Study On The Preparation And No_x Storage Reduction Mechanism Of The Mn-based Rare Earth Perovskite-type Catalysts

A series of the Mn-based catalysts was prepared by sol-gel method using citrateand EDTA as the complexing agent. The catalysts obtained in this way possessed bigBET surface area, as well as complete and simplex perovskite structure. This paperdisplays many of experiments for studying the impact of calefactive velocity,calcinations temperature and pH on the NO_x storage activity of the La1-xSrxMnO3catalysts. The catalytic tests were carried out with the simulated exhaust. When the Asite element La was partially substituted by Sr, the perovskite lattice expansionappeared. The NO_xstorage capacity （NSC） showed the tendency to increase when theratio of the Sr doping increased. An integrated and simplex structure of perovskitewas obtained when catalysts were calcinated at700C with a temperature rate of2C/min. The best pH of precursor solution is8. The NSC of samples increased afterseveral NO_xStorage Reduction （NSR） cycles, meanwhile the NO conversationslightly reduced. The FT-IR results show that two new bands centered at1360and1438cm1appeared on the sample after six NSR cycles. It suggests that more NO_xstorage sites were generated after experiencing a reducing atmosphere.To enhance the sulfur resistance property of the Mn-based perovskite-typecatalysts, a series of the La_0.7Sr_0.3Mn_1-yFeyO₃（x=0.1,0.2,0.3,0.4） catalysts had beenprepared with the same preparation method. The Fe doping catalysts not onlyimproves the resistance to sulfur, but also increased the NO_xstorage capacity. Of allthe catalysts, the La0.7Sr0.3Mn0.4Fe0.6O3sample presented the best performances onthe aspects of sulfur resistance and NSC. The mechanisms of the NO_xstorage and thesulfur poisoning on the perovskite catalysts were investigated by the characterizationsof H2-TPR, FT-IR, EXAFS, and XPS under different experimental conditions. thenumber of oxygen vacancies in catalyst structures increase when Mn⁴⁺substituted byFe2+, led to the improvement of the oxidative capabilities of the catalysts. Thepresence of Fe compounds inhibits the deposition or growth of sulfate on the catalystsurface, which enhanced the sulfur resistance property of the Mn-based perovskitecatalysts.The NSR mechanisms of the catalysts with or without Pd doping for treatingnitrogen oxides were investigated in the simulated exhaust. To the perovskite-typecatalyst without precious metal doping, oxygen vacancies play an important role inthe process of elimination of NO_x. The La_0.63Sr_0.3MnO₃, SrMnO3and other catalysts were prepared to better investigate the NSR mechanisms. Here, we found oxygenvacancies played a key role not only in the process of NO_xstorage, but also in theprocess of storage reduction cycle. The La_0.7Sr_0.3Mn_0.97Pd_0.03O₃catalyst showed anexcellent NO_xeliminate efficiency with the ratio that NO_xconverted to N₂reaching100%. Kind of reductants were investigated in the cycles, and the orders of thereducing ability follows the sequency of H₂> C₃H₆> CO. Pd in the structure of theperovskite crystal lattices can separate out of the lattice after a reducing treatment toeleminate NO_xefficiently.