The Chemistry of Alkyl Iodides on Copper Surfaces. 2. Influence of Surface Structure on Reactivity

The thermal chemistry of iodomethane, iodoethane, 1-iodopropane, 1-iodobutane, and 2-iodohexane on copper (100), (110), and (111) single-crystal surfaces was characterized in this and previous studies by temperature-programmed desorption (TPD) spectroscopy. The main decomposition pathway available to the methyl surface moiety that results from C-I bond activation in adsorbed iodomethane is α-hydride elimination to methylene, a step that occurs around 460-470 K on all three surfaces. Some methylene dimerization to ethane is also seen at higher coverages, at a rate that depends significantly on surface structure; ethane desorption peaks at 400 K on Cu(110), but only above 440 K on Cu(100) and Cu(111). Ethyl groups produced by iodoethane decomposition react at much lower temperatures and mostly undergo β-hydride elimination to ethylene. The ethyl dehydrogenation reaction is structure sensitive as well, a fact illustrated by the different ethylene desorption peak maxima observed in the TPD experiments, at 225, 247, and 255 K on Cu(110), Cu(111), and Cu(100), respectively (at saturation). Perhaps the more telling observations are the difference in feasibility of H-D scrambling in the ethylene resulting from conversion of a 1:1 mixture of normal and perdeutero iodoethane, a reaction viable on Cu(100) but not on Cu(110), and the 10-fold difference in ethane yield between those two crystals. Additional studies with 1-iodopropane and 1-iodobutane provided some information on the effect of chain length on reactivity, and experiments with 2-iodohexane attested to the high selectivity for removal of internal hydrogen atoms during β-hydride elimination from alkyl species.