Cyclohexene adsorption and reactions on clean and bismuth-covered Pt(111)

Abstract

The chemisorption and dehydrogenation of cyclohexene on clean and bismuth-covered Pt(111) was studied using thermal desorption mass spectrometry (TDS), X-ray photoelectron spectroscopy (XRS), and Auger electron spectroscopy. Four different molecular cyclohexene desorption states appear in the thermal desorption spectra of cyclohexene on clean Pt(111). The temperatures and activation energies for desorption of these states are 158 K (9.1 kcal/mole), 190 K (11.0 kcal/mole), 255 K (15.0 kcal/mole), and 300 K (17.9 kcal/mole). The XPS data indicate that about 4.5 Pt(111) surface atoms are required on average to accommodate each adsorbed cyclohexene molecule. At low coverages ( Θ C 6 H 10 < 0.05 molecule per Pt atom) all the adsorbed cyclohexene dehydrogenates upon heating to produce absorbed benzene (at temperatures below 350 K), which further decomposes on the surface by 800 K to give graphitic carbon and H 2. At high coverages of either cyclohexene or coadsorbed bismuth, the adsorbed cyclohexene and the product benzene molecularly desorb from the surface. A different intermediate in cyclohexene dehydrogenation is stabilized at these high coverages (perhaps π-allyl c-C 6H 9,a). During the TDS experiments on clean or Bi-dosed Pt(111), neither cyclohexadiene nor products of CC bond breaking were detected with the mass spectrometer. The blocking of Pt surface sites with coadsorbed Bi adatoms, which have only minor electronic influences on the Pt sites, showed that the rate constants for diffusion and dehydrogenation of cyclohexene on Pt(111) are considerably larger than that for desorption. As a consequence, the effective ensemble requirement for cyclohexene dehydrogenation is relatively small, especially compared to benzene and cyclohexane.