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

Six samples of platinum-promoted tungstated zirconia catalysts (Pt_0.5/WO_x/ZrO₂) with W loadings between 6.5 and 12.5 wt.% are investigated using X-ray 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 °C. In the as-prepared samples, the absence of any lines due to WO₃, irrespective of W loading, suggests the highly dispersed state of WO₃. However, the dispersed WO₃ appears to be necessary to stabilize the tetragonal phase of zirconia (t-ZrO₂). Ex-situ XRD studies show that on heating the samples to 1000 °C, the fraction of m-ZrO₂ (monoclinic) increases, with the simultaneous appearance of crystalline m-WO₃. This leads us to infer that the dispersed WO_x species are associated with t-ZrO₂ only. By comparing the magnitude and the temperature variations of the magnetic susceptibility χ of the samples with those of Pt, α-PtO₂, Pt₃O₄, WO₃ and ZrO₂, we infer that Pt in the as-prepared catalysts is primarily in the oxidized form, α-PtO₂ and/or Pt₃O₄, relative magnitudes of the two oxides being dependent on the preparation procedures, thermal treatments and aging. The oxides are converted to Pt in reducing atmosphere.