Catalytic cyclopropane hydrogenation on platinum(111) using in situ soft X-ray methods

As part of a continuing program focused on the role of hydrogen in catalyti c reactions, cyclopropane hydrogenation on the Pt(lll) surface has been cha racterized using in situ soft X-ray studies above the carbon K edge. In sit u soft X-ray methods provide interesting new information regarding concentr ations, stoichiometries, bonding, and reactivities of adsorbed carbon-conta ining species under reaction conditions. Ar low temperature, cyclopropane: is weakly adsorbed and tilted up from the Pt(lll) surface. The saturation c overage is 4.4 x 10(14) molecules/cm(2) at 100 K. Catalytic hydrogenation o f cyclopropane to form propane is observed during batch reactivity studies in the 350 K range. No methane or ethane products are observed. Approximate ly 2.9 x 10(14) C-3 molecules/cm(2) of adsorbed carbonaceous species are ob served on the surface at 350 K under reaction conditions. The concentration of these species decreases above 350 K in excess hydrogen. In situ isother mal reactivity studies in hydrogen near 350 K indicated chat a significant fraction of these species can be removed from the surface with a thermal ac tivation energy of 15.2 kcal/mol. Taken together the observation of catalyt ic propane formation and the estimated activation energy suggests that the surface species are directly involved in propane formation. In situ charact erization of this species, using soft X-ray C-H intensities to determine st oichiometry, indicates that a C3H6 species is dominant up to 320 K. In situ glancing and normal spectra taken at 320 K indicate that the C3H6 species is a platinacylobutane intermediate adsorbed in an upright configuration re lative to the surface.;Increasing temperature to 350 K under reaction condi tions increases the average C-H stoichiometry to C3H7 This hydrogen additio n suggests formation of adsorbed propyl in the 350 K temperature range. Tak en together these experiments indicate that the dominant mechanism for C-C bond breaking is associated with insertion of the Pt surface into the adsor bed cyclopropane reactant to form a metallocycle intermediate. This metallo cycle is strongly bound and stable up to 320 K in large excesses of hydroge n. With increasing temperature this C-3 platinacyclobutane intermediate is hydrogenated to form propane in the 350 K range. Observation of a hydrogena ted C3H7 intermediate suggests that propane formation may involve sequentia l hydrogen addition and a transient propyl intermediate.