AVS 50th International Symposium
    Surface Science Friday Sessions
       Session SS1-FrM

Paper SS1-FrM7
Probing the Chemistry of CH@sub 3@I on Pt-Sn alloys

Friday, November 7, 2003, 10:20 am, Room 327

Session: Catalysis IV: Reactivity of Complex Systems
Presenter: E.C. Samano, CCMC-UNAM, Mexico
Authors: E.C. Samano, CCMC-UNAM, Mexico
C. Panja, USC
N.A. Saliba, USC
B.E. Koel, USC
Correspondent: Click to Email
Adsorption and reaction of CH@sub 3@I (methyl iodide) on Pt(111) and the (2x2) and (@sr@3x@sr@3)R30° Sn/Pt(111) surface alloys was investigated primarily by using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). CH@sub 3@I adsorbs molecularly on Pt(111) at 100 K, and 34 % of the adsorbed CH@sub 3@I 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 CH@sub 3@I. Alloyed Sn slightly reduces the CH@sub 3@I adsorption bond energy from 13.4 kcal/mol on Pt(111) to 11.4 kcal/mol on the (2x2) alloy with @theta@@sub Sn@ = 0.25 and 9.3 kcal/mol on the (@sr@3x@sr@3)R30° Sn/Pt(111) alloy with @theta@@sub Sn@ = 0.33. More notably, the Sn-Pt alloy surface strongly suppressed CH@sub 3@I decomposition. Only 4% of the adsorbed CH@sub 3@I monolayer decomposed on the (2x2) Sn/Pt(111) surface, and no decomposition of CH@sub 3@I occurred on the (@sr@3x@sr@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" three-fold hollow sites as reactive sites for CH@sub 3@I decomposition. Finally, we note that CH@sub 3@I, 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 CH@sub 3@(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 CH@sub 3@-radical source or non-thermal activation of adsorbed precursors via photodissociation or electron-induced dissociation (EID).