Sulfate Ions and Boehmite Crystallization in a Sol Made with

J. Phys. Chem. C 2007, 111, 103-107

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Sulfate Ions and Boehmite Crystallization in a Sol Made with Aluminum Tri-sec-butoxide and 2-Propanol Xim Bokhimi,*,† Antonio Morales,† and Jaime S. Valente‡ Institute of Physics, The National UniVersity of Mexico, A.P. 20-364, 01000 Me´ xico D. F., Mexico, and Instituto Mexicano del Petro´ leo, A.P. 14-805, 07730 Me´ xico D. F., Mexico ReceiVed: February 23, 2006; In Final Form: NoVember 3, 2006

The properties of colloidal particles in a sol prepared with aluminum tri-sec-butoxide and 2-propanol containing different concentrations of sulfuric acid were studied. The particles were nanocapsules with diameters between 10 and 50 nm and shells 3.5 nm thick, made of noncrystalline ordered Al-O polynuclear species. When the nanocapsules interacted with each other, their atoms reordered into the crystalline structure of boehmite. This interaction was diminished when the nanocapsule shells were covered with sulfate ions. The concentration of sulfate ions allowed control of boehmite crystallization, which was completely hindered when the concentration was high enough to cover all the nanocapsules in the sample. The initial crystallization of boehmite was analyzed in detail; this showed that the formation of crystalline boehmite bars was a consequence of the contours formed by the interaction between neighboring nanocapsules. This interaction and its control via the sulfate ions determined the texture of the sample when it was calcined, which is important for the applications of this system. The study helps to understand the properties of aluminum oxide prepared by the sol-gel technique, especially when sulfuric acid is the hydrolysis catalyst.

Introduction γ-Alumina is an oxide widely used in catalysis as the active phase or as the support1-4 and is characterized by having acidic sites.5,6 This acidity determines the activity and selectivity of the catalyst for specific catalytic reactions. Therefore, with the idea of changing and controlling alumina acidity, many research groups have doped it with sulfate ions.7-11 Alumina doping can be performed by impregnating its solid material with sulfuric acid or with another compound containing sulfate ions,8-11 such as ammonium sulfate.10 This impregnation occurs on the solid’s surface which is predetermined by the synthesis method and the conditions used to prepare the alumina. Another way of bringing sulfate ions on alumina is to add them via the solutions used for the synthesis of its precursor.12,13 Among the alumina precursors, boehmite is especially interesting, because its transformation into γ-alumina is pseudomorphic,14 which preserves boehmite’s texture.15 Since this work focuses on interest in the properties of sulfated γ-alumina derived from boehmite, this alumina precursor was synthesized via the sol-gel method and sulfuric acid was used as the hydrolysis catalyst in order to simultaneously provide the sulfate ions that will give rise to sulfated alumina. During the synthesis of boehmite under the above conditions, it was found that, prior to the formation of crystalline boehmite, the sample consists of nanocapsules with shells made of noncrystalline ordered atomic clusters in which Al13 tridecamers are the main component.16 These nanocapsules should be, then, the colloidal particles of the sol made during the synthesis. Therefore, the properties of these colloidal particles and the effect of sulfate ions on them and on the interaction between them was studied in detail. * Corresponding author: e-mail [email protected]. † The National University of Mexico. ‡ Instituto Mexicano del Petro ´ leo.

In the present work we will show that the interaction between the colloidal particles gave rise to the crystallization of boehmite and that this interaction depended on the sulfate ion concentration in the sol. The interaction between the colloidal particles, which were nanocapsules, opened their closed structure, increasing the specific surface area of the material and creating conditions for the generation of crystalline boehmite when the sol was aged at room temperature in an open glass vessel. Experimental Section Synthesis. Nonsulfated Samples. Aluminum tri-sec-butoxide (AB) (Aldrich, 97%) was dissolved at room temperature in anhydrous 2-propanol for 4 h at a 2-propanol/AB molar ratio of 60. Sulfated Samples. Aluminum tri-sec-butoxide was dissolved and refluxed in anhydrous 2-propanol for 1 h at 85 °C; then the solution was cooled to room temperature and the sulfuric acid was added dropwise. Thereafter, this solution was heated to 85 °C and refluxed at this temperature for one more hour. After that, it was cooled down to room temperature and maintained there for 1 h, while the water was added dropwise. Then the temperature of the sample was raised again to 85 °C and held there for 3 h. The molar ratios of the components 2-propanol/AB ) 60 and H2O/AB ) 1 were constant, while the molar ratio H2SO4/AB was 0.01, 0.03, 0.06, or 0.09. The obtained gel was aged at room temperature for 30 days simply by depositing it in an open glass vessel. The corresponding aged and nonaged products were dried overnight in static air at 120 °C; one portion of each was calcined from room temperature up to 550 °C at 4 °C/min and maintained at this temperature for 4 h to transform the boehmite into γ-alumina. Experimental Techniques. X-ray Powder Diffraction. The diffraction patterns of the samples were measured in a θ-θ Bruker D-8 Advance diffractometer having Bragg-Brentano

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Figure 1. Micrographs of the sample without sulfate ions: (A) isolated colloidal particle; (B) several interacting colloidal particles.

Figure 3. X-ray diffraction patterns of nonaged samples prepared at different SO42-/Al molar ratios.

Figure 2. X-ray diffraction patterns of the sample without sulfate ions: (A) nonaged sample; (B) aged sample.

geometry, Cu KR radiation, a graphite secondary-beam monochromator, and a scintillation detector. Diffraction intensity was measured between 3° and 110°, with a 2θ step of 0.02° and a counting time of 9 s/point. The boehmite crystalline structure was refined via the Rietveld method by use of the Fullprof code.17 Crystallite morphology was modeled by using spherical harmonics;18 the background was modeled with a polynomial function that in addition to the constant, linear, quadratic, and cubic terms in 2θ also contained the terms (1/2θ) and (1/2θ)2. Transmission Electron Microscopy. Samples were analyzed with transmission electron microscopy in a Jeol JEM-100CX, a Jeol JEM-4000EX, and a Jeol JEM-2010F microscope. Samples were dispersed in ethanol before placing them in the copper grid with Formvar support. Texture. Specific surface area, pore volume, and pore diameter distribution were obtained from nitrogen adsorption-desorption isotherms measured in a Micromeritics ASAP-2100 analyzer. Nitrogen adsorption of the samples calcined at 550 °C was carried out in situ at liquid nitrogen temperature; before adsorption, the sample was outgassed at 400 °C for 4 h under a residual pressure of 10-5 Torr. Results and Discussion The dissolution of the aluminum tri-sec-butoxide in 2-propanol at room temperature gave rise to a transparent sol that presented the Tyndall effect. Transmission electron microscopic analysis of drops of this sol showed that the colloidal particles were nanocapsules of Al-O polynuclear species with diameters between 10 and 50 nm (Figure 1A) and shells with a noncrystalline atomic distribution that did not change when the sol was dried, as was shown in the X-ray powder diffraction analysis (Figure 2A). The nanocapsules in the sol aggregated when they interacted each other; this interaction also partially crystallized the atomic local distribution in the shells from the noncrystalline state into the atomic distribution of crystalline boehmite (Figures 1B and 2B).16

Aggregation was favored by aging the sample at room temperature in an open glass vessel, which slowly caused dissolvent evaporation. Because the diminishing of the solvent in the sol increased the number of interacting nanocapsules in the sample, the number of capsule shell crystallizing regions also increased with aging (Figure 2B). Particles aggregation in a sol is hindered when peptizing ions are present in the solvent19 because these ions surround the colloidal sol particles and hinder their interactions. This peptizing effect was observed when sulfuric acid was present during the synthesis of a sol prepared with aluminum tri-sec-butoxide and 2-propanol.16 This effect of sulfate ions was more evident when their concentration was varied; sulfate ion concentration was quantified through the SO42-/Al molar ratio. For all SO42-/Al molar ratios, the nonaged samples, generated by drying the corresponding sol, produced an X-ray diffraction pattern with only a few broad peaks (Figure 3), which is the typical diffraction pattern for molecules or atomic clusters.20 The patterns were similar to the one obtained for the fresh sample without sulfate ions (Figure 2A). Because aluminum solutions are characterized by consisting of Al-O polynuclear species,21-24 of which Al13 tridecamer is an example,21 the diffraction patterns described in Figures 2A and 3 can be interpreted as produced by objects made up of these kinds of polynuclear species. Recently,16 we showed that the peaks in the above diffraction patterns, at angles above 15°, are reproduced if one assumes that they are generated by the X-ray scattering produced by Al13 tridecamers, which then provide a first approximation regarding the atomic local order of the nanocapsules that constitute the sol. The Al-O polynuclear species are ordered with a noncrystalline symmetry in the nanocapsule shells. This ordering gives rise to an X-ray diffraction peak below 2θ ) 15°, which, in the present case, had an intensity that depended on the SO42-/Al molar ratio (Figure 3). That is, the intensity of this peak increased with the molar ratio; in the following paragraphs we will show that the intensity increase was related to the diminished interaction between the nanocapsules. Samples with different sulfuric acid concentrations produced similar X-ray diffraction patterns since their peaks were in the same positions. Due to the fact that these peaks corresponded to the local atom distribution determined by the atomic clusters, the coincidence of the peak positions for all diffraction patterns indicated that this distribution did not change with sulfuric acid concentration. The intensities of these peaks, however, diminished as the density of sulfate ions in the sample decreased, itself favored by the decrease in the number of sulfate ions in the sol.

Boehmite Crystallization in Aluminum Alkoxide Sol

J. Phys. Chem. C, Vol. 111, No. 1, 2007 105

Figure 5. X-ray diffraction patterns of the aged samples prepared at different SO42-/Al molar ratios; the Miller indices correspond to boehmite.

Figure 4. Micrographs of nonaged samples prepared at different SO42-/ Al molar ratios: (A) 0.09, (B) 0.06, (C) 0.03, and (D) 0.01.

TABLE 1: Textural Properties of the Nonaged Samples Calcined at 550 °C SO42-/Al molar ratio

specific BET surface area (m2/g)

pore volume (cm3/g)

avg pore diameter (nm)

0.01 0.03 0.06 0.09

511 431 380 269

1.17 0.68 0.72 0.51

8.5 6.1 7.4 7.3

The intensity of the diffraction peak below 15° also decreased as the SO42-/Al molar ratio decreased. Since this peak is associated with the ordering of the Al-O polynuclear species in the capsules shell,16 the decrease in its intensity indicates that the number of ordered Al-O polynuclear species decreased as a consequence of strong interaction between the nanocapsules. This means that increasing the number of sulfate ions during the synthesis decreased the number of nanocapsules interacting strongly, which also hindered their aggregation. Micrographs of the nonaged samples (Figure 4) clearly show that they were made of nanocapsules when the SO42-/Al molar ratios were 0.03, 0.06, and 0.09 (panels A-C). These nanocapsules, however, were not detected in the micrographs of the sample prepared at a molar ratio of 0.01 (Figure 4D); instead, they showed images more similar to those reported previously for the samples prepared at a molar ratio of 0.03 and aged at room temperature for 20 days,16 which correspond to the capsule openings produced by the interaction between capsules. The above results show that the interaction between the nanocapsules in the sol decreased as the sulfate ion concentration in it increased. The capsule opening augmented the specific surface area and the pore volume of the samples calcined at 550 °C (Table 1); therefore, in the sample prepared at a SO42-/Al molar ratio of 0.01, these texture parameters were 511 and 1.17 cm3/g, respectively; while they were only 269 and 0.51 cm3/g for the sample prepared at a SO42-/Al molar ratio of 0.09. These results clearly show that the specific surface area of the sample was determined by interaction between the colloidal particles in the sol. It is important to remark here that texture was measured in the samples calcined at 550 °C, whose phase was γ-alumina, obtained after the pseudomorphic transformation of the boehmite phase. This kind of transformation maintains particle morphol-

ogy and texture.15 To avoid confusion in the following paragraphs, it must be remembered that texture was measured in the samples calcined at 550 °C for 4 h. The specific surface area of the sample containing almost exclusively nonopened capsules was determined by the capsules’ external surface area, while in the sample made up of opened capsules the internal surface area of the capsules also contributed to the total specific surface area. Since the capsules shell was 3.5 nm thick, when capsules collapsed, the cross section of the broken shells contributed to this specific surface area as well. The average pore diameter of the samples was almost independent of the SO42-/Al molar ratio (Table 1): It varied between 6.1 and 8.5 nm, with the largest value corresponding to the sample with opened capsules. In the nonopened capsules, the pore volume was determined by the interstitial space between capsules, which was smaller than the capsules’ diameters. In order to analyze the time evolution (at a fixed temperature) of the samples for different sulfate ion concentrations, samples were aged at room temperature for 30 days. Although the aging temperature was low, changes were observed not only in the atomic local order but also in the texture of the corresponding calcined sample. The magnitude of the changes depended on the SO42-/Al molar ratio. The largest changes were observed in the sample prepared at a molar ratio of 0.01, where the number of interacting capsules was also the largest. After this sample was aged at room temperature for 30 days, most of it was crystalline (Figure 5). In contrast, the sample prepared at a SO42-/Al molar ratio of 0.09 never crystallized; the corresponding micrograph (Figure 6A) shows that the texture of this sample was essentially the same as the one observed in the sample without aging (Figure 4A). In spite of aging, this sample remained made up of noninteracting (or weakly interacting) nanocapsules. The X-ray diffraction patterns of the aged samples prepared at molar ratios of 0.01, 0.03, and 0.06 contained crystalline boehmite peaks (Figure 5). In fact, these samples contained a mixture of two phases: boehmite and the phase obtained in the nonaged samples, which was made of Al-O polynuclear species condensates. The phase concentrations, however, could not be quantified because the software used for the Rietveld refinement did not contain a proper model to simulate the phase made up of polynuclear species. The X-ray diffraction patterns (Figure 5) show that the boehmite concentration decreased when the SO42-/ Al molar ratio was increased. Crystallization of the nanocapsule shells, made up of Al-O polynuclear species, into boehmite depended not only on the interaction between capsules but also on sample aging, which induced atom reordering. Initially, the interaction only opened the capsules (Figure 4D) while maintaining the noncrystalline

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Figure 6. Micrographs of aged samples prepared at different SO42-/ Al molar ratios: (A) 0.09, (B) 0.06, (C) 0.03, and (D) 0.01.

Figure 7. Micrograph at different magnifications of an aged sample prepared at a SO42-/Al molar ratio of 0.03.

atomic local distribution of the capsules’ shells, as is concluded from Figure 3 for the SO42-/Al molar ratio of 0.01, where the diffraction pattern did not show any boehmite diffraction peak even though the corresponding micrograph shows that the capsules were opened (Figure 4D). With aging, the atomic local order in the capsules’ shells was transformed, via the interaction between them, into the atomic local order of crystalline boehmite (Figure 5). The interaction between the Al-O polynuclear species of two different nanocapsules in contact initiated the transformation of their atomic order: Eventually, the Al-O octahedra representative of the atomic local order in the Al-O polynuclear species rearranged into Al-O octahedra representative of the atomic local order of boehmite. Since the transformation of Al-O polynuclear species’ atomic local order into that of boehmite required several days at room temperature, the interaction between two capsules should be very weak. The above results indicate that the interaction between two different capsules was a necessary but insufficient condition for shell crystallization. Time was the other parameter that must be taken into account to explain the observed crystallization. Of course, the aging temperature would be another physical parameter that should be considered for this transformation; in the present case, however, it was maintained constant at room temperature. The TEM micrographs of the nanocapsules opened by their mutual interaction show a contrast, giving the appearance that samples were made of fibers (Figures 4D, 6B-D, and 7). A careful analysis of these samples by high-resolution transmission electron microscopy shows that the fiberlike appearance was formed by the contrast produced by those regions of the shells of adjacent interacting capsules that crystallized through the interaction (Figure 7C,D). These were the first regions where the atomic local order was transformed into that of boehmite during aging (Figure 5); therefore, when the electron beam of the microscope interacted with these regions, the contrast they produced was different from the contrast produced by the noncrystallized regions of the interacting capsules, because they had a different atomic local order. It is worthwhile to note that these apparent fibers had a significant curvature when they were produced in the nonaged

sample at a SO42-/Al molar ratio of 0.01 (Figure 4D), in contrast with the apparent fibers observed in the aged samples, which were straight (Figure 6B-D). This can be explained in terms of the atomic local order: In the apparent fibers of the nonaged sample with a molar ratio of 0.01, the atomic local order was similar to that of the Al-O polynuclear species, the Al13 tridecamers (Figure 2A),16 which was noncrystalline and can give rise to curved objects such as capsules. In the aged samples, however, the interacting regions of the capsules crystallized, producing an atomic order with translational symmetry that is energetically incompatible with curved objects. Therefore, the objects generated during crystallization were straight. The interaction between particles in a sol is softened, and in some cases hindered, when ions are attached to their surface, as occurs in peptized samples.19 In the present case, similar behavior occurred because the capsules were covered by the sulfate ions used in the synthesis. Therefore, as the concentration of these ions was increased, the number of capsules, with shells free of sulfate ions, that could interact with other capsules was reduced, thus diminishing the number of crystalline regions in the sample because crystallization occurred only between capsules not covered with sulfate ions. The above results show that the sulfuric acid used in the sol reported in the present work peptized the sample, isolating the colloidal particles and hindering the interaction between them. The above results describe the initial steps of boehmite crystallization in a sol prepared with tri-sec-butoxide and 2-propanol; they also show the initial stages that generate boehmite samples’ texture, particularly the generation of boehmite crystallites with a bar morphology.25-28 The contact between interacting capsules produced the first crystalline regions with undefined morphology and with dimensions smaller than the shell thickness, 3.5 nm (Figure 8A). The next step in the crystalline morphology was generated by the extension of this crystallization between adjacent nanocapsules, which produced the apparent fibers described above (Figure 7). Because of the rigidity associated with the crystalline phase when crystallite grew, these apparent fibers were eventually transformed into straight crystallites with a bar morphology (Figure 8B), a fact frequently reported by other authors in micrographs of boehmite.25-28

Boehmite Crystallization in Aluminum Alkoxide Sol

J. Phys. Chem. C, Vol. 111, No. 1, 2007 107 colloidal particles. In the initial stage, these contours were curved because their atomic arrangement did not have translation symmetry, but by aging the sample, they became straight when the atoms reordered into a structure with translation symmetry to build the crystalline structure of boehmite. The present study provides a methodology for understanding the initial steps of crystallization in samples of aluminum oxide prepared by the sol-gel method, as well as the formation of the corresponding sample texture, which is very important for the different applications of the system, for example, catalysis.

Figure 8. Micrographs of (A) an aged sample and (B) a boehmite sample prepared under hydrothermal conditions.

TABLE 2: Textural Properties of the Aged Samples Calcined at 550 °C SO42-/Al molar ratio

specific BET surface area (m2/g)

pore volume (cm3/g)

avg pore diameter (nm)

0.01 0.03 0.06 0.09

386 317 456 435

1.82 0.97 1.44 1.62

14.8 9.4 9.3 11.7

Aging notoriously increased the specific surface area and pore volume of the calcined samples prepared with high sulfate ion concentrations (Table 2); in this case the interaction between nanocapsules was weak because they were covered with sulfate ions. This increase suggests a reordering of capsule aggregation to produce more and larger pores (Table 2). The decrease of the surface area of the aged samples with low SO42-/Al molar ratios (Tables 1 and 2) was caused by their crystallization; as the crystalline regions grew, the atoms reordered into more compact objects, thus decreasing the number of surface to bulk atoms. The above results obtained for the sol prepared with aluminum tri-sec-butoxide and 2-propanol are of great interest when boehmite is synthesized by the sol-gel technique with the aforementioned two compounds as aluminum and dissolvent precursors and sulfuric acid as the hydrolysis catalyst, which implies a low sulfuric acid concentration. These results provide the basis for understanding the origin of the final boehmite crystallite size and morphology as well as the sample texture. Conclusions The properties of the colloidal particles of a sol prepared with aluminum tri-sec-butoxide and 2-propanol and different concentrations of sulfate ions were studied. The colloidal particles were nanocapsules with diameters between 10 and 50 nm and a shell with a thickness of 3.5 nm, made of noncrystalline ordered Al-O polycations. The interaction between nanocapsules crystallized their shells into crystalline boehmite. This interaction was diminished when the nanocapsules were covered with sulfate ions, which occurred by introducing sulfuric acid into the sol. In other words, the sulfate ions peptized the sample. In the present study, the manner in which the interaction, and consequently the formation of boehmite, depended on the sulfate ion concentration in the sol is demonstrated. Also shown is that the origin of boehmite crystallites as bars occurred due to the formation of crystalline contours between adjacent interacting

Acknowledgment. We thank M.Sc. Manuel Aguilar and Mr. L. Rendo´n for technical assistance. This work was financially supported by The National University of Mexico (UNAM) “Proyecto Universitario de Nanotecnologia” and the Mexican Petroleum Institute (IMP), Project D.00285. References and Notes (1) Misra, C. Industrial Alumina Chemicals. ACS Monogr. 1986, 184. (2) Kno¨zinger, H.; Ratnasamy, P. Catal. ReV.sSci. Eng. 1978, 17, 31. (3) Vaudry, F.; Khodabandeh, S.; Davis, M. E. Chem. Mater. 1996, 8, 1451. (4) Kotanigawa, T.; Yamamoto, M.; Utiyama, M.; Hattori, H.; Tanabe, K. Appl. Catal. 1981, 1, 185. (5) Coster, D.; Blumenfeld, A. L.; Fripiat J. J. J. Phys. Chem. 1994, 98, 6201. (6) Valente, J. S.; Bokhimi, X.; Herna´ndez, F. Langmuir 2003, 19, 3583. (7) Marczewski, M.; Jakubiak, A.; Marczewska, H.; Frydrych, A.; Gontarz, M.; Sniegula, A. Phys. Chem. Chem. Phys. 2004, 6, 2513. (8) Matsuhashi, H.; Sato, D.; Arata, K. React. Kinet. Catal. Lett. 2004, 81, 183. (9) Nicholas, C. P.; Ahn, H.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 4325. (10) Yang, T. S.; Chang, T. H.; Yeh, C. T. J. Mol. Catal. A 1997, 123, 163. (11) Guzma´n, M. L.; Lo´pez, E.; Fripiat, J. J.; Valente, J. S.; Herna´ndez, F.; Rodrı´guez, A.; Navarrete, J. J. Catal. 2003, 220, 317. (12) Pradhan, J. K.; Bhattacharya, I. N.; Das, S. C.; Das, R. P.; Panda, R. K. Mater. Sci. Eng. B 2000, 77, 185. (13) Balankin, A.; Lopez, T.; Alexander-Katz, R.; Cordova, A.; Susarrey, O.; Montiel, R. Langmuir 2003, 19, 3628. (14) Lippens, B. C. Structure and Texture of Aluminas. Ph.D. Thesis, Technical University of Delft, Amsterdam,1961. (15) Guzma´n-Castillo, M. L.; Bokhimi, X.; Salmones-Bla´zquez, J.; Toledo-Antonio, A.; Herna´ndez-Beltra´n, F. J. Phys. Chem. B 2001, 105, 2099. (16) Bokhimi, X.; Lima, E.; Valente, J. J. Phys. Chem. B 2005, 109, 22222. (17) Rodriguez-Carbajal, J. Laboratoire Leon Brilloin (CEA-CNRS), France; e-mail [email protected]. (18) Kara, M.; Kurki-Suonio, K. Acta Crystallogr. A 1981, 37, 201. (19) Pottier, A.; Chaneac, C.; Tronc, E.; Mazerolle, L.; Jolivet, J.-P. J. Mater. Chem. 2001, 11, 1116. (20) Warren, B. E. X-ray Diffraction; Addison-Wesley: Reading, MA 1969; p 116. (21) Singhal, A.; Keefer, K. D. J. Mater. Res. 1994, 9, 1973. (22) Turner, R. C. Can. J. Chem. 1976, 54, 1528. (23) Akkit, J. W.; Farthing, A. J. Chem. Soc., Dalton Trans. 1981, 1617. (24) Seichter, W.; Mo¨gel, H. J.; Brand, P.; Salah, D. Eur. J. Inorg. Chem. 1998, 6, 795. (25) Buining, P. A.; Pathmamanoharan, C.; Jansen, J. B. H.; Lekkerkerker, H. N. W. J. Am. Ceram. Soc. 1991, 74, 1303. (26) Zhang, Z.; Pinnavaia, T. J. J. Am. Chem. Soc. 2002, 124, 12294. (27) Lee, H. C.; Kim, H. J.; Chung, S. H.; Lee, K. H.; Lee, H. C.; Lee, J. S. J. Am. Chem. Soc. 2003, 125, 2882. (28) Zhu, H. Y.; Gao, X. P.; Song, D. Y.; Bai, Y. Q.; Ringer, S. P.; Gao, Z.; Xi, Y. X.; Martens, W.; Riches, J. D.; Frest, R. L. J. Phys. Chem. B 2004, 108, 4245.