Morphology and structure of CuOx/CeO2 nanocomposite catalysts produced by inert gas condensation:: An HREM, EFTEM, XPS, and high-energy diffraction study

Inert gas condensation (IGC) has been employed to produce nanoparticles of the low-temperature combustion catalyst CuOx/CeO2. For the first time we have used a multiple heating crucible setup to tailor various morphologies over the whole compositional range (2-98% Cu). The factors that control the growth, structure, and morphology of the nanocomposite have been studied. A powerful combination of complementary characterization methods has been used to elucidate the catalytic synergistics of this material. Investigations by high-resolution transmission electron microscopy (HRTEM) and energy-filtered TEM (EFTEM) are supported by X-ray photoelectron spectroscopy (XPS) and high-energy diffraction (HED) measurements. The nonstoichiometric CuOx/CeO2 composite displays an amorphous character consisting of aggregated CeO2 (ceria) nanocrystallites over which amorphous copper clusters (or a thin film of a solid solution) are finely dispersed. In the range 6similar to20% Cu, copper is predominantly located at the surface, which can give the material optimum catalytic properties. Development of crust structures, for example, core-shells, are formed in the 30similar to70% Cu concentration range and is attributable to a sequential oxidation of Ce followed by Cu and an ideal proportion of lattice expansion for the oxides. We suggest a model that illustrates the formation of the crust structure and may explain the observed extreme dispersion of copper on ceria. The helium gas pressure during the thermalization controls the crystal size and the degree of crystallite aggregation. Rounded particle shapes consisting of epitaxially interfaced nanocrystallites exhibit an X-ray amorphous character, while block-shaped crystals displaying sharp edges and distinct flat surfaces give rise to a higher X-ray crystallinity. Bulk CuO crystals were detected by high-energy diffraction above a 30% Cu content. However, the extreme copper dispersion is preserved even for higher copper contents, showing no limit of surface saturation.