Enhanced soot combustion over partially substituted hydrotalcite-drived mixed oxide catalysts CoMgAlLaO

Abstract

A series of hydrotalcite-derived multicomponent oxide catalysts Co2.5Mg0.5Al1–x%Lax%O (denoted as CMALax, x=0, 5, 8, 10 or 15) with different La contents were synthesized by co-precipitation, and employed for catalytic soot combustion and NOx storage at lean condition. It is found that the La-substituted catalysts are more active than the one without La for soot combustion and NOx storage. Among all the catalysts CMALax, CMALa10 (x=10) exhibits the highest catalytic performance, showing the largest NOx storage capacity (300μmol/g) and the lowest temperature for the maximal soot oxidation rate (Tm=388°C). The results of H2-TPR and O2-TPD indicate that the La-substitution can enhance the redox properties and increase the amount of surface lattice oxygen species of the catalysts. The results of soot combustion kinetics show that the apparent activation energy of CMALa10 is the lowest (92.75kJ/mol). Based upon the catalytic performance and in situ DRIFTS characterization, a nitrate and/or NO2 enhanced soot combustion mechanism over CMALa10 was proposed. It is revealed that at low temperature the stored nitrate can directly promote soot ignition, while at high temperature the NO2 species arising from NO oxidation or nitrate decomposition act as the main active species for soot combustion.