Catalytic activity of Cu and Co supported on ceria-yttria-zirconia oxides for the diesel soot combustion reaction in the presence of NOx

Abstract

Copper and cobalt oxides supported on ceria-yttria-stabilized zirconia (Ce-YSZ) have been studied as an alternative to noble metals for the removal of soot from diesel engines. The Ce-YSZ support was activated in order to obtain a catalyst with a high redox performance. The resulting catalysts were characterised by N2 physisorption, ICP, XRD, TPR-H2, TPR-CO, HAADF-STEM and XEDS. The catalytic performance of the materials was evaluated in the oxidation of NO to NO2 and in the combustion of soot in the presence/absence of NOx. The Co/Ce-YSZ catalyst showed a maximum NO2 production activity higher than 70%, i.e., very close to that obtained with a commercial Pt catalyst used as a reference and significantly better than that of Cu/Ce-YSZ. A similar trend was observed in the combustion of soot in the presence of NOx, thus indicating the prevalence of the NO2-assisted mechanism for the oxidation of soot when NO2 is present. Results obtained with unsupported Co3O4 and CuO oxides suggest that a Co-containing catalyst is not only a better NO2 generator but also utilises NOx more efficiently. Co3O4 entities would act as a pseudo-platinum phase that would provide a fresh supply of NO2 along the catalytic bed, thus accelerating the soot combustion reaction. However, if NOx is absent, the order of the soot combustion rate matches the performance in reducibility (‘active oxygen’ mechanism), with the Cu catalyst being more active than Co or Pt. Under these conditions the Ce-YSZ supported catalysts benefit from the excellent oxygen-exchange properties of surface Ce-Y-Zr oxide nanostructures.