Health & Environmental Research Online (HERO)


Print Feedback Export to File
1756755 
Journal Article 
Genesis and structure of WOx/ZrO2 solid acid catalysts 
Scheithauer, M; Grasselli, RK; Knozinger, H 
1998 
1 
Langmuir
ISSN: 0743-7463
EISSN: 1520-5827 
14 
11 
3019-3029 
WOS:000073888500014 
Zirconia-supported tungsten oxide catalysts were prepared by
suspending hydrous zirconium oxide in aqueous solutions of ammonium metatungstate at their
natural pH. The suspensions were refluxed at 383 K followed by evaporation of the water, drying
and calcination. The WO3 loadings were chosen between 3.6 and 32 wt % and the calcination
temperatures were in the range 773 less than or equal to T less than or equal to 1273 K. The
resulting materials were structurally characterized by X-ray diffraction, differential thermal
analysis, and surface area measurements, by laser Raman and Fourier transform infrared (FTIR)
spectroscopy, and by UV-vis diffuse reflectance spectroscopy. Their acidic properties were tested
by FTIR spectroscopy using carbon mono;dde as a probe molecule. The experimental results indicate
that the presence of WO, retards the crystallization of the zirconia material and stabilizes the
tetragonal ZrO2 phase and the intrinsic BET surface area. W-O-W linkages are detected even at the
lowest WO3 loadings indicating that oligomeric WOx clusters are initially anchored to the ZrO2
surface which grow in extension with increasing loading. Near the theoretical monolayer coverage
a relatively dense overlayer has formed that eventually forms a three-dimensional network of
interconnected WO6 octahedra. W=O groups are present and presumably connected with peripheral WO6
octahedra. This structural model is supported by the number of nearest W neighbors, which varies
between two and five depending on the WO3 loading as determined from the edge position in W-Vis
diffuse reflectance spectra. It is inferred that the WO6 network may be structurally described as
a pseudo-heteropolytungstate that might have some Zr4+ polyhedra incorporated. FTIR spectroscopy
showed an extremely broad quasi-continuous absorption in the O-H stretching region, which is
suggestive of the presence in the overlayer network of delocalized protons. These are considered
to be responsible for the detected protonic acidity of the materials.