Variation of catalytic activity over platinum-rhenium/.gamma.-alumina

Hydrogen isotope exchange (at 100/sup 0/C) and hydrogenolysis (at 240/sup 0/C) were studied over ..gamma..-Al/sub 2/O/sub 3/-supported Pt, Re, and a series of PtRe catalysts, where the composition and pretreatment conditions were varied. Sites catalyzing the rate-limiting step of the former reaction appear to be monoatomic, while those for the latter reaction are multiatomic. Alloys behave as physical mixtures for the isotope-exchange reaction, since the strength of the adsorption bond depends little on the atomic environment of the site. For hydrogenolysis, however, the PtRe catalysts behave differently than the Pt or Re monometallic catalysts. This is attributed to the formation of ensembles of atoms that contain both Pt and Re on the bimetallic catalyst and thus exhibit a heat of adsorption which is intermediate between the two. The most ensemble-specific reaction is total hydrogenolysis to methane. This rate is highest for sites on which adsorption is stronger than on Pt but weaker than on Re. This reaction, therefore, probes the extent of alloying on the supported particles.