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Nanocrystalline perovskites for catalytic combustion and oxygen separation

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dc.contributor.advisor Jackie Y. Ying. en_US Sangar, Neeraj, 1974- en_US
dc.contributor.other Massachusetts Institute of Technology. Dept. of Chemical Engineering. en_US 2005-06-02T16:11:33Z 2005-06-02T16:11:33Z 2002 en_US 2002 en_US
dc.description Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2002. en_US
dc.description Includes bibliographical references. en_US
dc.description.abstract Nanocrystalline perovskites (Lal-xAMnl-yByO3) were successfully synthesized with higher surface area and smaller grain size by chemical co-precipitation compared to solid-state and complexation/combustion synthesis routes. The choice of solvent, base and suspension pH in co-precipitation was found to strongly affect the chemical stoichiometry of the resulting material. Stoichiometric La0.5Sr0.sMnO3 was successfully obtained at a high pH using isopropanol as the solvent and tetraethylammonium hydroxide as the base. La0.sSr0.sMnO3 was derived with a ultrafine grain size of 13 nm and a high surface area of 43 m2/g at 650⁰C, and maintained its nanocrystalline microstructure on heating to 1000⁰C, with a grain size of 25 nm and a surface area of 19 m2/g. The catalytic activity of these perovskites was investigated for different A- and B-site substitutions. Among LaBO3 perovskites, the catalytic activity was found to decrease in the order: Mn > Fe [approx.] Ni > Co, with LaMnO3 showing the lowest light-off temperature of 420⁰C. The intrinsic catalytic activity at 650C decreased in the order: Ni > Co > Fe > Mn. Substitution of Group IIA metals for La3+ was found to increase the reaction rate of LalxAxMnO3, while higher valency dopants did not change or decreased catalyst activity. In the case of Ca2+ and Sr +dopants, intrinsic activity of Lal-xAxMnO3 was found to increase with doping level until x = 0.4 and 0.6, respectively. La0.4Sr0.6MnO3 exhibited the lowest light-off temperature of 3800C, with a reaction rate that was 2.5 times higher than LaMnO3. Methane TPR experiments showed that methane oxidation over the perovskites occurred by methane adsorption on the catalyst surface via hydrogen abstraction. en_US
dc.description.abstract (cont.) Substitution of Group IIA metals for La3+ enhanced catalytic activity by increasing the rate of methane activation, but lowered activity at high doping levels due to slow carbonate decomposition. Mixed conducting BalxSrCol-yMyO3- perovskite membranes were developed for oxygen separation applications. Ba0.75Sr0.25Coo.8Feo.203- showed a very high oxygen flux of [approx.] 3.8 cm3[STP]/min/cm2 at 900⁰C. Bao.25ro.75Co0sTio.2036 exhibited an oxygen flux of [approx.] 1.4 cm3[STP]/min/cm2 at 750⁰C with excellent stability over time. These oxygen fluxes were [approx.] 2 times higher than those reported for the best existing membrane materials. High oxygen fluxes were obtained by creating a high oxygen vacancy concentration ([approx.] 15% of oxygen lattice sites) via extrinsic doping, and by increasing the unit cell free volume to allow facile oxide ion hopping. The challenge in developing these membranes was to prevent the phase transformation of the vacancy-disordered perovskite to a poorly conductive vacancy-ordered structure in the desired temperature range of 750-900⁰C. This was accomplished by doping various cations in place of cobalt at the B site. Iron was found to be the most effective dopant for stabilizing the perovskite phase, followed by titanium and tin. A novel approach was developed to stabilize the vacancy-disordered perovskite phase of BaCoo.8M0.203 on cooling to room temperature, so that significantly higher oxygen fluxes could be achieved at low temperatures with excellent stability. When a single type of dopant cation was introduced at the B site, the vacancy-disordered phase could not be ... en_US
dc.description.statementofresponsibility by Neeraj Sangar. en_US
dc.format.extent 172 leaves en_US
dc.format.extent 8200848 bytes
dc.format.extent 8200654 bytes
dc.format.mimetype application/pdf
dc.format.mimetype application/pdf
dc.language.iso eng en_US
dc.publisher Massachusetts Institute of Technology en_US
dc.rights M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission. en_US
dc.subject Chemical Engineering. en_US
dc.title Nanocrystalline perovskites for catalytic combustion and oxygen separation en_US
dc.type Thesis en_US Ph.D. en_US
dc.contributor.department Massachusetts Institute of Technology. Dept. of Chemical Engineering. en_US
dc.identifier.oclc 52298584 en_US

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