| Recently,diesel has gradually become the main driving force for automobile,however,due to its own nature and unique lighting methods,the exhaust emissions characteristics of diesel vehicles are that content of PM is particularly high.The kinetic diameter of these ultrafine particles is less than 2.5?m,suspended in the air for a long time and inhaled by the human causing respiratory infections,lung cancer or other diseases.It is also the direct culprit of PM2.5 in the urban atmosphere.At present,C-DPF technology is the most direct and effective treatment method for the PM purification in diesel vehicle exhaust.The key of this technology is to find a suitable catalyst that can significantly reduce the temperature of soot combustion.Ag/CeO2 based catalysts show excellent catalytic performance for the abatement of diesel exhaust using C-DPF technology.However,the various tests show that Ag/CeO2 based catalysts still need to been improved in terms of thermal and hydrothermal stability.It is mainly due to the SMSI between Ag and CeO2 leads to the interfacial oxidation of silver by CeO2 at high temperatures,which results in an obvious deacti-vation of catalysts.This research is to explore the Pd effect on the surface oxidation of metal silver at high temperature.At the same time,the active phase and mechanism of soot oxidation catalyzed by Pd modified catalyst were also studied.Mg2+ is further modified cerium-zirconium support to investigate the effect of Mg2+ on the PdAg alloy formation.Firstly,a series of Pd modified Ag/LCZ catalysts was prepared by successi-ve impregnation method.As proven by XRD,HAADF-STEM-EDX,CO-DRIFTS,palladium is apt to alloy with the supported silver component.XPS analysis further confirmed that Ag component alloyed with palladium maintain its reduction state,which is crucial for the catalytic activity for soot combustion.And the alloying effectively inhibit silver particles from getting oxidized through SMSI route.the significant improvements on both the initial activity and stability are observed on the palladium modified Ag/LCZ.The alloying effect is even prominent for aging test,the combustion activity remains increasing after hydrothermal treatment at 850 ? for 8h.Secondly,Pd0.1Ag/LCZ catalysts with Pd introduced by different addition order wear prepared by stepwise impregnation method to explain the specific mechanisms of Pd and Ag alloying.When Pd and Ag precursors introduced into the cerium-zirconium support by successive impregnation method,the PdAg alloys are formed by synchronous way,and degree of alloying is significantly higher than that catalysts prepared by stepwise impregnation,which is mainly due to the strong interaction between the metal and the support to hinder the progress of alloying.At the same time,we also explore the active phase and mechanism over Pd modified catalyst for soot combustion.The EPR results show that the addition of Pd promotes the formation of more anion defect sites,and as proved by O2-TPD and XPS the content of reactive oxygen species on Pd modified catalysts is obviously higher than that of single metal Ag/LCZ catalyst,and O2-has a direct relationship with the ignition temperature of soot combustion,however O-contributes to most of soot oxidation.Finally,foundly that in cerium-zirconium based catalysts modified by Mg,the loading of Mg has a significant effect on the metal dispersion,redox properties,activity of the catalyst.The low loading of Mg can facilitate the metal dispersion,while the existence most of MgO hinder the metal dispersion,resulting that migration of active oxygen being slower.Found in the activity test that Pd0.1Ag/0.2wt%Mg-LCZ was most active for soot combustion.On the one hand,an appropriate amount of Mg can weak the interaction between metal and support,on the other hand to provide more alkaline sites,promoting the activation of oxygen molecules and speeding up soot oxidation. |
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