| Diesel engine have been widely used because of it’s high thermal cycle efficiency, good fuel economy and high durability. However, the NOx and PM of exhaust emissions have adverse effects on air pollution and human health. The after-treatment technology must be added to meet increasingly stringent emissions regulations, and catalyst is the key for exhaust purification in the after-treatment technology. Now, the catalyst to be used is expensive, easy to poisoning, poor thermal stability, so the study of the catalyst with low prices, good activity, strong thermal stability is very urgent and necessary. The present paper intends to make use of rare earth resources in our country, develop low cost, high efficiency rare earth A2B2O7 pyrochlore composite oxide catalyst for catalyzing and removing soot and NOx efficiently in the range of engine exhaust gas temperature.In this paper, pyrochlore type composite oxide as catalyst, and study amount of doping, calcination time, doped with transition metals impact on the catalytic activity. The structure, micro morphology, pore structure property, oxygen vacancy concentration and redox properties of prepared materials were characterized by XRD, FT-IR, TPR, BET, PL, DLS and SEM techniques. The catalytic activities of soot and NOx removal for pyrochlore catalytic were determined by temperature-programmed oxidation and catalytic mechanism was discussed by isothermal anaerobic titrations.First, a series of pyrochlore-type Nd2Sn2-XMn XO7 catalysts were successfully synthesized by co-precipitation method. The study found small doses of Mn does not change the crystal phase of pyrochlore, and the prepared catalyst crystal phase single, good crystallinity. Mn-doped samples changed the Sn-O bond energy, redox properties, aliovalently doping to further promote the generation of oxygen vacancies, improved the rate of migration of oxygen. According to the results of catalytic activity test, all catalysts improved CO2 selectivity to aboved 70 %, the removal rate of NOx increased to about 12 %. The ignition temperature is further reduced under NO + O2, because of NO exists to promote the role of NOx auxiliary mechanism. Among all samples, Nd2Sn1.8Mn0.2O7 sample has the best catalytic activity and selectivity, T5 = 346 oC, T50 = 449 oC and Sco2 = 85.7 %, and the ignition temperature is reduced to 124 oC. This is due to Mn0.2 sample have the appropriate concentration of oxygen vacancies and redox properties. The oxygen spillover mechanism and redox mechanism of soot oxidation occur simultaneously, thereby increasing the soot combustion activity and selectivity.Taking into account the oxygen vacancies of pyrochlore composites affected by particle size, the heat treatment temperature or time, Sn-O bond energy changes, La2Sn2O7 pyrochlore oxide compound prepared by co-precipitation 900 oC temperature calcinations, and investigated the effect of calcination time on the oxygen vacancy concentration, grain size, pore structure properties, redox properties to establish relationship between the catalytic activity and calcination time. The study found that prolonged calcination, the catalyst sample still remained single pyrochlore structure. The increase of calcination time makes the infrared vibration of of Sn-O bond shifted to the lower wavenumber region, so Sn-O bond energy reduced, thereby changed the redox properties of the material. When the catalyst is excited by 310 nm wavelength, the fluorescence intensity is low, which indicates that the catalyst has more oxygen vacancy concentration. The sample of the calcination time of 6 h has the largest specific surface area 11.3 m2/g. Calcination time too long or too short may cause sample structure collapse or agglomerate, reducing the exposed active sites. Catalytic activity tests indicated that the pyrochlore sample of 6h calcination time has high activity, soot ignition temperature dropped to about 333 oC, CO2 selectivity increased to about 86.6% in NO + O2. The good catalytic activity of this sample is related to its larger surface area, more exposured oxygen vacancies and better reduction performance.In order to increase the surface area and active sites of the catalyst, transition metal doped La2Sn1.8TM0.2O7(TM = Sn, Mn, Fe, Co and Cu) catalyst were synthesized by CTAB assisted sol-gel method. The prepared pyrochlore sample crystal phase complete and single, indicating transition metal completely into the pyrochlore structure. The catalyst was calcined at 900 oC and showed a spherical morphology with a relatively large surface area(~20 m2/g). the Sn4+ is replaced by transition metal in order to maintain the charge balance, that leads to the generation of oxygen vacancies on the surface, promoted the mechanism of oxygen spillover, and then improves the catalytic activity of the catalyst. These oxygen vacancies can adsorb and activate NO or O2 molecules, improving to the transformation of NO to NO2, and it acilitated the elimination of NOx and soot. The incorporation of transition metal greatly influence the reduction of pyrochlore oxide in low temperature range, improved soot combustion redox mechanism. The existence of NO promoted the role of NOx assistant mechanism, which is beneficial to the catalytic oxidation of soot. In all catalysts, the Co-LSO sample shows the best activity. The ignition temperature of catalytic soot combustion is 314 °C and CO2 selectivity close to 100 % in NO + O2 atmosphere, which can be attributed its high oxygen mobility, good low-temperature redox activity and large specific surface area. Since the catalyst has a strong oxidation, the NO is converted to NO2 which has a stronger oxidation, and the NOx is reduced to N2 under the action of soot and catalyst, achieving the purpose of removing NOx. The intrinsic activity(TOF) of the catalyst was calculated by isothermal anaerobic titrations. The study found that intrinsic activity(TOF) is consistent with the apparent activity results. The TOF of Co-LSO was 3.20×10-3 s-1 which indicates that the change of B site in the Ln2Sn2O7 pyrochlore could improve oxygen vacancies concentration and the amount and mobility of active oxygen species, thus modulating catalyst activity. |
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