Preparation of MnOx–CeO2–Al2O3 mixed oxides for NOx-assisted soot oxidation: Activity, structure and thermal stability

Abstract

Three kinds of MnOx–CeO2–Al2O3 mixed oxides were prepared by introducing Al2O3 to MnOx–CeO2 at different stages of a sol–gel method. MnCeAl and MnCe/Al were received by adding aluminum nitrate and alumina powders to the Mn–Ce precursor solution, respectively, while MnCe+Al was obtained by mixing the sol–gel-synthesized MnOx–CeO2 and Al2O3 powders mechanically. The catalysts were calcined in air at 800°C for 20h to obtain the thermally aged samples. The catalysts were characterized by soot and NO temperature-programmed oxidation (soot-TPO and NO-TPO), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), extended X-ray absorption fine structure (EXAFS), energy dispersive spectroscopy (EDS) and H2 temperature-programmed reduction (TPR). The introduction of Al2O3 degrades the soot oxidation activity of the fresh catalysts compared with MnOx–CeO2 mixed oxides, but it helps to maintain relatively high catalyst surface area and high MnOx dispersion after the rigorous aging treatment. Among the catalysts prepared, MnCe/Al exhibits the highest thermal stability with a small increase of Tm by 17°C for soot oxidation after aging. The active surface area, which refers to the exposure area of MnOx on the catalyst, is suggested to determine the NO oxidation and soot oxidation activities of the aged catalysts.