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Competition between Acetaldehyde and Crotonaldehyde during Adsorption and Reaction on Anatase and Rutile Titanium Dioxide James E. Rekoske and Mark A. Barteau* Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delware, Newark, Delaware 19716 Received May 1, 1998. In Final Form: December 24, 1998 The adsorption of acetaldehyde and crotonaldehyde on the anatase and rutile polymorphs of TiO2 has been investigated with Fourier transform infrared spectroscopy (FTIR). Chemisorption of acetaldehyde on TiO2 involves a strong interaction between the surface and the carbonyl oxygen, causing a significant shift in the location of the ν(CdO) vibrational mode to lower frequencies; no interaction with surface hydroxyl groups was observed. Capacities for acetaldehyde and crotonaldehyde adsorption under conditions relevant to aldolization reactions were determined in a novel reactor system providing simultaneous mass measurements and mass spectral analysis of gas-phase products. The coverage of acetaldehyde irreversibly adsorbed on TiO2 was similar to values previously reported for the adsorption of alcohols; coverages of crotonaldehyde were approximately 60% of those for acetaldehyde. Both gas-phase and surface analyses indicate that formation of crotonaldehyde by aldol condensation of acetaldehyde occurs on rutile TiO2 at temperatures as low as 313 K. This reaction was not observed on anatase at these conditions; higher temperatures were required. The production of crotonaldehyde on rutile at 313 K diminished with increasing exposure of acetaldehyde. Acetaldehyde and crotonaldehyde adsorbed in a similar fashion on both anatase and rutile, and either aldehyde could displace the other from the surface layer. Accordingly, the surface concentrations of adsorbed acetaldehyde and crotonaldehyde mirror those in the gas phase. Upon heating an adsorbed layer of acetaldehyde, small amounts of ethoxide and acetate species were formed, possibly from a Cannizzaro-type disproportionation reaction. The similarity of these results to those of studies on TiO2 single crystals illustrates the applicability of properly chosen metal oxide single-crystal surfaces as models for polycrystalline powders. Both demonstrate that the chemistry of aldehydes on TiO2 can be successfully explained in terms of the reactions of a few key surface species.
Introduction The extensive scope of oxygenate surface reactions on metal oxides provides a testament to the versatility of these materials as catalysts. Indeed, many catalytic processes exist which occur either directly on or through the intimate involvement of a metal oxide phase; several (e.g., methanol synthesis, automotive exhaust combustion, etc.) are of great practical significance.1-8 The breadth of oxygenate catalysis arises in part from the slate of adsorbed species which may be found on metal oxides. Alcohols typically form surface alkoxides9-11 while carboxylic acids often result in carboxylate formation12-14 via simple dissociative adsorption. Surface intermediates on metal oxides may undergo reduction and/or oxidation reactions to convert the adsorbates between these and (1) Bart, J. C. J.; Sneeded, R. P. A. Catal. Today 1987, 2, 1. (2) Bhattachaharyya, S. K.; Nag, N. K.; Ganguly, N. D. J. Catal. 1971, 23, 158. (3) Campbell, C. T.; Daube, K. A.; White, J. M. Surf. Sci. 1987, 182, 458. (4) Cheng, W.-H. Catal. Lett. 1996, 36, 87. (5) Imai, H.; Murakami, Y.; Irikawa, H. Stud. Surf. Sci. Catal. 1993, 75C, 2003. (6) Hattori, T.; Inoko, J.; Marakami, Y. J. Catal. 1976, 42, 60. (7) Centi, G.; Fornasari, G.; Trifiro, F. J. Catal. 1984, 89, 44. (8) Lashier, M. E.; Moser, T. P.; Schrader, G. L. Stud. Surf. Sci. Catal. 1990, 55, 573. (9) Cunningham, J.; Morrissey, D. J.; Goold, E. L. J. Catal. 1978, 53, 68. (10) Kim, K. S.; Barteau, M. A. Surf. Sci. 1989, 223, 13. (11) Kim, K. S.; Barteau, M. A. J. Mol. Catal. 1990, 63, 103. (12) Kim, K. S.; Barteau, M. A. Langmuir 1990, 6, 1485. (13) Kim, K. S.; Barteau, M. A. J. Catal. 1990, 125, 353. (14) 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; Vol. 327.
other adsorbed forms. Aldehydes and ketones, which are between alcohols and carboxylic acids in their extent of oxidation/reduction, often produce a combination of these surface species.15 Through the application of modern surface sensitive spectroscopies and model catalyst systems, the fate of such intermediates can be more clearly defined. The acid- or base-catalyzed homogeneous-phase aldol condensation of aldehydes and ketones is a reaction of great synthetic utility because of its ability to form C-C bonds.16 Industrial scale syntheses have often focused on exploiting the acid/base properties of metal oxides to effect the heterogeneously catalyzed versions of this chemistry. Examples of heterogeneous catalysis of aldol condensation include2-ethyl-2-hexenalproductionfrombutyraldehyde,17-19 mesityl oxide and isophorone production from acetone,20-23 and acrolein production from acetaldehyde/formaldehyde mixtures.24-26 While heterogeneous catalysis of aldol (15) Idriss, H.; Diagne, C.; Hindermann, J. P.; Kiennemann, A.; Barteau, M. A. J. Catal. 1995, 155, 219. (16) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry. Part A: Structure and Mechanism; Plenum Press: New York, 1990. (17) Zhang, G.; Hattori, H.; Tanabe, K. Bull. Chem. Soc. Jpn. 1989, 62, 2070. (18) Tsuji, H.; Yagi, F.; Hattori, H.; Kita, H. J. Catal. 1994, 148, 759. (19) Swift, H. E.; Bozik, J. E.; Massoth, F. E. J. Catal. 1969, 15, 407. (20) Reichle, W. T. J. Catal. 1980, 63, 295. (21) Reichle, W. T. J. Catal. 1985, 94, 547. (22) Zhang, G.; Hattori, H.; Tanabe, K. React. Kinet. Catal. Lett. 1987, 34, 2. (23) Di Cosimo, J. I.; Diez, V. K.; Apesteguia, C. R. Appl. Catal. A: Gen. 1996, 137, 149. (24) Dumitriu, E.; Bilba, N.; Lupascu, M.; Azzouz, A.; Hulea, V.; Cirje, G.; Nibou, D. J. Catal. 1994, 147, 133. (25) Dumitriu, E.; Hulea, V.; Bilba, N.; Carja, G.; Azzouz, A. J. Mol. Catal. 1993, 79, 175.
10.1021/la9805140 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/20/1999
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condensation imparts the typical benefits associated with product/catalyst separation, a primary disadvantage of solid-catalyzed condensations is catalyst deactivation. Metal oxide-catalyzed condensations generally produce R,β-unsaturated aldehydes and ketones, which can themselves undergo additional condensation reactions. The result may be the formation of large, nonvolatile organic species which deposit on the catalyst surface, blocking active sites and causing deactivation. Understanding the reactive competition between the initial carbonyl reactant and the R,β-unsaturated product is an important issue in eliminating deactivation. This competition begins with the adsorption of reactants and products on the catalyst surface. There have been relatively few investigations of the adsorption of aldehydes on oxide surfaces. Most have concentrated on materials which can be classified as acidic, with emphasis on adsorption modes rather than quantitative coverage determinations. Robinson and Ross27 examined the mode, quantity, and energetics of adsorption of acetaldehyde between 294 and 324 K on silica gels of varying aluminum content. Using infrared spectroscopy, the authors determined that acetaldehyde adsorbs through a strong hydrogen-bonding interaction between surface OH groups and the carbonyl oxygen; no interaction between the methyl group of acetaldehyde and the silica surface was observed. With added aluminum, the adsorption of acetaldehyde was accompanied by condensation reactions. Additional studies have been performed on silica,28 γ-alumina,29 and silica-alumina30 and more recently on several other oxide materials.15,31,32 Each study concluded that the initial adsorption of acetaldehyde occurs in one of two modes: a hydrogen-bonded complex between surface OH groups and the carbonyl oxygen or a weak coordination complex between surface cations and the carbonyl compound. We report here comparisons of the adsorption of acetaldehyde and its R,β-unsaturated condensation product, crotonaldehyde, on oxidized surfaces of anatase and rutile TiO2 powders, studied through Fourier transform infrared spectroscopy (FTIR) and quantitative molecular coverage determinations. Anatase and rutile TiO2, aside from being quite active materials for aldol condensation reactions,33-36 also provide the opportunity to probe the effect of bulk crystal structure on the adsorption and condensation chemistry. Finally, we compare the present results with previous studies of the adsorption and reaction of acetaldehyde on surfaces of TiO2 single crystals; such comparisons are essential to assess the performance of single crystals as models of higher surface area metal oxide catalysts. Experimental Section Apparatus for Quantitative Determination of Molecular Coverage. All adsorption measurements were performed in a novel flow-through microbalance reactor which has been de(26) Ai, M. Appl. Catal. 1991, 77, 123. (27) Robinson, E.; Ross, R. A., J. Chem. Soc. (A) 1968, 2137. (28) Young, R. P.; Sheppard, N. J. Catal. 1967, 7, 223. (29) Sokolskii, D. V.; Vozdvizhenskii, V. F.; Kuanyshev, A. S.; Kobets, A. V. React. Kinet. Catal. Lett. 1976, 5, 163. (30) Fabbri, G.; Gusmundo, F. Ann. Chim. (Rome) 1962, 52, 1327. (31) Ivanov, V. A.; Bachelier, J.; Audry, F.; Lavalley, J. C. J. Mol. Catal. 1994, 91, 45. (32) Li, C.; Domen, K.; Maruya, K.-I.; Onishi, T. J. Catal. 1990, 125, 445. (33) Idriss, H.; Barteau, M. A. Catal. Lett. 1996, 40, 147. (34) Idriss, H.; Kim, K. S.; Barteau, M. A. J. Catal. 1993, 139, 119. (35) Idriss, H.; Libby, M.; Barteau, M. A. Catal. Lett. 1992, 15, 13. (36) Rekoske, J. E., The Kinetics and Selectivity of Oxygenate Transformations on Oxidized and Reduced Metal Oxides. Ph.D. Thesis, University of Delaware, 1998.
Rekoske and Barteau scribed in detail previously.37,38 The mass measurement device (TEOM 1500, Rupprecht and Patashnick Co., Albany, NY) utilizes inertial principles to provide microgram resolution of catalyst mass changes, rapid temporal response, and a flow pattern analogous to that for conventional down-flow microreactors. All flows within the upstream gas-handling manifold were controlled by electronic mass flow controllers (Tylan FC-260). The temperatures of the preheat zone and the catalyst were controlled by a two-zone electric furnace built into the TEOM 1500. The reactor effluent was connected both to a gas chromatograph (HP 5890 Series II+) equipped with both thermal conductivity (TCD) and flame ionization (FID) detectors and to a quadrupole mass spectrometer (UTI 100C). Analyses were performed either in stand-alone gas chromatography (GC) and mass spectrometry (MS) modes or in a combined GC/MS operation. All lines, both prior to and following the reactor zone, were heat traced to 413 K to eliminate the condensation within these lines of feed or product species. Fourier Transform Infrared Experiments. The adsorption and reaction of acetaldehyde and crotonaldehyde on anatase and rutile TiO2 were investigated by transmission-mode Fourier transform infrared (FTIR) spectroscopy; a Nicolet Model 510M FTIR spectrometer connected to an all Pyrex gas-handling manifold was utilized. Samples of the rutile and anatase TiO2 powders (described below) were pressed at 5 tons in-2 into selfsupporting disks less than 0.1 mm thick and mounted in a quartz cell attached to the gas-handling system. The manifold consisted of two identical but separate gas-handling systems which could be evacuated to less than 1 × 10-6 Torr. One side of the manifold was connected to a gas flow system equipped with mass flow control valves. The manifold was also equipped with an ion gauge for background pressure measurement and two MKS Baratron capacitance manometer pressure gauges, with ranges to 1 and 100 Torr. The cell, capable of operating at temperatures from ambient to over 900 K, was equipped with calcium fluoride windows and could be evacuated to a base pressure of 4 × 10-5 Torr. All spectra reported, recorded at 4 cm-1 resolution in the full spectral range allowed by transmission through TiO2 (typically 1000-4000 cm-1), are the result of 512 coadded scans and are ratioed to either the pretreated TiO2 material or the empty cell at background pressure, as appropriate. All spectra were recorded after evacuation to a base pressure of at least 1 × 10-4 Torr for 1 h, unless otherwise indicated. Materials. Acetaldehyde (Aldrich, 99.5%) and crotonaldehyde (Aldrich, 99%) were subjected to at least four freeze-pumpthaw cycles prior to use in either uptake or infrared investigations. The purity of each was verified prior to each use by combined GC/MS. In uptake experiments, the aldehyde was fed to the microbalance by saturation of a stream of helium carrier (Matheson, 99.999%) in a constant-temperature saturator (MGW/ Lauda RM6 thermostated bath). The partial pressure of the feed was determined by GC analysis using several quantitative standards (Scott Specialty Gas) ranging from 1 to 400 Torr. In infrared investigations, two aldehyde delivery methods were utilized. In flow-mode experiments, the acetaldehyde was fed by saturation of helium in a manner similar to that for the uptake measurements. In the majority of the experiments (static mode), acetaldehyde or crotonaldehyde was kept immersed in the thermostated bath and expanded into an evacuated, heat-traced Pyrex manifold and then exposed to the self-supporting disk. Two commercially available titanium dioxide samples were used in this investigation: an anatase TiO2 obtained from Aldrich (99.9+%, BET surface 10 m2/g) and a rutile TiO2 obtained from Alfa-Aesar (Puratronic grade, 99.995%, BET surface: 2 m2/g). Prior to use, these materials were identically pretreated in situ for both uptake and infrared measurements. The materials were initially subjected to oxidation at 673 K in a flow of 50 cm3 min-1 ultrahigh-purity oxygen (Matheson) for 12 h, followed by a calcination period of 2 h in 30 cm3 min-1 flowing helium (99.999%, Matheson), also at 673 K. (37) Rekoske, J. E.; Barteau, M. A. J. Phys. Chem. B 1997, 101, 1113. (38) Rekoske, J. E.; Barteau, M. A. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1995, 40, 187.
Adsorption on Anatase and Rutile Titanium Dioxide
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Figure 1. Uptake determination and dynamics for the adsorption of aldehydes on oxidized TiO2: (a) acetaldehyde on anatase; (b) acetaldehyde on rutile; (c) crotonaldehyde on anatase; (d) crotonaldehyde on rutile.
Results Uptake of Acetaldehyde and Crotonaldehyde on Oxidized TiO2. The coverage of acetaldehyde or crotonaldehyde adsorbed at saturation is dependent upon the bulk crystal structure of TiO2. Figure 1 shows the adsorption dynamics for both anatase and rutile by monitoring the transient mass change of the TiO2 catalyst with aldehyde exposure at 313 K. Each catalyst was oxidized and calcined using the standard method described previously, and the baseline was established with the catalyst in the oxidized form, in flowing helium at 313 K. After 30 min of baseline mass measurement, the adsorption of the aldehyde was begun. After exposure at the indicated partial pressure at a flow rate of 30 cm3 min-1 for 30 min, the catalyst was purged until a stable mass was reached; the purge period required to reach a stable mass never exceeded 40 min. The saturation coverage was determined from the net mass gain remaining after this purge period. The coverage values per unit surface area are listed on the right-hand axes in Figure 1. For acetaldehyde, values of 2.2 and 3.4 molecules nm-2 were determined on anatase and rutile, respectively, while values for crotonaldehyde were 1.3 and 2.0 molecules nm-2 for anatase and rutile, respectively. Thus, the saturation coverage per unit area was over 50% greater for the rutile surface than for anatase for both aldehydes, and the crotonaldehyde coverage was about 60% of the values determined for acetaldehyde. The observed rutile-toanatase molecular coverage ratio for both aldehydes of approximately 1.5 is in agreement with the known cation densities of the predominant low-index planes for these
two polymorphs.39,40 The values obtained were nearly independent of the partial pressure of exposure, suggesting the surfaces were indeed saturated. This is also indicated by the stabilization of the catalyst mass during the adsorption process, as the catalyst mass was essentially constant after the first 15 min of exposure. During the adsorption process, the gas-phase effluent of the reactor was also monitored by mass spectrometry and gas chromatography. Adsorption of crotonaldehyde produced no volatile products from either surface. For adsorption of acetaldehyde on anatase, no species other than helium, acetaldehyde, and traces of water were observed. For acetaldehyde adsorption on the rutile surface at 313 K, however, significant generation of crotonaldehyde was observed. The mass spectral traces for acetaldehyde and crotonaldehyde during adsorption at 313 K and 151.5 Torr are shown in Figure 2. A standard analysis of the integrated intensities of these signals39 indicates that, in addition to the 3.4 molecules nm-2 saturation coverage of acetaldehyde as determined in Figure 1b, approximately 1.3 molecules nm-2 of acetaldehyde were converted to crotonaldehyde during the adsorption process. Water desorption was also observed (by mass spectrometry) during the adsorption process; however, the amount of water desorbed was only about 25% of the stoichiometric amount expected for the amount of crotonaldehyde formed. (39) Lusvardi, V. S.; Barteau, M. A.; Farneth, W. E. J. Catal. 1995, 153, 41. (40) Rekoske, J. E.; Barteau, M. A. J. Catal. 1997, 165, 57.
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Figure 2. Gas-phase species observed upon adsorption and reaction of acetaldehyde on the oxidized surface of rutile TiO2 at 313 K. Acetaldehyde partial pressure is 151.5 Torr. The signals correspond to the mass spectrometer traces for m/e ) 43 (acetaldehyde) and m/e ) 70 (crotonaldehyde). Table 1. Vibrational Assignments for Gas-Phase and Adsorbed Acetaldehyde frequency (cm-1) mode νas(CH3) νs(CH3) 2ν6(A′) resonance ν(CH) ν(CdO) δ(CH3), δ(CH)
gas
phase64
2967 2926 2840 2736 1735 1352
SiO228 anatase TiO2 rutile TiO2 2974 2924 2852 2762 1724 1352
2969 2914 2846 2759 1718 1355
2963 2915 2848 2759 1703, 1725 1348
FTIR Spectra following Acetaldehyde Adsorption on Oxidized TiO2. The static adsorption of acetaldehyde on oxidized anatase TiO2 at 313 K proceeded to saturation in under 60 min at an initial partial pressure of 10 Torr, producing a surface-coordinated form of acetaldehyde and no other detectable adsorbed species. Figure 3 presents a series of FTIR spectra of the oxidized anatase TiO2 surface after exposure to 10 Torr of acetaldehyde for differing lengths of time followed by evacuation to 1 × 10-4 Torr; no change in the spectrum was observable after 60 min of exposure at 10 Torr. At low exposure (5 min), absorbance bands were observed at 2969, 2914, 2759, 1718, and 1342 cm-1, and through comparison to literature spectra 28,41 (see Table 1) are assigned to the νas(CH3), νs(CH3), ν(CH), ν(CdO), and δs(CH3) modes of adsorbed acetaldehyde, respectively. With increasing exposure, a sharp band developed at 1355 cm-1, a slightly higher frequency than that of the original δ(CH3) mode. While this new feature continued to increase in intensity with increasing exposure up to 60 min, the original band at 1342 cm-1 did not increase after the appearance of this new feature. In general, no other shifts in band position were observed with increasing exposure, though the bands within the C-H stretching region appeared to sharpen (41) Sullivan, D. L.; Roark, R. D.; Ekerdt, J. G.; Deutsch, S.; Gates, B. C. J. Phys. Chem. 1995, 99, 3678.
slightly. Further, no bands other than those which could be ascribed to adsorbed acetaldehyde were observed at any exposure. Adsorption of acetaldehyde at 10 Torr and 313 K on oxidized rutile TiO2 was more complex. Figure 4 shows FTIR spectra, analogous to those of Figure 3, of the adsorbed phase formed upon static exposure of oxidized rutile TiO2 to acetaldehyde at 313 K. Exposures were performed for differing lengths of time followed by evacuation to the base pressure. As was observed for adsorption on anatase, the FTIR spectrum of the adsorbed phase on rutile TiO2 remained unchanged after 60 min of exposure at an initial partial pressure of 10 Torr. In contrast to the spectra of the adsorbed phase on anatase, however, significant changes between 5 and 60 min of exposure were observed for acetaldehyde on rutile. Numerous absorbance bands were observed at low exposure. The C-H stretch region of the infrared spectrum shows bands at 3035, 2950, 2915, 2905, 2759, and 2740 cm-1, while features at 1703, 1656, 1602, 1406, 1285, and 1156 cm-1 were apparent at low frequencies. When the exposure of acetaldehyde was increased to 15 min, the intensities of the features at 3035, 2905, 1656, 1602, 1406, and 1156 cm-1 decreased, while those of the features at 2950, 2915, 2759, 1703, and 1285 cm-1 increased. Further increasing the exposure to 30 min completely eliminated the bands at 3035, 2905, 2740, 1656, 1602, 1406, and 1156 cm-1, while the features at 2950, 2915, and 2759 cm-1 continued to increase in intensity. These changes were accompanied by a slight broadening of the absorbance band at 1703 cm-1 and the appearance of a new feature at 1348 cm-1. Meanwhile, the intensity of the band at 1285 cm-1 remained unchanged upon increasing the exposure from 15 to 30 min. All remaining bands continued to increase with increasing exposure up to 60 min, with the exception of the features at 1703 and 1285 cm-1, which were saturated. Additionally, a new absorbance feature became apparent at 1735 cm-1; the growth of this band was most likely responsible for the previously mentioned broadening of the 1703 cm-1 feature. The resulting spectrum of the adsorbed layer on oxidized rutile TiO2 after adsorption of acetaldehyde to saturation (spectrum d, Figure 4) contained features at 2963, 2915, 2759, 1735, 1703, 1348, and 1285 cm-1. FTIR Spectra of Crotonaldehyde Adsorption on Oxidized TiO2. As shown by the mass spectrometry results in Figure 2, adsorption of acetaldehyde at 313 K and 151.5 Torr on oxidized rutile TiO2 resulted in the formation of crotonaldehyde during acetaldehyde uptake. Accordingly, we examined the adsorption of crotonaldehyde on the oxidized surfaces of anatase and rutile TiO2 to (1) assist the assignment of absorbance bands for acetaldehyde adsorption on rutile at 313 K and (2) confirm the absence of bands associated with crotonaldehyde following adsorption of acetaldehyde on oxidized anatase at this temperature. Spectra a and b of Figure 5 show the adsorption of crotonaldehyde on oxidized anatase at an initial partial pressure of 10 Torr and 313 K and exposures of 5 and 60 min, respectively. Absorbance bands at 3038, 2979, 2954, 2920, 2845, 2745, 1686, 1636, 1394, and 1165 cm-1 were observed, with additional shoulders at 1577 and 1105 cm-1. The position of the bands was independent of the length of exposure, and all bands were saturated by exposure to 10 Torr of crotonaldehyde for 60 min. These bands were all assigned to various vibrational modes of crotonaldehyde by comparison to literature spectra;28 the assignments are given in Table 2. Though significant overlap of much of the C-H stretch region existed between crotonaldehyde
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Figure 3. FTIR spectra of the adsorbed phase following the exposure of the oxidized surface of anatase TiO2 at 313 K to 10 Torr of acetaldehyde for (a) 5, (b) 15, (c) 30, and (d) 60 min. Samples were evacuated to 1 × 10-4 Torr prior to collecting spectra.
Figure 4. FTIR spectra of the adsorbed phase following the exposure of the oxidized surface of rutile TiO2 at 313 K to 10 Torr of acetaldehyde for (a) 5, (b) 15, (c) 30, and (d) 60 min. Samples were evacuated to 1 × 10-4 Torr prior to collecting spectra.
and acetaldehyde, the strong bands at 1686, 1636, 1394, and 1165 cm-1 were indicative of crotonaldehyde adsorbed on anatase. None of these bands were observed in the spectra of Figure 3, indicating the absence of crotonaldehyde in the adsorbed layer on oxidized anatase following acetaldehyde exposure at 313 K. Similar experiments were performed for adsorption of crotonaldehyde on the surface of oxidized rutile; the FTIR spectra from 5 and 60 min exposures at 313 K and an initial partial pressure of 10 Torr are shown in spectra a and b of Figure 6, respectively. Initial exposure resulted in the appearance of resolved bands at 3036, 2952, 2907,
2831, 2738, 1664, 1605, 1401, 1151, and 1098 cm-1, with a shoulder at 1559 cm-1. As for anatase, prolonged exposure did not produce any observable shift in the absorbance features, and all bands were saturated by exposure to 10 Torr of crotonaldehyde for 60 min. Again, these bands can be ascribed to the various modes of adsorbed crotonaldehyde, as illustrated in Table 2. In contrast to the case for anatase TiO2, the adsorption modes of crotonaldehyde on oxidized rutile match nicely with many vibrational modes observed on rutile following low exposures of acetaldehyde. Therefore, we may ascribe the absorbance bands at 2950, 2905, 2830, 2740,
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Figure 5. FTIR spectra of the adsorbed phase following the exposure of the oxidized surface of anatase TiO2 at 313 K to (a) 10 Torr of crotonaldehyde for 5 min and (b) 10 Torr of crotonaldehyde for 60 min. Spectrum c was collected after exposure of the sample in spectrum b to 10 Torr of acetaldehyde for 60 min. Samples were evacuated to 1 × 10-4 Torr prior to collecting spectra. Table 2. Vibrational Assignments for Gas-Phase and Adsorbed Crotonaldehyde frequency (cm-1) mode ν(CdCsH) νas(CH3) νs(CH3) resonance ν(CH) ν(CdO) ν(CdC) δ(CH3) F(CH3), ν(CsC)
gas phase66 SiO228 anatase TiO2 rutile TiO2 3058 2963 2938 2728 1720 1649 1391 1147
3042 2951 2924 2850 2744 1688 1642
3038 2954 2920 2845 2745 1686 1636 1394 1165
3036 2950 2905 2830 2740 1656 1602 1406 1156
1656, 1602, 1406, and 1156 cm-1 of spectra a and b in Figure 4 to the vibrational modes of adsorbed crotonaldehyde. To assess the relative stability of the adsorbed layers formed by exposure to acetaldehyde and crotonaldehyde, sequential adsorption experiments were performed. Oxidized anatase and rutile TiO2 were exposed to crotonaldehyde to saturation, and the FTIR spectra collected were as shown in spectrum b of Figures 5 and 6, respectively, as previously discussed. These surfaces were then exposed to 10 Torr of acetaldehyde for 60 min, and the FTIR spectra were collected following evacuation. The results are presented in spectrum c of Figures 5 and 6 for anatase and rutile TiO2, respectively. In both cases, the spectra reveal only adsorbed acetaldehyde; spectra of both surfaces are completely analogous to those previously obtained for saturation exposure of acetaldehyde on the clean surfaces of anatase and rutile. This is verified by comparing spectrum c in Figures 5 and 6 to spectrum d of Figures 3 and 4, respectively. When either anatase or rutile as prepared in spectrum c of Figures 5 and 6 was once again exposed to 10 Torr of crotonaldehyde for 60 min, the spectra closely resembled those of spectrum a in Figures 5 and 6, respectively. Thus, acetaldehyde and crotonaldehyde were capable of displacing each other from the adsorbed layer
on both anatase and rutile TiO2, and the adlayer composition is thus a reflection of the gas phase to which the surface has been most recently exposed. This result explains the transient nature of the adsorbed crotonaldehyde vibrational modes on rutile observed in Figure 4. During prolonged exposure, surface deactivation is also observed, since crotonaldehyde production ceases and further exposure to acetaldehyde generates a pure acetaldehyde adsorbed layer. Comparison of Surface Layers Resulting from Different Adsorption Conditions. While uptake measurements were carried out in a flow mode with no bypass, the infrared studies used a static system to effect adsorption onto the TiO2 surfaces. To determine what impact, if any, this difference may have on the adsorbed layer formed upon exposure of oxidized TiO2 to acetaldehyde, we collected a limited number of FTIR spectra in which the adsorption of acetaldehyde was performed in a flowing stream of helium. The experimental procedure used in these FTIR experiments was identical to that utilized in the uptake measurements. An initial helium purge was conducted at 313 K for 30 min, followed by exposure to 151.5 Torr of acetaldehyde in flowing helium for 30 min and completed with a purge period of 40 min, also in helium at 313 K. The spectra were then collected in flowing helium. Thus, the state of the TiO2 surface was as similar to that obtained in the uptake measurements as is reasonably possible. FTIR spectra of the adsorbed layer resulting from this procedure for acetaldehyde exposure to oxidized anatase and rutile TiO2 surfaces are shown in spectra a and c of Figure 7, respectively. Both qualitatively and quantitatively, these spectra are quite similar to those obtained in the static adsorption experiments previously discussed, as illustrated by comparing spectra a and c in Figure 7 with spectrum d in Figures 3 and 4, respectively. Thus, the mode of adsorption and/or removal of weakly adsorbed species has no noticeable effect on the content of the adsorbed layer resulting from the saturation exposure of acetaldehyde to anatase and rutile TiO2.
Adsorption on Anatase and Rutile Titanium Dioxide
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Figure 6. FTIR spectra of the adsorbed phase following the exposure of the oxidized surface of rutile TiO2 at 313 K to (a) 10 Torr of crotonaldehyde for 5 min and (b) 10 Torr of crotonaldehyde for 60 min. Spectrum c was collected after exposure of the sample in spectrum b to 10 Torr of acetaldehyde for 60 min. Samples were evacuated to 1 × 10-4 Torr prior to collecting spectra.
Figure 7. FTIR spectra of the adsorbed phase following the exposure of oxidized TiO2 at 313 K to acetaldehyde at 151.5 Torr via saturation in 30 cm3 min-1 helium: (a) oxidized anatase after a 40 min purge; (b) surface in part a ramped to 373 K and cooled to 313 K in helium; (c) oxidized rutile after a 40 min purge; (d) surface in part c ramped to 373 K and cooled to 313 K in helium.
Finally, the suggestion that crotonaldehyde formation occurs by reaction of acetaldehyde molecules within the adsorbed phase was also examined. Acetaldehyde adsorbed on the surface of anatase and rutile, as prepared in spectra a and c, respectively, of Figure 7, was heated at a rate of 10/min to 373 K and subsequently cooled to 313 K under flowing helium. Spectra b and d in Figure 7 show the FTIR spectra of the resulting adsorbed layers for anatase and rutile, respectively. While adsorbed acetaldehyde is still clearly present in the adsorbed layer on both anatase and rutile, additional species have also
developed. One of these additional species can be easily identified as crotonaldehyde by comparing the absorbance bands in spectra b and d of Figure 7 with those of the previously reported spectra for adsorbed crotonaldehyde in Figures 5 and 6 as well as the frequencies reported in Table 2. After the presence of acetaldehyde and crotonaldehyde is accounted for, unassigned bands remain at 1562, 1418, 1376, and 1063 cm-1 on anatase and at 1544, 1421, 1383, and 1062 cm-1 on rutile. These bands may be ascribed to the presence of ethoxy and acetate species with the assignments as indicated in Table 3. Only very
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Table 3. Vibrational Assignments for Adsorbed Acetate and Ethoxy Species on Metal Oxides frequency (cm-1) mode acetate νas(COO) νs(COO) ethoxy δs(CH3) ν(C-O)
15
rutile TiO251
anatase TiO2
rutile TiO2
1559 1423
1550 1440
1562 1418
1544 1421
1376 1067
1380
1376 1063
1383 1062
CeO2
small amounts of these species are observed, with the dominant fraction of the surface being occupied by either acetaldehyde or crotonaldehyde. Discussion Adsorption of acetaldehyde onto the anatase and rutile forms of TiO2 proceeds through a rich set of processes, complete with multiple adsorbed forms, transient surface species, and shifts of band positions with changing exposure. However, much of the surface chemistry of acetaldehyde can be explained in terms of a few simple surface species and the interaction among them. Adsorption occurs via interaction between the carbonyl oxygen and the surface, as indicated by the shift toward lower frequency of the carbonyl CdO vibrational mode in Figures 3 and 4. This red-shift, and thus the O-surface interaction, is strongest for low exposures of acetaldehyde on rutile TiO2, with the ν(CdO) band being shifted to lower frequencies by 25-30 cm-1 when compared to that for gas-phase acetaldehyde.28,30,41 The magnitude of this shift is indicative of a moderate O-surface interaction42-44 and a small loss in CdO double-bond character. At saturation exposures, one adsorbed form of acetaldehyde is present on the surface of anatase while two distinct forms are observed for adsorption on rutile. Two separate forms of adsorbed acetaldehyde, with relative populations of these forms varying with coverage, have also been reported previously for vanadia-titania catalysts.45 From comparison of Figures 3 and 4, it is clear that the adsorbed form which exists only after saturation exposures on rutile is similar to the form which is present at all exposures on anatase TiO2. This species is characterized by bands at 1720-1730 cm-1 for the ν(CdO) mode and 1348-1355 cm-1 for the δ(CH3) mode of adsorbed acetaldehyde. As indicated by the smaller red-shift of the ν(CdO) mode with respect to that for gas-phase acetaldehyde, this form involves loss of less of the CdO doublebond character than observed for adsorption on rutile TiO2 at low exposure (spectrum b, Figure 4). Adsorption on rutile TiO2 initially proceeds through a form exhibiting stronger interactions with the surface; this species saturates, and another more weakly bound form occurs. The C-H stretch region of the adsorbed layer on both anatase and rutile at all coverages indicates little or no interaction between the surface and the carbons of acetaldehyde. Similar conclusions have been reached previously for the adsorption of acetaldehyde on metal oxides.28-30 While the C-H stretches are unaffected, the δ(CH3) mode is perturbed upon adsorption of acetaldehyde, especially at low exposures on rutile TiO2 (spectrum a, Figure 4). The mode is shifted to lower frequencies and is broadened from its characteristic sharp appearance;44 (42) Bell, A. T. ACS Symp. Ser. 1980, 137, 13. (43) Hair, M. L. ACS Symp. Ser. 1980, 137, 1. (44) Little, L. H. Infrared Spectra of Adsorbed Species; Academic: New York, 1966. (45) Sanchez Escribano, V.; Busca, G.; Lorenzelli, V. J. Phys. Chem. 1990, 94, 8945.
it sharpens with increasing exposure as the more weakly bound state is populated. Previous investigations of the adsorption of acetaldehyde on the surfaces of silica,27,28 alumina,27,29 and silicaalumina catalysts30 have also demonstrated the existence of a strongly bound form of acetaldehyde. This species generally retains vibrational modes similar to that of gasphase acetaldehyde, though it cannot be removed by prolonged evacuation near room temperature.29 On these materials this state has been attributed to a strong hydrogen-bonding interaction between surface hydroxyl groups and the carbonyl oxygen of acetaldehyde.27,28 Although the materials above are associated with acidcatalyzed reactions, such interactions are possible on all metal oxides containing hydroxyl groups. On TiO2 however, strong hydrogen bonding does not appear to be the primary means of aldehyde adsorption, as discussed below. The anatase and rutile TiO2 powders used in the present investigation showed no indications of either isolated or hydrogen-bonded hydroxyl groups after being subjected to the standard pretreatment procedure described above. This is consistent with several previous observations of the dehydroxylation of TiO2 at 673 K.46-48 Further, upon adsorption of acetaldehyde, no change in the OH stretch region of the infrared spectrum was observed. This is in spite of the fact that acetaldehyde was strongly bound on these surfaces; evacuation to less than 1 × 10-4 Torr for more than 1 h at 313 K could not remove the features shown in Figures 3 and 4. Further, earlier studies28-30 have reported a ν(CdO) red-shift of 5-10 cm-1 on other metal oxides, far less than the 25-30 cm-1 observed in the present investigation. Recently, Escribano et al.45 studied the adsorption of several C2 oxygenates on the surface of a vanadia-titania material. These authors observed a significant shift of the ν(CdO) band to lower frequencies upon adsorption of acetaldehyde; this feature occurs at 1682 cm-1 in the adsorbed form, compared to 1746 cm-1 in the gas phase. This shift was attributed to significant coordination of acetaldehyde to the surface cations through one of the lone pairs on the carbonyl oxygen. Further, no perturbation of the surface hydroxyl groups was observed upon the adsorption of acetaldehyde. Thus it appears that transition-metal oxides (e.g., vanadia, titania, etc.) may coordinate aldehydes more strongly through the carbonyl oxygen than materials such as silica and alumina, and do so without requiring hydroxyl interactions. Though the downward shift observed by Escribano et al.45 on vanadia-titania is larger than that in the present investigation on pure titania, the basic conclusion remains. As illustrated by the similarities between spectra a and c of Figure 7 and spectrum d in Figures 3 and 4, respectively, the methods of adsorption (static or flow) and removal of weakly bound species (evacuation or helium purge) do not substantially affect the composition of the adsorbed layer on anatase and rutile TiO2. Thus the acetaldehyde uptake measurements can be directly related to the FTIR spectra of the adsorbed phase. Since these spectra show no signs of any adsorbed species other than acetaldehyde at saturation coverage, the uptake measurements reported in Figure 1 are representative of the acetaldehyde adsorption capacity of TiO2. Since the primary mode of acetaldehyde adsorption on TiO2 is through the oxygen of the carbonyl group, (46) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1216. (47) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1221. (48) Suda, Y.; Morimoto, T.; Nagao, M. Langmuir 1987, 3, 99.
Adsorption on Anatase and Rutile Titanium Dioxide
Langmuir, Vol. 15, No. 6, 1999 2069
Table 4. Comparison of Acetaldehyde and Ethanol Molecular Coverages on TiO2 (molecules nm-2) anatase
rutile
ref 39 ref 52 ref 65 acetaldehyde 2.2 crotonaldehyde 1.3 ethanol
ref 39 ref 48 3.4 2.0
2.89
3.03
2.25
4.56
3.75
similarities to the adsorption of other oxygen-containing probe molecules may be expected. Alcohols, commonly used probe molecules for adsorption9,49-53 and reaction54-60 on the surfaces of metal oxides, may adsorb on TiO2 dissociatively, resulting in the formation of surface-bound alkoxide fragments.50-52,58 Interestingly, the chemisorption capacities of acetaldehyde and ethanol on oxidized TiO2 are quite similar, as indicated by the data in Table 4. Ethanol molecular coverages are larger than those of acetaldehyde, but only by about 30-40%. Further, the ratio of the alcohol coverage on rutile to that on anatase was found to be approximately 1.6.39 A rutile/anatase coverage ratio of 1.5 was observed for acetaldehyde in the present study, again consistent with the known difference of the cation densities of the low-index surfaces of these two TiO2 polymorphs.39 While the infrared spectra indicate ethoxide species are not formed upon adsorption of acetaldehyde on oxidized TiO2 at 313 K, the surface interactions with acetaldehyde and ethanol are similar from the viewpoint of surface capacity. The formation of adsorbed crotonaldehyde upon limited exposure of oxidized rutile TiO2 to acetaldehyde is apparent when spectrum a of Figure 4 is compared to the spectrum for adsorbed crotonaldehyde (spectrum b, Figure 6). In general, the positions of all bands for crotonaldehyde adsorbed on rutile were similar to those for adsorption on anatase; further, these bands match conclusively with those features observed after low exposure of acetaldehyde to oxidized rutile. The ν(CdO) and ν(CdC) vibrational modes for crotonaldehyde on rutile, however, are significantly red-shifted compared to those for adsorption on anatase, appearing well below their expected frequencies from gas-phase spectra.15,27,41,61 As was observed for acetaldehyde, these results indicate a stronger interaction between crotonaldehyde and rutile than between crotonaldehyde and anatase. Previous results33,36 have conclusively demonstrated that the surface of oxidized rutile TiO2 contains sites which are capable of initiating the aldol condensation of acetaldehyde to crotonaldehyde at relatively low temperatures. This is confirmed by the appearance of crotonaldehyde in the gas phase during the adsorption of acetaldehyde at 313 K on rutile TiO2, demonstrated in Figure 2. However, (49) Dai, Q.; Gellman, A. J. Surf. Sci. 1991, 257, 103. (50) Gamble, L.; Jung, L. S.; Campbell, C. T. Surf. Sci. 1996, 348, 1. (51) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans. 1991, 87, 2661. (52) Kim, K. S.; Barteau, M. A.; Farneth, W. E. Langmuir 1988, 4, 533. (53) Zaki, M. I.; Sheppard, N. J. Catal. 1983, 80, 114. (54) Cunningham, J.; Hodnett, B. K.; Ilyas, M.; Tobin, J.; Leahy, E. L.; Fierro, J. L. G. Faraday Discuss. 1981, 72, 283. (55) Davis, B. H. In Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G. C., Eds.; Elsevier: Amsterdam, 1985; Vol. 309. (56) Freidlin, L. K.; Sharf, V. Z.; Abdumavlyanova, V. S. Izv. Akad. Nauk SSSR, Ser. Khim. 1971, 10, 2308. (57) Gercher, V. A.; Cox, D. F. J. Phys. Chem., submitted. (58) Groff, R. P.; Manogue, W. H. J. Catal. 1984, 87, 461. (59) Larson, S. A.; Widegren, J. A.; Falconer, J. L. J. Catal. 1995, 157, 611. (60) Mostafa, M. R.; Youssef, A. M.; Hassan, S. M. Mater. Lett. 1991, 12, 207. (61) Klaassen, A. W.; Hill, C. G., J. Catal. 1981, 69, 299.
no vibrational features associated with adsorbed crotonaldehyde were observed after continued exposure of the rutile surface to acetaldehyde (spectrum c, Figure 6). Also, the desorption rate of crotonaldehyde into the gas phase ceased to be measurable after a short period of time. Both results suggest a termination of the reaction. Repeated sequential dosing cycles with acetaldehyde and crotonaldehyde were performed, as shown in Figures 5 and 6, to attempt to elucidate the reason for the activity loss at 313 K. These results illustrate the ability of either aldehyde to displace the other from the surface upon saturation exposure. Thus, adsorption of the two aldehydes occurs in a competitive fashion on similar surface sites of both anatase and rutile. Competition alone, however, could not cause the observed transient surface compositions, as crotonaldehyde is formed via reaction of adsorbed acetaldehyde,34-36 which is always present after exposure of any length. It is likely that the loss of adsorbed crotonaldehyde may indicate the sites on rutile TiO2 active for aldol chemistry at 313 K are limited in number and not capable of “turnover”, at this temperature. These may be poisoned by retention of reaction products such as OH, H2O, or the higher molecular products of multiple aldolization events. However, none of these are revealed by the infrared spectra. Upon heating to 373 K, adsorbed acetaldehyde is converted into crotonaldehyde on both anatase and rutile surfaces, as shown in Figure 7. The relative amounts of adsorbed acetaldehyde converted on the two surfaces are nearly equal, with a slightly higher activity observed on rutile compared to anatase. Since aldolization occurs at comparable rates on anatase and rutile at 373 K, yet rutile is significantly more active than anatase at 313 K, the activation energy on anatase must be slightly larger than that on rutile in this temperature range. This result is consistent with our observations showing a slightly higher activation energy (37 vs 34 kJ mol-1) on anatase in steadystate aldolization studies36 as well as a higher peak temperature for crotonaldehyde (400 K on anatase versus 380 K on rutile) in temperature programmed desorption experiments for acetaldehyde on anatase and rutile.34,35,62 Adsorption of aldehydes at room temperature on several metal oxides, particularly CeO2 and Al2O3, results primarily in the formation of reduced and oxidized surface species. The absence from the FTIR spectra of features associated with acetate and ethoxide species on acetaldehyde adsorption on TiO2 is noteworthy, as it illustrates the relatively low rates of adsorbate oxidation and reduction, respectively, on TiO2.34,58,63 This result also demonstrates that the Cannizzaro reaction of adsorbed acetaldehyde is much less favorable than aldol condensation, consistent with the results of temperature-programmed desorption experiments on TiO2 rutile single crystals and polycrystalline anatase.33,34 Upon heating an adsorbed layer of acetaldehyde on either anatase or rutile, aldol condensation and desorption predominate; only small amounts of oxidation and reduction products are observed. Similar results have been reported for the adsorption of acetaldehyde on the {114}-faceted (001) surface of TiO2;34 oxidation and reduction were minor reaction pathways on this surface, while aldolization was extensive. Previous work13,34 has suggested the {114}-faceted surface of TiO2, (62) Rekoske, J. E.; Barteau, M. A. in preparation. (63) Groff, R. P.; Manogue, W. H. J. Catal. 1983, 79, 462. (64) Hollenstein, H.; Gu¨nthard, H. H. Spectrochim. Acta 1971, 27A, 2027. (65) Carrizosa, I.; Munuera, G. J. Catal. 1977, 49, 174. (66) Oelichmann, H.-J.; Bougeard, D.; Schrader, B. J. Mol. Struct. 1981, 77, 179.
2070 Langmuir, Vol. 15, No. 6, 1999
which contains a fairly broad range of coordination environments, provides a good model for high-surfacearea polycrystalline materials; the present results reinforce this conclusion. Conclusions The chemistry of acetaldehyde on rutile and anatase powders was found to be quite similar to that observed previously on the {114}-faceted rutile TiO2(001) surface. Both anatase and rutile are active for the aldolization of aldehydes at 373 K. However, only a small fraction of the surface sites on rutile are capable of carrying out aldolization at 313 K, while anatase is inactive at this temperature. Crotonaldehyde formed from the aldolization reaction on rutile is readily displaced through continuing
Rekoske and Barteau
exposure to acetaldehyde. Activity increases dramatically at slightly elevated temperatures, with the relative activity on anatase and rutile surfaces becoming comparable by 373 K. This suggests a slightly lower activation energy for aldolization on rutile; adsorption at 313 K falls within the proper temperature window to produce activity on rutile but not on anatase. The rich chemistry of carbonyl compounds on these materials can easily be related to the reactivity and stability of a small number of common adsorbed intermediates. These results again illustrate the substantial links that can be constructed between properly selected single-crystal model surfaces and polycrystalline, high-surface-area materials. LA9805140
