AVS 53rd International Symposium
    Surface Science Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM9
Mechanisms of Isomerization and Dehydrocyclization of n-Hexane on Pt(100) and Pt(111) Surfaces Studied with Sum Frequency Generation

Wednesday, November 15, 2006, 10:40 am, Room 2004

Session: Electronic and Vibrational Excitations and Dynamics
Presenter: K.M. Bratlie, University of California, Berkeley
Authors: K.M. Bratlie, University of California, Berkeley
G.A. Somorjai, University of California, Berkeley
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Using sum frequency generation (SFG) vibrational spectroscopy the adsorption geometries and surface reactions of various C@sub 6@ hydrocarbons (n-hexane, 2-methylpentane, 3-methylpentane, and 1-hexene) on a Pt(100) single-crystal surface was investigated. The adsorptions and reactions were carried out under pressures of 1.5 Torr of C@sub 6@ hydrocarbon in the presence and absence of hydrogen (15 Torr) and in the temperature range of 300 - 450K. At 300K and in the presence of excess hydrogen, n-hexane, 3-methylcyclopentane, 2-methylpentane, and 1-hexene adsorbed molecularly on Pt(100) in flat-lying geometries. Upon heating the surface temperature to 450K, both n-hexane and 3-methylpentane underwent dehydrogenation to form a surface species in a standing-up geometry: hexylidyne. 2-Methylpentane, however, was dehydrogenated to @pi@-allyl c-C@sub 6@H@sub 9@ at 375K. Further heating of the surface hydrogenated the adsorbed molecules to hexylidyne. Adsorbed 1-hexene remained unreacted as the temperature was increased. Subsequently cooling the surface temperature to 300K returned all the flat-lying hydrocarbons. The absence of excess hydrogen impeded surface reactions. Both 2-methylpentane and 1-hexene adsorbed intact and in flat-lying geometries. 3-Methylpentane and n-hexane dehydrogenate to form metallacyclohexane and flat-lying 1-hexene. All four adsorbates remained intact as the surface temperature was increased to 450K and, afterward, decreased to 300K. On the basis of these results, the mechanisms for catalytic isomerization and dehydrocyclization of n-hexane were elucidated.