Removal of NOx and diesel soot over catalytic traps based on spinel-type oxides

Nanostructured spinel-type oxides catalysts AB2O4 (where A = Co and Mn, and B = Cr and Fe), prepared by the solution combustion synthesis method and characterized by BET, XRD, SEM, TEM, and catalytic activity tests, proved to be effective in the simultaneous removal of soot and NOx, the two prevalent pollutants in diesel exhaust gases in the temperature range 350–450 °C. The activity order for soot combustion was found to be CoCr2O4 > MnCr2O4 > CoFe2O4, while the activity order for NOx reduction CoFe2O4 > CoCr2O4 > MnCr2O4. The best compromise between soot and nitrogen oxide abatement was therefore shown by CoCr2O4 catalyst; it could promote soot combustion and appreciable NOx reduction below 400 °C, the maximum temperature reached in the exhaust line of a diesel engine. On the grounds of transient thermal analysis studies (TPD), the prevalent activity of the chromite catalysts could be explained by their higher concentration of suprafacial, weakly chemisorbed oxygen, which contributes actively to soot combustion by spillover in the temperature range 300–500 °C. Further studies should be aimed to better elucidate the role of the various elements in the spinel-type catalytic activity towards NOx reduction.