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 tec hniques of different sampling depths (ultraviolet photoelectron spectroscop y (UPS), V (2p) X-ray photoelectron spectroscopy (XPS), and valence-band XP S excited by Mg K alpha) in order to investigate the distribution of V4+ sp ecies in the near-surface region. Reduction was performed by evacuation in ultrahigh vacuum or treatment with flowing hydrogen (10% H-2 in Ar) at temp eratures between 373 and 923 K. Average reduction degrees wore derived from V4+ (3d) and O (2p) signals in the valence-band region and from the V (2p( 3/2)) signal (V4+/V5+ ratio). Comparison of average reduction degrees obtai ned from different sampling depths showed that pronounced depth profiles of the V4+ concentration develop during reduction. At low reduction temperatu res, the reduction degree decreases monotonically with the distance from th e surface. At higher temperatures, when oxygen mobility in the solid compet es with oxygen removal from the surface, the depth profile of the reduction degree changes significantly. During reduction in flowing hydrogen, the ma ximum 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 m ay lend to significant errors when the reduction state of the outermost sur face 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.