The hydrogenation and dehydrogenation of isobutene on Pt(111) monitored by IR-visible sum frequency generation and gas chromotography

The hydrogenation of isobutene (2-methylpropene) has been monitored near ambient pressure over a Pt(111) single crystal at 295 K by SFG vibrational spectroscopy. During hydrogenation tert-butyl groups and pi-bonded isobutene species were the dominant surface species detected. Only small amounts of dehydrogenated species such as isobutylidyne could be observed unless the reaction conditions were very hydrogen poor. The 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-bonded isobutene through the slow kinetic step of isobutyl formation. Vacuum studies of isobutene dehydrogenation show evidence for a stable intermediate during the dehydrogenation of di-sigma-bonded isobutene to isobutylidyne.