| Popis: |
Model Pt/WO 3 /SiO 2 catalysts, prepared by “impregnation” of a silica-covered TEM grid using the same precursor salts and pretreatment steps as those used for supported catalysts, were characterized with TEM by analyzing the same catalyst regions through the series of pretreatment steps. Overlayers appeared after a second calcination step following Pt impregnation. By a series of control experiments the overlayers were shown not to arise from carbon contamination, from impurities in the silica or Pt precursor, or from aging under ambient conditions. On the other hand, the mobility of WO 3 was indicated. These observations qualitatively correlate to previous characterization of the real catalysts and suggest that the SMSI state in Pt/WO 3 /SiO 2 catalysts arises from mobile, monodisperse species of WO 3 formed during the second, low-temperature calcination step. |
