THE HYDROGENATION AND DEHYDROGENATION OF ISOBUTENE ON PT(111) MONITORED BY IR-VISIBLE SUM-FREQUENCY GENERATION AND GAS-CHROMATOGRAPHY

The hydrogenation of isobutene (2-methylpropene) has been monitored ne ar ambient pressure over a Pt(111) single crystal at 295 K by SFG vibr ational spectroscopy. During hydrogenation tert-butyl groups and pi-bo nded isobutene species were the dominant surface species detected. Onl y small amounts of dehydrogenated species such as isobutylidyne could be observed unless the reaction conditions were very hydrogen poor. Th e hydrogenation rate for isobutene was at least one order of magnitude lower than for but-1-ene and cis-but-2-ene. Vacuum studies indicated that the rate of tert-butyl hydrogenation was much slower than that of isobutyl, which may force isobutane production to proceed from x-bond ed isobutene through the slow kinetic step of isobutyl formation. Vacu um studies of isobutene dehydrogenation show evidence for a stable int ermediate during the dehydrogenation of di-sigma-bonded isobutene to i sobutylidyne.