Hydrotalcite-Derived MnxMg3−xAlO Catalysts Used for Soot Combustion, NOx Storage and Simultaneous Soot-NOx Removal

Abstract

The hydrotalcite-based Mn(x)Mg(3-x)AlO catalysts with different Mn:Mg atomic ratios were synthesized by coprecipitation, and employed for soot combustion, NOx storage and simultaneous soot-NO(x) removal. It is shown that with the increase of Mn content in the hydrotalcite-based Mn(x)Mg(3-x)AlO catalysts the major Mn-related species vary from MnAl(2)O(4) and Mg(2)MnO(4) to Mn(3)O(4) and Mn(2)O(3). The catalyst Mn(1.5)Mg(1.5)AlO displays the highest soot combustion activity with the temperature for maximal soot combustion rate decreased by 210 degrees C, as compared with the Mn-free catalyst. The highly reducible Mn(4+) ions in Mg(2)MnO(4) are identified as the most active species for soot combustion. For NO(x) storage, introduction of Mn greatly influences bulk NO(x) storage, with the adsorbed NO(x) species varying from linear nitrites to ionic and chelating bidentate nitrates gradually. The coexistence of highly oxidative Mn(4+) and highly reductive Mn(2+) in Mn(1.0)Mg(2.0)AlO is favorable to the simultaneous soot-NO(x) removal, giving a NO(x) reduction percentage of 24%. In situ DRIFTS reveals that the ionic nitrate species are more reactive with soot than nitrites and chelating bidentate nitrates, showing higher NO(x) reduction efficiency.