm was 10.8 nm,11 which was smaller than 25 and 59.7 nm in cases of using a mixer with a movable needle8 and a swirl mixer,10 respectively. From the viewpoint of increasing mixing rate, i.e., heating rate, these mixers worked well. Here, it should be noted that the starting solution was heated to reaction temperature with contacting not only the preheated water but also the inner wall of the mixer, which were marked as dots (•) in Figure 1. During the particle nucleation, the existence of the inner walls induces heterogeneous nucleation in addition to homogeneous nucleation and has a large possibility of producing larger particles and particles with multimodal distribution, especially at high molality condition of the starting materials. In this work, we focus on development of a simple micromixer with a new structure for preventing heterogeneous nucleation, which is called a central collision-type micromixer (CCM). A schematic diagram of CCM is shown in Figure 1h,i. The preheated water was supplied to the vertical flow of the starting solution from four horizontal directions. The starting solution was surrounded by the preheated water. For this structure, contact of the starting solution with the inner wall of the mixer can be minimized. As described above, AlOOH synthesis from Al(NO3)3 was selected to evaluate new mixers in previous works.8,10 However, evaluating nucleation and growth behavior in AlOOH synthesis is difficult because of the anisotropic platelike form and the coexistence of Al2O3, which is a dehydrated form of AlOOH, in some cases.8,10,11 Considering this fact, synthesis of Fe2O3 from Fe(NO3)3 is selected as a model reaction for evaluating CCM in this work. A continuous hydrothermal synthesis of Fe2O3 is performed using CCM and TM having an inner diameter of 0.3 mm at 673 K and 30 MPa. Effects of Fe(NO3)3 molality and residence time on the average

10.1021/ie1008597  2010 American Chemical Society Published on Web 08/02/2010

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Figure 1. Mixers developed for continuous hydrothermal synthesis of metal oxide nanoparticles: (a) a vertical cross section of a conventional T-type mixer I, (b) a vertical cross section of a conventional T-type mixer II, (c) a vertical cross section of a mixer with a movable needle,8 (d) a horizontal cross section along the dashed line in (c),8 (e) a vertical cross section of a nozzle-type mixer,9 (f) a vertical cross section of a swirl-type mixer,10 (g) a horizontal cross section along the dashed line in (f),10 (h) a vertical cross section of a central collision-type mixer proposed in this work, and (i) a horizontal cross section along the dashed line in (h).

particle size and the particles size distribution are examined in each mixer. Through the comparison between CCM and TM, an advantage of CCM is discussed on the basis of the mixer structure and the estimated Fe2O3 solubility. 2. Experimental Section 2.1. Materials. Solutions were prepared by dissolving precise amounts ((1 mg) of Fe(NO3)3 · 9H2O (purity >99.9%) in distilled and deionized water (resistivity >0.18 MΩ · m). The material was purchased from Wako Pure Chemical Industries, Ltd. and was used without further purification. The molalities of the nitrate in the starting solutions were 0.05, 0.25, and 0.50 mol/kg. 2.2. Procedure. Hydrothermal synthesis was performed using a flow-through apparatus as shown in Figure 2a. The Fe(NO3)3 solution was fed with a high-pressure pump at a flow rate of 20 g/min. In the case of using CCM, the preheated water was fed at a flow rate of 80 g/min from another line, was divided into four streams using three TMs as shown in Figure 2b, and then flowed into CCM. Details of a vertical cross section of the developed CCM are shown in Figure 2c. A diameter of all flow channels was 0.3 mm. The Fe(NO3)3 solution was mixed in CCM with the preheated water and then the mixture was rapidly heated to reaction temperature. A 1/16 in. SUS316 tube having the same inner diameter of 0.3 mm as CCM was used for keeping a smooth flow of the preheated water stream. A 1/16 in. SUS316 tube having a smaller inner diameter of 0.2 mm than CCM was used for preventing preheating in the tube before mixing by increasing flow velocity, i.e., decreasing residence time. A 1/8 in. SUS316 tube of 1.6 mm i.d. was used for the reactor and the cooler sections. All tubes, unions, and mixers were made of SUS316. Reaction temperature was set to 673 K by controlling the temperature of the preheated water at 702 K, which was estimated on the basis of an enthalpy balance. Residence time was calculated using the total flow rate [kg/s], the reactor volume [m3], and water density [kg/m3] at 673 K and 30 MPa and was managed by varying the reactor length.

After passing through the reactor, the mixed fluid was cooled to room temperature by mixing cooling water fed at a flow rate of 100 g/min and by an external water jacket. System pressure was controlled to be 30 MPa by a back pressure regulator. Products in recovered slurry solutions formed some aggregates more than 25 nm, although the average size of the products was less than 25 nm as described in section 5 and were separated with a membrane filter with a pore size of 25 nm. Then the separated products were dried at 333 K in an electric oven for 24 h. Experiments using TM having a 0.3 mm diameter of all flow channels was also performed for comparison. 2.3. Analyses. Crystal structures of the products were analyzed by powder X-ray diffractometry (XRD, Ultima IV, Rigaku) using Cu KR radiation. Observations of the products were performed by transmission electron microscopy TEM, JEM-2010, JEOL). Average particles size and standard deviation were determined on the basis of the size of about 200 particles measured from TEM images. The Fe3+ molality remaining in the recovered aqueous solutions was measured by UV-visible spectroscopy (V-660, JASCO) using the 1,10-phenanthroline method to obtain conversion [%] of Fe3+ into solid products. 3. Computational Fluid Dynamics Simulation Computational fluid dynamic (CFD) simulation was carried out for understanding the heating profiles in CCM. GAMBIT ver. 2.4.6 and FLUENT ver. 6.3.26 (ANSYS, Inc.) were used for CFD simulation. In this simulation, all solutions are treated as pure water at given temperature and pressure for simplifying estimation of fluid properties. Simulation conditions and methods are the same as those used in previous studies.12,13 Temperature contour diagram in CCM is shown in Figure 3. The colors represent temperature from 298 K (blue) to 711 K (orange). It was clearly shown that the Fe(NO3)3 solution colored as a blue area was heated with the surrounding preheated water supplied from four horizontal directions. Maximum and minimal temperatures in each cross section of the x-y surface along the z-axis were obtained from CFD results at 0.1 mm intervals from

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Figure 2. (a) Schematic diagram of the experimental apparatus. (b) Details around CCM. (c) Details of the central collision-type micromixer.

Figure 4. Maximum and minimal temperature at each cross section along the z-axis as a function of the distance from the mixing point: Circle and triangle denote CCM and TM.

mm in comparison with TM. The longest heating up time to reaction temperature for CCM was estimated to be 1.4 × 10-4 s on the basis of the minimal temperature from z ) 0 to z ) 1.5 mm and was shorter than that for TM, 8.7 × 10-4 s.13 These results indicate that the use of CCM promises rapid and homogeneous nucleation. Figure 3. Temperature contour diagram in the central collision-type micromixer.

z ) 0 (mixing point, see Figure 3) to z ) 1 mm, 0.5 mm intervals from z ) 1 mm to z ) 5 mm, and 1 mm intervals from z ) 5 mm to z ) 10 mm. The result for CCM is shown in Figure 4 together with the result for TM reported in a previous study.13 In the case of using CCM, the difference between the maximum and minimal temperatures drastically decreased with increasing the distance from the mixer center, z ) 0, to 2.0

4. Fe2O3 Solubility and Degree of Supersaturation Fe2O3 solubility is a key factor in discussing nucleation and growth behavior. The solubility at temperatures from 298 to 673 K and constant pressure of 30 MPa was calculated in the presence of NO3- of 0.03-0.30 mol/kg that correspond to Fe(NO3)3 molalities in the reactor, 0.01-0.10 mol/kg. Details of the calculation procedure are described in a previous study.11,14

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Figure 5. (a) Estimated Fe2O3 solubility. (b) Degree of supersaturation. Circle, triangle, and square denote 0.01, 0.05, and 0.10 mol/kg of the Fe(NO3)3 molality after mixing. Figure 6. XRD profiles of the products synthesized from Fe(NO3)3 at (a) 0.05 mol/kg and 0.1 s using TM (run 1), (b) 0.50 mol/kg and 0.1 s using TM (run 5), (c) 0.05 mol/kg and 1.0 s using TM (run 2), (d) 0.50 mol/kg and 1.0 s using TM (run 6), (e) 0.05 mol/kg and 0.1 s using CCM (run 7), (f) 0.50 mol/kg and 0.1 s using CCM (run 11), (g) 0.05 mol/kg and 1.0 s using CCM (run 8), and (h) 0.50 mol/kg and 1.0 s using CCM (run 12). Circle and triangle denote Fe2O3 crystals of tetragonal (No. 25-1402) and rhombohedral (No. 33-0664) structures, respectively.

The solubility curves are shown in Figure 5a as a function of temperature ranging from 298 to 673 K and Fe(NO3)3 molality ranging from 0.01 to 0.10 mol/kg at 30 MPa. The solubility decreases with increasing temperature and decreasing Fe(NO3)3 molalities. The difference in solubility at each temperature decreased at higher temperatures. Degree of supersaturation (DS) was estimated as the natural logarithm value of the ratio of the Fe(NO3)3 molalities after mixing to the calculated Fe2O3 solubilities. The curves of DS are shown in Figure 5b as a function of temperature ranging from 298 to 673 K and Fe(NO3)3 molality ranging from 0.01 to 0.10 mol/kg at 30 MPa. DS increases with increasing temperature because of the decrease in the solubility. In the following, the effect of Fe(NO3)3 molality on DS is discussed. An increase in Fe(NO3)3 molality results in an increase in HNO3 molality and also in Fe2O3 solubility. At lower temperatures below TC, HNO3 dissociates completely and an increase of 1 order of magnitude in Fe(NO3)3 molality results in an increase of more than 3 orders of magnitude in Fe2O3 solubility. As a result, DS decreases with increasing Fe(NO3)3 molality. In contrast, at higher temperatures above TC, HNO3 strongly associates because of the low permittivity.15 Therefore, the Fe2O3 solubility changes only slightly within two times with the increase of 1 order of magnitude in the Fe(NO3)3 molality. As a result, DS increases with increasing Fe(NO3)3 molality.

5. Results Experimental results of Fe3+ conversion by UV-vis, and average particle size by TEM with experimental conditions of Fe(NO3)3 molality and residence time are summarized in Table 1. Experimental data at Fe(NO3)3 molality of 0.05 mol/kg in the case using TM (runs 1 and 2) were from previous work.11,13 In all cases, Fe3+ conversions were more than 96% and average particle sizes less than 16 nm. Typical XRD results of the obtained particles are shown in Figure 6. All peaks of the obtained crystals were assigned to Fe2O3 with rhombohedral (R-Fe2O3, No. 33-0664) and tetragonal (γ-Fe2O3, No. 25-1402) structures. It was found that the peak width of γ-Fe2O3 was narrower than that of R-Fe2O3. This means a smaller crystallite size of γ-Fe2O3 compared to that of R-Fe2O3. At low Fe(NO3)3 molality, the product was γ-Fe2O3 at 0.1 s in both cases using CCM and TM. With increasing residence time to 1.0 s, part of the product changed to R-Fe2O3. At high

Table 1. Summary of Experiments on Continuous Hydrothermal Synthesis of Fe2O3 from Fe(NO3)3 at 673 K and 30 MPa: Experimental Conditions, Conversion by UV-Vis, and Average Particle Size by TEM

a

run no.

mixer type

Fe(NO3)3 molality [mol/kg]

residence time [s]

conversion [%]

particle size [nm]

coefficient of variation

1a 2b 3 4 5 6 7 8 9 10 11 12

TM TM TM TM TM TM CCM CCM CCM CCM CCM CCM

0.05 0.05 0.25 0.25 0.50 0.50 0.05 0.05 0.25 0.25 0.50 0.50

0.1 1.0 0.1 1.0 0.1 1.0 0.1 1.0 0.1 1.0 0.1 1.0

96.9 ( 0.2 98.3 ( 0.1 97.4 ( 0.1 99.2 ( 0.2 99.6 ( 0.1 99.7 ( 0.2 97.1 ( 0.2 98.5 ( 0.1 96.9 ( 0.2 98.7 ( 0.2 98.9 ( 0.2 99.4 ( 0.1

4.0 ( 1.0 5.5 ( 1.3 6.0 ( 3.5 8.2 ( 5.7 9.5 ( 8.2c 15.1 ( 16.6c 4.9 ( 0.9 6.2 ( 0.9 5.5 ( 0.7 6.7 ( 1.0 4.9 ( 0.9 10.1 ( 3.4

0.24 0.24 0.58 0.70 0.86c 1.10c 0.18 0.14 0.12 0.16 0.19 0.34

Data from previous work.13 b Data from previous work.11 c Bimodal distribution.

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Figure 7. TEM images of the products synthesized from Fe(NO3)3 using TM at (a) 0.25 mol/kg and 1.0 s (run 4) and (b) 0.50 mol/kg and 1.0 s (run 6) and using CCM at (c) 0.05 mol/kg and 0.1 s (run 7), (d) 0.05 mol/kg and 1.0 s (run 8), (e) 0.25 mol/kg and 1.0 s (run 10), and (f) 0.50 mol/kg and 1.0 s (run 12).

Figure 8. Particle size distributions of the products synthesized from Fe(NO3)3 using TM at (a) 0.05 mol/kg and 0.1 s (run 1), (b) 0.05 mol/kg and 1.0 s (run 2), (c) 0.25 mol/kg and 1.0 s (run 4), and (d) 0.50 mol/kg and 1.0 s (run 6) and using CCM at (e) 0.05 mol/kg and 0.1 s (run 7), (f) 0.05 mol/kg and 1.0 s (run 8), (g) 0.25 mol/kg and 1.0 s (run 10), and (h) 0.50 mol/kg and 1.0 s (run 12).

Fe(NO3)3 molality, the product obtained using TM was a mixture of γ-Fe2O3 and R-Fe2O3 at 0.1 s and varied to dominant phase of R-Fe2O3 at 1.0 s. In contrast, the product obtained using CCM was γ-Fe2O3 single phase at 0.1 s and varied to a mixture of γ-Fe2O3 and R-Fe2O3 at 1.0 s. Fe3+ conversions slightly increased with increasing residence time and Fe(NO3)3 molality as shown in Table 1. It was found

that the conversions using CCM were slightly lower than that using TM at high Fe(NO3)3 molality in spite of shorter heating up time as described in section 3. Typical TEM images of the obtained crystals are shown in Figure 7. Nanoparticles in the sizes ranging from ca. 4 to ca. 80 nm were produced. The particle size distributions (PD) and average particle size (AS) with coefficient of variation (CV)

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evaluated on the basis of the TEM images are summarized in Figure 8 and Table 1. AS and CV increased with increasing Fe(NO3)3 molality in both cases of CCM and TM. CV using CCM remarkably decreased compared with CV using TM. Especially at 0.50 mol/kg and 1 s, PD was changed from bimodal to monomodal by using CCM in addition to the decreases in AS and CV from 15.1 nm and 1.10 (run 6) to 10.1 nm and 0.34 (run 12), respectively. 6. Discussion Nucleation and growth behavior are discussed on the basis of CFD simulation, DS, and experimental results and then an advantage using CCM is clarified. In both cases using CCM and TM, the heating of the starting solution to 673 K was finished, even at the shorter residence time of 0.1 s on the basis of CFD simulation. The increases in AS and conversion with increasing residence times from 0.1 to 1.0 s imply that the crystal growth slightly proceeds within a very short residence time of 0.9 s. With increasing residence time, CV at 0.25 and 0.5 mol/ kg increased in both cases using CCM and TM. At 0.05 mol/ kg, CV using CCM decreased and CV using TM did not change. The remarkable increase in AS and CV at high Fe(NO3)3 molality was probably led by the aggregation growth of a large number of the nucleated amorphous or low-crystallinity Fe2O3 nanoparticles. In contrast, it is considered that Ostwald ripening is a dominant growth reaction compared with the aggregation growth due to the low particle density condition at low Fe(NO3)3 molality. The increases in AS and CV with increasing Fe(NO3)3 molality, i.e., increasing DS at 673 K, is a opposite trend on the basis of classical nucleation theory. However, the trend can be explained by decreasing DS at lower temperatures with Fe(NO3)3 molality. The trend means that the characteristics of the products are still affected by the heating up history (i.e., the history of solubility change as shown in Figure 5a), even at the very short heating up time of less than 10-3 s on each mixer. Then, on the basis of XRD results, it seems reasonable to suppose that metastable and smaller γ-Fe2O3 nanoparticles are produced by the homogeneous nucleation. In addition, stable and larger R-Fe2O3 nanoparticles are produced by the heterogeneous nucleation and growth on the inner wall of the mixer and also the phase transition during the aggregation growth of the primary nucleated γ-Fe2O3 nanoparticles. Most important of all, the use of CCM results in the production of smaller and narrow size distribution particles as shown in Table 1 and Figure 8. This means that CCM effectively works for rapid heating of the starting solution and preventing heterogeneous nucleation, especially at higher molality. Further, the prevention of heterogeneous nucleation is assumed to be one possible reason for lower conversion in spite of higher heating rate of CCM than TM since heterogeneous nucleation easily occurs compared with homogeneous nucleation because of the lower energy barrier to nucleation. 6. Conclusions Continuous hydrothermal synthesis of Fe2O3 nanoparticles was carried out at 673 K and 30 MPa using a central collisiontype micromixer (CCM). CCM has a structure that four preheated water channels are arranged from a horizontal direction to downstream channel for the starting solution and the starting solution was heated to reaction temperature by surrounding the preheated water. The use of CCM resulted in

the production of smaller Fe2O3 nanoparticles with narrow and monomodal distribution compared with a conventional T-type micromixer and worked well for preventing heterogeneous nucleation on the inner wall of the mixer. Acknowledgment This research was partially supported by the Organization for Small & Medium Enterprises and Regional Innovation and the Ministry of Education, Culture, Sports, Science and Technology. TEM observation was performed using facilities of the Institute for Solid State Physics, the University of Tokyo. We thank M. Ichihara, D. Hamane, and M. Koike for their assistance in the TEM observation. Literature Cited (1) Galindo, I. R.; Viveros, T.; Chadwick, D. Synthesis and Characterization of Titania-Based Ternary and Binary Mixed Oxides Prepared by the Sol-Gel Method and Their Activity in 2-Propanol Dehydration. Ind. Eng. Chem. Res. 2007, 46, 1138. (2) Widiyastuti, W.; Balgis, R.; Iskandar, F.; Okuyama, K. Nanoparticle Formation in Spray Pyrolysis under Low-pressure Conditions. Chem. Eng. Sci. 2010, 65, 1846. (3) Tsumura, T.; Matsuoka, K.; Toyoda, M. Formation and Annealing of BaTiO3 and SrTiO3 Nanoparticles in KOH Solution. J. Mater. Sci. Technol. 2010, 26, 33. (4) Chung, J. W.; Yang, H. K.; Moon, B. K.; Choi, B. C.; Jeong, J. H.; Bae, J. S.; Kim, K. H. The dependence of temperature synthesis of GdVO4: Eu3+ nanoparticle phosphors by solvothermal method. Curr. Appl. Phys. 2009, 9, S222. (5) Adschiri, T.; Kanazawa, K.; Arai, K. Rapid and continuous hydrothermal crystallization of metal oxide particles in supercritical water. J. Am. Ceram. Soc. 1992, 75, 1019. (6) Adschiri, T.; Hakuta, Y.; Sue, K.; Arai, K. Hydrothermal synthesis of metal oxide nanoparticles at supercritical conditions. J. Nanopart. Res. 2001, 3, 227. (7) Adschiri, T.; Hakuta, Y.; Arai, K. Hydrothermal synthesis of metal oxide nanoparticles at supercritical conditions. Ind. Eng. Chem. Res. 2000, 39, 4901. (8) Mae, K.; Suzuki, A.; Maki, T.; Hakuta, Y.; Sato, H.; Arai, K. A new micromixer with needle adjustment for instant mixing and heating under high pressure and high temperature. J. Chem. Eng., Jpn. 2007, 40, 1101. (9) Lester, E.; Blood, P.; Denyer, J.; Giddings, D.; Azzopardi, B.; Poliakoff, M. Reaction engineering: The supercritical water hydrothermal synthesis of nano-particles. J. Supercrit. Fluids 2006, 37, 209. (10) Wakashima, Y.; Suzuki, A.; Kawasaki, S.-I.; Matsui, K.; Hakuta, Y. Development of a new swirling micro mixer for continuous hydrothermal synthesis of nano-size particles. J. Chem. Eng., Jpn. 2007, 40, 622. (11) Sue, K.; Suzuki, M.; Arai, K.; Ohashi, T.; Ura, H.; Matsui, K.; Hakuta, Y.; Hayashi, H.; Watanabe, M.; Hiaki, T. Size-controlled synthesis of metal oxide nanoparticles with a flow-through supercritical water method. Green Chem. 2006, 6, 634. (12) Kawasaki, S.-I.; Sue, K.; Ookawara, R.; Wakashima, Y.; Suzuki, A.; Arai, K. Engineering study of continuous supercritical hydrothermal method using a T-shaped mixer: Experimental synthesis of NiO nanoparticles and CFD simulation. J. Supercrit. Fluids 2006, 54, 96–102. (13) Sue, K.; Kawasaki, S.-I.; Suzuki, M.; Hakuta, Y.; Hayashi, H.; Arai, K.; Takebayashi, Y.; Yoda, S.; Furuya T. Continuous hydrothermal synthesis of Fe2O3, NiO, and CuO nanoparticles by superrapid heating using a T-type micro mixer at 673 K and 30 MPa. Submitted to Chem. Eng. J. (14) Sato, T.; Sue, K.; Suzuki, W.; Suzuki, M.; Matsui, K.; Hakuta, Y.; Hayashi, H.; Arai, K.; Kawasaki, S.-I.; Kawai-Nakamura, A.; Hiaki, T. Rapid and continuous production of ferrite nanoparticles by hydrothermal synthesis at 673 K and 30 MPa. Ind. Eng. Chem. Res. 2008, 47, 1855. (15) Chlistunoff, J.; Ziegler, K. J.; Lasdon, L.; Johnston, K. P. Nitric/ Nitrous Acid Equilibria in Supercritical Water. J. Phys. Chem. A 1999, 103, 1678.

ReceiVed for reView April 12, 2010 ReVised manuscript receiVed July 5, 2010 Accepted July 13, 2010 IE1008597