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

摘要

In-situ EXAFS spedtroscopy was used for identifying the surface species involved during sintering and redispersion of Pt/Al_2O_+3 catlysts under oxidizing atmospheres. Pt-Pt metallic distances were alwasy detected during the oxidizing treatments. Sintering involved three successive steps. In the first step, a continuous increase of both Pt-O and Pt-Cl coordinations was observed as temperature increased, indicating the progressive formation of hydroxychlorided complexes such as (Pt_s(OH)_xCl_(6-x))~2-.The formation of Pt(OH)_4Cl_2 species was detected at the end of the first step. The retranformation of (Pt(OH)_4Cl_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 platium core coated by oxychlorinated species. Redispersion experiments indicated that Pt(OH))4Cl_2~2- species were responsible for Pt redispersion. At the end of the redispersion treatment, Pt was present onthe support in two differnt environements: Pt atoms in a raft arrangement containing Cl and O in an octahedral environemtn and relatively alrge Pt metallic particles covered by Pt(OH)_4Cl_2~2- species.