In recent years, the formation of thin, well-ordered but complex surface oxides on late transition metals has been discovered. The driving force for this line of research has been the strong incentive to increase the partial pressure of oxygen from ultra-high vacuum to conditions more relevant for heterogeneous catalysis. Here we review the present status of the research field. Compared to oxygen adatom superstructures, the structure of the surface oxides has proven to be extremely complex, and the investigations have therefore relied on a combination of several experimental and theoretical techniques. The approach to solving the structures formed on close-packed surfaces of Pd and Rh is presented in some detail. Focusing on the structures found, we show that the surface oxides share some general properties with the corresponding bulk oxides. Nevertheless, of all surface oxide structures known today, only the two-dimensional surface oxides on Pd(100) and Pt(110) have the same lattice as the bulk oxides (PdO and PtO, respectively). In addition to two-dimensional oxides, including the O-Rh-O trilayers found on Rh, one-dimensional oxides were observed at ridges or steps of open surfaces such as (110) or vicinal surfaces. Finally, we briefly report on a few studies of the reactivity of surface oxides with well-known structure.
- Atom- och molekylfysik och optik