Low Temperature Molten Salt Synthesis And Morphology Control Of Perovskite-type Composite Oxides Of La-Mn-O System

According to previous studies,it can be suggested that the reason why there were few reports on high orderly mesoporous perovskite-type LaMnO₃ oxide was that the formation of perovskite phase LaMnO₃ required a higher calcination temperature,which may destroyed the existed orderly mesoporous structure.Therefore,in order to prepare ordered mesoporous perovskite LaMnO₃,firstly,a molten salt method was developed to prepare a series of perovskite-type composite oxides of La-Mn-O system,using three kinds of manganese oxides with different morphologies as precursors in eutectic NaNO3-KNO3.Secondly,in order to explore whether samples with morphologies similar to precursors can be obtained and the optimum synthesis condition.The as-prepared samples were characterized by XRD,FESEM,SEM,TEM,N₂ adsorption-desorption,FT-IR and XPS to determine the relationship between synthesis conditions and the phase composition,morphologies,structural properities,the surface Mn⁴⁺/Mn³⁺molar ratio and adsorbed oxygen species.Catalytic performances for CO oxidization on the as-prepared samples were also examined to determine their catalytic activity and stability.The results showed that:?1?By adopting the molten salt method,the spherical porous LaMn0.8Fe0.2O3 with a good crystallinity,higher specific surface area?55.73m²/g?and better catalytic performance for CO oxidization can be obtained at 450? for 4 h in eutectic NaNO₃-KNO₃,using hierarchical porous?-MnO₂ microshperes assembled from interleaving thin nanosheets as the precursor.The growth processes of porous LaMn_0.8Fe_0.2O₃ microspheres included threel stages.In the initial stage,La?NO₃?₃ and Fe?NO₃?₃ dissolved into the salt and then diffused onto the surface of nanosheets.Then the LaMn_0.8Fe_0.2O₃ phase was formed in situ and the primal morphology was semblable to that of?-MnO₂.At last,nanosheets gradually transformed into nanoparticles.La_0.55Sr_0.45Mn_0.8Fe_0.2O₃ and La_0.4Sr_0.6Mn_0.8Fe_0.2O₃obtained by the same method exhibited a relatively high catalytic activity as well as a better a catalytic stability at 500? under the simulative environment.T₅₀temperatures were both far below 125? and compared with La_0.55Sr_0.45Mn_0.8Fe_0.2O₃,the T₉₀0 temperature of La_0.4Sr_0.6Mn_0.8Fe_0.2O₃ was still less than 125?,while the T₉₀temperature of La_0.55Sr_0.45Mn_0.8Fe_0.2O₃ was 181?;?2?By adopting the molten salt method,sample with morphology similar to precursor S90 but less perovskite phase LaMnO₃ can be prepared at 450? for 4 h in eutectic NaNO₃-KNO₃.The spherical porous LaMnO3 with a little amount of La?OH?3 formed by nanorods and nanocrystals can be obtained at 550? for 4 h through this method,using the spherical porous?-MnO₂ and KMn₈O₁₆ consisted of the lamellae and nanorods as the precursor and it presented a higher catalytic activity(T₅₀=239°C)but a worse catalytic stability;?3?By adopting the molten salt method,with morphology resemble to sea urchins KMn₈O₁₆,the spherical porous LaMnO₃ with a certain amount of La?OH?₂NO₃ can be prepared at 500? for 4 h in eutectic NaNO₃-KNO₃,using the sea urchins KMn₈O₁₆consisted of nanotubes as the precursor.The spherical porous LaMnO₃ with rare La?OH?₃ constituted by nanorods and nanocrystals can be obtained at 550°C and600°C for 4 h,respectively.They exhibited a higher catalytic activity and worse catalytic activity and the T50 temperatures were 268°C and 262°C,respectively.When the calcining temperature was 500°C,it can also reduce the content of La?OH?2NO3 to improve the catalytic activity through decreasing the molar ratio of La?NO₃?·6H₂O and the precursor.So,it is feasible to prepare the samples with morphologies resemble to the precursors through the molten salt method at under appropriate conditions while the structure of precursors were not destroyed.Based on above,at last,combining the advantages of the molten salt method and the sol-gel method,ordered mesoporous perovskite-type LaMnO₃ oxide can be prepared at 500°C for 4 h,using SBA-15 as a sample and compared to previous studies,it has a relatively high catalytic activity(T₅₀=212? and T₉₀=325?).