Synthesis And Properties Of La-Mn-O System Perovskite-type Catalyst

The preparation method,micromorphology and components regulation may have a significant effect on the activity of catalyst.At present,most of researchers improve the activity of catalysts by doping,loading and increasing specific surface area and so on,but,there are still few studies on La-Mn-O catalysts with special micromorphology,such as spheres and cubes.According to the latest dynamics in the field of catalytic materials and the accumulation of relevant research in our group,the purpose of this study is to prepare La-Mn-O catalysts with excellent activity at low temperature,good high temperature stability and specific micromorphology.Exploring the new technology of in-situ synthesis of catalysts using MnCO₃?containing Mn element that required for the target catalyst?as template,The morphologies and activity of catalysts prepared by solid phase method,sol-gel method,and molten salt method respectively were studied systematically by using MnCO₃ with different micromorphology as template,which combine with synergistic substitution of the B-site and A-site elements to solve the technical problems of low specific surface area,low-temperature activity and poor high-temperature stability of the perovskite catalyst.?1?The spherical MnCO₃ prepared by precipitation method was used as template and manganese source,La_0.8Sr_0.2MnO₃ catalyst was prepared by sol-gel method,solid phase method and molten salt method respectively at different calcination temperature,to explore the influence of preparation technology on the morphology and properties of the catalyst.The phase analysis showed that the sample synthesized by solid phase method and sol-gel method contained a little of monoclinic La?OH?2NO3 phase,while the sample synthesized by molten salt method only contained La_0.8Sr_0.2MnO₃.Furthermore La_0.8Sr_0.2MnO₃ perovskite phase was synthesized at lower temperature?550°C?by molten salt method.Moreover,the catalyst prepared by solid phase method just contained a few spherical structures,and it has the lowest specific surface area and catalytic activity.No globular structure is observed in the catalyst prepared by the sol-gel method,It's a massive structure with many pores,and its specific surface area was next to the catalysts prepared by molten salt method,and it showed the best fresh catalytic activity(T₅₀=138.13°C,T₉₀=149.42°C)but the lowest stability.The catalyst corresponding to the molten salt method exhibited more globular structure,and its fresh catalytic activity is only second to the catalytic activity of the catalyst prepared by the sol-gel method,but it has the supreme specific surface area?49.65 m²/g?,cyclic catalytic activity and catalytic stability.?2?Spherical and cubic La0.8Sr0.2MnO3 catalysts were prepared by the molten salt method at 550°C for 2 h,using the spherical and cubic MnCO₃ as template and manganese source,and explore the influence of the microstructure of the template on catalyst morphology and properties.Both catalysts are pure phase and have good crystallinity.The spherical size of spherical catalyst is approximately equal to the spherical size of the spherical template,but the size of the cubic catalyst is nanoscale,and the size of the cubic template is micron size,which is due to the low structural stability of the cube.The cubic catalyst exhibited better fresh catalytic activity(T₅₀=137.43°C,T₉₀=148.5°C)than the spherical catalyst(T₅₀=145.14°C,T₉₀=148.5°C),but the cubic catalyst has lower catalytic stability.Based on various factors,it is considered that the spherical morphology is a more ideal catalyst structure.In addition,the effect of Cu²⁺doping at the B site of ABO₃ on the activity and stability of the spherical catalyst was also investigated when the Sr²⁺doping at the A site is 0.2.The results show that CuO and La?OH?2NO3 additional phases were observed when the amount of Cu²⁺was over 0.2,and when the Cu²⁺doping amount is0.2,the catalyst exhibits the best CO catalytic activity(T₅₀=83.63°C,T₉₀=97.40°C),but doping Cu²⁺at the B site decrease the stability of the catalyst.Overall,the appropriate amount of Cu²⁺doping can increase CO catalytic activity,but excessive doping will weaken the catalytic activity of the catalyst.