Surface vibrational sum frequency generation (SFG) has been used to identify surface species in situ during catalytic reactions of 1.5 Torr cyclohexene (C6H10) in the presence and absence of 15 Torr H2 on Pt(111) crystal surfaces. Various surface species were observed during the reactions in the range 303−483 K. Upon adsorption at 303 K in the absence of H2, cyclohexene is dehydrogenated to form 1,4-cyclohexadiene (C6H8). At 323 K, this species converts to π-allyl c-C6H9 intermediate. At temperature higher than 423 K, benzene coexists with C6H9 on the surface. Similar results were obtained in the presence of 15 Torr H2 except that in this circumstance 1,3-cyclohexadiene coexists with 1,4- cyclohexadiene at 303 K, and with C6H9 at temperature higher than 403 K. This study reports, for the first time, the existence of π-allyl c-C6H9 as a major surface species in the presence of high-pressure cyclohexene and H2 on Pt(111). In addition, adsorbed 1,3-cyclohexadiene has been observed as a reactive intermediate in the process of dehydrogenation to benzene. Finally, it has been found that excess hydrogen increases the disordering and decreases the surface coverage of the C6H9 on the surface.

The adsorption geometries and surface reactions of various C(6) hydrocarbons (n-hexane, 2-methylpentane, 3-methylpentane, and 1-hexene) adsorbed on Pt(111) were investigated using sum frequency generation (SFG) surface vibrational spectroscopy. The adsorptions and reactions were carried out in 1.5 Torr of C(6) hydrocarbons in the absence and presence of excess hydrogen (15 Torr) and in the temperature range 296-453 K. At 296 K and in the presence of excess hydrogen, n-hexane and 3-methylpentane adsorbed molecularly on Pt(111) mostly in "flat-lying" geometries. Upon heating the sample up to 453 K, the molecules underwent dehydrogenation to form new surface species in "standing-up" geometries, such as hexylidyne and metallacyclic species. However, 2-methylpentane and 1-hexene were dehydrogenated to metallacyclobutane and hexylidyne, respectively, at 296 K in the presence of excess hydrogen. The dehydrogenated species remained unreacted on the surface upon heating the sample up to 453 K. The absence of excess hydrogen enhanced dehydrogenation of n-hexane and 3-methylpentane to form pi-allyl c-C(6)H(9) and metallacyclohexane, respectively, at 296 K. Upon heating to 453 K, the pi-allyl c-C(6)H(9) species underwent irreversible dehydrogenation, while hexylidyne and metallacyclic species remained unreacted. On the basis of these results, the mechanisms for catalytic isomerization and dehydrocyclization of n-hexane, which are the important "reforming" reactions to produce high-octane fuels over platinum, were discussed.

No abstract