Pd ÜLyuXJCgG}Ì_»EÒ³MãÌ\¢Àè« O ÷EcTvEã¼^¢EàqöÇÞ Eä §çHÞ _CncHAE()Þ¿ZpC5202593 ê § ¶S³¤¬RVã Þ k»»wHAE()»¬i¤C2430023 _Þì §úØsËcWe have reported that the Pdperovskite catalyst regenerates itself through the solid solution and segregation of Pd in and out of the perovskite crystal during the inherent fluctuation of engine exhaust gases. We named this in novative ceramic thegintelligent catalyst.hIn this study, the structural stability and the catalytic activity of Pd perovskite catalysts such as LaFePdO 3 , LaCoPdO 3 and LaMnPdO 3 prepared by the alkoxide method are com pared. Although it was reported that the catalytic activity of perovskite catalysts without Pd is, in the order, LaCoO 3 LaMnO 3 âLaFeO 3 , the catalytic activity of perovskite catalysts containing Pd after heat treatment un der redox conditions was superior and in the order LaFePdO 3 âLaMnPdO 3 LaCoPdO 3 . It was confirmed that the LaFePdO 3 catalyst's superior catalytic activity was due to a highly efficient suppression of particle growth. We found out that the structural stability of perovskite crystal was the key to suppressing the particle growth of Pd, and thereby maintaining the catalytic activity of Pdperovskite.[ Received September 9, 2004; Accepted October 26, 2004] Keywords : Pdperovskite catalyst, Structural stability, Alkoxide method, Intelligent catalyst, Selfregeneration 1. Introduction In order to guarantee cleaner air for the environment, emis sion regulations are becoming stricter throughout over the world. The Japanese government has introduced cleaner emis sion standards designated as G (good), E (excellent) and U (ultra) LEV (lowemission vehicles), which respectively represent 25÷, 50÷ and 75÷ reductions in HC and NOx in comparison with the standards for emission regulation set in the year 2000. Since then, it has introduced a new emission standard designated as SU (superultra)LEV, which repre sents 75÷ reductions in HC and NOx in comparison with standards for emission regulation set for the year 2005.In a conventional catalyst, the deterioration of catalytic ac tivity is mainly due to the particle growth of precious metals during vehicle use. To maintain catalytic activity in such a way as to continue to meet the stricter emission regulations after heat treatment under redox conditions, larger amounts of precious metals must be used. In order to solve the problems created by the excess use of precious metals, we have been developing a Pdperovskite catalyst that regenerates itself through the solid solution and segregation of Pd in and out of the perovskite crystal during the lifetime of a vehicle without any auxiliary treatment. A common automotive gasoline en gine is operated close to the stoichiometric airtofuel ratio (by using an oxygen sensor and a sophisticated feedback control system linked to the catalyst) in order to convert simultane ously three pollutant emissions into harmless gases. A time lag associated with adjusting the airtofuel ratio results in redox fluctuations between reductive and oxidative atmospheres at ...
Mullite ceramic has been recognized as a promising material for high-temperature structural applications, because of its high mechanical strength [1, 2], low thermal expansion coefficient [3] and high creep resistance [4]. However, only a few papers have been published with regard to the optical properties of mullite ceramics. Prochazka and Klug [5] studied infrared transmittance of mullite ceramics which were prepared by hot-pressing and hot-isostatic pressing by using the starting powders derived from alkoxides.In the present study, visible spectral to infrared translucent mullite ceramic has been fabricated by pressureless sintering without additives. Because the elimination of pores was considered to be essential for the achievement of translucency, a vacuum-sintering process was adopted. As the presence of the second phase usually affects the degree of transmittance, mullite powder compacts have to be sintered in the solid-solution range to be a single-phase mullite body free from second phase (corundum or glassy phase). In the case of optically anisotropic materials such as mullite, the in-line transmittance is considered to be improved by the achievement of the sufficiently smaller grain size compared with the wavelength of the transmitted radiation. However, the fabrication of a fully sintered body which has smaller grain size than visible ray wavelength is practically impossible. On the other hand, the presence of grain boundaries has been recognized to cause reflection and refraction of the irradiated beam. Therefore, the enlargement of the mullite grains up to the appropriate grain size was considered to improve the total transmittance of the visible ray. For this purpose, mullite ceramics with different grain sizes were prepared and the optical property of those materials has been examined.Mullite powders (Hokko Chemical Industry Co. Ltd, Japan) were synthesized by hydrolysis of aluminium isopropoxide and methylsilicate mixed solution. The chemical compositions of the synthesized powders ranged from 71.8wt % A1203 to 76.5wt % A1203 (analysed by X-ray fluorescence). Typical impurities of these powders in wt % were Na20 , 0.003; K20 , 0.004; CaO, 0.001 (analysed by ICP). These powders were calcined at 1250 ° C for 1 h, ground for 50h in methanol, dried and passed through a 150-mesh screen. Specific surface areas of the calcined and subsequently ground powders measured by BET were about 60m 2 g-~. The powder compacts were formed by cold isostatic pressing under 200 MPa, and were sintered in the temperature range 1600 to 1800 ° C for 0 to 8h in vacuum (10 -3 to 10 -4 Pa). In the firing process, the heating and cooling rates were 10 and 20 ° C min-~, respectively. The phases present and the microstructure of the sintered bodies were characterized by XRD and SEM. Bulk density of the specimens was measured by the Archimedes' method using distilled water. Grain-size measurements were made on photomicrographs of polished and thermally etched surfaces of the sintered bodies by the intercept method [6]. In-lin...
Synthesis of lanthanoid cobaltates, LnCo0 3 (Ln: La, Pr, Dy), via the mechanochemically prepared complex ( oxy-)hydroxides has been studied. The hydroxides are prepared by grinding Co(OH)z or Co304 with Ln20 3 (Ln: La, Dy) or Pr 6 0 11 in acetone containing a very small amount of H 2 0 in LnCo0 3 • 4H 2 0 composition. The starting hydroxides act as the reactive grinding aids to each other. La 2 0 3 and Co(OH) 2 form the complex hydroxide, LaCo(OH) 5 , that is a sort of inorganic polymers having La-0-Co bond. The hydration affinity of trivalent Co3+ ions is not high enough to form complex hydroxide LaCo(OHk Then, the addition of H 2 0 2 to the grinding liquid results in the formation of oxyhydroxide, LaCoO(OH)4. These ( oxy-)hydroxides are directly converted to pseudotetragonal LaCo0 3 at 600°C and rhombohedral LaCo0 3 above 800°C without forming any by-product. The redox interaction between Co(OH) 2 and Pr 6 0 1 I. which shows the poor reactivity with Fe 2 0 3 , enables the formation of PrCoO(OH)4. Contrary, the reaction with Dy 2 0 3 becomes poor.
Highly-crystallized fine LiMn2O4 powders have been synthesized by a combination of the mechanochemical (MC) and solvothermal (ST) treatments of LiOH and MnO2 in Acetone. The MC product is amorphous, but, shows the formation of Mn-O-Li bond. The ST post treatment promotes the nucleation of LiMn2O4 by the chemical reduction of Mn4+ to Mn3+. The estimated chemical compositions are LiMn (III)0.11Mn (IV)1.89O4.45 (MC) and LiMn (III)0.76Mn (IV)1.12O3.88 (MC/ST). The MC/ST powder crystallized at 800°C indicates the cell capacity of 119mA.h.g-1. The dilution of Acetone with Tetrahydrofuran is effective to reduce the particle size. The MC/ST product using the diluent shows the fine particle size of 150nm and the excellent cell capacity of 134mA.h.g-1.
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