CATALYTIC DEHYDROGENATION AND THERMAL CHEMISTRY OF CYCLOHEXENE AND 1-METHYL-1,4-CYCLOHEXADIENE ON SI(111)7X7

Recently, we reported a thermal desorption study on the evolution of a n intense mass 78 profile for the room-temperature exposure of cyclohe xene to Si(111)7 x 7 surface, which was believed to give rise to the f ormation of benzene by a surface dehydrogenation reaction. Because mas s 78 was also found to be the base ion in the gas-phase cracking patte rns of both 1,3- and 1,4-cyclohexadiene, the dehydrogenation of cycloh exene on clean, sputtered and oxidized Si(111)7 x 7 surfaces has been re-examined in order to determine the origin of the intense mass 78 de sorption profile; i.e. whether it was in fact due to the evolution of benzene or cyclohexadiene, or both. Moreover, a similar dehydrogenatio n reaction giving rise to toluene desorption between 350 and 600 K has been observed for the room-temperature exposure of 1-methyl-1,4-cyclo hexadiene to clean and sputtered Si(111)7 x 7 surfaces. The effects of methyl substitution on the reactivity of these cyclic olefins towards Si(111)7 x 7 can be inferred from these studies. Furthermore, the cat alytic activity of Si(111)7 x 7 was found to be enhanced significantly by extending the thermal desorption cycles to a higher temperature of 925 K. The dehydrogenation of these olefins on Si(111)7 x 7 also gave rise to a unique 7 x 1 low energy electron diffraction pattern. Possi ble factors that may play a role in any proposed model for the dehydro genation reaction are discussed. Finally, evidence of other surface re actions including cyclohexene hydrogenation to cyclohexane will also b e presented.