Catalytic Soot Elimination Over Monolithic Transition Metal Oxide Catalysts

Author:Cao Chun Mei

Supervisor:li xin gang


Degree Year:2017





Soot particulates emitted by diesel engines cause serious health and environment problems.Nowadays,the catalytic combustion technique combined with diesel particulate filters(DPFs)is regarded as an efficient after-treatment for soot elimination through a relatively complex gas-solid-solid reaction.Since soot particulates have a large size(25100 nm),they hardly access the inner surface of micropores and mesopores of catalysts,leading to very low contact efficiency between soot and catalytic sites.As a result,efficiency of soot catalytic oxidation is low.This paper developed a seriers of monolithic metal oxide catalysts with the open macroporous stucture to improve the contact efficiency between catalysts and soot particles,and thus to improve the catalytic activity of the catalyst for soot combustion.The CeO2 nanobelt monolithic catalyst(CeO2-NA)was successfully prepared by a facile hydrothermal method,the space between nanobelts increased the contact chance between catalysts and soot particulate.The contact chance(Cc)between the CeO2-NA and soot particulates is 122 times higher than that between the power catalysts and soot particulates through calculation.Moreover,the CeO2-NA catalyst possesses the higher redox and intrinsic activity(TOF),the larger amount of oxygen vacancies and active oxygen species,and thus shows the better catalytic activity.In order to further improve the contact chance between catalysts and soot particulates,crossed nanosheets of transition metal oxide(TMO-NS:Co-NS,Mn-NS and Fe-NS)were synthesized by a facile hydrothermal method and employed for soot combustion in the NO/O2/N2 and O2/N2 atmosphere under gravitational contact mode(GCM).They show high catalytic activities for soot combustion due to the macroporous structure of the as-prepare catalysts increasing the soot-catalyst contact efficiency.Among these catalysts,the Co-NS catalyst shows the best activity for soot combustion,especially in the presence of NO,whose catalytic activity of T50(391℃)and SCO2(100%)is as good as that of the Pt/Al2O3 catalyst.Crossed Fe2O3 nanosheets supported cobalt oxide nanoparticles on three-dimensionally macroporous nickel foam substrate(xCo/Fe-NS)were designed and successfully prepared through a facile hydrothermal and impregnation route.These catalysts showed high catalytic soot combustion acivities under self-capture contact mode.The three-dimensional macroporous structures of Ni foam and the crossed Fe2O3 nanosheets constituted macroporous“little rooms”can greatly increase the contact efficiency between soot particulates and catalysts.The interaction between Co and Fe facilitated the activation of Fe-O bond and increased the amounts of active oxygen species,thus improving the redox property of the catalysts.The potassium-promoted Co3O4 nanowires supported on the monolithic 3-DM nickel foam substrate(xKCo-NW)were synthesized through a facile hydrothermal and impregnation method.Loading of potassium provides the new active sites for soot oxidation,improves catalyst-soot contact as a molten salt and increases the amount of the surface adsorbed oxygen species.Thus,the xKCo-NW catalysts show the enhanced catalytic performance,especially for the catalyst loaded with 5%potassium(5KCo-NW).In the NO/O2 atmosphere,the catalytic activity of the 5KCo-NW catalyst is higher than that of conventional Pt-based catalysts.It is because potassium promotes the activity for NO oxidation to NO2 over Co3O4,further inducing the enhanced catalytic performance for soot combustion.Moreover,our results show that the chemisorbed NOx species on the 5KCo-NW catalyst are more active than the gaseous NO2 for soot combustion.In the presence of O2,soot combustion is accelerated to react with the chemisorbed NOx species;whereas the formed nitrate/nitrite species are catalytically inert.