Catalytic removal of NOx and diesel soot over nanostructured spinel-type oxides

Nanostructured spinel-type oxide catalysts AB2O4 (where A = Co and Mn, and B = Cr and Fe), prepared by the solution combustion synthesis method and characterized by BET, XRD, FESEM, TEM, FTIR, 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 of 350–450 °C. The activity order for soot combustion was found to be CoCr2O4 > MnCr2O4 > CoFe2O4, whereas the activity order for NOx reduction was CoFe2O4 > CoCr2O4 > MnCr2O4. The best compromise between simultaneous abatement of soot and nitrogen oxide 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 basis of oxygen temperature-programmed desorption, the prevalent catalytic combustion 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 of 300–500 °C. Tentative reaction pathways for the simultaneous reduction of NOx are outlined as well.