Effects of surface defects and coadsorbed iodine on the chemistry of alkyl groups on copper surfaces: evidence for a cage effect

Abstract

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 @-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 @-hydride elimination reaction could be monitored by TPR. Neither surface defects nor low coverages of coadsorbed iodine significantly affect the @-elimination rate. For high coverages of iodine, however, the rate of @-elimination by 5-10% of the adsorbed alkyl groups is decreased by over five orders of magnitude ( Trxn = 385 K versus 230 K). The reaction kinetics together with observations from low-energy electron diffraction studies ‘suggest that the dramatic inhibition of the @-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. Try as we may, no transition-metal single crystal is perfect. Scanning tunneling microscopy studies have shown that steps, kinks, adatoms, and vacancies are present at a fraction of a percent on even the most carefully prepared surfaces.’ Because surface defects contain atoms that are coordinatively unsaturated relative to terrace. planeatoms, reactivity at defect sites is often enhanced, leading to dramatic effects on surface reactions despite their small numbers. For example, polyolefin formation using Ziegler-Natta type catalysts is thought to occur exclusively at defect sitesO2 The important role of defect sites on surface reactions has recently been reviewed by Wandelt.3 Like surface defects, elemental coadsorbates can dramatically affect surface reactions. The promotion of the Haber process by alkali additions‘ and the poisoning of hydrocarbon reforming by small amounts of sulfurs are well-known examples. The chemical mechanisms for these effects remain controversial, and the question of whether steric (site-blocking) or electronic effects are dominant continues to be debated.6 In the present work, we examine the effects of surface defects and coadsorbed iodine atoms on alkyl dehydrogenation by @-hydride elimination on single-crystal copper surfaces using temperature-programmed reaction (TPR) experiments. TPR is particularly useful for this system, because the rate of dehydrogenation determines the rate of product evolution, Le., for most conditions, desorption is not rate-determining. The rateof product evolution can thus be used to monitor the rate of the surface dehydrogenation reaction. Previous studies of this system have shown that linear alkyl iodides of two to four carbons in length dissociate below 150 K on copper surfaces to form adsorbed alkyl groups and iodine atoms.’J As shown in Scheme 1, these alkyl moieties dehydrogenate by 8-hyride elimination above 200 K to form adsorbed hydrogen atoms and the corresponding olefin, which desorbs from the surface.8.9