Contributions of surface and bulk heterogeneities to the NOoxidation activities of ceria–zirconiacatalysts with composition Ce0.76Zr0.24O2 prepared by different methods

Abstract

The study of the catalytic activity towards NO oxidation to NO₂ was approached by using ceria–zirconia mixed oxides with the same nominal composition (Ce_0.76Zr_0.24O₂) but prepared by different routes of synthesis: coprecipitation, solid combustion synthesis with urea, citrate complexation route, reversed microemulsion and template synthesis. The characterisation of the catalysts was performed by N₂ adsorption at −196 °C, XRD, Raman Spectroscopy, H₂-TPR and XPS in order to ascertain the relationships between their catalytic activities and their bulk and surface properties. The results showed that the preparation method is critical for the physico-chemical properties of the mixed oxides, exhibiting very different BET surface areas, crystalline phase/s contributions and bulk oxygen mobility. The distribution of Ce and Zr on the surface with regard to the bulk is very much influenced by the preparation method as well. The NO₂ production from NO oxidation was shown to be mostly correlated with the Ce/Zr surface atomic ratio and the proportion of Ce⁴⁺ (presumably in a doped cubic phase) in the uppermost layers.