IODOMETHANE DISSOCIATION ON CU(111) - BONDING AND CHEMISTRY OF ADSORBED METHYL-GROUPS

The chemistry of iodomethane on a Cu (111) surface has been studied by a combination of work function measurements, Auger electron spectroscopy, high-resolution electron energy-loss spectroscopy, and temperature programmed reaction spectroscopy. Our results show that iodomethane dissociates below 200 K to form iodine atoms and methyl groups, which are stable up to 400 K. At low coverages, the methyl groups bond with C(s) symmetry and at high coverages with C(3-nu) symmetry. The methyl group stretching frequency on the surface is redshifted by approximately 150 cm-1 compared to that of iodomethane. Possible bonding sites as well as reasons for the stretching mode softening and coverage-dependent orientation of methyl groups are discussed. The surface reactions of methyl groups on Cu (111) are coverage dependent. At low coverages, CH3(a) decomposes at 450 K to evolve methane, ethylene, and propylene. a elimination from methyl groups to produce CH2 and H is the rate-determining step in forming these products. At higher coverages, methyl coupling also occurs to evolve ethane.