Mesostructured CeO₂ and Pd/CeO₂ nanophases: Templated synthesis, crystalline structure and catalytic properties

This work reports a ceria solid and Pd/ceria catalyst prepared through a surfactant-templated synthesis route used for simultaneous abasement of NO and CO emissions. The surface features, textural properties and crystalline structure of ceria and Pd/ceria catalyst were studied by means of thermogravimetric analysis (TGA), N₂ physisorption isotherms and in situ Fourier transform infrared (FT-IR) spectroscopy, high resolution electron transmission microscopy (TEM) and X-ray diffraction (XRD) techniques. In the calcination procedure, part of the adsorbed water on the surface of the solid was derived into unidentate and bidentate hydroxyls associated with surface cationic ions of ceria. The surfactant cations were strongly interacted with the solid during the preparation, which induces defects formation in the crystalline structure of the annealed ceria. The retained surfactant in the solid could be combusted to yield CO₂, water and organic molecules with a small amount of coke-like deposits. The resultant ceria showed mesoporous texture and cubic phase containing lattice defects in the crystalline structure. The Pd/CeO₂ catalyst was very active for NO reduction via CO with a high selectivity to N₂. A 100% NO conversion with a selectivity to 100% N₂ was achieved over the Pd/ceria catalyst at a reaction temperature of 300 °C. The catalytic activity and selectivity of this catalyst are much superior to the catalysts of Pt or Rh supported on TiO₂, Al₂O₃, TiO ₂-Al₂O₃ and ZrO₂-Al ₂O₃ prepared by a sol-gel method. A possible reaction mechanism of NO reduction by CO over the Pd/CeO₂ catalyst was discussed.