锰负载量对NSR催化剂Mn-K2CO3/γ-Al2O3催化性能的影响

摘要

A series of noble metal-free Mn-K2CO3/γ-Al2O3 catalysts for NOx storage and reduction (NSR) with different Mn loadings were prepared via dry impregnation.The effects of manganese loading on the structure and catalytic activity of the NSR catalysts were investigated,and the effects of H2O and CO2 on the NOx storage capacities were also investigated.The techniques,including X-ray diffraction (XRD),extended X-ray absorption fine structure (EXAFS),temperature-programmed of H2 reduction (H2-TPR) and temperature-programmed desorption of CO2 (CO2-TPD),were used to characterize the structure of the catalysts.The results showed that manganese was mainly in the forms of MnO2 and Mn3O4,and there is a small amount of Mn2O3.With the increase of Mn loading,the NOx-redox ability of the catalysts was enhanced.However,with the increase of Mn loading,the specific surface area of the catalysts decreased,resulting in the decrease of the K2CO3 distribution and the surface K2CO3 gradually turned into bulk K2CO3.The NOx storage capacity (NSC) was affected by the two factors of catalyst redox ability and K2CO3 dispersion.When the mass ratio of Mn/Al2O3 is 0.10,the catalyst has the largest NSC (1.30 mmol· g-1).After 10 cycles under lean-burn/fuel-rich conditions,the NOx reduction efficiency reached as high as 99％.The NSC of the catalyst decreased when H2O and CO2 were added to the reaction gas,and the effect of CO2 was larger than that of H2O.%采用等体积浸渍法制备了一系列非贵金属NOx储存还原(NSR)催化剂Mn-K2CO3/γ-Al2O3,考察了不同锰的负载量对NSR催化剂的结构和催化活性的影响,同时考察了水和CO2对NOx储存量的影响.应用X射线衍射分析(XRD)、X射线吸收精细结构(EXAFS)、H2程序升温还原(H2-TPR)、CO2程序升温脱附测试(CO2-TPD)等技术,对催化剂的结构进行表征.结果表明,锰主要是以MnO2和Mn3O4的形式存在,还有少量的Mn2O3.随着锰负载量的增加,催化剂对NOx的氧化还原能力增强,但催化剂的比表面积降低,使K2CO3的分散性降低,逐渐由表相K2CO3向体相K2CO3转变.NOx的储存量受到催化剂氧化还原能力和K2CO3的分散性两方面因素的影响.当Mn/Al2O3的质量比为0.10的时候,NOx的储存量最大,达到1.30 mmol·g-1,稀燃/富燃条件下,10个循环后,对NOx的还原效率达到99％以上.向反应气中加入水和CO2后,储存能力下降,其中CO2对储存量的影响比水更大.