Sequential Dehydrogenation of Unsaturated Cyclic C₅ and C₆ Hydrocarbons on Pt(111)

Abstract

The structures and reactivities of various cyclic C 5 and C 6 hydrocarbons (cyclopentene, cyclopentadiene, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene) adsorbed on Pt(111) have been examined by means of reflection-absorption infrared (RAIR) spectroscopy. At temperatures below 200 K, these molecules bind intact to the Pt(111) surface by means of strong interactions with CdC double bonds. Steric interactions between the surface and certain CH 2 groups on the ring systems figure prominently in determining the conformation adopted in the bound states of several of the molecular adsorbates (e.g., cyclopentene and cyclohexene). These bonding habits are identified both by observing significant electronic interactions that weaken certain C-H bonds (so-called mode softening) and also by developing analogies with trends seen with similar ring systems and complexes. At higher temperatures above 200 K, the C 5 species are dehydrogenated in high yield to a planar, surface-bound pentahaptocyclopentadienyl species (η 5 -C 5 H 5 ) while the C 6 cyclic hydrocarbons react to give benzene; these surface-bound products, which are stable to temperatures >400 K, have been identified in earlier studies as well. The present work adds to the understanding of the nature and energetics of the sequential C-H bond activation processes involved in their formation. In addition, several intermediates lying along the reaction pathways to the respective planar intermediates have been identified and spectroscopically characterized for the first time. Most notably, we observe that 1,3cyclohexadiene loses one hydrogen between 200 and 250 K to give a stable η 5 -cyclohexadienyl intermediate. An efficient partial dehydrogenation of cyclopentene at 250 K to give the corresponding diene is also observed. Our data also demonstrate the importance of heretofore unappreciated hyperconjugation effects in the vibrational spectroscopy of the C-H stretching modes of metal-surface-bound π systems. The insights developed in this study regarding such electronic interactions are used to develop an understanding of the binding sites and conformational states adopted by the various adsorbates and intermediates formed during their decomposition.

Molecular surface science of C–H bond activation and polymerization catalysis

“…At low pressures of cyclohexene (£0.5 Torr) and low surface temperatures p-allyl c-C 6 H 9 is the major surface species on the Pt(100) surface as seen in figure 10 [33]. Increasing pressures to 1.5 Torr of cyclohexene shows that 1,4-cyclohexadiene is co-adsorbed with p-allyl c-C 6 H 9 .…”

“…Parameters such as the pressure of cyclohexene, presence of excess hydrogen, and surface temperature are varied. p-Allyl c-C 6 H 9 and cyclohexyl have been found to co-exist on the Pt(100) surface until the pressure of cyclohexene is increased to 1.5 Torr whereupon 1,4-cyclohexadiene becomes co-adsorbed with p-allyl c-C 6 H 9 and cyclohexyl as seen in figure 10 [33]. The dominant surface species on the Pt(111) surface is p-allyl c-C 6 H 9 until the cyclohexene pressure is increased to 1.5 Torr as a result of which p-allyl c-C 6 H 9 dehydrogenates to 1,4-cyclohexadiene.…”

Scanning Tunneling Microscopy (STM) at High Pressures. Adsorption and Catalytic Reaction Studies on Platinum and Rhodium Single Crystal Surfaces

“…At 2Â10 )6 Torr both cyclohexane and cyclohexene form the same structure on the surface (figure 6a, b). Spectroscopic [14] and thermal desorption [15] studies tell us that the species present at room temperature is the partially dehydrogenated p-allyl C 6 H 9 . The periodicity of $5.5-6 Å suggests that the structure may in fact be a (2Â2), but further studies are necessary to determine the orientation of the underlying platinum lattice.…”

Investigation of the Deactivation Phenomena Occurring in the Cyclohexane Photocatalytic Oxidative Dehydrogenation on MoOx/TiO2 through Gas Phase and in situ DRIFTS Analyses

“…In particular, by using dichloromethane as solvent, the presence of cyclohexene and cyclohexenone was found. Thus it is not possible to exclude the formation of an unsaturated cyclic hydrocarbon in the presence of molybdenum, taking into account also that the cyclohexene adsorbs on the surface through the C=C double bond, and the contribution of the -HC=CH-stretching would therefore be absent [36].…”