Probing the chemistry of CH₃I on Pt-Sn alloys

Adsorption and reaction of CH₃I (methyl iodide) on Pt(111) and the (2×2) and (√3×√3)R30° Sn/Pt(111) surface alloys was investigated primarily by using temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). CH₃I adsorbs molecularly on Pt(111) at 100 K, and 34% of the adsorbed CH ₃I monolayer decomposes during heating above 200 K in TPD. Competition occurs during heating within the chemisorbed layer between hydrogenation to produce methane and dehydrogenation that ultimately leads to adsorbed carbon. Alloying Sn into the Pt(111) surface decreases the heat of adsorption and the amount of decomposition of CH₃I. Alloyed Sn slightly reduces the CH₃I adsorption bond energy from 13.4 kcal/mol on Pt(111) to 11.4 kcal/mol on the (2×2) alloy with θ _Sn=0.25 and 9.3 kcal/mol on the (√3×√3)R30° Sn/Pt(111) alloy with θ_Sn=0.33. More notably, the Sn-Pt alloy surface strongly suppressed CH₃I decomposition. Only 4% of the adsorbed CH₃I monolayer decomposed on the (2×2) Sn/Pt(111) surface, and no decomposition of CH₃I occurred on the (√3×√3)R30° Sn/Pt(111) surface during TPD. Methane was the only hydrocarbon desorption product observed during TPD. These results point to the importance of adjacent "pure Pt" threefold hollow sites as reactive sites for CH₃I decomposition. Finally, we note that CH₃I, and presumably the other short-chain alkyl halides, are not reactive enough on Pt-Sn alloys to serve as convenient thermal precursors for preparing species small alkyl groups such as CH₃(a) for important basic studies of the reactivity and chemistry of alkyl groups on Pt-Sn alloys. Another approach is required such as the use of a CH₃-radical source or non-thermal activation of adsorbed precursors via photodissociation or electron-induced dissociation (EID).