Gas Phase Atomic Hydrogen Induced Carbon−Carbon Bond Activation in Cyclopropane on the Ni(100) Surface

Abstract

Carbon−carbon bond activation in adsorbed cyclopropane is observed following exposure to gas phase atomic hydrogen on the Ni(100) surface for temperatures as low as 100 K. Exposure to either gas phase atomic hydrogen or deuterium results in formation of adsorbed propyl. In both cases subsequent reaction between adsorbed propyl and coadsorbed hydrogen/deuterium produces propane at 121 K. The activation of a single C−C bond in adsorbed cyclopropane dominates as indicated by the fact that propane is the only product observed. No multiple C−C bond activation which would result in methane or ethane formation was ever observed. These reactions and their mechanisms have been investigated using temperature-programmed reaction (TPR) and vibrational spectroscopy using high-resolution electron energy loss spectroscopy (HREELS). The reactivities of hydrogen and deuterium were indistinguishable during these experiments so we have used the generic term hydrogen or gas phase atomic hydrogen to describe the reactions of both hydrogen and deuterium. The vibrational spectrum of adsorbed cyclopropane indicates weak interaction with the Ni(100) surface at 100 K. This weak interaction results in molecular cyclopropane desorption at 123 K with only a small amount of dehydrogenation. After atomic hydrogen exposure, a new loss feature appears at 383 cm-1 in the vibrational spectrum. This new mode corresponds to the Ni−C bond stretching mode of adsorbed propyl, the primary reaction intermediate. Carbon−carbon bond activation in adsorbed cyclopropane also causes substantial reduction in the intensities of cyclopropane's ring deformation modes at 821 and 1006 cm-1. These results clearly indicate that C−C bond activation occurs during exposure to gas phase atomic hydrogen. Isotopic labeling studies reveal that the adsorbed propyl intermediate is hydrogenated by labelled surface hydrogen. Carbon−carbon bond activation in adsorbed cyclopropane has never been observed during adsorption on a surface with preadsorbed hydrogen nor during exposure to nascent hydrogen formed by dissociating molecular hydrogen. A detailed potential energy diagram for the reactions of adsorbed cyclopropane on the Ni(100) surface is developed based on results from these experiments and the literature.

Gas Phase Atomic Hydrogen-Induced Hydrogenation of Cyclohexene on the Ni(100) Surface

“…We propose that competition between adsorption of gas phase atomic hydrogen on the Ni surface and abstraction from the adsorbed organic appears to play an important role in controlling the reaction selectivity. Adsorption of gas phase atomic hydrogen on the Ni surface is a nonactivated exothermic process (Δ H ∼ −64 kcal/mol) . However, hydrogen abstraction from cyclohexene is activated by more than 9 kcal/mol of activation energy and is less exothermic (Δ H ∼ 23 kcal/mol)…”

“…The experiments were performed in an ultrahigh-vacuum chamber which has been described previously in detail …”

“…We have found that small strained cycloalkanes react readily with gas phase atomic hydrogen on the Ni(100) surface …”

“…On the other hand, atomic hydrogen addition is not observed for cyclohexane, which has unstrained C−C bonds similar to saturated n -alkane's. Instead, hydrogen abstraction dominates in the adsorbed cyclohexane, leading to cyclohexene and cyclohexadiene formation during exposure to gas phase atomic hydrogen …”

Interaction of gas phase atomic hydrogen with Pt(111): Direct evidence for the formation of bulk hydrogen species

“…Because gas phase atomic hydrogen is 218.4 kJ/mol more energetic than molecular hydrogen (1/2 dissociation energy of H 2 ), when interacting with Pt(111), gas phase atomic hydrogen is energetic enough to overcome any activation energy to form all likely adsorbed H ad species (surface H ad species and bulk H ad species), surely including the active species for practical catalytic reactions. Gland et al [21][22][23][24][25] found that gas phase atomic hydrogen can undergo ring-opening reaction directly with cycloalkanes adsorbed on metal single crystal surfaces and that the ring-opening reaction follows Eley-Rideal mechanism. Therefore, even under UHV conditions, interaction of gas phase atomic hydrogen with Pt(111) can lead to the formation of detectable bulk H ad species.…”