EFFECTS OF SURFACE-DEFECTS AND COADSORBED IODINE ON THE CHEMISTRY OF ALKYL-GROUPS ON COPPER SURFACES - EVIDENCE FOR A CAGE EFFECT

The effects of defect sites and coadsorbed iodine atoms on the chemistry of alkyl groups with two to four carbon atoms on copper surfaces have been studied by temperature-programmed reaction (TPR). The primary reaction pathway for the adsorbed alkyl group both in the presence and absence of defects and iodine atoms is beta-hydride elimination. Because desorption is not (under most conditions) the rate-determining step in the evolution of the product from the surface, the rate of the surface beta-hydride elimination reaction could be monitored by TPR. Neither surface defects nor low coverages of coadsorbed iodine significantly affect the beta-elimination rate. For high coverages of iodine, however, the rate of beta-elimination by 5-10% of the adsorbed alkyl groups is decreased by over five orders of magnitude (T-rxn = 385 K versus 230 K). The reaction kinetics together with observations from low-energy electron diffraction studies suggest that the dramatic inhibition of the beta-elimination rate for high iodine coverages is due to cages of immobile iodine atoms that surround the alkyl groups and prohibit hydrogen transfer to the surface.