Effects of O2 pressure on the oxidation of VOx/Pt(111)

Abstract

Vanadium oxide (VO_x) has been extensively used in many oxidation and selective oxidation reactions. In this study, VO_x 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 × 10⁻⁷ Torr O₂, and further oxidized in the ‘high-pressure’ reaction cell with 1 Torr O₂. 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, VO_x forms isolated OVO_x (x = 0–3) species, surface two-dimensional (2D) (2 × 2)-V₂O₃ domains, a bi-layer structure with a (3√3 × 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 O₂, VO_x can be oxidized to higher oxidation states, mainly V₂O₅, as evidenced by the shifts of the core-level binding energies and presence of VO. These results indicate that thorough oxidation of VO_x requires sufficiently high O₂ pressure, and that vanadium-based catalysts may possess higher oxidation states under most reaction conditions in the presence of O₂.