Adsorption and Reactions of C₆Hydrocarbons at High Pressures on Pt(111) Single-Crystal Surfaces Studied by Sum Frequency Generation Vibrational Spectroscopy:  Mechanisms of Isomerization and Dehydrocyclization ofn-Hexane

Abstract

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.

Influences of Mesoporosity Generation in ZSM-5 and Zeolite Beta on Catalytic Performance During n-Hexane Isomerization

“…However, as is clear from Fig. 2, 27 Al NMR spectrum of the deslicated sample, the treatment did result in dealumination and about 6% of Al was present in extra-frame position d = 0.82 ppm). This observation is in corroboration with that by Chal et al [18].…”

“…A study on location of Pt with respect to the acid sites within the channel as well as on the external surface viz a viz within the channels would be of interest particularly when many workers report the Pt loading (wt%) and then referring to the dehydrogenation-isomerization-hydrogenation mechanism necessitating the proximity of acidand metal-functions at molecular level [25]. Skeletal isomerization of alkanes can proceed solely on metals [26,27] as well as solely on acid catalyst, zeolite Beta [25] following different mechanisms reflected in product distributions. In case of the former only mono-branched paraffins have been observed where as in the latter case, additionally di-branched isomers were also observed.…”

Sub-10 nm Platinum Nanocrystals with Size and Shape Control: Catalytic Study for Ethylene and Pyrrole Hydrogenation

“…Under the catalytic reaction condition, SFG is a much more effective technique for interfacial chemical analysis than conventional linear spectroscopy, such as infrared absorption or Raman. [36][37][38][39][40][41] In linear spectroscopy, the absorption is mainly contributed by bulk gases, and therefore the absorption generated by adsorbed species at the catalyst surface is relatively small. In contrast, due to the second-order nonlinear optical process of SFG, the absorption of isotropic bulk gases do not appear in the SFG spectrum and the absorption of adsorbed species, the symmetry of which is broken at the surface, is solely enhanced.…”

Thiolate‐Protected Aun Nanoclusters as Catalysts for Selective Oxidation and Hydrogenation Processes

“…On the other hand, a great deal of work on model catalysts, such as on single-crystal surfaces, have been investigated under ultrahigh vacuum (UHV) conditions. [11] These model catalysts are, however, largely deviated from the real-world catalytic conditions. Herein, we report our effort in creating a well-defined catalytic system by preparing truly monodisperse, thiolate-capped Au n nanoclusters and utilizing them as homogeneous (or supported) catalysts for certain chemical reactions, including selective oxidation and hydrogenation processes.…”