Li-modified MnO₂ catalyst and LiMn₂O₄ 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.