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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 hydrogen atoms add to adsorbed cyclohexene at 100 K on the Ni(100) surface, resulting in cyclohexane formation during subsequent TPR (temperature-programmed reaction) experiments. No C-C bond activation is observed after exposure to gas phase atomic hydrogen. Vibrational and isotope studies indicate that a cyclohexyl intermediate is formed by the addition of gas phase atomic hydrogen to adsorbed cyclohexene. This adsorbed cyclohexyl is hydrogenated primarily by surface hydrogen to form cyclohexane during subsequent heating. Cyclohexane yields increase with increasing atomic hydrogen exposure, and yields in the 90% range have been observed with large atomic hydrogen exposures. In the presence of only coadsorbed hydrogen, no significant hydrogen addition to adsorbed cyclohexene is observed, and cyclohexene desorption dominates during subsequent TPR experiments. Vibrational spectroscopy indicates that π-bonded cyclohexene with the CdC bond parallel to the surface is the dominant surface species in the presence of coadsorbed surface hydrogen. In contrast, di-σ-bonded cyclohexene is the dominant species in the absence of coadsorbed hydrogen. In the absence of coadsorbed hydrogen, dehydrogenation of the adsorbed cyclohexene results in benzene formation with increasing temperature. Adsorbed benzene formed by dehydrogenation has been identified after heating to 390 K in the absence of hydrogen using vibrational spectroscopy.

Gas-phase atomic hydrogen induces C-C bond activation in adsorbed cyclopropane on the Ni(111) surface, while coadsorbed hydrogen does not. Propane is the only desorbing product observed during subsequent temperature-programmed desorption experiments. Three propane formation pathways are observed. Gasphase atomic hydrogen reacts with adsorbed cyclopropane to form intermediates at 105 K, which are hydrogenated by coadsorbed hydrogen to form propane at 116 and 210 K. The 116 K pathway is similar to previous results obtained on the Ni(100) surface where propyl was determined to be the primary intermediate. The 210 K pathway has no analogue on the Ni(100) surface and is thought to involve a more stable form of propyl on the Ni(111) surface. The reaction of subsurface hydrogen with adsorbed cyclopropane leads to propane formation at 170 K on the Ni(111) surface. The absence of methane and ethane formation indicates that no multiple C-C bond activation processes occur. In contrast, cyclopropane desorption occurs before sufficient thermal energy is available to induce C-C bond breaking with coadsorbed hydrogen.