Study On Catalytic Combustion Of Soot Particles Over Lanthanum/Praseodymium-based Rare Earth Composite Oxide Catalysts

Catalytic particulate filter trap?CDPF?is one of the most promising technologies for the purification of fine particulate matter?PM?emitted from diesel vehicles.One of the core technologies of CDPF is the development and optimization of PM oxidation catalyst.Based on the limitations of traditional precious metal catalysts and commercial application catalysts,it is still necessary to develop cheap and efficient catalysts.Rare earth complex oxide catalysts are a kind of oxide materials formed by the combination of rare earth elements and other elements?alkali?earth?metals,transition metals,etc.?.They generally have excellent thermal stability and catalytic oxidation ability.Pyrochlore?A₂B₂O₇?and perovskite?ABO₃?are two kinds of common rare earth composite oxides.They can withstand extensive chemical substitution and structural modification.They can also be used as support materials to load precious metals or alkali metals and other active metal components in order to achieve highly dispersed active components and improve the stability of active metals.La/Pr-based pyrochlore catalysts and La-based perovskite catalysts were synthesized by various methods in this study.They were modified with precious metals Pd,alkali?earth?metals and transition metals as dopants.The purpose was to analyze the changes of physicochemical properties as well as catalytic activity of the catalysts before and after modification.The reaction mechanism of catalytic oxidation of soot was preliminarily analyzed.The specific contents are as follows.Two kinds of Pd modified La₂Sn₂O₇ pyrochlore oxide catalysts were synthesized by sixteen alkyl three methyl ammonium bromide?CTAB?assisted sol-gel process combined with impregnation method.It was found that the existence of Pd strongly affected the oxygen vacancy concentration and redox property of the catalysts.Pd/LS catalyst supported by surface Pd species had the highest catalytic activity for NO oxidation and soot combustion.Surface Pd species brought a large number of surface and bulk oxygen vacancies to the catalyst,and the presence of Pd/PdO redox pair on the surface caused the interaction between metal oxide and support,which improved the low temperature redox performance of the catalyst.For Pd-LS sample doped by bulk Pd phase,most of Pd was immobilized in pyrochlore structure,which was difficult to participate in surface reaction.Therefore,Pd-LS exhibited moderate activity in catalytic reaction.Based on all the characterization results,several reaction pathways of Pd-modified pyrochlore catalyst for catalytic oxidation of soot were revealed,including direct oxidation of activated adsorbed oxygen,rapid oxidation of Pd/PdO and assisted oxidation of NO₂.In order to further adjust the structure and properties of pyrochlore catalysts,a series of lanthanum tin pyrochlore catalysts were prepared by partial substitution of Ca and Co for A and B sites of pyrochlore.The double substitution of Ca and Co improved the texture properties and redox performance of the catalysts.XPS and PL analysis showed that the charge imbalance caused by double substitution resulted in a large number of surface and bulk oxygen vacancies,which increased the adsorption and activation rate of reactant molecules?O₂,NO₂?,and then promoted the catalytic oxidation process.Compared with monosubstituted catalysts,the ignition temperature of LCSC catalyzed soot oxidation was significantly lower,which revealed the synergistic effect of Ca and Co in the catalytic soot combustion reaction.In addition,the process of NO adsorption and oxidation on catalysts was investigated by DRIFT technology.There were two main stages:low temperature stage?<250-300°C?and high temperature stage?>300°C?.The low temperature adsorption stage was mainly related to the weakly adsorbed nitrites.The decomposition product NO₂ of these nitrites participated in the ignition process of soot combustion.K-promoted Pr₂Sn₂O₇ pyrochlore catalysts with different loads were synthesized by CTAB-assisted hydrothermal method and impregnation method.It is found that K species can be highly dispersed on the surface of Pr₂Sn₂O₇ nanospheres when K loading is not more than10 wt%.The main forms of K are highly dispersed K₂CO₃,highly dispersed KO₂ and bulk KO₂.The results of H₂-TPR showed that low-dose surface dispersed K species improved the reduction behavior of oxygen on the catalyst surface.When K loading was too high,the reduction ability decreased due to the coverage of active ingredients.10K/PrSn has the best catalytic activity for soot oxidation and good thermal stability.The comparative experiments of loose contact and tight contact show that K species on the surface reduced the sensitivity of the catalyst to soot-catalyst contact mode.The reaction mechanism of soot combustion catalyzed by K/PrSn was inferred through Soot-TPR experiments.La-based transition metal perovskite catalysts?LaMO₃,M=Mn,Fe,Co,Ni?as well as the catalysts doped with alkaline earth metal Sr were synthesized by solution combustion method.Sr dopants had different effects on the redox properties of perovskite oxides.For LM and LN,although the valence distribution of metal ions in the lattice changed after Sr doping,the redox ability did not improve.For LF and LC,the redox performance at low temperature was improved due to the increase of oxygen adsorption on the surface and the improvement of bareness of active metal composition.Among all catalyst samples,LSC showed the best catalytic activity of NO oxidation and soot oxidation,which was attributed to Sr doping that increased the specific surface and the exposure of surface active metal,and induced a large number of oxygen vacancies,which then accelerated the catalytic oxidation rate.This study provides a scientific basis for the optimization and modification of La-based perovskite catalysts.