二氧化锰在低温NH_3-SCR催化反应上的形貌效应

Morphological Effects of Manganese Dioxide on Catalytic Reactions for Low-Temperature NH_3-SCR

利用水热法制备了三种不同形貌的MnO_2催化剂,分别为α-MnO_2纳米棒,γ-MnO_2纳米片和δ-MnO_2纳米薄膜组装的微球,考察了纳米材料的形貌结构对催化剂低温选择性催化还原(SCR)反应性能的影响,并利用X射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)、N_2吸附-脱附、H_2程序升温还原(H_2-TPR)、NH3程序升温脱附(NH3-TPD)、X射线光电子能谱(XPS)以及拉曼(Raman)光谱等表征手段对催化剂的结构进行了分析。结果表明,在50-250°C的温度范围内,γ-MnO_2纳米片表现出最优的SCR催化性能,不仅NOx转化率最高而且N_2选择性也最好。表征结果表明,比表面积并不是影响MnO_2催化性能的主导因素,纳米材料的晶型结构与表面暴露的活性晶面共同决定着催化剂的SCR性能。γ-MnO_2纳米片表面暴露的(131)晶面上不仅存在着大量的配位不饱和Mn离子,从而形成了较多的强酸性位点;而且还存在着较多的活性氧物种。这些活性位点可以使得NH_3和NO_x的活化过程在较低温度下进行。高浓度的流动性氧以及高价态的Mn^(3+)和Mn^(4+)也使得催化剂的氧化还原反应更易发生。

α-MnO_2 nanorods,γ-MnO_2 nanosheets,and δ-MnO_2 nanofilm-assembled microspheres were prepared using a hydrothermal method and evaluated as catalysts for the selective catalytic reduction（SCR）of nitrogen oxides（NOx）.They were also structurally characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）,N_2adsorption-desorption,temperatureprogrammed reduction with hydrogen（H_2-TPR）,temperature-programmed desorption of ammonia（NH_3-TPD）,X-ray photoelectron spectroscopy（XPS）,and Raman spectroscopy.The γ-MnO_2 nanosheets performed the best for the reduction of NOxand selectivity of N_2,while the α-MnO_2 nanorods performed the worst.Structural analysis indicated that the main factor determining the catalytic activities of the nanomaterials was not the specific surface area but the crystal structure and the exposed active crystals.The γ-MnO_2 nanosheets performed best because their exposed（131） planes contained multiple Mn cations in coordinatively unsaturated environments,which formed numerous strongly acidic sites.They also benefited from active oxygen species.The active sites allowed the activation of NH_3 and NO_x at lower temperatures.Moreover,high concentrations of liquid oxygen and Mn cations at high oxidation states facilitated the redox reactions.