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On the Mechanism for the Hydrogenation of Olefins on Transition-Metal Surfaces: The Chemistry of Ethylene on Pt(111)† Francisco Zaera Department of Chemistry, University of California, Riverside, California 92521 Received September 1, 1994. In Final Form: May 4, 1995X The chemistry of ethylene on Pt(111) single-crystal surfaces was chosen here to represent olefin hydrogenation reactions on transition-metal catalysts. In vaccum the hydrogenation of ethylene was proven to proceed via a stepwise incorporation of hydrogen atoms on the clean surface, but under high pressures the catalyst was shown to become covered with carbonaceous deposits soon after exposure to the reactant gases. The species that compose the strongly bonded hydrocarbon fragments were identified as ethylidyne, a C2 moiety where one carbon atom sits on a 3-fold hollow site on the surface and is singlebonded to a methyl group directly above it. In order to better understand the role of the ethylidyne layer in the hydrogenation reaction, the mechanism of the conversion of ethylene to ethylidyne was studied in some detail. Even though no simple scheme has been found to explain this surface process so far, our studies have led to the rejection of previously suggested two-step pathways involving either vinyl or ethylidene intermediates. Vinyl moieties were shown to undergo a series of reactions and to form a family of intermediates, including ethylene, before ultimately transforming into ethylidyne. The involvement of ethylidene in any simple two-step mechanism was shown to also be inconsistent with results from kinetic studies using trideuteroethylene. Finally, the participation of ethyl groups was discarded on the grounds that they decompose readily via a β-hydride elimination step into ethylene. The participation of any of those intermediates in the mechanism for ethylene conversion could nevertheless be possible if they were to reach a fast pre-equilibrium with the chemisorbed ethylene and to then decompose slowly to ethylidyne. Unfortunately, further testing of this hypothesis is hindered by the added complications due to the nonfirst-order kinetics of the ethylidyne formation and the competition of that reaction with other H/D exchange and hydrogenation steps. It is also not clear yet what role these ethylidyne moieties may play in the mechanism of the high-pressure catalytic reactions, but several options are discussed here, including the possibility of them acting either as hydrogen transfer agents or simply as site blockers on the surface.
1. Introduction Since its discovery by Sabatier and Senderens in 1897, the hydrogenation of olefins over metal catalysts has been one of the most studied chemical processes.1,2 Several mechanisms have been proposed for this reaction over the years. It was suggested early on that olefins hydrogenate after chemisorption by incorporating two surface hydrogen atoms into the double bond, either in a stepwise fashion, forming an alkyl intermediate first and hydrogenating to the corresponding alkane later,3 or simultaneously in a concerted step.4 Alternatively, it was proposed that the alkenes could react directly with molecular hydrogen5 or that the reaction scheme should include steps for the disproportionation and H-D exchange of ethyl intermediates.1 All these mechanisms were supposed to take place directly on the clean metal surface, but even that notion was challenged by Thomson and Webb, who proposed a pathway where the hydrogenation occurs by hydrogen transfer between an adsorbed (and unspecified) hydrocarbon species and the adsorbed olefin.6 There has been a pleiad of papers in this field,7-13 but a consensus on a complete microscopic picture to † Presented at the symposium on Advances in the Measuring and Modeling of Surface Phenomena, San Luis, Argentina, August 24-30, 1994. X Abstract published in Advance ACS Abstracts, January 1, 1996.
(1) Bond, G. C. Catalysis by Metals; Academic Press: London, 1962. (2) Horiuti, J.; Miyahara, K. Hydrogenation of Ethylene on Metallic Catalysts; NSRDS-NBS, No. 13; US Government Printing Office: Washington, DC, 1968. (3) Polanyi, M.; Horiuti, J. Trans. Faraday Soc. 1934, 30, 1164. (4) Bond, G. C.; Wells, P. B. In Advances in Catalysis; Erley, D. D., Pines, H., Weisz, P. B., Eds.; Academic Press: New York, 1964; Vol. 15; pp 91-226. (5) Twigg, G. H. Discuss. Faraday Soc. 1950, 8, 152.
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account for the steps involved in the catalytic hydrogenation of olefins by transition metals has still not been reached. Recent studies using modern surface sensitive techniques have provided further insight into the mechanism for catalytic hydrogenation reactions. We here summarize work done by us and others on the characterization of the chemistry involved in ethylene hydrogenation on platinum (111) single crystal surfaces. This is not meant to be a comprehensive review, but rather a critical and personal overview of the field. First, the hydrogenation of ethylene over platinum under ultrahigh vacuum (UHV) will be discussed. Next the results from experiments on single crystal surfaces under catalytic conditions will be summarized. In the following section the studies addressing the conversion of ethylene to ethylidyne will be presented. Finally, some tentative conclusions will be suggested. 2. Ethylene Reactivity under Vacuum Several surface science experiments have shown that below 200 K the adsorption of ethylene on Pt(111) surfaces is molecular. It has also been determined that those ethylene molecules first adsorb as flat π-bonded species and then rehybridize above 52 K into another flat di-σ (6) Thomson, S. J.; Webb, G. J. Chem. Soc., Chem. Commun. 1976, 526. (7) Sinfelt, J. H. J. Phys. Chem. 1964, 68, 856. (8) Morrow, B. A. J. Catal. 1969, 14, 279. (9) Gardner, N. C.; Hansen, R. S. J. Phys. Chem. 1970, 74, 3298. (10) Schlatter, J. C.; Boudart, M. J. Catal. 1972, 24, 482. (11) Hattori, T.; Burwell, R. L., Jr. J. Phys. Chem. 1979, 83, 241. (12) Briggs, D.; Dewing, J.; Berden, A. G.; Moyes, R. B.; Wells, P. B. J. Catal. 1980, 65, 31. (13) Goddard, S. A.; Cortright, R. D.; Dumesic, J. A. J. Catal. 1992, 137, 186.
© 1996 American Chemical Society
Olefin Hydrogenation
Figure 1. Hydrogen, ethylene, and ethane temperature programmed desorption spectra from an ethylene-saturated Pt(111) surface. Of particular interest to the discussion in this paper is the appearance of the ethane desorption peak around 294 K, which is indicative of the feasibility of self-hydrogenation reactions under vacuum.
configuration.14-19 Upon further heating of the sample, however, the chemisorbed ethylene decomposes by following a series of surface reactions that lead to the desorption of hydrogen, ethylene, and ethane into the gas phase around room temperature. Figure 1 shows typical temperature programmed desorption spectra for this system. The detection of ethane is of particular interest to the discussion presented in this paper, because it demonstrates that a small fraction of the chemisorbed ethylene is capable of undergoing a self-hydrogenation reaction even under the vacuum conditions of these experiments. This self-hydrogenation is believed to occur via the incorporation of surface hydrogens produced by a previous decomposition of some of the ethylene molecules on the surface (see below). H2/D2 coadsorption and other labeling experiments have shown that the presence of hydrogen atoms on the surface facilitates the hydrogenation reaction. An example of the results obtained from the experiments used to reach this conclusion is illustrated in Figure 2. There it is shown that all C2D6, C2D5H, and C2D4H2 can be produced by thermal activation of C2D4 on Pt(111) in the presence of a small amount of coadsorbed normal hydrogen. The thing to notice here is the fact that while C2D4H2 originates from direct incorporation of two H atoms into the original ethylene molecule, the formation of C2D5H and C2D6 requires the presence of deuterium atoms on the surface, which must come from other ethylene molecules. Moreover, the production of C2D5H proves that hydrogen incorporation is stepwise, namely, that the hydrogen atoms (14) Demuth, J. E. Surf. Sci. 1979, 84, 315. (15) Steininger, H.; Ibach, H.; Lehwals, S. Surf. Sci. 1982, 117, 685. (16) Albert, M. R.; Sneddon, L. G.; Eberhardt, W.; Greuter, F.; Gustaffson, T.; Plummer, E. W. Surf. Sci. 1982, 120, L17. (17) Horsley, J. A.; Sto¨hr, J.; Koestner, R. J. J. Chem. Phys. 1985, 83, 3146. (18) Hugenschmidt, M. B.; Dolle, P.; Jupille, J.; Cassuto, A. J. Vac. Sci. Technol. 1989, A7, 3312. (19) Cassuto, A.; Kiss, J.; White, J. M. Surf. Sci. 1991, 255, 289.
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Figure 2. Temperature programmed desorption spectra for molecular ethylene (C2D4) and for the products of hydrogenation (C2D6, C2D5H, and C2D4H2) and H-D exchange (C2D3H) reactions from a mixture of normal hydrogen and fully deuterated ethylene coadsorbed on a Pt(111) surface. The detection of all three types of ethanes points to the stepwise nature of the hydrogenation mechanism under vacuum.
add to the double bond one at a time.20,21 Finally, the total yield for ethane production increases considerably in the presence of coadsorbed hydrogen. The TPD peak maximum for ethane formation in the experiments reported above shifts to higher temperatures for the more deuterium-substituted molecules, from 298 K for C2D4H2 to 304 and 309 K for C2D5H and C2D6, respectively. This implies that the rate for the selfhydrogenation of ethylene is controlled by the ethylene decomposition step that provides the extra hydrogens (deuteriums) required for the formation of ethane. Hydrogen coadsorption bypasses such a bottleneck and accelerates the overall hydrogenation reaction. The data displayed in Figure 3 prove this point by showing how the presence of surface deuterium reduces the temperature of the ethane TPD maxima for C2D4 from 311 to 302 K, which corresponds to a reduction in activation energy of about 2 kcal/mol from the initial value of around 15 kcal/ mol.21 This same figure also helps illustrate the fact that the slow step in the coadsorbed case is the formation of an ethyl intermediate: it shows how ethane formation from ethyl moieties occurs readily at much lower temperatures, around 255 K, which corresponds to an energy barrier of only about 6 kcal/mol.22 Ethyl formation was shown to be responsible for the initiation of H-D exchange reactions within the ethylene chemisorbed molecules as well. The occurrence of this reaction can be seen in the upper TPD traces of Figures 2 and 4, which correspond to the formation of C2D3H from C2D4 on Pt(111) surfaces predosed with normal hydrogen. The fact that the shapes of the TPD traces for C2D3H (the exchanged ethylene) and C2D4H2 (the hydrogenation product) desorption are, within experimental error, iden(20) Godbey, D.; Zaera, F.; Yates, R.; Somorjai, G. A. Surf. Sci. 1986, 167, 150. (21) Zaera, F. J. Phys. Chem. 1990, 94, 5090. (22) Zaera, F. J. Phys. Chem. 1990, 94, 8350.
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Figure 5. Energy diagram for the chemistry of C2 moieties on clean Pt(111) surfaces under vacuum. Shown are the heats of formation of the relevant species as well as the activation energies of the interconnecting reactions. All values are reported in kcal/mol.
elimination step to produce ethylene and by a reductive elimination reaction with coadsorbed hydrogen to yield ethane, and that both those steps are much faster than the initial ethylene hydrogenation that produces the ethyl groups.22-24 A diagram summarizing the energetics of all these reactions is provided in Figure 5. Figure 3. Ethane (C2D6 and C2D4H2) temperature programmed desorption spectra from both ethylene (C2D4) and ethyl iodide (CD3CH2l) adsorbed on clean and deuterium predosed Pt(111) surfaces. The shift of the ethane desorption peak to lower temperatures when switching from ethylene to ethyl iodide indicates that the rate limiting step in the hydrogenation of ethylene is the formation of surface ethyl moieties.
Figure 4. Ethane (C2D6 and C2D4H2) and H-D exchanged ethylene (C2D3H) temperature programmed desorption spectra from both D2 + C2D4 and H2 + C2D4 pairs coadsorbed on Pt(111). The similar shapes of the traces for C2D4H2 and C2D3H in the H2 + C2D4 case suggest that both hydrogenation and H-D exchange reactions go through a common rate limiting step.
tical points to the fact that both exchange and hydrogenation reactions are limited by the same initial step, namely, the formation of ethyl groups.21 Independent studies using ethyl iodide have shown that the ethyl intermediates can indeed react both via a β-hydride
3. Catalytic Hydrogenation of Ethylene The results from the vacuum surface science studies reported above support a mechanism for olefin hydrogenation where the incorporation of hydrogen atoms into the double bond takes place in a stepwise manner directly on the clean surface (by following the so-called PolanyiHoriuti mechanism). The efficiency of this reaction under vacuum, however, is quite low; even on surfaces presaturated with hydrogen, ethane formation only accounts for a few percent of the total initial ethylene. Instead, most of the ethylene molecules chemisorbed on the surface dehydrogenate around 250 K to form a surface moiety with C2H3 stoichiometry. After some initial controversy, this hydrocarbon fragment was identified as ethylidyne, a species where one carbon atom sits on a 3-fold hollow site and has a methyl group bonded directly above it.25-27 Ethylidyne has since proven to be so stable that it can in fact be produced by decomposition of other C2 precursors as well. Figure 6 shows the signature infrared spectra of the ethylidyne fragments produced by thermal activation of chemisorbed ethylene, ethyl iodide, vinyl iodide, and 1,1-diiodoethane.28-30 Experiments on single crystal surfaces under atmospheric pressures have suggested that ethylidyne may play an important role in catalytic ethylene hydrogenation reactions.31 For one, the platinum(111) crystals used for this reaction were found to become completely covered within the first few seconds of the reaction with strongly bonded surface hydrocarbon fragments, which a post(23) Zaera, F. Surf. Sci. 1989, 219, 453. (24) Zaera, F. J. Am. Chem. Soc. 1989, 111, 8744. (25) Skinner, P.; Howard, M. W.; Oxton, I. A.; Kettle, S. F. A.; Powell, D. B.; Sheppard, N. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1203. (26) Salmero´n, M.; Somorjai, G. A. J. Phys. Chem. 1982, 86, 341. (27) Zaera, F.; Somorjai, G. A. In Hydrogen Effects in Catalysis; Fundamentals and Practical Applications; Paa´l, Z.; Menon, P. G., Eds.; Marcel Dekker: New York, 1988; pp 425-447. (28) Zaera, F.; Bernstein, N. J. Am. Chem. Soc. 1994, 116, 4881. (29) Hoffmann, H.; Griffiths, P. R.; Zaera, F. Surf. Sci. 1992, 262, 141. (30) Janssens, T. V. W.; Zaera, F. Abstracts of Papers; 209th National Meeting of the American Chemical Society, Anaheim, April 1995; American Chemical Society: Washington, DC, 1995; paper No. COLL 45. (31) Zaera, F.; Somorjai, G. A. J. Am. Chem. Soc. 1984, 106, 2288.
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Table 1. Turnover Frequencies (TOF) for C2 Conversion Reactions on Pt(111) TOF/monolayer·s-1 reaction
conditions
250 K
300 K
A. C2H4(g) + H2(g) f C2H6(g) B. C2H4(g) + D2(g) f C2H3D(g) + HD(g) C. tCCH3(ad) + H2(g) f C2H4(g), C2H6(g) D. tCCH3(ad) + D2(g) f tCCH2D(ad) + HD(g) E. C2H4(ad) f tCCH3(ad) + H(ad) F. C2H4(ad) f C2H4(g) G. C2H4(ad) + D(ad) f C2H3D(g) + H(ad) H. C2H4(ad) + 2H(ad) f C2H6(g)
10 Torr C2H4, 10 Torr H2 10 Torr C2H4, 10 Torr H2 saturation C2H3 1 atm H2 saturation C2H3 1 atm D2 vacuum saturation C2H4 vacuum saturation C2H4 vacuum, sat. C2H4 0.5 langmuir predosed D2 vacuum, sat. C2H4 0.5 langmuir predosed H2
0.01 0.002 7 × 10-5
