Generation and Reaction of Vinyl Groups on a Cu(100) Surface

Abstract

The adsorption and reactions of vinyl bromide and vinyl iodide on a Cu(100) surface have been studied by temperature-programmed desorption in conjunction with near-edge X-ray absorption fine structure (NEXAFS) and work function change measurements. Vinyl bromide adsorbs molecularly on the surface at 100 K. The polarization dependence of the π*CC resonance indicates that the molecules lie with their π bond within 28 ± 5° of parallel to the surface. Upon heating, both vinyl bromide and vinyl iodide decompose to generate surface vinyl groups, which adopt a tilted orientation on the surface. Both the molecular halides and the surface vinyl groups show a splitting of the π*CC NEXAFS resonance due to the inequivalence of the carbon atoms in these species. The position of the σ*CC shape resonances for these species indicates little change (<0.05 Å) in CC bond length due to adsorption and dissociation to form vinyl groups. Chemical displacement studies show that the CBr bond cleavage in vinyl bromide occurs at 160 K. This dissociation temperature is confirmed by complementary NEXAFS and work function change measurement results. At 250 K, vinyl groups couple to yield 1,3-butadiene with 100% selectivity.

Reactions of vinyl groups on a model chromia surface: Vinyl chloride on stoichiometric α-Cr2O3

“…A similar assignment has been reported for multilayer vinyl chloride and surface vinyl groups on Cu(1 0 0) [13,14]. In the mixed monolayer following adsorption at 130 K both peaks are evident but they display different polarization dependence.…”

“…This interpretation is consistent with the photoemission data which indicates that most of the C-Cl bonds are broken after heating to 200 K. The remaining C 1s ? p* transition at 284.5 eV is similar to the reported spectrum for surface vinyl groups on Cu(1 0 0) [13], and serves as a fingerprint for surface vinyl groups while unambiguously demonstrating the presence of an intact p-system in the surface intermediate responsible for the observed reaction chemistry. The polarization dependence of this peak is the same as observed at 130 K indicating that the vinyl groups are still oriented primarily along the surface normal.…”

“…r à ðC-CÞ and C 1s ! r à ðC-HÞ , should appear at higher photon energies in the range of about 288.0-305.0 eV [13,14].…”

“…C 1s ? p* transitions are expected in the photon energy range from about 284.0-288.0 eV when there is a species containing an intact p-system on the surface [13,14,23] and the polarization vector of the light has a non-zero projection on the p-orbitals in the p-system. Resonances for vinyl species with C 1s transitions corresponding to carbon sigma bonds, such as C 1s !…”

“…The use of halogenated reactants to simulate the formation of surface intermediates occurring in hydrocarbon surface reactions has found increasing application in surface science experiments [13][14][15], and vinyl chloride is expected to provide a low temperature route to the formation of surface vinyl groups in UHV on a-Cr 2 O 3 ð1 0 1 2Þ. Thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure 0039-6028/$ -see front matter Ó 2008 Elsevier B.V. All rights reserved.…”