Langmuir 1995,11, 3048-3053
3048
Ethyne Cyclization to Benzene over Cu(ll0) Julian R. Lomas, Christopher J. Baddeley, Mintcho S. Tikhov, and Richard M. Lambert* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 lEW, U.K. Received February 23, 1995. I n Final Form: May 22, 1995@ Ethyne cyclization to benzene over Cu(ll0) is an efficient reaction that proceeds at low temperatures with close to 100% efficiency. On the clean surface, C2H2 adsorbs into islands, there is no threshold coverage for the onset of reaction, and benzene evolution into the gas phase occurs in a single TPR peak due to a surface reaction rate limited process. In each of these four respects the behavior is very different from that found on Pd(ll1). Isotope tracing experiments show that cyclization occurs by an associative mechanism, and the use ofcis-1,2-dichlorocyclobutene indicates that C4H4is the key reaction intermediate, as it is on Pd(111).Additional data, includingresults of experiments with CeHe and with CdH4C12 C2D2, demonstrate that cyclooctatetraene is not a reaction intermediate in this system, and the possible scheme 2CzH2(a) C4H4(a); 2C4H4(a) CeHda); C8Hda) CsHs(a) + C2H2(a) is ruled out. The mechanism 2C2H2(a) C4H4(a);C4H4(a) C2H2(a) CsHs(a)is established, and it is shown that the first step is rate limiting overall.
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Introduction Ethyne coupling reactions over Pd surfaces constitute a n interesting class of processes which, depending on the conditions, can yield a variety of linear and cyclic They display close similarities with alkyne coupling catalyzed by transition-metal cluster compounds and provide a striking example of the cluster/extended surface analogy. Furthermore they can be carried out over a variety of dispersed Pd catalysts a t atmospheric The key process involves formation of a C4H4 metallocycle common intermediate which then react further: benzene formation, the prototype reaction, involves addition of a third ethyne molecule. Experimental and theoretical studies, covering many structural,kinetic, and mechanistic aspects have appeared,8-16and the topic has been reviewed.l7J8 More recently the reaction has been Abstract published in Advance ACS Abstracts, July 15, 1995. (1)Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. J.Chem. SOC.,Chem. Commun. 1983,623. (2)Ormerod, R. M.; Lambert, R. M. Catal. Lett. 1990,6,121. (3)Gellman, A. J. J.A m . Chem. SOC.1991,113,4435. (4)Abdelrehim, I. M.; Thornberg, N. A.; Sloan, J. T.; Land, D. P. Surf. Sei. 1989,208,93. (5)Ormerod, R.M.;Lambert, R. M. J. Chem. Soc., Chem. Commun. 1990,1421. (6)Lee, A. F.;Baddeley, C. J.;Hardacre, C.; Ormerod, R. M.; Lambert, R. M.; West, H.; Schmid, G. J. Phys. Chem. 1996,99,6096. (7)Lee, A. F.;Hardacre, C.; Baddeley, C. J.;Ormerod, R. M.; Candy, J. P.; Basset, J.-M.; Lambert, R. M. i n preparation. (8)Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. Su$. Sci. 1983,110, 6871. (9) Patterson, C. H.; Lambert, R. M. J.A m . Chem. SOC.1988,110, 6871. (10)Hoffman, H.; Zaera, F.; Ormerod, R. M.; Lambert, R. M.; Wang, L. P.; Tysoe, W. T. Surf. Sei. 1990,232,259. (11)Ormerod, R.M.; Lambert, R. M.; Horrman, H.; Zaera, F.; Yao, J. M.; Saldin, D. K.; Wang, - L. P.; Bennett, D. W.; Tysoe, W. T. Surf. Sci. 1993,295,277. (12)Ormerod, R.M.; Lambert, R. M. J.Phys. Chem. 1992,96,8111. (13)Pacchioni, G.;Lambert, R. M. Surf. Sei. 1994,304, 208. (14)Cuests. A. R.:Valladares. D.: Velasco, A.: Zarablich, G.: Tvsoe, W. T.; Ormerod, R. M.; Lambert, R. M. J.Phys.: Co&!ens. Matter 1993, 5 , 239. (15)Gentle, T. M.; Muetterties, E. L. J.Phys. Chem. 1983,87,2469. Baddeley, C. J.;Lambert, R. M. Surf. Sci. 1991, (16)Ormerod, R.M.; 251,L709. (17)Ormerod, R.M.; Lambert, R. M. Muter. Chem. Phys. 1991,29, 105. @
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observed over Au(l11)/Pd,l9 Pd/Au(l 11),20 Pt(l11)/Sn,21 Pd( 111)/Sn,22and reduced T i 0 2 ( O O l ) 2 3 surfaces. Ethyne cyclization over Cu(ll0)was reported in a short paper by Avery which appeared in 1985.24This presented only limited data and appeared to show that in a n earlier study of the same system, Outaka et al.25had wrongly attributed the 27 amu fragment ion of benzene to ethene. Here, we confirm the correctness of Avery's observation, and obtain rather detailed insight into the kinetics and mechanism of the trimerization reaction over Cu(ll0). It is shown that the reaction is very efficient and that C4H4 is the likely reaction intermediate, as in the case of Pd. However, the behavior on Cu(ll0) differs from that on Pd surfaces in regard to (i)the kinetics ofbenzene desorption, (ii) the nature of the rate determining step, and (iii) the absence of a critical threshold coverage for reaction.
Experimental Section Experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 1 x Torr equipped with three-grid LEED optics, a collimated VG Q7 (1-120 amu) quadrupole mass spectrometer (QMS) for temperature programmed reaction (TPR)experiments,and a Varian single pass cylindrical mirror analyzer (CMA) with coaxial electron gun for Auger electron spectroscopy (AES). A heating rate of 6 K s-l was used for all TPlWDS experiments,and the sample could be cooled to -120 K using liquid nitrogen. The Cu(110)surface was cleaned by sputteringwith Ar+ ions with the sample at -900 K, followed by 45 min of sputtering at room temperature. After being cleaned,the samplewas annealed at 750-800 K for 2 min and the cleanliness of the surface was checked using AES. The surface was considered t o be clean when intensity of the Auger peak due to the only detectable impurity (C 273 eV) was ~0.01%of the Cu 60 eV signal. Gas dosing was (18)Ormerod, R. M.; Lambert, R. M. In Surface Reactions; Madix, R. J.,Ed.; Springer Series in Surface Science; Springer-Verlag: Berlin, lQQ4.Vnl -- -, . --. 24 - -. (19)Baddeley, C. J.; Ormerod, R. M.; Stephenson, A. W.; Lambert, R. M. J.Phys. Chem. 1996,99,5146. (20)Baddeley, C. J.; Hardacre, C.; Tikhov, M. S.; Lomas, J. R.; Lambert, R. M.: in preparation. (21)Xu,C.;Peck,J.W.;Koel,B.E.J.Am.Chem.Soc.1993,115,751. (22)Lee, A. F.;Baddeley, C. J.; Lambert, R. M. Submitted to J.Phys. Chem. (23)Pierce, K.G.; Barteau, M. A. J. Phys. Chem. 1994,98,3882. (24)Avery,N. A. J.Am. Chem. Soc. 1986,107,6711. (25) Outka, D.A,; Friend, C. M.; Jorgenson, S.; Madix, R. J. J.Am. Chem. Soc. 1983,105,3468.
0743-746319512411-3048$09.00/00 1995 American Chemical Society
Langmuir, Vol. 11, No. 8, 1995 3049
Ethyne Cyclization to Benzene over Cu(ll0)
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Figure 1. TPR spectra for varying exposures of CzHz dosed at a crystal temperature of 120 K on Cu(ll0): (a) 26 amu (C2Hz); (b)78 a m u (CsH6). Inset shows the variation of ethyne coverage with ethyne exposure.
performed by back-filling the chamber, except for large doses (> 15 langmuirs)when a tube doser was used. All gas exposures were carried out at a crystal temperature of -120 K.
Results The C2H2 and C6H6 TPD and TPR spectra obtained aRer dosing the clean surface with C2H2 (Figure 1)are in good quantitative agreement with Avery’s limited data.24 (The 26 amu fragment ofbenzene only contributes -20% ofthe 325 K state in the ethyne desorption spectra.) The evolution of these spectra with initial CzH2 exposure is interesting and has not been reported before. In particular, the C2H2 desorption state at -280 K grows in as the benzene state a t 325 K saturates (between 3 and 4 langmuirs C2H2 dose). It is also interesting to note that there is a C2H2 desorption feature that almost coincides with the desorption of reactively formed benzene. The C2H2 dose required to saturate the chemisorbed layer (Le., both 280 and 325 K C2H2 desorption states) is -6 langmuirs, higher exposures resulted in multilayer ad-
sorption signaled by continuous growth of a desorption peak at -130 K. Note that there appears to be no threshold coverage for the onset of benzene formation, in marked contrast with the behaviour found on P d ( l l 1 ) where benzene formation does not occur for coverages
