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J . Phys. Chem. 1987, 91, 5224-5228
understood. Work is under way to improve our understanding of these intensities. The spectral subtraction techniques used herein provide useful information about vibrational and electronic transitions not immediately observable in the original sample and reference spectra. Further, they are required for obtaining accurate representations of the band shapes and intensities of electronic transitions studied by inelastic electron tunneling spectroscopy.
Acknowledgment. We gratefully acknowledge the National Science Foundation and the Division of Materials Research for their support in the form of Grants DMR-8414566 and DMR8320556. We also thank S. N. Sarkar and Alana Riste for their assistance. Registry No. (Me,N)NCS, 15597-46-3; (Me,N),Co(NCS),, 1405214-3: (Et4N)CI,56-34-8: CoBr:-, 14493-02-8;(Et,N),CoCI,, 6667-75-0: COCI,~-,14337-08-7:(Et,N),CoBr,, 2041 -04-5.
Reaction of Niobium Clusters with Benzene-h6 and -d6: Evidence for Cluster-Induced Dehydrogenation M. R. Zakin, D. M. Cox, and A. Kaldor* Corporate Research Laboratory, Exxon Research and Engineering Company, Annandale, New Jersey 08801 (Received: March 6, 1987: In Final Form: May 13, 1987)
The gas-phase reaction of niobium clusters with benzene-h, and -d6 has been studied by using a fast-flow reactor. The products obtained not only depend dramatically on both cluster size and benzene isotope but also depend on the photon energy and fluence used for photoionization detection. With three notable exceptions, the major reaction channel observed with low-fluence ( G O pJ/cm2), single-photon 6.42-eV ionization is “molecular” addition to the metal clusters. Clusters containing 5, 6, and 1 1 niobium atoms, however, also give rise to a significant yield of products exhibiting loss of hydrogen for the reaction with benzene-h,, but not for that with benzene-d,. This kinetic isotope effect is primarily attributed to C-H(D) bond activation but may reflect some aspects of hydrogen desorption from the clusters. These experiments suggest that substantial dehydrogenation of benzene occurs for specific size clusters in the fast-flow reactor. Depending on experimental conditions, the hydrogen (deuterium) may or may not remain bound to the clusters. At high ionizing laser intensities, further desorption of hydrogen is induced by the ionization process, except for complexes containing less than three niobium atoms.
Introduction The study of gas-phase niobium clusters (Nb,) has revealed a number of size-dependent chemical and physical properties. These include the discovery that certain size clusters are relatively unreactive with molecular and nitrogen.’ The adsorption of benzene3 was found to be accompanied by conversion of Nb,C6H6 into Nb,C6, with a threshold for conversion at x = 4 and maxima in conversion probability at x = 5 , 6, 9, and 11. The reactivity pattern for Nb,C, formation coincides fairly well with that for the adsorption of H 2 and N2; e.g., clusters that dehydrogenate benzene the least are also those that are relatively unreactive with hydrogen and nitrogen. Photoionization threshold measurements2 for Nb, indicate a highly nonmonotonic dependence of IP on the number of constituent atoms, even out to x = 28. The variation in IP with cluster size inversely correlates with the observed reactivity toward D, and N,.2 Additionally, the size-selective reactivity of the cluster ions Nb,+ ( x = 7-10) toward H24aand N24bhas been reported to be quite similar to that observed for the neutral clusters. In and 0; are facile but show contrast, reactions of Nb, and C01b35 no significant cluster size selectivity. In this paper we examine further the reaction of gas-phase niobium clusters with benzene, (1) (a) Geusic, M. E.; Morse, M. D.; Smalley, R. E. J . Chem. Phys. 1985, 82, 590. (b) Morse, M . D.; Geusic. M. E.; Heath, J. R.; Smalley, R . E. J . Chem. Phys. 1985, 83, 2293. (2) (a) Whetten, R. L.; Zakin, M. R.; Cox, D. M.; Trevor, D. J.; Kaldor, A. J . Chem. Phys. 1986, 85, 1697. (b) Cox, D. M.; Whetten, R. L.; Zakin, M. R.; Trevor, D. J.; Reichmann, K. C.; Kaldor, A. In Adcances in Laser Science; Stwalley, W . C.; Lapp, M., Eds.; AIP Conf. Proc. No. 146; AIP: New York, 1986; p 527. (3) SI. Pierre, R . J.; El-Sayed, M. A. J . Phys. Chem. 1987, 91, 763. (4) (a) Alford, J. M.; Weiss, F. D.; Laaksonen, R. T.; Smalley, R. E. J . Phys. Chetn. 1986, 90, 4480. (b) Brucat, P. J.; Pettiette, C. L.; Yang, S.; Zheng, L.-S.;Craycraft, M. J.; Smalley, R. E. J . Chem. Phys. 1986,85, 4747. ( 5 ) Cox, D. M.;Reichmann, K. C.; Trevor, D. J.; Kaldor, A,, submitted for publication in J . Chem. Phys. (6) Zakin, M. R.; Cox, D. M.;Kaldor, A,, unpublished results.
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with special emphasis on the mechanism of the dehydrogenation reaction. Benzene chemisorption has been studied on small gas-phase clusters of platinum’ and rhodium.8 The reactions are facile, producing a rich spectrum of products, including such novel organometallic species as mono- and biarene complexes for atomic Pt and Rh. With low fluence photoionization detection the major product^^^^^^^ correspond to “molecular” addition of benzene to the metal clusters; i.e., hydrogen loss from the cluster-benzene complexes is not observed. Benzene chemisorbs9 with the molecular ring a-bonded and parallel to the surface on most hexagonal metal surfaces, though some irreversible, dissociative adsorption is also observed on Ru(001) as well as on stepped and highly disordered surfaces. Decomposition on most metals, for example, Ni, Pd, Pt, and Rh, accompanies and competes with desorption, with barriers to both processes comparable in the 25-35 kcal/mol range. Ruthenium is very much an exception with up to 50% irreversibly, i.e., dissociatively, adsorbed benzene coadsorbed with molecular benzene. The upper limit for this dissociation barrier is 9-1 1 kcal/mol. We are not aware of any studies of benzene chemisorption on niobium surfaces. The issue of competing reactions on heterogeneous catalysts is an important one, since carbonaceous products formed on the catalyst from benzene may be crucial in determining yields and product distributions in hydrogenation and dehydrogenation reactions of cyclic C6 hydrocarbons. The observation of size-selective branching of (7) (a) Trevor, D. J.; Whetten, R. L.; Cox, D. M.; Kaldor, A . J . A m . Chem. SOC.1985, 107, 518. (b) Trevor, D. J.; Kaldor, A. In High-Energy Processes in Organometallic Chemistry; Suslick, K. S . , Ed.; ACS Symposium Series 333; American Chemical Society: Washington, DC, 1987; p 43. (c) Kaldor, A.; Cox, D. M.; Trevor, D. J.; Zakin, M. R. Z . Phys. D: At., Mol., Clusters 1986, 3, 195. (8) Zakin. M. R.; Cox, D. M.; Kaldor, A,, manuscript in preparation. (9) Excellent discussion of this topic can be found in two recent articles and references therein: Polta, J. A.; Thiel, P. A. J . A m . Chem. SOC.1986, 108, 7560; Koel, B. E.; Crowell, J. E.; Bent, B. E.; Mate, C. M.; Somorjai, G . A . J . Phy.s. Chem. 1986, 90, 2949.
0 1987 American Chemical Society
Niobium Clusters with Benzene-h6 and -d,
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
5225
the chemisorption reaction of benzene to dehydrogenation on niobium clusters offers a model system to study “surface carbon” formation. In this report we consider the reaction of gas-phase niobium clusters, x = 1-1 2, with both benzene-h6 (C6H6) and benzene-d6 (C,D,) under reaction conditions where kinetics dominates. The product distribution obtained not only depends dramatically on both cluster size and benzene isotope but also depends on the photon energy and fluence used for photoionization mass spectrometric detection. Under the most gentle ionization conditions the major reaction channel is “molecular” addition of benzene to the metal clusters. The exceptions under our experimental conditions are Nb5, Nb,, and N b l l , for which products exhibiting loss of hydrogen are also observed in significant yield upon reaction with benzene-h,. Reaction with benzene-d,, however, produces primarily the “molecular” addition products for all clusters. This kinetic isotope effect suggests that these specific clusters chemically dehydrogenate benzene in the fast-flow reactor. Such an effect may arise in the C-H bond activation step or as a result of hydrogen desorption from the clusters. The desorption of hydrogen may take place in the reactor, if the cluster temperature is high enough; or it may be induced by the ionization process, presumably because sufficient excess energy is deposited into the cluster-adduct complexes by the ionizing photons. The interpretation that C-H activation occurs in the reactor is also supported by recent observation of size-selective conversion reactions of niobium cluster ions with benzene-h, and d 6 . I ’ The size-selective C-H activation is discussed in terms of electronic and geometrical factors, as well as in terms of steric effects on the initial bond formation mechanism.
Experimental Section Details of the experimental apparatus have been described in a series of earlier publications.11,12Briefly, clusters are synthesized via pulsed laser vaporization of a niobium target and detected by time-of-flight photoionization mass spectrometry in a molecular beam. The photoionization source is an excimer laser operating on either the 6.42- (ArF) or 7.87-eV (F2) transition. Ionizing laser fluence is limited to
