Li改性MnO_2及LiMn_2O_4催化NH_3-SCR反应性能研究

采用高温固相反应法、Pechini合成方法和柠檬酸配位法,制备了系列锂锰复合氧化物LiMn2O4催化剂,应用于NH3-SCR反应,并与固相反应法合成的MnO2进行了比较。采用N2吸附-脱附、扫描电镜、X射线衍射、H2程序升温还原、NH3程序升温脱附、NO程序升温脱附和X射线光电子能谱对LiMn2O4催化剂进行表征。结果表明,引入Li有利于提高锰基催化剂的SCR活性和抗硫性。Pechini法制备LiMn2O4的NO转化率可在130~260℃达到90%以上;固相反应法制备LiMn2O4的NO转化率大于90%的温度为90~310℃;MnO2的温度窗口则仅为140~280℃。与MnO2相比,引入Li可形成LiMn2O4结构,因此,催化剂中更多的锰离子保持在相对较低的价态Mn3+,并调整表面活性氧含量;同时,Li的存在调变了LiMn2O4表面的酸位,从而减少高温下MnO2表面容易发生的NH3非选择性氧化,改善其催化NH3-SCR反应的温度窗口,也增强了抗硫性。

Li-modified MnO2 catalyst and LiMn2 O4 for selective catalytic reduction of NO with NH3

LiMn₂O₄ prepared by high temperature solid state reaction, pechini, and citric acid coordination methods was applied in selective catalytic reduction (SCR) of NO with NH₃. MnO₂ prepared by high temperature solid state reaction method and the activity was tested as a comparsion. The catalysts were characterized by N₂ adsorption-desorption, scanning electron microscopy, X-ray diffraction, H₂ temperature-programmed reduction, NH₃ temperature-programmed desorption, NO temperature-programmed desorption, and X-ray photoelectron spectroscopy. The results showed that high-temperature activity of SCR were improved after the introduction of Li into MnO₂. NO conversion on the LiMn₂O₄ prepared by pechini method was above 90% in the range of 130~260 ℃; NO conversion on the LiMn₂O₄ by high temperature solid state reaction method could be kept above 90% in the range of 90~310 ℃; while the temperature window of MnO₂ was only 140~280 ℃. Compared with MnO₂, LiMn₂O₄ crystal structure not only keeps more manganese cations at a relatively low valence of Mn³⁺, but also adjusts surface active oxygen. Meanwhile, the existence of Li adjusts surface-acid sites of LiMn₂O₄, thus alleviates the unselective oxidation of NH₃ in the high temperature, broadens the operating temperature window of NH₃-SCR reaction, and improves the catalyst tolerance of SO₂.