Langmuir 1998, 14, 1345-1347
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Carbon-Carbon Bond Activation of Cyclopropane by Subsurface Hydrogen on the Ni(111) Surface Adam T. Capitano and John L. Gland* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055 Received July 3, 1997. In Final Form: September 24, 1997 Carbon-carbon bond activation by subsurface hydrogen has been observed for adsorbed cyclopropane at 170 K on the Ni(111) surface. Propane was the dominant product, and no multiple C-C bond activation processes were observed. Cyclopropane desorbs at 124 K from both the clean and the hydrogen-coadsorbed Ni(111) surface. Neither coadsorbed hydrogen nor disproportionation induces propane formation. These observations are consistent with previous results that energetic forms of hydrogen are necessary to activate the C-C bond even in strained cyclopropane under ultrahigh-vacuum conditions.
Introduction The generation of surface intermediates under ultrahigh-vacuum (UHV) conditions is an area of growing interest. Although many catalytic and surface studies are based on a variety of adsorbed hydrocarbon intermediates, only recently have many of these intermediates been isolated and characterized.1 Brian Bent and his research group have done a series of definitive studies regarding the generation and understanding of these intermediates. Bent created and studied alkyl, vinyl, and cycloalkyl intermediates on the Cu(111) and Cu(100) surfaces through the dissociation of carbon halide bonds.2-6 Bent’s group also explored the use of energetic hydrogen for surface intermediate generation through Eley-Rideal mechanisms. Hydrogen atom induced abstraction from cyclohexane was used in an interesting series of studies which were focused on the surface chemistry of the cyclohexyl radical.7 Energetic hydrogen addition to ethylene, cyclohexene, and benzene has revealed the mechanisms of these interesting hydrogen addition reactions.8 Energetic forms of hydrogen can also be used to generate surface intermediates through carbon-carbon bond activation. On the Ni(100) surface, propyl was generated at low temperatures by carbon-carbon bond activation in cyclopropane with energetic forms of hydrogen.9 Both gas phase atomic hydrogen as well as subsurface hydrogen induced this reaction. Several reactions of subsurface hydrogen have been previously discussed by Ceyer.10 On the Ni(111) surface, subsurface hydrogen has an extra 15 kcal/mol of potential energy compared to surface hydrogen.11 Bulk hydrogen desorbs in the 180-250 K temperature range from the Ni(111) surface and has a high (1) (a) Gault, F. G. Gazz. Chim. Ital. 1979, 109, 255. (b) Rooney, J. J. J. Mol. Catal. 1985, 31, 14. (2) Chao, M. C.; Wentzlaff, T. H.; Bent, B. E. J. Phys. Chem. 1992, 96, 1836. (3) Lin, J. L.; Bent, B. E. J. Phys. Chem. 1992, 96, 8529. (4) Yang, X. M.; Kash, P. W.; Dong-Hong, S.; Flynn, G. W.; Holbrook, M. T.; Fischer, D. A.; Gland, J. L.; Bent, B. E. Submitted to be published. (5) Yang, X. M.; Kash, P. W.; Eng, J., Jr.; Flynn, G. W.; Holbrook, M. T.; Bare, S. R.; Fischer, D. A.; Gland, J. L.; Bent, B. E. J. Phys. Chem. 1996, 100, 12431. (6) Teplyakov, A. V.; Bent, B. E. J. Am. Chem. Soc. 1995, 117, 10076. (7) Ming, X.; Bent, B. E. J. Phys. Chem. 1993, 97, 4167. (8) Ming, X.; Bent, B. E. J. Vac. Sci. Technol., B 1992, 10, 2440. (9) Son, K. A.; Gland, J. L. J. Am. Chem. Soc. 1995, 117, 5415. (10) Johnson, A. D.; Daley, S. P.; Utz, A. L.; Ceyer, S. T. Science 1992, 257, 223. (11) Alefeld, G.; Vokl, J. In Hydrogen in Metals I; Alefeld, G., Vokl, J., Eds.; Springer-Verlag: Berlin, 1978; p 326.
translational energy as evidenced by a strongly forward peaked angular distribution.12-14 Subsurface hydrogen has been used to hydrogenate adsorbed methyl and ethylene on Ni(111),10,14,15 cyclohexene on Ni(100),16 and cyclopropane on Ni(100).8 In this paper, carbon-carbon bond activation in cyclopropane by subsurface hydrogen on the Ni(111) surface is reported. Cyclopropane is an interesting strained molecule which has been widely used as a model to study C-C bond activation in high-pressure studies.17 Although extensively studied in catalytic systems, experiments focused on hydrogen addition to cyclopropane under UHV are limited to the Ni(100) surface.8 Cyclopropane adsorption and bonding has also been studied on Ru(100),18 Ir(110),19 and Pt(111).20 Experimental Section All experiments were performed in an ultrahigh-vacuum system with a base pressure
