REDISPERSION OF SINTERED PT AL2O3 NAPHTHA REFORMING CATALYSTS - AN IN-SITU STUDY MONITORED BY X-RAY-ABSORPTION SPECTROSCOPY/

Redispersion of sintered Pt (0.6-1.0%)/Al2O3 catalysts at 773 K under HCl/H2O/O-2/N-2 atmospheres was studied by hydrogen chemisorption, tem perature-programmed reduction (TPR), transmission electron microscopy (TEM) and in situ X-ray absorption spectroscopy (EXAFS) experiments. T PR and H-2 chemisorption results showed that the concentration of hydr oxychlorided Pt species on the alumina carrier and, as a consequence, the metallic dispersion constantly augmented during the 8 h redispersi on treatment. In situ EXAFS experiments indicated that Pt(OH)(4)Cl-2(2 -) species, which are formed by the attack of gaseous/ surface chlorid ed species to partially oxidized metallic Pt particles, are responsibl e for Pt redispersion. EXAFS studies also suggested that once the Pt-C l/Pt-O coordination number ratio was stable, the Pt atoms were present on the alumina carrier either in the form of small rafts containing C l and O atoms in a slighly distorted octahedral environment or in rela tively large metallic particles covered by Pt(OH)(4)Cl-2(2-) species. This suggestion was supported by the bimodal distribution of metal pla tinum particles observed by TEM when the redispersed samples where exa mined after its reduction with H-2.