Surface characterization of WO3/ZrO 2 catalysts
2002
Vaidyanathan, Nithya | Hercules, David M. | Houalla, Marwan
A series of WO₃/ZrO₂catalysts with tungsten (W) loadings ranging from 0.5 to 11.4 wt% was prepared by incipient wetness impregnation on a preformed ZrO₂support. The oxidic catalysts were characterized using XRD, Raman spectroscopy, XPS, ISS, and IR spectroscopy. XRD and Raman results showed that the ZrO₂support was predominantly present in the monoclinic form. XPS and Raman measurements indicated the formation of increasing amounts of W interaction species for catalysts with W loadings up to 8.8 wt% WO₃. In addition to the W interaction species, bulk WO₃was also observed for catalysts with W loadings ≥ 3.0 wt% WO₃. Comparison of the XPS results with coverage measurements by ISS and CO adsorption suggests that the W surface phase is in the form of two-dimensional polymeric patches for catalysts with W loadings 3.0 ≤ wt% WO₃≤ 4.5. For catalysts with W loadings >4.5 wt% WO₃, the results indicated an additional build-up of a bilayer (or multilayer) polymeric W species. Analysis of the hydroxyl region of ZrO₂by IR spectroscopy showed that initial additions of W occur on the high frequency hydroxyl group. A schematic for the structure of the catalysts has been proposed based on the above observations.
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