AVS 52nd International Symposium
    Surface Science Monday Sessions
       Session SS2-MoA

Paper SS2-MoA3
Structural Characterisation and Reactivity of V@sub2@O@sub3@(0001) Thin Films

Monday, October 31, 2005, 2:40 pm, Room 203

Session: Oxide Surfaces Structure and Reactivity
Presenter: S. Guimond, Fritz Haber Institute of the Max Planck Society, Germany
Authors: S. Guimond, Fritz Haber Institute of the Max Planck Society, Germany
M.A. Haija, Fritz Haber Institute of the Max Planck Society, Germany
A. Uhl, Fritz Haber Institute of the Max Planck Society, Germany
H. Kuhlenbeck, Fritz Haber Institute of the Max Planck Society, Germany
H.J. Freund, Fritz Haber Institute of the Max Planck Society, Germany
Correspondent: Click to Email
Vanadium oxides are used as catalysts for several oxidation reactions, including the selective oxidation of hydrocarbons and the oxidative dehydrogenation of alkanes to olefins. In spite of their importance, many questions about the molecular structure and the reaction mechanisms at their surfaces remain unanswered. For instance, a range of vanadium oxidation states exists (from 2+ to 5+) and it is often argued that lower oxidation states like V@sub2@O@sub3@ could take part in some of the reactions taking place at the surface of the V@sub2@O@sub5@-based catalysts. In the present work, we prepared well ordered V@sub2@O@sub3@(0001) thin films and studied their interaction with different molecules. The films were grown on Au(111) by evaporation of vanadium in a partial pressure of oxygen. Under typical UHV conditions, the films are terminated by vanadyl groups which are not part of the V@sub2@O@sub3@ bulk structure. As indicated by XPS and vibrational spectroscopy, the oxygen atoms contained in the V=O groups can be removed by electron irradiation, resulting in a surface terminated by metal atoms. This reduction process was followed with STM. The chemical activity of the two surfaces with respect to the adsorption of O@sub2@, H@sub2@O, CO@sub2@ and propane was investigated with XPS, IRAS and TDS. While the surface terminated by vanadyl groups was found to be chemically not very active, a rather high activity was observed for the V-terminated surface: O@sub2@ adsorption re-establishes the V=O groups via a charged precursor at low temperature, similar to the case of oxygen on Cr@sub2@O@sub3@(0001). After thermal desorption of an adsorbed CO@sub2@ layer, the surface is partially re-oxidized, meaning that part of the CO@sub2@ oxygen remains on the surface and forms vanadyl groups. Water dissociates on the surface, forming a layer of hydroxyl groups which is stable up to 500 K. Propane is partially transformed into propene, possibly via an oxygen-containing intermediate.