Research On Rare Earth Pyrochlore Oxides For Catalytic Combustion Of Soot Particulates

Diesel engines have been generally accepted owing to their high efficiency of the engines, their low-operating costs, high durability and reliability, but one of the emissions of their pollutants—particulate materials(PM) have caused severe environmental problems and threatened our living conditions. The control of soot emission has become a challenging issue in recent years. Nowadays, catalytic combustion of soot is the most efficient method to remove the soot, and the key is catalyst. Pyrochlores with open structure have been applied in catalytic and electronic conductors field due to their excellent thermal stability and catalytic activities at high temperatures.Spherical nanocrystalline La2Sn2O7 and La2Sn1.8Co0.2O7 with a phase-pure pyrochlore structure were synthesized by a hydrothermal method, and they exhibited larger surface area and pore volume and uniform particle size. Compared with co-precipitation, the surface area was increased from 7.6 to 39.0 m2/g and pore volume from 0.012 to 0.057 m3/g for La2Sn2O7 prepared by hydrothermal method. SEM observation revealed that the hydrothermal catalysts show nearly spherical particle with rough surfaces and particle size of 200-500 nm, while co-precipitation samples demonstrate aggregation of particles. The ?(Sn-O) of hydrothermal samples based on IR spectra shifts to high frequency compared to the co-precipitation samples. Small amounts of Sn replaced by cobalt metals facilitate the formation of oxygen vacancies, thus improved oxygen migration and reducibility of samples. Under O2 atmosphere, the samples by hydrothermal method are more active than that by co-precipitation method, which can be ascribed to the following reasons:(1) the change of the pore structure property will favor of gas molecule adsorption, reaction and desorption;(2) the spherical morphology and better dispersed particles with smaller size can increase the contact points with soot;(3) hydrothermal environment can promote the formation of oxygen vacancies on the surface of catalysts, thus improve oxygen migration.A series of nanosphere Nd2Sn2O7 catalysts with a phase-pure pyrochlore structure was successfully prepared by polyethylene glycol(PEG)-assisted hydrothermal method, which possess higher crystallinity, larger surface areas and smaller crystallite size. The surface area of hydrothermal samples range from 40 to 70 m2/g and particles dispersed better. The increase of hydrothermal time makes the particles aggregation, crystallite size larger and surface area and pore volume smaller due to cross-linking effect enhancement of PEG. There are a lot of oxygen vacancies on the surface of nanoparticles. Generally, the smaller particle size is, the more oxygen vacancies are. TPR combined with IR studies confirmed that oxygen mobility of catalyst prepared by hydrothermal method was improved. The ignition temperature(T5) for catalytic soot combustion is 350 °C which decreases by about 50 °C compared to that of co-precipitation sample. The active of samples has a trend of decline with increase of hydrothermal time(6h, 12 h, 24h), while the selectivity to CO2 formation are higher than 98%. All pyrochlore catalysts prepared by the hydrothermal method display high catalytic soot oxidation activity and selectivity towards CO2 formation, which may be related to the spherical morphology, high surface area and pore volume, small crystallite size and improved oxygen migration.Rare-earth stannate pyrochlore catalysts Ln2Sn2O7(Ln=La, Nd, Sm) were prepared by co-precipitation method. The oxygen vacancies of the rare earth element substitution were investigated and the relationship between oxygen vacancies concentration modulation and apparent activity and intrinsic activity of catalytic soot combustion was established. The Sn-O bond energy decreased with the increase of the rare earth elements ionic radius(from Sm to La). The reducibility of catalysts enhanced which was related with Sn-O bond energy decreased, which was confirmed by TPR. The release of a amount of lattice oxygen at 300-600 °C was beneficial to the redox mechanism for soot combustion. The existence of oxygen vacancy was demonstrated by photoluminescence(PL). After excitated at 280 nm, the catalyst exhibited an emission peak around 408 nm, which was attributed to the luminescent centers due to oxygen vacancies on the surface. The emission intensity increased with ionic radius of rare earth elements, which may be caused by an increase in oxygen vacancy concentration. The oxygen vacancies concentration was carried on semi-quantitative analysis according to the XPS peak of adsorbed oxygen, which confirmed that the oxygen vacancies concentration of catalysts increased with rare earth elements ionic radius. The existence of oxygen vacancy can accelerate the migration rate of bulk oxygen ion and enhanced the capacity of adsorption and release of O2 on the surface of catalysts. In general, the higher oxygen vacancies concentration is, the lower the activation barrier of oxygen migration is, which benefit to the oxygen spillover mechanism in catalytic soot combustion.The results of catalytic soot combustion under O2 atmosphere suggest that the apparent activity for catalytic soot combustion(the ignition temperature, T5) was in according with oxygen vacancy concentration. La2Sn2O7 possessed the best activity with soot ignition temperature of 376 °C and high selectivity to CO2 formation of 85.1%. The kinetics analysis of catalytic soot combustion was carried out by the differential reactor model and turnover frequency(TOF) were calculated. The results suggested that TOF was also consistent with oxygen vacancy concentration, TOF of La2Sn2O7 was 2.33×10-3 s-1. It showed that the change of A site in the Ln2Sn2O7 pyrochlore could improve oxygen vacancies concentration and the amount and mobility of active oxygen species, thus, made the active of catalysts change regularly. At the same time, the stability of the catalyst was investigated with simulating a variety of operating conditions in order to provide a scientific basis for the design of a new catalyst for diesel soot combustion.