AVS 46th International Symposium
    Surface Science Division Monday Sessions
       Session SS2-MoM

Paper SS2-MoM4
Reactions of Methyl Groups on Sn/Pt(111) Alloys

Monday, October 25, 1999, 9:20 am, Room 607

Session: Catalysis on Metals
Presenter: B.E. Koel, University of Southern California
Authors: B.E. Koel, University of Southern California
H. He, University of Southern California
Correspondent: Click to Email
Reactions of alkyl intermediates are important in many catalytic hydrocarbon reactions over metal and metal alloy surfaces. Reactions of adsorbed CH@sub 3@ (methyl) groups, formed using a pyrolytic azomethane source to produce incident methyl radicals, on two ordered Sn/Pt(111) surface alloys have been studied using HREELS, UPS, TPD, AES and XPS. Chemisorbed CH@sub 3@ species are identified at low doses but longer chain hydrocarbons can be formed at high doses on both Sn/Pt(111) surfaces at 100 K. Chemisorbed methyl groups are characterized by a prominent peak in UPS at 8 eV BE (CH@sub 3@ HOMO) and a strong peak in HREELS at 1240 cm@super -1@ from the @delta@@sub s@(CH@sub 3@) mode. These studies show that alloyed Sn decreases the dehydrogenation rates of akyl groups on Pt(111) surfaces and enables other C-C bond coupling reactions to occur. Alkane and alkene products were desorbed below 200 K in TPD for both Sn/Pt(111) surface alloys. On the (2x2) alloy, only CH species are stable at 300 K, and these react to desorb CH@sub 4@ at 432 K. TPD after methyl dosing on the (@sr@3x@sr@3)R30° alloy at 300 K gave only one very sharp CH@sub 4@ desorption peak at 453 K, and we propose that this arises from decomposition of CH@sub 3@CH species. The chemistry of Pt-Sn alloy surfaces leads to much lower carbon buildup than on Pt surfaces from thermal reactions of alkyl adsorbates.