The Preparation Of SnO₂-Based Catalysts Materials For Reduction Of NO_x By Propylene

Nitrogen oxides?NO_x?coming from different sources are harmful to both human health and environment.Selective catalytic reduction?SCR?of NO_x is one of the most efficient ways to eliminate their pollution from both mobile and stationary sources.For gasoline car emissions,NO_x can be reduced into N₂ by unburned hydrocarbons in the presence of oxygen on the three way catalytic systems.Pt,Pd and Rh supported on ceria zirconia solid solution supports are the major active components for the three way catalysts.Due to the limited source and high prices of noble metals,there is always strong incentive to find cheaper but competitive catalysts based on non-noble metal oxides,or at least reduce the amount of noble metals.Therefore,in the past several decades,non-noble metal oxide catalysts have attracted much attention and been extensively investigated.In this thesis,based on monolayer dispersion theory,SnO₂-based catalysts with high activity and good stability have been designed and studied for selective reduction of NO_x.Both XRD and XPS extrapolation methods have been used to quantify the monolayer dispersion capacity of SnO₂ on different supports.The physicochemical properties of the catalysts have been characterized by means of XPS?SEM?ICP?NH₃-TPD?H₂-TPR?In site DRIFT.The structure-reactivity relationship of the catalysts has been investigated and elucidated.The main results are summarized here:A series of SnO₂/Beta catalysts with different SnO₂ loadings on Beta zeolite support have been fabricated by using deposition-precipitation method and evaluated for NO_x-SCR by C₃H₆.With XRD extrapolation method,the monolayer dispersion capacity of SnO₂ on Beta was revealed to be 0.29 mmol 100m^-2,which equals 16%?wt.?SnO₂ loading on Beta surface.Based on the reaction performance,it is concluded that there presents an apparent monolayer dispersion threshhold effect on SnO₂/Beta catalysts for NO_x-SCR by C₃H₆,among which 15%SnO₂/Beta shows the best activity.Characterization results have demonstrated that the catalysts with SnO₂ loadings close to the monolayer disopersion capacity possesses the largest amount of surface acidic sites.Therefore,it is concluded that the abundance of surface acidic sites determins the reaction performance on SnO₂/Beta catalysts.To elucidate structure-reactivity relationship and promote the reaction performance of SnO₂/Al₂O₃ for NO_x-SCR,a series of catalysts have been prepared by impregnation method and treated by non-thermal dielectric barrier discharge?DBD?plasma.It is quantified that SnO₂ disperses on?-Al₂O₃ with a monolayer capacity of0.262 mmol 100 m^-2.The catalyst with a SnO₂ loading close to the capacity displays the best activity,testifying the presence of a monolayer threshold effect.Plasma treating can improve SnO₂ dispersion evidently,thus expanding the monolayer capacity to0.340 mmol 100 m^-2.As a result,the surface Sn cations are enriched,which improves the surface acidity of the catalysts,benefits the adsorption and activation of NO and C₃H₆ molecules,and eventually enhances the activity.It is concluded that the surface acidic site amount determines the reaction activity of catalysts.Over SnO₂/Al₂O₃catalysts,NO_x-SCR by C₃H₆ follows a Langmuir-Hinshelwood mechanism.Surface monodentate nitrate and formate are found to be the major active intermediates for the reaction.