Hydrogenation and dehydrogenation of cyclohexene on Pt(100): A sum frequency generation vibrational spectroscopic and kinetic study

Sum frequency generation (SFG) vibrational spectroscopy and kinetic measurements were performed during cyclohexene hydrogenation/dehydrogenation over a range of pressures (10(-S)-5 Torr) and temperatures (300-500 K) on the Pt(I 00) surface. Upon adsorption at pressures below 1.5 Torr and at 300 K, cyclohexene dehydrogenates to form pi-allyl c-C6H9 and hydrogenates to form cyclohexyl (C6H11) surface intermediates. Increasing the pressure to 1.5 Torr produces adsorbed 1,4-cyclohexadiene, pi-allyl c-C6H9, and cyclohexyl species. These adsorbed molecules are found both in the absence and presence of excess hydrogen on the Pt(I 0 0) surface at high pressures and up to 380 K and 360 K, respectively. pi-Allyl c-C6H9 and cyclohexyl are adsorbed on the surface up to 440 K in the absence of excess hydrogen and 460 K in the presence of excess hydrogen, at which point they are no longer detectable by SFG. Kinetic studies in the absence of excess hydrogen show that the apparent activation energy for the dehydrogenation pathway (14.3 +/- 1.2 kcal/mol) is similar to that of the hydrogenation pathway (12.9 +/- 0.6 kcal/mol). Different apparent activation energies are observed for the dehydrogenation pathway (22.4 +/- 1.6 kcal/mol) and the hydrogenation pathway (18.8 +/- 0.9 kcal/mol) in the presence of excess hydrogen. (c) 2005 Elsevier B.V. All rights reserved.