Reactions of vinyl groups on a model chromia surface: Vinyl chloride on stoichiometric α-Cr₂O₃(1 0 over(1, ̄) 2)

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.