An electron spectroscopy and x-ray diffraction study of nickel oxide/alumina and nickel-oxide-tungsten trioxide/alumina catalysts

A series of NiO-WO/sub 3//Al/sub 2/O/sub 3/ catalysts were studied by UV-vis diffuse reflectance spectroscopy (DRS) and x-ray diffraction (XRD). Both the metal content and the temperature of calcination of the catalysts were varied over a wide range. It was found that W is present as a W/sup 6 +/ surface phase regardless of the temperature of calcination and W content. In NiO/Al/sub 2/O/sub 3/ samples several Ni/sup 2 +/ species were detected: an octahedral species located at the surface and both an octahedral and a tetrahedral species located in the Al/sub 7/O/sub 3/ support. In NiO-WO/sub 3//Al/sub 2/O/sub 3/ samples a different octahedral Ni species was also found, in which Ni is present in a mixed phase containing Ni, W, Al, and O. The Ni content has some influence on the speciation of Ni, but the temperature of calcination is the major factor which determines the location and the coordination of Ni. Calcination below 775 K results in catalysts which contain only octahedral Ni at the surface. At higher temperatures diffusion of Ni into the support is detected by XRD, and at 1175 K an equilibrium distribution throughout the support particles is established. In the support most of themore » Ni is tetrahedrally coordinated, and above 925 K the distribution of Ni over tetrahedral and octahedral sites is hardly affected by a change in calcination temperature or Ni content. A model is presented which can quantitatively describe the diffusion process using independently determined parameters. It is concluded that Ni diffusion into the support is negligible under hydrodesulfurization (HDS) reaction conditions, which stresses the importance of the structure of the oxidic precursor for the structure and HDS activity of sulfided catalysts.« less