A Sum Frequency Generation Vibrational Spectroscopic Study of the Adsorption and Reactions of C₆Hydrocarbons at High Pressures on Pt(100)

Sum frequency generation (SFG) vibrational spectroscopy was used to investigate the adsorption geometries and surface reactions of various C6 hydrocarbons (n-hexane, 2-methylpentane, 3-methylpentane, and 1-hexene) on Pt(100). At 300 K and in the presence of excess hydrogen, n-hexane, 3-methylpentane, and 2-methylpentane adsorb molecularly on Pt(100) mostly in "flat-lying" conformations. Upon heating the metal surface to 450 K, the molecules underwent dehydrogenation to form new surface species in "standing-up" conformations, such as hexylidyne and metallacyclic species. 1-Hexene, however, dehydrogenated to form metallocycle Pt3⋮C-(CH2)5-Pt at 300 K in the presence of excess hydrogen and remained unreacted on the surface upon heating the metal surface to 450 K. Dehydrogenation was enhanced in the absence of excess hydrogen in the cases of n-hexane, 2-methylpentane, and 3-methylpentane to form metallocycle Pt3⋮C-(CH2)5-Pt; 2-methyl-1-pentene and 4-methyl-1-pentene; and metallacyclohexane, respectively, at 300 K. These surface species remained unreacted after increasing the surface temperature to 450 K. The mechanisms for catalytic isomerization and dehydrocyclization of n-hexane were discussed on the basis of these results.