Unusual hydrogen effect in olefin dehydrogenation: 1-methylcyclohexene dehydrogenation initiated by excess hydrogen over Pt(111) surfaces, a combined sum frequency generation spectroscopy and kinetic study

Dehydrogenation and hydrogenation of 1-methylcyclohexene (C7H12) at 1.5 Torr on Pt(111) in the presence and absence of excess hydrogen were studied using gas chromatography and sum frequency generation vibrational spectroscopy. Kinetic studies showed that the turnover rate (TOR), in molecules per Pt atom per second, for the dehydrogenation to toluene (C7H8) in the presence of 15 Torr of H-2 increased continuously, starting at about 350 K. This onset temperature matches the onset temperature of the formation of the methylcyclohexenyl (C7H10 or C7H11) intermediates observed by SFG experiments, indicating that methylcyclohexenyl is a reactive surface intermediate in the dehydrogenation to toluene. In the absence of hydrogen, the TORs of the hydrogenation/dehydrogenation reactions were negligible at 403 K. When 15 Torr of H-2 was added, the dehydrogenation as well as the hydrogenation reactions began with constant rates (TOR = 0.042 and 0.016, respectively). This observation is consistent with an SFG measurement in which the methylcyclohexenyl intermediate was not detected until 5 Torr of H-2 was added to 1.5 Torr of 1-methylcyclohexene. In light of these results, we discuss the rate-limiting step and the role of excess hydrogen in the 1-methylcyclohexene dehydrogenation reaction.