Sn-Based Functional Materials For Efficient NO_X Storage At Low Temperatures

Low temperature NOX storage has become one of the most urgent and challenging issues in lean burn vehicle emission control. Thisworkaimed to investigate the NOX storage properties of Sn-based materials. NOX storage capacities of modified Mn-Sn and Sn-Co materials were studied. Meanwhile, physical and chemical characterization methods were used to analyze the physical structures, redox properties and NOX storage performance of the materials. The main conclusions of this thesis are:The NOX storage capacity(NSC) of Mn-Sn dramaticallyincreased after incorporating Ce. The catalyst with 4:5:1molar ratioof Mn/Sn/Ce(denotedMn0.4Sn0.5Ce0.1) exhibited a NSC of 435.37 μmol/g at 100 oC.Combinedwith physical and chemical characterizations, three main functions of Ce incorporationwere found.Firstly, it reduced the crystal size and increased BET area of Mn-Sn-Ce, leading to more adsorption sites during NOX storage. Secondly, NO oxidation ability of Mn-Sn-Ce was enhanced because Ce incorporation increased the concentration of active oxygen species on the surface.Last but not least, the Sn-Ce synergy produced more NOX adsorption sites.K was doped into Mn0.4Sn0.5Ce0.1 by co-precipitation and wet impregnation methods. Experimental results revealed a decrease in NOX capacity in K/MnSnCe prepared by impregnation method at 100 oC. However, 0.2K-MnSnCe prepared by co-precipitation method demonstrated a much higher NOX capacity of 479.6 μmol/g. Combined with other characterization results, I deduced that large amounts of KNO3 and K2 O were formed on the surface when impregnation method was used. Consequently, the pores and channels in the material were blocked, specific surface area was lowered, NOX storage sites were lost. Nevertheless, potassium can be more evenly dispersed in Mn Sn Ce prepared by co-precipitation method. Accordingly, the original NOX storage sites were not affected. In addition, potassium also provided some basic sites for NOX storage. Moreover, this work investigated the influence of actual reaction atmosphere on the materials ’ low temperature NOX storage performances. Research results showed that factors such as reaction temperature, oxygen concentration, steam and hydrocarbons in the exhaust all affected the NOX storage properties.Since Mn tends to react with SO2 to form sulfates with low solubility, this work did a preliminary study on the NOX storage performance of Sn-Co-X(X=Fe or La or Ce). Experimental results exhibit ed that Sn-Co-Fe still had a NOX capacity of 308.83μmol/g at 100 oC after pretreatment with SO2. The four materials demonstrated a sulfur resistance property of Sn-Co-Fe> Sn-Co-La >Sn-Co-Ce >Sn-Co. XPS results showed that the Sn-Co-Fe material still had a high concentration of active oxygen species after SO 2 pretreatment, which facilitated NOX storage processes.