Oxidation of some Late Transition Metal Surfaces: Structural Studies from UHV to Atmospheric Pressure

Sammanfattning: The oxidation of Rh, Pd and Pt is studied under conditions ranging from ultra high vacuum to ambient pressures, with emphasis on a complete understanding of the geometry of the various oxide structures found. The investigations use a combination of various experimental methods, including high resolution core level spectroscopy, low energy electron diffraction, surface x-ray diffraction and scanning tunneling microscopy, as well as density functional theory calculations. It is shown that two-dimensional so-called surface oxides are formed on Rh(111), Rh(100) and Pd(100) at intermediate pressures. While the surface oxide on Pd(100) resembles an ultra-thin film of PdO(101), the surface oxides on Rh do not correspond to any known bulk Rh oxide. Instead a hexagonal tri-layer of O-Rh-O was found on both the (111) and the (100) surface despite the difference in substrate structure. These surface oxides are found to be surprisingly stable, and only transform into bulk oxides when significantly higher pressures have been reached. The effects of defects in the form of steps on the oxidation process are studied using vicinal surfaces, consisting of flat areas separated by periodically ordered monatomic steps. While no surface oxides are found on Pt surfaces, a one-dimensional oxide structure is found along the step edges on Pt(332). On Rh(553) a similar one-dimensional oxide structure is found at pressures slightly lower than those needed to form the surface oxide on the flat surfaces. In conjunction with the formation of the one-dimensional oxide, the Rh(553) surface undergoes a major rearrangement into (331) facets, where the oxide resides, and larger (111) terraces covered by chemisorbed oxygen. The catalytic activity of the Rh(111) surface oxide and the one-dimensional Pt(332) oxide is studied through reduction by CO. In both cases the activity is found to be higher than on the same surface covered by chemisorbed oxygen. In the Rh(111) case, however, a certain incubation period is needed before the reaction starts, due to low amount of available adsorption sites for CO on the well-ordered oxide film.