Redispersion of sintered Pt
(0.6−1.0%)/Al2O3 catalysts at 773 K under
HCl/H2O/O2/N2 atmospheres
was
studied by hydrogen chemisorption, temperature-programmed reduction
(TPR), transmission electron
microscopy (TEM) and in situ X-ray absorption spectroscopy (EXAFS)
experiments. TPR and H2
chemisorption results showed that the concentration of hydroxychlorided
Pt species on the alumina carrier
and, as a consequence, the metallic dispersion constantly augmented
during the 8 h redispersion treatment.
In situ EXAFS experiments indicated that
Pt(OH)4Cl2
2-
species, which are formed by the attack of gaseous/surface chlorided species to partially oxidized metallic Pt particles,
are responsible for Pt redispersion. EXAFS
studies also suggested that once the Pt−Cl/Pt−O coordination number
ratio was stable, the Pt atoms were
present on the alumina carrier either in the form of small rafts
containing Cl and O atoms in a slighly distorted
octahedral environment or in relatively large metallic particles
covered by
Pt(OH)4Cl2
2-
species. This
suggestion was supported by the bimodal distribution of metal platinum
particles observed by TEM when the
redispersed samples where examined after its reduction with
H2.
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