ADSORPTION AND THERMAL-DECOMPOSITION OF CH3SH ON THE PT(111) SURFACE

Adsorption, desorption and thermal decomposition of methanethiol (CH3SH) on a clean and on a (2 x 2)-S covered Pt(111) surface have been studied using temperature programmed desorption, high resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy as a function of temperature and coverage on the Pt(111) surface. Vibrational spectroscopy has been used to characterize the structure and bonding of surface intermediates formed during dehydrogenation of adsorbed methanethiol. The identity of surface intermediate species which form during thermal dehydrogenation is determined by both temperature and the availability of free Pt sites. Low coverages strongly favor low temperature dehydrogenation and decomposition reactions. Initial S-H bond activation does not occur at 110 K on crowded surfaces while complete decomposition of methanethiol is observed for 5% of a monolayer on a clean surface. For a saturated monolayer of methanethiol, XPS results indicate that about 60% of the carbon remains on the surface after heating to 750 K indicating that 40% of the carbon desorbs as CH4, and C2H4. The fractional yield of volatile organic products increases with increasing initial coverage of methanethiol up to monolayer coverage. The structure and geometry of the adsorbed intermediates formed by CH3SH decomposition also depends on coverage and temperature. Thermal dehydrogenation generates first CH3S, then CH2S as previously reported. In addition a new intermediate sigma-bonded SCH is identified in this work. Similar species form on the (2 x 2)-S pre-sulfided surface; however, in the pre-sulfided case the S-C bonds tend to be oriented more closely along the surface normal than they are on the clean surface. Methanethiol molecules remain intact on the pre-sulfided surface up to 180 K.