The Preparation Of SnO₂ Catalytic Materials For Soot Combustion:Study On The Structure-reaction Relationship

Catalytic soot combustion is of great significance for both human health and increasingly serious environmental problems.It is currently a hot research topic to prepare low cost and high-performance catalysts for this reaction.In this thesis,the preparation and properties of SnO₂-based catalysts have been studied for soot combustion.The physicochemical properties of the prepared catalysts have been characterized by means of XRD,N₂ adsorption-desorption,H₂-TPR,O₂-TPD,XPS,SEM,TEM,ICP,and other characterization.The structure-reactivity relationship of the catalysts has been investigated and elucidated.The main results are summarized here:First,SnO₂-based catalysts promoted by different alkali metal oxides with a Sn/M?M=Li,Na,K,Cs?molar ratio of 9/1 have been prepared for soot combustion under tight contact.In comparison with the un-modified Sn O₂support,the activity of the modified catalysts has been evidently enhanced,following the sequence of Cs Sn1-9>KSn1-9>NaSn1-9>LiSn1-9>Sn O₂.As testified by Raman,H₂-TPR,soot-TPR-MS,XPS and O₂-TPD results,the incorporation of various alkali metal oxides can induce the formation of more abundant and mobile oxygen species on the surface of the catalysts.Moreover,quantified results have proved that the amount of the surface acti ve oxygen species is nearly proportional to the activity of the catalysts.Cs Sn1-9,the catalyst promoted by cesium oxide,owns the largest amount of surface mobile oxygen species,thus having the highest activity among all the studied catalysts.It is concluded that the amount of surface active and mobile oxygen species is the major factor determining the activity of the catalysts for soot combustion.Second,a series of three-dimensionally ordered macroporous?3DOM?SnO₂-based catalysts modified by Ce⁴⁺,Mn³⁺,Cu²⁺cations have been prepared by using a colloidal crystal templating method and tested for soot combustion under loose contact.XRD and STEM-mapping results testify that all the secondary metal cations have entered the lattice matrix of tetragonal rutile SnO₂ to form non-continuous solid solutions,thus impeding the crystallization and improving the surface areas and pore volumes of the modified catalysts.In comparison with regular nanoparticle SnO₂,the 3DOM SnO₂ displays evidently improved activity,testifying that the formation of macro porous 3DOM structure can anchor the soot particulates in the macropores,which makes the contact of the soot particles with the active sites on the 3DOM skeleton become easier,thus benefiting the target reaction.With the incorporation of the secondary metal cations,the activity of the catalysts can be further improved due to the formation of more abundant mobile oxygen species.In summary,these effects are believed to be the major factors responsible for the activity of the catalysts.Third,0.5%Pd/SnO₂ catalysts with Pd supported on SnO₂ possessing different morphologies,that is,nanocubes,nanoparticles and nanorods,have been prepared and used for soot combustion under tight contact condition.XRD results showed tetragonal rutile SnO₂ is the only crystalline phase detected for the catalysts.H₂-TPR,O₂-TPD and XPS results showed that the amount of the active oxygen species follows the sequence of 0.5%Pd/SnO₂-NP Cube>0.5%Pd/SnO₂-NP>0.5%Pd/SnO₂-Rod,which affects the valence states of the supported Pd species.On the surface of SnO₂-NP cube with the highest amount of active oxygen,both Pd²⁺and Pd⁴⁺are present;On the surface of regular SnO₂-NP with an medium amount of active oxygen,only Pd²⁺is present;However,on the surface of the SnO₂-Rod with the lowest amount of active oxygen,metallic Pd⁰ are present together with Pd²⁺.The soot oxidation activity of the catalysts follows the order of 0.5%Pd/SnO₂-NP Cube>0.5%Pd/SnO₂-Rod>0.5%Pd/SnO₂-NP,indicating the presnece of metastable Pd⁴⁺or metallic Pd⁰ is beneficial to the activity of the catalysts.