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.