Temperature-programmed reaction/desorption, Auger electron spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure in combination of calculations based on density functional theory have been employed to investigate adsorption and reaction of 1,3-C 6 H 4 I 2 on Cu(100). At 100 K, the surface species after 1,3-C 6 H 4 I 2 adsorption are found to be 1,3-C 6 H 4 I 2 , C 6 H 4 I, and 1,3-C 6 H 4 . The formation of these adsorbates is dependent on the adsorption sites of 1,3-C 6 H 4 I 2 . 1,3-C 6 H 4 I 2 adsorbed with the ring at a hollow site and parallel to the surface is predicted to be unstable and preferentially leads to CÀI bond dissociation. 1,3-C 6 H 4 , the intermediate from 1,3-C 6 H 4 I 2 decomposition, has a tilted adsorption geometry with a distorted ring. H 2 is the only reaction product observed after 550 K in the 1,3-C 6 H 4 I 2 decomposition on Cu(100), with all of the carbon atoms left on the surface. Dimerization of 1,3-C 6 H 4 molecules on Cu(100) has been described computationally, showing an activated and exothermic process. With the theoretically obtained activation energy of 28.2 kcal/mol and estimated surface coverages, coupling of 1,3-C 6 H 4 can occur by second-order kinetics before H 2 evolution. Dimerization of 1,3-C 6 H 4 on Cu(100) shows a different intermolecular interaction behavior from those of 1,2-C 6 H 4 and 1,4-C 6 H 4 on copper single crystal surfaces.
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.
The chemistry of 2-iodoacetic acid on Cu(100) has been studied by a combination of reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reaction/desorption (TPR/D), and theoretical calculations based on density functional theory for the optimized intermediate structures. In the thermal decomposition of ICH(2)COOH on Cu(100) with a coverage less than a half monolayer, three surface intermediates, CH(2)COO, CH(3)COO, and CCOH, are generated and characterized spectroscopically. Based on their different thermal stabilities, the reaction pathways of ICH(2)COOH on Cu(100) at temperatures higher than 230 K are established to be ICH(2)COOH --> CH(2)COO + H + I, CH(2)COO + H --> CH(3)COO, and CH(3)COO --> CCOH. Theoretical calculations suggest that the surface CH(2)COO has the skeletal plane, with delocalized pi electrons, approximately parallel to the surface. The calculated Mulliken charges agree with the detected binding energies for the two carbon atoms in CH(2)COO on Cu(100). The CCOH derived from CH(3)COO decomposition has a CC stretching frequency at 2025 cm(-1), reflecting its triple-bond character which is consistent with the calculated CCOH structure on Cu(100). Theoretically, CCOH at the bridge and hollow sites has a similar stability and is adsorbed with the molecular axis approximately perpendicular to the surface. The TPR/D study has shown the evolution of the products of H(2), CH(4), H(2)O, CO, CO(2), CH(2)CO, and CH(3)COOH from CH(3)COO decomposition between 500 and 600 K and the formation of H(2) and CO from CCOH between 600 and 700 K. However, at a coverage near one monolayer, the major species formed at 230 and 320 K are proposed to be ICH(2)COO and CH(3)COO. CH(3)COO becomes the only species present on the surface at 400 K. That is, there are two reaction pathways of ICH(2)COOH --> ICH(2)COO + H and ICH(2)COO + H --> CH(3)COO + I (possibly via CH(2)COO), which are different from those observed at lower coverages. Because the C-I bond dissociation of iodoethane on copper single crystal surfaces occurs at approximately 120 K and that the deprotonation of CH(3)COOH on Cu(100) occurs at approximately 220 K, the preferential COOH dehydrogenation of monolayer ICH(2)COOH is an interesting result, possibly due to electronic and/or steric effects.
Temperature-programmed reaction/desorption, reflection-absorption infrared spectroscopy, and density functional theory calculations have been employed to investigate the adsorption and thermal reactions of ClCH2CH2OH on clean and oxygen-precovered Cu(100) surfaces. On Cu(100), ClCH2CH2OH is mainly adsorbed reversibly. The ClCH2CH2OH molecules at a submonolayer coverage can change their orientation with increasing temperature. However, on oxygen-precovered Cu(100), all of the adsorbed ClCH2CH2OH molecules below 0.5 langmuir exposures completely dissociate to generate ethylene and acetaldehyde via the intermediate of ClCH2CH2O-. The computational studies predict that the ClCH2CH2O- is most likely to be adsorbed at the 4-fold hollow sites of Cu(100), with its C-O bond only slightly titled away from the surface normal and with a gauche conformation with respect to the C-C bond. The hollow-site ClCH2CH2O- has an adsorption energy that is 4.4 and 19.2 kcal x mol(-1) lower than that of the ClCH2CH2O- bonded at the bridging and atop sites, respectively. No significant effect of precovered oxygen on the ClCH2CH2O- bonding geometry and infrared band frequencies has been observed, as compared with the case without oxygen.
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