Structure, properties and roles of the different constituents in Pt/WOx/ZrO2 catalysts

Six samples of platinum-promoted tungstated zirconia catalysts (Pt-0.5/WOx/ ZrO2) with W loadings between 6.5 and 12.5 wt.% are investigated using X-ra y diffraction (XRD) and magnetic susceptibility measurements (4 to 350 K). Studies are also carried out on some of these samples after annealing them in air at temperatures up to 1000 degreesC. In the as-prepared samples, the absence of any lines due to WO3, irrespective of W loading, suggests the h ighly dispersed state of WO3. However, the dispersed WO3 appears to be nece ssary to stabilize the tetragonal phase of zirconia (t-ZrO2). Ex-situ XRD s tudies show that on heating the samples to 1000 degreesC, the fraction of m -ZrO2 (monoclinic) increases, with the simultaneous appearance of crystalli ne m-WO3. This leads us to infer that the dispersed WO, species are associa ted with t-ZrO2 only. By comparing the magnitude and the temperature variat ions of the magnetic susceptibility X of the samples with those of Pt, alph a -ptO(2), Pt3O4, WO3 and ZrO2, we infer that Pt in the as-prepared catalys ts is primarily in the oxidized form, alpha -PtO2 and/or Pt3O4, relative ma gnitudes of the two oxides being dependent on the preparation procedures, t hermal treatments and aging. The oxides are converted to Pt in reducing atm osphere. (C) 2001 Elsevier Science B.V. All rights reserved.