MnOx/SAPO-11催化剂的制备、表征及其低温NH3-SCR活性

分别采用浸渍法、柠檬酸络合法以及沉淀法在SAPO-11分子筛上负载MnO_x,制备了一系列MnO_x/SAPO-11催化剂。考察了催化剂的低温NH₃选择性催化还原(SCR) (NH₃-SCR) NO_x的性能。结果表明,沉淀法制备的负载量为20%(w)的MnO_x/SAPO-11催化剂表现出最优异的低温NH₃-SCR性能及N₂选择性。通过X射线衍射(XRD)、扫描电镜(SEM)、能量散射谱(EDS)、原子吸收光谱(AAS)、N₂吸附-脱附、X射线光电子能谱(XPS)、H₂程序升温还原(H₂-TPR)、NH₃程序升温脱附(NH₃-TPD)以及NO/O₂程序升温脱附-质谱(NO/O₂-TPD-MS)等多种表征手段对催化剂的结构及表面性质进行分析。表征结果显示,采用不同方法制备催化剂时,其表面MnO_x的存在形式和晶相结构不同。沉淀法制备的催化剂表面存在无定型态MnO_x以及MnO₂晶型,具有较大的介孔及外表面积、更多比例的Mn⁴⁺和化学吸附氧,同时表面存在对反应有利的中强酸以及强酸。因此,催化剂在低温SCR反应阶段能够生成重要中间产物NO₂,从而表现出最佳低温活性。同时,三种制备方法均能使MnO_x相对均匀分散在SAPO-11表面。SAPO-11对催化剂表面MnO_x物种的形成具有一定的影响,从而影响催化剂的酸性,拓宽了MnO_x的活性温度窗口,提高了催化剂的N₂选择性。

Preparation,Characterization and Low-Temperature NH3-SCR Activity of MnOx/SAPO-11 Catalysts

MnO_x/SAPO-11 catalysts were prepared by impregnation, citric acid, and precipitation methods for low-temperature selective catalytic reduction (SCR) of NO with NH₃. The results indicated that the MnO_x/SAPO-11 catalyst with 20%(w) Mn loading prepared by the precipitation method showed the best SCR activity and N₂ selectivity. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), atomic absorption spectrometry (AAS), N₂ adsorption-desorption, X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (H₂-TPR), NH₃ temperature-programmed desorption (NH₃-TPD), NO/O₂ temperature-programmed desorption, and mass spectrometry (NO/O₂-TPD-MS) were used to analyze the structural properties and catalytic performance of the catalysts. The results indicated that different manganese oxides were formed on the surface of SAPO-11 by the three different preparation methods. MnO_x loaded via the precipitation method existed as MnO₂ phase and amorphous MnO_x. The advantages of the catalyst prepared via this method were a large mesoporous and external surface area, the highest content of chemisorbed oxygen and Mn⁴⁺ as well as more favorable medium and strong acid sites. Thus, more NO₂ was produced on the catalyst during low-temperature SCR, which was a primary goal. MnO_x prepared by all three methods could be well-dispersed on the surface of SAPO-11. The dispersive action of SAPO-11 could affect the formation of MnO_x, which could affect the acidity of the catalysts. Thus, the temperature window was widened and N₂ selectivity was improved compared with pure MnO_x, with SAPO-11 acting as an excellent carrier.