Model Studies on Rare Earth Oxide Thin Films : Surface Chemistry and Catalytic Properties

Abstract

Rare earth oxides (REOs) have shown considerable capacities for performing certain catalytic reactions. However, only ceria (CeOx) has been systematically studied regarding the surface chemistry and the factors for catalytic behavior. The fundamental understanding with respect to the connections between the surface chemistry and catalytic properties of other REOs is still in a very limited scope. Major part of this PhD dissertation reports on surface science studies of samaria (SmOx) conducted in ultra-high vacuum. In addition, the dissertation briefly introduces recent studies on praseodymia (PrOx), PrOx-CeOx mixed oxides, and terbia (TbOx) which were carried out by my working group (AG Bäumer) and our collaborators. Sm2O3 thin films grown on a Pt(111) substrate were employed as SmOx model systems. Structural and film morphologic studies were conducted by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Such thin films grow as a defective-fluorite structure, i.e., Sm atoms arrange into a hexagonal sublattice whereas surface vacancies randomly distribute on the thin films. A film wetting/dewetting behavior responding to the SmOx reduction/oxidation conditions was observed. To learn about the chemical/catalytic properties of the SmOx model system, carbon monoxide (CO), water (D2O), and methanol (MeOH) were dosed onto the system for adsorption-reaction experiments conducted by temperature programmed desorption spectroscopy (TPD). The MeOH-TPD study was further combined with infrared absorption-reflection spectroscopy (IRRAS) to gain the whole picture of the MeOH reaction mechanism on the model system. The influence of the SmOx reduction/oxidation conditions on the chemical responses of these adsorbates were also investigated. The existence of the perimeter sites between SmOx islands and uncovered Pt areas obviously promotes the reactivity and alters chemical properties of the SmOx model system. A spillover of adsorbed species from the SmOx surface to exposed Pt areas was observed. It is particularly important in increasing the SmOx reactivity towards the MeOH decomposition.