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Reactions of Glutaric Acid on the TiO2(001) Single Crystal. Effect of Surface Reduction on the Reaction Pathway J. N. Wilson and H. Idriss* Department of Chemistry, The University of Auckland, New Zealand Received April 10, 2005. In Final Form: June 8, 2005 The detailed reaction of glutaric acid (a C5 dicarboxylic acid) has been studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Upon adsorption at 300 K, both carboxylic acid functions are deprotonated to give adsorbed glutarate species. The reaction of these species differs significantly on the oxidized surface from that on the reduced surface. On the oxidized surface, two competing reactions are seen: (i) decomposition to two molecules of CO and one molecule of propene and (ii) dehydration to ketene. Upon sputtering with hydrogen ions (reducing the surface states of Ti ions and implanting hydrogen atoms in the lattice), the main observed reaction is reduction to the dialdehyde and the dialcohol. The yield of these two products, not seen on the oxidized surface, reaches 80% on the highly reduced surface. Another compound is seen on the reduced surface with m/z ) 70. The analysis of its fragmentation pattern tends to assign it to cyclopentane that is formed by an intramolecular reductive coupling reaction on the O-defect sites.
I. Introduction Metal oxides are commonly at the heart of heterogeneous catalysis either as a support for highly dispersed metals or as the active material. The titanium dioxide (TiO2) semiconductor is of great interest and has been studied in great detail for several decades.1 The rutile form has until recently been the only polymorph (anatase and brookite being the others) available in the single crystal and is referred to here. The surfaces of TiO2(001) single crystal are interesting to surface science and catalysis in the way that they facet upon thermal annealing at moderate temperatures2 and because their atomic positions are known precisely. Figure 1 shows the bulk termination of TiO2(001) and the surface upon annealing to 750 K, the {011}-faceted surface. The reconstruction is driven by the minimization of the relaxed surface energy from 2.40 J/m2 on the (001) surface to 1.85 J/m2 on the (011) surface3 by increasing the titanium coordination from 4-fold to 5-fold. These surfaces have been shown to be extremely reactive toward organic molecules.1,4,5 Moreover, TiO2 is an ideal material to use in studying the effects of surface defects. Ion bombardment has been used as a reproducible means of introducing defects by the preferential removal of oxygen, and it produces a large number of reduced Tix+ states, where x ) 4, 3, 2, and 1 as seen in Ti(2p3/2) and Ti(2p1/2) via X-ray photoelectron spectroscopy (XPS).6 Monocarboxylic acid decomposition has widely been used as a probe of the catalytic properties of metals and metal oxides. On metal oxide surfaces, carboxylic acids are adsorbed as carboxylates on the 5-fold titanium cations at room temperature in bidentate and/or bridging forms as evidenced with a variety of techniques including XPS, scanning tunneling microscopy (STM), and low-energy * Corresponding author. E-mail:
[email protected]; Fax: 64 9 373 7422. (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (2) Firment, L. E. Surf. Sci. 1982, 116, 205. (3) Oliver, P. M.; Watson, G. W.; Kelsey, E. T.; Parker, S. C. J. Mater. Chem. 1997, 7, 563. (4) Barteau, M. A. Chem. Rev. 1996, 96, 1413. (5) Idriss, H.; Barteau, M. A. Adv. Catal. 2000, 45, 261. (6) Idriss, H.; Barteau, M. A. Catal. Lett. 1994, 26, 123.
Figure 1. Top and side views of the {011}-faceted TiO2 rutile surface. Ti atoms are 5-fold coordinated, whereas surface O atoms are 2-fold coordinated.
Figure 2. Orientation of chemisorbed carboxylates on a titanium surface, where R ) H, CH3, etc.
electron diffraction (LEED);1,4,7-10 see Figure 2. On the TiO2 surface, carboxylic acids prefers a dehydration pathway,11 for example, the dominant reaction of acetate (7) Kim, K. S.; Barteau, M. A. Langmuir 1988, 4, 945. (8) Gutierrez-Sosa, A.; Martinez-Escolano, P.; Raza, H.; Lindsay, R.; Wincott, P. L.; Thornton, G. Surf. Sci. 2001, 471, 163. (9) Onishi, H.; Sasahara, A.; Uetsuka, H.; Ishibashi, T.-a. Appl. Surf. Sci. 2002, 188, 257. (10) Uetsuka, H.; Henderson, M. A.; Sasahara, A.; Onishi, H. J. Phys. Chem. B 2004, 108, 13706. (11) Morikawa, Y.; Takahashi, I.; Aizawa, M.; Namai, Y.; Sasaki, T.; Iwasawa, Y. J. Phys. Chem. B 2004, 108, 14446.
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species on the {011}-faceted surface is unimolecular dehydration to ketene.7,12 There have been several studies of the adsorption of a dicarboxylic acid, bi-isonicotinic (2,2′-bipyridine-4,4′dicarboxylic acid), on the single-crystal surface of anatase TiO2(101)13 and rutile TiO2(110)14-16 to date. The evidence, both experimental and theoretical, points to the dissociation of both ends of the acid functional group forming carboxylates and binding to the surface in a bidentate configuration. Glutaric acid (GA), pentanedioic acid (HOOC(CH2)3COOH), is a dicarboxylic acid and is part of an important group of biological/organic molecules including glutaric anhydride, both of which are closely related to maleic anhydride. Because of the presence of two carboxylic groups in the molecule, the dicarboxylic acids form two series of salts, esters, or other derivatives depending on whether one or both of the carboxylic groups is saponified, esterified, or otherwise modified.17 The dicarboxylic acids are, therefore, excellent synthetic agents and have been employed in the preparation of many fatty derivatives. In addition, dicarboxylic acids are important ligands for several applications such as solar cell devices and the surface passivation of semiconductors.18 In this work, we focus on two aspects of the reaction of the dicarboxylic acid. First, we study the reaction product of GA on the stoichiometric surface to gain an understanding of the decomposition pathway. Second, we see the possibility of selectively reducing this dicarboxylic acid to the corresponding dialdehyde and/or the dialcohol and track the change with TPD and XPS. II. Experimental Section All TPD experiments were conducted in an ultrahigh vacuum (UHV) chamber described in detail previously.19 The UHV chamber is pumped with ion, turbo molecular, and titanium sublimation pumps to a base pressure of approximately 2 × 10-10 Torr and is equipped with a quadrupole mass spectrometer (up to 300 m/z), with a Pyrex shroud (having an orifice smaller than the crystal face; this design largely disregard desorption from the crystal mount), ion sputter gun, heating stage (x, y, r motion feed through), ion gauge, and two dosing lines with a dosing needle positioned a few millimeters away from the crystal face. The single crystal of TiO2(001) is prepared with sputter and annealing cycles, a typical Ar pressure of 1 × 10-5 Torr, 5-kV beam voltage, and 25-mA emission current, followed by several “flashes” at 750 K under oxygen pressure of 1 × 10-6 Torr for 10-min intervals until no detectable CO, CO2, or H2O is seen in blank TPD runs. X-ray photoelectron spectroscopy (XPS) analyses are carried out in a separate stainless steel UHV chamber from the one described above for the TPD experiments.20 The main chamber is pumped with a Perkin-Elmer Ultek 220 L/s ion pump, titanium sublimation pump, and sorption pump where the ultimate pressure is in the 10-10 Torr range. It is equipped with a Perkin-Elmer dual-anode (magnesium and aluminum, KR (12) Libby, M. C.; Watson, P. C.; Barteau, M. A. Ind. Eng. Chem. Res. 1994, 33, 2904. (13) Persson, P.; Lunell, S. Sol. Energy Mater. Sol. Cells 2000, 63, 139. (14) Persson, P.; Lunell, S.; Paul, A. Bru¨hwiler, P. A.; Schnadt, J.; So¨dergren, S.; O’Shea, J. N.; Karis, O.; Siegbahn, H.; Mårtensson, N.; Ba¨ssler, M.; Patthey, L. J. Chem. Phys. 2000, 112, 3945. (15) Odelius, M.; Persson, P.; Lunell, S. Surf. Sci. 2003, 529, 47. (16) Patthey, L.; Rensmo, H.; Westmark, K.; Vayssieres, L.; Stashans, A.; Bru¨hwiler, P. A.; Siegbahn, H.; Lunell, S.; Mårtensson, N. J. Chem. Phys. 1999, 110, 5913. (17) Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 1991. (18) Cohen, R.; Kronik, L.; Vilan, A.; Shanzer, A.; Cahen, D. Adv. Mater. 2000, 12, 33. (19) Wilson, J. N.; Titheridge, D. J.; Kieu, L.; Idriss, H. J. Vac. Sci. Technol., A 2000, 18, 1887. (20) Wilson, J. N.; Idriss, H. Langmuir 2004, 20, 10956.
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Figure 3. TPD of glutaric acid, saturation coverage, on the stoichiometric surface of the TiO2(001)-{011}-faceted single crystal. 1253.6 and 1486.6 eV, respectively) X-ray source, angle-resolved double-pass cylindrical mirror analyzer (containing a coaxial electron gun for AES), and an autoscaling ion gauge (GranvillePhillips series 271) for the pressure reading. The XP spectra shown were collected in pulse-counting mode under the following conditions: accelerator voltage, 14 kV; emission current, 20 mA; 0.1 eV/step; 350 ms/step; and pass energies of 25, 15, and 50 eV for Ti (2p), O (1s), and C (1s), respectively. All spectra were normalized to Ti (2p3/2) at 459.2 ((0.1) eV corresponding to Ti4+ to correct for shifts due to charging.21 Peak fitting of the curves obtained by XPS was conducted using the XPSPEAK4.1 program written by R. W. M. Kwok.22 The background used was Shirleytype and Gaussian peak fitting. Glutaric acid was purified by heating gently prior to use in the dosing line for a few hours and then dosed (vapor pressure at 300 K ≈ 0.3 Torr22a) onto the single crystal through a variable-leak valve with a stainless steel dosing needle. All dosing was performed at room temperature. A dosing pressure of 5 × 10-8 Torr for 2 min or more was judged to be large enough for monolayer formation, as evidenced from the TPD data and in line with similar work with other organic compounds on this surface.23,24 A heating rate of 1 K/s was used in the TPD experiments. The correction factors for the reaction products were calculated following the method of Ko et al.25 and were as follows: glutaric acid (m/z 86) 3.0; ketene (m/z 42) 2.3; carbon monoxide (m/z 28) 1; propene (m/z 39) 4.3; glutaralcohol (m/z 31) 2.8; and glutaraldehyde (m/z 29) 5.9.
III. Results and Discussion A. Stoichiometric Surface. A. i. TPD. TPD of glutaric acid on the stoichiometric surface of the TiO2(001)-{011}faceted single crystal shows desorption domains in three temperature regions (Figure 3). The molecular yields (%) are shown in Table 1. Glutaric acid, ∼39%, desorbs in two temperature domains, 350 and 550 K; this is comparable to the many monocarboxylic acids studied on the stoichiometric surface of the TiO2(001) single crystal.7,23,24,26-28 (21) Titheridge, D. J.; Barteau, M. A.; Idriss, H. Langmuir 2001, 17, 2120. (22) http://sun.phy.cuhk.edu.hk/∼surface/XPSPEAK/XPSPEAK usersguide.doc. (a) Bilde, M.; Pandis, S. N. Environ. Sci. Technol. 2001, 35, 3344. (23) Idriss, H.; Lusvardi, V. S.; Barteau, M. A. Surf. Sci. 1996, 348, 39. (24) Idriss, H.; Kim, K. S.; Barteau, M. A. Stud. Surf. Sci. Catal. 1991, 67, 327. (25) Ko, E. I.; Benziger, J. B.; Madix, R. J. J. Catal. 1980, 62, 264. (26) Wilson, J. N.; Idriss, H. J. Am. Chem. Soc. 2002, 124, 11284. (27) Kim, K. S.; Barteau, M. A. J. Catal. 1990, 125, 353. (28) Wilson, J. N.; Idriss, H. J. Catal. 2003, 214, 46.
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Table 1. Molecular Yield (%) of Glutaric Acid on the Stoichiometric Surface of the TiO2(001) Single Crystal
glutaric acid H2O CO propene ketene
peak temp (K)
% molecular yield
350, 550 350 550 550 620
39
Scheme 1. Different Modes for Dissociative Adsorption of Glutaric Acid on Ti Ions
15 7 39
Water, 18 m/z, desorption is concomitant with the initial glutaric acid peak at 350 K and is attributed to the disproportionation of surface hydroxyl species
OH(ads) + OH(ads) f H2O(g) + O(l) + VO
(1)
where ads, g, l, and VO denote adsorbed, gas, lattice, and oxygen vacancy in the lattice, respectively. OH(ads) is formed upon dissociative adsorption of glutaric acid (details of adsorption are presented in the XPS C(1s) section later). Whether the dissociative adsorption results in a bridging or bidentate configuration is not known. However, it is known that the monocarboxylic acids formic acid, acetic acid, and benzoic acid are adsorbed in a bridging configuration on the (110) surface of TiO2.29,30 Thus, glutaric acid can be adsorbed in a 1:1, 1:2, or 1:4 ratios with respect to Ti atoms on the surface (as seen in the scheme below): The minimum distance between two Ti atoms on the bulklike terminated (011) surface is 3.57 Å. We recently performed DFT periodic repetition of the (011) surface upon formic acid adsorption and found that the bidentate mode is more favored than the monodentate, whereas the chelating mode is unfavored.31 This indicates that the relatively large distance between two Ti atoms of 3.57 Å can accommodate a bridging configuration of the same carboxylic function. The 1:1 ratio (Scheme 1a) would require the presence of surface-only 4-fold-coordinated Ti atoms prior to adsorption; these are not present on the {011}-facetted surface in this work. Thus, one can neglect configurations 1a and 1b in Scheme 1. Ketene (CH2dCdO), 14 and 42 m/z, desorption occurs at 620 K in a large quantity, 39%. Ketene formation from carboxylic acids proceeds via dehydration and is the principle reaction product (C2-C5 ketenes are produced in 35-38% yield) on the stoichiometric surfaces of many metal oxides.12 Although ketene formation from a molecule such as acetic acid is achieved via dehydration, that from GA cannot be formed without a carbon-carbon dissociation. Coincident peaks of CO, 28 m/z, and propene, 39 and 41 m/z, desorb at 550 K, indicating that they are formed in a concerted process. The reactions of maleic anhydride on this surface have shown analogous products, namely, ketene, CO, and acetylene.19 Ketene formation, a relatively well-understood reaction in the case of acetic acid,24-27 has been seen from maleic anhydride,19 bromoacetic acid,32 iodoacetic acid,33 and proline34 on this surface before. The reaction requires the splitting of the hydrocarbon chain and the (29) Chambers, S. A.; Thevuthasan, S.; Kim, Y. J.; Herman, G. S.; Wang, Z.; Tober, E.; Ynzunza, Morais, R. J.; Peden, C. H. F.; Ferris, K.; Fadley, C. S. Chem. Phys. Lett. 1997, 267, 51. (30) Guo, Q.; Williams, E. M. Surf. Sci. 1999, 433, 322. (31) McGill, P.; Idriss, H. To be submitted for publication. (32) Titheridge, D. J.; Wilson, J. N.; Idriss, H. Res. Chem. Int. 2003, 29, 553. (33) Wilson, J. N.; Idriss, H. To be submitted for publication. (34) Fleming, G. J.; Idriss, H. Langmuir 2004, 20, 7540.
formation of surface carbon and/or CHx:
TiOC(O)CH2CH2CH2C(O)OTi f 2CH2dCO + C(s) + 2 H(s) + 2O(s) (2) Surface carbon (eq 2) may combine with surface oxygen (also formed in eq 3, see below), giving CO. The formation of propene can be seen as a decarbonylation reaction:
TiOC(O)CH2CH2CH2C(O)OTi f CH3CHdCH2 + 2CO + 2O(ads) (3) The ratio of propene to CO should be equal to 1 to 2 and it is indeed what is obtained, within experimental error (Table 1). No detectable traces of hydrogen were observed but cannot be ruled out because of the high background of this signal in the chamber. In addition, no formation of CO2 was evidenced. The dehydrogenation of formates (from formic acid) to CO2 has been shown to be unfavorable with a ratio of CO/CO2 of 3:1.35 A. ii. XPS. O(1s) of the clean and glutaric acid-saturated TiO2(001)-{011}-faceted surface is presented in Figure 4A and B, respectively. The spectra show differences between the chemically distinct lattice and adsorbate oxygen atoms. Both sets of data show the lattice oxygen at 530.5 eV, fwhm 1.4 eV, (region I); however, in B the maximum is less intense (0.90), the width is larger, and the tail is skewed. These results are indicative of a surface that is attenuated upon the addition of glutaric acid. Curve fitting (see Experimental Section for more details) of the peaks shows region II at ∼1.7 eV above the lattice oxygen peak (region I), in good agreement with several other studies signifying deprotonation of the carboxylic functional group and chemically indistinguishable oxygen atoms that may indicate a bidentate configuration. For monolayer spectra, the splitting between the lattice and carboxylic peak is from 1.4 eV for benzoic acid to 1.5 eV for picolinic acid, 1.6
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Figure 5. Ti(2p) XPS spectra of the clean (b) (×0.90) and glutaric acid-dosed (0) TiO2(001)-{011}-faceted surface (saturation coverage).
Figure 4. O(1s) XPS spectra of the clean (A) and glutaric acid-dosed (B) TiO2(001)-{011}-faceted surface (saturation coverage).
eV for the isonicotinic, nicotinic, and bi-isonicotinic acids,36 1.7 eV for formic acid,29 and 1.6 eV for proline37 and trimethylacetic acid38 on TiO2(110) single-crystal surfaces, whereas on the TiO2(001)-{011}-faceted single-crystal surfaces it is 1.9 eV for acetic acid27 and acrylic acid.21 The Ti(2p) spectra of the clean and glutaric acid-dosed surfaces are also informative and consistent with the O(1s) spectra; the peak is attenuated by a similar amount, 0.90 (Figure 5). Trimethylacetic acid had a value of 0.89.38 From the 0.9 attenuation, we found the size of glutaric acid (volume after energy minimization using Spartan39 ) 124.98 Å3 giving 2r ) 6.2 Å) and the kinetic energy of the Ti(2p) photoelectron (795 eV). (The estimated surface coverage at saturation was found to be equal to 0.44 with respect to Ti atoms of the surface. It is thus highly likely that one molecule of glutaric acid is adsorbed on two Ti atoms; see Scheme 1 structures 1c and 1d). Figure 6A shows a typical C(1s) of the room-temperature glutaric acid-dosed TiO2(001)-{011}-faceted surface. Two peaks centered at 289.6 and 285.8 eV are observed. The C(1s) peak at 289.5 eV was assigned to the carboxyl carbon, and the peak at 285.8 eV, to the CH2-CH2-CH2 groups. These values are consistent with previous studies of organic acid adsorbates on the TiO2 single-crystal surfaces (Table 2). The ratio of the carboxylate to carbon chain species, 2:3.1, is in very good agreement with the expected ratio of 2:3. This ratio indicates that total decomposition of part of the glutaric acid during the introduction into the chamber does not occur and gives further confidence of the TPD data. (35) Kim, K. S.; Barteau, M. A. Langmuir 1990, 6, 1485. (36) Schnadt, J.; O’Shea, J. N.; Patthey, L.; Schiessling, J.; Krempasky´, J.; Shi, M.; Mårtensson, N.; Bru¨hwileret, P. A. Surf. Sci. 2003, 544, 74. (37) Adib, K. G.; Feliming, J. A.; Rodriguez, J. A.; Barteau, M. A.; Idriss, H. 51st American Vacuum Society Meeting, November 2004, Anaheim, CA. (38) White, J. M.; Szanyi, J.; Henderson, M. A. J. Phys. Chem. B 2004, 108, 3592. (39) Spartan 02 for Windows; Wavefunction, Inc.
Figure 6. C(1s) of the room-temperature glutaric acid-dosed TiO2(001)-{011}-faceted surface (A) (saturation coverage), heated to 450 K (B), heated to 550 K (C), and heated to 750 K (D).
Heating the glutaric acid-dosed surface to 450 K, trace B, results in a decrease in intensity, approximately indicating the removal of adsorbates from the surface. Further heating to 550 K, trace C, results in the complete loss of the carboxylate peak at 289.5 eV and broadening of the carbon peak at 285.8 eV, whereas heating to 750 K, trace D, is necessary to remove remaining carboncontaining species completely. B. Substoichiometric Surface. Initial work was conducted using Ar ions to sputter the surface. The most striking result was the formation of glutaraldehyde and glutaric alcohol. It was thought that the hydrogen required for the reduction process is provided from some hydrogen traces in the chamber that are inevitably ionized during sputtering. Intentionally mixing H2 with Ar has resulted in a considerable increase of the yields of the reduction
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Table 2. C (1s) Peak Position of Acid Adsorption on TiO2 Single Crystal Surfaces peak position (eV)
acid glutarica formicb aceticc propionic acrylicd benzoice picolinice isonicotinice nicotinice bi-isonicotinice trimethylaceticf a
surface
carboxylate
(001)-{011}-faceted (001)-{011}-faceted (001)-{011}-faceted (001)-{011}-faceted (001)-{011}-faceted (110) (110) (110) (110) (110) (110)
289.5 289.3 289.6 289.5 289.2 288.5 288.8 288.8 288.8 288.7 289.1
This work. b Reference 23. c Reference 27. e Reference 38. f Reference 36.
carbon (functional group) 285.8 285.8 285.7 285.6 284.7 285.6 285.1 285.4 285.2 285.8
d
Reference 21.
Figure 8. Reaction products, glutaraldehyde and glutaralcohol, as a function H2 sputtering time on the TiO2(001) single crystal.
Figure 9. Reaction products, CO, propene, and ketene, as a function of H2 sputtering time on the TiO2(001) single crystal.
Figure 7. TPD of glutaric acid, saturation coverage, on the 60-min hydrogen ion-sputtered surface of the TiO2(001) single crystal.
products. We are presenting below the results of pure hydrogen ion sputtering prior to glutaric acid adsorption. B.i. TPD. TPD of glutaric acid on the substoichiometric surface (60 min hydrogen sputtering) of the TiO2(001) single crystal shows rich chemistry in three temperature domains (Figure 7). The first, at around 360 K, consists of unreacted glutaric acid. At ∼460-480 K, several interesting products, glutaraldehyde (m/z 29, 72, and 82) and glutaralcohol (m/z 31, 56, and 68), are seen desorbing. The formation of glutaraldehyde, a dialdehyde, is analogous to the formation of monoaldehydes from monocarboxylic acids. These products increase as a function of sputtering reaching a plateau at approximately 30 min of hydrogen sputtering time (Figure 8). Formic acid, acetic acid, and acrylic acid give formaldehyde, acetaldehyde, and acrolein (2-propenaldehyde), respectively. The reactions of these and other carboxylic acids have been studied in detail elsewhere4,7,23,24,27,35 on the reduced surfaces of TiO2. As for the stoichiometric surface, there are two competing reaction pathways: the formation of propene and CO and the formation of ketene. Carbon monoxide, 28 m/z, and propene, 41 and 39 m/z, are seen desorbing together at around 460-480 K. This indicates that they are formed in a concerted process (i.e., the decarbonylation of both ends of the molecule). Trace amounts of the ketene, 14 and 42 m/z, desorb in the high-temperature domain at ∼620 K, as with the stoichiometric surface. The peak in the mid-temperature domain of ketene is from other CH2
Table 3. Uncorrected Area of 70 m/z as a Function of H-Ion Sputter Time on the TiO2(001) Single Crystal Surface H2 sputter time (min)
uncorrected area 70 m/z
0 15 30 45 60
0 2.2 × 10-8 3.6 × 10-8 5.2 × 10-8 1.2 × 10-7
Table 4. Fragmentation Patterns of Cyclopentane and Cyclobutanone cyclopentane
cyclobutanone
m/z
intensity (%)
m/z
intensity (%)
42 70 41 55 39 27 40 29
100 30 29 29 22 15 7 5
42 70 38 28 40 43 71 29
100 30 10 9 9 8 6 5
fragments. Ketene production drops significantly as a function of hydrogen sputtering time (Figure 9). In addition, there is a small desorption of 70 m/z at 475 K (Figure 7 and Table 3). Although this number is small, it is worth discussing because of the nature of the chemistry. Two possible products have been identified that may correspond to this mass, cyclopentane and cyclobutanone. Their fragmentation patterns are shown in Table 4. Glutaraldehyde may undergo an intramolecular reductive coupling reaction giving cyclopentene (m/z 68).
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This reaction is similar to the intermolecular reductive coupling of acrolein to hexatriene. This molecule, now hydrogenated, has the correct mass at 70 m/z, giving cyclopentane. Because of the significant contribution of glutaralcohol to the m/z 68 signal, the presence or absence of cyclopentene could not be conclusive.
Cyclopentane is in fact a starting material for the production of glutaric acid. Its formation is thus not surprising as such (principle of microreversibility). The other possibility is the formation of cyclobutanone that may occur via an intramolecular ketonization reaction of glutarate species, a mechanism that is very similar to the formation of acetone from acetic acid27 and of divinyl ketone from acrylic acid.21
This reaction requires the formation of the {114}-faceted surface accommodating Ti ions with double vacancies.40,41 Although recent STM studies show that this surface might be more complex, it is nevertheless ordered.39 It is thus very unlikely that such a surface is present upon sputtering, and this implies that the proposed product (cyclobutanone) is highly unlikely. The cracking patterns of both molecules, cyclopentane and cyclobutanone, are unfortunately very similar (Table 4). Discerning between one or the other is difficult without the use of tracer atoms such as C13 or O.18 Because their desorption is in all cases relatively small, the identification of the product was not judged to be worth further effort. The fact that it tracks the extent of reduction of the surface tends to assign it to the reductive coupling product followed by hydrogenation (cyclopentane). B. ii. XPS. XPS of the Ti(2p) region is shown in Figure 10. Trace A shows the stoichiometric surface for comparison from Figure 5, whereas traces B and C show 60 min of sputtering with hydrogen and argon, respectively. The reduced surface of this single crystal has been studied in detail previously.4,5 In brief, upon sputtering preferential removal of lattice oxygen atoms takes place via momentum transfer, reducing about 3/4 of the Ti4+ XPS signal to lower oxidation states. This is accompanied by the loss of the LEED pattern. Another feature of the surface is its higher uptake for adsorbates. Upon adsorption of glutaric acid at room temperature, an important (40) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; Wiley: Chichester, U.K., reprint 1996. (41) Kieu, L.; Boyd, P.; Idriss, H. J. Mol. Catal. A: Chem. 2001, 176, 117.
Figure 10. XPS of the Ti(2p) region as a function of sputtering conditions on the TiO2(001) surface. (A) Stoichiometric surface, from Figure 5, (B) 60 min of hydrogen sputtering, and (C) 60 min of argon sputtering. (D) Glutaric acid-dosed (saturation coverage) 60-min argon-sputtered surface. Traces are stacked for clarity.
Figure 11. C(1s) of the room-temperature glutaric acid-dosed TiO2(001) 60-min argon-sputtered surface (A) (saturation coverage), (B) heated to 450 K, (C) heated to 550 K (×3), and (D) heated to 750 K.
observation is made (trace D). In the Ti(2p3/2,1/2) regions, there is an increase in the intensity of the Ti4+ and Ti3+ envelopes and a corresponding decrease in the intensity of the contributions from Ti ions in lower oxidation states, approximately 455-456 eV. Thus, glutaric acid adsorption reoxidizes the reduced surface. It is worth indicating that sputtering affects deeper layers (because of H ions penetrating into the oxide material). Ti(2p) lines of Figure 10 represent the surface and near surface (up to 4 to 5 layers) as determined by the escape depth of the Ti(2p) photoelectron. In other words, the change in Ti(2p) lines attributed to Ti atoms in lower oxidation states on the surface is far higher than what is actually seen in Figure 10D. Figure 11A shows XPS C(1s) of the room-temperature glutaric acid-dosed surface after it has been Ar+ sputtered for 60 min. As with the stoichiometric surface, two distinct
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the surface to 550 K, trace C, results in attenuation and broadening of the C(1s) peak at 285.8 eV, whereas a temperature of 750 K, trace D, is necessary to desorb all remaining carbon species from the surface. Figure 12 compares the surface coverage as determined from XPS and TPD. It is clear that in both cases desorption from the reduced surface occurs earlier. This is due to the reaction of the COO groups with the reduced Ti ions and is a result of the destabilization of the remaining adsorption species. The stability of formic acid with respect to that of formaldehyde (as determined by their adsorption energy) on a model (011) rutile TiO2 surface has been computed previously. Formic acid is more stable than formaldehyde by about 0.8 eV.41 By analogy, glutaric acid is more stable then glutaraldehyde on the same surface, and that explains the earlier desorption on the reduced surface. Figure 12. Comparison between the surface coverage as determined by XPS C(1s) total peak areas following glutaric acid adsorption at 300 K to that determined by TPD. Smooth line, TPD; broken lines, XPS.
peaks at 289.5 and 285.8 eV are obtained and attributed to the carboxyl carbon and carbon chain group, respectively. Trace B shows the surface heated to 450 K where the carboxylate peak, 289.5 eV, decreases nearly to the level of the baseline, indicating decomposition (reaction) of the adsorbed glutaric acid species. This is in large contrast to the stoichiometric surface and indicates that the reduced surface reacts differently vis-a`-vis the carboxylic function. The loss of the carboxylic group is concomitant with the broadening of the C(1s) peak at 285.8 eV. Curve fitting gives an extra peak centered at 287 eV. The formation of both the dialcohol and the dialdehyde during TPD tends to attribute this peak to a stable intermediate, most likely a C-O intermediate because alkoxides have their XPS C(1s) at 286-287 eV whereas the aldehydes are usually seen close to 288 eV.40 Heating
IV. Conclusions Glutaric acid reaction on the TiO2(001) single-crystal surfaces has different pathways depending on the surface states. On the stoichiometric surface, decomposition of glutarate species occurs and gives ketene, CO, and propene. This reaction pathway is analogous to maleic anhydride decomposition on the same surface (to ketene, CO, and acetylene).19 On the reduced surface, the reaction pathway has been found to be very different. The main products are those of the reduction to the corresponding dialdehyde and dialcohol with a high selectivity (reaching 80%). In addition, an m/z 70 compound is seen. The selectivity of this product increases with increasing surface reduction. It is attributed to cyclopentane, formed by intramolecular reductive coupling of glutaraldehyde to cyclopentene that is hydrogenated to cyclopentane. Because glutaric acid is manufactured by cylcopentane oxidation, the formation of cyclopentane during TPD can be incorporated within the concept of microreversibility. LA050951W
