HIGH-RESOLUTION ELECTRON-MICROSCOPY CHARACTERIZATION OF SMALL PT-PD SIO2 PARTICLES IN OXIDE-REDUCING CYCLES/

In this work we characterized a commercial Pt-Pd alloy (Pt80%-Pd20%) s upported on amorphous silica using conventional electron microscopy, h igh resolution electron microscopy and gas chromatography. The samples were maintained on an oxide-reducing cycles, in order to observe the changes in morphology, crystalline structure and chemical phases forme d during these treatments, A Pt-Pd alloy wire was evaporated on a SiO2 planar film and reduced in H-2 to form the bimetallic particles, whic h we call a 'model' catalyst. The same particles were observed after e ach chemical treatment. We did not observe segregation of any metal us ing high resolution electron microscopy (HREM) when the sample was red uced in H-2 at 673 K at least at this atomic concentration, We correla ted the changes in morphology, and crystalline structure of the sample maintained at different chemical treatments with the catalytic activi ty. The monometallic 'real' catalysts (Pt/SiO2 and Pd/SiO2) prepared b y the impregnation method on a comercial silica (Aerosil) were tested in benzene hydrogenation at 373 K and were compared with the bimetalli c 'real' catalysts also prepared by this method and maintained on an o xide-reducing cycles. We observed differences in catalytic activity fo r each catalyst maintained at different oxide reducing cycles which we attributed to variations in particles sizes and morphologies and also to the different chemical phases formed in each treatment.