Langmuir 1994,10, 3693-3700
3693
Reactions of p-Benzoquinone on Ti02(001)Single-Crystal Surfaces: Oligomerization and Polymerization by Reductive Coupling H. Idriss+ and M. A. Barteau* Center for Catalytic Science a n d Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received June 13, 1994@ Reduction and coupling reactions of benzoquinone on sputter-reduced Ti02(001)surfaces yield benzene, biphenyl, terphenyl, and phenol as volatile products. Like the reductive couplingofmonofunctionalaldehydes and ketones, reductive coupling of benzoquinone requires titanium cations in oxidation states below +4; activity for this chemistry is strongly quenched by oxidation of reduced surfaces by annealing at 750 K and above. Unlike monofunctional carbonyl compounds, benzoquinone can undergo multiple coupling events, as shown by the production of terphenyl. Both TPD and XPS results suggest that substantial amounts of higher phenylene oligomers are also produced and remain on the surface even at high temperatures. Thus reductive coupling of benzoquinone may provide a route for synthesis of a surface layer of p-polyphenylene, a conductive polymer.
Introduction Reductive coupling of carbonyl compounds with titanium reagents has proven to be a useful synthetic tool since its discovery 2 decades A recent intramolecular example is a critical ring-closure step in the synthesis of t a x ~ l .Reductive ~ carbonyl coupling, commonly referred to as the McMurry reaction, was first observed with reduced titanium halides as a liquid-solid stoichiometric r e a ~ t i o n . This ~ reaction, which yields symmetric olefins by coupling of aldehydes or ketones, is not exclusive to titanium; other metal halides including early transition metals, lanthanides, and actinides, in the presence of a strong reducing agent (LiAlH4,LiB&, or metal hydrides) are active.6 It takes place as a two-step reaction; the first is the reductive dimerization of the carbonyl compoundvia formation of a carbon-carbon bond to produce a pinacolate or diolate intermediate. The second step involves deoxygenation of the diolate to form the olefin, resulting in oxidation of the reduced metal. The intermediate pinacolate species have been isolated; when these intermediates are treated with reduced titanium, deoxygenation to the symmetric olefin OCCUIS.~ Reductive coupling can be extended to the formation of high molecular weight oligomeric products if dialdehydes and/or diketones are used, provided that both carbonyl groups couple to different molecules. In order to avoid intramolecular coupling in such cases, small or rigid molecules are referr red.^ I t has been reported, for example, that p-formylbenzaldehyde (OCH-Ph-CHO) reacts in slurries containingTiClhiAlH4 to form polymers by carbonyl coupling.' We have recently shown that reductive coupling of aldehydes and ketones can be carried out as a gas-solid
* To whom correspondence should be addressed. t Present address: Department of Chemical Engineering, University of Illinois, Urbana, IL 61801. Abstract published in Advance ACS Abstracts, September 15, @
1994. (1) Tyrlik, S.;Wolochowicz, I. Bull. SOC.Chim. Fr. 1972,2147. (2)Mukaiyama, T.;Sato, T.; Hanna, J. Chem. Lett. 1973,1041. (3)McMuny, J.E.; Fleming,M. P. J.Am. Chem.SOC.1974,96,4708. (4)Nicolaou, K.C.; Yang, 2.;Liu, J. J.; Veno, H.; Nantermet, P. G.; Guy, R. K.; Claiborne,C. F.; Renaud,J.;Couladouros,E. A.; Paulvannan, IC; Sorensen, E. J. Nature 1994,367,630. (5)McMurry, J. E. Chem. Rev. 1989,89,1513. (6)Kahn, B. E.; Rieke, R. D. Chem. Rev. 1988,88,733. (7)Rajaraman, L.; Balasabramanian, M.; Nanjan, M. J. Curr. Sci. 1980,49(3), 101.
reaction on reduced surfaces of a Ti02(001) single crystal, even in ultrahigh vacuum.8-10 Although i t had been proposed that Ti(0) centers are the active sites for this reaction in liquid-solid slurries,l' XPS results from singlecrystal experiments in UHV clearly indicated that no Ti(0)was present on reduced TiOZ(001) surfaces active for this reaction.12 Thus, carbonyl coupling, a four-electron reduction, must occur at ensembles of Ti+xcations (1i x i 3) capable of collectively undergoing a four-electron oxidation.1°J3 In addition to its advantages with respect to spectroscopic characterization, noted above, gas-solid carbonyl coupling offers advantages with regard to the possibility of carrying out this chemistry catalytically. First, it lends itself to more efficient fured-bed, flow-reactor operation. Second,it may obviate the need for strong reducing agents such as LiAlH4; reduction by dihydrogen was sufficient to activate TiOa, CeO2, and iron oxide powders for reductive cross-coupling of two different a1deh~des.l~Third, a reduced oxide catalyst for a gas-phase reaction would be expected to be less expensive and easier to manipulate (through reduction, reaction, and regeneration) than the metal halide reagents in liquid-solid slurries. Beyond the possibility of catalysis, carbonyl coupling reactions on surfaces of reduced oxides may open the door for other applications. Since the reaction occurs on the surface of the oxide, a formation of a monolayer of olefin may result. If bifunctional reagents are utilized, this monolayer may be polymeric in nature. Such polymeric overlayers, particularly if highly conjugated, might have wider applications. Coating of insensitive materials by conjugated polymers can turn them into sensitive sensor devices.14 Formation of organic films, if highly stable, (8) Idriss,H.; Pierce, IC;Barteau,M. A. J.Am. Chem.Soc. 1991,113,
715. (9) Idriss, H.; Barteau, M. A. In Heterogeneous Catalysis and Fine Chemicals III; Guisnet, M., Barbier, J., Barrault, J., Bouchoule, C., Duprez, D., Perot, G., Montassier,C., Eds.; Elsevier: Amsterdam, 1993; p 463. (10)Idriss, H.; Pierce, K.; Barteau, M. A. J . Am. Chem. SOC.1994, 116,3063. (11)Dams, R.;Malinowski, M.; Westdorp, I.; Geise, H. J . Org. Chem. 1982,47,248. (12)Idriss, H.; Barteau, M. A. Catal. Lett. 1994,26,123. (13)Idriss, H.; Libby, M.; Barteau, M. A. Catal. Lett. 1992,15, 13. (14)Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarty, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.;Yu, H. Langmuir 1987,3,932.
0 1994 American Chemical Society 0743-7463/94/2410-3693$04.50/0
Idriss and Barteau
3694 Langmuir, Vol. 10, No. 10,1994 might be used for sealing the surface of electronic devices for chemical protection. As an example, the linear combination of benzene rings to producep-polyphenylene results in a polymer with remarkable resistance to thermal decomposition or 0xidati0n.l~There is a need for stable polymers in optoelectronic devices, as well as in electronic devices such as field effect transistors, diodes, and h e t e r o j u n c t i o n ~for ; ~example, ~ doping TiOzwith polypyrol produces an increased surface state density, enhancing its diode quality factor.16 Moreover, one can propose carbonyl coupling of dialdehydes or diketones to form polymers directly on the surface as an alternative to the usual sequence involving condensation followed by deposition. Some polymers such as polyimides suffer from the fact that their synthesis by condensation releases water, which can cause significant processing difficulties, e.g., corrosion.l4 In contrast, carbonyl coupling reactions to form conjugated polymers would result in restoration of part of the oxygen vacancies on the surface and might increase the stabilityof the semiconductor. Thus, coating a semiconductor with a polymer produced by reaction between the semiconductor and a dialdehyde or a diketone would appear to be a direct and eEcient route. We report, in this work, the reactions ofp-benzoquinone (1,Cbenzoquinone) on the reduced and oxidized surfaces of a TiOz(001)single crystal. These reactions, which yield benzene, phenol, biphenyl, terphenyl, and very probably p-polyphenylene, were studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS)under ultrahigh vacuum conditions. The formation by thermal reactions of volatile oligomeric products, including those containing as many as 18carbon atoms, is without precedent under such conditions.
Experimental Section All experiments were performed in a VG ESCALAB system operating at a base pressure of 1x 10-loTorr. Ultrahigh vacuum was maintained using two diffusion pumps and a sublimation pump. This instrument, described previously,17J8is equipped with an ultraviolet lamp, LEED optics, ion gun, electron gun, UTI mass spectrometer, and a twin-anode X-ray source. The (001)-orientedT i 0 2 single-crystal sample (10 mm x 9 mm x 1.5 mm) was prepared from a rutile boule (99.9%, Atomergic Chemetals Corp.). It was aligned by the Laue method, cut, and mechanically polished to a mirror finish with 1 pm diamond paste. After installation in the vacuum chamber, the single crystalwas cleaned by repeated cyclesof argon ion bombardment and annealing. The crystal amount was described in detail previously;'9 briefly, the sample was mounted on a tantalum foil sample holder (0.127 mm thick), the back of the tantalum foil was spot-weldedto two tantalum wires used to heat the crystal, the wires were spot welded to two molybdenum rods of arotatable samplemanipulator, and the sampletemperature was monitored using a chromel-alumel thermocouple attached to the side of the crystal. p-Benzoquinone, contained in a glass-metal tube attached to a dosing manifold, was purified by freeze-pumpthaw cycles until a reproducible fragmentation pattern was obtained. Table 1 presents the fragmentation pattern of 1,4benzoquinone measured in the same vacuum systemat a pressure of 2 x Torr. The highest intensity observedwas that of mle 26 (-CH=CH-) followed by mle 54 (O=C-CH=CH-); the intensityof the signal of the parent moleculeat mle 108was 50% of that of mle 26. The sensitivityfactor relative to CO calculated (15)Skothem, T. A. Handbook of Conducting Polymers; Marcel Dekker Inc.: New York and Basel, 1986. (16)Inganas, 0.;Skotheim, T.; Lundstrom, I. J.Appl. Phys. 1983,
54,3636. (17) Idriss,H.; Kim, K. S.; Barkau, M. A. Surf. Sci. 1992,262,113. (18)Peng, X.D.;Barteau, M. A.Langmuir 1989,5,105. (19)Idriss,H.; Kim, K. S.; Barteau, M. A. J. Cutal. 1993,139,199. (20) Stenhagen, E., Abrahamsson, S., McLafferty, F. W., Eds.;Atlus of Muss Spectral Datu; Interscience: New York, 1969.
Table 1. Fragmentation Pattern of 1,4-Benzouuinone mle
26 54 28 108 53 82 52 80 25 50 18 12
re1 intens (measured) 100 99
78
50
28 27 27 21
12 10 10 3
re1 intens20 96.7 100 2.5 82.6 22.6 32 22.5 22.5 14.6 8 not given not given
by the method of KOet aLZ1was 7.2 for the mle = 108 signal of benzoquinone. Sputtering the surface with argon ions was carried out using a 2 keV beam, for a tixed time, with the Ar+ flux at about 70" from the plane ofthe surface. Argon (Mathesson ultrapure)was introducedto the vacuum chamber through the gun and controlled by a variable leak valve. The focus, beam size, gun current, and crystal position were adjusted until a current of about 300 nA cm2was obtained at the crystal during sputtering. X-ray photoelectron spectroscopy W S ) experiments were conducted using the Al anode with a power of 600 W (15 kV x 40 mA). Before adsorption ofp-benzoquinone,the Ti(2p),O(ls), and C(1s) regions were scanned for each pretreated surface (sputtered or annealed). Typically 15, 5, and 30 scans were collected for the Ti(2p), O(ls), and C(1s) regions, respectively. Adsorption of p-benzoquinone (at room temperature or below) was carried out until saturation of the surface. Saturation was determined fromthe C(ls)signal after different exposures.Direct dosingofp-benzoquinonefromthe dosingneedle at a background pressure of 10-8 Torr for ca. 100 s was sufficient to saturate the surface. In the case of experiments in which XPS spectra were collected as a function of temperature, the first set in the series was collected as describedabove,and the surfacewas then heated to the desired temperature and then cooled to room temperature before collection of the spectra, without turning off the X-ray source to avoid further degassing. Temperature-programmeddesorption (TPD) was performed as follows. M e r the appropriate clean surfaceofthe singlecrystal was prepared (either by sputtering and/or annealing),p-benzoquinone was dosed on the surface to saturation. M e r the chamber was pumped down to a base pressure of ca. 5 x 10-10 Torr, the crystal was turned toward the mass spectrometer and positioned about 1mm from the orifice in the quartz envelope around the mass spectrometer probe (more details about the experimental hardware are givenelsewherelg). The temperature ramp (1.2 Ws)was started. As many as 100 masses were monitored during a single run, using a UTI 100 C mass spectrometer. Three identical runs were carried out on each surface to check for all masses from 2 to 300 amu (the operating range of the mass spectrometer). An IBM-XT computer was used for data acquisition and data analysis routines in these experiments.
Results The surfaces prepared by sputtering and annealingthe Ti02(001) singlecrystal were studied in detailpreviously.lZ Briefly, the surface of the sputtered Ti02(001) single crystal is disordered (no LEED pattern) and reduced; the distribution of surface cations is approximately 30% Ti+4 and 70%TifX(0 < x < 4). Annealing this surface to 750 Kresults in the complete oxidation of all Tifx to Tif4 cations and the formation ofthe first ordered structure, the (011)faceted surface on which all Ti+4cations are ideally fivefold ~ ~ reactions ~~~ of p-benzocoordinated to ~ x y g e n .The quinone were studied on both surfaces by TPD and are presented below. (21) KO,E. I.; Benziger, J. B.; Madix, R. J. J . Catal. 1980,62,264. (22) Firment, L. E. Surf. Sci. 1982,116,205. (23) Kim, K. S.; Barteau, M. A.J.Cutal. 1990,125,353.
Reactions of p-Benzoquinoneon Ti02(001)
Langmuir, Vol. 10, No. 10, 1994 3695
TPD Studies of p-Benzoquinone Adsorption on the Reduced (Sputtered) Surface. Temperatureprogrammed desorption of 1,Cbenzoquinone on the reduced Ti02(001) surface produced a variety of monomeric and oligomeric products. The TPD spectrum following benzoquinone adsorption on the reduced surface a t 300 K is presented in Figure 1. All masses from 2 to 300 were monitored in several runs. The absence of a measurable signal for mle 108 indicated that all p-benzoquinone molecules reacted with the surface during the TPD run; none desorbed. Trace amounts of hydrogen a t ca. 650 K were observed; water desorbed with a maximum at ca. 550 K and continued to rise up to 800 K. At ca. 650 K benzene (mle 78, 77; sensitivity factor 2.51, phenol (mle 94; sensitivity factor 2.31, biphenyl (mle 154; sensitivity factor 5.81, and terphenyl (mle 230; sensitivity factor 12.3) desorbed from the surface. The highest mass accessible with the UTI 100 C quadrupole mass spectrometer (300 AMU) is lower than that of tetraphenyl (mle 306);however, no signals above mle 230 were observed, suggesting the absence of desorbing products with higher molecular weight than terphenyl. The expected reductive coupling product of two molecules of benzoquinone (C6H402) has the molecular formula C12Ha02;4,4'-diphenoquinone with mle 184 was also not observed among the products. The desorption of biphenyl and terphenyl indicated that the coupling of two or more molecules ofp-benzoquinonefrom one end is followed by reduction of the other end to finally yield these non-oxygenated products. This can be represented as follows:
Benzene (m/e 78)
Phenol (m/e 94) X 16
A
Biphenyl (m/e 154) X 32
Terphenyl (m/e 230) X 64
4,4'-biphenoquinone(m/e 184) X 64
p-Benzoquinone (m/e 108) X 64 I
I
I
I
I
400
500
600
700
800
K
Figure 1. TPD obtained aRer 1,4-benzoquinoneadsorption at room temperature on the sputtered surface of the TiOZ(001) single crystal; product spectra are not corrected for mass d e 184. no desorption
d e 108, no desorption
spectrometer sensitivity.
Table 2. Product Distribution for p-Benzoquinone TPD on the Reduced TiOd001) Single-CrystalSurface' ~
~~
d e 154, desorbs product
Phenol production from benzoquinoneinvolves abstraction of one of the oxygen atoms of the quinone and addition of two hydrogen atoms. Although both carbonyl scission and carbonyl hydrogenation reactions have been observed previously on reduced TiOz(001) surfaces,8-10~17~24~25 only for diketones or dialdehydes is it possible for both reactions to occur for a n individual molecule. The quantitative distribution of the different products from benzoquinone TPD is presented in Table 2. The fractional yield of benzene (which results from reduction of both ends of the molecule) was 0.54, that of biphenyl was 0.25, and that of terphenyl was 0.11, based on the molar yield of carbon-containing products desorbed. The decrease of the product yield with increasing number of coupling events required for each product can be fit by the Flory distribution (or the most probable distribution).26 This permits one to estimate the fraction of the benzoquinone which reacted to form higher oligomers not desorbed or detected in the TPD experiment. If n is the number of monomer units, X, coupled according to
nX-q (24) Idriss, H.; Kim, K. S.; Barteau, M. A. In Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis; Grasselli, R. K., Sleight, A. W., Eds.; Elsevier: Amsterdam, 1991; p 327. (25) Idriss, H.; Kim, K. S.; Barteau, M. A. MRS, Extended Abstract (EA-24). Proc. Symp. S 1990,127. (26)Flory, P. J. Principles ofpolymer Chemistry; Cornel1 University Press: Ithaca, 1953.
hydrogen water benzoquinone benzene phenol biphenyl p-terphenyl '3 rings a
~
adjusted desorption fractional yield, fractional carbon yield fraction temp (K) desorbed species
600-700 500-800
650 650 650 650
n. q. n. q.
n. q. n. q.
0
0 0.50 0.09
0.54 0.10 0.25 0.11
n. d.
0.23 0.10
0.08"
0.28
0.05 0.26 0.18 0.23a
n. q. = not quantified; n. d. not detected; a = calculated.
and with a probability for monomer addition, p , which is independent of chain length, then the probability of forming an n-mer is
Pn = p n - (1 - p )
(1)
The probability of monomer addition, p , can therefore be determined from the ratio of yields of oligomers differing by one unit in length. (2) Thus, from the fractional yields of benzene (0.54))biphenyl (0.25))and terphenyl(O.11) in Table 1, values ofp can be calculated: p 2 = 0.2510.54 = 0.46 and
3696 Langmuir, Vol. 10, No. 10,1994
Idriss and Barteau
p3 = 0.1110.25 = 0.44
thus the average value ofp for this data is 0.45. Since the
P, values from (1)must sum to unity, one can therefore calculate the total yield of higher oligomers not detected:
Pn>,= 1 - P I
- P, - P, =p3
(3)
F o r p = 0.45, Pn,3 = 0.092. By including this value and renormalizing the sum of all fractional yields (including those of other species not included among the phenyl oligomers)to unity, one obtains a n estimated fractional yield of 0.082 for the higher oligomers. Table 2 presents the fractional yield (as observed from TPD) and the fractional yield adjusted by assuming the production of higher oligomers following the Flory distribution. The value above represents a n estimate of the number fraction of oligomers with n 2 4. The carbon content of higher oligomers is essentially the corresponding weight fraction, given by
n
Phenol (m/e 94) X 32 Biphenyl (m/e 154) X 128
Terphenvl (m/e 230) X 128
p-Benzoquinone (m/e 108) X 32
400 500 600 700 800 K Figure 2. Temperature-programmed desorption after 1,4benzoquinone adsorption, at room temperature, on the 750 K-annealed surface of the “i02(001) single crystal (the (011)faceted structure); product spectra are not corrected for mass
i=l
The closed form solution can be represented by
Wn,, = 1-
+ + 3p2)= 4 p 3 - 3p4
(1 2p
( L I ” ‘1 -p’
spectrometer sensitivity.
(5)
For p = 0.45 the calculated weight fraction of phenylene oligomerswith n > 3 is 0.24. Correcting for the production of phenol not included in the calculation of the oligomer distribution, one obtains a n estimate of the fraction of the total adsorbed carbon contained in higher (n > 3)oligomers of 0.23. As shown by the XPS results below, this value underestimates the fraction of carbonaceous species remaining after desorption of all volatile products. Temperature-ProgrammedDesorption of p-Benzoquinone on the Oxidized Ti02(001)Surface. TPD after p-benzoquinone adsorption at room temperature on the fully oxidized surface was significantly different from that obtained on the reduced surface. Figure 2 presents the different desorption products following 1,lbenzoquinone adsorption at room temperature on the (011)faceted surface. Unlike its absence on the reduced surface, 174-benzoquinonedesorbed from the surface in a broad peak centered a t ca. 500 K, water desorption was also observed over a similar temperature range. At higher temperatures, ca. 650 K, benzene, phenol, and very small amounts of biphenyl were observed. No masses above mle 155were detected, indicating the absence ofterphenyl. These results are quantified and presented in Table 3. The major product desorbed was 1,Cbenzoquinone (fractional yield = 0.73) followed by benzene (fractional yield = 0.175 compared to 0.54 on the reduced surface), phenol (0.05 compared to 0.1 on the reduced surface), and biphenyl (0.04 compared to 0.25 on the reduced surface). The comparison between these results and those obtained on the sputtered surface (Table 2) clearly shows the dramatic difference between the activities of the two
Table 3. Product Distribution for p-Benzoquinone-TPD on the Oxidized (011)-Faceted‘I%O2(001)Single-Crystal Surface
Droduct
desorption temD (K)
fractional vield ~
water
p-benzoquinone benzene
phenol biphenyl terphenyl
400-700 ca. 500 650 650 650 650
carbon fraction ~~
n. g. 0.73 0.18 0.05 0.04
0
~
0.70 0.17
0.05 0.08 0
surfaces. None of the benzoquinone adsorbed on the reduced surface desorbed, but 70%of that on the oxidized surface did, and the yields of all of the reduction products decreased accordingly. Oxidation of the surface strongly diminishes its activity for reduction of organic adsorbates, as expected. X-rayPhotoelectronSpectroscopyFollowing 1,4Benzoquinone Adsorption at 260 K. Ti(2p) Spectra. Previous analyses ofthe Ti(2p) core levels of reduced TiOz(001) surfaces12 indicated that the Ti2p312 XPS binding energy of Ti+4cations, representing ca. 30% of the overall Ti2p3,~signal, is located a t 459-459.1 eV. Annealing the surface above 750 K resulted in total oxidation of (0 < x < 4) to Ti+4by oxygen migration from the bulk to the surface, and the Ti+4binding energy position shifted to 459.3 eV. This 0.2-0.3 eV shiR was previously observed12 and is consistent with UPS results as ~ e l l . No ~ ~sample 3 ~ ~ (27) McKay, J. M.; Henrich, V. E. Surf. Sci. 1984,137,463. (28)GCipel, W.; Anderson, J. A,; Frankel, D.; Jaehnig, M.; Phillips, K.; Schafer, J. A.; Rocker, G.Surf. Sci. 1984,139,333.
Langmuir, Vol. 10, No. 10,1994 3697
Reactions of p-Benzoquinoneon TiOdOOl)
Table 5. Change in Degree of Oxidation of Ti Cations, Determined by XPS, After 1,4-Benzoquinone Adsorption at 250 K on the Surface of the 650 K-Annealed Ti02(001) Single Crystal before
.... .*.. . -. .-. *..,.. .. . . . ...-* . . .*.. . .: . . ... . . . . . . ...... . . .
*
TifX
adsorption (%)
Ti+4 Ti+3
Tif2
50.4 37.9 12.2
Ti+'
0
after adsorption (%) 58.4 32.4 9.2 0
% change
+16 -15 -25
a .
*a
*
Y
