| Abstract
| - Using density functional theory and statistical thermodynamics, we obtained the phase diagram of thin VnOmfilms of varying thickness (∼2−6 Å, 1−6 vanadium layers) supported on α-Al2O3(0001). Depending on thetemperature, oxygen pressure, and vanadium concentration, films with different thickness and terminationmay form. In ultrahigh vacuum (UHV), at room temperature and for low vanadium concentrations, an ultrathin(1 × 1) OV-terminated film is most stable. As more vanadium is supplied, the thickest possible films form.Their structures and terminations correspond to previous findings for the (0001) surface of bulk V2O3 [Kresseet al., Surf. Sci.2004, 555, 118]. The presence of surface vanadyl (OV) groups is a prevalent feature. Theyare stable up to at least 800 K in UHV. Vanadyl oxygen atoms induce a V2p core-level shift of about 2 eVon the surface V atoms. The reducibility of the supported films is characterized by the energy of oxygendefect formation. For the stable structures, the results vary between 4.11 and 3.59 eV per 1/2O2. In contrast,oxygen removal from the V2O5(001) surface is much easier (1.93 eV). This provides a possible explanationfor the lower catalytic activity of vanadium oxides supported on alumina compared to that of crystallinevanadia particles.
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