No abstract

Temperature-programmed reaction/desorption and reflection−absorption infrared spectroscopy have been employed to investigate the thermal reactions and adsorption geometry of FCH2CH2OH molecules on clean and oxygen-preadsorbed Cu(100) surfaces. Molecular desorption predominates in heating FCH2CH2OH adsorbed on clean Cu(100). However, ∼20% adsorbed FCH2CH2OH molecules at about half-monolayer coverage dissociate on the surface to form water, ethylene, and 1,4-dioxane. On the other hand, monolayer FCH2CH2OH completely dissociates on oxidized Cu(100) to form 1,4-dioxane and the surface intermediate of FCH2CH2O(a), which further decomposes to evolve FCH2CHO(g) at temperatures higher than ∼350 K. The decomposition of FCH2CH2OH to form FCH2CH2O(a) on oxidized Cu(100) begins at ∼160 K and is completed by 220 K. On clean Cu(100), FCH2CH2OH molecules at ∼0.25 monolayer coverage are adsorbed with the C−C−O skeleton approximately parallel to the surface. The C−C−O skeleton tilts away from the surface as the exposure is increased to a half-monolayer coverage. However, the parallel C−C−O orientation is not observed on the oxidized surface, even at the FCH2CH2OH exposure for a 0.25 monolayer coverage.

The reactions of BrCH 2 CH 2 OH were investigated on clean and oxygen-precovered Cu(100) surfaces under ultrahigh vacuum conditions. Reflection-absorption infrared spectroscopy (RAIRS) studies were performed to examine the surface intermediates that were generated from BrCH 2 CH 2 OH decomposition. Density functional theory calculations were employed to predict the infrared spectra, assisting in the identification of the reaction intermediates. On Cu(100), -CH 2 CH 2 O-, formed from the simultaneous scission of the bromine-carbon and oxygen-hydrogen bonds of BrCH 2 CH 2 OH at ∼190 K, decomposed and evolved into C 2 H 4 between 210 and 310 K in temperature-programmed reaction/desorption (TPR/D) experiments. A small amount of CH 3 CHO desorption was also observed. On oxygen-precovered Cu(100), -CH 2 CH 2 O-was also generated at lower exposures (<1.5 L) but at the BrCH 2 CH 2 OH dosing temperature of 115 K. The TPR/D study showed that C 2 H 4 with minor amounts of CH 3 CHO evolved between 210 and 310 K. However, at higher BrCH 2 CH 2 OH exposures (g1.5 L), BrCH 2 CH 2 O-was the major intermediate formed at ∼200 K. The formation temperature of C 2 H 4 and CH 3 CHO was extended to ∼400 K in this case.

X-ray photoelectron spectroscopy has been employed to study the surface intermediates from the thermal decomposition of HSCH2CH2OH on Cu(111) at elevated temperatures. On the basis of the changes of the core-level binding energies of C, O, and S as a function of temperature, it is found that HSCH2CH2OH decomposes sequentially to form -SCH2CH2OH and -SCH2CH2O-. Theoretical calculations based on density functional theory for an unreconstructed one-layer copper surface suggest that -SCH2CH2OH is preferentially bonded at a 3-fold hollow site, with an adsorption energy lower than the cases at bridging and atop sites by 15.6 and 47.5 kcal x mol(-1), respectively. Other structural characteristics for the energy-optimized geometry includes the tilted C-S bond (14.1 degrees with respect to the surface normal), the C-C bond titled toward a bridging site, and the C-O bond pointed toward the surface. In the case of -SCH2CH2O- on Cu(111), the calculations suggest that the most probable geometry of the adsorbate has its S and O bonded at hollow and bridging sites, respectively. With respect to the surface normal, the angles of the S-C and O-C are 27.9 and 34.0 degrees.