Influence of pretreatment temperature on the bimetallic interactions in Pt-Re/Al2O3 reforming catalysts studied by X-ray absorption spectroscopy

The influence of pretreatment temperature on the metal function of a commercial Pt-Re/Al2O3 reforming catalyst was studied by X-ray absorption spectroscopy. By simultaneously examining the rhenium L-III and platinum L-III EXAFS data, the bimetallic interaction and the metal-support interaction can be distinguished from the overall spectrum. The results show that if the catalyst is dried in air at temperatures less than or equal to 500 degreesC before reduction at 480 degreesC, bimetallic particles of platinum and rhenium are formed. Drying at higher temperatures and in absence of air inhibits the transport of mobile (rhenium) species on the surface causing no intimate contact between the two metals. Platinum L-III EXAFS data show that the average particle size of the bimetallic particles on the alumina surface is less than 10 Angstrom. The results from the rhenium L-III EXAFS analysis confirm that rhenium is not completely reduced to metallic rhenium after reduction, with a significant fraction of the rhenium present in low, positive oxidation states and in intimate contact with the support. The EXAFS data are consistent with a structural model of rhenium metal particles 1-3 run wide with smaller platinum particles situated within or at the boundary of the rhenium particles, and that moderate heating in presence of air (i.e., moist) provides the best conditions for transport of mobile rhenium species on the surface, and hence alloy formation. (C) 2001 Elsevier Science.