Adsorption and reaction of cyclohexene on a Ni(111) surface

We studied the adsorption and reaction of cyclohexene (C6H10) on Ni(111) at different temperatures with high-resolution in-situ X-ray photoelectron spectroscopy (HR-XPS). For exposure at 125 K, we find intact cyclohexene with two distinct C 1s signals at 283.3 and 284.2 eV, due to the nonequivalent carbon atoms in the molecule. The energetic separation is significantly increased relative to the gas-phase value, due to the interaction with the substrate. Upon exposure at 210 K, complete dehydrogenation of cyclohexene to benzene (C6H6) and hydrogen is observed; coverage-dependent changes of the benzene adsorption site occur in a way similar to those for pure benzene layers, which indicates a phase separation in benzene and hydrogen islands. The thermal evolution of the adsorbed layers was studied by temperature-programmed (TP-) XPS and temperature-programmed desorption spectroscopy (TPD). Upon heating, the benzene + hydrogen layer formed at 210 K shows a coverage-dependent reorientation of the benzene molecules during partial desorption. The cyclohexene layer adsorbed at 125 K only shows partial conversion of cyclohexene to benzene and hydrogen upon heating to 185 or 210 K, with the remaining cyclohexene being stable up to approximately 300 K. We propose that upon heating these molecules are stabilized by coadsorbed benzene and hydrogen; furthermore, the mobility of benzene and hydrogen in this coadsorbed layer is reduced, so that no phase separation can occur.