| Abstract
| - 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 cyclohexenewith two distinct C 1s signals at 283.3 and 284.2 eV, due to the nonequivalent carbon atoms in the molecule. Theenergetic 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 layerswas 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 benzenemolecules during partial desorption. The cyclohexene layer adsorbed at 125 K only shows partial conversion ofcyclohexene to benzene and hydrogen upon heating to 185 or 210 K, with the remaining cyclohexene being stableup to ∼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 separationcan occur.
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