Performance of K-promoted hydrotalcite-derived CoMgAlO catalysts used for soot combustion, NOx storage and simultaneous soot-NOx removal

A series of K-promoted hydrotalcite-derived CoMgAlO catalysts were synthesized by coprecipitation. Their catalytic performance for soot combustion, NOx storage and simultaneous soot-NOx removal was evaluated, respectively. The techniques of TG/DTA, BET, XRD, EXAFS, XPS and in situ DRIFTS were employed for catalyst characterization. When K is added to the catalyst CoMgAlO, soot combustion is largely accelerated, with the temperature (T_m) for maximum soot conversion lowered by at least 50 °C. Moreover, the NOx reduction by soot over the catalysts calcined at 500-700 °C is also facilitated. The K-containing catalyst calcined at 600 °C shows not only the highest soot combustion rate, but also the maximum NOx reduction percentage of 32%, which is attributed to its high surface K/Co atomic ratio and the strong interaction between K and Co. In situ DRIFTS results reveal that NO is readily oxidized to NO₂ and stored as nitrates over K-promoted catalysts, which could be reduced by soot more efficiently. Based on these investigations, a reaction pathway for soot combustion, NOx storage and simultaneous soot-NOx removal is proposed.