The thermal reaction of vinyl chloride on nearly-stoichiometric α-Cr2O3(1 0 over(1, ̄) 2) produces gas phase acetylene, ethylene, 1,3-butadiene, and dihydrogen, and results in the deposition of chlorine adatoms. No surface carbon or combustion products (CO, CO2, H2O) are observed. Thermal desorption and spectroscopic studies indicate that the surface reaction of vinyl chloride proceeds through C-Cl bond cleavage to form Cl adatoms and surface vinyl groups which dehydrogenate to acetylene, hydrogenate to ethylene, and couple to form butadiene. Cl adatoms affect surface reactivity in two ways: (1) by increasing the barrier to vinyl dehydrogenation from 145 to 160 kJ/mol, and (2) by blocking Cr3+ sites which shuts down the surface chemistry. Selectivity to butadiene is dependent on Cl adatom coverage, where the increased stability of vinyl intermediates at lower Cl coverages gives rise to more butadiene coupling product. At higher Cl coverages, Cl appears to inhibit the mobility of surface vinyl and decreases the reaction probability for coupling. Photoemission and near edge X-ray absorption fine structure (NEXAFS) spectra show that a mixed monolayer of molecular and dissociated vinyl chloride is formed at 130 K. The polarization dependence of the NEXAFS indicates that vinyl chloride π-bonds with the molecular plane nominally parallel to the surface, while vinyl intermediates σ-bond at Cr centers with the molecular plane nominally perpendicular to the surface, in agreement with DFT predictions of the adsorption geometries.
Bibliographical noteFunding Information:
MAM, QM and DFC gratefully acknowledge financial support by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy through Grant DE-FG02-97ER14751. The efforts of DRM are sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy, under Contract No. DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. Special thanks are offered to NSLS staff members Steven Hulbert and Qing-Li Dong for their assistance. We also thank Qingfeng Ge of the Department of Chemistry and Biochemistry, Southern Illinois University for his help in setting up the original Cr 2 O 3 input file for the DFT calculations. The Laboratory for Advanced Scientific Computing and Applications (LASCA) at Virginia Tech is acknowledged for computational support.
- Chromium oxide
- Near edge extended X-ray absorption fine structure (NEXAFS)
- Single crystal
- Synchrotron radiation photoelectron spectroscopy
- Thermal desorption
- Vinyl chloride