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 chemist ry of alkyl groups with two to four carbon atoms on copper surfaces ha ve 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. Beca use desorption is not (under most conditions) the rate-determining ste p in the evolution of the product from the surface, the rate of the su rface beta-hydride elimination reaction could be monitored by TPR. Nei ther surface defects nor low coverages of coadsorbed iodine significan tly affect the beta-elimination rate. For high coverages of iodine, ho wever, the rate of beta-elimination by 5-10% of the adsorbed alkyl gro ups is decreased by over five orders of magnitude (T-rxn = 385 K versu s 230 K). The reaction kinetics together with observations from low-en ergy electron diffraction studies suggest that the dramatic inhibition of the beta-elimination rate for high iodine coverages is due to cage s of immobile iodine atoms that surround the alkyl groups and prohibit hydrogen transfer to the surface.