Iodomethane Decomposition on Cu(110): Surface Reactions of C1 Fragments

Article abstract of DOI:10.1021/j100183a061

Methyl groups can be generated on Cu(110) surfaces under ultrahigh-vacuum conditions by dissociatively adsorbing iodomethane at temperatures above 180 K.We report here on the surface reactions of adsorbed CH3 as studied by a combination of Auger electron spectroscopy (AES), temperature-programmed reaction experiments, and isotope labeling.The reactions of methyl groups on Cu(110) are coverage-dependent.For θ_CH3 less than 0.015 (number of methyl groups per surface copper atom), methyl groups disproportionate to produce methane and ethylene in a 2:1 ratio.At higher coverages, methyl groups also couple to form ethane.Studies in which the surface coverage of iodine is varied suggest that the major effect of this coadsorbate is to block surface sites.Mechanistic studies in which H, CH3, and CH2 (formed by CH2I2 dissociation) were coadsorbed and/or isotopically labeled show that the rate-determining step in methyl disproportionation on Cu(110) is α-elimination to produce adsorbed H and CH2.Methane is then formed by reductive elimination (CH3 + H -> CH4) while ethylene is formed primarily by methylene insertion coupled with β-hydride elimination (CH2 + CH3 -> C2H5 -> C2H4 +H).Computer simulations using the experimentally determined kinetic parameters show that H recombination to form H2 as well as CH2 coupling to form C2H4 can also occur, but these are minor reaction pathways under the conditions of methyl decomposition.The experimentally determined activation energies have been applied in Born-Haber cycles to determine the heats of formation for adsorbed CH3, CH2, and H.The bond enthalpy for Cu(110)-CH2 has also been determined to be 75 +/- 10 kcal/mol, more than twice the nominal Cu(110)-CH3 bond energy of 33 +/- 8 kcal/mol.