Study On The Synthesis And Catalytic Properties Of Perovskite-type Mixed Oxides With Large Surface Area For Methane Combustion

Perovskite mixed oxides have been extensively studied due to its particular physical and chemical properties, but the perovskite mixed oxides prepared via conventional synthesis routes have relatively low specific surface areas, pore volume and show low catalytic activity in the reactions, and its applications were limited. So it is urgent affairs to develop a new method to prepare the kind of perovskites with large specific surface areas(SSA) and high catalytic activity. In this dissertation, the perovskite La_0.95Ce_0.05CoO₃ mixed oxides were synthesized by solide state reaction method, citric-acid method, glycine method, citric-acid combustion method and glycine combustion method in order to study the effects of preparation methods on specific surface area and catalysts performances. It was discussed the effects of the preparation condition, such as glycine-to-metal ratio, pH value of the precurors solution, calcination temperature and calcination time, on the specific surface area and catalytic activity for methane combustion. La_0.95Sr_0.05Ni_0.05Co_0.95O₃ was prepared by citric acid combustion method, and the effects of composite doping on structure, surface patterns, pore size, surface area and pore volume of the catalysts were investigated. Catalytic materials M/La_0.95Ce_0.05CoO₃ were prepared by metal M(M=Pt, Pd, Ni) supported on the perovskite mixed oxides with large SSA , the effects of loaded M on the structure and properties of the catalysts were discussed. La_0.95Ce_0.05Co_0.95M_0.05O₃ were prepared and the effects of doped M on the structure and performances of the catalysts were also discussed, its internal causes were discussed. The effects of rare earths and stoichiometric ratio of organic fuel to oxidizer ((?)) on structure, surface properties and catalytic activities of Ln_0.8Sr_0.2CoO₃ catalysts were studied.All catalysts were used successfully for CH4 combustion. The properties of these catalysts, such as crystal structures, particle sizes, surface patterns, pore sizes, surface areas and pore volumes, were characterized by X-ray diffraction (XRD), electron microscopy (TEM/SEM), thermal analysis (TG-DTA), nitrogen adsorption experiments (BET), (FT-IR Spectrometer) IR, temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR), respectively.The experiment results show that all catalysts synthesized by different methods were perovskite-type mixed oxides. Doping Ce³⁺ ions on A sites entered the crystal lattices of LaCoO₃ in the place of La³⁺. At the order of La_0.95Ce_0.05CoO₃ prepared by solidc state reaction method, citric-acid method, glycine method, citric-acid combustion method and glycine combustion method, oxygen vacancies, mobile lattice oxygen, the porous structures and SSA of the La_0.95Ce_0.05CoO₃catalysts increase gradually, which enhance the catalytic activity of the catalysts. La_0.95Ce_0.05CoO₃ prepared by glycine combustion method has higher catalytic activity for methane combustion, whose BET specific surface area and crystallite size were 32.4 m²/g and10～20 nm, respectively. It can be explained in terms of its more mobile chemical-adsorped oxygen and lattice oxygen.Preparation conditions of La_0.95Ce_0.05CoO₃ catalyst were investigated, the results indicate that the structure and properties of catalyst can remarkably affect by glycine/metal molar ratio, calcined temperature, calcined time and pH value of the precurors solution. The increase of glycine/metal molar ratio is propitious to form perovskite structure, increases the BET specific surface area and improves the the catalytic activity of La_0.95Ce_0.05CoO₃. The BET specific surface area, pore volue and average pore diameter of La_0.95Ce_0.05CoO₃ perovskite-type catalyst decrease with the increase of calcined temperature and calcined time, which decrease adsorption oxygen, mobile lattic oxygen and the catalytic activity for methane combustion. When pH value of precurors solution is about 7, the performance of the catalysts is improved.The results indicate that M/La_0.95Ce_0.05CoO₃,La_0.95Ce_0.05Co_0.95M_0.05O₃ (M=Pt, Pd) synthesized by the modified glycine combustion method are porous catalysts. BET surface areas are large, which are favorable for their catalytic activities. The activity of M/La_0.95Ce_0.05CoO₃ is better than that of La_0.95Ce_0.05Co_0.95M_0.05O₃(M=Pt, Pd), because Pt, Pd disperse in La_0.95Ce_0.05CoO₃ surface, which increases the catalytic activities of the catalysts. The activitie of Ni/La_0.95Ce_0.05CoO₃ and La_0.95Ce_0.05Co_0.6Ni_0.4O₃ do not change much compared with La_0.95Ce_0.05CoO₃. The effects of composite doped Sr ions and Ni ions on the properties of La_1-xSr_xNi_xCo_1-xO₃ were investigated. The BET specific surface area can be increase and the catalytic performance of La_1-xSr_xNi_xCo_1-xO₃ can be improved when part La ions of A-sites and Co ions of B-sites were replaced by Sr ions and Ni ions, respectivly. When x=0.3 perovskite and other phases coexite, which lead to lower catalytic activity of the catalysts.The effects of different rare earths (La, Nd, Ce) on structure and catalytic activities of the catalysts are different , the sequences of catalysts activities are La_0.8Sr_0.2CoO₃>Ce_0.8Sr_0.2CoO₃>Nd_0.8Sr_0.2CoO₃. Rare earths on site A lead to lattice distortion of the CoO₆ octahedral, The activities of the catalysts are relative to the distortion of BO₆ octahedra. With the enlarging of distortion of BO₆, catalytic activity of the catalyst increases. All La_0.8Sr_0.2CoO₃ mixed oxides with the different coefficient of (?) have perovskite structure, whose structure and catalytic activities alter regularly with the change coefficient of (?). The catalytic activities of La_0.8Sr_0.2CoO₃ ((?)=0.76-1.52) for CH₄ combustion improve gradually with the increase of (?) When (?) is equal to 1.52, the catalytic activity of La_0.8Sr_0.2CoO₃ is the best, which can be explained in terms of the smaller of average crystal size, the higher of specific surface area, the bigger of lattice distortion, the lower of activation energy and more mobile chemical-adsorped oxygen and lattice oxygen.The reaction mechanism of the catalytic combustion of methane over the prepared catalysts was studied, the results indicate that the reaction of the catalytic combustion of methane accords approximately with features of first order kinetics equation. A probable Eley-Rideal mechanism is considered to interprete the observed kinetic results. The activation energy for the catalytic combustion of methane varies with the catalysts prepared by varied methods.