Effects of O-2 pressure on the oxidation of VOx/Pt(111)
- 化学化工－已发表论文 
Vanadium oxide (VOx) has been extensively used in many oxidation and selective oxidation reactions. In this study, VOx thin films were prepared in an ultra-high vacuum (UHV) chamber by evaporating V onto a Pt(111) surface followed by subsequent oxidation at 623 K in 1 x 10(-7) Torr O-2, and further oxidized in the 'high-pressure' reaction cell with 1 Torr O-2. The film quality and structure were investigated by high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), low energy ion scattering spectroscopy (LEIS), Auger electron spectroscopy (AES), and in situ infrared reflection absorption spectroscopy (IRAS). On the Pt(111) surface, VOx forms isolated O=VOx (x = 0-3) species, surface two-dimensional (2D) (2 x 2)-V2O3 domains, a bi-layer structure with a (3 root 3 x 6) arrangement, and a complicated tri-layer structure as the coverage increases from submonolayer to multilayer. Under the UHV conditions, the oxidation state of V is mainly +3 and the stability was found to be surface V2O3 > bi-layer V2O3 > tri-layer one. After exposing to 0.3-1 Torr O-2, VOx can be oxidized to higher oxidation states, mainly V2O5, as evidenced by the shifts of the core-level binding energies and presence of V=O. These results indicate that thorough oxidation of VOx requires sufficiently high O-2 pressure, and that vanadium-based catalysts may possess higher oxidation states under most reaction conditions in the presence of O-2.