1,3- and 1,4-cyclohexadiene reaction intermediates in cyclohexene hydrogenation and dehydrogenation on Pt(111) crystal surface: a combined reaction kinetics and surface vibrational spectroscopy study using sum frequency generation
Xc. Su et al., 1,3- and 1,4-cyclohexadiene reaction intermediates in cyclohexene hydrogenation and dehydrogenation on Pt(111) crystal surface: a combined reaction kinetics and surface vibrational spectroscopy study using sum frequency generation, J MOL CAT A, 141(1-3), 1999, pp. 9-19
The hydrogenation and dehydrogenation reactions of cyclohexene on Pt(lll) s
urface were investigated by surface vibrational spectroscopy via sum freque
ncy generation (SFG) both under ultrahigh vacuum (UHV) and high pressure co
nditions with 10 Torr cyclohexene and various hydrogen pressures up to 590
Torr. Under UHV, cyclohexene on Pt(lll) undergoes a change from pi/sigma-bo
nded, sigma-bonded, and c-C6H9 surface species to adsorbed benzene when the
surface was heated. A site-blocking effect was observed at saturation cove
rage of cyclohexene and caused the dehydrogenation to shift to higher surfa
ce temperature, At high pressures, however, none of the species observed in
UHV condition were seen. 1,4-cyclohexadiene (CHD) was found to be the majo
r species on the surface at 295 K even in the presence of nearly 600 Torr o
f hydrogen. Hydrogenation was the only detectable reaction at the temperatu
re range between 300-400 K with I,3-CHD on the surface as revealed by SFG.
Further increasing surface temperature results in a decrease in hydrogenati
on reaction rate and an increase in dehydrogenation reaction rate with both
1,3-CHD and 1,4-CHD detectable on the surface simultaneously. Monitoring t
he reaction kinetics and the chemical nature of surface species together al
lows us to postulate a reaction mechanism: cyclohexene hydrogenates to cycl
ohexane via a 1,3-CHD intermediate, and dehydrogenates to benzene through b
oth 1,4-CHD and 1,3-CHD intermediates. Both 1,3- and 1,4-CHD dehydrogenate
to benzene at sufficiently high temperature on Pt(111). (C) 1999 Elsevier S
cience B.V. All rights reserved.