13) Scong, H. S.; Kim, U. S.; Kim, Y.; Seff, K. J. Phys. Chem., submitted (14) Swng, H. S.; Kim, Y.; Seff, K. J. Phys. Chem. 1991,95,9919-9924. (15) Sun, T.; Heo, N. H.; Seff, K., unpublished work. (16) Stamovlasis, D.; Wilson, J. R.; Seff, K., unpublished work.The interactions of cyclohexene with a Pt( 1 11) surface have been studied using a combination of bismuth postdosing thermal desorption mass spectroscopy (BPTDS) and high-resolution electron energy loss spectroscopy (HREELS). BPTDS is a technique which utilizes vapor-deposited Bi (at a surface temperature of -100 K) to passivate a previously prepared adlayer against intraadsorbate bond-breaking reactions. After such passivation, the surface is heated and adsorbed intermediates desorb intact for mass spectral identification, provided they have stable gas-phase analogues. Here we show that BPTDS is a useful technique for monitoring the coverages of adsorbed intermediates produced during the dehydrogenation of cyclohexene on Pt(ll1). At 95 K, cyclohexene adsorbs molecularly in a di-a fashion. By 200 K (E, z 13.7 kcal/mol) this species converts to another form of di-a, molecularly adsorbed cyclohexene, which itself converts to r-allyl c-C6H9, (plus adsorbed hydrogen) at 200-240 K (E, z 14.4 kcal/mol). At about the same temperature, part of the cyclohexene decomposes, producing small amounts of adsorbed benzene and cyclohexadiene. At about 340 K, the C-CgHg,, species converts to adsorbed benzene (E, = 20.8 kcal/mol) and the adsorbed hydrogen desorbs as H2 The C-CgHg, intermediate is identified in BPTDS by its deuteration to c-CsH9D gas at -190 K when coadsorbed with deuterium. No significant H-D exchange occurs between coadsorbed deuterium and molecularly adsorbed hydrocarbons when probed by BPTDS.
