|Thema:||Untersuchungen zur Struktur-Reaktivitäts-Beziehung von Vanadiumoxid-Aggregaten auf geordneten Übergangsmetalloxidoberflächen|
|Fachgebiete und Arbeitsrichtung:||Chemische Physik, Oberflächen, Modellkatalyse|
During the previous period, we focused on the
preparation and structural properties of vanadia nanoparticles deposited on
thin oxide films such as alumina and silica. For these studies, we have employed
a unique experimental setup combining LEED, STM, XPS, TPD and IRAS facilities
in one UHV chamber. In particular, we showed that the vanadia/support interface
might play an important role in the reactivity of the vanadia based catalysts.
It has turned out however, that preparation of the well ordered silica films
is extremely difficult and time-consuming, therefore, to date, the studies on
adsorption properties of the vanadia particles were limited to CO as a probe
molecule. In the forthcoming period, two main goals will be pursued: a) investigation
of the reactivity of the supported vanadia particles towards catalytically relevant
molecules such as methanol and propane; b) the studies on the origin of the
support effects, reported in the literature, on the properties of vanadia particles.
Adsorption behaviour of vanadia will be studied by infrared spectroscopy in
combination with a temperature programmed desorption. We anticipate that the
reactivity of the vanadia/alumina(silica) systems under the UHV conditions is
low, therefore, the studies under the higher pressure conditions become necessary.
Currently, we are building a high-pressure cell for performing IRAS studies
at the mbar-pressure range. The cell will be also used for studying effects
of high O2 pressure on the surface structure of the vanadia particles.
It is well documented that catalytic properties of vanadia strongly depend on
the oxide support. For example, turnover numbers for vanadia catalysts in methanol
oxidation are about two orders of magnitude higher when supported on zirconia,
titania and niobia as compared to alumina and silica. In other words, the reducible
oxide support dramatically changes the reactivity of vanadia particles in reactions
involving oxygen. We are going to study these effects by comparing structure
and reactivity of vanadia/alumina system with vanadia deposited on the reducible
oxide films. It has been reported that well-ordered titania and ceria films
can be grown, for example, on a Ru(0001) substrate.
The results on the supported vanadia particles will be compared to those obtained for the thick vanadia films studied in project C1 (H.-J. Freund, H. Kuhlenbeck). The intense collaboration with the project C5 (J. Sauer, V. Ganduglia-Pirovano) will continue?