Co–Mn–Al Nonstoichiometric Spinel-Type Catalysts Derived from Hydrotalcites for the Simultaneous Removal of Soot and Nitrogen Oxides

A series of Co-Mn-Al nonstoichiometric spinel-type oxides were synthesized from hydrotalcites precursors prepared through co-precipitation method, and their catalytic activities for the simultaneous removal of soot and NOx were investigated. The solids were characterized by XRD, BET, H2-TPR, in situ FTIR and TPO techniques. Manganese carbonate was formed during the hydrotalcite synthesis for Mn containing precursors. Thermal decomposition at 800 °C led to nonstoichiometric spinel-type oxides for Co containing mixed oxides with specific surface areas ranged from 16 to 25 m2/g. CoMnAl ternary oxides exhibited much higher NOx storage capacities than CoAl and MnAl binary oxides, which may be due to the improved reducibility. The presence of NO in the feed gas significantly enhanced the soot oxidation activity. A cooperative effect between Co and Mn in the spinel oxides was found for the simultaneous catalytic removal of soot and NOx. The in situ FTIR spectroscopy over catalyst-soot mixtures in O2 indicated the presence of surface oxygen complexes (SOC), existing mainly as carboxyl species and phenol groups, in soot oxidation process. In the case of O2/NO atmosphere, NO was easily oxidized to NO2 over the mixed oxides. NO2, a more powerful oxidant, acts as an intermediate which oxidizes soot to CO2 at a lower temperature with itself reduced to NO or N2, resulting in the high catalytic performances in simultaneous soot-NOx removal.