| The NOx storage and reduction （NSR） technology will be the most promising way to reduce the NOx emission from lean-burn engines. In this work, we have designed a series of NSR catalysts based on Ba-Mn, including BaMnO3 perovskite oxides and Ba/Mn/TiO2-X （X=Al2O3/SiO2/ZrO2） supported catalysts. Their structures and properties were characterized by the techniques of XRD, TG-MS, BET, H2-TPR and NOx-TPD.Their NOx storage capacity, thermal stability and sulfur resistance were also investigated. The mechanism of NOx adsorbing species were discussed by in-situ DRIFT.BaMnO3 perovskite oxides were prepared by citric acid complexation method. At 300oC, the rate of nitrite to nitrate transformation is higher than that of nitrate and nitrite decomposition. As a result, the BaMnO3 perovskite oxide exhibits the highest NOx uptake at 300oC. No difference in the structure and NSR activity is found in the case of partial substitution of Mn by Cu/Mg. However, Fe substitution in B site leads to generation of oxygen vacancies. The highest NOx storage capacity is achieved on BaMn0.6Fe0.4O3. Adsorption and storage of NOx on BaMnO3 proceeds in three steps: （1） the formation of nitrites at low temperature （≤200oC）; （2） its transformation to nitrates at temperature higher than 200oC, but lower than 300oC; （3） the formation of nitrates at high temperature （≥300oC）. On the Fe-substitution catalysts, the NOx adsorption and storage processes are similar to the BaMnO3 catalyst.The TABM6ã€TSBM6 and TZBM6 catalysts were prepared by spreading Ba-Mn precursors as a thin layer on the TiO2-Al2O3, TiO2-SiO2 and TiO2- ZrO2 supports calcined at 600 oC. The NOx storage capacity depends strongly on the relative abundance of BaCO3 phases. Compared with the TSBM6 catalyst, more active BaCO3 phases are detected in the TABM6 catalyst due to less acidity, which results in a higher NOx storage capacity. Although the BaCO3 concentration was relatively lower, the TZBM6 catalyst still shows a high NSC of 105.7μmol/g due to the presence of a large amount of BaMnO3 perovskite phase. During adsorption of NOx under lean burn condition, surface nitrates are formed on the TABM6 and TSBM6 catalysts, while surface nitrites are the dominant species on the TZBM6 catalyst. The transformation of nitrites to nitrates is observed on the TABM6 catalyst by in-situ DRIFT.Thermal aging at 800oC leads to the conversion of dispersed BaCO3 in the TABM6 and TSBM6 catalysts to crystalline BaAl2O4 and Ba2TiSi2O8 respectively. The TZBM6 catalyst is quite stable against thermal treatment. NOx storage capacities of TABM6ã€TSBM6 and TZBM6 catalysts decline by the percentages of 52.6%,19.7% and 47.6% respectively, after treated in 200ppm SO2 atmosphere for 30 min. The order of NSC of sulfated catalysts is TABM6>TSBM6>TZBM6. The TSBM6 catalyst displays the best sulfur resistance ability due to the highest acidity of TiO2-SiO2 support.The Pt-Mn/Ba/TiO2-ZrO2 catalysts were prepared by a successive incipient wetness impregnation using TiO2-ZrO2 as support. Higher Mn loading resulted in aggregation of Mn2O3 crystallites on TiO2-ZrO2 support. After calcination at 800oC, a part of Mn species diffuses into the lattice of the support and forms a new phase Mn0.33Ti0.33Zr0.33O1.67. The dispersion and phase of Pt species is the key factor for the NOx storage capacity of the catalyst. As the calcinations increases, the dispersion of Pt species is higher, while some PtO2 species decompose to Pt metal phase. The results of EXAFS and NSC measurements showed that the maximum NO conversion and NOx storage capacity were achieved on the sample Pt-C1-800 due to the highest dispersion of Pt species. |
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