Zr改性MnO_x/MWCNTs催化剂的结构特征与低温SCR活性

Structural Properties and Low-Temperature SCR Activity of Zirconium-Modified MnO_x/MWCNTs Catalysts

以ZrO(NO3)2·2H2O为前驱体对多壁碳纳米管(MWCNTs)进行了改性并负载MnOx制备了MnOx/ZrO2/MWCNTs催化剂.考察了Zr对催化剂低温选择性催化还原(SCR)反应活性的影响,并通过多种分析手段对催化剂的结构进行了表征.结果表明Zr的添加对催化剂的低温SCR活性具有显著的促进作用,当Zr负载量为30%时,催化剂活性最佳.X射线衍射(XRD)、拉曼(Raman)光谱、透射电镜(TEM)、N2吸附-脱附的表征结果分析表明,适量的Zr改性促进了MnOx在载体表面的分散,增强金属氧化物与MWCNTs之间的作用,也能增加催化剂的比表面积、孔容和孔径.X射线光电子能谱(XPS)、H2程序升温还原(H2-TPR)和NH3程序升温脱附(NH3-TPD)的分析结果则显示,Zr能提高催化剂表面化学吸附氧浓度,促进Mn3+转化为Mn4+,从而使催化剂表面的活性位点增多,氧化还原能力增强,同时还提高了催化剂表面酸性位点的数量和强度,促进了NH3的吸附,是MnOx/ZrO2/MWCNTs催化剂低温SCR活性提高的主要原因.

A series of ZrO2/MWCNTs were prepared, using ZrO（NO3）2.2H2O as a precursor, by the surface modification of multiwalled carbon nanotubes （MWCNTs）. Manganese oxides were supported on the ZrOJ MWCNTs to prepare MnOx/ZrO2/MWCNTs catalysts. The effect of zirconium on the selective catalytic reduction （SCR） activity of the catalysts was investigated. Furthermore, the structural properties of the catalysts were comprehensively characterized by a suite of analytical methods. The results show that the addition of zirconium improved the SCR activity of the MnOJMWCNTs significantly and the catalyst with 30% Zr loading was found to have the highest activity. X-ray powder diffraction （XRD）, Raman spectroscopy, transmission electron microscopy （TEM）, and N2 adsorption-desorption results revealed that the modification of zirconium could enhance the dispersion of MnOx on the support as well as enhance the interaction between the metal oxides and the MWCNTs. Additionally, zirconium could also increase the specific surface area, the total pore volume, and the average pore size of the catalysts. Moreover, from the results of X-ray photoelectron spectroscopy （XPS）, H2 temperature-programmed reduction （H2-TPR）, and temperature-programmed desorption of NH3 （NH3- TPD）, zirconium increased the atomic concentration of the chemisorbed oxygen on the catalysts surface and promoted the conversion of Mn^3＋ to Mn^4＋. Therefore, the surface-active sites increased and the redox ability of the catalysts improved. Additionally, the amount and strength of acid on catalyst surface increased. These factors are the main reason for the MnOx/ZrO2/MWCNTs catalysts having better low-temperature SCR activity.