Langmuir 1999, 15, 7615-7620
7615
Investigation of Cyclooctatetraene on Reduced TiO2(001) as a Possible Intermediate in Alkyne Cyclotrimerization Adrian B. Sherrill, Victor S. Lusvardi,† and Mark A. Barteau* University of Delaware, Department of Chemical Engineering, Center for Catalytic Science and Technology, Newark, Delaware 19716 Received February 4, 1999. In Final Form: July 6, 1999 Cyclotrimerization of alkynes to form aromatics can be carried out catalytically with low valent transition metal complexes in solution, with supported and single-crystal metal catalysts, and with partially reduced TiO2 surfaces. In each of these cases the reaction has been proposed to proceed via formation of a metallacyclopentadiene intermediate, i.e., by association of a pair of acetylene molecules to form a C4 ligand bound to the catalytic site. At least one group of authors has proposed that on late-transition metal surfaces a pair of C4 ligands combine to form the C8 ligand, cyclooctatetraene (COT). In this alternative mechanism, benzene is then formed by the elimination of a C2 fragment from the C8 intermediate. This reaction channel was previously examined on Pd(111) and Cu(110), and was found to be a minor or nonexistent channel on each, reinforcing the case for direct addition of a third acetylene molecule to the metallacyclopentadiene intermediate to form the aromatic product. We have examined the reactions of 1,3,5,7cyclooctatetraene on reduced TiO2 surfaces to determine whether this species can be converted to benzene, and if so, whether the kinetics of this reaction is compatible with a COT intermediate in alkyne cyclotrimerization. It was found that the reduced surface of TiO2(001) does convert COT into benzene, although at a higher temperature (550 K) than that at which acetylene cyclotrimerizes on similarly reduced surfaces (400 K). This suggests that the COT channel is, at best, a minor route to benzene in the course of acetylene cyclooligomerization on this oxide.
Introduction The heterogeneously catalyzed cyclotrimerization of acetylene and substituted acetylenes has been investigated extensively on single-crystal metal, metal oxide, and bimetallic surfaces.1-12 One of the attractive aspects of this chemistry is that it offers an opportunity to study catalytic carbon-carbon bond formation under ultrahigh vacuum (UHV) conditions. Catalytic syntheses typically must be studied by monitoring the decomposition of an adsorbate and invoking the principle of microscopic reversibility. Alkyne cyclotrimerization is sufficiently facile, even in UHV, so that microkinetic measurements can be made directly. The cyclotrimerization of acetylene on reduced surfaces of TiO2(001) has been characterized previously, and the results of these studies strongly support the proposal that the reaction proceeds through a metallacyclopentadiene formed at Ti2+ centers on the † Present address: DuPont Central Research and Development, Wilmington, DE 19880. * To whom correspondence should be addressed.
(1) Pierce, K. G.; Barteau, M. A. J. Phys.. Chem. 1994, 98, 3882. (2) Lusvardi, V. S.; Pierce, K. G.; Barteau, M. A. J. Vac. Sci. Technol. A 1997, 15, 1586. (3) Janssens, T. V. W.; Volkening, S.; Zambelli, T.; Wintterlin, J. J. Phys. Chem. B 1998, 102, 6521. (4) Abdelrehim, I. M.; Pelhos, K.; Madey, T. E.; Eng Jr., J.; Chen, J. G. J. Mol. Catal. A. 1998, 131, 107. (5) Dvorak, J.; Hrbek, J. J. Phys. Chem. B 1998, 102, 9443. (6) Patterson, C. H.; Lambert, R. M. J. Am. Chem. Soc. 1988, 110, 6871. (7) Lomas, J. R.; Baddeley, C. J.; Tikhov, M. S.; Lambert, R. M. Langmuir 1995, 11, 3048. (8) Baddeley, C. J.; Ormerod, R. M.; Stephenson, A. W.; Lambert, R. M. J. Phys. Chem. 1995, 99, 5146. (9) Lee, A. F.; Baddeley, C. J.; Hardacre, C.; Lambert, R. M. J. Am. Chem. Soc. 1995, 117, 7719. (10) Holmblad, P. M.; Rainer, D. R.; Goodman, D. W. J. Phys. Chem. B 1997, 101, 8883. (11) Tysoe, W. T. Langmuir 1996, 12, 78. (12) Abdelrehim, I. M.; Caldwell, T. E.; Land, D. P. J. Phys. Chem. 1996, 100, 10265.
surface.1,2 Metallacycles are a recurring theme in alkyne cyclization chemistry. Studies on Pd(111), Cu(110), and Au/Pd(111) all claim a surface metallacycle as the important catalytic intermediate, even though selectivities to the trimer and the proposed rate-limiting steps differ among these metals.6-8 Homogeneously catalyzed cyclotrimerization reactions (with Ni, Co, and Rh complexes, for example) also proceed via metallacycle intermediates. The metal center oxidatively couples two unsaturated ligands, formally increasing the oxidation state of the metal center but retaining the coordination number.13-15 In solution, the reaction requires that the metal center be able to undergo a two-electron oxidation, the same condition observed for the reaction on reduced TiO2(001) surfaces. The cluster-surface analogy of Muetterties proposes that multinuclear organometallic complexes are appropriate models for reactions on metal surfaces; for an oxide surface lacking metal-metal bonds, the most appropriate analogues may be mononuclear complexes.16 It was proposed, therefore, that alkyne cyclotrimerization on reduced TiO2(001) follows a mechanism analogous to that in homogeneous catalysis:
This reaction mechanism also resemblances that proposed for metal surfaces6,7: (13) Colborn, R. E.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1986, 108, 5470. (14) Wakatsuki, Y.; Kuramitsu, T.; Yamazaki, H. Tetrahedron Lett. 1974, 51/52, 4549. (15) Collman, J. P.; Kang, J. W.; Tuttle, W. F.; Sullivan, M. F. Inorg. Chem. 1968, 7, 1298. (16) Muetterties, E. L.; Rhodin, T. N.; Brucker, C. F.; Pretzer, W. R. Chem. Rev. 1979, 79, 91.
10.1021/la990119p CCC: $18.00 © 1999 American Chemical Society Published on Web 08/21/1999
7616
Langmuir, Vol. 15, No. 22, 1999
Sherrill et al.
2 C2H2ad f C4H4ad C4H4ad + C2H2ad f C6H6ad Recent kinetic studies on Pd(111) using laser-induced thermal desorption/Fourier transform mass spectrometry have shown that the second step, addition of acetylene to a metallocyclopentadiene intermediate, is rate determining on this metal.12 In contrast, Hostetler et al.17 previously proposed an alternative mechanism on Pt(111). They showed that 1,3,5,7-cyclooctatetraene could ring-contract to form benzene and acetylene on Pt(111) surfaces. A second reaction channel was advanced which involves condensation of two metallacyclopentadiene intermediates to cyclooctatetraene (COT), followed by alkyne elimination from this intermediate:
2 C2H2ad f C4H4ad 2 C4H4ad f C8H8ad C8H8ad f C6H6ad + C2H2ad This reaction channel was examined on Pd(111) and Cu(110) and was found to be a minor or nonexistent channel on each, reinforcing the case for a ring closure via acetylene addition to the metallacyclopentadiene.7,9 It is unclear, however, which mechanism is more important on oxide surfaces. In the present work, we examine the reaction of COT on TiO2(001) surfaces of varying extents of reduction. We will demonstrate that, although COT will form benzene on reduced surfaces of TiO2(001), it does so at temperatures greater than that at which benzene is formed when acetylene is used as a precursor. Although important similarities exist between the active sites required and the nature of the interaction of these with acetylene-derived and COT-derived intermediates, these reactants do not appear to generate benzene by a common mechanism. Experimental Section All experiments were performed in a VG ESCALAB Mark II equipped with a UV lamp, low-energy electron diffraction optics, an ion gun, an electron gun, a twin-anode X-ray source, and a Hiden HAL 3F PIC RC mass spectrometer. The TiO2(001) sample was cut from a boule aligned to the (001) face and polished with diamond paste, finishing with 0.25 µm. The crystal was clipped to a tantalum foil backing plate to which tantalum wire power feeds had been spot-welded. Temperature was measured by a thermocouple glued to the side of the crystal with Aremco Ultratemp 516 ceramic cement. The thermocouple was connected to an Omega PID controller interfaced with a Hewlett-Packard power supply to generate linear temperature ramps for temperature-programmed desorption (TPD) experiments. The crystal was then repeatedly sputtered and annealed in a vacuum until clean and consistent X-ray photoelectron spectroscopy (XPS) spectra were obtained, indicating a stable population of surface cations. The crystal was a gun-metal blue color throughout the experiments. The background pressure of the chamber was typically 6 × 10-11 mbar, although outgassing of the X-ray anode during XPS experiments would raise the chamber pressure as high as 2 × 10-10 mbar. The COT (>98%) was purchased from Aldrich. The sample as delivered was inhibited with hydroquinone and was vacuum distilled to remove as much inhibitor as possible. The COT was kept frozen when not being used to dose the crystal. Subsequent mass spectrometer cracking patterns did not show the presence of COT-dimer products or hydroquinone in the sample. The COT (17) Hostetler, M. J.; Nuzzo, R. G.; Girolami, G. S.; Dubois, L. H. J. Phys. Chem. 1994, 98, 2952.
Figure 1. Ti (2p) spectra of reduced TiO2 surfaces as a function of the prior annealing temperature. The top spectrum (830 K) shows the doublet associated with Ti4+ (TiO2). The bottom spectrum shows a surface fully reduced by Ar+ ion bombardment. The curves fitted to the spectrum represent the 2p3/2 and 2p5/2 doublets of Ti1+, Ti2+, Ti3+, and Ti4+. The middle spectrum shows a surface that has been partially reoxidized by annealing the surface to 600 K. was admitted to the chamber through a leak valve to a needle directed at the crystal face. The valve and needle were mounted on a linear translator to position the needle reproducibly in front of the crystal face during dosing. Whereas this technique raises the pressure of the adsorbate at the crystal face above that in the chamber, the increase in chamber pressure is proportional to the flux; thus, the uncorrected ionization gauge reading was used as an indication of the exposure. The mass spectrometer ionizer source was enclosed within a differentially pumped quartz shroud. The shroud aperture was smaller in diameter than the crystal face so that any species desorbed from the crystal mounting hardware were masked from the detector. X-ray photoelectron spectra were collected with a Mg source operated at 300 W (15 kV, 20 mA). The crystal was sputtered for 60 min with 2 keV Ar+ ions. After surface reduction, XPS spectra of the Ti(2p), O(1s), and C(1s) regions were collected. The crystal was then annealed to the temperature of interest for 20 min and allowed to cool to ambient temperatures. XPS spectra were collected again. The population of Tix+ cations was determined by curve fitting the Ti(2p3/2) region of the spectrum. Figure 1 shows such an analysis. After XPS analysis, the crystal was flashed to 600 K and allowed to cool below room temperature for adsorption of COT. The crystal was exposed to 2.4 × 10-9 mbar of COT for 4 min at 273 K. For TPD experiments, the dosed crystal was further cooled to 260 K, and the crystal was moved to face the aperture of the mass spectrometer shroud. The temperature ramp was initiated at 1 K/s and the TPD data were collected by a PC controlling the mass spectrometer. Twenty mass fragments were monitored during each TPD experiment. The TPD data were scaled to account for the different ionization sensitivities of the mass spectrometer and the product yields (Figures 8 and 9) were quantified by deconvoluting the spectra using literature and experimental cracking patterns.18,19 X-ray photoelectron spectra of adsorbed COT were collected in separate experiments to minimize radiation damage to the adsorbed layer before conducting TPD experiments.
Results Titanium (2p) XPS. Because previous studies of alkyne cyclotrimerization on reduced metal oxide surfaces have shown a direct relationship between the availability of Ti2+ surface cations and the trimer yield, XPS was used to track the populations of surface cations as a function of the prior annealing temperature of the surface. Figure (18) NIST Standard Reference Database; http://webbook.nist.gov/ chemistry. (19) Ko, E. I.; Benziger, J. B.; Madix, R. J. J. Catal. 1980, 62, 264.
COT as Intermediate in Alkyne Cyclotrimerization
Figure 2. Normalized cation populations of the lower-valent titanium surface as a function of prior annealing temperature. Maximum surface populations for the species (and the temperature at which they are most abundant): Ti1+, 14% (300 K); Ti2+, 23% (450 K); Ti3+, 34% (600 K).
Figure 3. TPD spectra of desorbing COT (m/e ) 104) from surfaces prepared at different prior annealing temperatures.
2 illustrates the population of each of the lower oxidation states (Ti1+, Ti2+, and Ti3+) present on our reduced surfaces at different annealing temperatures; the balance in each case is composed of Ti4+. The populations of the surface cations are reasonably stable for surfaces annealed up to 550 K. Surfaces annealed to higher temperatures show decreases in the populations of lower valent cations, consistent with healing of the surface by diffusion of titanium and/or oxygen species between the surface and the bulk. The most reduced sites on the surface are extinguished in sequence; surfaces annealed to 750 K are almost totally reoxidized and exhibit photoelectron spectra characteristic of TiO2.20,21 The average oxidation state of the surface changes dramatically over a small span, suggesting that the reactivity of the surface might also change substantially. Cyclooctatetraene TPD. Temperature-programmed desorption experiments in which COT was adsorbed on fully reduced surfaces of TiO2(001) gave rise to desorption of COT and the production of benzene and 1,3,6-cyclooctatriene (a hydrogenation product). Figure 3 depicts the desorption spectra of the COT parent mass (m/e ) 104) as a function of the prior annealing temperature of the surface. The bottom spectrum shows the desorbing species (20) Idriss, H.; Barteau, M. A. Catal. Lett. 1994, 26, 123. (21) Lusvardi, V. S.; Barteau, M. A.; Chen, J. G.; Eng Jr., J.; Fruhberger, B.; Teplyakov, A. Surf. Sci. 1998, 397, 237.
Langmuir, Vol. 15, No. 22, 1999 7617
Figure 4. TPD spectra of benzene (m/e ) 78) formed from COT. These spectra correspond to those in Figure 3; the initial desorption peak is associated with fragmentation of COT within the mass spectrometer.
from a fully reduced surface; representative desorption spectra from surfaces annealed at higher temperatures before exposure to COT are plotted on the same figure in order of increasing average surface oxidation state. The lower temperature (280 K) desorption peak is characteristic of weakly bound COT and coincides with the initiation of the temperature ramp. This peak is common to all TPD spectra on the titanium oxide surfaces examined, regardless of the average oxidation state of the surface. A second desorption peak (centered at about 500 K) is most intense on highly reduced surfaces, is diminished in intensity on more oxidized surfaces, and is totally absent on surfaces annealed to 650 K and above before COT adsorption. As discussed below, the disappearance of this peak coincides with the extinction of Ti2+ cations from the surface. Benzene Formation. Figure 4 demonstrates the production of benzene (m/e ) 78) from COT on surfaces exhibiting different extents of reduction. An initial desorption peak for m/e ) 78 appears at the same temperature as the lower temperature COT peak. This signal results from fragmentation of the desorbing COT in the mass spectrometer. A higher temperature desorption peak for m/e ) 78 is also evident on reduced surfaces; two features of the spectrum demonstrate that this peak is primarily due to benzene and is not merely the result of fragmentation in the mass spectrometer. First, the higher temperature peak in the m/e ) 78 benzene trace appears 50 K higher (at 550 K) than in the COT (m/e ) 104) trace. Second, the higher temperature peak on the benzene trace is much more intense than the low-temperature peak. The opposite behavior is apparent in the COT desorption spectrum. After deconvolution of the m/e ) 78 signal to account for COT cracking, the area not attributable to COT fragmentation was therefore assigned as the benzene product. Just as with the COT high-temperature desorption state, the benzene product state vanishes for surfaces annealed to temperatures of 650 K or higher, indicating that the production of benzene depends on the presence of Ti2+ cations at the surface of the crystal. 1,3,6-Cyclooctatriene Formation. Earlier studies of the cyclotrimerization of alkynes on reduced TiO2(001) surfaces showed that these reactants could be hydrogenated on the surface. The source of hydrogen has been attributed to surface hydroxyls, background hydrogen gettered by the reduced surface (in the manner of a Ti sublimation pump), or hydrogen implanted in the surface during the sputtering process. In the present experiments,
7618
Langmuir, Vol. 15, No. 22, 1999
Sherrill et al.
Table 1. Literature and Experimental Cracking Patterns for the Isomers of Cyclooctatrienea fragment (m/e)
1,3,5-COTriene18
1,3,6-COTriene18
experimental
106 104 91 78 77 76 52
21 2 48 100 24 1 11
10 1 68 100 30 2 10
8 1 68 100 36 4 17
a The intensity of the most abundant mass fragment (m/e ) 78) has been normalized to 100 in each case.
Figure 6. X-ray photoelectron spectra for COT adsorbed on surfaces reduced and annealed to different temperatures. Carbon (1s) spectra were obtained following COT adsorption at 273 K on the reduced TiO2(001) surface previously annealed to 550 K.
Figure 5. TPD spectra of a representative mass of the hydrogenation product, 1,3,6-cyclooctatriene (m/e ) 91) following COT adsorption.
COT formed the hydrogenation product 1,3,6-cyclooctatriene on reduced surfaces. This product is formed selectively; its isomer (1,3,5-cyclooctatriene) has a distinct fragmentation pattern that does not agree with the measured desorption spectra, whereas that for 1,3,6cyclooctatriene is in good agreement (Table 1). Figure 5 depicts the TPD spectra of a representative fragment (m/e 91) of 1,3,6-cyclooctatriene. COT does not give rise to a fragment at m/e 91, so the low-temperature peak that is associated with weakly bound COT is not present in these spectra. A high-temperature peak centered at 550 K is apparent for reduced surfaces in these spectra, but disappears as the surface of the crystal is reoxidized. This peak is coincident with the evolution of benzene from the surface. That behavior, and the disappearance of this feature as the surface was reoxidized (as was also observed for the benzene product) indicate that the hydrogenation product, 1,3,6-cyclooctatriene, and the ring-contraction product, benzene, may proceed from a common adsorbed intermediate on the surface. Carbon (1s) XPS. X-ray photoelectron spectra were collected from surfaces prepared and dosed identically with those in the TPD experiments described. Figure 6 shows C(1s) spectra for COT on surfaces reduced and subjected to the same prior annealing temperatures as those in the TPD spectra shown previously. The spectra on all of the surfaces are characterized by a peak centered at about 284.8 eV, with a full width at half-maximum (fwhm) in each case of about 2.5 eV. The spectrum on the most highly reduced surface is extended to illustrate the absence of any signal at higher binding energies. COT is antiaromatic and normally tub-shaped. It can be made to lie flat upon aromatization, which involves the formation
Figure 7. Reduced TiO2(001) surfaces dosed with COT. Before TPD, the spectrum shows a peak at about 284.8 eV. After TPD, no carbon remains on the surface.
of the dianion.22 Aromatic molecules typically display π f π* shake-up satellites about 6-7 eV higher in binding energy than the carbon peak, and between 5 and 10% of the primary peak23; no such features are evident here. Spectra of the adsorbed layer were also collected after TPD to 670 K to determine whether in the course of benzene formation the remaining C2 unit was deposited as surface carbon (with hydrogen from these molecules possibly scavenged for the hydrogenation product, 1,3,6cyclooctatriene). Figure 7 shows that negligible amounts of carbon were deposited by the reactions of COT, implying that the residual C2 fragment is returned to the reactant pool intact. Discussion Temperature programmed desorption experiments indicated that the adsorption of COT was strongly influenced by the presence of lower valent surface cations; as the population of these sites decreased, the amount of COT desorbing at 500 K decreased. The availability of lower valent surface cations controlled the amount of benzene and 1,3,6-cyclooctatriene formed as well. The metallacyclopentadiene-mediated mechanism proposed earlier to explain the cyclotrimerization of acetylene on reduced TiO2 (22) Fray, G. I.; Saxton, R. G. In The Chemistry of Cyclo-octatetraene and Its Derivatives; Cambridge University Press: New York, 1978; pp 4-18. (23) Briggs, D. In Practical Surface Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; Wiley & Sons: New York, 1990; p 448.
COT as Intermediate in Alkyne Cyclotrimerization
Langmuir, Vol. 15, No. 22, 1999 7619
Figure 10. (a) Proposed reaction sequence for COT reacting on reduced titania. Note that the proposed bicyclic intermediate contains the elements of a metallacyclopentadiene as suggested by the exclusive formation of the 1,4-hydrogenation product 1,3,6-cyclooctatetraene. (b) Proposed reaction sequence for the formation of benzene from acetylene on reduced titania. Benzene production from this reaction is desorption limited at 400 K. Figure 8. Maps of COT product yield and the population of Ti2+ surface cations versus prior annealing temperature. Benzene yield, 9; 1,3,6-cyclooctatriene yield, B; Ti2+ fraction, [.
Figure 9. Plot of the total yield of COT-derived products versus the population of Ti2+ cations.
surfaces requires that the metal center be able to cycle between two oxidation states two units apart, thus tying the cyclotrimerization activity to the availability of Ti2+ centers. Figure 8 illustrates the yields of benzene and 1,3,6-cyclooctatriene as a function of the prior annealing temperature of the surface. Figure 8 also shows the fractional population of Ti2+ surface cations as measured by XPS. The dramatic decrease in the production of benzene between 550 and 650 K apparently parallels the extinction of Ti2+ cations on the surface. The abrupt quenching of surface activity with small changes in surface annealing temperature for this reaction is less evident in the formation of 1,3,6-cyclooctatriene. The gradual decrease shown for the production of the hydrogenation product likely reflects the reduced availability of hydrogen with annealing at progressively increasing temperatures. The key point, however, is that the production of 1,3,6cyclooctatriene stops at the same time that benzene production is quenched, consistent with the participation of a common intermediate that depends on the availability of Ti2+ active sites. Plotting the total yield of products as a function of the fractional population of Ti2+ cations (Figure 9) shows that a good correlation exists between the population of Ti2+ cations and the cumulative yield of COT-derived products. Photoelectron spectra of the C(1s) region after heating to a moderate temperature (670 K) show that no carbon is left on the surface. This suggests that the hydrogenation product scavenges hydrogen from sources other than adjacent COT molecules. Furthermore, the ring contraction of a COT intermediate to form benzene plus acetylene would conserve hydrogen. Earlier experiments showed
that reduced TiO2 surfaces exposed to acetylene desorbed benzene at 400 K during TPD experiments.1 Therefore any acetylene formed from the surface as a result of ring contraction should readily cyclize and desorb from the surface. The common peak temperatures for benzene and cyclooctatriene production in COT TPD experiments, and the common dependence of these products on the presence of surface Ti2+ sites strongly suggest that ring contraction and 1,4-hydrogenation of COT share a common surface intermediate. Figure 10 illustrates the proposed pathways for these reactions and depicts the proposed C8-intermediate species on the surface. The elements of the metallacyclopentadiene intermediate that was proposed for alkyne cyclotrimerization on reduced metal oxide surfaces exist within this intermediate. This intermediate is also reminiscent of one of the a valence tautomers of COT, bicyclo[4.2.0]octa-2,4,7-triene. COT can be photolyzed (in both gas and condensed phases) to give benzene, acetylene, and styrene, and it is thought that the intermediate species in this reaction is bicyclo[4.2.0]octa-2,4,7-triene.22 Although there was no evidence of styrene in our product slate (and presumably the acetylene is returned to other Ti2+ sites to produce benzene), this chemistry provides an analogue for our proposed intermediate: insertion of a metal center into the bridging carbon-carbon bond of that bicyclic tautomer would form the metallapentacycle in Figure 10. Figure 10 also highlights previous experimental information about alkyne cyclotrimerization on reduced surfaces of titanium oxide. It has been shown that acetylene will cyclotrimerize to form benzene, and that above 300 K benzene desorption from the surface is rate determining. In TPD experiments in which acetylene was adsorbed on reduced TiO2 surfaces, benzene desorbed at about 400 K, 150 K below the benzene peak resulting from the ring contraction of COT. It seems obvious that C4 fragments on the surface do not dimerize to form COT; some other mechanism appears to control cyclotrimerization of alkynes on reduced surfaces of this oxide. It even appears from the product slate and surface cation dependence that COT molecules on the surface form a bound species that is effectively a metallacyclopentene. (Note that the proposed bicyclic intermediate consists of 7- and 5-membered metallacycles). Conclusions Cyclooctatetraene reacts on reduced surfaces of TiO2(001) to form benzene and a hydrogenation product, 1,3,6cyclooctatriene. The reaction depends on the availability of Ti2+ sites on the surface; when no such sites are
7620
Langmuir, Vol. 15, No. 22, 1999
available, the reaction is quenched. Both products are formed at the same temperature, suggesting that a common intermediate leads to the formation of both. The selective formation of the 1,3,6-isomer of the hydrogenation product and the dependence on the surface cation population are consistent with the formation of a bicyclic
Sherrill et al.
intermediate. Comparison with previous studies of acetylene on reduced surfaces of this oxide indicates that COTmediated cyclotrimerization is an unlikely path to benzene from acetylene. LA990119P
