The NSR Catalytic Activities Of The Rare-earth Perovskite-type La_1-xSr_xCo_0.8Fe_0.2O₃Catalysts

The NO_xstorage-reduction （NSR） technique is regarded as an effective approachto remove NO_xfrom lean-burn automotive engine. The La_0.7Sr_0.3Co_0.8Fe_0.2O₃perovskites have been synthesized by a sol-gel method with citric acid and EDTA ascomplexing agent. The best reduction temperature of the stored nitrate was300oC,which has been confirmed by the TG and H₂-TPR analysis of the fresh and storedcatalysts. At this temperature, the stored catalysts were reduced with differentreductants （CO, C3H6and H₂） for different times. Amoung these reductants, COexhibited the fastest reduction rate and the strongest reductive capacity. After3cyclesof “60min NO_xstorage/30min reduction” tests, only the sample with CO asreductant always kept the increasing NO_xstorage capacity （NSC）. The Sr₃Fe₂O₇phasewas detcted by the XRD test in the CO-reduced sample and the structure of theperovskite was partially destroyed. However, the process of NO_xstorage-reductioncycles had no obvious influence on the morphology of the catalysts.The La_0.7Sr_0.3Co_0.8Fe_0.2O₃catalyst presented the best sulfur-resistanceperformance after the pre-sulfated treatment. The H₂-TPR, XPS and EXAFS resultsconfirmed that the iron in the perovskite crystal lattice inhibited the sulfation of theneighboring strontium by forming Fe2（SO4）3. The sulfated catalysts were desulfuratedby H₂reduction in high temperature and washing in solution respectively. Thehigh-temperature reduction could remove the sulfate effectively. Simultaneously, thestructure of perovskite-type catalysts was destroyed. The structure and NO_xstorageproperties of the catalysts could be recovered by calcination in air at700oC. Notably,the regeneration ratio of the pre-sulfated catalyst after10%H₂reduction at600oCwas up to100%.The best experimental conditions were optimized through exploring theconditions of NSR cycles for the La_0.7Sr_0.3Co_0.8Fe_0.2O₃catalyst. However, theLa_0.7Sr_0.3Co_0.8Fe_0.2O₃catalyst itself possessed lower De-NO_xactivity, which the NO_xconversion was only about25%. There was also no obvious improvement on NO_xremoval efficiency for the perovskite catalyst mixed with the Pt/Al2O3catalyst.Interestingly, we discovered the De-NO_xactivities of catalysts were improved significantly through increasing the substituting proportion of the Sr element （0.4-0.7）and the main perovskite phases of the catalysts got maintained. Among these catalysts,the Sr50catalyst presented the100%NO_xconversion and100%N2selectivity. TheH₂-TPR, O₂-TPD and XPS results confirmed that a lot of adsorption oxygen speciesexisted in the Sr50catalyst, which were considered to be the active ones in the NSRcycles. They could not only oxidize NO in the presence of O₂but also desorb andactivate reductant to reduce the stored nitrate effectively. The Sr50catalyst alsoexhibited excellent sulfur-resistant performance. Its De-NO_xefficiency reached97%after the pre-sulfated treatment with380ppm SO₂balanced by air, dropping only3%compared with the fresh one. Moreover, the NO_xcould be fully removed with thereactant gases containing50ppm SO₂.