Abstract

Objective To enhance the NO_x storage and reduction capacity of NO_x catalysts under medium and low temperature conditions and achieve efficient NO_x storage and reduction performance， the development of an efficient and cost-effective catalyst for medium and low temperature NO_x storage and reduction is crucial.

Methods In this study， the perovskite La_0.5Sr_0.5CoO₃（LSC） catalyst was synthesized utilizing the glycine-assisted solution combustion 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 NO_x storage performance of the catalyst was systematically investigated. Furthermore， the sulfur resistance， hydrothermal stability， and NO_x storage mechanism of the LSC catalyst during NO_x storage were thoroughly examined.

Results and Discussion Based on the aforementioned characterization and experimental findings， the NO_x 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 NO_x species. The order of NO_x adsorption capacity （A） and NO_x storage capacity （S） of the catalyst was as follows： LSC-1.6 > LSC-2.4 > LSC-0.8. Upon reaching equilibrium adsorption of NO_x，the concentrations of NO and NO₂ in the atmosphere remained stable. The relative NO₂ reduction （R_NO2） 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 R_NO2， attributed to its large specific surface area， robust NO oxidation capacity， and the presence of appropriate SrCO₃ species. Furthermore， the NO_x desorption curve in Fig. 10 revealed a shift of the catalyst desorption peak towards lower temperatures with increasing calcination temperature， indicating decreased stability of nitrate species at higher calcination temperatures. Specifically， the catalyst prepared at a calcination 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 R_NO2 values were observed in the following order：700 ℃（63%），800 ℃（44%）， and 600 ℃（41%）. The NO_x storage phase in the LSC catalyst comprised perovskite and SrCO₃， with an appropriate amount of SrCO₃ species favoring NO_x adsorption and storage. However，an excessive presence of SrCoO_x could inhibit the active Sr-Co sites， thereby diminishing the NO_x storage capacity of the catalyst.Hydrothermal aging resulted in an increased SrCoO_x phase and a decreased SrCO₃ phase on the catalyst surface， consequently reducing its NO_x storage performance. Nonetheless， it's noteworthy that nitrate species formed on the surface of SrCO₃ exhibited high thermal stability， thereby maintaining excellent NO_x 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 SrCoO_x crystalline phases. Notably， when φ was set to 1. 6 and calcination temperature to 700 ℃， the catalyst demonstrated the highest capacity for NO_x adsorption and storage. The catalysts displayed a loose and porous structure with a spongy morphology. 2） The NO_x · storage phase in the LSC catalyst primarily comprised perovskite and SrCO₃ . The NO_x storage capacity was significantly influenced by the presence of SrCO₃ species within the perovskite structure， with an optimal quantity of SrCO₃ species favorable for NO_x adsorption and storage. However， excessive SrCoO_x content could obstruct active Sr-Co sites， reducing contact with the reaction gas， and diminishing NO_x 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 SrCoO_x heterophase. While hydrothermal aging promoted the growth of the perovskite structure and SrCoO_x phase， it inhibited SrCO₃ phase growth. Despite declines in NO_x adsorption capacity （A） and relative NO₂ reduction （R_NO2） post-treatment， the LSC catalyst maintained strong resistance， and retained high NO_x storage capacity， with A values of 1 434 and 1 262 µmol·g^-1 after vulcanization and hydrothermal aging， respectively.

Keywords：solution combustion method； perovskite； NO_x storage； glycine