Redispersion of Sintered Pt/Al₂O₃Naphtha Reforming Catalysts: An<i>in Situ</i>Study Monitored by X-ray Absorption Spectroscopy

Abstract

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)4Cl22- 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)4Cl22- 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.