Preparation of La0.5Sr0.5CoO3 perovskite catalyst and its performance for NOx storages
Objective To enhance the NOx storage and reduction capacity of NOx catalysts under medium and low temperature conditions and achieve efficient NOx storage and reduction performance.The development of an efficient and cost-effective catalyst for medium and low temperature NOx storage and reduction is crucial.Methods In this study,the perovskite La0.5Sr0 .5CoO3(LSC)catalyst was synthesized utilizing the glycine-assisted solution com-bustion method.The physicochemical properties of the catalyst were comprehensively characterized through various analytical techniques.The impact of preparation parameters,including the molar ratio of glycine to nitrate and calcination temperature,on the NOx storage performance of the catalyst was systematically investigated.Furthermore,the sulfur resistance,hydrothermal stability,and NOx storage mechanism of the LSC catalyst during NOx storage were thoroughly examined.Results and Discussion Based on the aforementioned characterization and experimental findings,the NOx desorption curve depicted in Fig.9 illustrated that altering the amount of glycine led to a shift in the temperature of the catalyst desorption peak towards higher values,consequently enhancing the stability of nitrate species.Specifically,at a glycine-to-nitrate ratio(φ)of 1.6,the catalyst exhibited the lowest desorption peak temperature,indicative of less stable nitrate species prone to releasing NOx.The order of NOx adsorption capacity(A)and NOx storage capacity(S)of the catalyst was as follows:LSC-1.6＞LSC-2.4＞LSC-0.8.Upon reaching equilibrium adsorption of NOx,the concentrations of NO and NO2 in the atmosphere remained stable.The relative NO2 reduction(RNO2)of the catalyst followed the sequence:φ=1.6(65％)＞φ=2.4(51％)＞φ=0.8(49％).Notably,the LSC catalyst synthesized with φ=1.6 exhibited the highest S,A,and RNO2,attributed to its large specific surface area,robust NO oxidation capacity,and the presence of appropriate SrCO3 species.Furthermore,the NOx desorption curve in Fig.10 revealed a shift of the catalyst desorption peak towards lower temperatures with increasing calcination temperature,indi-cating decreased stability of nitrate species at higher calcination temperatures.Specifically,the catalyst prepared at a calcina-tion temperature of 700 ℃ exhibited reduced SrCO3 content but possessed a larger specific surface area,pore volume,strong NO oxidation capacity,and effective reduction performance,thereby demonstrating good activity.The RN02 values were observed in the following order:700 ℃(63％),800 ℃(44％),and 600 ℃(41％).The NOx storage phase in the LSC catalyst comprised perovskite and SrCO3,with an appropriate amount of SrCO3 species favoring NOx adsorption and storage.However,an excessive presence of SrCoOx could inhibit the active Sr-Co sites,thereby diminishing the NOx storage capacity of the catalyst.Hydrother-mal aging resulted in an increased SrCoOx phase and a decreased SrCO3 phase on the catalyst surface,consequently reducing its NOx storage performance.Nonetheless,it is worth noting that nitrate species formed on the surface of SrCO3 exhibited high ther-mal stability,thereby maintaining excellent NOx storage performance even after hydrothermal aging.Conclusion 1)LSC catalysts prepared under various φ values and calcination temperatures predominantly exhibited perovskite crystalline phases,with minor traces of SrCO3 and SrCoOx crystalline phases.Notably,when φ was set to 1.6 and calcination temperature to 700 ℃,the catalyst demonstrated the highest capacity for NOx adsorption and storage.The catalysts displayed a loose and porous structure with a spongy morphology.2)The NOx storage phase in the LSC catalyst primarily comprised perovskite and SrCO3.The NOx storage capacity was significantly influenced by the presence of SrCO3 species within the perovskite structure,with an optimal quantity of SrCO3 species favorable for NOx adsorption and storage.However,excessive SrCoOx content could obstruct active Sr-Co sites,reducing contact with the reaction gas,and diminishing NOx storage capacity.3)After vulcanization,all LSC catalysts exhibited a pure perovskite structure without sulfur-containing species.Following hydrothermal aging,the catalyst primarily comprised the perovskite crystal phase,with a small amount of SrCO3 and SrCoOxhet-erophase.While hydrothermal aging promoted the growth of the perovskite structure and SrCoOx phase,it inhibited SrCO3 phase growth.Despite declines in NOx adsorption capacity(A)and relative NO2 reduction(RNO2)post-treatment,the LSC catalyst maintained strong resistance,and retained high NOx storage capacity,with A values of 1 434 and 1 262 μmol·g -1 after vulcaniza-tion and hydrothermal aging,respectively.
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