High Performance of Mn-Doped MgAlOx Mixed Oxides for Low Temperature NOx Storage and Release

Abstract

NOx storage-reduction (NSR) is a potential approach for the effective removal of NOx under the lean conditions in lean-burn engines. Herein, manganese-doped mixed oxides (Mn/MgAlOx) with high performance for low temperature NOx storage and release were derived from hydrotalcites precursors prepared by a facile coprecipitation method. The catalysts were characterized by X-ray diffraction (XRD), SEM, N2 adsorption-desorption, H2-TPR, FT-IR, and X-ray photoelectron spectroscopy (XPS) techniques. The Mn-doped MgAlOx catalysts exhibited high NOx storage capacity (NSC) at low temperature range (150–300 °C), which was related to their increased surface area, improved reducibility and higher surface Mn3+ content. The largest NSC measured, 426 μmol/g, was observed for NOx adsorption at 200 °C on Mn15 catalyst (the sample containing 15 wt% of Mn). The in situ DRIFTS spectra of NOx adsorption proved that the Mn-doped hydrotalcite catalysts are preferred for low temperature NOx storage and release due to their ability to store NOx mainly in the form of thermally labile nitrites. NSR cycling tests revealed the NOx removal rate of Mn15 sample can reach above 70% within the wide temperature range of 150–250 °C. Besides, the influence of CO2, soot, H2O and SO2 on NOx storage performance of Mn15 catalyst was also studied. In all, owning to their excellent NOx storage capacity, NSR cycling performance, and resistance to CO2, soot, SO2 and H2O, the Mn-doped MgAlOx NSR catalysts have broad application prospects in NOx control at low temperatures.