The hydrogenation and dehydrogenation reactions of cyclohexene on Pt 111 surface were investigated by surface ลฝ . ลฝ . vibrational spectroscopy via sum frequency generation SFG both under ultrahigh vacuum UHV and high pressure ลฝ . conditions with 10 Torr cyclohexene and various hydrogen pressures up to 590 Torr. Under UHV, cyclohexene on Pt 111 undergoes a change from prs-bonded, s-bonded, and c-C H surface species to adsorbed benzene when the surface was 6 9 heated. A site-blocking effect was observed at saturation coverage of cyclohexene and caused the dehydrogenation to shift to higher surface temperature. At high pressures, however, none of the species observed in UHV condition were seen.
ลฝ . 1,4-cyclohexadiene CHD was found to be the major species on the surface at 295 K even in the presence of nearly 600 Torr of hydrogen. Hydrogenation was the only detectable reaction at the temperature range between 300-400 K with 1,3-CHD on the surface as revealed by SFG. Further increasing surface temperature results in a decrease in hydrogenation reaction rate and an increase in dehydrogenation reaction rate with both 1,3-CHD and 1,4-CHD detectable on the surface simultaneously. Monitoring the reaction kinetics and the chemical nature of surface species together allows us to postulate a reaction mechanism: cyclohexene hydrogenates to cyclohexane via a 1,3-CHD intermediate, and dehydrogenates to benzene through both 1,4-CHD and 1,3-CHD intermediates. Both 1,3-and 1,4-CHD dehydrogenate to benzene at sufficiently high ลฝ . temperature on Pt 111 .