Study On The La-Mn Perovskite-type Catalysts For The Catalytic Oxidation Of Diesel Exhaust Soot

Series of La-Mn perovskite-type catalysts have been prepared by a newly-developed sol-gel method, and the activities for soot oxidation over these catalysts have been evaluated in an activity evaluation system. Furthermore, the micro-reaction mechanism for soot oxidation has been established by means of XRD, SEM, BET, FT-IR, XPS, O₂-TPD, OSC, H₂-TPR, NO-TPD, in situ DRIFTS and other characterization methods. La_0.8Ce_0.2Mn_0.7Bi_0.3O₃, which exhibits the highest activity among these catalysts, has evaluated for the removal of soot under real engine operation conditions through the engine bench test. This study will benefit designs for commercial perovskite-type catalysts used in the purification of diesel exhaust theoretically and practically. Several conclusions have been drawn:（1） Compared with the conventional sol-gel method, the newly-developed sol-gel method can help compose the LaMnO₃ catalyst with a higher BET specific surface area, a less crystalline size and a higher activity for soot oxidation. The 10 % soot conversion rate temperature（T₁0）, maximum soot conversion rate temperature（T_m） and 90% soot conversion rate temperature（T₉0） over the newly-developed LaMnO₃ decrease by 32℃, 33℃ and 24℃, respectively, compared with the conventional LaMn O₃.（2） For Ce-substituted catalysts, the poor solubility of Ce nitrates induces the formation of a small amount of CeO₂ over the catalysts. Among these catalysts, La_0.8Ce_0.2MnO₃ exhibits a small crystalline size, a fluffy surface, the largest BET specific surface area and highest low-temperature reducibility. In the atmosphere of air, the maximum soot oxidation rate temperature T_m over La_0.8Ce_0.2MnO₃ is 60℃ lower than that over LaMnO₃.（3） The substitution of Mn by Bi at B-site in perovskites inhibits the growth of the crystal particle, increases the BET specific surface area and the amount of surface adsorbed oxygen, promotes the absorption and activation of NO over the surface oxygen sites and improves the reducibility of the catalyst. Among these catalysts, LaMn_0.7Bi_0.3O₃ exhibits the highest activity. In the atmosphere of air, the T₁0, T_m and T₉0 over it are 319, 394 and 425℃, respectively. The activation energy E for soot oxidation over LaMn_0.7Bi_0.3O₃ is 87.3kJ/mol.（4） For A- and B-site substituted catalysts, La_0.8Ce_0.2Mn_0.7Bi_0.3O₃ possesses the highest activity for soot oxidation. In the atmosphere of air, the T₁0, T_m and T₉0 over it are 314, 375 and 393 ℃, respectively. The oxidation of soot over La_0.8Ce_0.2Mn_0.7Bi_0.3O₃ follows the redox mechanism and spill-over mechanism simultaneously. Therefore, La_0.8Ce_0.2Mn_0.7Bi_0.3O₃ exhibits a high catalytic activity for soot oxidation even under loose contact condition. The experiment in the atmosphere of NOx suggests that soot and NOx can be reduced by La_0.8Ce_0.2Mn_0.7Bi_0.3O₃ simultaneously.（5） The La_0.8Ce_0.2Mn_0.7Bi_0.3O₃ catalyst has been coated in several POCs with different weight ratios, and then evaluated for soot removal through engine bench tests. When the engine is connected with both POC and DOC, the removal efficiency for HC and CO reaches 90%. At the engine load of maximum power and 2510r/min, the removal efficiency and oxidation efficiency of POC20(the POC with 20g/cm-3La_0.8Ce_0.2Mn_0.7Bi_0.3O₃ catalyst supporting) for particulate matters（PM） are 89.8% and 38.7%, respectively. Meanwhile, the removal efficiencies for SOF and soot are 98.0% and 92.9%, respectively. The removal efficiency of POC20 for PM reaches the maximum of 94.2% at the engine load of 75% of maximum power. At the engine load of medium power, only PM with large particle size can be effectively trapped by POC20. At the high engine load, however, both the PM with large particle size and small particle size can be removed by POC20. In addition, a decrease in the density of NOx in the emission after flowing through POC is observed, suggesting that the NOx and PM can be removed by POC20 simultaneously.