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In Situ Investigation of Solid-Liquid Catalytic Interfaces by Attenuated Total Reflection Infrared Spectroscopy Ivelisse Ortiz-Hernandez and Christopher T. Williams* Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, South Carolina 29208 Received September 20, 2002. In Final Form: December 13, 2002 It is demonstrated that attenuated total reflection infrared (ATR-IR) spectroscopy can be effectively used for in situ investigation of supported catalyst-liquid interfaces. Thin (ca. 10 µm) films of a 5 wt % Pt/γ-Al2O3 catalyst were deposited onto the surface of a germanium waveguide. The very small support particles (average size, 37 nm) form a stable film that does not peel from the waveguide when exposed to flowing liquid. The catalyst film is thick enough so that essentially all of the IR electric field is attenuated within the film, thus decreasing bulk liquid signals. The applicability of this technique for studying adsorption and reaction of molecules on supported catalysts has been tested using several probe molecules (carbon monoxide, formaldehyde, ethanol, butyronitrile) and solvents (water, ethanol, hexane). Examination of adsorption of CO from aqueous and ethanolic solutions reveals that CO resides in both atop and bridged configurations on the catalyst surface in both solvents. A 10-fold decrease in the oxidation rate of adsorbed CO in ethanol was observed. This is attributed both to the lower solubility of O2 in ethanol compared to water and the likely presence of trace ethanol dissociation products that may block O2 adsorption. The dissociation of formaldehyde and ethanol in water was studied by following the formation of adsorbed CO. The extent of dissociation appeared considerably larger for formaldehyde than for ethanol as determined by comparing absorption intensities and peak frequencies. Finally, adsorption of butyronitrile from hexane was examined with a view toward extending this approach to the study of more complex systems. Butyronitrile was found to adsorb on the catalyst by σ-bonding of the CN group with the platinum. The prospects of using this approach to examine solid-catalyzed liquid-phase reactions are discussed in light of these findings.
Introduction In recent years there has been an increased desire to implement heterogeneous catalysis in fine chemicals and pharmaceuticals industries.1-3 This is being driven by the goal of developing environmentally friendly (“green”) approaches for the production of such specialty products. Sheldon et al. have analyzed applications of heterogeneous catalysis in the fine chemical industry and concluded that there are several advantages over other approaches.3 Heterogeneous catalysts facilitate separations and can decrease the amount of solvent required, thus minimizing waste produced. Furthermore there are possibilities for combining multiple process steps as well as increasing selectivity toward the desired product. One of the main obstacles in the establishment of heterogeneous catalysis in these industries is the difficulty in reproducing homogeneous catalyst properties, such as selectivity and product quality. Monitoring surfaces of solid catalysts in the liquid phase can provide much-needed insight into these reactions and help lead to implementation of heterogeneous catalysis in these industries. To enable the investigation of catalytic surface reactions in general, much effort has been directed toward the development of in situ vibrational spectroscopic techniques. Fourier transform infrared (FTIR) spectroscopy has been successful for the characterization of gas-solid interactions leading to increased understanding of reaction mechanisms.4,5 The transmission and diffuse reflectance * To whom correspondence should be addressed. Fax: (803)777-8265. E-mail:
[email protected]. (1) Carpenter, K. J. Chem. Eng. Sci. 2001, 56, 305. (2) Mills, P. L.; Chaudhari, R. V. Catal. Today 1997, 37, 367. (3) Sheldon, R. A.; Downing, R. S. Appl. Catal., A 1999, 189, 163. (4) For a review see: Ryczkowski, J. Catal. Today 2001, 68, 263. (5) Hirschmugl, C. J. Surf. Sci. 2002, 500, 577.
modes are the most successful and often used. However, while FTIR spectroscopy has been used extensively for gas-solid catalytic systems, the study of liquid-phase catalysis has been limited due to large spectral interference caused by the liquid bulk phase. Much of the experimental vibrational spectroscopic data available on solid-liquid catalytic interfaces has been obtained using reflectionabsorption infrared spectroscopy (RAIRS) and surfaceenhanced Raman spectroscopy (SERS) of electrochemical interfaces. RAIRS requires a cell with a very short infrared path length, which is achieved by a “thin layer” configuration. Combined with potential-difference techniques, the approach has been powerful for studying electrochemical systems.6 However, in heterogeneous catalytic systems there is usually not potential control, and the thin-layer geometry induces large mass-transfer limitations. While SERS is more applicable under typical reaction conditions, it is largely limited to polycrystalline metal surfaces.7,8 A fairly recent development has been the application of sum-frequency spectroscopy (SFS) to catalytic interfaces.9 However, this nonlinear optical technique has not yet been applied to solid-catalyzed liquid-phase reactions. One approach that has not been utilized extensively for surface studies of heterogeneous catalysts is attenuated total reflection infrared (ATR-IR) spectroscopy. Nevertheless, ATR-IR has been used to analyze different types of solid-liquid interfaces related to heterogeneous cata(6) For review see: Weaver, M. J.; Zou, S. In Advances in Spectroscopy, Vol. 26; Clark, R. J. H., Hester, R. E., Eds.; Wiley: Chichester, U.K., 1998; p 219. (7) Weaver, M. J. J. Raman Spectrosc. 2002, 33, 309. (8) For a review see: Campion, A. J.; Kambhampti, P. Chem. Soc. Rev. 1998, 27, 241. (9) Somorjai, G. A.; Ruprechter, G. J. Phys. Chem. B 1999, 103, 1623.
10.1021/la020799n CCC: $25.00 © 2003 American Chemical Society Published on Web 02/27/2003
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lysts.10 For example, Harris et al. studied adsorption of ethyl acetate at an n-heptane/SiO2 interface.11 The silica film was grown by using sol-gel deposition. Multidimensional least-squares analysis was used to correct for solvent peaks when subtracting the unscaled spectrum from the others containing the adsorbate. Using chemometrics,12 adsorption isotherms of ethyl acetate on SiO2 in n-heptane were generated. McQuillan et al. have studied adsorption of carboxylic acids and surfactants on different oxide thin layers, such as TiO2, ZrO2, Al2O3, and Ta2O5.13-15 In these studies sol-gel methods were also used to grow the films. More directly related to catalysis is the work of Baiker and co-workers, who recently studied the adsorption of carbon monoxide on Pt/Al2O3 using ATR-IR with CH2Cl2 as a solvent.16 In this case, the Pt/Al2O3 film was deposited by physical vapor deposition. A more recent study17 examined this vapor-deposited platinum catalyst modified with cinchonidine, which is a very well-known modifier for facilitating chiral hydrogenation reactions. It was shown that the cinchonidine adsorption orientation depends on surface coverage and is affected by solvent interactions with the active sites. Other related work was performed by Zippel et al.18 who studied the adsorption of CO in platinum films in electrochemical environments. They compared CO adsorption under dry conditions with the effect of the presence of water both in vapor and liquid phase. Their efforts in getting a clean spectrum for the adsorption of CO in aqueous solution were not successful due to undesired peeling of the Pt film. Clearly there is a potential for ATR-IR to become an important tool for examining solid-liquid catalytic interfaces in situ. In the present study, we extend the ATRIR approach to examine adsorption and dissociation of several probe molecules (CO, CH2O, C2H5OH, C4H7N) in various liquid solvents (H2O, C2H5OH, C6H14) onto a Pt/ Al2O3 powder catalyst. The method employs thin films of the catalyst deposited on the surface of an ATR crystal. Results clearly demonstrate the feasibility of studying supported catalysts in this fashion. Theoretical and experimental considerations are discussed, along with the prospects of using this approach to examine solid-catalyzed liquid-phase reactions. Theory of ATR-IR of Thin Catalyst Films Attenuated total reflection infrared spectroscopy is a powerful vibrational spectroscopic approach that provides useful information regarding molecular properties of monolayers and thin films.13-20 The full theory of ATR-IR (10) See review articles: (a) Hind, A. R.; Bhargava, S. K.; McKinnon, A. Adv. Colloid Interface Sci. 2001, 93, 91. (b) Johnson, B. W.; Bauhofer, J.; Doblhofer, K.; Pettinger, B. Electrochim. Acta 1992, 37(12), 2321. (11) (a) Poston, P. E.; Rivera, D.; Uibel, R.; Harris, J. M. Appl. Spectrosc. 1998, 52, 1391. (b) Rivera, D. A. Ph.D. Thesis, Department of Chemistry, University of Utah, 2000. (12) (a) Otto, M. Chemometrics; Wiley-VCH: New York, 1999. (b) Beebe, K. R.; Pell, R. J.; Seasholtz, M. B Chemometrics: A Practical Guide; Wiley: New York, 1998. (c) Haaland, D. M.; Melgaard, D. K. Appl. Spectrosc. 2001, 55, 1. (d) Wold, S.; Sjo¨stro¨m, M.; Erikson, L. Chemom. Intell. Lab. Syst. 2001, 58, 109. (13) Dobson, K. D.; McQuillan, A. J. Spectrochim. Acta, Part A 1999, 55, 1395. (14) Dobson, K. D.; Roddick-Lanzilotta, A. D.; McQuillan, A. J. Vib. Spectrosc. 2000, 24, 287. (15) Dobson, K. D.; McQuillan, A. J. Spectrochim. Acta, Part A 2000, 56, 557. (16) Ferri, D.; Bu¨rgi, T.; Baiker, A. J. Phys. Chem. B 2001, 105, 3187. (17) (a) Ferri, D.; Bu¨rgi, T. J. Am. Chem. Soc. 2001, 123, 12074. (b) Ferri, D.; Bu¨rgi, T.; Baiker, A. J. Chem. Soc., Chem. Commun 2001, 1172. (18) Zippel, E.; Breiter, M.; Breiter, W.; Kellner, R. J. Chem. Soc., Faraday Trans. 1991, 87(4), 637. (19) Harrick, N. J. Internal Reflection Spectroscopy; Wiley: New York, 1967.
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has been discussed thoroughly,19 and modern applications in all areas of surface science have been reviewed.10,20 We limit our discussion here to the considerations relevant to the present study. In ATR-IR an infrared beam is directed through a crystal at (or above) the critical angle for total internal reflection. The resulting localized evanescent electric field in the rare medium can then be used to probe material close (within a few micrometers) to the crystal surface. A variety of materials is commercially available for use as internal reflection elements (IREs), with the most commonly used being silicon, zinc selenide, and germanium. The selection of the material depends on its refractive index (n), the pH of the sample of interest, and the desired spectral range. In the present study, a germanium element was chosen because of its relatively wide spectral window (900-4000 cm-1) and a very large refractive index (4.00). A large difference in refractive index between the crystal, n1, and the rare medium, n2, ensures that total internal reflection is achieved at the angle of incidence, θ (in this case 60°). In this study, the rare medium is a solution-deposited catalyst powder film. The goal is to have a film thick enough so that the IR electric field will not extend outside the film. The catalyst film thickness that is required depends on the depth of penetration, dp, which is defined as the depth required for the electric field amplitude to decrease to e-1 of its value at the surface. This penetration depth is given by19
dp )
λ/n1 2
2π(sin θ - n212)1/2
where λ is the wavelength of light, n21 ) n2/n1, and θ is the incidence angle. Thus, if all (>99%) of the infrared electric field is to be attenuated within the catalyst film, the thickness should be around 5dp. For the case of an Al2O3 film with a refractive index between 1.2 and 1.7 (the range of indices corresponding to infrared wavelengths of interest), dp is well below 1 µm. Figure 1 provides an overview of the ATR-IR approach as applied to the study of catalyst thin films. Experimental Procedure Reagents. Ethyl alcohol (99.5+%) and formaldehyde (37 wt % in water) were obtained from Aldrich and used without further purification. The gases used for the experiments were ultrahigh purity hydrogen, oxygen, helium, and carbon monoxide from National Welders Supply. H2PtCl6 (99.5%, Premion) was obtained from Alfa Aesar. Water was deionized (18 MΩ) and purified of organic contaminants using a Barnant B-pure dual filter with a Millipore 4-filter system. Catalyst Preparation. The catalyst samples consisted of 5 wt % Pt/γ-Al2O3 prepared using standard wet (aqueous) impregnation with H2PtCl6 as the precursor. The support is γ-Al2O3 powder from Alfa Aesar with a mean particle size of 37 nm and a surface area of 45 m2/g as given by the manufacturer (and measured in our laboratory using the BET method). While larger Al2O3 support particles were tested, it was found that the resulting films adhered poorly to the ATR element (see below). The dried impregnated support was calcined in O2 at 500 °C for 3 h and reduced in H2 for 2 h at 300 °C. The resulting catalyst had around 50% dispersion (obtained by H2 chemisorption) with a mean platinum particle size of 3 ( 2 nm, which was verified by highresolution transmission electron microscopy. Thin Film Preparation. To deposit the catalyst onto the ATR element, a suspension of catalyst particles in either water or ethanol was prepared. The suspension consisted of 100 mg of catalyst in 20 mL of solvent and was placed in an ultrasonic bath for at least 12 h in order to obtain a uniform suspension. A 60° (20) Mirabella, F. M. Appl. Spectrosc. Rev. 1985, 21, 45.
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Figure 1. Description of the main concepts of ATR-IR spectroscopy applied to the study of catalyst thin films. See text for details. germanium ATR element (Spectra Tech) was coated with a thin film of catalyst using the following procedure. A thin layer of solution was spread into the ATR element and dried under room conditions (25 °C and 1 atm). This procedure was repeated six times, giving an average film thickness of 10 ( 2 µm, as found using optical microscopy (Olympus BX50 microscope and Simple PCI image analysis software by Compix). Infrared Spectroscopy. All spectra were acquired using a Nicolet 670 FTIR spectrometer with a liquid nitrogen-cooled MCT detector. A horizontal ATR accessory (Spectra Tech) was used in conjunction with a home-built aluminum flow cell. The flow cell consisted of a small channel with a volume of 1.10 cm3 that allows liquid contact with the gasket-sealed (Garlock gylon, style 3540) ATR element. A schematic of the flow cell design is provided in the Supporting Information (Figure S1). The solvent was stored in a separate reservoir equipped with glass frit-capped gas inlets to allow saturation or purging of the liquid with gases. The liquid was pumped through the flow cell at a flow rate of 36 cm3/min using a Vera Varistaltic Manostat pump (Barnant Co.). Either Tygon or Viton tubing was used in this study as appropriate based on their resistances to the solvents and reactants employed. The equipment schematic is provided in the Supporting Information (Figure S2). The flow rate used for each of the gases (CO, O2, H2, and He) was 100 cm3/min. For each experiment, the ATR accessory optics were aligned and optimized, and the sample was left under a flow of solvent until the system reached equilibrium (usually at least 2 h, as determined by achieving a consistent, unchanging absorbance spectrum). A catalyst pretreatment was then performed by flowing O2-saturated solvent for 1 h, purging the flowing solvent with He for 15 min, and finally flowing H2-saturated solvent for 1 h. Scans were taken every 5 min at each stage to see the behavior of the catalyst under these conditions. Data collection consisted of 128 scans per spectrum with a resolution of 4 cm-1. The collection time for each spectrum was 69 s. All the experiments were performed at room temperature (25 °C).
Results and Discussion Film Thickness and Quality. The first step in the study of the Pt/Al2O3 films was to characterize the effect of film thickness on the liquid-phase signal. As discussed previously, the evanescent electric field decays exponentially from the surface of the germanium element into the catalyst film. Thus, by increasing the film thickness, the bulk solvent signal will be diminished while the sampled catalyst surface area will be increased. The key parameter is the void fraction (i.e. fraction of volume not occupied by catalyst particles) within the film, since this space will be filled with liquid during flow experiments and thus contribute unwanted infrared absorption. It is instructive to consider the possible range of void fractions if one assumes uniform catalyst particle size and packing arrangement within the film. For example, a face-centered cubic packing geometry would produce a void fraction of
Figure 2. Dependence of the 2974 cm-1 adsorbance of ethyl alcohol on number of catalyst film coatings. Intensities have been normalized to the value obtained for ethyl alcohol on the uncoated element.
0.26, followed by a body-centered cubic structure at 0.32 and simple cubic packing at 0.48. Thus, films having a void fraction within this range would be expected to be the best for use in catalyst experiments. In an attempt to estimate the void fraction within the film, the dependence of a liquid-phase infrared signal on the number of catalyst coatings was examined. The film was prepared in the usual fashion, but after each added coating the film was saturated with ethyl alcohol and an ATR-IR spectrum was acquired. Figure 2 shows the intensity dependence of the 2974 cm-1 band of liquid ethyl alcohol as a function of the number of catalyst film coatings. The band intensities have been normalized to that obtained from ethanol on the uncoated element. The data reveal that by three coatings the ethanol signal drops to a value that is around 55% of the initial value with no film present. Additional coatings of the element do not result in any further decrease in this band intensity. While it is tempting to immediately assign this value to the void fraction within the film, we must first consider two possible optical effects. These effects arise from the fact that the refractive index of the rare medium (n2) changes from that of bulk ethanol to that of a physical ethanol/catalyst mixture as the film is grown. Since the refractive index of Al2O3 at 2974 cm-1 (i.e., a wavelength of 3.362 µm) is larger than that of ethanol, growth of the film results in an increase in the refractive index of the material next to
ATR-IR Investigation of Catalytic Interfaces
the germanium. This induces an increase in both the transmitted electric field strength and the penetration depth of the evanescent field. The enhanced electric field will increase the number of photons absorbed per ethanol molecule while the increased penetration depth will enlarge the volume of material that is being sampled. The very high refractive index of Ge (n ) 4.0) and large incident angle (60°), coupled with the relatively small ethanol absorbance at this wavelength, conspire to limit these effects in the present case. Indeed, the magnitude of these effects can be estimated by performing calculations using standard optical theory as presented in refs 19-21 and using reported optical constants of crystalline Al2O3 (n ) 1.700)22 and ethyl alcohol (n ) 1.38)23 at 2974 cm-1. Such calculations suggest that the final relative intensity in Figure 2 overestimates the void fraction by around 5-10%. This places our estimated void fraction for the film at around 0.50, suggesting a film packing quality that is similar to the simple cubic arrangement discussed above. As discussed in the theoretical section, the thickness of the film should be equal to at least 5dp to avoid sampling liquid that is outside the film. The film thickness measured after six coatings of the element is around 9-10 µm, suggesting that each coating is equivalent to ca. 1.5 µm. At the IR frequency considered in Figure 2, dp will be equal to ca. 0.20 µm and thus 5dp will be around 1.00 µm. Thus, we would predict that only one coating of catalyst should be sufficient to the minimize liquid signal. However, it is clear from Figure 2 that it requires three coatings to achieve this. We believe that this discrepancy arises from increased packing that occurs in the portion of the film closest (within 1 µm) to the element as more coatings are applied (i.e., elimination of islands). CO Adsorption and Oxidation. Infrared spectroscopic characterization of supported and unsupported transition metal catalysts using CO as a probe molecule has been performed extensively in gas-phase investigations.24 In the liquid phase, CO adsorption studies have been largely limited to metal electrode-solution interfaces.6 In general, two bands associated with adsorbed CO have been observed on platinum. Linearly (i.e., terminal, atop) adsorbed CO typically exhibits νCO frequencies between 2000 and 2090 cm-1, while 2-fold and 3-fold bridging CO typically lies between 1780 and 1900 cm-1. The frequency of these bands is significantly affected by surface coverage, coadsorbates, and electric field effects. As a result, CO makes a perfect choice for initial probing of catalyst films in liquid solvents using ATR-IR spectroscopy. The adsorption of CO from water was the first system considered, with typical results shown in Figure 3. A background spectrum was taken prior to beginning the pretreatment procedure and was used for all other spectra acquired during the experiment. The CO region is flat and featureless during and after completing pretreatment. In contrast, small changes in the C-H stretching region (not shown) are observed to occur during pretreatment. Such changes indicate that carbonaceous impurities are present on freshly grown films. These species can be minimized by performing multiple oxidation-reduction pretreatment cycles, although only one cycle was used in (21) Born, M.; Wolf, E. Principles of Optics, 7th ed.; Cambridge University Press: Cambridge, U.K., 1999. (22) Gervais, F. In Handbook of Optical Constants of Solids 2; Palik, E. O., Ed.; Academic Press: New York, 1991; p 771. (23) Sethna, P. P.; Williams, D. J. Phys. Chem. 1979, 83, 405. (24) (a) Weaver, M. J. Top. Catal. 1999, 8, 65. (b) Matyshak, V. A.; Krylov, O. V. Catal. Today 1995, 25, 1. (c) Sheppard, N.; Nguyen, T. T. Adv. Infrared Raman Spectrosc. 1978, 5, 67.
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Figure 3. ATR-IR spectra of adsorption and oxidation of CO on Pt/Al2O3 in water. Spectra were acquired (a) after O2/H2 pretreatment, (b) after bubbling CO through the solution, (c) after purging with He and flowing O2 for 30 min, (d) after purging with He and flowing H2 for 30 min, and (e) after readsorption of CO. See text for details.
the present study. Following pretreatment, the CO flow was turned on and adsorption was followed for at least 30 min or until the system reached steady state, which never required more than 1 h. At full CO adsorption an intense absorption band at 2052 cm-1 is observed that corresponds to atop CO adsorbed on platinum (Figure 3, spectrum b). There is also a very broad and weak peak at ca. 1814 cm-1, indicating the presence of CO adsorbed on 2-fold bridging sites. Similar results have been obtained by Ferri et al. in their ATR-IR study of CO adsorption on vapordeposited Pt/Al2O3.16 Following carbon monoxide adsorption, the flowing water was purged with He for 15 min. The system was then saturated with flowing O2 for a period of 30 min, which resulted in removal of most of the CO from the surface (Figure 3, spectrum c). Following another 15 min He purge, the water was saturated with H2 for 30 min, which resulted in essentially complete removal of the remaining adsorbed CO (Figure 3, spectrum d). Thus, a clean surface was achieved by the oxygen/hydrogen treatment. Such regeneration appears to have no detrimental effect on the film quality, as readsorption of CO results in an spectrum identical to that obtained previously (Figure 3, spectrum e). A similar procedure was also followed to test CO adsorption in ethyl alcohol. A typical set of spectra is shown in the Supporting Information (Figure S3). The spectra look very similar to the case of CO adsorption in water, although the adsorption peak for atop CO was located at 2056 cm-1 rather than at 2052 cm-1. However, oxidizing the CO from the catalyst surface proved to be difficult in ethanol. Even after 3 h of O2 saturation there was a significant amount of CO remaining (Figure S3, spectrum c). To aid in the understanding of this interesting resistance to CO oxidation, it is instructive to examine timedependent spectral intensity changes of atop CO during adsorption (Figure 4a) and oxidation (Figure 4b) in both solvents. The band intensities have been normalized to their value at maximum coverage to ease the comparison.
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Figure 5. ATR-IR spectra of CO on Pt/Al2O3 via (a) adsorption of CO in the gas phase, (b) adsorption of CO in ethyl alcohol, and (c) adsorption of CO in water. See text for details.
Figure 4. (a) Time-dependent infrared signal for adsorbed atop CO during adsorption on Pt/Al2O3 from CO-saturated water (triangles) and ethyl alcohol (squares). The infrared absorbance values have been normalized to the value obtained at full CO adsorption. (b) Time-dependent infrared signal for adsorbed atop CO during oxidation on Pt/Al2O3 from O2-saturated water (triangles) and ethyl alcohol (squares). The infrared absorbance values have been normalized to the value obtained at full CO adsorption. See text for details.
It is apparent from Figure 4a that the build-up of adsorbed CO is largely unaffected by solvent. This is not unexpected since there is no significant CO desorption at this temperature to compete with the CO adsorption process. In contrast, there is a ca. 10-fold decrease in the rate of CO oxidation when moving from water to ethyl alcohol. The main explanation for this decrease is likely the reduced solubility of O2 in ethanol as compared with water.25 An additional possibility is that in the case of ethyl alcohol there are probably decomposition products (e.g., ethoxy) that are also present on the catalyst surface. While direct spectroscopic evidence of dissociation has not been obtained, even partial and undetectable levels of ethyl alcohol fragments could restrict the adsorption of O2 that is necessary for CO oxidation to take place. After examining CO adsorption in solution, it is worth comparing such results with those obtained in the gas phase. In this case the film was pretreated directly with gaseous O2 for 3 h to remove any solvent residues. Then it was purged for 15 min using N2 followed by reduction for 1 h by flowing H2. Pure CO was introduced into the cell for 30 min followed by purging with N2. The resulting spectrum is shown in Figure 5, along with the spectra obtained in CO-saturated water and ethyl alcohol. The gas-phase result (Figure 5, spectrum a) shows the atop peak at a frequency of 2063 cm-1, while the CO bridging (25) (a) Cargill, R. W. J. Chem. Soc., Faraday Trans. 1 1976, 72, 2296. (b) Tokunaga, J. J. Chem. Eng. Data 1975, 20, 41.
peak (located at 1814 cm-1 in water) was not detected. The absence of this species on the surface is likely the result of a larger surface coverage, which drives the adsorption of CO toward atop sites. There is a 7-11 cm-1 shift of the atop peak to higher frequency in the spectrum of CO adsorbed from the gas phase. This is not surprising, since it is well-known that the frequency of atop CO vibrations will blue-shift as a result of dipole-dipole coupling when surface coverage increases.26 Dissociation of Formaldehyde and Ethyl Alcohol in Water. In addition to studies of simple molecular adsorption, it is also desirable to be able to examine cases of dissociative chemisorption. Two molecules that provide a useful test case are formaldehyde and ethyl alcohol. Both molecules have been extensively studied to determine their dissociative chemistry on platinum surfaces.27-31 In the case of formaldehyde, decomposition proceeds rapidly to form adsorbed CO and H. While ethyl alcohol dissociation is considerably more complex, involving intermediates such as adsorbed ethoxy and acetaldehyde, CO is also a major surface product. Given the results obtained for CO adsorption above, these molecules were therefore chosen to test the utility of ATR-IR for examining dissociative chemisorption in the liquid phase. Figure 6 shows typical ATR-IR results for formaldehyde adsorption on Pt/Al2O3 in water. For these experiments the film was pretreated following the same procedure as described earlier for the CO experiments. After the standard O2/H2 pretreatment, 5 mL of 37% formaldehyde (26) (a) Hollins, P. Surf. Sci. Rep. 1992, 16, 51. (b) Browne, V. M.; Fox, S. G.; Hollins, P. Mater. Chem. Phys. 1991, 29, 235. (c) Hollins, P. Adsorpt. Sci. Technol. 1986, 2, 177. (27) Iwasita, T.; Pastor, E. Electrochim. Acta 1994, 39, 531. (28) Beden, B.; Morin, M. C.; Hahn, F.; Lamy, C. J. Electroanal. Chem. 1987, 229, 353. (29) Morallon, E.; Huerta, F.; Cases, F.; Vazquez, J. L.; Aldaz, A. Curr. Top. Electrochem. 1997, 5, 1. (30) Henderson, M. A.; Mitchell, G. E.; White, J. M. Surf. Sci. 1987, 188, 206. (31) (a) Olivi, P.; Bulhoes, L. O. S.; Leger, J. M.; Hahn, F.; Beden, B.; Lamy, C. J. Electroanal. Chem. 1994, 370, 241. (b) Olivi, P.; Bulhoes, L. O. S.; Beden, B.; Hahn, F.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. 1992, 330, 583.
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Figure 6. ATR-IR spectra for the dissociation of formaldehyde on Pt/Al2O3 in water. Spectra were acquired (a) after O2/H2 pretreatment, (b) after adding formaldehyde into the solution, (c) after purging the system with pure water, and (d) after flowing O2 for 1 h. See text for details.
was injected into the system (resulting in a 0.6 M formaldehyde solution) and spectra were taken until no further changes were observed (ca. 15 min). A sharp peak in the CO stretching region at a frequency of 2030 cm-1 (Figure 6, spectrum b) is observed and clearly arises from adsorbed atop CO. It is accompanied by a broad band around 1800 cm-1 that clearly suggests the presence of a bridge-bound species. In an attempt to oxidatively remove the adsorbed CO, oxygen was flowed for 1 h. No significant change was observed in the CO band intensity. This observation is not difficult to interpret, since formaldehyde is in great excess of O2 in solution. Thus, formaldehyde may continuously dissociate to replenish CO on the surface faster than oxidation can occur. To test this hypothesis, formaldehyde was flushed from the system with pure water, during which the intensity of the CO peak decreased slightly (Figure 6, spectrum c). This decrease was likely the result of trace O2 being present in the pure water. Oxygen was then flowed to try to remove the remaining CO on the surface, with complete removal occurring within 1 h (Figure 6, spectrum d). This result is very similar to that obtained for oxidation of adsorbed CO formed by direct molecular adsorption. As a comparison, ethyl alcohol interactions with platinum in dilute aqueous solutions were also examined. Following pretreatment, ethyl alcohol was injected in several steps to examine the concentration dependence of dissociation. Results are shown in Figure 7 for concentrations of 0.086, 0.172, and 0.345 M. After each injection the spectra were acquired continuously until reaching steady state. As with formaldehyde, the results show that ethyl alcohol chemisorbs and dissociates on the catalyst with a small feature observed at 2007 cm-1, indicating the presence of adsorbed CO (Figure 7, spectra b-d). The very small intensity and lowered frequency of this vibrational band suggest that the coverage of CO is likely very small, and thus the degree of dissociation is not as extensive as
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Figure 7. ATR-IR spectra for the dissociation of ethyl alcohol on Pt/Al2O3 in water. Spectra were acquired (a) after O2/H2 pretreatment, (b) after exposure to a 0.086 M ethanol solution, (c) after exposure to a 0.172 M ethanol solution, and (d) after exposure to a 0.345 M ethanol solution. See text for details.
for formaldehyde. The spectra were also examined for bands that could be associated with adsorbed ethoxy. One characteristic band for ethoxy on Pt is a downshifted νCO stretch at a frequency to 1020 cm-1 (as opposed to 1050 cm-1 for bulk C2H5OH). We are unable to discern this band in the present spectra. However, Iwasita and Pastor27 have shown that C-O stretching due to ethoxy adsorption is very unfavorable for detection with infrared spectroscopy. Also, the significant bulk C2H5OH signal at 1050 cm-1, combined with increased absorption of infrared by Al2O3 in this spectral region, make detection of this band difficult. Butyronitrile Adsorption in Hexane. Having demonstrated the suitability of ATR-IR spectroscopy for examining decomposition of small molecules on a catalyst thin film, it is desirable to begin to look at systems more relevant to liquid-phase organic synthesis. As a starting point, the study of nitrile adsorption has been chosen with a view toward developing an understanding of the surface chemistry involved in heterogeneous hydrogenation of nitriles. In perhaps the most extensive review on the subject, DeBellefon and Fouilloux trace the development of the reaction mechanisms for nitrile hydrogenation over the years.32 Fairly recent results from Sachtler and coworkers have shed considerably more light on nitrile hydrogenation in the gas phase.33,34 The use of ATR-IR spectroscopy can likely shed some light on the surface chemistry occurring under liquid-phase reaction conditions. In this section we present some preliminary results on the adsorption of butyronitrile on Pt/Al2O3. The catalyst film was first pretreated with gaseous O2 followed by saturation with hexane. Oxygen was not flowed through the hexane because of undesired spectral changes (32) De Bellefon, C.; Fouilloux, P. Catal. Rev.-Sci. Eng. 1994, 36, 459. (33) Huang, Y.; Sachtler, W. M. H. Appl. Catal., A 1999, 182, 365. (34) (a) Huang, Y.; Sachtler, W. M. H. Stud. Surf. Sci. Catal. 2000, 130, 527. (b) Huang, Y.; Sachtler, W. M. H. J. Catal. 2000, 190, 69. (c) Huang, Y.; Sachtler, W. M. H. J. Catal. 1999, 188, 215.
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the band observed with the Pt/Al2O3 catalyst after hexane purging (Figure 8, spectrum d) clearly arises from adsorbed butyronitrile on Pt. This vibrational frequency is consistent with a σ-bonded butyronitrile species. This experiment (as well as those for formaldehyde) also provides a useful method of determining how fast the liquid in the film is replaced upon switching the flow to a new mixture. In this case, the νCN band was found to disappear within around 5 min, indicating that the liquid within the film was completely switched during this time. This result shows that the ATR method will likely be capable of examining slow transient surface phenomena on the catalyst free of external mass-transfer limitations when the time scales of the changes are of the order of a few minutes. Concluding Remarks
Figure 8. ATR-IR spectra for the adsorption of butyronitrile on Pt/Al2O3 in hexane. Spectra were acquired (a) after pretreatment, (b) after adding butyronitrile into the system to form a 0.225 M solution, (c) after flowing H2 for 30 min and purging with He, and (d) after purging with pure hexane to remove butyronitrile from solution. See text for details.
that were observed to occur, likely as a result of some reaction that takes place. The film was reduced with H2 for 30 min while flowing hexane with the final subtraction results shown in spectrum a of Figure 8. Pretreatment was followed by injection of 2 mL of butyronitrile to give a 0.225 M solution. The system was allowed to equilibrate, revealing a feature at 2253 cm-1, which is indicative of νCN butyronitrile stretching (Figure 8, spectrum b). The solution was then saturated with H2, while data were collected to follow possible hydrogenation effects. In the resulting spectra, no clear changes were observed in the CN bands (Figure 8, spectrum c) or in the N-H stretching region. This suggests that under this condition it is difficult to hydrogenate adsorbed butyronitrile to amines. The system was then purged with pure hexane in order to verify whether the 2253 cm-1 band indeed comes from adsorbed butyronitrile. (Figure 8, spectrum d). The resulting spectra show a smaller peak remaining at around the same frequency. Given that this band has almost the same frequency as that of bulk butyronitrile, an experiment was performed to confirm that this feature was not arising from liquid still trapped in the film. A film of Al2O3 support (i.e., without platinum) was grown on the germanium element in the same fashion as that described for the catalyst. After introducing butyronitrile to flowing hexane the νCN stretch was clearly observed. However, this feature was completely removed upon switching to pure hexane. Thus,
This study provides persuasive evidence that attenuated total reflection infrared (ATR-IR) spectroscopy can be an effective in situ probe of surface chemistry occurring at supported metal catalyst-solution interfaces. In the current approach, sample preparation is fairly straightforward, involving deposition of a catalyst film on an ATR crystal by successive coatings of a suspension. Thus, the approach does not require any specialized thin film coating devices or synthetic procedures other than what is required for catalyst synthesis. One important factor is the size of the catalyst particles chosen to make up the film. It was found that small particles were necessary to form stable films on the ATR elements. Thus, when extending this approach to other catalytic materials (e.g., zeolites, oxides), small
