X-ray Absorption Spectroscopy: a powerful tool to investigate intermediate species during sintering-redispersion of metallic catalysts.

In-situ EXAFS spectroscopy was used for identifying the surface species involved during sintering and redispersion of Pt/Al2O3 catalysts under oxidizing atmospheres. Pt-Pt metallic distances were always detected during the oxidizing treatments. Sintering involved three successive steps. In the first step, a continuous increase of both Pt-O and Pt-Clcoordinations was observed as temperature increased, indicating the progressive formation of hydroxychlorided complexes such as [Pt-s(OH)(x)Cl(6-x)](2-). The formation of Pt(OH)(4)Cl-2 species was detected at the end of the first step. The retransformation of [Pt(OH)(4)Cl-2](2-) into a disordered non-stoichiometric oxychlorinated species occurred in the second step. During sintering, the Pt particles were made up of a metal platinum core coated by oxychlorinated species. Redispersion experiments indicated that Pt(OH)(4)Cl-2(2-) species were responsible for Pt redispersion. At the end of the redispersion treatment, Pt was present on the support in two different environments: Pt atoms in a raft arrangement containing Cl and O in an octahedral environment and relatively large Pt metallic particles covered by Pt(OH)(4)Cl-2(2-) species.