Structure, Morphology, and Catalytic Properties of CuOx/CeO2 Model Catalysts

Abstract: Cerium dioxide, or ceria, is a chemically stable oxygen ion conducting material with a capacity to store and release oxygen, and is therefore extensively used in combustion catalysts. The Cu-Ce-O system has been identified as one of the most active catalysts for the combustion of carbon monoxide. To study the catalytic oxidation of CO, we have synthesised model CuOx/CeO2 catalysts by gas phase techniques. PVD rf-magnetron sputtering was used to produce thin film catalysts. Inert gas condensation (IGC) with multiple thermal heating sources was employed to produce nanocomposite particles. This allowed us also to study the growth of thin films and particles, respectively. The catalysts were characterised by a powerful complementary combination of methods, combining small-area and whole-sample characterisation of local structure and long-range order; High resolution electron microscopy, atomic force microscopy, nitrogen adsorption, and analytical X-ray techniques, such as energy-dispersive spectroscopy, and photoelectron spectroscopy. High-energy diffraction, and X-ray absorption fine structure spectroscopy using synchrotron radiation were employed to collect information on crystallinity and the local environment of Cu. The thin CeO2 films grown on surfaces of á-Al2O3 formed extremely sharp ridges exposing exclusively {111}-type surfaces despite the nominally (001)-orientation. We found that a well-defined portion of (001) could be created by annealing. Thin layers of CuOx were deposited on top of both the as-prepared and annealed ceria thin films. The annealed films with copper on top exhibit a markedly higher activity than the others, indicating a favourable synergistic effect between the copper oxide and CeO2(100) surfaces (i.e. catalytic anisotropy). In addition, it was realised that the as-prepared films are excellent for quickly evaluating AFM tips (i.e. as tip characteriser), and for studying artefacts and tip-sample phenomena occurring during tapping mode AFM imaging. Nanocomposite CuOx/CeO2 particles were produced over the whole compositional range. A change of the copper content clearly altered the nanostructured morphology, forming e.g. crust structures between 30 and 70 at.% Cu. A growth mechanism for the formation of crust structures is suggested. The dispersion of copper that controls the number of active sites and, hence, the catalytic activity was shown to be dependent on the: i) total copper content, ii) the nanostructured morphology, and iii) the crystallinity of ceria. A copper content between 5 and 30 at.% Cu was found to be beneficial for the combustion of carbon monoxide.