Preparing Highly Efficient Catalysts For Soot Combustion For Diesel Cars

Due to their high efficiency and low operating cost,diesel engines have achieved a wide range of applications.Nevertheless,soot particulate materials?PM?emitted from diesel engines can cause severe environmental and health problems for human being.Catalytic combustion is currently an effective way to eliminate the pollution of PM.Seeking highly efficient non-noble metal catalysts to replace noble catalysts is always the important research topic in this field.In this thesis,the reaction performance of CuO-based catalytic materials for PM combustion has been studied.First,the structure-reactivity of pure CuO for PM combustion has been systematically investigated.On the basis of this,several supported CuO catalyst systems have been explored.By using different characterization techniques,the bulk and surface properties of the catalysts have been elucidated,which are correlated with the reaction performance.The major parts of the thesis are summarized here:1.To gain fundamental understanding on the catalytic property and the structure-reactivity relationship of individual CuO for soot combustion,a series of CuO samples have been prepared with a simple precipitation method by using different Cu precursors and precipitants.By changing the precursors and precipitants,the morphology and texture property of the CuO samples can be influenced.Furthermore,the redox property of the CuO catalysts can also be altered.Soot-TPR results have testified that four types of active sites with varied activity are present on the surface of all the samples,but only the two types of active sites that can be reduced by soot particles below 500 ^oC are believed to contribute effectively to soot combustion.The results of quantitfied XPS analysis show that the surface oxygen mobility and abundance are the crucial factors to determine the soot combustion activity of CuO catalysts.Compared with other catalysts,CuO-Cu?NO₃?₂-Na₂CO₃,a CuO catalyst prepared by the combination of Cu?NO₃?₂ precursor and aqueous Na₂CO₃ precipitant,not only exists the surface peroxide(O₂^2–),but also generates a large number of superoxide?O₂^–?.Because of the optimal balance of these two factors,CuO-Cu?NO₃?₂-Na₂CO₃,displays the highest soot combustion activity among all the catalysts.2.With the objectives to design and prepare catalysts with better performance for soot combustion,on the base of the monolayer dispersion theory,the catalysts with different CuO loading were prepared by impregnation method in this part.The monolayer dispersion threshold measured by XRD and XPS extrapolation method is nearly consistent within the experimental error range,which is 2.09 mmol 100 m^-2,that is,4.8 wt.%CuO loading.Below this loading,no crystalline CuO can be detected by XRD due to the formation of sub-monolayer or monolayer dispersed CuO.When the CuO loading increased to 5 wt.%,CuO crystal phases could be observed on the surface of SnO₂ by XRD,which was also confirmed by stem-mapping analysis.The TPO results of soot combustion showed that,comparing with the unmodified SnO₂support,the loading of the active component CuO accelerated the combustion of soot.The catalysts with CuO loading above the monolayer dispersion capacitydisplay the highest soot combustion activity,an apparent monolayer dispersion threshold effect is observed over CuO/SnO₂ catalysts.Raman results have testified that the interactions of monolayer CuO and crystalline CuO with SnO₂ support are different.It is concluded that the amount of surface active oxygen sites is the major factor to decide the activity of the catalysts.TPO tests testify that the SnO₂ support can obviously improve the stability of CuO.3.Based on the results achieved in the last two parts,and considering the fact that the surface of CeO₂ owns a large amount of active oxygen species,a series of CuO/CeO₂ catalysts with different loadings have been prepared,with the target to design and prepare more applicable catalysts for PM combustion.The structures of the catalysts have been characterized by different techniques,which are correlated with their reaction performance.By using XRD and XPS extrapolation methods,it is disclosed that CuO disperses finely on the CeO₂ support to form a monolayer with a capacity of 1.09 mmol 100 m^-2,which equals to 2.9 wt.%CuO loading.When CuO loading is below the capacity,it is in a sub-monolayer state.However,when the loading is above the capacity,CuO micro crystallites will be formed and co-exist with the CuO monolayer.Soot combustion activity of the catalysts increases with the CuO loading until it reaches the monolayer dispersion capacity.Further increasing the CuO loading has no evident influence on the activity.Raman results have testified that with the addition of CuO onto CeO₂ support,surface active oxygen species can be formed,whose amount also increases significantly with the increasing of CuO loading until it reaches the monolayer dispersion capacity.Further increasing the CuO loading has no evident impact on the surface oxygen amount.Therefore,an apparent monolayer dispersion threshold effect is observed for soot combustion over CuO/CeO₂ catalysts.It is concluded that the amount of surface active oxygen sites is the major factor to decide the activity of the catalysts.