Conductivity Mechanism Of Pervoskite-Stuctured Oxygen Storage Materials

The world wide introduction of new automobile emission standards, such as Euro Vand Euro VI becomes highly stringent and demanding. The commercially availablethree way catalysts thus encounter increasing chanllenge because the catalysts arerequired to light off at even lower temperature with higher oxygen storage ability andlonger time durability at high temperature. Consequently, great interest is focused onexploring three way catalysts with fast light off at low temperature （such as closecoupled catalysts）, large oxygen storage and release capability and long-term durability.So far, three kinds of crystalline-structured catalysts become the centered interest,represented by the fluorite-type amd perverskite-type catalysts and molecular sifts. Ithas been well recognized that the catalytic properties are intimately related to thecrystal’s structural defect types and their concentrations. As such, the objective of thisstudy is to investigate the conductivity of the perverskite-structured catalysts, and thusestablish the internal mechanism between catalytic behavior and the structural defects.Perovskite-structured oxygen storage materials, La_1-xSr_xMnO₃（x=0,0.2）, La1-xSrxCoO₃（x=0,0.2）, and La_1-xCo_xMnO₃（x=0,0.2） were prepared by a typical ureadecomposition method. The as-prepared samples were then sintered at temperature of1050℃for3hours. After furthermore standardized treatment, the oxygen storageproperties were examined using TG and DSC. The chemical composition, morphologyand crystal structure were investigated using SEM, EDX, XRD and TEM. Pelletsamples were prepared by pressing the powdered samples at high temperature andpressure. The conductivity of the pellets was measured using AC impedance techniquein a single gas, each with oxygen and hydrogen.The study reveals that the as-prepared powdered samples are nano-scaled particleswith averaged diameters less than100nm. All the samples show characteristicperovskite crystal structur with good dispersion. Compared to the lattice constant ofLaCoO₃and LaMnO₃, the cell constants are increasing after the two chemicals aredoped with Sr2+in their lattices. This is because a group of large sized cations Sr2+replace small-sized La³⁺in the lattice, resulting in expansion of the lattice. Even thougha portion of the cations of Mn³⁺in LaMnO₃maybe transformed into Mn4+after theformer are doped with Sr2+which decreases the lattice constant, however, the compensated effect eventually expand the cell. Thermal analyses to the oxygen storagematerials LaCoO₃, LaMnO₃, La_0.8Co_0.2MnO₃, La_0.8Sr_0.2CoO₃and La_0.8Sr_0.2MnO₃,show that the oxygen storage capability ranks from high to low with the order asLa_0.8Sr_0.2CoO₃> La_0.8Sr_0.2MnO₃> LaCoO₃> La_0.8Co_0.2MnO₃> LaMnO₃. The electricalconductive performance indicates the impedance of oxygen storage materialsLa_0.8Sr_0.2MnO₃decreases with increment in temperature as well as with thetransformation with the gas changing from oxygen to hydrogen. The impedance ofLa_0.8Sr_0.2MnO₃reduces with an increase in partial pressure of hydrogen. In oxygen, theconductivity behavior of LaMnO₃shows two platform in the impedance Bode plot, andin hydrogen at temperature above400℃, the conductivity does not change withfrequency. The conductivity of LaCo0.8Mn0.2O₃measured in hydrogen shows to beslightly greater than that in oxygen. The La_0.8Sr_0.2CoO₃and LaCoO₃samples exhibitsimilar conductivity values in hydrogen and oxygen.