3882
J. Phys. Chem. 1994,98, 3882-3892
Cyclotrimerization of Alkynes on Reduced Ti02 (001) Surfaces Keith G. Pierce and Mark A. Barteau. Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received: December 9, 1993; In Final Form: February 7 , 1994’
The reactivity of simple alkynes (acetylene, propyne, and 2-butyne) on various reduced Ti02 (001) surfaces was investigated using temperature-programmed desorption (TPD). The predominant reaction pathway in each case was cyclotrimerization to form aromatic compounds (benzene from acetylene, trimethylbenzene from propyne, and hexamethylbenzene from 2-butyne) with selectivities between 5 1% and 86%. Other less prevalent reactions included reductive dimerization to open chain dienes and hydrogenation to olefins. The selectivity for cyclization increased with reactant carbon number for the series above, while those of the olefin and diene products decreased, as did the fraction of the alkyne adlayer converted. By analysis of the cracking fractions of the trimethylbenzene produced during methylacetylene TPD, it was determined that the ratio of the 1,2,4to 1,3,5-isomers was = 3:l. This is the isomeric ratio expected if cyclotrimerization proceeds through a metallacyclopentadiene intermediate, with no preferential orientation of the methylacetylene molecules inserted. The yield of trimethylbenzene can be directly correlated with the population of Ti(2+) cations, quantified by XPS, on reduced Ti02 (001) surfaces. This surface site requirement is analogous to that for formation of metallacyclopentadiene complexes in solution; the latter reaction with mononuclear complexes requires transition metal cations capable of undergoing a two-electron oxidation. Direct precedents exist for formation of titanium(IV) cyclopentadienyl complexes from Ti(I1) species in solution. Although the cyclotrimerization of alkynes has been reported on several single crystal metal surfaces, this is the first example of this reaction on a singlecrystal metal oxide surface under ultrahigh-vacuum (UHV) conditions.
Introduction Under industrial process conditions it is difficult to determine the identity and nature of the active sites required for heterogeneously catalyzed reactions. The great advantage of studying chemistry on well-defined single-crystal surfaces is the ability to relate reactivity directly to the state of the surface. A number of examples of structure-sensitive reactions on metals exist in both catalysis and surface science studies, with the latter providing insights into the geometric arrangement of atoms required to effect the reaction of interest. Although the concept of structure sensitivity is less developed for oxides, single-crystal studies promise to play a similar role. Recent studies on oxidized Ti02 single-crystal surfaces have demonstrated relationships of the activity and selectivity of surface reactions to the coordination environment of the surface Ti cations.’V2 For example, the bimolecular ketonization of carboxylic acids occurs only on fully oxidized surfaces exposing Ti(4+) cations which are doubly coordinatively unsaturated; only these sites allow the binding of the two surface carboxylate species necessary for the coupling reaction to occur. Another manner in which surface characteristics can be related to reactivityon single crystal metal oxides is through the oxidation states of surface cations. In most studies of the oxidation state of functioning catalysts, the manner of measurement is typically indirect and yields an average value over a range of potential oxidation states, rendering direct correlations between activity and specificsurfacesites difficult. This difficulty can be addressed by use of well-characterized single crystal surfaces prepared under carefully controlled conditions. Recent studies in our group have demonstrated the ability to generate a number of reduced T i 0 2 surfaces and to quantify the population of Ti cations in various oxidation states (+I, +2, +3, +4) using X-ray photoelectron spectroscopy (XPS).3 The ability to control the oxidation state of the surface Ti cations enables one to relate activity to oxidation To whom correspondence should be addressed. *Abstract published in Aduonce ACS Absmcts, March 15, 1994.
state of the active site. This work explores that connection using alkyne cyclotrimerization as the probe reaction. The cyclotrimerization of alkynes was first reported by Berthelot over 125 years ago and involved heating acetylene in a glass bulb to produce b e n ~ e n e ;this ~ must certainly be one of the earliest examples of perfect atom e c o n ~ m y .Since ~ that time a variety of systems have been found to catalyze this reaction, including homogeneous transition metal complexes of a number of different metals (e.g., CO,~?’ Ni,* Ir9) and supported metal heterogeneous systems (NiSi02’O and Pd on charcoal and on a variety of different supports, such as Si02, TiO2, and A120311). More recently, the cyclotrimerizationof alkynes has been studied on a number of single crystal metals under UHV conditions (e.g., Ni,l0 C U , ’Pdl3). ~ Detailed studies of the cyclotrimerization of acetylene on Pd surfaces have elucidated the mechanism for this reaction in great detail.”24 The mechanism for the reaction of alkynes to form aromatic compounds with organometallic complexes25 as well as that on single crystal metal s u r f a ~ e s I ~ 2 ~ is thought to proceed through a metallacyclopentadiene intermediate (11):
where M represents a metal site or surface. This mechanism involves the coordination of two alkynes to the active site (I), cyclization to the five-membered metallacycle (11), and final addition of a third alkyne monomer to form the aromatic product
(In).
Also of direct relevance to this work are titanium-based systems and their reactivity toward alkynes. Some Ziegler-Natta systems, such as TiC14/A1Et2Cl, in addition to catalyzing olefin polymerization, also promote the cyclization of alkynes.26 Other
0022-365419412098-3882%04.50/0 0 1994 American Chemical Society
Cyclotrimerization of Alkynes on Ti02 (001)
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3883
Ziegler-Natta catalysts, like TiC14/Al(i-C4H9)3, polymerize acetylene while showing no activity for ~ y c l i z a t i o n .It~ has ~ been proposed based on catalyst weight gain during steady-state reaction experiments28and on results from Raman ~pectroscopy2~ that Ti02 powders promote the oligomerization of alkynes. Benzene has been reported to be formed from acetylene on Ti02 powders both exposed and not exposed to UV light,30 as well as in FTIR studies.31 However, none of the powder studies were able to relate the activity for polymerization or cyclization to specific surface sites. Working with low-valent titanium species (TiCla/Zn-Cu) in liquid-solid slurries, McMurry has reported the ability to cyclize internal alkynes.32 That work attributed the activity for alkyne cyclization to Ti(0). In this paper we demonstrate thecyclotrimerizationof a number of simple alkynes (acetylene, propyne, and 2-butyne) on reduced Ti02 (001) surfacesanddirectlyrelate the formationof cyclization products to the presence of surface Ti cations in specific oxidation states. Experimental Section Experiments were performed in a modified Physical Electronics Model 548 surface analysis system described p r e v i o u ~ l y , ~ ~ equipped with a UTI lOOC quadrupole mass spectrometer, LEED optics, a cylindrical mirror analyzer for Auger electron spectroscopy, and a sputter ion gun. Base pressures of (1 X Torr were produced using ion and titanium sublimation pumping, while a supplementary turbomolecular pump was used to rough pump the system and remove argon from the chamber after sputtering. A (001)-oriented single crystal sample with rectangular shape (10 mm X 9 mm X 1.5 mm) of Ti02 (rutile) was prepared from a crystal boule (99.9%Atomergic Chemetals Corp.) as described previously.33 After being cut, the crystal was mechanically polished using sequentially finer grades of diamond pastes, concluding with 0.25 pm, and ultrasonically rinsed in acetone after each polishing step. Thecrystal was mountedin thechamber using a holder made of 0.127-mm tantalum foil spot-welded to a 0.5" tantalum wire connected to a high-current power supply (Lambda LES-EE-01-OV) for sample heating. The sample temperature was monitored using a chromel-alumel thermocouple attached to the side of the crystal with high-temperature cement (Aremco Ultra-Temp 5 16). The crystal was cleaned by repeated cycles of argon ion bombardment and high-temperature (950 K) annealing until no noticeable impurities (mainly carbon prior to cleaning) were detected using Auger electron spectroscopy. For each TPD experiment the surface was prepared as follows. After initial sample cleaning, the surface was bombarded using a 1.5-keV Ar+ beam at normal incidence for 1 h. The chamber was back-filled with argon (Matheson Grade 5) to a pressure of 5 X 10-5 Torr before sputtering. During sputtering the chamber pressure was maintained by use of a turbomolecular pump. Following sputtering the surface was annealed at the desired temperature for a minimum of 45 min. When preparing surfaces that were annealed at temperatures less than 600 K, the surface was flashed to 600 K prior to each TPD experiment to remove any contaminants adsorbed from the background (predominantly water) while producing no detectable oxidation of the surface. Annealing for longer periods at elevated temperatures causes oxygen diffusion from the bulk, oxidizing the surface. Recent detailed studies in our laboratory have quantified the populations of Ti cations as a function of annealing temperature by using XPS;, these results are utilized below. After the surface had been prepared through sputtering and annealing at the desired temperature, the reagent of interest was adsorbed to saturation coverage at room temperature by dosing through a stainless steel needle. After dosing, the chamber pressure was allowed to return to a 2 X 10-10 Torr before beginning the TPD heating ramp. Control of the mass spectrometer and power supply for generating
-7;
i j n
m / O 27
H-C=C-H I
I
I
I
(x,)
m/e26
3884
Pierce and Barteau
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994
ponents. Both products are consistent with the cyclotrimerization mechanism for alkynes with organometallic complexes in solution. This reaction involves association of two acetylenes attached to a metal center to form a metallacyclopentadiene intermediate.25 Insertion of a third acetylene yields the cyclotrimerization product; reductive elimination yields the hydrogenated dimer, 1,3-butadiene. Desorption peaks a t 400 and 525 K with a substantial mle 27 signal could not be accounted for by contributions from benzene and 1,3-butadiene. (Less than 10% of this peak was assignable to these two compounds.) Based on the remaining mle 27 signal as well as a corresponding peak at mle 14,this contribution was attributed to ethylene. (The mle 28 signal was unusable because of a significant CO background in the vacuum system.) The remaining peaks at mle 26, 25,24,and 13 could be assigned to the parent molecule, acetylene. The hydrogen required by the unimolecular hydrogenation reaction to produce ethylene and by the reductive dimerization to produce 1,3-butadiene is most likely generated through complete decomposition of acetylene to surface carson and hydrogen species. Deposition of carbon and the production of surface hydrogen from acetylene are a common reaction on a number of metal surfaces, including Pd(l1 l),13. Cu( 1 10),34and Ni( 1 1O).35 XPS studies in our lab examining the reactivity of various oxygenated compounds on the Ar+-bombarded Ti02 (001) surface indicated that small amounts of carbon deposition are c0mmon.2,~~J~ Using the area of the C( 1s) peak as an indicator, the amount of carbon deposited in these cases was around 10%of the initial adsorbate coverage. Auger electron spectra taken before and after TPD of alkynes on the reduced surface showed small amounts of carbon deposition, although exact amounts could not be quantified reproducibly. Assuming that the hydrogen required for ethylene and butadiene formation is supplied through the complete decomposition of acetylene to surface carbon and hydrogen, about 29% of the initial coverage of acetylene would have to be atomized, based on the yields of these various products discussed below. This result should be viewed as an upper limit on the amount of surface carbon deposited, however. Electron-stimulated desorption (ESD) studies of the Ti02 (001) surface have shown that reactive hydrogen species exist in the near-surface regions on Ar+-bombarded ~urfaces.3~If these hydrogen impurities participate in the reduction reactions, the amount of hydrogen needed from reactant decomposition processes would be reduced. Three distinct peak positions were observed among the four major products. Acetylene desorbed from the surface at 390 K. At a slightly higher temperature, around 400 K, contributions from the three main hydrocarbon products were observed: the unimolecular hydrogenation product ethylene, the dimerization product 1,3-butadiene, and the cyclotrimerization product benzene. Benzenedesorption exhibitedan extended high-temperature shoulder ending at about 700 K. Ethylene and butadiene also desorbed in a second, broad peak at around 5 15 K. Benzene and acetylene desorbed in a third, high-temperature state at 720 K. No H2 was observed to evolve from the reduced surface. The high-temperature peaks of acetylene and benzene can be explained in terms of the reactionsof surfaceacetylide ( H - C e - ) species produced by acetylene dissociation at lower temperatures. The formation of stable acetylides has been demonstrated on ZnO single crystal surface@ as well as on other oxides. Decomposition of these species a t elevated temperatures would produce carbon and hydrogen species, with the latter reacting with other acetylides to re-form acetylene molecules which desorbor cyclize to benzene. The high-temperature acetylene peak has a broad shoulder starting at 560 K and culminating in a peak a t 720 K,coincident with the high-temperature benzene peak. This temperature range agrees well with that reported by Vohs and Barteau for acetylide decomposition on ZnO; they observed stable acetylides at 500 K which decomposed between 500 and 715 K.38
TABLE 1: Product Yields for Acetylene TPD on the Ar+-BombardedTi02 (001) Surface m /e of principal fragment used product for yield determination peak temp (K) molar yielda 0.047 C2H2 26 390 78 400 0.397 CsHs 0.106 C4H6 54 400,510 0.348 C2H4 27 400,525 26 720 0.031 C2H2 0.070 CsH6 78 720 a Yields are based on the total amount of carbon in each product, normalizing the sum of volatile products to unity.
JA
Ethylene (ex Awlylone) we n (x 3)
Benzene
we 78 (X 2) Benzene (ex Acetylene) I
I
I
I
I
400
500
600
700
800
Temperature (K)
Figure 2. TPD spectra of the various products formed during acetylene
TPD experiments and the desorption spectra resulting from the molecular adsorption of those products. Products formed during acetylene TPD are labeled (ex acetylene).
The yields of volatile products from acetylene TPD are presented in Table 1. Yields shown represent the fraction of the total carbon desorbed contained in each product and are normalized so that the sum of all the volatile products is unity. These have been calculated using sensitivity factors which account for differences in ionization efficiencies, mass spectrometer gain, mass fraction transmission, and cracking among the product molecules.39 The calculation of these sensitivity factors requires a cracking pattern for each product. In order to determine whether the evolution of each product from acetylene TPD was limited by the kinetics of its formation, experiments were carried out in which each of the three major products was adsorbed individually on an Ar+-bombarded Ti02 (001) surface to saturation coverage. The results of these TPD experiments are shown in Figure 2. All products exhibited a prominent desorption state at or below 400 K (benzene a t 380 K, 1,3-butadieneat 395 K, and ethylene at 400K). Ethyleneand 1,3-butadiene also exhibited high-temperature shoulders at 500 and 490 K,respectively. Although benzene desorbs a t a slightly lower temperature than that produced from acetylene TPD (380 K vs 400 K),this most likely reflects the variability inherent in the sputtering process and the difference in the surface coverage of benzene formed from acetylenevs that directly adsorbed. Since no desorption state for adsorbed benzene at high temperatures (720 K) was observed, we conclude that the low-temperature benzene state from acetylene is desorption limited while the hightemperature state is reaction limited. Characterization of the
Cyclotrimerization of Alkynes on TiOz (001)
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3885
A Trlmeth:lB
(x 1)
Jl \ \
Annoallng Temperature
Prior Annealing Temperature 300 K
Jj+625 K
-
650 K 750 K 850 K
850 K 950 K
950K
I
I
I
I
I
400
500
600
700
400
Temperature (K)
TPD spectra of trimethylbenzene ( m / e 105, X 1) following methylacetylene adsorption at room temperature as a function of prior annealing temperature of the Ti02 (001) surface. desorption states for ethylene and 1,3-butadiene was not as clear. Both ethylene and 1,3-butadiene desorb in two peaks at roughly the same temperatures, whether adsorbed directly or produced from acetylene (although the second peak of the products from the acetylene TPD is at a slightly higher temperature). However, the relative populations of the two states are dependent on the source of these products; those produced from acetylene TPD were roughly equal, while the lower-temperature state produced by direct adsorption was significantly larger than the highertemperature state. This difference reflects in part the different coverages produced by adsorption rather than reaction; adsorption of molecular species at saturation coverage fills the lowertemperature state. 1,3-Butadiene adsorbs in much larger amounts than are produced during acetylene TPD, making it difficult to draw any conclusions. The lower-temperature desorption peaks for ethylene agree reasonable well in temperature and magnitude, while the second desorption peak for ethylene from acetylene TPD is significantly larger, suggesting that the first ethylene peak (at 400 K) is limited by the kinetics of its desorption while the second (at 500 K) is reaction limited. Methylacetylene TPD on Reduced Ti02 (001) Surfaces. Methylacetylene (propyne) TPD experiments were carried out on a number of Ti02 (001) surfaces prepared at different extents of reduction. As demonstrated previously, the extent of reduction of the near-surface region can be controlled by the temperature at which the surface is annealed after Ar+ bombardment prior to each TPD experiment3 TPD spectra for the products desorbed from the various surfaces examined are presented in Figures 3-6. As for acetylene, four principal desorption products were observed. The highest molecular weight product exhibited significant intensity at m l e 105 and 120, indicating a probable molecular formula of CgH12. Coincident peaks at m / e 51, 65, and 77 indicated the presence of an aromatic ring, and the dominant peak at mle 105 suggested that the main fragmentation pathway involved loss of a methyl group. The ratio of m / e 105 to m / e 120 agreedvery closely with that of trimethylbenzene, theexpected cyclotrimerization product. A much smaller signal was observed at higher temperatures (ca. 520 K) for fragments with m / e 67 and 82. Since the largest fragment observed in this region of the TPD spectrum was m / e 82, a product with molecular formula C6H10 is implicated. Analogous to the identification of 1,3butadiene as a product of acetylene TPD, these fragments were
500
1
1
600
700
Temperature (K)
Figure 4. TPD spectra of dimer product ( m / e 67,
X 14) following methylacetylene adsorption at room temperature as a function of prior annealing temperature of the Ti02 (001) surface.
&
850K
+A/
I
400
I
I
500
600
950K I 700
Temperature (K)
Figure 5. TPD spectra of methylacetylene ( m / e 38, X 3.5) following methylacetylene adsorption at room temperature as a function of prior annealing temperature of the Ti02 (001) surface.
attributed to an open-chain diene, a product of methylacetylene dimerization. Assuming that no methyl shifts occur, the three possible dimeric products are 2,4-hexadiene, 2-methyl-l,3pentadiene, and 2,3-dimethyl- 1,3-butadiene. Attempts to identify the isomer produced by comparison of the TPD spectra with pure component cracking patterns were unsuccessful because of the small signal strength of the dimer product and the resulting high variability of the ratios of peak areas. Significant peaks in the methylacetylene TPD spectrum for m / e 41 and 42 were assigned to the unimolecular hydrogenation product, propylene. The remaining peaks at m/e 39,38, and 37 (mle40could not be used due to the high argon background pressure in the chamber as a result of sputtering) were assigned to the parent molecule, methylacetylene. The peakat m / e 38 was chosen for calculations
Pierce and Barteau
'he Journal of Physical Chemistry, Vol. 98, No. 14, 1994
t
Anneallng Temperature
/b /Trimethyl
i
Benzene\ I
300 K
550 K
600K
200
625 K
I
I
I
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600
700
Temperature (K)
TABLE 2: Product Yields for Methylacetylene TPD on the Ar+-BombardedTi02 (001) Surface m / e of principal fragment used peak molar product for yield determination temp (K) yieldo 31 42 105
400 415,510 395 505
500
800
700
800
900
1000
Figure 7. Molar yield of products from methylacetylene TPD (based on total carbon desorbed) as a function of prior annealing temperature after Ar+bombardment of the Ti02 (001)surface. The yields have been scaled such that the total yield of carbon-containing species from the Artbombarded (300 K) surface sums to unity.
Figure 6. TPD spectra of propylene ( m / e 42, X 6) following methylacetylene adsorption at room temperatureas a function of prior annealing temperature of the Ti02 (001) surface.
HCEC-CH~ HzC=CH--CH, C&MCH3h
400
Prior Annealing Temperature (K)
850 K
400
300
0.196 0.134 0.611 0.059
67 Yields are based on the total amount of carbon in each product, normalizing the sum of volatile products to unity. of the yield of methylacetylene because it had fewer contributions from other products (28% of the m / e 42 signal from propylene) than other masses (such as m / e 39, which has a largercontribution from propylene, as well as a contribution from trimethylbenzene). Qualitatively, the TPD spectra for methylacetylene on the Ar+bombarded T i 0 2 (001) surface were similar to those of acetylene TPD on the same surface. All four products desorbed in a lowtemperature state around 400 K, with the hydrogenated monomer and dimer products also desorbing around 510 K. Unlike acetylene, however, no high-temperature state for either monomer or trimer was observed. Molar product yields for methylacetylene TPD on the Ar+-bombarded T i 0 2 (001) surface are presented in Table 2. The dependence of product yields on the prior annealing temperature (and hence on the extent of reduction of the surface) is illustrated in Figures 3-6 and summarized in Figure 7. The yield of trimethylbenzene increased slightly from the 300 to the 450 K surface, remained constant on surfaces annealed between 450 and 600 K, and then decreased drastically for surfaces annealed between 600 and 650 K. For surfaces annealed above 650 K the trimethylbenzene yield was quite low, with a value on the 950 K-annealed surface of only 2%of the value obtained on the 450 K surface. The peak position of trimethylbenzene in TPD spectra was constant at 395 K on surfaces annealed up to 600 K: it increased monotonically to 420 K on the 750 K surface and then decreased to 385 K on surfaces annealed at higher temperature. The trimethylbenzene desorption peak observed during methylacetylene TPD was identical to that produced from the adsorption of trimethylbenzene (vide infra), implying a firstorder desorption-limited process. Assuming first-order desorption
and a preexponential factor of 1013s-I, the energy of activation determined by Redhead's method40 for trimethylbenzene desorption is 25.3 kcal/mol for surfaces annealed below 600 K, 27.0 kcal/mol on the 750 K surface, and 24.6 kcal/mol on the high-temperature annealed surfaces, giving a maximum variation of activation energies of 2.4 kcal/mol. It has been shown previously2J that the Ar+-bombarded, reduced surface is completely oxidized to the (011)-faceted surface after heating to 750 K and reconstructs to the oxidized 1114)-faceted surface upon being heated from 750 to 950 K. The trimethylbenzenedesorption peak temperatures (but not the yields) seem to track this surface variability, reaching a maximum on the first fully oxidized surface and decreasing thereafter. The dimer product also exhibited a profound decrease in yield between the 600 and 650 K annealed surfaces. Considering the dimer desorption peak to consist of a major peak at 505 K and a small low-temperature shoulder around 41 5 K, one can observe from Figure 4 that the major peak was almost completely extinguished on the 650 K surface, while the low-temperature peak was not eliminated on surfaces annealed below 750 K. Methylacetylene desorbed in a sharp peak at 400 K with a significant high-temperature tail. The yield of methylacetylene decreased to a minimum a t a prior annealing temperature of 750 K and then increased slightly on surfaces annealed at higher temperatures. Methylacetylene desorption peaks from surfaces annealed above 750 K appeared a t a slightly higher temperature (415 K) and exhibited no high-temperature tail. Propylene exhibited similar behavior to methylacetylene: a drastic yield decrease on surfaces previously annealed between 600 and 650 K, reaching a minimum on the 750 K surface followed by a slight increase on higher-temperature surfaces. It is interesting to note that the two unimolecular products reached a minimum yield on the 750 K surface while the dimer and cyclotrimerization products, after a sharp decrease between 600 and 650 K, showed a monotonic decrease on the surfaces prepared at higher temperatures. The coverage of methylacetylene was relatively constant up to a prior annealing temperature of 600 K and then decreased sharply on the moreseverely annealed surfaces. On the basis of the amounts of products desorbed (determined on a total carbon basis), the coverage of methylacetylene decreased more than 11-fold from the 450 K surface to the 750 K surface. To determine whether the products of methylacetylene TPD were desorption or reaction limited, TPD experiments for 1,2,4and 1,3,5-trimethylbenzene, 2-methyl-1,3-pentadiene (a representative dimer), and propylene were performed on the Ar+bombarded Ti02 (001) surface (Figures 8 and 9). Both 1,2,4and 1,3,5-Trimethylbenzene desorbed a t 395 K in virtually identical peaks. This peak temperature was identical to that
Cyclotrimerization of Alkynes on Ti02 (001)
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3887
TABLE 3 Experimental Mass Spectrometer Cracking Patterns for 1,2,4- and 1,3,5-Trimetbylbenzen~ intensity intensity 1.2.4-isomer 1.3.5-isomer m/e 1,2,4-isomer 1,3,5-isomer 27 60 61 91 125 122 39 103 106 103 82 84 51 83 80 105 1000 1000 58 56 106 83 83 65 146 119 93 93 146 17 78 57 58 120 382 403 19 140 131 Cracking fractions have been normalized so the largest peak has a value of 1000. All fragments with intensities >5% of the most abundant are included.
* (I
J
Trlmethyl Benzene (ex Methyl Acetylene) d e 105 (x 8 ) I
4w
I
500
I
600
700
800
Temperature (K)
Figure 8. TPD spectra of trimethylbenzene formed during methylacet-
ylene TPD experiments and the desorption spectra resulting from the molecular adsorption of 1,2,4- and 1,3,5-trimethylbenzene.
n Propylene mle 42
I
I
400
500
I
600
1
I
700
800
Temperature (K)
Figure 9. TPD spectra of the reduction products formed during methylacetylene TPD experiments and the desorption spectra resulting from the molecular adsorptionof those products. Products formed during methylacetylene TPD are labeled (ex methylacetylene).
observed during methylacetylene TPD experiments, supporting the conclusion that trimethylbenzene production from methylacetylene was desorption limited. Note, however, that the coverage of trimethylbenzene produced during methylacetylene TPD was roughly 8 times less than that of direct adsorption of the products. 2-Methyl-l,3-pentadiene also desorbed at 395 K but exhibited a significant high-temperature shoulder around 490 K. Similar to 1,3-butadiene, however, the amount of dimer produced from methylacetylene TPD was a small fraction of that adsorbed directly, making it difficult to draw any conclusions. Propylene desorbed in a peak around 400 K with a high-
temperature tail, although saturation coverage of propylene was much lower than that of the alkylbenzenes, as demonstrated by the relative signal intensities in Figures 8 and 9. The desorption spectrum for propylene suggests a desorption-limited lowtemperature peak with a reaction-limited high-temperature peak, similar to the results for ethylene from acetylene TPD. Identification of Trimethylbenzene Isomers Produced by Methylacetylene TPD. Detailed analysis of the cracking pattern of trimethylbenzene produced during methylacetylene TPD was conducted in order to identify the isomers formed. Trimethylbenzene exists in three isomeric structures: 1,2,3-, 1,2,4-, and 1,3,5-trimethylbenzene. If one assumes that the surface cyclotrimerization reaction proceeds by association of monomers, and that no methyl shifts occur, then 1,2,3-trimethylbenzenecan be eliminated from consideration as a possible product. (For further discussion of these assumptions, see below.) Pure component cracking patterns of the 1,2,4- and 1,3,5-isomers were obtained experimentally in the same vacuum system as used for the TPD experiments, and the results are presented in Table 3. The relative intensities of all but a few fragments are essentially indistinguishable for the two isomers. However, normalized against the intensity of the most abundant fragment, m / e 105, the intensity of the signal associated with the parent mass, m / e 120, is approximately 5% greater for the 1,3,5-isomer than for the 1,2,4isomer. This observation is consistent with patterns of poly(alky1)benzene cracking reported previously. It has been demonstrated that closer proximity of side chains in these molecules accentuates the principal decomposition path in the mass ~pectrometer.~~ The main decomposition path of trimethylbenzene is the scission of a single methyl group from the ring to give m / e 105 as the principal fragment. Since the 1,2,4-isomer has a closer proximity of side chains, the main decomposition path in the mass spectrometer is accentuated relative to that of the 1,3,5-isomer, giving smaller amounts of the parent m l e 120 relative to m / e 105 for the 1,2,4-isomer. A comparison of the ratio of peak areas of mle 120 to mle 105 for the pure component isomers as well as the product of propyne TPD is presented in Figure 10. Each bar involves three experiments: the range of the three experiments is shown in the clear section of each bar, and the line within that section is the average of the three runs. The ratio of m / e = 120 to 105 for the TPD product of methylacetylene cyclotrimerization is between the values observed for the two isomers but closer to the value for the 1,2,4-isomer. Explanation of the preference for production of the 1,2,4-isomer over the 1,3,5-isomer requires closer examination of the mechanistic details of the cyclotrimerization reaction. As stated earlier, the mechanism for the cyclotrimerization reaction with organometallic complexes2sas well as single crystal metal surfaces14Js proceeds through a metallacyclopentadieneintermediate. If this is the active mechanism, and no methyl shifts occur, it is impossible to produce 1,2,3-trimethylbenzene. One possible mechanism that would produce 1,2,3-trimethylbenzene involves an q4-cyclobutadiene complex as an intermediate?
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Pierce and Barteau
3888 The Journal of Physical Chemistry. Vol. 98, No. 14, 1994
1.2.4 TMB Cracking Pattern
;r
C n ct.3.S-TMB k l n g Paltsrn
However, no cyclobutadienes (or other cyclic C4 dimers) were observed from methylacetylene T P D only open chain dimers were produced. Thissupportsthe premise that thesurfacereaction proceeds through a metallacyclopentadieneintermediate and not an r14-cyclobutadiene. Studies of the aromatic products of unsymmetric alkynes produced from complexes of Cr, Co, Ni, and Ti also indicated no 1,2,3-substituted products: only AICI, generated 1,2,3-isomers in the amount expected if an $cyclobutadiene intermediate were present?) If one therefore assumes a metallacyclopentadiene intermediate and random orientation of the methylacetylene molecules inserted to form thearomatic product, the ratioof 1.2.4- to 1.3,S-isomer produced would be 3:l. Again, one must make the assumption that no methyl shifts occur. This assumption is supported by a number ofpoints. The presenceoftrimethylbenzeneas thesolearomatic product and theabsenceof dimethylbenzene, toluene, or methane production support the premise that no carbon+arbon bond scission occurs. The low temperature of trimethylbenzene formation ( HIC-HC~H-CH) CizHis
CsHii
fragment used pak molar for yield determination temp (K) yield. 54 56 147 95
390 410 435 485
0.305 0.078 0.597 0.019
a Yields are based on the total amount of carbon in cash product, normalizing the sum of volatile products to unity.
starting reagent, only one structural isomer is possible in each case for the dimerization and cyclotrimerization products. All peaksabovem/e56couldbeaccountedfor by thesetwoproducts. Significant peaks at m/e 56 and 41 were identified as belonging to 2-butene. Pure component cracking patterns for cis-2-butene and trans-2-butene were measured in an attempt to identify the geometric isomer produced, but both cracking patterns were identical within the limits of experimental error. This result is not surprising, for although structural isomers can sometimes be distinguished using mass spectroscopy, the similarity of the mass spectra for geometric isomers makes distinguishingcis and trans isomers difficult.& The remaining masses at m/e 39,53, and 54 wereconsistent with theparent molecule, 2-butyne. Thesimplicity ofthe productdistribution producedduring 2-butyneTPDshows that no significant rearrangement of methyl groups, either intraor intermolecular, takes place and supports the conclusion
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3889
Cyclotrimerization of Alkynes on Ti02 (001)
r
loo
I
I
,
I100
4
4
25
Product
P Unlmolecuiar
20
-
-
. Dimerization . Product
0
1
Acetylene
Propyne
2-Butyne
20
0
Figwe 12. Selectivity and conversionfrom TPDexperimentsas a function of identity of the alkyne. All TPD experiments were performed on the Ar+-bombardcd(300 K) Ti02 (001) surface.
above in the identification of trimethylbenzene isomers that such processes are not significant. 2-Butyne desorbs in a low-temperature state at 390 K and exhibits a discernible high-temperature shoulder extending to 600 K. The cyclotrimerization product, hexamethylbenzene, desorbs in a broad state at 435 K. This desorption temperature is the highest observed for any of the cyclotrimerization products but is expected for such a massive C12 product. 2-Butene desorbs in a broad state with a peak at 410 K and a second smaller peak arond 500 K. Very small quantities of the dimeric product 3,4dimethyl-2,4-hexadienewere observed in a broad peak at around 485 K. As with methylacetylene, no high-temperature stateabove 600 K was observed.
Discussion Alkynes, both internal and terminal, react by three main pathways on reduced Ti02 (001) surfaces. The first involves unimolecular hydrogenation to the corresponding olefin. The hydrogen for this reaction is supplied in part by decomposition and carbon deposition from the parent alkyne. The second decomposition pathway involves the association of two alkynes to form a diene. Open chain dimer products also require hydrogen to be supplied. This dimer product is not surprising if the cyclization mechanism involves a metallacyclopentadiene intermediate because an open, conjugated diene product would result from the hydrogenation of this surface species. Finally, the predominant reaction pathway for alkynes is the cyclotrimerization to form aromatic products. This is the first example of this reaction on a single crystal oxide surface in UHV. The selectivities and yields of products from thevarious reaction paths depend moderately on the identity of the alkyne reagent but more strongly on the state of the surface. The trends in selectivityand yield data vs carbon number are presented in Figure 12 for the reaction of each alkyne on the sputtered (unannealed) surface. The selectivity toward the cyclization product on the sputtered surface increased with increasing carbon number (5 1% for acetylene to 76% for methylacetylene to 86% for 2-butyne) while the dimeric product decreased (1 1% to 7% to 3%) as did the unimolecular hydrogenation product (38% to 17% to 11%). Since the selectivities toward the reductive dimerization product and the unimolecular hydrogenation product change in parallel, the controlling factor for these two reactions is probably the hydrogen supply available on the surface. However, the trend toward fewer hydrogenated products does not parallel the relative hydrogen content of the molecules examined; Le., one observes a decreasing yield of hydrogenated products as the carbon-tohydrogen ratio is changed from 1:l for acetylene to 1:1.33 for propyne to 1:1.5 for 2-butyne. The hydrogenated product selectivity trend does track the number of acetylenic hydrogens on the alkyne, exhibiting a distinct decrease in selectivity with
+---
-B
8
\A
o ~ ~ ' ~ ~ " ' ' ' ~ ' ' ' ' " ' ' " ' ' 200
300
400
500
600
700
800
0 900
1000
Prior Anneellng Temperature (K)
Figure 13. Molar yield of trimethylbenzenefrom methylacetylene TPD based on total carbon (scaled as in Figure 7) and the population of Ti(+2) cations from XPS measurements3as a function of prior annealing
temperature.
the decrease in the number of acetylenic hydrogens, from two for acetylene to one for propyne to none for 2-butyne. The activity of the surface for the production of oligomerization products is closely related to the oxidation state of the Ti cations of the surface. Recent XPS results have mapped the Ti cation oxidation states on the (001) surface as a function of Ar+ bombardment and reoxidation through annealing of the sample at different temperat~res.~ These results may be summarized as follows. Ion bombardment of the stoichiometric Ti02 (001) surface produces a reduced surface which gives rise to a broad envelope of peaks in XPS containing contributions from the Ti 2~312and 2~112lines of the +1, +2, +3, and +4 but not 0 oxidation states. Quantitative population distributions for each of these oxidation states were obtained by curve-fitting the Ti 2~312region of the spectrum following the analysis of Rocker and Gbpel,'s utilizing characteristic positions for each oxidation state: Ti+4, 459.0 eV; Ti+3,457.4 eV; Ti+*,455.9 eV; Ti+*,454.8 eV. Since the fraction of the cations in each oxidation state must sum to unity, changes in the population of individual states of the order of 10%can be determined3936sincetheother states will be affected as well. As the surface is annealed at higher temperatures, the lower oxidation states disappear in roughly sequential fashion, as the near-surface region of the sample is oxidized by diffusion of oxygen from the bulk of the crystal. Angle-resolved XPS measurements, however, show a minor dependence of the cation population on depth within the region sampled by XPS,' indicating that the cation populations determined are characteristic of the surface layer. The yield of trimethylbenzene produced during methylacetylene TPD as a function of prior annealing temperature of the surface exhibits quite distinctive behavior as shown in Figure 13. The trimethylbenzene yield increases slightly from the 300 K surface to the 450 K surface, is constant for surfaces annealed between 450 and 600 K, and drastically decreases in a narrow temperature range between 600 and 650 K before reaching a minimum value on the 950 K surface. This yield behavior is closely duplicated by the population of Ti(+2) cations on the surface. From a comparison of the trimethylbenzene yield and Ti(+2) cation population, one sees that both the slight decrease on the 300 K surface and the sharp decrease between 600 and 650 K are common to both data. Only the population of Ti(+2) cations is consistent with the trimethylbenzene yield behavior. The populations for the other titanium cations observed on the reduced surfaces (Ti(+l) and (+3)) are similar to the shape of the curve for Ti(+2): stable up to a certain temperature range where the population drops to roughly zero in a monotonic fashion. The important difference is the characteristic temperature range in which this decrease occurs, as illustrated by Figure 14. For Ti(+2), the sharp decrease occurs between 600 and 650 K: for
Pierce and Barteau
3890 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994
0.4
1 1
4
0.2
c
4
0.6
i=
300
200
400
500
600
800
700
900
1000
Prior Annealing Temperature (K)
Figure 14. Normalized titanium cation populations for reduced states)
as a function of prior annealing temperatureof the surface. These have been normalized against the maximum population observed for each oxidation state. These maximum values are 17.5% Ti+' on the 300 K surface, 28% Ti+2on the 560 K surface, and 33% Ti+' on the 630 K surface.
5
m
0.7 0.6 0.5 0.4
0.3 0.2
implies that the production of trimethylbenzene on Ti(+2) cations is nearly stoichiometric, Le., 2.6/3.0 = 0.87 molecules of trimethylbenzene would be produced on average per Ti(+2) site. If one includes the dimer products in order to estimate the population of surface metallacycle intermediates, one obtains a value of 1.0 metallacycles per Ti(+2) site, of which 87% add a third alkyne molecule and 13% are reductively eliminated. If one ascribes the remaining desorbing products (methylacetylene and propene) to Ti cations in other oxidation states, one obtains fractional populations of those sites ranging from 0.5 on the reduced surface to 0.05 on the 750 K annealed surface. The minimal yields of all hydrocarbons desorbed from surfaces annealed at 750 K and above (Figure 7) indicate that the Ti(+3) and (+4) sites are virtually inactive for alkyne adsorption, as well as alkyne conversion. To the extent that the assumption of monolayer coverage of methylacetylene on the reduced surface represents an overestimate, the coverage of only 5% of the Ti(+3) and Ti(+4) sites on the 750 K annealed surface likewise is overestimated by the same factor. Thus, alkyne adsorption and reaction both clearly require highly reduced surface sites. A number of precedents in organometallic chemistry support the correlation between the cyclotrimerization product yield and the population of Ti(+2) cations on the surface. It is known that some Ti-based Ziegler-Natta systems promote the cyclotrimerization of alkynes.26 Although Ti(+3) and Ti(+4) are usually thought to produce the active catalytic sites for olefin polymerization, early workers found evidence that Ti(+2) sites were responsible for some Ziegler-Natta chemistry.4 Crystalline, ballmilled Tic12 has also been shown to be an effective ZieglerNatta catalyst in the polymerization of olefins.47 Dicyclopentadienyl titanium dicarbonyl also forms metallacyclopentadienyl intermediates from alkynes (as well as catalyzing their hydrogenation to olefins):4*
0.1
z;+2
Ph 0 0
0.05
0.1
0.15
0.2
0.25
0.3
Ti(+2) fraction of surface cations
Figure 15. Molar yield of trimethylbenzene as a function of the population of Ti(+2) cations.
Ti(+l) andTi(+3),itis300-63OKand630-750K,respectively.3 For a weighted-average extent of reduction of the surface (calculated based on the populations of the individual cations), the decrease occurs between 560 and 700 K.' The temperature at which each of these states is completely removed from the surface is well defined by the low binding energy edge of the Ti(2p) XPS spectra.' None of these measures match the trimethylbenzene yield as well as the Ti(+2) population. The yield of the dimer product during methylacetylene TPD also supports the assignment of Ti(+2) as the active site for cyclotrimerization. From Figure 7 it is apparent that the yield of dimer follows the same trend with surface oxidation as the cyclization product yield. In contrast, the yields of methylacetylene and propylene from the surface are distinctly different, reaching minimum values on the 750 K surface before increasing at higher prior annealing temperatures. These results imply that the dimerization and cyclotrimerization products share a common active site, in agreement with the earlier assumption that the dimer is produced by hydrogenation of the metallacyclopentadiene intermediate common to both products. To emphasize the correlation of the trimethylbenzene yield to thefractionofTi(+2) on thesurface, data for these twoquantities a t similar prior annealing temperatures were plotted (Figure 15). The agreement is excellent, exhibiting a correlation coefficient of 0.99. The molar yields in Figures 7, 13, and 15 have been scaled so that the total yield from the reduced unannealed surface hasa value of unity. If one assumes that this represents monolayer coverage, then the slope of the straight line in Figure 15, 2.6,
Cp,TI*'(C0)2
+ 2 PhCECPh a C p 2 T I * '
co
Ph
Note that the formation of this metallacyclopentadienyl complex requires a two-electron transfer, so that Ti must be in an oxidation state of +2 or lower for this reaction to occur on a single site. The formation of metallacyclopentadienyl complexes by a two-electron oxidation process is not limited to Ti complexes; C0,6*' Rh, and Ir9exhibit similar behavior. Although Ti metallacyclopentadienyl complexes have not been shown to be intermediates in the production of aromatic compounds from alkynes, it should be noted that the metallacyclopentadienyl complexes formed from Co, Rh, and Ir are indeed intermediates involved in cyclotrimerization. Weconcludefrom the correlationspresented in Figures 13 and 15 that the cyclotrimerization of alkynes on reduced Ti02 surfaces also requires individual Ti cations in the +2 oxidation state. The surface oxidation state requirements for the cyclotrimerization reaction of alkynes provides an interesting contrast with the requirement for reductive coupling of aldehydes to form olefins on similar reduced Ti02 surface^.^^^^^ The reductive coupling of aldehydes also requires titanium cationsin a lower oxidation state than +4. The activity of reduced surfacesfor this reaction appears to track the extent of reduction of the surface from Ti02 and exhibits a less steep dependence on surface annealing, although the activity of oxidized and reduced surfaces differs by more than an order of magnitude as it does for alkyne cyclotrimerization. Since reductive carbonyl coupling involves a four-electron reduction of a pair of aldehydes, this result implies that ensembles of titanium cations capableof undergoing a four-electron oxidation process are active sites for the reductive coupling reaction. In sharp contrast to this reduced ensemble requirement for reductive
Cyclotrimerization of Alkynes on Ti02 (001) carbonyl coupling is the surface found to be effective for promoting the cyclotrimerization of alkynes. The two-electron process necessary for cyclization occurs at individual cation sites, while the four-electron reductive coupling occurs on ensemble sites. The latter reaction persists as long as a significant population of Ti(+3) cations is availablein the near-surface region, while alkyne cyclotrimerization is quenched as soon as Ti(+2) is eliminated. Ti(+l) cations, although surely able to undergo a two-electron oxidation, do not seem effective in promoting cyclotrimerization, possibly because of a higher tendency of these sites to promote decomposition reactions (such as carbon deposition) vs cyclization. It appears that individual Ti(+2) cations are the sole active and selective sites for the cyclotrimerization of alkynes. The results presented in this work may also be compared with the most thoroughly investigatedsingle crystal metal surface that shows activity for the cyclotrimerization of alkynes, Pd( 111). The mechanism of the cyclotrimerization of acetylene on Pd( 111) is thought to involve a tilted C4H4 metalla~ycle'~ similar to the metallacyclopentadieneintermediate invoked in this study. However, the maximum conversion of acetylene to benzene observed on clean Pd(ll1) was never greater than 25%,50 significantlyless than the value obtained on reduced TiOz. When N O was coadsorbed with acetylene on Pd( 11l), the coverage threshold for the production of benzene was substantially lowered and the cyclotrimerization conversion approached Ormerod and Lambert attributed the promotion of acetylene cyclization to the segregation of the acetylene and nitric oxide into separate compressed domains. One could also interpret this result in terms of site isolation; restriction of the number of surface metal atoms with which each molecule of acetylene can interact would favor cyclizationover unselective rearrangement and bond scission reactions. On reduced Ti02 surfaces the active Ti(+2) sites are already dispersed. The oxide surface is inherently binary: cation sites are separated from each other by the intervening oxygen anions. Also, the population of Ti(+2) sites on the reduced surfaces in this study never exceeded 30% of the surface cations; thus, they are likely to be further isolated from each other although one cannot rule out the possibility that they are not randomly distributed, owing to the method of surface reduction. In any case, the heterogeneity of the reduced oxide surface may lead to alkyne cyclization selectivities achievable on metals only with the addition of coadsorbates. The lack of stereoselectivityin the cyclotrimerization reaction, demonstrated by the indistinguishability from the statistics of random insertion of the ratio of 1,2,4- to 1,3,5-isomers of trimethylbenzene produced from methylacetylene, suggests that the methyl substituent groups are small enough to provide little steric hindrance in these cyclization reactions. Additionally, if steric hindrance were significant, one would expect inhibition of the formation of hexamethylbenzene from 2-butyne, which was not observed. Thus, there appears to be no surface-induced stereoregularitythrough interactionswith the methyl substituents of reactants or products. This result is in contrast to the important steric effects that exist in the Ti-based Ziegler-Natta polymerization of molecules with methyl substituents (Le., propylene). The production of stereoregularpolypropylene on a-Tic13involves edge sites where significant interactionsbetween the methyl group and surface C1 atoms favor binding configurationsof the monomer that lead to isotactic polymer formation.51 This implies the reduced Ti02 surfaces (and specifically the Ti(+2) sites) are moreopen than the a-TiC13 sites active in producing polypropylene and that the reduced Ti02 surface has no significant influence on the cyclization product structural form. Other studies of trisubstituted products produced from alkyne cyclotrimerization on oxide catalysts have exhibited little stereoselectivity for reactions of alkynes with relatively small substituent groups. The cyclization of propyne, 1-butyne, and 1-pentyne over supported tungsten oxide showed ratios of 1,2,4- to 1,3,5-trialkylbenzenes
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3891 of 3.7, 3.6, and 1.2, respectively, using mass, NMR, and IR spectroscopies to identify the products.52 In contrast, Hoover et al. observed 90% 1,2,4-trivinylbenzene vs 10% 1,3,Strivinylbenzene by infrared and ultraviolet spectroscopies from the trimerization of monovinylacetylene with homogeneous trialkylaluminum-titanium tetrachloride catalysts.53 By providing evidence for random insertion of methylacetylene on reduced Ti02 surfaces, the present study represents one of the few occasions where the isomer distribution of hydrocarbon products in a TPD experiment can be established using mass spectrometry alone.
Conclusions Alkynes adsorbed on partially reduced Ti02 (001) surfaces decompose via t h e e different pathways: (1) unimolecular hydrogenation to form olefins, (2) reductive dimerization to form dienes, and (3) cyclotrimerization to form aromatic compounds. The dominant product by far resulted from cyclotrimerization; the selectivity toward the aromatic product reached 86% for 2-butyne. All three product classes and the unreacted alkyne desorb in each case in a low-temperature state around 400 K, with the two hydrogenation products exhibiting additional peaks around 500 K. The selectivity to the cyclotrimerization product increased with higher alkynecarbon number in theseries acetylene, methylacetylene, and 2-butyneI while the selectivity toward the two hydrogenation products, as well as the conversion, decreased. Trimethylbenzene produced from methylacetylene exhibited a = 3:l ratio of 1,2,4- to 1,3,5-isomers, implying random insertion of acetylenic units. The yield of trimethylbenzene was strongly dependent on the annealing temperature of the surface prior to alkyne adsorption: a sharp decrease in cyclotrimerizationactivity was observed for prior annealing temperatures between 600 and 650 K. The cyclotrimerization product yield correlates extraordinarily well with the Ti(+2) population on the surface,implying the active sites for the cyclization reaction are individual Ti(+2) cations. This surface requirement for the cyclotrimerization reaction provides a clear example of a discrete oxidation state requirement for a surface reaction under UHV conditions.
Acknowledgment. We gratefully acknowledge the support of the National Science Foundation, Grant CTS 9 100404, for this research. References and Notes (1) Barteau, M. A. J. Yac. Sci. Technol. A 1993,11, 2162. (2) Kim, K.S.;Barteau, M. A. J. Cafal.1990, 125, 353. (3) Idriss, H.;Barteau, M. A. Carol. Lerf., in press. (4) Berthelot, M. Ann. Chem. 1867,141, 173. (5) Trost, B. M. Science 1991, 254, 1471. (6) Wakatsuki, Y .; Kiramitsu, T.;Yamazaki, H. Tetrahedron Lett. 1974, 4549. (7) Yamazaki, H.;Hagihara, N. J. Orgammer. Chem. 1967,7, P22. (8) Reppe, W.; Schweckendieck, W. J. Uebigs Ann. Chem. 1948,560, 104. (9) Collman, J. P.; Kang, J. W.; Little, W. F.;Sullivan, M. F. Inorg. Chem. 1968. 7. 1298. (10) Beiolini, J. C.; Massardier, J.; Dalmai-Imelik, G. J . Chem. Soc., Faraday Trans. I 1978, 74, 1720. (11)Ormerod, R. M.; Lambert, R. M. J. Chem. Soc., Chem. Commun. 1990, 1421. (12) Avery, N. R.J. Am. Chem. Soc. 1985,107,6711 (1 3) Tysoe, W. T.;Nyberg, G. L.; Lambert, R. M. J. Chem.Soc., Chem. Commun. 1983,623. (14) Ormerod, R. M.; Lambert, R. M. J. Phys. Chem. 1992.96,s1 1 1 and references therein. (15) Patterson, C. H.;Mundenar, J. M.;Timbrell, P. Y.; Gellman, A. J.; Lambert, R. M. Surf.Sci. 1989,208,93. (16) Tysoe, W. T.;Nyberg, G. L.; Lambert, R. M. Surf. Sci. 1983,135, 128. (17) Sesselmann, W.;Woratschck, B.; Ertl, G.; Kuppers, J. Sur/. Sci. 1983, 130, 245. (18) Gentle, T. M.; Muetterties, E. L. J . Phys. Chem. 1983,87, 2469. (19) Marchon, B. Surf. Sci. 1985, 162,382. (20) Rucker, T. G.; Logan, M. A.; Gentle, T.M.; Muetterties, E. L.; Somorjai, G. A. J . Phys. Chem. 1986,90,2703. (21) Patterson, C. H.;Lambert, R. M. J. Phys. Chem. 1988,92, 1266.
3892 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 (22) Hoffmann, H.; Zaera, F.; Ormerod, R. M.; Lambert, R. M.; Wang, L. P.;Tysoe, W. T. Surf.Sci. 1990,232,259. (23) Ormerod, R. M.; Lambert, R. M. Catal. Lett. 1990,6,121. (24) Lambert, R. M.;Ormerod, R. M. Mater. Chem. Phys. 1991,29,105. (25) Collman, J. P.; Hegedus, L. S.;Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry, 2nd ed.; University Science Books: Mill Valley, 1987;p 870. (26) Furlani, A,; Moretti, G.; Guerrieri, A. Polym. Lett. 1967, 5, 523. (27) Watson, W. H.,Jr.; McMordie, W. C., Jr.; Lands, L. G. J . Polym. Sci. 1961. 55. 137. (28) Halliday, M. M.; Kemball, C.; Leach, H. F. J. Chem. Soc.,Faraday Trans. I 1974, 70, 1743. (29) Rives-Arnau, V.; Sheppard, N. J. Chem. Soc., Faraday Trans. I .. 1980,76, 394. (30) Boonstra, A. H.;Mutsaen, C. A. H. A. J . Phys. Chem. 1975,79, 2025. 131) Sakata. Y.: Liu.. 2.: , Imamura. H.:Tsuchiva. S . J . Chem.SOC..Chem. commun. 1991;1392. (32) McMurry, J. E. Chem. Reo. 1989,89, 1513. (33) Kim,K. S.; Bartcau, M. A. Surf. Sci. 1989,223, 13. (34) Outka, D. A,; Friend, C. M.; Jorgensen, S.; Madix, R. J. J. Am. Chem. Soc. 1983,105, 3468. (35) Stroscio, J. A.; Bare, S. R.; Ho, W. Surf.Sci. 1984,148, 499. (36) Idriss, H.; Pierce, K. G.; Barteau, M. A. J. Am. Chem.Soc., in press. (37) Knotek, M. L. Surf. Sci. 1980, 91, L17. (38) Vohs, J. M.; Bartcau, M. A. J . Phys. Chem. 1987.91, 4766.
Pierce and Bartcau (39) KO, E. 1.; Benziger, J. B.; Madix, R. J. J . Catal. 1980, 62,264. (40) Redhead, P. A. Vacuum 1962, 12,203. (41) Grubb, H. M.;Meyerson,S.In Mass Spectrometry of Organic Ions; McLafferty, F. W.,Ed.; Academic ReM: New York, 1963;Chapter 10. (42) Collman, J. P.;Hegedus, L.S.;Norton,J. R.; Finke, R. G. Principles ; and Applications of OrganorransirionMetal Chemistry, 2nd 4.University Science Books: Mill Valley, 1987;p 509. (43) Whitesides, G. M.; Ehmann, W. J. J. Am. Chem. Soc. 1969, 91, 3800. (44) McLafferty, F.W. InterpretationofMassSprcrrcr,3rd ed.;University Science Books: Mill Valley, 1980; p 179. (45) Rocker, G.; G-1, W. Surf.Sei. 1987,181, 530. (46) Ludlum, D. B.; Anderson, A. W.; Ashby, C. E. J. Am. Chem. Soc. 1958,80, 1380. (47) Werber, F. X.;Benning, C. J.; Wszolek, W. R.; Ashby, G. E. J. Polym. Sci., Part A-1 1968,6, 743. (48) Sononashira. K.: Hanihara. N.Bull. Chem.Soc.Jan. 1966.39.1178. (49j 1dri.s; H.; Pier&, K I Bart&, M. A. J. Am. C h e k Soc. i991,113, 1671. (50) Ormerod, R. M.; Lambert,R. M.Surf.Sci. 1990, 225,L20. (51) Cosscc, P. In The Stereochemistry of Mocromolecuies;Ketley, A. D., Ed.; Marcel Dekker: New York, 1967;Vol. 1, Chapter 3. (52) Moulijn, J. A.; Reitsma, H. J.; Boelhouwer, C. J . Catal. 1972, 25. 434. (53) Hoover, F. W.; Webster, 0. W.; Handy, C. T. J . Org. Chem. 1961, 26, 2234.
