In Situ Construction Of Macroporous Metal Oxide Monolithic Catalysts And Their Soot Combustion Performances

Diesel engine plays a leading role in transportation because of its advantages of low energy consumption,high driving power,and stable performance.However,soot particulates emitted from diesel engine have become an important source of mobile source air pollution.Diesel particulate filter（DPF）coupled with catalytic combustion technology is an effective strategy to capture and eliminate soot particulates.The difficulty lies in how to design high-performance catalysts to decrease the soot combustion temperature and realize the DPF regeneration within the exhaust temperature range.Due to the high combustion temperature and large particulate size（25～100 nm）of soot,powder catalysts coated on DPF are difficult to form efficient contact with soot,which greatly limits the utilization of catalytic active sites.To solve this problem,in this dissertation,nanostructured catalysts with open macroporous space were in situ synthesized on industrial ceramic/metal-based DPFs for soot combustion,which improves the soot-catalyst contact efficiency.A large number of oxygen vacancies were constructed on the monolithic catalysts via the strategies of interface effect,defect engineering,and strong metal-support interactions to promote soot combustion.Through summarizing the activation of gaseous oxygen and the migration of surface active oxygen in soot combustion,the reaction mechanism of soot combustion was discussed in detail.Co₃O₄ decorated manganese oxide octahedral molecular sieve（OMS-2）catalysts with cactus-like nanorod morphology were in situ constructed on industrial cordierite monoliths for soot combustion.The specific structure of the catalysts provides sufficient macroporous space to improve the soot-catalyst contact efficiency.Compared with traditional powder-coated catalysts,the consumption of the nanorod catalyst is only～1/13 with the same thickness,and the contact area is enlarged by～4 times with the same weight.The study shows that the interface interaction between Co₃O₄ and OMS-2 promotes the generation of oxygen vacancies and Mn³⁺cations through electron transfer from Co²⁺to Mn⁴⁺.The isotopic ¹⁸O₂ results reveal that the number of active sites for soot combustion on the Co₃O₄ modified catalyst is 3 times larger than that on OMS-2 alone.Additionally,this interaction accelerates the activation of gaseous oxygen,and especially enhances the intrinsic activity（TOF）,as well.The above advantages improve the apparent soot combustion activity by～11 times.To further improve the catalytic soot combustion performance of OMS-2monolithic catalyst,the defects of the nanorod catalyst were constructed by surface modification,and the noble metal Ag was introduced by photochemical deposition.The results show that surface defects on OMS-2 promote the increase of Mn³⁺content,accompanied by the formation of a large amount of surface oxygen species.The tunnel structure of[Mn O₆]octahedron in OMS-2 and the existence of surface vacancies induce the migration of Ag during photodeposition and calcination,resulting in highly dispersed Ag⁺species.Compared with the different spatial distribution of Ag species caused by defect construction,it is found that defective OMS-2 possesses electron-rich Ag⁺cations.It contributes to the migration ability of lattice oxygen on OMS-2,as well as the adsorption and dissociation of gaseous oxygen on OMS-2 surface,resulting in more active oxygen species for soot combustion.Besides,TiO₂ nanowire catalysts with a low content of Pt（0.4 wt%）were in situ synthesized on titanium monolith for soot combustion.TiO₂ nanowires provide accessible macroporous space for the contact between soot and catalysts.It is discovered that Pt nanoparticles（Pt NPs）are gradually encapsulated by amorphous TiO_x layers via H₂/N₂ treatment,forming strong metal-support interaction（SMSI）.It increases the number of active oxygen species on TiO_x layers and promotes soot combustion.With the rise of reduction temperatures,the coverage of TiO_x layers on Pt NPs increases,and the enhanced metal-support interaction promotes the activation of gaseous oxygen.When the reduction temperature reaches 800°C,the catalyst achieves the highest soot combustion activity and intrinsic activity among the as-synthesized catalysts.