Preparation And Characterization Of Mesoporous La_0.8Sr_0.2CoO₃ Catalysts By COK-12 Nanocasting Method

Perovskite-type catalysts have great application potential in automobile exhaust purification,and increasing the specific surface area and A/B substitution doping in ABO₃ perovskite structure are the main means to improve the catalytic activity of perovskite catalysts.The perovskite-type catalyst prepared by nano-casting method combined with sol-gel method has high purity and high specific surface area.COK-12 is an ordered mesoporous material with large specific surface area.At the same time,its silicon source is cheap and easy to be prepared in neutral solution.There is no study on the preparation of perovskite-type catalyst after modification as template agent.This paper intends to prepare mesoporous La-Co-O catalysts with high catalytic activity and high stability.The effect of partial substitution of A-site elements with modified COK-12 as the template on the catalytic activity of the catalyst was explored to solve the problem of low activity at low temperature and instability at high temperature.（1）Ordered silicon-based mesoporous material COK-12 with a specific surface area of 738 m²/g and hexagonal pore structure was prepared by using P123 surfactant as soft template and sodium silicate as silicon source in neutral buffer solution.The modification of COK-12 with n-hexane showed that the structure of H-COK-12 was similar to“ink bottle”.The specific surface area of H-COK-12-1 was the largest（660.5 m²/g）,while H-COK-12-4 not only had a large specific surface area（619.2m²/g）,but also had regular and orderly channels（the most probable pore diameter was 7.91 nm）,which could be used as a template for the preparation of mesoporous perovskite catalysts.（2）Mesoporous La Co O₃ catalysts were prepared by nanocasting combined with sol-gel method,and the optimal preparation conditions for preparing mesoporous perovskite catalysts with COK-12 as the template were explored.The catalyst prepared with n _COK-12:n _catalyst=2:1 has larger specific surface area(75.8m²·g^-1)and better catalytic activity(T₅₀=163°C,T₉₀=173°C).（3）Mesoporous La_0.8Sr_0.2Co O₃（LSC）catalysts were prepared by nanocasting combined with sol-gel method using mesoporous material H-COK-12-4 with a“ink bottle”structure as template and n _H-COK-12-4:n _catalyst=2:1.The change of catalytic performance of LSC catalysts prepared at different temperatures was investigated.LSC catalyst has high purity and no impurity phase,La,Co,Sr and O elements are uniformly dispersed in it.With the increase of temperature,the specific surface area of mesoporous catalysts decreased,and LSC-550 had the largest specific surface area(140.51 m²·g^-1).LSC-650 had the highest O_ads/O_lat（2.50）,followed by LSC-550(O_ads/O_lat=2.04),indicating that there were more oxygen vacancies on the surfaces of LSC-650 and LSC-550,which was conducive to the adsorption of CO for surface catalysis at low temperature.The partial substitution of Sr enhances the redox ability of the catalyst.LSC-650 has the highest Co⁴⁺/Co³⁺（0.680）,followed by LSC-550(Co⁴⁺/Co³⁺=0.529),so LSC-650 and LSC-550 have stronger catalytic activity.The catalytic conversion of CO over LSC catalyst at low temperature was tested.Among the initial catalysts,LSC-650 had the best catalytic activity(T₅₀=145°C,T₉₀=149°C).The catalytic activity of each catalyst was found to be enhanced to varying degrees by reusing the catalysts.The catalytic activity of LSC-550-U was prominent(T₅₀=105°C,T₉₀=107°C),which was directly related to its relatively large specific surface area(140.51 m²·g^-1).At 600°C for 10h,LSC-550,600,650 can always maintain the complete conversion of CO,indicating that LSC prepared at 550-650°C has excellent high temperature stability.