Six Pt SiO2 and four Pt Al2O3 catalysts were characterized by CO chemisorption and H2O2 titration and by electron microscopy. Five of the Pt SiO2 catalysts originated at Northwestern University and had been similarly characterized before. Over these catalysts the hydrogenation reaction of the complex molecular probe (+)-apopinene (6,6-dimethyl-1R,5R-bicyclo[3.1.1]hept-2-ene) was used to evaluate the change in the mix of active sites as a function of percentage exposed Pt atoms (percentage dispersion, %D). The ratio of isomerization to addition, ki ka, goes through a maximum at approximately 60% D, which corresponds to a maximum in edge sites (C7 sites) on fcc octahedra. These sites are identified as Siegel type 2M sites which catalyze essentially isomerization whereas adatoms and vertices (C4 sites), Siegel type 3M sites, catalyze addition and isomerization.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry