The Catalytic Performance And Reaction Mechanism Of Alkali Metals As Active Components For Soot Combustion

Soot particulates are one of the main pollutants emitted from diesel engines,which pose a significant threat to environment and human health.Many researchers have paid much attention to study how to effectively control the diesel particulate emissions.Currently,diesel particulate filters?DPF?are considered to be the most efficient way to eliminate soot from diesel engine exhaust.One of the great challenges for DPF is to find a robust catalyst to decrease ignition temperatures of the deposited soot.The alkali metal catalysts have been widely reported due to their excellent catalytic activities for soot combustion.However,the reaction mechanism of alkali metal species is controversial.It is commonly accepted that the alkali metals can improve soot/catalyst contact efficiency and thus catalytic activity due to the mobility of alkali metal species.But in our previous work,the alkali metal species are suggested to be catalytically active and not only contact promoters.In order to prove this view,the catalysts have been designed in this work.We confined the alkali metal ions in the channel of catalysts in order to isolate the soot.ZSM-5 possesses a well-defined three-dimensional micropore structure and a capability for cation exchange.M-ZSM-5-25?M=Na/K/Cs?have been prepared by ion exchanged methods.The physicochemical properties of these catalysts were characterized by XRD,BET,SEM,ICP,FT-IR,Raman,XAFS and NH₃-TPD.It was shown that the K⁺is located at the ion-exchanged sites.The apparent and intrinsic activities of these catalysts for soot combustion were evaluated by O₂-TPO and isothermal reaction,respectively.The order of catalytic activity is attributed to the nature of alkali metals.In addition,in order to study the mechanism of the soot oxidation catalyzed by alkali metals catalysts,the effect of K amounts in ZSM-5 has also been researched.The activity for soot combustion was improved by increasing K⁺-exchanged amounts.¹⁸O₂ isotopic isothermal reactions suggested that the gaseous oxygen can be activated by K-ZSM-5,attributing to the roles of active K component.By confining alkali metal ions into the ZSM-5 catalysts with the three-dimensional micropore structure,the alkali metal species have been proved to be active components for soot combustion.In order to further confirm this view,another kind of catalysts have been designed for soot combustion,with the potassium in the channel of the Ti O₂.Firstly,the rod-like KTi₈O₁₆ catalyst was prepared by solid phase method.Due to introduction of the K,the catalytic activity of Ti O₂ was increased to a large degree.In order to further improve the activity by increasing K amounts,a series of K_x Ti₈O₁₆ particles were prepared by a different method.The status of the potassium were characterized by XRD,FT-IR,XAFS.Besides,combined with XPS and O₂-TPO,the effect of the oxygen vacancy on soot combustion activity was eliminated.It is noted that the intrinsic activities were consistent with the apparent activities of the catalysts.It is further confirmed that the alkali metal species can catalyze the soot combustion as active components.