Synthesis And Reaction Mechanism Of Catalysts With Confined K Cations For Soot Oxidation

Diesel technology has tremendously advanced over the past decade owing to the efficient energy conversion of the diesel engine. However, emission of a particulate matter(PM) from diesel vehicles, which consists mostly of carbonaceous soot as a major pollutant, causes acute health problems to humans. Consequently, an effective technology needs to be exploited to reduce the emission of PM from diesel vehicles. Diesel Particulate Filter(DPF) has been the most popular way to reduce PM by trapping PM on the ceramic beehive carrier. It can collect more than 90% of diesel soot. The most challenging task is the in situ simultaneous regeneration of the DPF by combusting trapped PM. Hence, there is a strong incentive to develop a catalyst for continuous regeneration of the DPF.The identification and characterization of catalytic active sites are essential for understanding of catalytic mechanism from microscopic and designing excellent heterogeneous catalysts. Catalysts containing potassium(K) were considered to be the best soot oxidation catalysts since the K-species can improve catalysts-soot contact and/or build strong interaction between K and supporter. Indirect evidence in our previous reports suggests that K-species can enhance the catalytic active for soot combustion by servicing as active sites. Catalysts with confined active sites have been designed to separate active sites and reactants with special structure. Along this way, cryptomelane with confined K~+ were designed to purely characterize the relationship between catalytic activity and K content by eliminating the contact between soot and K~+. K~+ of the cryptomelane were confirmed to be confined in the tunnel of the cryptomelane with mobility. The nonentity of surface K-species were proved by XRD, TEM, ICP, BET, FT-IR, XPS, XAFS. The attribution of strong interaction between Mn and K for soot combustion was eliminated. K was finally proved to be the active sites for soot oxidation by activity tests such like O2-TPO, isothermal reaction.Although the concept that K~+ can act as active sites to take part in soot oxidation reaction has been proved, the contribution of Mn in cryptomelane for soot oxidation could not be totally eliminated since Mn can oxidate soot itself. ZSM-5 is a kind of molecular siever with micropore. In this paper, ZSM-5 were employed to prove our ideal since all Al and Si in ZSM-5 were proved to be inertial for soot oxidation. M-ZSM-5(M=K, Na, Cs, H) were preapred by ion exchange after ZSM-5 were synthesised througt hydrothermal method. XRD, FT-IR were performed to make sure that the molecular sieves were successfully prepared and no surface alkali metal species exist; The surface areas and pore structure were given by BET test; SEM was employed to characterize the morphology of the catalysts. The apparent activity and intrinsic activity of these catalysts for soot combustion were measured by O2-TPO and isothermal reaction, respectively. The order of catalytic activity was determined to be Cs-ZSM-5>K-ZSM-5>Na-ZSM-5 which is attributed to the nature of alkali metals.