| AVS 60th International Symposium and Exhibition | |
| Surface Science | Tuesday Sessions |
| Session SS+AS-TuM |
| Session: | Synthesis, Structure and Characterization of Oxides |
| Presenter: | A. Schaefer, University of Bremen, Germany |
| Authors: | A. Schaefer, University of Bremen, Germany J.-H. Jhang, University of Bremen, Germany W. Cartas, University of Florida S.R. Epuri, University of Florida M.H. Zöllner, IHP, Germany H. Wilkens, University of Osnabrück, Germany T. Schroeder, IHP, Germany J.F. Weaver, University of Florida M. Bäumer, University of Bremen, Germany |
| Correspondent: | Click to Email |
The rare earth oxides (REOs) have potentially versatile applications in heterogeneous catalysis. Catalysis of reactions like dehydrogenation and selective oxidation of organic compounds, methane conversion to syntheses gas or applications in three way catalysis may be mentioned as examples. The versatility of the REOs is based on the ease of valency changes of rare earth metals (change of f-electron configuration) as compared to, e.g., that of d-block elements. In particular, the possible dioxides in the REO series (ceria, praseodymia and terbia) are highly interesting materials for oxidation catalysis due to their ability to easily give off oxygen and switch to different oxidation states. As lattice oxygen participates directly in the molecule-surface reactions (cf. the Mars-van Krevelen mechanism), the selectivity of REO catalysts in oxidation chemistry depends strongly on their oxygen storage/release capabilities. The oxide of samarium for instance, which only forms the sesquioxide Sm2O3, seems to be the most effective REO catalyst for partial (selective) oxidation of methane.
In the recent years considerable progress has been made in the study of ceria thin films and oxide structures on a variety of transition metal substrates. Surface Science studies of REO films other than ceria are however fairly limited. In this context we will present two approaches to study other complex REOs like samaria, praseodymia and also terbia in ultra high vacuum (UHV). The presentation concerns a more classical approach for the preparation of samaria and terbia on a Pt(111) single crystal by reactive physical vapor deposition. Praseodymia thin films on the other hand were prepared on a Si(111) substrate by molecular beam epitaxy and new routes for adjusting the oxidation state of those films will be presented. By means of plasma treatment, e.g., we achieved to prepare even the dioxide of praseodymium (PrO2), a complex oxide which has not been available for UHV studies up to now. In combination with temperature-programmed desorption of oxygen a range of different oxidation states from PrO2 to Pr2O3 can be adjusted and made accessible for model studies. This concept has been expanded to ceria on Si(111) as well, making it possible to even stabilize the surface of the iota-phase (Ce7O12), which contains ordered oxygen vacancies, for UHV model studies.
While it is a common concept in microelectronics and adjacent fields to dope and mix
oxides to tune their properties this concept has not yet entered the world of surface
catalytic studies of thin films to a large extend. The talk will close with an outlook and first results on the mixture of REOs for UHV model studies.