Structural Design And Application Of CeO₂-based Catalysts In Soot Combustion

Diesel particulate filter（DPF）combined with catalytic combustion of renewable technology is considered to be an efficient after-treatment of soot oxidation;an effective catalyst with high activity and high thermal stability is extremely important for DPF regeneration.With high-activity CeO₂-based catalysts as our mission,CePr and CeMn catalysts have been synthesized in the paper by inserting Pr and Mn atoms into CeO₂,in order to study the influence of CeO₂-based catalysts doped with different ions on catalytic performance.Subsequently,CePr catalyst has been selected for structural design.After a series of characterizations had been carried out,the main research results we got are as follows.（1）Cubic fluorite structure solid solution can be formed both Pr and Mn doped into CeO₂ lattice.Three oxygen species exist in the solid solution structure,which are lattice oxygen OI（O₂-）,defect oxygen species OII（O₂-,O₂2-and O-）and hydroxyl-like groups and adsorbed molecular water OIII.Generally,defect oxygen species OII is responsible for soot combustion.The three oxygen species exist in CePr,and the channel of oxygen migration（OI?OII?OIII）of which is open.Thus,the active oxygen OII could be replenished continuously,resulting in a fast oxidation rate.In addition,the oxygen channel remains intact after high temperature aging,leading to a high thermal stability.CeMn possesses a large number of surface active oxygen,resulting in a high low-temperature catalytic activity.However,only OI and OII exist in the solid solution structure,the oxygen channel is inaccessible.Therefore,it presents a poor oxidation rate,due to the unfilled active oxygen species after running out.Furthermore,phase separation occurs after high temperature aging,with Mn2O3 releases from solid solution surface.（2）Four CePr mixed oxides with different structures were designed and synthesized by a solid-phase grinding method: CePr–NN（uniform solid solution of Ce and Pr）,CePr–NO（Pr6O11-coated CeO₂）,CePr–ON（CeO₂-coated Pr6O11）,and CePr–OO（particle packing of CeO₂ and Pr6O11）.CeO₂ is considered as the active phase in CePr catalysts.This phase located on the catalyst surface efficiently reduce the ignition temperature of soot combustion.In addition,doping Ce O₂ with Pr can greatly enhance its thermal stability,because of the high stability of Pr6O11,which was used as the supporting phase of the catalyst for soot oxidation.Furthermore,incorporation of Pr atom into CeO₂ with nitrates as precursors mainly improves its OSC and the mobility of active oxygen at 200 °C to 400 °C,which leads to a higher combustion rate without affecting the soot ignition temperature.The reason is because the ―molten state‖ appeared during the heat treatment of CePr synthesized by solid-phase grinding method using nitrates as precursors,which rapidly dispersed Ce and Pr.It indicated that homogeneous distribution of Ce and Pr in solid solution structure results in more defects and active oxygen species,and thus forming high-activity oxide.Meanwhile,the catalytic activity is independent of the grinding time.