Decomposition of cyclohexene on Pt(111) : a BPTDS and HREELS study

• Published in: Journal of physical chemistry (1952) 1992-07, Vol.96 (14), p.5965-5974
• Main Authors: HENN, F. C, DIAZ, A. L, BUSSELL, M. E, HUGENSCHMIDT, M. B, DEOMAGALA, M. E, CAMPBELL, C. T
• Format: Article
• Language: English
• Subjects: Catalysis; Catalytic reactions; Chemistry; Exact sciences and technology; General and physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0022-3654; 1541-5740
• DOI: 10.1021/j100193a059

The interactions of cyclohexene with a Pt(111) 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 B sub 1 (at a surface temperature of approx 100K) 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. It is shown that BPTDS is a useful technique for monitoring the coverages of adsorbed intermediates produced during the dehydrogenation of cyclohexene on Pt(111). At 95K, cyclohexene adsorbs molecularly in a di- sigma fashion. By 200K (E sub a approx = 13.7 kcal/mol) this species converts to another form of di- sigma , molecularly adsorbed cyclohexene, which itself converts to pi -allyl c-C sub 6 H sub 9,a (plus adsorbed hydrogen) at 200-240K (E sub a approx = 14.4 kcal/mol). At about the same temperature, part of the cyclohexene decomposes, producing small amounts of adsorbed benzene and cylcohexadiene. At approx 340K, the c-C sub 6 H sub 9,a species converts to adsorbed benzene (E sub a = 20.8 kcal/mol) and the adsorbed H desorbs as H sub 2 . The c-C sub 6 H sub 9,a intermediate is identified in BPTDS by its deuteration to c-C sub 6 H sub 9 D gas at approx 190K when coadsorbed with D. No significant H--D exchange occurs between coadsorbed D and molecularly adsorbed hydrocarbons when probed by BPTDS.