The Synthesis Of Nanocrystals Mesoporous Alumina With High Thermal Stability And Its Application In CO And C₃H₆ Oxidation Reaction

The number of automobiles markedly increased over recent decades which brought a series of environmental problems. At present, the most effective way to control automobile exhaust pollution is installing the catalytic converters, such as three-way catalyst, in the engine exhaust system to convert carbon monoxide ?CO?, hydrocarbons ?HC? and nitrogen oxides ?NOx? to innocuous gas carbon dioxide ?CO₂?, nitrogen ?N2? and water vapor ?H₂O?. Three-way catalyst usually use precious metal such as Pt, Rh, Pd as active component,, cordierite with 1350 ? melting point as the first carrier, y-Al₂O₃ as second carrier for the active component coating. When the working temperature of catalysts higher than 1000 ?, phase transformation of ?-Al₂O₃ easily occur, causing the loss of specific surface area, and the loading active metal will concentrated, resulting in the decrease of catalytic activity as well as catalyst life. A number of studies have been conducted to improve the thermal stability of ?-Al₂O₃ in order to meet the increasingly stringent requirements of automobile exhaust emissions.This paper analyzes the main factors influencing on the catalyst performance ?composition, structure and morphology?, improve the synthesis methods of ?-Al₂O₃ and focus on the synthesis of nanocrystals mesoporous alumina with high thermal stability and its application in CO and C₃H₆ oxidation reaction. The main work is list as follows:?1? Employing a modified sol-gel process with hydrochloric acid as pH regulators in the presence of aluminum isopropoxide and Pluronic P123, the influence of species of interfacial protector and synthesis temperature ?Ts:the formation and aging temperature of sol? were studied in the morphology, texture properties and thermal stability of nanocrystals mesoporous alumina.?2? Investigating the influences of bi-interfacial protector under different synthesis temperature on the morphology, texture properties and thermal stability of nanocrystals mesoporous alumina.?3? On the basis of above, cerium and zirconium were introduced into alumina and the influence of Ce and Zr on the morphology and thermal stability of alumina.?4? Taking obtained alumina as carriers, the Pd supported catalysts was synthesized by incipient-wetness impregnation method. The catalytic activity and properties of catalysts were explored under the CO and C₃H₆ reactant gas.Through the researchs of above, the following results was achieved:?1? Employing a modified sol-gel process with the assistance of volatile acetic acid as interfacial protector and controlling the synthesis temperature to control the the formation of liquid crystal phase and solvent volatilization rate. When n ?AA?/n ?Al? =0.10, synthesis temperature ?the formation and aging temperature of the sol-gel? Ts=30 ?, the nanocrystalline spherical mesoporous alumina was obtained with the particle size between 400 to 600 nm; when Ts=20 ?, ordered mesoporous alumina with high specific surface area was obtained.?2? It is found that adding different amount of citric acid ?CA? in the n ?AA?/n ?Al?= 0.10 system was adverse to the formation of uniform spherical when Ts = 30 ?, but the obtained aluminas possess high specific surface area when ?CA?/n ?Al?<= 0.10. When Ts = 20 ?, n ?AA?/n ?Al?) = 0.10, n ?CA?/n ?Al? = 0.05 the product present ordered mesoporous alumina and the specific surface area reach to 137 m2/g after calcined at 1000 ?.?3? In the study of cerium or cerium-zirconium doped alumina in the single interfacial protector system of n ?AA?/n ?Al? = 0.10 and bi-interfacial protector system of n?AA?/n?Al?=0.10, n?CA?/n?Al?=0.05, we found that both can get spherical morphology mesoporous alumina with Ts=30 ? and oldered mesoporous alumina with Ts=20 ? after calcinated at 1000 ?. The formation of CeO₂ and CexZr_1-xO₂ significantly improve the thermal stability of alumina?4? Taking the obtained alumina as carriers, Pd supported catalysts were prepared by incipient wetness impregnation method. In the process of researching, it is found that both morphology and specific surface area of aluminas impact dispersion of active components significantly, and then influence the catalytic activity of CO and C₃H₆ oxidation reaction. The catalytic activity cerium doping catalyst increased significantly compared with no-doping. And catalyst doped cerium-zirconium, because the Zr⁴⁺ doped into the lattice of CeO₂ forming the CexZr_1-xO₂ which improve the resistance of the CeO₂ sintering properties and catalytic activity.