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Infrared Spectroscopic Study of Surface Diffusion to Surface Hydroxyl Groups on Al2O3: 2-Chloroethylethyl Sulfide Adsorption Site Selection Douglas B. Mawhinney, Joseph A. Rossin,† Karl Gerhart,‡ and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received May 18, 1999. In Final Form: November 15, 1999 The interaction of the various hydroxyl groups on the Al2O3 surface with a bifunctional adsorbate, 2-chloroethylethyl sulfide, was studied by transmission infrared spectroscopy. Al2O3 has five unique AlOH groups, differentiated by their coordination to the surface, which give independent AlO-H stretching modes. Monitoring intensity changes in these modes as 2-chloroethylethyl sulfide (CEES) molecules diffuse into the porous structure of alumina shows distinctly different stages of surface diffusion and reaction at the Al-OH groups. Three sequential steps have been separated by studies at low temperature, where the rate of the processes was kinetically retarded to allow observation. The final stage of the CEES interaction with the surface occur when the CEES molecules adsorbed on the surface via the sulfide moiety at Al3+ Lewis acid sites undergo a hydrolysis reaction with the neighboring basic Al-OH groups.
I. Introduction The surface properties of catalytically important amorphous oxide powders1,2 such as Al2O3 are inherently difficult to characterize. The surfaces of the particles are typically inhomogeneous, owing to the presence of surface defects, multiple hydroxyl groups, and different crystalline faces that terminate the surface. Transmission infrared spectroscopy can be applied to help characterize these surfaces by resolving different hydroxyl group stretching modes3-5 and adsorbate vibrations on different adsorption sites.6-11 For example, by employing CO as an adsorbate on the Al2O3 surface, Zaki and Kno¨zinger were able to distinguish acidic and basic OH groups9 and octahedrally and tetrahedrally bound Al3+ sites.12 Using CO as a probe molecule, Ballinger and Yates were able to correlate the formation of two different types of Al3+ sites with the thermal removal of the hydroxyl groups.13 Two main models of the alumina surface exist.5,14 The model proposed by Kno¨zinger and Ratnasamy14 to explain the difference in the surface Al-OH groups is the most widely accepted. The Kno¨zinger-Ratnasamy model takes † Guild Associates, Inc., 5750 Schier-Rings Road, Dublin, OH 43017. ‡ U.S. Army ERDEC, Building E-3549 SCBRD-ENP-E, Aberdeen Proving Ground, MD 21010-5423.
(1) Tanabe, K. Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1981; Vol. 2, p 231. (2) Kno¨zinger, H. Adv. Catal. 1976, 25, 184. (3) Peri, J. B.; Hannan, R. B. J. Phys. Chem. 1960, 64, 1526. (4) Peri, J. B. J. Phys. Chem. 1965, 69, 211. (5) Peri, J. B. J. Phys. Chem. 1965, 69, 220. (6) Little, L. H.; Amberg, C. H. Can. J. Chem. 1962, 40, 1997. (7) Parry, E. P. J. Catal. 1963, 2, 371. (8) DeRosset, A. J.; Finstrom, C. G.; Adams, C. J. J. Catal. 1962, 1, 235. (9) Zaki, M. I.; Kno¨zinger, H. Mater. Chem. Phys. 1987, 17, 201. (10) Crowell, J. E.; Beebe, T. P., Jr.; Yates, J. T., Jr. J. Chem. Phys. 1987, 87, 3668. (11) Little, L. H., Infrared Spectra of Adsorbed Species; Academic: London, 1966. (12) Zaki, M. I.; Kno¨zinger, H. Spectrochim. Acta 1987, 43, 1455. (13) Ballinger, T. H.; Yates, J. T., Jr. Langmuir 1991, 7, 3041. (14) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev.sSci. Eng. 1978, 17, 31.
into account all possible crystal faces, concentrating on the coordination of the Al3+ ion (octahedral or tetrahedral) and the O2- ion (terminal or bridging coordination) of the hydroxyl group. This model was used to calculate the net charges of the hydroxyl groups on the basis of these coordinations. These net charges cause the acidity of the hydroxyl groups to range from the most acidic (most positive) to the most basic (least positive). The Kno¨zingerRatnasamy model has been shown to agree with the varied behavior of the Al-OH groups and their AlO-H stretching frequencies. The results of this model are shown in Figure 1. 2-Chloroethylethyl sulfide (CEES) is an interesting molecule because it contains a sulfur atom and a chlorine atom; both are capable of interactions with adsorptive surface sites. The geometrical location of the sulfur atom relative to the chlorine atom allows the molecule to undergo a hydrolysis reaction at room temperature with the isolated hydroxyl groups on the Al2O3 surface.15 In this reaction, the sulfur atom in CEES adsorbs strongly on the Al2O3 surface Al3+ sites, allowing the carbonchlorine moiety to undergo interaction with neighboring isolated Al-OH groups,16 ultimately leading to reaction and HCl elimination.15 CEES, 2-chloroethylethyl sulfide, is a simulant for mustard, 2,2′-dichlorodiethyl sulfide. This paper will report on CEES transport properties through the porous structure of alumina as monitored by intensity changes in the various AlO-H stretching modes. Sequential adsorption processes at increasing temperature that involve various Al-OH groups ultimately lead to bonding of CEES to Al3+ Lewis acid sites and then to hydrolytic reaction of the C-Cl moiety. II. Experimental Section A. Vacuum System. The stainless steel vacuum system employed in these studies has a base pressure of 1 × 10-7 Torr. The system is pumped sequentially with a liquid N2-cooled zeolite (15) Mawhinney, D. B.; Rossin, J. A.; Gerhart, K.; Yates, J. T., Jr. Langmuir 1999, 15, 4789. (16) Mawhinney, D. B.; Rossin, J. A.; Gerhart, K.; Yates, J. T., Jr. Langmuir 1999, 15, 4617.
10.1021/la990604k CCC: $19.00 © 2000 American Chemical Society Published on Web 01/22/2000
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Figure 1. Correlation of the υ(OH) vibrational frequency with the net OH charge and the coordination of the Al3+ and O2- ions. sorption pump, a Pfeiffer-Balzers 60 L/s turbomolecular pump, and a Varian 20 L/s ion pump. The system base pressure is measured with an ionization gauge, while reactant gas pressures are measured with a MKS capacitance manometer (Type 107). Helium leak checking is performed with a UTI 100C quadrupole mass spectrometer. The infrared cell used in these experiments has been described previously.17 The powdered Al2O3 sample is pressed into the openings of a flat tungsten grid18 held by nickel clamps that connect directly to a combination thermocouple/electrical power feedthrough mounted on the end of a reentrant Dewar in the cell. Sample cooling is achieved through the power leads of the electrical feedthrough that pass through the liquid N2-cooled Dewar. These power leads connect to a 0-50 A, 0-100 V power supply controlled by a Honeywell UDC 500 digital controller, which heats the sample by resistive heating. This setup allows temperature control from 97 to 1300 K with an accuracy of (2 K via feedback from the thermocouple spot-welded to the top of the tungsten grid sample support. The sample is positioned in the cell between two differentially pumped KBr infrared windows. The sample cell is mounted on a computer-controlled precision translation system from the Newport Corporation. The translation system is capable of moving the cell to (1 µm accuracy in the horizontal and vertical directions. This system allows the infrared beam to move reproducibly between sample and empty positions on the grid, where background spectra are recorded. B. Materials and Sample Preparation. The Al2O3 powder (Type A-200-N-01) was obtained from Guild Associates, Inc. This Al2O3 is of the γ-phase with a surface area of 273 m2/g. The samples, typically weighing 1.4-1.6 mg (yielding a density of 2.3-2.7 mg/cm2), were pressed at 12 000 lbs/in2 into a tungsten grid (0.0508 mm thick, with 0.22 mm holes, obtained from Buckbee-Mears, St. Paul, MN) which is held by nickel clamps. The Al2O3 sample and the grid support assembly were then placed into the infrared cell, evacuated for 24 h, and heated in a vacuum at 773 K for 5 min to partially dehydroxylate the Al2O3. 2-Chloroethylethyl sulfide (98%) was obtained from Aldrich. The liquid was purified by five freeze-pump-thaw cycles after being transferred to a glass bulb under nitrogen gas. The vapor was transferred directly to the Al2O3 sample at 140 K from the (17) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. Rev. Sci. Instrum. 1988, 59, 1321. (18) Ballinger, T. H.; Wong, J. C. S.; Yates, J. T., Jr. Langmuir 1992, 8, 1676.
Figure 2. Schematic representation of the low-temperature experiments, showing the onset of CEES diffusion for a condensed overlayer into the Al2O3 pore structure. glass bulb through the stainless steel vacuum system to the infrared cell. C. IR Measurements. All spectra were measured with a Mattson Research Series I FTIR spectrometer. The spectrometer employs a liquid N2-cooled HgCdTe detector and operates in the infrared region from 4000 to 500 cm-1. All spectra were recorded at 4 cm-1 resolution and averaged using 2048 scans. The spectrometer was controlled from a personal computer using WinFIRST software supplied by Mattson.
III. Results A. First Stage of CEES Surface Diffusion on Al2O3. A schematic representation of the experiment is shown in Figure 2. A thin layer of CEES was frozen on the outer surfaces of the Al2O3 sample at 140 K. The temperature was incrementally raised to the desired temperatures
Surface Diffusion to Surface Hydroxyl Groups on Al2O3
Figure 3. Behavior of the intensity of the isolated (AlO-H) stretching modes as sequential thermally activated processes occur.
momentarily and then cooled back to 140 K, where the spectra were measured under vacuum. This procedure allows each spectrum to serve as a snapshot of the diffusion interactions between the CEES molecules and the AlOH groups. The onset of diffusion at 153 K corresponds to the lowest activation energy process available to the CEES molecule on the Al2O3 surface. Three sequential processes are observed as the temperature is raised from 140 K. These processes are summarized in Figure 3 by plotting the integrated intensity of the isolated Al-OH groups (integrated between 3810 and 3660 cm-1) versus temperature. Initially at 140 K, the CEES exists as a frozen layer on the outer geometrical surface of the Al2O3, and little effect is observed on the isolated Al-OH intensity. In the temperature range 153-163 K, the two isolated Al-OH infrared bands begin to lose intensity as they associate with CEES molecules at the onset of CEES diffusion into the porous Al2O3. As will be shown later, this association of CEES with the isolated Al-OH groups causes their frequency to decrease and results in the development a broad, intense associated AlO-H infrared feature centered near 3590 cm-1. This behavior continues up to 243 K, where the decrease of intensity of the two isolated Al-OH bands comes to an end. The second thermally activated process begins at 273-278 K. Here the intensity of the infrared band of the most acidic isolated Al-OH groups (centered at 3730 cm-1) begins to increase as desorption of CEES takes place. In this temperature range, the infrared band of the more basic isolated Al-OH groups (centered at 3770 cm-1) remains attenuated as a result of continued association with CEES molecules. Finally, the third stage occurs at 298 K, where the basic AlO-H intensity remains attenuated and the more acidic AlO-H intensity has returned to become dominant. At about 298 K, the onset of hydrolysis of the CEES begins to occur, as discussed elsewhere.15,16 B. Isothermal Behavior at 198 and 273 K. To clearly separate the different stages of interaction of CEES with the isolated Al-OH groups, two different isothermal experiments were undertaken in two different temperature regions. In each experiment, the sample was dosed with CEES at 140 K. The temperature was then raised quickly to the desired temperature, and spectra were collected as a function of time at constant temperature. Each experiment shows a distinct kinetic region of the CEES interaction with the isolated Al-OH groups. Figure 4 shows the isothermal experiment at 198 K. The CEES molecule migrates into the porous network of
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Figure 4. Infrared spectra of the isolated and associated (AlOH) stretching modes during the 198 K isothermal experiment. Spectra were measured at t ) 0 (no CEES), 0, 16, 28, 35, 57, 75, 96, 116, and 270 min, respectively.
Figure 5. Infrared spectra of the isolated and associated (AlOH) stretching modes during the 273 K isothermal experiment. Spectra were measured at t ) 0 (no CEES, upper spectrum), 0, 4, 8, 16, 30, 46, 71, 113, and 180 min, respectively. The lack of redevelopment of the most basic Al-OH infrared mode indicates that this species has participated in the hydrolysis of the adsorbed CEES.
the Al2O3, indiscriminately interacting with both the acidic and more basic types of isolated hydroxyl groups. In Figure 4 it can be seen that both of the isolated AlO-H stretching bands are converted into an associated Al-OH band that is broader and more intense and develops at lower frequencies. This effect is expected when interactions such as hydrogen bonding19 occur between the isolated Al-OH groups and the CEES molecules.16 Figure 5 presents the spectra collected during the second isothermal experiment at 273 K. The initial warm-up from 140 K causes the conversion of the isolated Al-OH groups to associated Al-OH groups, as in the 198 K experiment. However, with increasing time, this trend reverses itself for some of the isolated AlO-H stretching modes. It can be seen that, after the initial spectrum, the associated AlO-H stretching modes begin to lose intensity as the isolated acidic AlO-H infrared band begins to increase in intensity. An isobestic point can be seen in the spectra, lending further proof that the associated Al-OH species are converted directly back into the acidic Al-OH species. In sharp contrast, the basic AlO-H stretching modes never (19) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H. Freeman: San Francisco, 1960.
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Figure 6. Schematic of the acid-base pair site responsible for the hydrolysis reaction in this study.
return, as may be seen by comparison to the spectrum of the surface before exposure, where the most basic, highest frequency Al-OH groups are shown crosshatched. IV. Discussion A. Reactivity of CEES with Al2O3sBackground Information. Hydroxyl groups on the alumina surface are known to hydrogen bond to alkyl chorides10 and to react20 with alkyl chlorides at high temperatures to produce adsorbed alkoxy species and HCl gas.20 The C-Cl bond in CEES also undergoes a hydrolysis reaction with the isolated Al-OH groups (in experiments conducted at 303 K).15 The reactive CEES molecules are first adsorbed onto the alumina surface by means of a strong interaction with the surface. This most likely occurs through the sulfur atom, a Lewis base, donating lone pair electrons to the Al3+ Lewis acid sites. The C-Cl moiety then associates with the nearby isolated hydroxyl groups, followed by the hydrolysis reaction to produce HCl gas and surface-bound C2H5SC2H4-OAl species.15 Some fraction of the interaction with Al-OH groups also probably occurs through the sulfur atom of CEES.21 The initial bonding of CEES to Al2O3 is summarized schematically in Figure 6, showing the bonding of CEES through the two functionalities to the Lewis acid and Bro¨nsted base sites on Al2O3. B. Sequence of Thermally Activated Events in the CEES/Al2O3 Interaction. Figure 7 shows a schematic potential energy diagram describing the sequence of thermally activated processes found in these experiments. Initially, at T < 153 K, the CEES molecules self-associate with each other in forming the condensed layer on the outer geometric surface of Al2O3. In the temperature range 153-243 K, CEES molecules diffuse into the porous Al2O3 structure and associate with the basic and the acidic AlOH groups by interactions such as that schematically shown in Figure 6. Above 243 K, the CEES begins to desorb from the Al2O3 surface, causing the return of intensity for the more acidic of the isolated Al-OH groups as these sites are vacated. In addition, the hydrolytic reaction of the Cl moiety in CEES with the more basic isolated AlOH groups begins to occur at about 298 K. C. Reactive Site for Hydrolysis. Two main adsorption sites for CEES exist on the alumina surfacesisolated hydroxyl groups and Lewis acid sites. By infrared spectroscopy, we can directly monitor the interactions with the isolated hydroxyl groups. The acidic Al-OH groups would be expected to form stronger hydrogen bonds than the basic Al-OH groups. However, the more acidic sites are the first to lose CEES by desorption. Therefore, the (20) Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J. Phys. Chem. 1988, 92, 1296. (21) Mereiter, K.; Preisinger, A.; Zellner, A.; Mikenda, W.; Steidl, H. J. Chem. Soc. Dalton Trans. 1984, 1275.
Figure 7. Schematic representation of the diffusion and reaction events on a potential energy diagram.
interaction leading to hydrolysis under these conditions may be thought to occur through the oxygen of the more basic Al-OH group and the carbon attached directly to the chlorine, as shown in Figure 6. The activation of Al2O3 at 773 K has been shown to produce two Lewis acid sites by CO adsorption.13,22-23 The formation of these two sites occurs as H2O is lost during thermal activation.2,3,5,13-14,24 Ballinger and Yates,13 in the course of correlating Al-OH removal with the production of Lewis acid sites, reported the production of a second Al3+ Lewis acid site by heating to 800 K that strongly perturbed the stretching mode of adsorbed CO on this site. The high-frequency, basic AlO-H stretching mode accompanied the production of this second type of Lewis acid site. Kno¨zinger23 also reported a stronger Al3+ Lewis acid site formed by heating to 870 K. He studied this site by CO adsorption and noted that adsorption of CO on this site also perturbed a basic AlO-H stretching mode. Hence, we postulate that these stronger Lewis acid sites are responsible for binding the molecules in the vicinity of the more basic isolated Al-OH groups until sufficient thermal energy can be gained to induce hydrolysis near 298 K, as shown schematically in Figure 6. A chemical reaction involving H2S and Al3+ Lewis acid site-hydroxyl group pairs formed at lower temperatures (673 K) has been reported.25 V. Conclusions Low-temperature CEES diffusion and reaction on partially dehydroxylated Al2O3 can be divided into four unique thermally activated stages: 1. 140-153 K: CEES is frozen on the outer surfaces of the Al2O3 powder, interacting primarily with itself and very little with the Al2O3 surface. 2. 153-273 K: CEES overcomes the barrier to diffusion and migrates into the porous Al2O3 network, interacting with both the acidic and basic Al-OH groups. (22) Della Gatta, G.; Fubini, B.; Ghiotti, G.; Moterra, C. J. Catal. 1976, 43, 90. (23) Kno¨zinger, H. In Surface Organometallic Chemistry: Molecular Approaches to Surface Catalysis; Basset, J. M., Gates, B. C., Eds.; Kluwer: Boston, 1988; p 35. (24) Cornelius, E. B.; Milliken, T. H.; Mills, G. A.; Oblad, A. G. J. Phys. Chem. 1955, 59, 809. (25) Datta, A.; Cavell, R. G. J. Phys. Chem. 1985, 89, 450.
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3. T g 273 K: CEES gains sufficient thermal energy to overcome interactions with the acidic Al-OH groups and desorbs from those sites. 4. T > 298 K: CEES is bound by the stronger Al3+ Lewis acid sites until sufficient thermal energy is introduced into the system to allow the hydrolysis reaction with the neighboring isolated basic Al-OH groups. 5. The sequence of thermally distinct surface processes
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experimentally separated at low temperature in these experiments is expected to occur also at higher temperatures. Acknowledgment. This work was supported by Guild Associates, Inc. LA990604K
