The NSR Catalytic Activities Of The Rare-earth Perovskite-type La_0.7Sr_0.3Co_1-xM_xO₃（M= Pd,Rh）catalysts

NO_x emission from lean-burn engines brings great hazards to atmosphere and human health. How to effectively eliminate NO_x has attracted great attention in the world. Perovskite-type catalysts have been previously reported for their low costs, adjustable redox properties and high thermal resistance. Recently, we reported that the La_0.7Sr_0.3CoO₃ catalyst showed the high activity for the NO_x oxidation and storage under lean-burn atmospheres even in the absence of a noble metal. However, its NO_x reduction efficiency in rich-burn atmospheres was still limited as compared to the Pt-based catalysts.Herein, we reported that the La_0.7Sr_0.3Co_0.97Pd_0.03O₃ catalyst showed the high NO_x storage-recution（NSR） activity in a wide operating temperature window（275 – 400 oC）. In order to further illuminate the effect of the noble metal Pd on the oxidative and reductive capacity in the perovskite catalysts, we synthesized the Pd-doped LSCP catalysts by a sol-gel process and the Pd supported P/LSC catalyst by an impregenation process to compare with the La_0.7Sr_0.3CoO₃（LSC） catalyst. The LSCP catalyst presented the highest NO oxidation ability under the lean-burn condition, while the Pd free LSC catalyst is lowest. After analysize of the structure of the LSC, LSCP and P/LSC by XRD, we found that the dope of Pd expanded the lattice of the perovskite lattice, while the supported Pd had no impact. H₂-TPR, O₂-TPD and XPS results indicate that the absorbed oxygen（Oads） is the active sites for the NO oxidation over the LSC perovskite catalysts. Kinetics results suggest that the NO oxidation reaction pathways follow the LH and ER models. The absorbed oxygen reacted with the NO firstly, and then a series of transformation on the oxygen and the lattice oxygen to replenish the adsorbed oxygen. The activation energy（Ea） of the LSCP is the lowest and the content of the adsorbed oxygen is the highest. Thus, its NO oxidation rate is the fastest. Furthermore, the LSCP catalyst displayed the much highest NO_x elimination efficiency in the alternative lean-burn/ fuel-rich atmospheres, as compared with the LSC; but the P/LSC showed little promoting effect. Our results show that the Pd of the P/LSC was supported on the surface of the LSC perovskite, and easily aggregated to the large PdO particles during the activity tests. As a result, its catalytic activity dramatically decreased. Contrarilly, the LSCP catalyst shows good stability in the NSR tests. It is because in lean-burn atmospheres, the Pd dopped into the perovskite lattice as cations, whereas in rich-burn atmospheres, the Pd was segregated out of the perovskite lattice as metallic Pd, which can dissociate the C3H6 to promote the reduction of the NO_x。Additionally, we investigated the influence of the calcinations temperature, preparation method and the Rh proportion doped into the LSC catalysts on the structures, properties and activities for removal of NO_x. The catalysts calcinated at 700 oC displayed the best NO_x storage capacity and NO oxidation ability, as the higher calcination temperature will lead to sintering of the caltalysts. H₂-TPR and O₂-TPD results show that the adsorbed oxygen species increased as the doping of Rh into the LSC crystal lattice. Moreover, the amount of the adsorbed oxygen species increased with the increase of the Rh incorporation, resulting in the improved NO oxidation ability.