REACTION OF DEUTERIUM ATOMS WITH CYCLOHEXANE ON CU(111) - HYDROGEN ABSTRACTION REACTIONS BY ELEY-RIDEAL MECHANISMS

Cyclohexane desorbs molecularly intact from Cu(111) and docs not react with deuterium atoms that are preadsorbed on the surface. By contrast, when deuterium atoms formed on a hot tungsten filament are impinged onto a Cu(111) surface precovered with cyclohexane, dehydrogenated products (cyclohexene, cyclohexadiene, and benzene) are evolved when the surface is heated in a subsequent temperature-programmed reaction (TPR) experiment. These D-atom-induced dehydrogenation products provide strong evidence for an Eley-Rideal mechanism where D atoms abstract hydrogen from cyclohexane prior to thermal accommodation with the surface. The kinetics of cyclohexene evolution indicate that both single and sequential H-atom abstractions occur by this mechanism. The cross section for abstraction is on the order of 0.5 angstrom2/cyclohexane, about an order of magnitude smaller than that for D-atom addition to pi-bonds, consistent with the relative cross sections for these reactions in the gas phase. In addition to these Eley-Rideal-type reactions, there is evidence for Langmuir-Hinshelwood hydrogenation and dehydrogenation reactions during the TPR experiment. Product distributions in the desorbing flux were quantified by mass spectrometry using an electron-impact ionization energy of 15 eV to accentuate the molecular ions.