Study On The NO_x Storage And Sulfur Resistance Of The Rare-Earth Containing La_0.7Sr_0.3Co_1-xFe_xO₃ Perovskite-type NO_x Traps

A series of perovskite-type catalyst La0.7Sr0.3Co1-xFexO₃ with different pH value (pH=4.6 and 9) was prepared by sol-gel method, as well as with the various calcination condition. Three phenomenon were aroused by the involvement of the element Fe: i) increasing the reunion degree of perovskite; ii) declining of the specific surface area; iii) the reduction of amount of SrCO₃. Especially, the sample of La_0.7Sr_0.3Co_0.8Fe_0.2O₃, after calcination in flowing air using tubular furnace and pH value of 9, showed much good NOx storage capacity and oxidizability to NO₂. It is because of its uniform particle size, i.e. diameter around 40～80 nm, the big specific surface area (21.9 m2/g), and the perfect perovskite structure.For the reason of security, a new calcined process, which is confirmed by the TG-DTA experiment, was employed to synthesize the La_0.7Sr_0.3Co_0.8Fe_0.2O₃ catalyst. The results of XRD and SEM show that, the sample preparing with the new method has bigger BET surface and better crystal structure. The diameter of the sample is round 80nm, and only the trace amount of the SrCO₃ phase exists on the catalyst. As shown in the results of the NOx storage tests, these samples presented both good NOx storage capacity and high NO convertion to NO₂. And, The NSC of the sample with pH=9 is 378μmol/g, and its NO conversion reached 69.8 %. After the pretreatment in SO₂/O₂, the crystal structure of the samples had not change, and their NSC decreased little, presenting good sulfur-resistance. Furthermore, the sample with ph=9 maintained the convertion of NO in a high level. Combined with the results of the FR-IR, H2-TPR, O₂-TPD, and XPS characterization, it can be concluded that the amount of carbonate formed by the new preparation method is less than before, and NOx can be stored as the bulk nitrates on the surface of the perovskite. After sulfation, only a small amount of sulfate were formed, the active site of the perovskite on the catalyst was remained without distroyment, which makes the sample keeping high NOx storage capacity and NO convertion.The final work in this study is to substitute various content of Co with Fe. It was found that the value of NSC decreased following the increase of the proportion of Fe. When the proportion of Fe was not less than 0.2, the catalyst presented a high sulfur resistance. The perovskite structure on the surface of the samples could be damaged by the deep sulfation degree such as exposing in SO₂/O₂ atmosphere more than 2 h. Considering that the NOx storage process happened at the interface of "gas/perovskite"phases, if the surface of the perovskite crystallites was fully sulfated and covered by sulfates, the active sites for NOx storage would drop sharply inducing in the serious sulfur poison.