GAS PHASE ATOMIC HYDROGEN INDUCED CARBON-CARBON BOND ACTIVATION IN CYCLOPROPANE ON THE NI(100) SURFACE

摘要

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.