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 O=VO(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 V=O. 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.