In situ-Studien zu Wachstum und Struktur seltenerdoxidbasierter inverser Modellkatalysatoren

Abstract

Rare earth oxides in combination with transition metals are under intense investigation due to their versatile catalytic properties. Even though especially ceria is already successfully used in today's catalytic converters, many fundamental aspects of their active role in catalytic processes are still unclear. Using well-defined metal-on-oxide and so-called inverse oxide-on-metal model catalysts is a widely accepted approach to gain insights into the active nature of rare earth oxides. Whereas the preparation of (111) oriented rare earth oxide inverse model catalysts are quite standard today, the synthesis of inverse model catalysts with different orientations, which are suspected to be different in activity and selectivity in chemical reactions, on well established transition metal surfaces is still challenging. Enabled by time-resolved low-energy electron microscopy the reactive growth of the rare earth oxides of cerium and terbium on Cu(111) as well as the growth of ceria on Ru(0001) are investigated. Both oxides grow on the Cu(111) surface in a Volmer-Weber growth mode but in two different orientations: ceria islands exhibit (100) and (111) oriented surfaces, terbia islands grow in (112) and (111) orientation. It is demonstrated that growth of CeO2 (100) on Cu(111) depends on the Ce/O ratio on the substrate surface, permitting the exclusive growth of CeO2(100) in the low Ce/O regime and exclusive growth of CeO2 (111) in the high regime. In case of terbia a favorable lattice matching enabled by a coincidence of the TbOx (112) unit cell with the Cu(111) unit cell was supposed to facilitate formation of the high- index surface. In addition, the emergence of CeO2 (100) on Ru(0001) is presented, which is also attributed to the local oxygen chemical potential. These findings illustrate that it is possible to manipulate the orientation of rare earth oxide surfaces, which can be applied to synthesize new kinds of model catalysts. The reactivity and selectivity of ceria not only depends on the orientation of the ceria surface but also on the presence of oxygen vacancies. Thus stable phases and phase transitions from CeO2 to Ce2O3 are of special interest to understand cerias catalytic properties. Observing structural changes during hydrogen reduction of ceria grown on ruthenium in real-time using low-energy electron microscopy and micro electron diffraction as well as micro x-ray absorption spectroscopy reveal the formation of ordered oxygen vacancies. Moreover three stable phases, which coexist for intermediate oxidation states, demonstrate a spatially varying stoichiometry of the ceria surface during reduction. These results shed new light on the changes of structure in chemical reactions, which have to be considered in view of structure-function relationship.