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

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 pr...