Simultaneous soot combustion and nitrogen oxides storage on potassium-promoted hydrotalcite-based CoMgAlO catalysts

A series of potassium-promoted hydrotalcite-based CoMgAlO mixed oxide catalysts used for simultaneous soot combustion and nitrogen oxides storage were prepared by impregnation method. The techniques of TG/DTA, XRD, H2-TPR and in situ DRIFTS were employed for catalyst characterization. Over the catalyst containing 7.5% or 10% K, the soot ignition temperature (Ti=260 degrees C) and total removal temperature (Tf=390 degrees C) are decreased by 180 degrees C and 273 degrees C, respectively, as compared with the uncatalyzed reaction. The results of kinetic calculation show that the presence of K-promoted catalysts decreases the activation energy of soot combustion from 207kJ/mol to about 160kJ/mol. When 400ppm NO is introduced, lower characteristic temperatures or higher reaction rate for soot oxidation is achieved. Simultaneously, relatively larger nitrogen oxides storage capacity is obtained. It is revealed by H2-TPR that the addition of K increases the amount of active Co sites and the mobility of bulk lattice oxygen due to the low melting point of K-containing compounds, the low valence of K+ and the strong interaction between K and Mg(Al). For nitrogen oxides storage, different routes via chelating bidentate nitrates, monodentate nitrates and ionic nitrates are confirmed by in situ DRIFTS over the CoMgAlO catalysts with potassium loadings of 0, 1.5 and 7.5%, respectively.