3. Results and discussion
The details of the preparation procedure as well as the average
sizes of Pt particles for all catalysts are summarized in Table 1. Vari-
ations in the preparation procedure allow one to obtain catalysts
containing the Pt particles with different sizes.
Fig. 1 shows fragments of X-ray powder diffraction patterns
of the Pt/?-Al 2 O 3 catalysts and ?-Al 2 O 3 support. The diffraction
peaks of the ?-Al 2 O 3 phase with a spinel-like structure (space group
Fd ¯ 3m, a = 0.793 nm) are predominant. The average crystallite size
of ?-Al 2 O 3 is 4.5 nm. The XRD patterns of the catalysts exhibit no
peaks corresponding to the platinum phases. The diffraction peaks
of the Pt 0 phase are detected only for sample V containing well
crystallized Pt 0 particles (Fig. 1, Table 1). The absence of reflections
attributable to the oxide or metal platinum phases implied a highly
dispersed state of platinum species. HRTEM data (Table 1) were in
agreement with this conclusion.
Additional information on the state and local structure of plat-
inum species in the catalysts was obtained using RDF of electron
density
analysis. Fig. 2 shows the RDF of the support in comparison
with model ones for the ?-Al 2 O 3 phase