Methyl radical adsorption on Cu(111): Bonding, reactivity, and the effect of coadsorbed iodine

Methyl groups have been generated on a Cu(111) surface by thermal dissociation of CH3I and by adsorption of methyl radicals produced by pyrolyzing azomethane (CH3N2CH3). High-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed reaction (TPR) have been used to study the bonding and chemistry of adsorbed CH3. Both in the presence and absence of coadsorbed iodine, methyl groups decompose by rate-determining CH bond scission above 400 K to produce methane and ethylene; ethane is also formed by rate-determining methyl coupling at high coverage. The product evolution temperatures are ∼ 15 K higher when iodine is coadsorbed on the surface. In addition, the maximum methyl coverage attained with CH3I is a factor of 3 to 4 less than that achieved by adsorbing methyl radicals. Surface vibrational spectra with and without coadsorbed iodine are quite similar. These results suggest that the predominate effect of iodine is to block surface sites.