ORIENTATION AND DECOMPOSITION KINETICS OF METHYL-IODIDE ON PT(111)

The thermal chemistry of methyl iodide adsorbed on Pt(111) at T-S greater than or equal to 21K was examined using temperature programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS). Both TPD and RAIRS indicate that saturation coverage for methyl iodide is 0.19 ML. The first 0.085 ML of adsorbate thermally dissociates at T-S greater than or equal to 190 K to form surface-bound methyl radicals and iodine. Coverages in excess of 0.085 ML follow competing molecular desorption and dissociation pathways with the probability of desorption increasing with coverage. The maximum coverage of adsorbed methyl radicals obtained by thermal dissociation of methyl iodide was 0.12 ML. Methyl radicals at T-S greater than or equal to 215 K both dehydrogenate and reactively scavenge adsorbed hydrogen to desorb as methane. Infrared surface dipole selection rules were employed to estimate the mean polar angle between the methyl iodide's symmetry axis and the surface normal. After annealing to 90 K, methyl iodide is tilted at 42 degrees for all coverages less than 0.085 ML. Further increases in coverage serve to linearly decrease the polar angle to 18 degrees at saturation. Arrhenius analyses of time-resolved RAIRS data indicate an activation barrier for H3C-I bond cleavage of 48 +/- 3 kJ mol(-1) and a pre-exponential factor of 8 x 10(10+/-13) 3 s(-1).