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    Surface Science Thursday Sessions
       Session SS1-ThA

Paper SS1-ThA2
Metal and Oxide Particles on Oxide Supports: Vanadium and Vanadia Deposits on Alumina

Thursday, November 1, 2001, 2:20 pm, Room 121

Session: Catalysis on Model Systems
Presenter: M. Baeumer, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
Authors: M. Baeumer, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
N. Magg, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
J.B. Giorgi, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
M. Frank, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
H.-J. Freund, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany
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Vanadium in its various oxidation states is a catalytically very interesting system. Oxide-supported vanadia deposits are, for example, used as catalysts for the selective oxidation and dehydrogenation of hydrocarbons. In order to study this multivalent system in more detail, we have carried out model catalytic studies by depositing vanadium under various conditions onto a thin alumina film grown on NiAl(110). Under UHV conditions metallic aggregates are formed. This is connected with extensive changes in the phonon spectrum of the support (intensity damping, frequency shifts and peak broadening). A comparison to other metals, such as Pd, Rh, Ir and Al, reveals that, especially in the low coverage regime, chemical effects at the particle-support interface play an important role. This is also corroborated by XPS. At high coverages, on the other hand, metallic screening seems to gain in importance. If the V deposition is carried out in an oxygen ambient, oxide particles are formed. According to photoelectron spectroscopic data, they have an average oxidation state of +3. Nevertheless, infrared spectroscopy points to the presence of vanadyl groups on the surface of the aggregates actually being a structural element of V@sub 2@O@sub 5@. For both situations, metal and oxide deposits, the CO adsorption and reaction behaviour will be discussed. IR spectra show that CO adsorption on the vanadia aggregates leads to a blue shift of the CO stretching frequency as compared to the gas phase. Interestingly, an interaction between CO and the vanadyl groups is observed. By contrast, CO adsorption on the vanadium aggregates is connected with a red shift. A comparison to the corresponding IR data of other metals (Pd, Rh, Ir)@footnote 1@ underscores the strong metal-support interaction. @FootnoteText@ @footnote 1@M. Frank and M. Baeumer, Phys. Chem. Chem. Phys. 2 (2000) 3723.