The Active Oxygen Species For Soot Combustion On Rare Earth Oxides

Diesel engines have been generally accepted in the world owing to their low oil consumption, high efficiency and durability. However, particulate materials (PM) released from these engines have caused severe health problems. The control of soot emission has become a challenging issue in recent years. Nowadays, catalytic combustion seems to be the best option to minimize soot discharge. The rare earth perovskite-type oxides behave high stability, and can easily activate oxygen molecules because they are full of oxygen vacancies, so they are widely used for soot combustion. What is more, studying of the active oxygen species on these catalysts will surely encourage the derivation of soot combustion mechanism.In this work, a series of perovskite-type La_1-xSr_xMO₃(x=0, 0.1; M=Mn, Co) catalysts were prepared by citric acid method and then characterized by X-ray powder diffraction (XRD), N2 adsorption/desorption and Transmission electron microscopy (TEM) for their crystal structures, specific surface areas and morphological features. The active oxygen species of the catalysts for soot combustion were studied using redox titration, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction with H₂ (H₂-TPR), temperature-programmed desorption of O₂ (O₂-TPD) and temperature-programmed reduction with soot (Soot-TPR). The catalytic activities of soot combustion for prepared mixtures of catalyst and soot were determined by temperature-programmed oxidation with O₂ (O₂-TPO) and isothermal anaerobic titrations. Then the apparent activation energy and turnover frequency (TOF) were calculated. Furthermore, the active oxygen species and TOF of the Sr-doped catalysts for soot combustion were also investigated.The La_1-xSr_xMO₃(x=0, 0.1; M=Mn, Co) samples are single phase perovskite-type oxides, with primitive rhombohedral cell. The catalysts possess high crystallinities, lattice defects and low surface areas. The specific surface areas of Mn-based catalysts are higher than that of Co-based catalysts. The doping of Sr induces a rise in surface area and a drop in crystallite size. The redox titration shows that there are over-stoichiometric oxygen in Mn-based catalysts, whereas there are oxygen vacancies in Co-based catalysts. The concentrations of the over-stoichiometric oxygen decrease and the oxygen vacancies rise after Sr doping. Mn⁴⁺ and Mn³⁺ as well as Co²⁺ and Co³⁺ exist in Mn-based catalysts and in Co-based catalysts, respectively. The concentrations of Mn⁴⁺ and Co³⁺ rise after Sr doping. The XPS spectra suggest that there are three kinds of oxygen species on the surface of catalysts, including surface lattice oxygen, adsorbed oxygen and adsorbed H₂O and/or surface carbonate. After Sr doping, the concentrations of adsorbed oxygen on Mn-based catalysts have no clearly change, but that rise on Co-based catalysts. Sr-doping enhances the mobility of surface lattice oxygen for Co-based catalysts.The profiles of H₂-TPR for all catalysts show that there are two reduction regions. The two sets of peaks can be attributed to the reduction of Mn⁴⁺ to Mn³⁺ and Mn³⁺ to Mn²⁺ for Mn-based catalysts, but the reduction of Co³⁺ to Co²⁺ and Co²⁺ to Co for Co-based catalysts. The results of O₂-TPD for Mn-based catalysts show that the adsorbed oxygen is released at lower temperature; the lattice oxygen on the surface and the over-stoichiometric oxygen in bulk are released at higher temperature. The weakly chemisorbed oxygen is composed of adsorbed oxygen and lattice oxygen on the surface. However, only the surface bulk oxygen released at higher temperature can be detected for Co-based catalysts. According to the results of Soot-TPR, the weakly chemisorbed oxygen that participates in the soot oxidation for Mn-based catalysts, while the surface bulk oxygen reacts with soot for Co-based catalysts.The TPO and the isothermal anaerobic titrations results suggest that all catalysts perform high activity for soot combustion and the selectivity CO₂ nearly 100 %. The T10 (the temperature of 10% soot conversion) and the TOF are 367℃and 2.24×10^-3 s^-1 for LaMnO₃, and the values change to 381℃and 0.63×10^-3 s^-1 for LaCoO₃. These suggest that the activity of the weakly chemisorbed oxygen is higher than that of the surface bulk oxygen. The doping of Sr results in an insignificant change in TOF for Mn-based catalysts and an increase of that for Co-based catalysts. There is an unconspicuous change in over-stoichiometric oxygen and Mn⁴⁺/Mn³⁺ molar ratio after Sr doping to the LaMnO₃ lattice. These lead to the result that the activity of La_0.9Sr_0.1MnO₃ is similar to that of LaMnO₃. The oxygen vacancies and Co³⁺/Co²⁺ molar ratio and TOF rise after Sr doping to the LaCoO₃ lattice. These induce the higher active of La_0.9Sr_0.1CoO₃ compared with LaCoO₃. Furthermore, the specific rates for Co-based catalysts are higher than that of Mn-based catalysts, which is due to the much higher oxygen density for the former. All these data suggest that soot combustion can be explained by a modified Mars and van Krevelen mechanism.TPO results show that the T10 for soot noncatalytic combustion (475℃) is higher than that for soot combustion over LaMnO₃. The apparent activation energy for soot noncatalytic combustion was calculated to be 128.9 kJ·mol^-1 according to the steady isothermal reactions, but it decreased to 97.5 kJ·mol^-1 for soot combustion over LaMnO₃. Both the activation energy and the characteristic temperature values decreased when using LaMnO₃ as the catalyst of soot combustion, suggesting that the catalytic methods can improve soot combustion at lower temperature.