Modified LaMnO₃ Perovskite For Catalytic Lean Burn Of CH₄

For catalytic lean burn of methane (diluted at 0.5vol.% by air), a series of perovskite powder catalysts were prepared, which were then washcoated onto -Al₂O₃ ceramic honeycomb substrates to make the monolithic catalysts.First, the methods for preparation of perovskite powders, such as coprecipitation, sol-gel, and solution combustion synthesis (SCS), were screened. XRD, BET, TGA, SEM were adopted to characterize and compare the crystalline type, specific area, pore structure, crystal sizes, powder morphology of different samples. LaMnO3 perovskite powders with pure crystalline phase, smallest particle sizes and the best catalytic activity were made by SCS method.For an even and firm coating of the catalyst powders on ceramic substrates, the substrate pretreatment and the introduction of doped ZrO2 as the second support were investigated. It was found that boiled in oxalic acid solution, the specific area of the -Al₂O₃ substrate improved from 0.34 m2/g to 18.94 m2/g, the activity of the washcoated monolith also increased, yet the mechanic strength of the substrate was meanwhile destroyed. Experiments indicated that tetragonal phase ZrO2 as the second support was beneficial to disperse perovskite, where 9mol% Y2O3 doped zirconia calcined at 800℃was found to be the most effective, due to its stabilized tetragonal phase zirconia and abundant oxygen defect.Next, La_0.8Sr_0.2Mn_0.5Ni_0.5O₃ perovskite(LSMN) was precipitated onto amorphous Yttrium Stabilized Zirconia(YSZ) powders at the molar ratio of 1:n to make the LSMN/nYSZ composite catalysts, for better investigation of the interaction between YSZ and LSMN. The temperature programmed reaction-mass spectroscopy was appled first to screen the composite ratio (n) and calcination temperature. It was found that for n between 5 to 9, the activity rationalized by mass of LSMN of the composite catalysts had increased by folds, and the best n value was between 7 to 8. The LSMN/7YSZ calcinated at 1000℃has demonstrated the best activity, with the apparent activition energy reduced to 64.0 kJ/mol. Through SEM, BET and Temperature Programmed-XRD characterization, it was found that being composited with YSZ, the transforming temperature of amorphous LSMN to crystalline was increased from 800℃to around 1000℃, meanwhile the resistance to sintering also enhanced. After calcination at 1000℃, perovskite was fast merged with YSZ, with the advent of a La2Zr2O7 phase. The monoliths washcoated by the composite catalyst powders were made and tested for methane combustion. The LSMN/7YSZ calcinated at 1000℃again demonstrated the best activity which was even a little higher than the optimized Pd (0.33wt.%) / -Al₂O₃ monolith at the same reaction conditions.For lean burn of methane in air, present work has systematically optimized the preparation methods, found the proper second support, and one step further, provided an effective perovskite YSZ composite catalyst, which have paved the way for real application.