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| - The Influence of Water on the Dispersion of Vanadia Supported on Silica SBA-15: A Combined XPS and Raman Study
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| - The influence of water on the dispersion and structure of silica SBA-15-supported vanadia model catalystshas been studied using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, which were combinedwithin one experimental setup, as well as UV−vis diffuse reflectance spectroscopy. By performing time-dependent XPS experiments, the influence of UHV/X-ray radiation could be eliminated by extrapolation ofthe observed temporal changes to t = 0. XPS characterization reveals that the V2p3/2 emission consists oftwo contributions, which are assigned to vanadia with distinctly different cluster size. Dehydration by treatmentin oxygen flow at elevated temperatures leads to a significant increase in total intensity and a substantialredistribution of spectral weight to higher binding energies as a result of an increase in the vanadia dispersion.The V/Si XPS intensity ratio of the dehydrated samples closely follows the corresponding bulk ratio over thewhole range of vanadium loadings (0−22 wt % V) studied. This observation renders possible a correlationof XPS and Raman results allowing for quantification of Raman features such as the relative cross sectionsof the vanadyl surface species. It is shown that the observed changes in vanadia dispersion are directly associatedwith the changes in the molecular structure of the surface vanadia species.
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