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要旨:
The structure of small platinum particles, in the standard Pt/SiO2 catalyst EuroPt-1, of 17 Å average size is investigated in situ by X-ray diffraction. For this analysis a numerical fit with Debye functions of a sequence of cuboctahedra build up by 13. 55, 147...., atoms is used. After interaction with oxygen the metallic phase was found to coexist with platinum oxide phases PtOx. the major fraction of which exhibits the local atomic arrangement as PtO and a smaller fraction that of Pt3O4. The maximum amount of Pt oxidized was found to be 65% under I bar of oxygen at 300 °C. Under particular conditions (oxidation at 400 °C and cooling to room temperature in oxygen) the particle size distribution of the residual metal is that of the starting sequence of cuboctahedra with their outer shells removed. For all oxidized samples the reduction in 1 bar of H2 for T a 300 K is fast (< 40 s), and in each case the initial particle size distribution is restored. Under hydrogen the Pt particles seem to relax to perfect fcc symmetry, but the Debye temperature is lower than that of the bulk metal. After evacuation, the reduced samples exhibit X-ray patterns very similar to those of weakly oxidized catalysts. The particles then show a high static disorder. In this study, the fraction of particles with icosahedral morphology could be restricted to ≤10 wt %.
