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Langmuir 2000, 16, 3042-3049
Reverse Microemulsion-Mediated Synthesis and Structural Evolution of Barium Hexaaluminate Nanoparticles Andrey J. Zarur, Henry H. Hwu, and Jackie Y. Ying* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307 Received June 23, 1999. In Final Form: September 8, 1999
Nanocrystalline barium hexaaluminate has been successfully synthesized through the use of a reverse microemulsion as a medium for controlled hydrolysis and polycondensation of barium and aluminum alkoxides. The nanoparticles derived were characterized with electron microscopy, X-ray diffraction, and nitrogen adsorption analysis. This novel material possessed a well-defined particle morphology and an ultrahigh surface area, and exhibited excellent catalytic performance in methane combustion. Its structural evolution was found to be strongly dependent on synthesis parameters, such as water/alkoxide ratio and aging period. Powder recovery and drying techniques also had an important impact on particle agglomeration and structural development. Through the unique synthesis approach described, barium hexaaluminate with superb thermal stability was achieved, with surface areas in excess of 100 m2/g retained even after calcination at 1300 °C.
Introduction Catalytic combustion has attracted considerable recent attention due to its potential for burning natural gas and methane in gas turbines with extremely low levels of NOx emissions. Catalytic oxidation can take place at lower temperatures than conventional homogeneous gas-phase combustion, avoiding the production of thermal NOx.1 It further allows ultralean fuel mixtures to be employed, providing for safer turbine operation and more complete hydrocarbon combustion.2,3 Traditionally, supported noble metals have been examined for catalytic combustion. However, these materials suffer from sintering and vaporization by 800 °C,4 leading to significant loss of catalytic activity at the high flame temperatures of combustion (∼1300 °C). Recent efforts have examined simple and complex metal oxides as alternative catalyst systems. Barium hexaaluminate (BHA) was of particular interest for its high thermal resistance at near-flame temperatures. It has been produced with surface areas of 15-20 m2/g through alkoxide sol-gel processing5 and supercritical drying.6 BHA systems may also be used as catalyst supports in automotive catalytic converters and in synthesis gas production via steam reforming or partial oxidation.7 Catalytic studies indicate that the high activity of BHA catalyst can be attributed to its high surface area available at the elevated reaction temperatures.8 The pore structure of the oxide is found to be an important parameter in catalytic combustion as well. Microporous catalysts with pores e2 nm appear to be less active than mesoporous catalysts (with >2-nm pores), perhaps due to mass* To whom correspondence should be addressed. (1) Pfefferle, L. D.; Pfefferle, W. C. Catal. Rev.sSci. Eng. 1987, 29, 219. (2) Zwinkels, M. F. M.; Jaras, S. G.; Menon, P. G. Catal. Rev.sSci. Eng. 1993, 35, 319. (3) Beer, J. M. J. Inst. Energy 1995, 68, 2. (4) Trimm, D. L. Catal. Today 1995, 26, 231. (5) Machida, M.; Eguchi, K.; Arai, H. Chem. Lett. 1987, 267. (6) Mizushima, Y.; Hori, M. J. Mater. Res. 1994, 9, 2272. (7) Machida, M.; Teshima, T.; Eguchi, K.; Arai, H. Chem. Lett. 1991, 231. (8) Machida, M.; Eguchi, K.; Arai, H. J. Catal. 1989, 120, 377.
transfer limitations at the high flow velocities (>50 cm/s) needed for flame stabilization.9 The research motivation of this study is to develop an approach for synthesis of complex oxides with ultrahigh surface areas for high-temperature catalytic applications. Through nanostructure processing, we seek to create BHA particles with thermal stability and catalytic activity superior to those of conventional materials. Nanostructured materials offer unique size-dependent properties,10-12 a large surface-to-volume ratio,13-15 and unusual chemical/ electronic synergistic effects from an ultrahigh component dispersion.16,17 The strategy here is to generate nonagglomerated materials with controlled nanoparticle morphology to attain superb surface area and sintering resistance. This study further seeks to achieve molecularlevel chemical homogeneity in complex oxides, so as to induce crystallization of the desired crystalline phase at lower temperatures to effectively suppress BHA grain growth. Synthesis of catalytic, ceramic, electronic, and biochemical systems in reverse emulsions has been explored recently in several studies.18-22 This synthetic approach allows particles to be obtained with well-defined geometry and size, since processing of materials is confined to the aqueous reverse micelles. With reverse microemulsion(9) Zarur, A.; Hwu, H. H.; Ying, J. Y. Unpublished results. (10) Cahn, R. W. Nature 1990, 348, 389. (11) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature 1996, 383, 802. (12) Alivisatos, A. P. Science 1996, 271, 933. (13) Karch, J.; Birringer, R.; Gleiter, H. Nature 1987, 330, 556. (14) Gleiter, H. Prog. Mater. Sci. 1989, 33, 223. (15) Siegel, R. W. Annu. Rev. Mater. Sci. 1991, 21, 559. (16) Ying, J. Y.; Tscho¨pe, A. Chem. Eng. J. 1996, 64, 225. (17) Ying, J. Y.; Tscho¨pe, A.; Levin, D. Nanostruct. Mater. 1995, 6, 237. (18) Moriya, Y.; Nakagawa, K.; Kawasaki, K.; Ozaki, Y. J. Ceram. Soc. Jpn. 1995, 103, 570. (19) Narita, T.; Nakagawa, K.; Kawasaki, K.; Ozaki, Y. J. Ceram. Soc. Jpn. 1996, 104, 644. (20) Chhabra, V.; Ayyub, P.; Chattopadhyay, S.; Maitra, A. N. Mater. Lett. 1996, 26, 21. (21) Khmelnitsky, Y.; Kabanov, A.; Klyachko, N.; Levashov, A.; Martinek, K. Structure Reactivity in Reverse Micelles; Elsevier: Amsterdam, 1989. (22) Lisieki, I.; Billoudet, F.; Pileni, M. P. J. Mol. Liq. 1997, 72, 251.
10.1021/la9908034 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/10/2000
Barium Hexaaluminate Nanoparticles
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mediated synthesis, nanoparticles can be derived from precursor hydrolysis within aqueous domains of 0.5-10 nm.23 Such an approach further provides a means to derive complex oxides in a controlled environment whereby chemical composition and particle morphology can be tailored with precision. This paper examines the flexibility of this novel synthesis and the importance of subsequent processing to deriving BHA materials with ultrahigh surface areas. This research shows that it is possible to obtain discrete BHA nanoparticles with superior chemical homogeneity by controlling the synthesis parameters. Furthermore, by using novel powder recovery and drying techniques, we were able to enhance the thermal resistance of the nanoparticles, so that surface areas >100 m2/g could be maintained at 1300 °C. The nanostructured BHA systems have demonstrated excellent catalytic activity and stability in methane combustion24 and are also of interest as a support material in various hightemperature catalytic applications. Experimental Section I. Synthesis of Reverse Microemulsion. Ionic surfactants, such as AOT (sodium bis(2-ethylhexyl)sulfosuccinate), are typically used in preparing reverse emulsions for particle synthesis. In our case, cations that may contaminate the BHA system have to be avoided. We have therefore examined a variety of nonionic surfactants in the derivation of reverse emulsions.25 Using polyethoxylated alcohols as surfactants and medium-chain linear alcohols as cosolvents, reverse microemulsions were achieved with a wide range of water contents. This provided a greater flexibility in synthesis control and yield in contrast to the case of the AOT-containing systems, which formed lamellar structures instead of bicontinuous micellar suspensions at high water contents (>10 wt %). The synthesis scheme of nanostructured BHA is illustrated in Figure 1. Isooctane (99.9% 2,2,4-trimethylpentane, Aldrich) was used as-received for the continuous oil phase of the reverse emulsion. Deionized water was added to isooctane to form mixtures containing 1-45 wt % H2O. The surfactant mixture consisted of 66.7 wt % polyethoxylated hexanol with 9-12 units of ethylene oxide per mole of hexanol (C9-12E6, Neodol 91-6, Shell Chemical) and 33.3 wt % 1-pentanol (99.9%, Aldrich). It was added to the water/isooctane mixture and stirred at room temperature until a clear, stable reverse microemulsion was obtained. II. Hydrolysis of Alkoxide Precursors. Barium isopropoxide was prepared by refluxing metallic barium (99.9%, Aldrich) in 2-propanol (99.9%, Mallinckrodt) at 70 °C for 12 h. Aluminum isopropoxide (99%, Aldrich) was dissolved in 2-propanol by refluxing at 80 °C for 24 h; the final alkoxide concentration in 2-propanol was varied between 1 and 10 wt %. The precursor solution was obtained by adding the solutions of barium and aluminum isopropoxides in a 1:12 molar ratio to previously degassed isooctane. No sign of hydrolysis was observed when isooctane was mixed with the alkoxide solutions, suggesting that the water content in the precursor solution was negligible. The precursor solution was then introduced to the reverse emulsion without exposure to air by use of a cannula. The water/alkoxide molar ratio was varied from 1 to 1000 times the stoichiometric value (0.2529 g of H2O/g of mixed alkoxide precursor). The hydrolysis reaction occurred slowly at room temperature, as reflected by the gradual reduction in the transparency of the reaction mixture. The mixture was aged at 25 °C with stirring for 1-48 h. The reverse microemulsions with low water contents (1-5 wt %) underwent phase separations during aging, and a very fine powder was precipitated to the bottom of the flask. The systems with high water contents (>10 wt %) remained stable even after prolonged aging. (23) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65. (24) Zarur, A. J.; Hwu, H. H.; Ying, J. Y. to be submitted to J. Catal. (25) Ying, J. Y.; Zarur, A. J.; Heibel, A. T.; Sun, T. U.S. Patent pending, 1997.
Figure 1. Reverse microemulsion-mediated sol-gel processing of BHA nanoparticles. BHA control samples were also prepared by conventional solgel processing. A similar water/alkoxide ratio was used to produce these samples as that in the reverse microemulsion-mediated synthesis. The sol-gel-derived BHA samples were also aged at 25 °C for 1-48 h. III. Particle Recovery and Processing. After aging, the particles were recovered from the reverse microemulsions using various techniques. One technique induced phase separation of the reverse microemulsion via cooling to -5 °C. Upon phase separation, the lighter phase containing isooctane and a fraction of the surfactants was removed by aspiration and recycled for future use. The heavier phase containing water, 2-propanol, the remaining surfactants, and the BHA powders was subjected to filtration with 0.1-µm vacuum quartz frits or filter paper. The filter cake was washed several times with dry 2-propanol to remove residual surfactants. Filtration was not effective at collecting ultrafine particles, particularly those generated from reaction mixtures of high water/alkoxide ratios. Centrifugation and rotoevaporation were also employed to recover the BHA powders from the reverse microemulsions. However, these two techniques in general failed to recover the particles without causing severe agglomeration. Freeze-drying was developed as an alternative particle recovery approach to the conventional techniques mentioned earlier. In this case, the aged system was sprayed or atomized into a cryogenically cooled vessel, which was then actively evacuated as it was slowly warmed to ambient temperature. This allowed effective removal of 2-propanol, isooctane, and water via sublimation, leaving behind BHA particles and residues of heavy surfactants. The BHA particles were recovered and washed with acetone or 2-propanol, and freeze-dried again repeatedly until a dry powder was obtained. Alternatively, the product from one cycle of freeze-drying was subjected to heat treatment at 500 °C under flowing N2. Typically, dry powder was obtained from one cycle of freezedrying for materials prepared from reverse microemulsions with low water contents (
