Fabrication of ultra-thin cerium oxide layers on Ru(0001) single crystal surfaces. Scanning tunneling microscopic and photoelectron spectroscopic studies on growth, structure and properties

The thesis at hand aims at a study of structure and properties of well-defined ultrathin CeO₂ films supported on Ru(0001). Such systems may serve as model systems in heterogenous catalysis. The epitaxial growth of ceria films on Ru(0001) surface has been achieved by electron beam evaporation of metal Cer at low background oxygen pressure of 10^-6 mbar under ultrahigh-vacuum conditions at room temperature. Cerium oxide qualifies for proper oxygen-storage in oxidation reactions, and hence it widely used in heterogenous catalysis. The oxidation begins with the adsorption of CO on the CeO₂(111) surface, and it ends with participation of lattice oxygen leading to vacancy formation and CO₂ desorption. We investigate the geometric structure by means of scanning tunneling microscopy and low energy electron diffraction. The coverage of 2.5 monolayers (ML) was sufficient to cover the substrate almost completely. We further analysed the interaction of CO with the CeO₂/Ru(0001) and the Pt/CeO₂/Ru(0001) systems. During the interaction process the ratio of Ce⁴⁺ and Ce³⁺ changes significantly. This ratio change as well as the effect of Pt evaporated onto the surface with respect to the reducibility of CeO₂/Ru(0001) in CO environment has been studied by X-ray photoemission spectroscopy and it has been confirmed with thermal desorption spectroscopy. It is revealed that the Pt-Nanoparticles with a height from 7.15 Aa to 9.73 Aa clearly enhances the reducibility of CeO₂.