Application of Ultraviolet Photoelectron Spectroscopy in the Surface Characterization of Polycrystalline Oxide Catalysts. 2. Depth Variation of the Reduction Degree in the Surface Region of Partially Reduced V2O5

Abstract

The surface of partially reduced V2O5 has been studied by photoemission techniques of different sampling depths (ultraviolet photoelectron spectroscopy (UPS), V (2p) X-ray photoelectron spectroscopy (XPS), and valence-band XPS excited by Mg Kα) in order to investigate the distribution of V4+ species in the near-surface region. Reduction was performed by evacuation in ultrahigh vacuum or treatment with flowing hydrogen (10% H2 in Ar) at temperatures between 373 and 923 K. Average reduction degrees were derived from V4+ (3d) and O (2p) signals in the valence-band region and from the V (2p3/2) signal (V4+/V5+ ratio). Comparison of average reduction degrees obtained from different sampling depths showed that pronounced depth profiles of the V4+ concentration develop during reduction. At low reduction temperatures, the reduction degree decreases monotonically with the distance from the surface. At higher temperatures, when oxygen mobility in the solid competes with oxygen removal from the surface, the depth profile of the reduction degree changes significantly. During reduction in flowing hydrogen, the maximum of the reduction degree shifts from the outermost surface layer into subsurface layers. A similar effect is likely to occur during reduction by thermal evacuation. The strong inhomogeneities in the near-surface region may lead to significant errors when the reduction state of the outermost surface layer is described on the basis of XPS alone under the assumption of a homogeneous sampling region. This indicates a need for further improvement of the methodology for surface analysis with reduced bulk oxides.