Effects of coadsorbed hydrogen (or d) on the dehydrogenation of cyclohexane on Pt(111): Observation of the production of adsorbed cyclohexyl (C6H11)

The effects of coadsorbed hydrogen (or deuterium) on the dehydrogenation of cyclohexane on Pt(lll) has been studied using laser induced thermal desorp tion-Fourier transform mass spectrometry (LITD-FTMS), Auger electron spectr oscopy (AES), and thermal desorption spectroscopy (TDS). With 0.25 of satur ation monolayer coverage of cyclohexane on the crystal surface, the onset f or the competing processes of desorption and dehydrogenation occurs at appr oximately 205 K. Our experiments show that coadsorbed hydrogen causes a shi ft to lower temperature of the onset of both cyclohexane dehydrogenation an d molecular desorption. In the presence of saturation amounts of coadsorbed hydrogen, the onset for the two precesses occurs at approximately 165 K. I n the presence of coadsorbed hydrogen or deuterium, LITD-FTMS experiments s how that adsorbed cyclohexyl (C6H11) is produced in the first step of the d ehydrogenation of cylclohexane. These experiments provide the first observa tion of cyclohexyl as a product of the thermal dehydrogenation of cyclohexa ne on Pt. The cyclohexyl species is stable on the Pt(111) surface at temper atures up to 205 K. In addition, H coadsorption thermal desorption spectra of cyclohexane show two desorption maxima, at 195 and 235 K. In the presenc e of coadsorbed H the total amount of C6H12 that desorbs is a factor of 2 m ore than in the noncoadsorption case. Deuterium coadsorption experiments in dicate that the higher temperature desorption process is the result of the rehydrogenation reaction: C6H11 + H (D) --> C6H12 (C6H11D) In the case of d euterium coadsorption with C6H12, both C6H12 and C6H11D are observed as reh ydrogenation products, the C6H12 resulting from surface hydrogen generated in the initial dehydrogenation process. In this case, both rehydrogenation products (C6H12 and C6H11D) are observed at 255 K. This observation is disc ussed in terms of a kinetic isotope effect for the rehydrogenation of C6H11 .