Reaction of Acrylic Acid on TiO2 (001) Single Crystal Surfaces

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Reaction of Acrylic Acid on TiO2(001) Single Crystal Surfaces. Evidence of Different Pathways for Vinyl and Carboxyl Groups David J. Titheridge,† Mark A. Barteau,‡ and Hicham Idriss*,† Materials Chemistry, Department of Chemistry, The University of Auckland, Auckland, New Zealand, and Department of Chemical Engineering, The University of Delaware, Newark, Delaware 19716 Received July 1, 2000. In Final Form: November 28, 2000 The TiO2(001) single crystal surface is unique in that surface structure, coordination number of titanium cations, and degree of reduction may be easily controlled by varying the annealing temperature in ultrahigh vacuum. In an effort to understand the chemical pathways of conjugated molecules over the surfaces of semiconductors, the complementary techniques of temperature-programmed desorption, X-ray photoelectron spectroscopy, and scanning kinetic spectroscopy have been applied to study the interaction and surface chemistry of CH2dCHCOOH with the TiO2(001) single crystal surface in detail. A clear difference is observed between the reaction of this unsaturated carboxylic acid and that of saturated analogues. Two different coupling pathways were observed. The first, involving the H2CdCH moiety, generated C4 and C6 hydrocarbon products (butene, butadiene, and benzene). The second results in the production of divinyl ketone, formed by ketonization of two adsorbed acrylate molecules at a doubly coordinatively unsaturated titanium center on the {114}-faceted TiO2(001) surface only.

Introduction Metal oxides are frequently at the core of modern heterogeneous catalytic processes, both as supports and as the active species itself. In these situations, the conditions used are usually high pressures (many times atmospheric) and high temperatures over polycrystalline materials. Although such conditions may produce good reaction yields, they render detailed molecular-level understanding of the chemical processes involved impossible, because polycrystalline forms expose many different crystal faces with many different surface structures and defects. Consequently, those interested in the more fundamental comprehension of the catalytic processes of oxides have turned to the use of single crystals under ultrahigh vacuum (UHV) conditions, allowing the application of powerful surface science techniques, such as those used here. Titanium dioxide (TiO2) is of great interest as a potential photocatalyst for decontamination of fluids through its semiconducting properties4 and also for protective surface coatings and other applications. Of several naturally occurring forms,4 rutile is the most suitable to the production of single crystals and is referred to here. A number of stable surface structures of TiO2 exist and have been characterized by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM).5-8 In * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 64 9 373 7422. † University of Auckland. ‡ University of Delaware. (1) Kim, K. S.; Barteau, M. A. Langmuir 1990, 6, 1485. (2) Kim, K. S.; Barteau, M. A. J. Catal. 1990, 125, 353. (3) 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. (4) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 3, 735. (5) Firment, L. E. Surf. Sci. 1982, 116, 205. (6) Poirier, G. E.; Hance, B. K.; White, J. M. J. Vac. Sci. Technol., B 1992, 10, 6. (7) Fan, F. F.; Bard, A. J. J. Phys. Chem. 1990, 94, 3761.

the bulk, titanium cations are octahedrally coordinated to six oxygen anions, and the more the surface structure resembles this situation, the more stable it is.9 Thus the most stable surface, and consequently the most studied, is the TiO2(110) plane, where titanium cations are alternately 5-fold and 6-fold coordinated to oxygen anions. The (100) surface is less stable as all titanium cations have a single coordination vacancy. The (001) surface studied in this work has each titanium cation only 4-fold coordinated in its unreconstructed form. Annealing to 700-800 K produces the {011}-faceted TiO2(001) surface, where all cations are 5-fold coordinated. Higher-temperature annealing to 900 K and above produces a further reconstruction: a {114}-faceted surface. On this surface, 1/3 of the surface cations are in fully 6-fold coordinated sites, 1/3 are in 5-fold, and 1/3 are in 4-fold coordinated sites. A third metastable phase has been recently found.10,11 This third reconstruction has been observed by STM and reflection high-energy electron diffraction (RHEED) after heating of the crystal to 1473 K followed by quenching at a rate of 100 K s-1. This structure has a two-domain network and is composed of ridges running in the [110] and [1 h 10] directions at a spacing 7 times the lattice constant (a0[110] ) 0.65 nm). The reconstruction appears to be a (7x2 × x2)R45°. Thus, by simply controlling the annealing temperature (following sputtering to disorder the surface)10 one may determine the degree of coordinative unsaturation at the surface. This unique property may be of great use in experimentally determining structure-activity relationships. A reduced, defected surface may be produced by sputtering, as the lighter oxygen atoms are preferentially removed upon ion bombardment, or by high-temperature (8) Cocks, I. D.; Quo, Q.; Williams, E. M. Surf. Sci. 1997, 390, 119. (9) Ramamoorthy, M.; Vanderbilt, D.; King-Smith, R. D. Phys. Rev. B 1994, 49 (23), 16721. (10) No¨renberg, H.; Dinelli, F.; Briggs, G. A. D. Surf. Sci. 1999, 436, L635. (11) No¨renberg, H.; Dinelli, F.; Briggs, G. A. D. Surf. Sci. 2000, 446, L83.

10.1021/la000931o CCC: $20.00 © 2001 American Chemical Society Published on Web 02/28/2001

Acrylic Acid on TiO2(001) Single Crystal Surfaces

annealing, especially in the presence of hydrogen.13 During annealing, oxygen rediffuses from the bulk, and the surface becomes stoichiometric again. The shift of X-ray photoelectron spectroscopy (XPS) Ti 2p3/2 and 2p1/2 peaks with oxidation states is a useful measure of the degree of reduction. We have conducted, in the past, detailed XPS,2,31,43,44 Auger,2 near-edge X-ray absorption fine structure (NEXAFS),45 and ultraviolet photoelectron spectroscopy (UPS)14 analyses of the surface of TiO2(001) upon Ar+-sputtering and exposure to annealing temperatures. A compilation of the majority of the work can be found in ref 15. Briefly, the Ar+-sputtered surface contains Ti cations in lower oxidation states than +4, besides Ti4+ cations. The extent of reduction is a function of ion energy and time. It appeared however that an asymptotic distribution of O/Ti of ca. 1.4 (AES,39 XPS)44 or 1.3 (NEXAFS)45 is reached by prolonged sputtering time. When this reduced surface is annealed at different temperatures (for 20 min at each temperature), the concentration of Tix+ cations decreases (x < 4) whereas that of Ti4+ and O anions increases, presumably by oxygen diffusion from the bulk to the surface. By 625 K, only Ti3+ and Ti4+ cations were present and O/Ti increases to 1.7, and by 750 K the surface was composed exclusively of Ti4+ cations (within the limits of XPS and UPS detection). Annealing at higher temperatures (up to 950 K) did not result in any observable change. Reduced surfaces containing oxygen vacancies are active for a number of carbon-carbon bond forming reactions by the coupling of reactant species.15 In the presence of a reduced ensemble of titanium cations able to undergo a four-electron reduction, aldehydes or ketones may reductively couple to give symmetric olefinssthe solid-gas analogy of the McMurry reaction of classical organic chemistry.16,17 A doubly coordinatively unsaturated Ti2+ is able to cyclotrimerize three alkyne species to form a benzene ring.18 Understanding the surface chemistry of carboxylic acids over TiO2 is of crucial importance to semiconductor technology as well as catalysis. Examples include solar cell devices,19 surface passivation,20 and synthesis of complex organic molecules.21,22 However, no detailed investigation of a conjugated carboxylic acid has been reported, with the exception of a preliminary work a few years ago,3 over any TiO2 single crystal. It has been shown that for dissociative adsorption of carboxylic acids on metal oxides (involving cleavage of the O-H bond of the hydroxyl group) a single coordination vacancy on a cation is required as well as an available oxygen to receive the hydrogen.23,24 Subsequent monomolecular reactions of carboxylic acids upon heating include dehydration, decarboxylation, and unselective deoxygenation as well as associative desorption of the parent species. Also observed under some (12) Watson, B. A.; Barteau, M. A. Chem. Mater. 1994, 6, 771. (13) Hoflund, G. B.; Yin, H. L.; Grogan, A. L.; Asbury, D. A. Langmuir 1988, 4, 346. (14) Idriss, H.; Lusvardi, V. S.; Barteau, M. A. Surf. Sci. 1996, 348, 39. (15) Barteau, M. A. Chem. Rev. 1996, 96, 1413. (16) McMurry, J. E. Acc. Chem. Res. 1983, 16, 405. (17) Pierce, K. G.; Barteau, M. A. J. Org. Chem. 1995, 60, 2405. (18) Pierce, K. G.; Barteau, M. A. J. Phys. Chem. 1994, 98, 3882. (19) Patthey, L.; Rensmo, H.; Person, P.; Westmark, K.; Vayssieres, L.; Petersson, A.; Bru¨hwiler, P. A.; Siegbhan, H.; Lunell, S.; Mårtenson, N. J. Chem. Phys. 1999, 110, 5913. (20) Cohen, R.; Kronik, L.; Vialan, A.; Cahen, D. Adv. Mater. 2000, 12, 33 and references therein. (21) Rajadurai, S. Catal. Rev.sSci. Eng. 1994, 36, 385 and references therein. (22) Dooley, K. M.; Randery, S. D. In 16th Mtg. of the North Am. Catal. Soc. Meeting, Boston, 1999; p II-041. (23) Vohs, J. M.; Barteau, M. A. Surf. Sci. 1988, 201, 481. (24) Vohs, J. M.; Barteau, M. A. Surf. Sci. 1986, 176, 91.

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conditions is bimolecular coupling, producing a symmetrical ketone (such as acetone from acetic acid). Single crystal UHV studies have shown the site requirement for this ketonization reaction to be a cation with two coordination vacancies, able to simultaneously coordinate two-reactant species.2 The behavior of simple C1-C3 carboxylic acids with saturated side chains (formic, acetic, and propanoic acids) over both TiO2 powder and the (001) single crystal surface has been well characterized in the past,1,2,25 and hydrocarbon side chains were found to be inert.26 These acids may potentially bind through the carboxylate group in a monodentate, bidentate, or bridging mode to surface cations. Acrylic acid was investigated as an example of a carboxylic acid with an unsaturated, π-bound side chain, which might allow further modes of interaction with the surface, producing more complex and interesting chemistry than that of its saturated analogue, propanoic acid. There are indeed many examples of solution π-bound complexes of titanium.27 However, investigation of the reactions of ethene and propene on the TiO2(001) single crystal surface showed mainly molecular desorption, indicating only weak binding of the olefin (see Discussion). The reactions of both acrylic and propanoic acids have been studied over the Cu2O(100) single crystal surface, where they showed little difference in behavior.26 Maleic anhydride, effectively the dicarboxylate analogue of acrylic acid closed to an anhydride, binds in a “lying down” π-configuration on Pd/Mo(110), though not on Mo(110).28 The reactivity of maleic anhydride on the TiO2(001) surface has been recently investigated.29 This species was chosen as the first example of a dicarboxylic acid (closed to an anhydride ring) over an oxide surface. The dominant products were CO and CO2 from decarboxylation of the maleate species, with the desorption of ethene and ethyne from its backbone. Production of benzene was also observed, presumably because of the cyclotrimerization of ethyne, as well as C4 coupling products butene and vinylacetylene. Preliminary work was conducted several years ago to investigate, by temperature-programmed desorption (TPD), the reaction of acrylic acid over TiO2 single crystal surfaces.3 The dominant products were CO and CO2 from decarboxylation, C4 products (butene, butadiene) due to coupling of the resulting fragments, acrolein due to reduction of the acid, and, only on the {114}-faceted surface, the ketonization product divinyl ketone (DVK).3 Our knowledge of the extent of the surface reactivity of oxides in general and of TiO2 in particular has increased considerably since then. For example, at that time neither the reductive coupling of carbonyls17 nor the cyclotrimerization of alkynes18 was known. This better understanding of the site requirement for the coupling reactions over defected surfaces has motivated us to reinvestigate the reactivity of this conjugated molecule. This work presents a detailed study of the reactions of acrylic acid, H2CdCHCOOH, by TPD, XPS, and scanning kinetic spectroscopy (SKS) over TiO2(001) surfaces. Experimental Section The work was conducted in several UHV chambers described briefly below. The 10 × 10 × 1 mm3 TiO2(001) single crystal (Princeton Scientific Corp.) was glued to tantalum foil, which (25) Kim, K. S.; Barteau, M. A. Langmuir 1988, 4, 945. (26) Schulz, K. H.; Cox, D. F. J. Phys. Chem. 1992, 96, 7394. (27) Elschenbroich, Ch.; Salzer, A. Organometallics: A Concise Introduction; VCH Verlagsgesellschaft: Weinheim, Germany, 1992. (28) Xu, C.; Goodman, D. W. Langmuir 1996, 12, 1807. (29) Wilson, J. N.; Titheridge, D. J.; Kieu, L.; Idriss, H. J. Vac. Sci. Technol., A 2000, 18, 1887.

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Titheridge et al. have been shown to vary at high temperatures.34 The relative yields given in this work may have estimated errors up to 1520%.

Results

Figure 1. Dosing geometry for scanning kinetic spectroscopy. was attached to the copper rods of an electrical feedthrough for resistive heating. Temperature measurement was by means of a type K thermocouple glued to the edge of the crystal surface. Reactant was dosed through a variable leak valve and a 0.8 mm i.d. stainless steel dosing needle situated approximately 1 cm from the crystal face. XPS and TPD techniques were used, as well as the more novel SKS method. XPS results were acquired in a VG ESCALAB, using an Al anode, previously described in detail.30,31 For all spectra, collected after saturation exposure to acrylic acid at room temperature, C(1s), O(1s), and Ti(2p) regions were scanned following heating to the indicated temperature and recooling. The spectra were normalized to the Ti 2p3/2 at 459.0 ( 0.1 eV, corresponding to Ti4+ cations, to correct for shifts due to charging (of lesser magnitude on the more metallic reduced surfaces), which were similar to those observed for XPS of acetic acid on TiO2(001).2 Some TPD experiments were conducted at the University of Delaware in a PHI model 548 surface analysis system equipped with a UTI 100C quadrupole mass spectrometer (QMS). These systems and the preparation of the rutile TiO2(001) single crystal used have previously been described in detail.2 Other TPD and SKS results were obtained at the University of Auckland in a UHV chamber described previously.29 Analytical grade acrylic acid was purified via several freezepump-thaw cycles prior to experimental work. It was then dosed onto the crystal surface at room temperature for several minutes, at ca. 2 × 10-8 Torr. After pumping down for 30-50 min, a linear 1 K/s temperature ramp was applied to the crystal, while monitoring 12 masses with the QMS. SKS was also applied to the acrylic acid system. This technique32 is similar to TPD, but in addition to a pre-experiment dosing of the surface a low flux of substrate (at 7.5 × 10-9 Torr, 5 × 10-9 Torr above background) was maintained onto the crystal surface during the heating ramp. After heating at 1 K/s to the maximum temperature, the crystal was cooled at 1 K/s as far as possible in the absence of cooling apparatus; below around 550 K, the baseline was exponentially approached (see Figure 9 below). Mass spectral monitoring continued throughout. Further information, not necessarily provided by the usual TPD technique, is gained. Initially, reactant is “reflected” off the saturated crystal surface into the QMS ionizer (Figure 1), and a constant reactant signal is seen. As the surface temperature is raised, reaction channels begin to open up and products are desorbed, freeing surface sites for the adsorption of reactant, so the reflected reactant signal decreases. At more elevated temperatures, where in a TPD experiment all reactant would be exhausted, the continuing supply of reactant enables the reaction chemistry to continue, potentially allowing the observation of higher-temperature reaction paths. As the crystal is recooled, the reactant signal rises again to near its original baseline value. Thus, both product formation and reactant consumption are visualized. To correct for mass spectrometer sensitivity, the method of Ko et al. was used.33 Note that this method was developed for C1-C2 organics only, specifically for the UTI 100C QMS. As such, the correction may not be extremely accurate for more complex organics, particularly as mass spectral fragmentation patterns (30) Peng, X. D.; Barteau, M. A. Langmuir 1989, 5, 1051. (31) Idriss, H.; Kim, K. S.; Barteau, M. A. Surf. Sci. 1992, 262, 113. (32) Gates, S. M.; Russell, J. N.; Yates, J. T., Jr. Surf. Sci. 1985, 159, 233. (33) Ko, E. I.; Benziger, J. B.; Madix, R. J. J. Catal. 1980, 62, 264.

TPD and XPS of acrylic acid were carried out over TiO2(001) annealed after sputtering at temperatures ranging from 550 to 950 K in order to study the structuredependence of the reaction chemistry. 1. Sputtered Surface. TPD. Over the 550 K annealed surface, the acrylic acid reactant desorbed in two clearly distinct domains: at ca. 400 and 600 K, some water desorbing simultaneously in both. As all carboxylic acids studied on TiO2 to date1,2,25 adsorb dissociatively, this is most likely for acrylic acid, especially in view of the production of water, indicating the presence of hydroxyl groups on the surface. Following dissociative adsorption (eq 1), the recombination of hydroxy groups with acrylate and other hydroxy groups produces acrylic acid (eq 2) and water (eq 3), respectively.

H2CdCHCOOH + O(s) f H2CdCHCOO(ad) + O-H (1) H2CdCHCOO(ad) + O-H f H2CdCHCOOH(g) + O(s) (2) 2O-H f H2O(g) + VO

(3)

where VO denotes a surface oxygen vacancy. A possible explanation for the two distinct acrylic acid desorptions is as follows. Initially, an acrylate species is bound through an oxygen to a 5-fold coordinate (singly coordinatively unsaturated) titanium center, for which desorption involves the reduction of coordination from six to five. The recombination of hydroxyls leaves oxygen vacancies. Upon migration of an acrylate to one of these vacancies, the acrylate oxygen is now bound to more than one titanium. Such a migration has been found for adsorbed ethoxides on TiO2(110).35 Desorption is now much more energetically demanding, meaning the reduction in coordination of two titanium centers from five to four. Thus, desorption will not occur until higher temperatures are reached, when decomposition routes leaving the oxygen on the surface may be favored, giving rise to other products. A number of products were seen from 600 to 700 K: substantial amounts of CO and CO2, butene and butadiene, acrolein, ethyne, and probably some ethene. Because of the similar overlapping fragmentation patterns of acrolein and the C4 products, exact quantification of these was difficult. TPD traces are shown in Figure 2, with quantitative yields in Table 1. The theoretical homolytic dissociation energy of acrylate species (splitting to a vinyl fragment plus CO2) was calculated to be 434 kJ/mol.36 The energy of activation for the TPD peaks may be estimated from the simple Redhead relation37 as approximately 160 kJ/mol. The reduction in energy is clearly due to stabilization of the products/transition states by the surface. The appearance of these products may be accounted for as follows. (34) Lohokare, S. P.; Crane, E. L.; Dubois, L. H.; Nuzzo, R. G. Langmuir 1998, 14, 1328. (35) Gamble, L.; Jung, L. S.; Campbell, C. T. Surf. Sci. 1996, 348, 1. (36) Work conducted by J. R. Brown, Department of Chemistry, University of Auckland, Feb, 1999. (37) de Jong, A. M.; Niemantsverdriet, J. W. Surf. Sci. 1990, 233, 355.


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Figure 2. TPD of acrylic acid on the 550 K annealed TiO2(001) surface. Heating rate ) 1.2 K/s. Table 1. Product Distribution for Acrylic Acid TPD on the 550 K Annealed Surfacea product

peak temp (K)

selectivity (% of products)

carbon yield (% of total C)

acrylic acid (1) acrylic acid (2) water carbon dioxide carbon monoxide ethyne acrolein butene butadiene

380 580 670 580 620 630 620 620 620

6.9 6.4 9.9 34 27 6 3.2 3.3 3.5

14 13

a

23 18 8 6 9 9

Carbon deposited nonselectively on the surface is neglected.

At higher temperatures, above 550 K, decomposition of the adsorbed acrylate species begins to occur. Hydrogen atoms released by this process are able to recombine with acrylates, producing further desorption of acrylic acid (eq 2). Desorptions of CO and CO2 in this domain indicated decarboxylation of the adsorbed acrylate (eq 4).

H2CdCHC(O)O(ad) f H2CdCH(ad) + CO2(g) or CO(g) + O(s) (4) Above 750 K, m/z 28 and 44 increased rapidly and m/z 18 increased slowly, whereas the other masses monitored decreased. This is due to burning off of carbon and hydrogen deposited on the surface by nonselective decomposition of hydrocarbon fragments. Signals at masses 56 (molecular weight) and 29 (CHO, the “signature” mass of an aldehyde) indicate the formation of acrolein in the 600 K peak. This may be formed by the reduction of adsorbed acrylate at oxygen anion vacancies (produced by the sputtering), as shown in eq 5. Hydrogen might be supplied by eq 7b (see below).

H2CdCHC(O)O(ad) + VO + H(a) f H2CdCHCHdO + O(s) (5) The production of DVK was not observed on the 550 K annealed surface.

Figure 3. XPS C(1s) after adsorption of acrylic acid over Ar+sputtered TiO2(001) single crystal surface: (a) 300 K, (b) surface in (a) heated to 400 K, (c) surface in (b) heated to 500 K, (d) surface in (c) heated to 650 K, and (e) surface in (d) heated to 750 K.

XPS. XPS of the sputtered surface showed the characteristic Ti(2p) signals of a disordered and reduced TiO2 surface, which have been studied in detail elsewhere14,31,43-45 (see Introduction). The C(1s) spectrum before dosing showed no signal above background, confirming the cleanliness of the surface. Figure 3 shows the raw spectra of the C(1s) XPS obtained upon adsorbing acrylic acid over the Ar+sputtered TiO2(001) surface and annealing to increment temperatures. Figure 4 shows the variation of the peak intensities of the different species (of Figure 3) with temperature. Two peaks are clearly resolved at 289.2 and 285.6 eV. The peak at the higher binding energy is assigned to the carboxyl carbon of adsorbed acrylate, and the lower binding energy peak is approximately twice as big, as expected for the C2 hydrocarbon side chain. These binding energies correspond well to values for acrylic acid on Cu2O(100)26 and acetic/propanoic acids on TiO2(001).2

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Figure 4. Uncorrected peak heights from XPS C(1s) of acrylic acid over Ar+-sputtered TiO2(001) single crystal surface as a function of flashing temperature. Dosing of acrylic acid was 2 L at 300 K.

When the sample was heated to 400 K, a 20% reduction in the carbonyl signal was seen, and the hydrocarbon signal dropped slightly. At 400 K on the TPD trace (Figure 2), it may be seen that some acrylic acid has desorbed, CO2 and CO signals have increased, and other products are not yet desorbing. This indicates a greater loss of the carboxyl group at this stage, as confirmed by the XPS. At 500 K, the carboxyl XPS signal has further decreased to approximately half its original value, whereas the hydrocarbon signal again is only slightly reduced. These results correlate well with those of the TPD, in which ongoing desorption of CO and CO2 is seen, without many other products. Upon increase to 650 K, the carboxyl signal continues to drop, and a dramatic reduction in the hydrocarbon signal is seen (40% decrease). This is as expected from the TPD results; at this temperature much of the product has desorbed. At 750 K, almost all of the carboxyl carbon signal is lost, with a further decrease in the hydrocarbon signal, corresponding to the completion of TPD desorption. The remaining “hydrocarbon” is probably carbon deposited on the surface by nonselective decomposition and burned off at higher temperatures as outlined above. This is also supported by the shift to a lower binding energy of this carbon signal. Trends can also be seen in the O(1s) spectra during heating (Figure 5). As the crystal is heated from room temperature, the intensity at the peak of the O(1s) signal steadily increases (Figures 5 and 6). Although some of the increase in the signal, particularly at 400 K, might be due to removing of the adsorbates (matrix effect), the decrease of the full width at half-maximum (fwhm) may indicate reoxidation of the reduced surface during heating (also indicated by increasing intensity and resolution of the two Ti(2p) signals, not shown). As shown in Figure 6, the narrowing of the peak is due to the disappearance of a shoulder at high binding energy with increased heating. Although this shoulder appears centered at about 1.9 eV above the central O(1s) peak (carboxylate oxygen of acetic acid on {011}-faceted TiO2(001) is 1.9 eV above that of lattice oxygen),2 the O(1s) spectrum before dosing acrylic acid was almost identical, indicating that this shoulder (also not seen on surfaces annealed to higher temperatures) must be mostly due to broadening of the oxygen of the disordered, that is, sputtered, surface (Doniach-Sˇ unjic broadening.)46 2. Annealed Surfaces. Previous Works. Previous TPD results were published for the 950 K annealed surface (the {114}-faceted form of TiO2(001), containing Ti4+4c cations).3 Briefly, a number of products desorb in the 350650 K range. As might be expected, the yield of acrolein (the reduction product) was greatly reduced on high-

Figure 5. XPS O(1s) of sputtered TiO2(001) surface as a function of flashing temperatures in the same conditions as in Figures 3 and 4.

temperature annealed surfaces, as these have been reoxidized following the sputtering process. The yield of coupling products, butene and butadiene, is also reduced, indicating that these are predominantly formed at oxygendeficient defect sites. Although benzene was not looked for in the original results,3 it was later detected by TPD (not shown) and SKS (see below). Recent results of maleic anhydride (closely related to acrylic acid) on the same crystal29 found the yield of benzene to be greatest on sputtered surfaces, less when annealed, and only very low on a surface annealed in the presence of oxygen (>103 L). This was presumably due to the lowered number of Ti2+ centers, which have been shown to be required for cyclotrimerization.18 Similar results might be expected from acrylic acid. The most important result on surfaces annealed above 900 K, however, is the formation of DVK by ketonization


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Figure 6. Intensity changes in the O(1s) XPS peak of sputtered surface with heating. The screening effect on the O(1s) signal, because of surface adsorbates, is not included.

of two acrylate species at a single doubly coordinatively unsaturated titanium center (eq 6).

2H2CdCHCOO(ad) f H2CdCH-C(O)CHdCH2 + CO2 + O(s) (6) The exact mechanism of the ketonization reaction is as yet not known, but it requires a transfer of the alkyl group from one adsorbed species to the other, resulting in a breaking of one carbon-oxygen bond. XPS. XP spectra were also acquired on the 950 K preannealed surface. Sharp, well-resolved Ti 2p1/2 and 2p3/2 peaks were observed, both their binding energy positions and widths indicating a stoichiometric, oxidized surface.2,12,31 Figures 7 and 8 present XPS C(1s) raw spectra as well as their peak intensity as a function of increment heating of the surface after dosing of acrylic acid at 300 K. As in the case of the sputtered surface, two carbon peaks were observed on the annealed surface, at 289.2 and 285.5 eV, attributed to -COO(a) and CH2dCH-, respectively. Similar trends were seen with increasing surface temperature with few slight differences. The CH2d CH- signal decreases monotonically with annealing temperature (although it did have a sharp decrease at 500 K on the sputtered surface). This may simply be because the sputtered surface contains Ti cations having 3d electrons (reduced states) that interact with the π bonds of the vinyl group, thus stabilizing the adsorbed species. SKS. This was carried out over a surface annealed at 800-850 K, giving an intermediate between the {011}and {114}-faceted surfaces. Figure 9a shows the strongest reactant fragments, m/z 72 (acrylic acid) and m/z 55 (loss of OH). The m/z 72 signal decreased as the temperature was raised to the maximum, and then increased again to near its starting value while the crystal was cooled. The m/z 55 signal (some contribution from acrylic acid) decreased less, indicating desorption of a product giving a fragment with mass 55, such as acrolein, butene, or DVK. Strong m/z 28 and 44 signals (not shown) indicated the formation of significant quantities of CO and CO2, from decarboxylation of the acrylic acid reactant at lower temperatures, and a greater increase due to burnoff of decomposed surface carbon at higher temperatures. A strong m/z 26 signal centered at 750 K indicated desorption of ethyne (Figure 9b) and an almost identically shaped m/z 27 signal indicated simultaneous production of ethene, the two alternative C2 products following decarboxylation. Cyclotrimerization of ethyne to benzene

Figure 7. XPS C(1s) after adsorption of acrylic acid over the {114}-faceted TiO2(001) single crystal: (a) 300 K, (b) surface in (a) heated to 400 K, (c) surface in (b) heated to 500 K, and (d) surface in (c) heated to 650 K.

Figure 8. Uncorrected peak heights from XPS C(1s) of acrylic acid over the {114}-faceted TiO2(001) single crystal surface.

was shown by a clear m/z 78 signal (Figure 9b). Benzene production decreases more rapidly than that of ethyne at higher temperatures (>750 K). This may be explained by

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the rationale that in order to form benzene, ethyne molecules must remain on the surface long enough to diffuse to the few defect sites and undergo cyclotrimerization. The half-life of a molecule on the surface decreases exponentially with temperature: t1/2 ) (ln 2/A) eE/RT. Thus, at high temperatures ethyne may desorb before further reaction can occur. A weak, noisy m/z 82 peak at 720 K (Figure 9c) signals the production of very small amounts of DVK (×10). This indicated the presence of a low degree of {114}-faceting in the surface. A m/z 54 signal shows butadiene, and a strong m/z 56 peak, with considerable desorption seen until low temperature is reached again, indicates production of acrolein and/or butene. For benzene, the signals for these C4 and C5 coupling products drop above 750 K (whereas ethene and ethyne production continue for longer), possibly because of insufficient residence time for coupling to occur. It might then be expected that production of these species would briefly increase again as the temperature is decreased. This is not observed, although a shoulder may be seen on the trailing edge of the m/z 54 and 56 peaks. This is most likely because some active sites have been blocked by deposition of carbon from the high-temperature dosing. An apparent shift of peaks to higher temperatures from those seen in TPD experiments is most likely due to the ongoing supply of substrate. Discussion The discussion focuses on the reaction/decomposition pathway of acrylate species. Decomposition Pathway. There are a number of possible fates for the vinyl fragment produced in eq 4. A stable C2 species may be formed by gain or loss of a hydrogen, giving ethene (eq 7a) or ethyne (eq 7b), respectively. We have calculated gas-phase homolytic bond dissociation energies shown for these reactions using the CBS-Q (complete basis set approximation) method.36,38

H2CdCH• + H• f H2CdCH2 H2CdCH• f HCtCH + H•

-460 kJ/mol -150 kJ/mol

(7a) (7b)

In the gas phase, formation of ethene would be 3 times more exothermic than that of ethyne. However, a much greater production of ethyne was observed in TPD, presumably because of the plausible two following reasons. (1) The hydrogen produced in eq 7b is stabilized by binding to oxygen, making this reaction more favorable, and the hydrogen consumed in eq 7a comes from the cleavage of a hydrogen-oxygen bond, making this route less exothermic. (2) Furthermore, ethene formation is essentially a “bimolecular” process requiring the presence of a “labile” hydrogen in the vicinity, whereas ethyne formation is “monomolecular”. Thus, ethene formation would be expected to show some second-order dependence on coverage not seen for ethyne, and at lower coverages the ethyne/ ethene ratio should be somewhat higher. A slowly increasing background desorption of hydrogen (H2) was possibly due in part to recombination and desorption of hydrogen generated via eq 7b. Other Pathways. Alternatively, two such H2CdCH fragments could couple to produce the stable C4 butadiene (eq 8a,b) or (with partial reduction, from eq 7b) butene. (38) Ochterski, J. W.; Petersson, G. A.; Montgomery, J. A., Jr. J. Chem. Phys. 1996, 104, 2598.

Figure 9. Scanning kinetic spectroscopy of acrylic acid on TiO2(001). Heating/cooling rate ) 1.0 K/s. The initial spike in each trace is desorption from mounting rods. (a) Acrylic acid reactant. (b) Ethyne and benzene. (c) C4 and C5 products.

m/z 54 and m/z 56 peaks indicated the presence of these two species.

2H2CdCH(ad) f H2CdCH-CHdCH2

(8a)

H2CdCH-CHdCH2 + 2[H] f H3CCHdCHCH3 (8b) We have conducted ethene TPD and XPS over TiO2(001) single crystal surfaces. XPS C(1s) spectra of both stoichiometric and sputtered ethene-dosed surfaces clearly show the presence of adsorbed species (Figure 9). The coverage at room temperature is about 10% that of acrylic acid as computed from XPS C(1s) lines. Moreover, TPD data indicated that with the exception of ethene desorption at 320-400 K and negligible amounts of C4 (less than 1%) no other products were observed (not shown). Propene did react similarly; that is, only traces of benzene (coupling) were seen. Thus, one is left to conclude that under UHV conditions ethene mainly adsorbs molecularly with no evidence of C-H bond dissociation. In other words, the reason for the reactivity of the vinyl group in the case of acrylic acid is that it is already bound to the surface. A detailed discussion of the possible fates of vinyl fragments is instructive.


Langmuir, Vol. 17, No. 7, 2001 2127

Following decarboxylation, the H2CdCH(ad) fragments on the surface might initially be bound via one of two ways: to an oxygen anion (route a), forming an O-CHd CH2 species, or to a titanium cation (route b), forming a metal-carbon bond. Some weak π-bonding may assist the species to remain coordinated long enough for the σ-bond to form. If bound to an oxygen anion (route a), a vinyl alcohol or enolate species results. The two primary decomposition reactions of alcohols on TiO2(001) are dehydration and dehydrogenation.39,40 Dehydration (route a1) by loss of a β-hydrogen and cleavage of the carbon-oxygen bond would generate ethyne. Dehydrogenation (route a2) by loss of the R-hydrogen with cleavage of the oxygen-titanium bond(s) would form ketene (eq 9). Small m/z 41 and 42 signals seen on more annealed surfaces were most likely due to some ketene. Considerable amounts of ethyne were observed. Ethyne formation might be expected to be more favorable as it avoids loss of a surface oxygen.

Cleavage of the oxygen-carbon bond could produce ethene with hydrogen gain (or the four-carbon products by coupling (eq 8)). Gain of a hydrogen might be expected to generate acetaldehyde (possibly via its enol tautomer). These two reactions were observed from an enolate species formed from 1-propanol over Cu2O(100).41 Acetaldehyde was not positively identified in our results. Binding of the H2CdCH(ad) fragment to a titanium cation (route b) may also be possible, as indicated by the existence of many organometallic compounds with titanium-carbon σ-bonds.42 Loss of hydrogen and release of ethyne might then occur via a β-hydride type elimination, a very common pathway for the decomposition of a metal alkyl to an olefin (eq 10). Upon the near approach of ethyne or a second C2 fragment, the formation of the C4 coupling products could occur.

Site Requirement for DVK. DVK is formed only on the surfaces annealed above 900 K. It is not formed on the Ar+-sputtered surface. It is also not formed on the {011}-faceted surface. The Ar+-sputtered surface is a defected25,43-45 surface that tends to abstract oxygen (to restore its surface oxygen) from oxygen-containing adsorbates, thus contributing to their reduction. As such, it is not expected to contribute much in the chemistry of the carboxyl (with the exception of their reduction and/or decomposition). The first faceted surface, the {011}(39) Kim, K. S.; Barteau, M. A. Surf. Sci. 1989, 223, 13. (40) Kim, K. S.; Barteau, M. A. J. Mol. Catal. 1990, 63, 103. (41) Schulz, K. H.; Cox, D. F. J. Phys. Chem. 1993, 97, 647. (42) Comprehensive Organometallic Chemistry; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: New York, 1995. (43) Idriss, H.; Pierce, K. G.; Barteau, M. A. J. Am. Chem. Soc. 1994, 116, 3063. (44) Idriss, H.; Barteau, M. A. Catal. Lett. 1994, 26, 123. (45) Lusvardi, V. S.; Barteau, M. A.; Chen, J. G.; Eng, J., Jr.; Fru¨berger, B.; Teplyakov, A. Surf. Sci. 1998, 397, 237.

Figure 10. XPS C(1s) after adsorption of ethene over (a) Ar+sputtered TiO2(001) single crystal surface and (b) {001}-faceted TiO2(001) single crystal surface.

reconstructed TiO2(001), contains, ideally, all Ti4+ cations as 5c. The second faceted surface, the {114}-reconstructed TiO2(001), contains 1/3 of Ti4+ as 4c. The fact that only on this latter surface was DVK observed indicates that the surface structure is essential. Because XPS showed that Ti cations are only in a +4 oxidation state (a very narrow fwhm of 1.1-1.2 eV of the XPS Ti 2p3/2), the oxidation state is not involved in this reaction but the coordination number is. Thus, it is most likely that Ti4+4c cations are responsible for the formation of DVK (because they are absent on the {011}-faceted surface and are most likely absent on the defected Ar+-sputtered surface). (It is important to mention that acetone from acetates and formaldehyde from formates were previously observed only on this surface.)1,2 Acetone was also observed from acetic over TiO2 (anatase), Cr2O3, MnO, Fe3O4, and other oxides.47,48 These sites (Ti4+4c), containing two vacancies per Ti4+ cation, may accommodate the two acrylate species required for the coupling reaction on one Ti4+4c. We have not conducted any work to investigate the reaction mechanism for this coupling reaction. Consequently, the rearrangement that takes place before making the ketone is unclear. Despite several reaction mechanisms proposed in the literature,21 from powder works, to explain the formation of ketones, we do not have data to explain the exact pathway for making DVK from acrylate species over the {114}-faceted surface. However, our work shows that the presence of Ti4+4c sites is essential.

(46) Doniach, S.; Sˇ unjic, M. J. Physica C 1970, 3, 285. (47) Gonzalez, F.; Munnuera, G.; Prieto, J. A. J. Chem. Soc., Faraday Trans. 1978, 174, 1517. (48) Swaminathan, R.; Kuriacose, J. C. J. Catal. 1970, 16, 357.

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Conclusions The reactions of acrylic acid over TiO2(001) single crystal surfaces provide a clear example of the complexity of the chemistry of conjugated molecules over the surface of semiconductor materials. Overall, there was little evidence that the olefin side chain affected binding of acrylic acid to the surface. However, once a decarboxylation has occurred, a range of products due to the CH2dCHfragment is seen. These products included ethene, ethyne, butene, and butadiene. These pathways are undoubtedly related to the conjugated nature of the adsorbate because they are absent during propanoic acid TPD2 (the saturated

Titheridge et al.

analogue). Scanning kinetic spectroscopy has been successfully applied on an oxide single crystal for the first time. The complementary nature of the technique to TPD has provided an in depth investigation of the reaction pathway of acrylic acid over the surfaces of TiO2(001) single crystal. On the other hand, acrylate species could couple on the {114}-faceted surface, the one containing Ti4+4c, making DVK. Switching the formation of DVK on and off could easily be obtained by Ar+-sputtering.

LA000931O

Reaction of Acrylic Acid on TiO2 (001) Single Crystal Surfaces

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