Adsorption of water on thin V2O3(0 0 0 1) films
Introduction
Vanadium oxides play an important role in catalysis where they act as part of catalysts for different reactions, most of them involving transfer of oxygen atoms [2]. Usually V2O5 is used together with other oxides to improve reactivity and selectivity [3], [4]. It is often claimed that vanadyl groups are the reactive centers [2]. However, details of the nature of the active centers under reaction conditions, including the vanadium oxidation state and the reaction pathways are still under active discussion (see Ref. [5] and references therein).
In oxidation reactions such as propane oxy-dehydrogenation () and methanol oxidation (), which both occur on vanadium oxide catalysts, formation of H2O plays an important role. In the context of an investigation of these reactions on V2O3(0 0 0 1) it appeared to be desirable to also study the interaction of water.
Water is a frequently studied adsorbate [6]. It often dissociates upon contact with a surface, forming adsorbed hydroxyl groups which may play a role in catalytic processes occurring on these surfaces. Dissociation is especially likely on defect sites, but there are also reports of dissociation on regular surfaces [7], [8]. A comprehensive overview of water adsorption of different surfaces has been published by Henderson [6].
As reported in Ref. [1], well ordered V2O3(0 0 0 1) layers can be grown on Au(1 1 1) and W(1 1 0). It was shown that these films are terminated by a layer of vanadyl groups under typical UHV conditions. Electron irradiation removes the oxygen atoms of the vanadyl groups, leading to a surface terminated by vanadium atoms (see Fig. 1). We have chosen to use these films for the adsorption studies described in this manuscript in order to learn about the interaction of water with vanadyl groups and, in the case of the reduced surface, with co-ordinatively unsaturated and reduced vanadium atoms.
Section snippets
Experimental
The experiments were performed using two different UHV systems. One system is located at the BESSY II electron storage ring in Berlin. It is attached to the UE52-PGM plane grating monochromator which delivers photons in the energy range from 90 eV to 1500 eV. The system consists of two chambers which may be separated by a gate valve. The upper chamber contains facilities for ion sputtering, gas dosing, low energy electron diffraction (LEED) and metal deposition using an Omicron EFM4 evaporator.
Results and discussion
Photoelectron spectra of the valence band and the O1s and V2p core level region of water on reduced and vanadyl terminated V2O3(0 0 0 1) are displayed in Fig. 3 for different annealing temperatures. At temperatures T ⩽ 165 K the typical levels of ice show up in the data. Further annealing leads to an O1s peak at around 533.5 eV in the case of the vanadyl terminated surface (Fig. 3(d)). This level may be attributed to molecular water (a list of O1s binding energies of water on different substrates may
Summary
We have shown that water interacts only weakly with the vanadyl terminated V2O3(0 0 0 1) surface. Only molecular water adsorption is observed and the water molecules desorb below 300 K, whereas on the vanadium terminated reduced surface water dissociates to form hydroxyl groups with a surface coverage of about 1.6 molecules per unit cell. The hydroxyl groups are removed completely from the surface upon annealing at T ⩾ 600 K. Reasons for the different reactivities of the two surfaces may be (1) the
Acknowledgements
This work was funded by the Deutsche Forschungsgemeinschaft through their Sonderforschungsbereich 546 ‘Transition Metal Oxide Aggregates’. The Fonds der Chemischen Industrie is gratefully acknowledged for financial support. We thank Phil Woodruff, Emily Kröger and David Sayago for helpful discussions.
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