Surface characterization of WO_3/ZrO_2 catalysts

摘要

A series of WO_3/ZrO_2 catalysts with tungsten (W) loadings ranging from 0.5 to 11.4 wt was prepared by incipient wetness impregnation on a preformed ZrO_2 support. The oxidic catalysts were characterized using XRD, Raman spectroscopy, XPS, ISS, and IR spectroscopy. XRD and Raman results showed that the ZrO_2 support 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 WO_3. in addition to the W interaction species, bulk WO_3 was also observed for catalysts with W loadings ≥3.0 wt WO_3. Comparison of the XPS results with coverage measurements by ISS and CO adsorption suggests that the W surface phase is in the from of two-dimensional polymeric patches for catalysts with W loadings 3.0 ≤ wt WO_3 ≤ 4.5. For catalysts with W loadings >4.5 wt WO_3, the results indicated an additional build-up of a bilayer (or multilayer) polymeric W species. Analysis of the hydroxyl region of ZrO_2 by 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.