A SURFACE SCIENCE STUDY OF THE HYDROGENATION AND DEHYDROGENATION STEPS IN THE INTERCONVERSION OF C-6 CYCLIC HYDROCARBONS ON NI(100)

The thermal chemistry of C-6 cyclic hydrocarbons (cyclohexane, cyclohe xene, benzene, 1,3- and 1,4-cyclohexadienes, 1-methyl-1-cyclohexene, a nd toluene) and halo hydrocarbons (iodo cyclohexane, iodo benzene, and 3-bromo cyclohexene) on Ni(100) surfaces has been studied under ultra high vacuum conditions by using temperature-programmed desorption (TPD ). Cyclohexane was found to only desorb molecularly from the surface, but this is because of the dynamic nature of TPD experiments, and Is t herefore not an indication of the inability of nickel to activate the adsorbed molecules. Indeed, cyclohexyl groups, which are the first exp ected intermediates after the initial C-H bond scission and which were prepared via the thermal treatment of adsorbed iodo cyclohexane, were shown to undergo a facile beta-hydride elimination step to yield firs t cyclohexene and ultimately benzene, and to concurrently follow a red uctive elimination path with surface hydrogen to produce cyclohexane. Cyclohexene was found to dehydrogenate easily to benzene and to not hy drogenate to any significant extent even if atomic hydrogen is present on the surface (again, because the dynamic nature of TPD favors molec ular desorption instead). The allylic intermediate expected to form du ring the dehydrogenation of cyclohexene to cyclohexadiene was prepared by thermal decomposition of 3-bromo cyclohexene and studied by TPD as well, and the remaining hydrogenation and dehydrogenation steps were probed by characterizing the thermal chemistry of the other compounds listed above. (C) 1996 Academic Press, Inc.