DRIFTS Study of Surface of γ-Alumina and Its Dehydroxylation

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DRIFTS Study of Surface of γ-Alumina and Its Dehydroxylation Xinsheng Liu* BASF Catalysis Research, R&D Center, 25 Middlesex-Essex Turnpike, Iselin, New Jersey 08830 ReceiVed: December 19, 2007; In Final Form: January 23, 2008

γ-Alumina surfaces pretreated at different temperatures (25 to 450 °C) and their rehydration after pyridine adsorption were studied using the conventional pyridine adsorption/DRIFTS technique. The results show that hydration at ambient temperature cannot convert all of the surface-coordinatived unsaturated Al sites to sixcoordination, and weak and medium strong Lewis acid sites can still be observed for the fully hydrated γ-alumina. Dehydration at above 200 °C starts to generate strong Lewis acid sites. The number of weak, medium strong, and strong Lewis acid sites changes with dehydration temperature and all types of Lewis acid sites have adjacent OH groups. A good correlation between the number of Lewis acid sites and the intensity of perturbed OH groups is observed. Rehydration of the pyridine-adsorbed 450 °C pre-dehydrated alumina leads to removal of the pyridine adsorbed on the strong and medium strong Lewis acid sites and recovery of the surface of the original alumina. Water cannot replace the pyridine adsorbed on the weak Lewis acid sites.

Introduction Transition aluminas of various forms are widely used in industry as adsorbents, catalysts, and catalyst supports.1-4 The wide range applications of aluminas have stimulated many detailed studies of their surface properties. Over the past 50 years, many attempts have been made to assign the hydroxyl bands observed from IR spectroscopic studies and to understand the nature of the surfaces. Recently, Morterra and Magnacca5 reviewed the field’s progress and Tsyganenko and Mardilovich6 discussed the assignment for hydroxyl spectral features and their changes upon dehydroxylation. Very recently, theoretical calculations have been shown to play an important role in revealing the nature of alumina structures and surface OH groups.7-8 Digne et al.7 proposed a nonspinel structure model for γ-alumina in contrast to the defective spinel model proposed by Kno¨zinger and Ratnasamy9 and assigned the OH groups observed from IR studies based on their theoretical calculations. For the structure of γ-alumina, whether the H exists in the bulk (spinel model) or not (nonspinel model) is still a topic of debate. Sun et al.’s10 DFT and XRD data Rietveld fitting work showed that the spinel-related model is better than the nonspinel model in describing the bulk structure of γ-alumina, while Paglia et al.’s11 atomic pair distribution function analysis of the powder diffraction data of γ-alumina supports the nonspinel model and also proposed a novel fine-scale nanostructure existing within a domain size of ∼1 nm in γ-alumina. Even though no final conclusions have been reached from these studies, progress has obviously been made toward a full understanding of the structure of γ-alumina. Lewis acidity is another aspect of the alumina surface. The Lewis acid sites are generated after dehydration/dehydroxylation of the surface and present in the form of coordinatively unsaturated aluminum ions. Studies on this aspect using techniques such as solid-state NMR,12-15 FT-IR,2,9,16-18 and theoretical calculations19-23 have revealed the presence of three-, * To whom correspondence should be addressed. Tel: (732) 205-7038. Fax: (732) 205-6900. E-mail: [email protected].

four-, and five-coordinate Al ions in aluminas as Lewis acid sites. Our previous studies24 on the surface of aluminas focused on the location of OH groups and Lewis acid sites on the surface. By using diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS) taking pyridine as a probe, we found that different Lewis acid sites have different adjacent OH groups. The pyridine molecules adsorbed on the Lewis acid sites simultaneously hydrogen-bond with their adjacent OH groups.24 We also found24,25 that Lewis acid sites control the interactions of foreign species with OH groups. These findings greatly helped us to understand how foreign species interact with the surface once they get onto it and explain why the ∼3775 cm-1 surface OH group is the “most reactive”. Despite a wealth of information about alumina surfaces and progress made in recent years, details of surface structures still need to be clarified. In this work, we report the results of our further studies on γ-alumina surfaces using the conventional pyridine adsorption/FT-IR spectroscopic technique. Here, we focus on the questions: (1) At ambient temperature and under hydrated conditions, does the γ-alumina surface still have Lewis acid sites? (2) Upon dehydration/dehydroxylation, does the Lewis acid sites on γ-alumina surface always have adjacent OH groups? (3) What happens when a dehydrated/dehydroxylated γ-alumina surface is rehydrated? The results reported here confirmed our previous findings and provided further details of arrangement of hydroxyls and Lewis acid sites on alumina surface. Experimental Section Chemicals and Sample Preparations. Pyridine was purchased from Aldrich and transferred from a newly opened bottle to a stainless steel bottle on the automation line (for adsorption and desorption) in a dry box under flowing dry N2. γ-Alumina was a product (SBA-150) of Sasol (surface area is ∼150 m2/ g). The different temperature-dehydrated/dehydroxylated γ-alumina was obtained in situ in the diffuse reflectance IR sample chamber (see below) by heating up the hydrated sample at each temperature for 2 h in flowing dry N2 (40 mL/min.).

10.1021/jp711901s CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008

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Figure 1. DRIFT spectra of the surface of hydrated γ-alumina, SBA-150, at 25 °C and after introduction of pyridine: (a) surface OH groups; (b) after introduction of pyridine (difference spectrum); and (c) expanded spectrum of pyridine ring vibration spectral region.

Instruments and Procedures for Acidity Measurements. DRIFT spectra were recorded on a Perkin-Elmer Paragon PC1000 spectrometer equipped with a MCT detector and a Spectra-Tech diffuse reflectance high-temperature chamber with KBr windows allowing gases such as N2 to flow through (spectral resolution, 2 cm-1). γ-Alumina was ground into a fine powder with an agate mortar and transferred evenly into an aluminum sample cup. The samples were first dehydrated at each temperature, 25, 50, 100, 200, 300, 400, and 450 °C, for 2 h under flowing dry N2 (40 mL/min), and then cooled to room temperature. For each temperature treatment, a new sample was used. Pyridine vapor was introduced in a N2 stream (pyridine ∼2%) for about 1 min and then shut off. The system was equilibrated at room temperature for 1 h under flowing N2, allowing removal of physisorbed pyridine. Single beam spectra were collected at room temperature before and after introduction of pyridine using an automation program. Difference spectra were obtained by subtracting the sample spectra without pyridine from those containing pyridine. Due to interactions of adsorbed pyridine with the surface, bands generated by adsorption are positive, while bands perturbed by adsorption are negative. For the spectra of aluminas without pyridine adsorption, a spectrum of finely ground KBr powder was used as a background. The reproducibility of the spectra collection under the same condition is quite good, within ∼5% deviation, due to the automation of the system. Results and Discussion We carried out the experiments in the following way: first, we examined the ambient temperature (25 °C) hydrated γ-alumina to understand whether the sample still has Lewis acidity. Then, we performed dehydration procedures at each temperature, 50, 100, 200, 300, 400, and 450 °C, for 2 h (new sample for each experiment) and measured their Lewis acidity and recorded the perturbation of surface OH groups after cooling to room temperature. We then correlate the Lewis acidity and the surface OH groups to understand the details of dehydration and dehydroxylation of the surface. Finally, we examined

rehydration of the pyridine adsorbed 450 °C pretreated alumina sample to see what happens on the surface upon rehydration. Surface of Fully Hydrated γ-Alumina. The structure of γ-alumina is built by AlO6 octahedra and AlO4 tetrahedra, but the most stable exposed surfaces of γ-alumina crystals are (100), (110), and (111) crystal planes with the (110) surface predominant (70-80%).7,8,26-27 Theoretical calculations7 have shown that on the (100) surface, only five-coordinate Al exists and surface relaxation does not drastically change the local geometry. On the (110) surface, four- and three-coordinate Al are present, which originate from bulk octahedral and tetrahedral Al, respectively. Surface relaxation introduces strong geometric modifications, leading to formation of pseudo-regular tetrahedral and planar AlO3 configurations. Different from the (100) and (110) surfaces, the (111) surface is built up of alternating stacking of oxygen and Al atoms and is not stable when fully dehydrated but stable when fully hydrated.7 Depending on temperature and water vapor pressure, the surfaces can have a variety of surface OH groups but when fully hydrated, all surface coordinatively unsaturated Al become saturated, i.e., sixcoordinated, AlVI.7 This means that no Lewis acid sites should exist on the surface of a fully hydrated alumina. To verify whether the theoretical results are true or not in the real world, we examined the surface of an ambient temperature hydrated γ-alumina using pyridine as a probe. Figure 1 shows DRIFT spectra of an ambient temperature hydrated γ-alumina, SBA150, and the alumina after pyridine adsorption. Since the sample is hydrated, the DRIFT spectrum in the hydroxyl spectral region (4000-2500 cm-1) of the sample prior to pyridine adsorption (see Figure 1a) gives a very broad band around 3510 cm-1. Such a spectral feature indicates that the surface OH groups and adsorbed water on the hydrated alumina are hydrogenbonded. Introduction of pyridine to this hydrated sample shows appearance of bands in its IR spectrum due to adsorbed pyridine. In Figure 1b, the positive bands in the spectral regions 17001400 cm-1 and 3200-2800 cm-1 are due to adsorbed pyridine and the negative bands at 3739 and 3699 cm-1 are associated with perturbed OH groups and the negative broad bands around

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Figure 2. (a) DRIFT spectra in the hydroxyl spectral region of the dehydrated γ-alumina after treatment at different temperatures (for 2 h) and (b) expanded spectra.

3450 cm-1 and relative narrow band at 1640 cm-1 are due to removal of adsorbed water. To view pyridine ring vibrations (8a, 8b, 19a, and 19b vibrations5) more clearly, we expanded the 1700-1400 cm-1 spectral region as given in Figure 1c. Two sets of bands, 1614 and 1593 cm-1 versus 1449 (shoulder) and 1443 cm-1 are clearly observed, corresponding to two types of Lewis acid sites of different acid strengths.5,24 From our previous studies,24,25 we know that the pyridine adsorbed on Lewis acid sites simultaneously interacts with their adjacent OH groups via hydrogen bonding. The observation of the perturbed OH groups at 3739 and 3699 cm-1 (Figure 1b) is consistent with our previous observations and indicates that the same has happened on the hydrated alumina. On the basis of our previous work, it is understood that the pyridine adsorbed on weak Lewis acid sites (bands at 1443 and 1593 cm-1) simultaneously hydrogenbonds to the 3739 cm-1 OH group, while that adsorbed on the medium strong Lewis acid sites (bands at 1449 and 1614 cm-1) to the 3699 cm-1 OH group. Estimation of the concentration of Lewis acid sites on the hydrated alumina using the bands at 1449 and 1443 cm-1 (Lewis acidity measurement) gives total Lewis acid sites, 0.35 mmol/g, which corresponds to ∼1.5 Lewis acid sites per nm2. Such low concentration of adsorbed pyridine on the hydrated alumina excludes direct adsorption of pyridine on the hydrogen-bonded OH groups. Direct interactions of adsorbed pyridine with OH groups via its N functional group (OH‚‚‚N) do not occur at ambient temperature, because if this was the case, a much high number of adsorbed pyridine would be observed [for hydrated alumina, much higher numbers of OH groups (10-20 times the Lewis acid sites observed) exist on the surface]. The results given above clearly demonstrate that surface hydration of alumina cannot completely convert all the surface Lewis acid sites to fully six-coordinate Al and that five- and four-coordinate Al Lewis acid sites can survive after hydration. An alternative explanation of the observation is that the water molecules adsorbed on the Lewis acid sites interact with the Lewis acid sites so weakly that pyridine can replace them upon adsorption. Estimation of the concentrations of the five- to fourcoordinate Al Lewis acid sites using the 1449 and 1443 cm-1 band intensities, assuming the same extinction coefficients of adsorbed pyridine on both sites, gives 0.24 mmole/g of weak Lewis acid sites and 0.11 mmole/g of medium strong Lewis acid sites on this ambient temperature hydrated alumina surface. The concentration ratio of these two types of Lewis acid sites is ∼2:1. Since it has been known that under hydrated conditions, the (100) and (111) surfaces do not have coordinatively unsaturated Al and only the (110) surface may have five- and

four-coordinate Al;7 therefore, the observed Lewis acid sites can only be located on the (110) surface. Compared to the dehydrated aluminas, it is noticed that the adjacent OH groups of Lewis acid sites perturbed by the adsorbed pyridine are not the same as those after dehydration/dehydroxylation (see below). This may be due to hydrogen bonding of the OH groups with other OH groups on the hydrated alumina. Surface of Dehydrated/Dehydroxylated γ-Alumina. Dehydration was performed at 50, 100, 200, 300, 400, and 450 °C each for 2 h in flowing dry N2. After each temperature treatment and cooling down to ambient temperature, pyridine vapor was introduced and the system was purged for 1 h to remove physisorbed pyridine. Difference spectra of chemisorbed pyridine were obtained by using the spectrum of the sample before introducing pyridine as background. For the spectra of the samples before pyridine adsorption, a spectrum of finely ground KBr powder was used as background. Figure 2 gives DRIFT spectra in the hydroxyl spectral region of the alumina dehydrated at different temperatures (Figure 2a). Similar to what were reported in the literature,5,6 dehydration removes adsorbed water, destroys the hydrogen bonding and dehydroxylates the surface. These changes are reflected in the spectra. With increasing temperature, the band at 3699 cm-1 disappeared and the hydrogen-bonding band around 3510 cm-1 was significantly decreased, and new bands at 3787, 3763, 3726, and 3674 cm-1 were developed (see Figure 2b for details). The appearance of new bands is a consequence of dehydroxylation and destruction of hydrogen bonding of the OH groups on the surface. Following the literature’s assignment5,6 and the general trend of hydroxyl band wavenumbers, I4 > I6 > II66 > II64 > III (the number represents the coordination of the Al),5,6,27,28 the 3787 cm-1 band is associated with type I4 OH group, the 3763 cm-1 band with type I6 OH group, the 3726 cm-1 band with type II66 OH group, and the 3674 cm-1 band with type III OH group. Note that the recent theoretical work by Digne et al.7 assigned the bands to OH groups located on different crystal planes. Following Digne et al.’s assignment, the 3787 cm-1 band is associated with type I4 OH group (HO-µ1-AlIV) on the (110) plane, the 3763 cm-1 band is with type I6 OH group (HO-µ1AlVI) on the (100) plane, the 3726 cm-1 band is a watermolecule-occupied Lewis acid site (H2O-µ1-AlV) on the (110) surface or is with type I6 OH group (HO-µ1-AlVI) on the (111) surface, and the 3674 cm-1 band is with type I66 OH group (HO-µ2-AlVI) on the (110) and (111) surfaces. Our present work (Figure 2) does not support the assignment of the 3726 cm-1 band to a water-molecule-occupied Lewis acid site (H2O-µ1-

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Figure 3. DRIFT spectra in the pyridine ring vibration region of the dehydrated γ-alumina after treatment at different temperatures (for 2 h). Temperature increases in the sequence of increasing band intensity.

Figure 4. Plot of pyridine 8a bands intensities as a function of pretreatment temperature (°C).

AlV) on the (110) surface because as we can see from Figure 2, the 3726 cm-1 band intensity increases upon dehydration. Figure 3 gives the spectra in the pyridine ring vibration spectral region of the samples after pyridine adsorption. It is clear from Figure 3 that with increasing the pretreatment temperature, more Lewis acid sites with different acid strengths are generated on the surface. The strong Lewis acid sites start to form at temperature ∼200 °C and increase in quantity with increasing temperature (see the 8a and 19b bands at 1621 and 1453 cm-1 in Figure 3). The changes of the Lewis acid sites with increasing temperature are shown in Figure 4 by plotting the 8a bands intensities versus the pretreatment temperature. From the plots, it is seen that with increasing temperature, the

weak Lewis acid site (represented by the 1592 cm-1 band) increases in number first and then decreases, passing through a maximum at temperature ∼200 °C. This implies that the weak Lewis acid sites can be converted to medium strong or strong Lewis acid sites with increasing pretreatment temperature. For the medium strong Lewis acid sites (the 1612 cm-1 band), the number increases rapidly with temperature and above 300 °C, the rate decreases. The strong Lewis acid sites (the 1621 cm-1 band) become obvious only at temperatures above 200 °C and increase with temperature. At lower temperatures, the weak Lewis acid sites dominate while at higher temperatures, the medium strong and strong Lewis acid sites become majority.

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Figure 5. DRIFT spectra in the hydroxyl spectral region of the dehydrated γ-alumina after treatment at different temperatures (for 2 h) showing the perturbation of the OH groups upon pyridine adsorption on the Lewis acid sites.

Figure 6. Plot of changes of the band intensities of different perturbed OH groups with the pretreatment temperature (°C).

The pyridine adsorbed on these Lewis acid sites perturbs their adjacent OH groups. Figure 5 shows the spectra in the hydroxyl spectral region of the samples after pyridine adsorption. The perturbation of OH groups is obvious by the negative bands and takes place via hydrogen bonding of the pyridine ring with the OH groups as evidenced by the positive broad bands around 3643 and 3535 cm-1 and a very weak broad band around 3350 cm-1. Different interaction strengths of pyridine with the Lewis acid sites of different acid strengths result in different electron densities on the pyridine ring, and therefore, the hydrogen bonding of the pyridine ring with their adjacent OH groups

varies in strength, giving rise to different hydrogen-bonding bands.24 Increasing pretreatment temperature increases the number of perturbed OH groups. Deconvolution of the perturbed bands separates the types of perturbed OH groups and plot of band intensities against temperature allows us to correlate the perturbed OH groups with different Lewis acid sites. Figure 6 gives plots of changes of different perturbed OH groups as a function of pretreatment temperature. Compared to Figure 4, it is found that with increasing temperature, the intensity change of the 3729 cm-1 band correlates well with that of the weak Lewis acid sites. This confirms our previous finding24 that this

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Figure 7. Plot of integrated perturbed OH band intensity against total Lewis acid sites for the samples pretreated at different temperatures.

band is adjacent to the weak Lewis acid site. The intensity change of the 3764 cm-1 band parallels that of the medium strong Lewis acid sites, indicating that this band is next to the medium strong Lewis acid site. The intensity change of the 3787 cm-1 band is similar to that of the strong Lewis acid sites, indicating that this band is adjacent to the strong Lewis acid site. In contrast to the bands discussed above, the intensity of the 3677 cm-1 band only exhibits a slight decrease with increasing the pretreatment temperature. By comparing Figures 4 and 6, it is understood that 200 °C is the temperature at which surface dehydroxylation starts to take place. Below 200 °C, the weak and medium strong Lewis acid sites are mainly surface five- and four-coordinate Al which are uncovered by dehydration, whereas above 200 °C, five- and four-coordinate Al starts to convert to four- and three-coordinate Al via dehydroxylation. From the surface structure (see above), it is known that the five-coordinate Al can only be generated from water removal or dehydroxylation of the six-coordinate Al while the four-coordinate Al can come from either the structural four-coordinate Al or the further dehydroxylated fivecoordinate Al. Due to the structural connection difference, the four-coordinate Al Lewis acid sites generated from these two routes may not be the same in acid strength. In the literature,29 medium-weak and medium-strong Lewis acid sites have been proposed. The three-coordinate Al can be formed via dehydroxylation of four-coordinate Al. The 3677 cm-1 band may be associated with intermediate four-coordinate Al medium strong Lewis acid sites whose number increases when the fivecoordinate Al dehydroxylates and decreases when further dehydroxylates. Correlating the integrated band intensity of all perturbed OH groups with the total Lewis acid sites for the samples pretreated at different temperatures gives a plot as shown in Figure 7. A straight line is clearly seen which passes through the origin. This strongly suggests that all OH groups perturbed by the adsorbed pyridine on the Lewis acid sites are adjacent OH

groups and that all Lewis acid sites have adjacent OH groups. Whether or not this conclusion holds for alumina dehydrated at even higher temperatures, >500 °C, needs to be clarified. We could not study it at the present time due to the limitation of our instrument. However, the observation has been reported in the literature30 that migration of surface OH groups takes place on the dehydrated alumina surface. Rehydration of the Surface of Pyridine Adsorbed 450 °CDehydrated γ-Alumina. Figure 8 shows DRIFT spectra of a pyridine-adsorbed 450 °C-dehydrated alumina before and after rehydration by exposure of the sample in ambient air for 30 min. Compared to the spectrum before rehydration, the spectrum after rehydration showed significant changes, indicating removal of adsorbed pyridine. The still disturbed OH groups after rehydration by the pyridine remaining on the Lewis acid sites are those having bands at 3739 and 3677 cm-1. The overall looking of the spectrum is very similar to that of the fully hydrated sample except for the 3677 cm-1 band. Rehydration of the surface removes the pyridine adsorbed on the strong and medium-strong Lewis acid sites and reconverts the original surface. The results demonstrate that the surface hydration preferentially takes place on the Lewis acid sites that are generated during the previous thermal treatment, i.e., the strong and medium-strong Lewis acid sites, and the weak Lewis acid sites can survive the rehydration of the dehydroxylated surface. Water reacts with the three and four coordinate Al ions and repels the adsorbed pyridine from these sites but does not react with the five coordinate Al and repels the pyridine on it. This confirms our observation that on the surface of the 25 °Cdehydrated alumina sample (see the first section above), coordinatively unsaturated Al sites (Lewis acid sites) cannot be completely converted to six-coordinate Al sites or water cannot replace the pyridine adsorbed on these weak Lewis acid sites. It needs to be noticed that the medium-strong Lewis acid sites generated at above 200 °C dehydroxylation are different from

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Figure 8. DRIFT spectra of a pyridine adsorbed 450 °C-dehydrated alumina before and after rehydration by exposure of the sample in ambient air for 30 min.

those observed on the fully hydrated surface. The existence of two types of medium-strong Lewis acid sites may be in line with the literature report29 that there are medium-weak and medium-strong Lewis acid sites on the γ-alumina surface. A complete surface relaxation after rehydration of the dehydrated sample is not achieved (at least within the 30 min exposure to ambient air), as evidenced by the band position, 3699 cm-1 versus 3677 cm-1 (see Figures 5 and 8). Conclusions The studies of surface of γ-aluminas dehydrated at different temperatures using the conventional pyridine/DRIFTS technique have shown that fully hydrated γ-alumina surface still has weak and medium-strong Lewis acid sites. They are covered by adsorbed water that can be replaced by strong base molecules such as pyridine. Upon dehydration/dehydroxylation, mediumstrong and strong Lewis acid sites are generated. The pyridine adsorbed on the Lewis acid sites hydrogen bonds simultaneously with their adjacent OH groups. A linear correlation of the total intensity of perturbed OH bands with the total Lewis acidity demonstrates that all of the Lewis acid sites have adjacent OH groups. Correlating weak, medium-strong, and strong Lewis acidity with different types of perturbed OH groups confirms our previous observations.24 The weak Lewis acid sites have type II66 or II64 neighboring OH groups, the medium strong Lewis acid sites have type III or I6 OH groups near by, while the strong Lewis acid sites have type I4 OH groups next to them. Due to the proximity of OH groups and Lewis acid sites on the surface, interaction of OH groups with adsorbed species is not determined by the OH group acidity itself or by their space accessibility, but by the strength of the Lewis acid sites. Our results further demonstrate that the three types and strengths of Lewis acid sites correspond to the three possible Al3+ coordination configurations, five-, four- and threecoordinate. The five-coordinate Al3+ sites are weak Lewis acid sites, while the four- and three-coordinate Al3+ sites are medium and strong Lewis acid sites. Due to different formation mechanisms of the four-coordinate Al, dehydration and further dehydroxylation of five-coordinate Al, medium strong Lewis acid sites with different acid strengths are present, which are reflected from the different perturbed OH groups. Hydration of the dehydrated/dehydroxylated surface of γ-alumina occurs preferentially on the strong and medium strong

Lewis acid sites, restoring the original alumina surface. Weak and some medium strong Lewis acid sites cannot be rehydrated if strong base molecules such as pyridine are adsorbed on them. References and Notes (1) (a) Tanabe, K. Solid Acids and Bases; Academic Press: New York, 1970. (b) Goldstein, M. S. In Experimental Methods in Catalytic Research; Anderson, R. B., Ed.; Academic Press: New York, 1968. (2) Wefers, K.; Misra, C. Oxides and Hydroxides of Aluminum, Alcoa Technical Paper 19 revised, Alcoa Laboratories, Aluminum Company of Ameraica, Pittsburgh, PA, 1987. (3) Schuth, F.; Unger, K. Handbook of Heterogeneous Catalysis; Ertl, G., Knozinger, H., Weitkamp, J., Eds.; Wiley, New York, 1997; Vol. 1, p 72. (4) Kabalka, G. W.; Pagni, R. M. Tetrahedron 1997, 53, 7999. (5) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497. (6) Tsyganenko, A. A.; Mardilovich, P. P. J. Chem. Soc., Faraday Trans. 1996, 92, 4843. (7) (a) Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P. Toulhoat, H. J. Catal. 2002, 211, 1. (b) Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. J. Catal. 2004, 226, 54. (c) Raybaud, P.; Digne, M.; Iftimie, R.; Wellens, W.; Euzen, P.; Toulhoat, H. J. Catal. 2001, 201, 236. (8) Sohlberg, K.; Pennycook, S. J.; Pantelides, S. T. J. Am. Chem. Soc. 1999, 121, 7493. (9) Kno¨zinger, H.; Ratnasamy, P. Catal. ReV., Sci. Eng. 1978, 4, 31. (10) Sun, M.; Nelson, A. E.; Adjaye, J. J. Phys. Chem. B 2006, 110, 2310. (11) Paglia, G.; Bozin, E. S.; Billinge, S. L. Chem. Mater. 2006, 18, 3242. (12) (a) Morris, H. D.; Ellis, P. D. J. Am. Chem. Soc. 1989, 111, 6046. (b) Majors, P. D.; Ellis, P. D. J. Am. Chem. Soc. 1987, 109, 1648. (c) Huggins, B. A.; Ellis, P. D. J. Am. Chem. Soc. 1992, 114, 2098. (13) (a) Coster, D.; Blumenfeld, A. L.; Fripiat, J. J. J. Phys. Chem. 1994, 98, 6201. (b) Blumenfeld, A. L.; Fripiat, J. J. Top. Catal. 1997, 4, 119. (14) Lunsford, J. H. Top. Catal. 1997, 4, 91. (15) Ripmeester, J. A. J. Am. Chem. Soc. 1983, 105, 2925. (16) Parry, E. P. J. Catal. 1963, 2, 371. (17) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garrone, E. J. Chem. Soc., Faraday Trans. 1, 1979, 75, 271. (18) Zecchina, A.; Platero, E. E.; Area´n, C. O. J. Catal. 1987, 107, 244. (19) Neyman, K. M.; Nasluzov, V. A.; Zhidomirov, G. M. Catal. Lett. 1996, 40, 183. (20) Fleisher, M. B.; Golender, L. O.; Shimanskaya, M. V. J. Chem. Soc., Faraday Trans. 1991, 87, 745. (21) Lindblad, M.; Pakkanen, T. A. Surf. Sci. 1993, 286, 333. (22) Tochikawa, H.; Tsuchida, T. J. Mol. Catal. A 1995, 96, 277. (23) Peri, J. B. J. Phys. Chem. 1965, 69, 220. (24) Liu, X.; Truitt, R. J. Am. Chem. Soc. 1997, 119, 9856. (25) Liu, X. J. Phys. Chem. B 1999, 103, 2647. (26) (a) Tsyganenko, A. A.; Filimonov, V. N. Spectrosc. Lett. 1972, 5, 477. (b) Tsyganenko, A. A.; Filimonov, V. N. J. Mol. Struct. 1973, 19, 579.

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J. Phys. Chem. C, Vol. 112, No. 13, 2008 5073 (29) Lundie, D. T.; McInroy, A. R.; Marshall, R.; Winfield, J. M.; Jones, P.; Dudman, C. C.; Parker, S. F.; Mitchell, C.; Lennon, D. J. Phys. Chem. B 2005, 109, 11592. (30) Huggins, B. A.; Ellis, P. D. J. Am. Chem. Soc. 1992, 114, 2098.