THERMAL-REACTIONS OF ALKYL IODIDES ON NI(100) SINGLE-CRYSTAL SURFACES

The chemistry of 1-iodopropane, 1-iodobutane, 2-iodobutane, 1-iodo-2-m ethylpropane, 2-iodo-2-methylpropane, 1-iodopentane, and 1-iodohexane on Ni(100) surfaces has been studied by using temperature-programmed d esorption and X-ray photoelectron spectroscopy. Below 100 K all the co mpounds adsorb molecularly through the iodine atom. The hydrocarbon ch ain orients parallel to the surface at first, but flips as the coverag e increases, and becomes perpendicular to the surface at saturation. T he C-I bond dissociates between 120 and 180 K to yield the correspondi ng alkyl fragment on the surface. At higher temperatures the alkyl gro ups decompose further, directly to carbon and hydrogen at low coverage s (below half-saturation), but mainly to a mixture of alkanes and alke nes at saturation. The change in the ratio between alkane and alkene p roduction was examined with respect to both the degree of substitution and the length of the carbon chain. It was found that the beta-hydrid e elimination that yields the alkenes is favored over the reductive el imination responsible for the alkane formation in alkyl species with a large number of beta-hydrogens, even in the presence of coadsorbed hy drogen on the surface. On the other hand, the hydrogenation steps were seen to dominate in the coadsorbed systems for alkyl species with a s mall number of beta-hydrogen atoms. An increase in chain length has th e effect of decreasing the alkene-to-alkane ratio further.