In situ EXAFS study of the effect of hydrocarbon deposition over Pt/Al2O3 and Pt–Re/Al2O3 catalysts

Abstract

The structural evolution of Pt/Al₂O₃ and Pt–Re/Al₂O₃ catalysts has been studied in situ by extended X-ray absorption fine structure (EXAFS) spectroscopy during the catalytic reforming of n-heptane at high pressures and temperatures. The results obtained for the monometallic catalyst show the formation of a platinum–carbon bond during the catalytic reaction at 733 K, without marked changes in the size of platinum particles. In contrast, in the case of Pt–Re/Al₂O₃, this new Pt–C bond is formed only at lower temperatures, and is accompanied by important changes in the structure of the metallic particles. These results show very clearly that the properties of platinum are modified by the presence of rhenium, and explain the higher stability of the bimetallic reforming catalysts in terms of the formation of a bimetallic phase, more resistant to deposition of well structured carbonaceous residues than the monometallic one.

In addition, the results obtained at the Re L_III edge show the presence of long Re–O distances (2.20 Å) in the reduced Pt–Re/Al₂O₃ catalyst, compatible with the existence of a rhenium phase incorporated into the alumina support. This phase is partially segregated to the surface and reduced after contact of the catalyst with an H₂–hydrocarbon mixture.