Langmuir 1997, 13, 1529-1544
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Active Site Nature of Pyrogenic Alumina/Silica and Water Bound to Surfaces V. M. Gun’ko,* V. V. Turov, V. I. Zarko, E. F. Voronin, V. A. Tischenko, V. V. Dudnik, E. M. Pakhlov, and A. A. Chuiko Institute of Surface Chemistry, 31 Prospect Nauki, Kiev, 252022, Ukraine Received May 6, 1996. In Final Form: September 3, 1996X The natures of the pyrogenic alumina/silica (AS) surface and water adsorbed on AS were studied by 1H NMR, infrared (IR), dielectric relaxation (DRS), and optical spectroscopies, thermally stimulated depolarization (TSD), and quantum chemical methods. An influence of the alumina content on the AS characteristics is nonlinear and stronger for the boundary of air/adsorbed water/oxide than for the liquid water/oxide interface. The 1H NMR study shows that the AS particles in aqueous suspension can strongly disturb 6-14 monolayers of interfacial water unfrozen at 200 K < T < 273 K. The alumina content does not have potent effects on the IR spectra independently of pretreatment temperatures, but other experiments exhibit the marked influence of the AS constitution on the oxide properties that is caused by both the alumina phase and the phase boundary of Al2O3/SiO2. The theoretical simulation demonstrates that adsorbed water can promote dissociative adsorption and associative desorption of water molecules especially in the regions of contact between the alumina and silica fragments in the AS particles, where the Brønsted acid sites are located and the adsorbed water clusters have a maximum size according to DRS and TSD data.
Introduction
* Author to whom correspondence should be addressed. Fax: 380 44 264 0446; Phone: 380 44 265 6731; E-mail: lena%silar.
[email protected]. X Abstract published in Advance ACS Abstracts, January 1, 1997.
for the liquid/solid interfaces. A method based on measurement of the 1H NMR signal intensity for adsorbed water at T < 273 K13-17 shows promise for an investigation of the characteristics of interfacial water in hydrated powders and suspensions of the dispersed particles. Not only a thickness of the boundary water layer disturbed by the surface but also the surface free energy for the interfaces of ice(water)/solids can be calculated using this method. Water molecules dissociatively adsorbed on oxides form the active sites as tMOH, tM-O(H)-Mt, etc., which are responsible for some physicochemical properties of the surface.18-20 Characteristics of the adsorbed water and active surface sites depend on the availability of impurities, point defects or dislocations, other phases in the matrix, pretreatment temperature, etc. The variety of factors having an influence on the surface states makes the interpretation of experimental data more difficult, especially for mixed oxides, for which the phase boundary is complicated and plays a key part in the integral properties of aqueous suspensions or hydrated powders. Alumina/silicas have the Brønsted acid sites (B-sites) tAl-O(H)-Sit, which (plus terminal tMOH) can be considered as one of a series of dissociatively adsorbed water molecules.20-23 Numerous studies of interaction of different compounds with these bridges were carried out hitherto.18,20,22,24-34 On a pure oxide surface water is easily
(1) Derjaguin, B. V.; Churaev, N. V.; Muller, V. M. Surface Forces; Plenum Press: New York, 1990. (2) Gee, L. M.; Healy, T. W.; White, L. R. J. Colloid Interface Sci. 1989, 131, 18. (3) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Evans, D. F. Science 1985, 229, 1088. (4) Barnet, M. K.; Zisman, W. A. J. Colloid Interface Sci. 1969, 29, 413. (5) Duncan, D.; Li, D.; Gayados, J.; Neumann, A. W. J. Colloid Interface Sci. 1995, 169, 256. (6) Schlangen L. J. M.; Koopal, L. K.; Stuart, M. A. C.; Lyklema, J. Langmuir 1995, 11, 1701. (7) Bilinski, B.; Chibowski, E. Powder Tecnol. 1983, 35, 39. (8) Bilinski, B. Mater. Chem. Phys. 1987, 18, 231. (9) Voelkel, A. Crit. Rev. Anal. Chem. 1991, 22, 411 (10) Conder, J. R.; Young, C. L. Physicochemical Measurements in Gas Chromatography; Wiley-Interscience: New York, 1979. (11) Adsorption on New and Modified Inorganic Sorbents; Dabrowski, A., Tertykh, V. A., Eds.; Studies in Surface Science and Catalysis; Amsterdam: Elsevier, 1996; Vol. 99. (12) Bogillo, V. I.; Voelkel, A. Polymer 1995, 36, 3503.
(13) Tabony, J. Prog. NMR Spectrosc. 1980, 14, 1. (14) Gorbunov, B. Z.; Lazareva, L. S.; Gogolev, A. Z; Hugilev, E. L. Kolloid. Zh. 1989, 51, 1062. (15) Gun’ko, V. M.; Zarko, V. I.; Turov, V. V.; Voronin, E. F.; Tischenko, V. A.; Chuiko, A. A. Langmuir 1995, 11, 2115. (16) Turov, V. V.; Bogillo, V. I.; Leboda, R. Exten. Abstr. EUROFILLERS’95, Sept. 11-14, 1995, Mulhouse, France, p 131. (17) Gross, R.; Boddenberg, B. Z. Phys. Chem. 1987, 152, 259. (18) Iler, R. K. The Chemistry of Silica; J. Wiley: Chichester, 1979. (19) Morrison, S. R. The Chemical Physics of Surfaces; Plenum Press: New York, 1977. (20) Tanabe, K. Solid Acids and Bases; Kodansha: Tokyo, 1970. (21) Kazansky, V. B. Zh. Fiz. Khim. 1985, 59, 1057. (22) Liebau, F. Structural Chemistry of Silicates; Springer-Verlag: Berlin, 1985. (23) Van Santen, R. A. In Adv. Zeolite Sci. Appl. Ser. Stud. Surf. Sci. Catal. 1994, 85, 273-294. (24) Ruelle, P.; Nam-Tran Ho, Buccmann, M.; Kesselring, U. W. J. Mol. Struct. Theochem. 1984, 109, 177.
Disperse oxides are widely used as sorbents, fillers, pigments, catalyst supports, etc. Their characteristics, determining interaction with surroundings (e.g., changes of the free energy in the surface layers upon adsorption), are connected with the nature of surface. The surface forces for flat solids can be measured by evaluation of disjoining pressure1 or by the ellipsometrical method,2 direct measurement of interaction between the flat surfaces,3 or determination of the contact angles.4,5 In the case of dispersed and porous solids, the surface free energy can be estimated from pressure of the adsorbed liquid films calculated from the adsorption isotherms for a high surface coverage using gravimetric6 or chromatographic measurements.7-12 Hydroxyl group dissociation, ion solvation, and their adsorption on surface, lateral and many-body interactions can give a significant contribution to total energy of particle interactions with surroundings in aqueous suspensions. Therefore, the value of the surface free energy obtained for the gas/solid interfaces at a small coverage of the particles can strongly differ from the magnitude obtained
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adsorbed dissociatively;20,34,35 therefore, under standard conditions the oxide surface is always hydrated but the amount of different types of OH groups is discussed.31,32,36,37 Interaction of the tM(1)-O-M(2)t bonds (M(1) and M(2) are different metal atoms) with H2O is of interest at the boundary with the gas phase33,38 and liquids,18,39,40 e.g., upon hydration or dissolution of mixed oxide and other processes. Mixed oxides obtained by chemical vapor deposition (CVD) of the second phase and pyrogenic mixed oxides have some distinctions caused by the difference in the nature of the interfaces and distribution of the second phase. For CVD oxides the tM(1)-O-M(2)t bonds are more hydrolyzable on frequent occasions; therefore, the interface for such oxides can be the boundary between hydrated and hydrogen-bonded surfaces of the particles with different phases that are supported by the absence of the IR bands corresponding to the tM(1)-O-M(2)t bridges in CVD oxides. However, for pyrogenic mixed oxides the tM(1)-O-M(2)t band appears in the IR spectra, but heating CVD oxides may lead to appearance of the tM(1)-O-M(2)t bonds due to dehydration.15,41-44 Characteristics of the active surface sites of parent and mixed oxides depend largely on the coordination numbers of the metal and oxygen atoms.20-22 For example, in zeolites the AlOn polyhedrons are typically individual (i.e., there are not the tAl-O-Alt bonds) and Al is mainly 4-fold O-coordinated and the B-sites tAl-O(H)-Sit have higher acidity for the smaller content of alumina (CA).20,21 Clearly the availability of water adsorbed on the oxide surface by different ways (molecular via the hydrogen and donor-acceptor bonds or dissociative, and in the clusters in different size) has a significant influence on the oxide surface properties.15,43-47 In spite of numerous studies of adsorbed water, many questions about inter(25) Pakhlov, E. M.; Voronin, E. F.; Bogillo, V. I.; Chuiko, A. A. Dokl. Akod. Nauk Ukr. SSR, Ser. B 1989, N8, 50. (26) Koller, H.; Lobo, R. F.; Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1995, 99, 12588. (27) Akratopulu, K. Ch.; Vordonis, L.; Lycourghiotis, A. J. Chem. Soc., Faraday Trans. 1 1986, 82, 3697. (28) Kassab, E.; Seiti, K.; Allavena, M. J. Phys. Chem. 1988, 92, 6705. (29) Kassab, E.; Seiti, K.; Allavena, M. J. Phys. Chem. 1991, 95, 9425. (30) Allavena, M.; Seiti, K.; Kassab, E. Chem. Phys. Lett. 1990, 168, 461. (31) Chuiko, A. A.; Gorlov, Yu. I. Chemistry of Silica Surfaces; Naukova Dumka: Kiev, 1992. (32) Tertykh, V. A.; Belyakova, L. A. Chemical Reaction Involving Silica Surface; Naukova Dumka: Kiev, 1991. (33) Rudzinski, W.; Charmas, R.; Borowiecki, T. Adsorption on New and Modified Inorganic Sorbents; In Studies in Surface Science and Catalysis; Dabrowski, A., Tertykh, V. A., Eds.; Elsevier: Amsterdam, 1996; Vol. 99, pp 357-410. (34) Kurtz, R. L.; Stockbauer, R.; Madey, T. E.; Roman, E.; De Segovia, J. L. Surf. Sci. 1989, 218, 178. (35) Refson, K.; Wogelius, R. A.; Eraser, D. G.; Payne, M. C.; Lee, M. H.; Milman, V. Phys. Rev. B: Condens. Mater. 1995, 52, 10823. (36) Heggie, M.; Jones, R. Philos. Mag. Lett. 1987, 55, 47. (37) Vogelsberger, W.; Seidel, A.; Rudakoff, G. J. Chem. Soc., Faraday Trans. 1992, 88, 473. (38) Humbert, B. J. Non-Crystal. Solid 1995, 191, 29. (39) Zhuravlev, L. T. Exten. Abstr. Int. Conf. Oxide Surf. Chem. Reaction Mechanisms, September 13-19, Kiev, 1992, Institute of Surface Chemistry, Kiev, p 4. (40) Dagan, G.; Tomkiewicz, M. J. Phys. Chem. 1993, 97, 12651. (41) Pakhlov, E. M.; Voronin,E. F.; Chuiko, A. A. Dokl. Akad. Nauk USSR 1991, 318, 148. (42) Voronin, E. F.; Pakhlov, E. M.; Chuiko, A. A. Colloid. Surf. A 1995, 101, 123. (43) Turov, V. V.; Gun’ko, V. M.; Zarko, V. I.; Bogatyr’ov, V. M.; Dudnik, V. V.; Chuiko, A. A. Langmuir 1996, 12, 3503. (44) Pakhlov, E. M.; Zarko, V. I.; Voronin, E. F. Ukr. Khim. Zh. 1993, 59, 373. (45) Zarko, V. I.; Gun’ko, V. M.; Chibovski, E.; Dudnik, V. V.; Leboda, R. Colloids and Surf, in press. (46) Tishchenko, V. A.; Gun’ko, V. M. Colloids Surf. A 1995, 101, 287.
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facial water on mixed oxides, an influence of the oxide synthesis method, the nature of the interfaces, and active sites on interaction with water still remain. The aim of the present work is an investigation of an influence of the alumina content in pyrogenic AS on water interaction with the surface, determination of the surface free energy, radial function of the free energy change for SiO2, Al2O3, and Al2O3/SiO2 in aqueous suspensions and in air, and estimation of a thickness of the liquid water layers strongly and weakly disturbed by the oxide surface. Experimental Section Materials. Highly disperse pyrogenic silica (Aerosil A-300 with specific surface area (S) of 300 m2 g-1), pyrogenic alumina (S ) 60 and 150 m2 g-1), and pyrogenic alumina/silica (99.5% purity) with 1.3 wt % of Al2O3 (S ) 219 m2 g-1), 3 wt % (181 m2 g-1), 8 wt % (180 m2 g-1), 23 wt % (171 m2 g-1), and 30 wt % (180 m2 g-1) were produced at PU “Chlorovinyl” (Kalush, Ukraine). The specific surface area value was measured using nitrogen at 77 K.
Methods Infrared Spectroscopy. The IR spectra of the AS (AS23 with 23 wt % of alumina) samples (8 × 28 mm) weighing 8-10 mg or 30 mg were recorded by a UR-20 (Germany) spectrophotometer. Samples weighing 30 mg give high-quality spectra in the 20004000 cm-1 range corresponding to the stretch bands of the surface free and hydrogen-bonded OH groups and adsorbed water molecules. The light samples were used for observation of the spectra below 1300 cm-1. Thermoevacuation of AS was carried out in special optical glass vessels up to 10-4 Torr for the 293923 K range. Optical Spectroscopy. (Dimethylamino)azobenzene (nDMAAB, pK ) 3.3) was chosen as an Hammet color indicator, and its diffuse reflection spectra have been recorded by a SF-18 (LOMO, Russia) spectrophotometer upon indicator adsorption on oxides, then they were recalculated to the absorption spectra according to the Kubelka-Munk formula.48 n-DMAAB adsorption on the samples previously evacuated and heated at 473 K for 1 h was performed from the gas phase at 338 ( 5 K for 2-4 h. Assignment of the n-DMAAB absorption bands has been done on the analogy of the spectra for this substance in neutral and acidic solutions.49,50 Four absorption bands of DMAAB adsorbed on mixed oxides can be detected:51,52 (1) 430-460 nm, d-DMAAB (physically adsorbed n-DMAAB via dispersive, nonspecific interaction); (2) 480-490 nm, H-DMAAB (hydrogen-bonded complex); (3) 520-545 nm, H+-DMAAB (complex with H+ transfer from the B-sites to DMAAB); (4) 555-560 nm, L-DMAAB (complex with the Lewis acid sites, L-sites). Dielectric Relaxation Spectroscopy (DRS) Method.53 Water adsorption was performed at 300 K with mean errors near (0.01 g of water/g of oxide. The dielectric characteristics (dielectric permittivity, ′, and dielectric loss, ′′) were measured by a Q-meter VM-560 (“Tesla”) at f ) 0.15, 0.25, 0.45, 0.8, 1.3, 3.0, 8.0, and 9.0 MHz at T ) 100-300 K. The measurements of ′(T) and ′′(T) were taken by a thermochamber with programmed temperature changes. The heating rate, β, was equal to 0.05 K/s with relative mean errors δβ ) (5%. The tgδ(f) ) ′′/′ function was detected at 300 K in the 104-107 Hz range. Thermally Stimulated Depolarization (TSD). The TSD method was described in detail in a few papers.15,46,54 The samples of 30 mg were polarized by the electrostatic field F ∼ 2 × 105 V/m at room temperature then cooled to 100 K with the field still applied and heated without the field with the linear heating rate β ) 0.05 K/s. The current evolving via depolarization is recorded (47) Gun’ko, V. M.; Voronin, E. F.; Zarko, V. I.; Pakhlov, E. M.; Chuiko, A. A. Submitted for publication in J. Adhes. Sci. Technol. (48) Kubelka, P.; Munk, F. Z. Tech. Phys. 1931, 12, 593. (49) Brazdil, J. F.; Yeager, E. B. J. Phys. Chem. 1981, 85, 1005. (50) Sivalov, E. G.; Tarasevich, Yu.I. Adsorp. Adsorb. 1982, N10, 49. (51) Tarasevich, Yu.I.; Sivalov, E. G. Dokl. Akad. Nauk Ukr. Ser. B 1978, N3, 252. (52) Zarko, V. I.; Kozub, G. M.; Sivalov, E. G.; Chuiko, A. A. Ukr. Khim. Zh. 1988, 54, 1144. (53) Digman, M. J.; Rao, B. Can. J. Chem. 1975, 53, 2252. (54) Bucci, C.; Fiechi, R. Phys. Rev. 1966, 148, 816.
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by an electrometer having the 10-15-10-5 A range. Relative mean errors on current measurement are δI ) (5%, δT ) (2 K for temperature, and δβ ) (5% for the temperature change rate. The TSD spectra parameters have been calculated in accordance with the equation describing dependence of the TSD current on temperature.55 NMR. The 1H NMR spectra were obtained by a high-resolution WP-100 SY (Bruker) NMR spectrometer with an operating frequency of 100 MHz and a bandwidth of 50 kHz. For temperature maintenance of (1 K a VT-1000 (Bruker) thermoadapter was used. The signal intensity was determined by an electronic integrator with relative mean errors (10%. The content of interfacial unfrozen water (CW) in frozen aqueous suspensions was measured as a comparison of its signal intensity (I) with that for water adsorbed on oxide powder using a calibrated function I ) f(CW). This dependence was obtained under measurement of I using leak-in of different amounts of water into an ampule with the oxide sample. The free energy (G) of the interfacial water layer was calculated using the dependence of the free energy of ice (Gi) on temperature.56 We assume that water is frozen at the interfaces when G ) Gi and the value of ∆G ) ∆Gi ) Gi|T)273K - Gi(T) corresponds to the free energy lowering caused by water adsorption.15,57,58 A capillary effect is absent for nonporous materials and the ∆G(CW) function can be used for determination of a radial dependence of the free energy of adsorption on a layer thickness. Assuming that an area occupied by one water molecule equals 0.09 nm2, we obtained the ∆G(d) function, where d is a thickness of the unfrozen water layer in terms of the number of the statistical monolayers of water (one layer corresponds to 0.3 nm). The 27Al magic angle spinning (MAS) NMR spectra were recorded on a CXP-200 (Bruker) spectrometer. The sample was rotated at the magic angle with a frequency of 3 kHz in a magnetic field of 4.7 T. To record the spectra of several samples, 6000 scans were needed. The chemical shifts were measured relative to aqueous solution of AlCl3 (i.e., Al3+(H2O)6). Quantum Chemicals Methods. Quantum chemical calculations were carried out using the cluster approach and considering the clusters from 2 up to 12 elementary units MOn by ab initio59 and semiempirical AM160 methods. The ab initio calculations were carried out using the Gaussian 92 program package59 and the Dunning/Huzinaga valence double-zeta basis set with the Los Alamos ECP (effective core potentials) (LANLDZ).61-64 The 6-31G(d) basis set was used for the calculations of one and two polyhedron models of oxides. Full geometry optimization has been performed for all ab initio and semiempirical calculations.
Results and Discussion Adsorption of water vapor on the AS surface was studied at two pretreatment temperatures (Tt) (for which an irreversible rearrangement of the surface does not occur) using the McBain-Bark scales.65 The isotherms weakly depend on Tt (Figure 1) that can be a result from a minor difference in Tt and due to those Tt values were below 673 K, as significant changes of the oxide surface, which lead (55) Reichle, M.; Nedetzka, T.; Mayer, A.; Vogel, H. J. Phys. Chem. 1970, 74, 2659. (56) Turov, V. V.; Leboda, R. Bogillo, V. I.; Skubishewska-Zieba, J. Langmuir, in press. (57) Turov, V. V.; Zarko, V. I.; Chuiko, A. A. Zh. Fiz. Khim. 1995, 69, 677. (58) Handbook of Thermodynamic Properties of Individual Substances; Nauka: Moscow, 1978. (59) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92, Revision G.3; Gaussian, Inc.: Pittsburgh, PA, 1992. (60) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. E.; Stewart, J. J. J. Am. Chem. Soc. 1985, 107, 3902. (61) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (62) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (63) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (64) Dunning, T. H.; Hay, P. J. Modern Theretical Chemistry; Plenum: New York, 1976; Chapter 1, pp 1-28. (65) McBain, J. W.; Bark, A. M. J. Am. Chem. Soc. 1926, 48, 690.
Figure 1. Isotherms of water adsorption on pyrogenic alumina/ silica with different content of Al2O3 at pretreatment temperatures: 430 K (a) and 615 K (b).
to major variations in the adsorption properties, can start at Tt g 673 K. In addition, perceptible changes in adsorption at Tt < 650 K can be observed for very small P/Ps ( 870 K the content of tSiOH is lower and the surface reactivity decreases. These changes of the surface are observed not only by IR and NMR spectroscopy but also by optical spectroscopy of adsorbed Hammet indicators. When Tt < 773 K, the absorbance spectra of DMAAB adsorbed on silica have only one d-DMAAB band (Figure 4a, 447 nm; Figure 5). If Tt > 773 K, the spectra have two bands (Figures 4 and 5) corresponding to d-DMAAB (440 nm) and H-DMAAB (488 nm). At Tt ) 1123 K, the band corresponding to L-DMAAB is absent as at lower Tt. The H+-DMAAB band is absent for silica at any Tt. The intensity of d-DMAAB has a maximum at Tt corresponding to removal of all water molecules until total dehydration of the surface (Figure 5, curve 1). The H-DMAAB band appears at Tt > 773 K, when only the hydroxyl groups remain on the surface and its intensity decreases as Tt increases (Figure 5, curve 2). Only one H-DMAAB band (480 nm) is observed upon DMAAB adsorption on pyrogenic Al2O3 at Tt < 773 K (Figures 4 and 5). At Tt > 773 K the H-DMAAB (488 nm) and L-DMAAB (556 nm) are detected, but the H+-DMAAB does not appear. The L-DMAAB band witnesses that the
from AS via the associative desorption mechanism than from silica. These results are in good agreement with literature data,66,67 according to which the IR spectra of amorphous Al2O3/SiO2 (e.g., clays and amorphous alumina/ silica catalysts) on frequent occasions do not have the separated, individual bands corresponding to the bonds
(66) Kiselev, A. V.; Lygin, V. I. IR Spectra of Surface Compounds and Adsorbed Materials; Nauka: Moscow, 1972. (67) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966 (Addition to Russian publication: Mir: Moscow, 1969). (68) Brey, V. V.; Guba, G. Ya.; Gulyanitskaya, N. E. Zh. Prikl. Khim. 1994, 67, 377.
Figure 2. IR spectra in the 2700-3900 cm-1 range of AS23 (a) and parent silica (b) after thermoevacuation at 293 K (1), 423 K (2), 523 K (3), 623 K (4), and 723 K (5) for a sample weighing 30 mg.
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Figure 4. Spectra of DMAAB adsorbed on the surfaces of SiO2 (a, c) and Al2O3 (S ) 150 m2 g-1) (b, d): pretreatment temperatures, 473 K (a, b), 773 K (d), 1123 K (c); bands, d-DMAAB (a, c, curve 1), H-DMAAB (b and c, curve 2; d, curve 1), L-DMAAB (d, curve 2).
surface is totally dehydrated and incompletely coordinated Al atoms appear. According to 27Al MAS NMR data (Figure 6,a), pyrogenic Al2O3 has both 4-fold (52 ppm) and 6-fold (0 ppm) O-coordinated Al atoms; therefore, AlIIIAlV can correspond to the L-sites appearing at elevated temperatures. Already at Tt ) 473 K the spectra have only the H-DMAAB (Figure 4); i.e., dehydration of Al2O3 occurs more easily (that was demonstrated as a weak effect in the IR spectra in Figures 2 and 3) than that for SiO2 and at such Tt only isolated hydroxyl groups are on the surface. Increase of the intensity of the H-DMAAB band at high Tt for Al2O3 shows that dehydration continues at Tt g 773 K (Figure 5). Already for CA ) 1.3 wt % (AS1) three bands d-DMAAB (454 nm), H-DMAAB (489 nm), and H+-DMAAB (532 nm) are observed (Figure 7). For all Tt the d-DMAAB intensity for AS1 is 10% less such band intensity for SiO2 (Figure 5). Consequently, the structure of the hydration layer on Al2O3/SiO2 has the major difference relative to parent SiO2 due to the variety of the surface sites, which can form the hydrogen bonds (hydroxyls) or d-DMAAB complexes (MO-M bonds). The adsorbed water clusters on AS are mainly located near the B-sites and are larger in size than those on SiO2 that can accelerate water desorption on heating. An analogous effect is observed for TiO2/SiO2.15 When Tt ) 473 K the DMAAB molecules form the hydrogen-bonded complexes dominantly with the surface hydroxyls (Figure 7). Comparison of the intensity of the H-DMAAB and H+-DMAAB at different Tt (Figures 5 and 7) shows that the amount of the active sites, interacting with DMAAB via the hydrogen bond or upon H+ transfer to DMAAB, are closely equal and decrease as Tt increases. At Tt ) 1123 K the L-DMAAB band (559 nm) appears, the H+-
DMAAB is absent, but H-DMAAB is observed as a result from DMAAB interaction with the tSiOH groups on the silica patches of AS. Characteristics of adsorbed DMAAB change as the Al2O3 content increases (Figures 5, 7, and 8). These changes can be caused by the difference in the coordination numbers of Al (AlVI appears for the high content of Al2O3 that has an influence on the surface structure, B- and L-sites), concentration of Al at the surface, and appearance of the individual Al2O3 phase. According to ref 68, the Al atoms are 4-fold O-coordinated for CA < 10 wt % in AS and for CA > 23 wt % 6-fold O-coordinated Al atoms are detected and for CA ) 30 wt % AlVI/AlIV ≈ 1 at room temperature (Figure 6). On heating and evacuation the AlVI atoms preponderate over AlIV (the alumina phase compacts). Analogous transformation is observed on heating of the samples in air.45 Change of the amount of the B-sites (H+-DMAAB band) depending on CA and Tt (Figure 5) shows the similarity of dehydration processes for AS with different content of Al2O3. A maximum of the B-site concentration is observed for CA ) 23 wt %, which is caused by formation of the individual alumina phase and change of the ratio between CA and area of the phase boundary, where the B-sites are located. An analogous picture was seen for TiO2/SiO2, when a maximum of the Ti-O-Si bridges was observed for Ctitania ) 20 wt %.15 Heating at Tt > 1000 K leads to total dehydration of SiO2 or Al2O3 and analogous treatment of AS gives a similar result. The DMAAB spectra at Tt ) 1123 K have the L-DMAAB band for any CA (Figures 5, 7, and 8). Dependence of the intensity of this band on CA has a minimum for a small content of Al2O3 and rises sharply for CA > 10 wt %, which can suggest the start of formation of the Al2O3 phase for such content of Al2O3.
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Figure 5. Dependencies of relative intensity of the DMAAB bands on pretreatment temperature (a, c) and alumina content (b, d) for Al2O3 (S ) 150 m2 g-1) (a), SiO2 (c), AS (b, d); bands: d-DMAAB (c, curve 1), H-DMAAB (a, b, curves 1, 2; c, curve 2), H+-DMAAB (d, curves 1 and 2), L-DMAAB (d, curve 3); Tt, 473 K (d, curve 1), 623 K (d, curve 2), 1123 K (b, curve 1; d, curve 3).
Figure 6. 27Al MAS-NMR spectra for pyrogenic Al2O3 (S ) 150 m2 g-1) (a) and AS30 ((b) curve 1) at Tt ) 300 K; AS30 ((b) curve 2) upon thermoevacuation at 473 K for 2 h; AS23 ((b) curve 3) on heating at 473 K in air for 2 h. Spectra were recorded at 300 K.
The availability of the B- and L-sites on AS and the possibility of dissociative adsorption of water molecules, dissociation of surface OH groups, and adsorption of ions from solution can have a strong influence on the characteristics of AS on the boundaries of air/adsorbed water/ oxide and liquid water/oxide. An influence of the surface on some characteristics of adsorbed water can be studied by the DRS and TSD methods. Dependence of dielectric relaxation on frequency for parent silica (A-300) and AS with the Al2O3 content of 1.3 and 3 wt % was studied for the content of adsorbed water near 5 wt % and for the 5 × 104 to 3.5 × 107 Hz range at room temperature. The tgδ(f) ) ′′/′ dependence for the dehydrated samples has a typical shape for dielectric
materials (Figure 9). The dielectric relaxation maxima observed for the hydrated samples are caused by the relaxation of adsorbed water. The relaxation maxima shift to the lower frequencies and the activation energy of the relaxation increases as the alumina content grows. Consequently, the increase of the alumina phase influence on adsorbed water is observed as CA grows. The activation energy can be estimated using dependence
τ ) (h/kT) exp(Ea/kT)
(1)
for time (τ) relaxation, where Ea is the activation energy of relaxation. Increase of the relaxation time and activation energy for AS in comparison with parent silica (Table
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Figure 7. The spectra of DMAAB adsorbed on AS1 (a, c, e) and AS8 (b, d, f): Tt, 473 K (a, b), 773 K (c, d), 1123 K (e, f); bands, d-DMAAB (a-f, curves 1), H-DMAAB (a-f, curves 2), H+-DMAAB (a-d, curves 3), L-DMAAB (e and f, curves 3).
1) may result from increasing interaction of the water molecule dipoles with the mixed oxide surface, which is more heterogeneous due to the contribution of the AlO-4/2 units to the electrostatic field of AS. For example, the tAl-O(H)-Sit structures upon dissociation of O-H and formation of the ion pairs can strongly interact with water molecules (according to the AM1 calculation, the energy of solvation of the cluster (O*3SiO)3AlO-Si(OSiO*3)3 after dissociation tAlO(H)Sit f tAlO-Sit + H+ is more -202 kJ/mol than that for the neutral cluster; in addition, formation of H3+O and its solvation gives near -900 kJ/ mol) and increase the barriers of the water molecule dipole rotation. In comparison with the dielectric relaxation spectra for parent pyrogenic silica69-71 having one band in the 160-
230 K range the ′′(T) function has two peaks for AS with CA ) 8 wt % (AS8) (Figure 10); i.e., the heterogeneity of the AS surface leads to appearance of the second type of adsorbed water. The first peak may be connected with the relaxation of the water molecules adsorbed on the silica matrix and the second is caused by water adsorbed near the AlOn polyhedrons embedded in the silica matrix and having the tAl-O(H)-Sit bridges. The activation energy of relaxation (Ea) estimated on the base of the temperature dependence ′′(T) shows the increase in size of the adsorbed water clusters near the B-sites and possible (69) Zarko, V. I.; Gun’ko, V. M. Functional Mater. 1995, 2, 110. (70) Zarko, V. I.; Belyakova, L. A.; Simurov, A. V.; Gulko, O. V. Zh. Fiz. Khim. 1995, 69, 2021. (71) Zarko, V. I.; Gun’ko, V. M. Adsorpt. Sci. Technol., in press.
1536 Langmuir, Vol. 13, No. 6, 1997
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Figure 8. Spectra of DMAAB adsorbed on AS30; Tt, 473 K (a), 623 K (b), 773 K (c), 1123 K (d); bands, d-DMAAB (curves 1), H-DMAAB (curves 2), H+-DMAAB (a-c, curves 3), L-DMAAB (d, curves 3).
Figure 9. Dependencies of tgδ on frequency (a) (the content of adsorbed water is 5 wt % and T ) 300 K in air) and temperature (b) in vacuum (10-3 Torr) for silica (a, curve 1; b, curves 1 and 2), AS1 (a, curve 2), and AS3 (a, curve 3; b, curves 3 and 4); f ) 0.05 MHz (curves 1 and 3) and 10 MHz (curves 2 and 4). Table 1. Time Relaxation and Activation Energy for Water Adsorbed on Parent Silica and AS oxide
τ, s
Ea, kJ/mol
SiO2 AS, 1.3 wt % AS, 3.0 wt %
5.3 × 10-8 1.56 × 10-7 2.27 × 10-7
32 34 35
dissociation of the O-H bonds from tAl-O(H)-Sit. A high-temperature (270 K) maximum of ′′(T) for water adsorbed on silica appears as CW grows up to 25 wt % (Figure 11) that is caused by formation of the tridimensional water clusters.15,43,46 In the case of AS a ′′(T) maximum is observed at room temperature for a small
content of adsorbed water; i.e., water on AS forms the large tridimensional clusters already for CW g 6 wt %. Using chemical modification of AS8 we can substitute the tSi-OH groups but the tAl-O(H)-Sit bridges remain, e.g., upon interaction of ethyl hydride cyclotetrasiloxane with AS (AS-M). The dielectric relaxation spectrum of AS-M has only one maximum in the 160230 K range. The Ea value has nonlinear dependence on CW as for CW ) 7, 12, and 16 wt % the peak does not shift (i.e., Ea is constant but for CW ) 30 wt % it increases two times). Thus the changes of the surface topography of AS only for the silica phase has an influence on the adsorbed water layers for the whole surface.
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Langmuir, Vol. 13, No. 6, 1997 1537
Figure 10. Dependence of ′′ on temperature for silica with 25 wt % (b) and 5 wt % (a, curve 1) of adsorbed water and AS8 with 6.2 wt % of adsorbed water (a, curve 2); f ) 8 MHz.
Figure 11. TSD spectra of silica (b, intensity is multiplied by 10) and AS (3 wt % of alumina). Dashed lines correspond to decomposition of the spectra.
The AS sample with 3 wt % of alumina was studied by TSD method. Two peaks of depolarization are observed in the TSD spectrum (Figure 11). The proximity of Ea for the second peak for AS at Tmax ) 277 K to Ea for water on parent silica (33 kJ/mol)46 as well as the τo values (for AS 8.6 × 10-4 and 6.2 × 10-4 s for silica) allow us to suggest that this peak for AS is connected with water relaxation on the silica phase. The first peak with Ea ) 40 kJ/mol and Tmax ≈ 250 K can be caused by the relaxation of water bound to the alumina fragments and tSi-O(H)-Alt bridges. The DRS and TSD studies show difference in an influence of the AS and parent silica surfaces on relaxation of water adsorbed at the boundary with air but it is of interest the changes of water behavior near the oxide surface at the boundary with liquid water that can be studied by the 1H NMR method. The NMR spectra dependence on temperature for frozen aqueous suspensions of AS is shown in Figure 12 for the alumina content of 23 wt %. The shape of these spectra is akin to that for suspensions of other oxides. Detected signals are caused by unfrozen water molecules disturbed by the oxide surface. The surface hydroxyls and OH groups of ice do not contribute to the spectra due to a small time ( 5 is small, the Al atoms are mainly 4-fold coordinated. According to literature data,68 for CA e 10 wt % only AlIV is observed in AS. On the other hand, for TiO2/SiO2 the individual TiO2 phase forms for Ctitania ≈ 5 wt %.69,70 Such a difference in formation of TiO2/SiO2 and Al2O3/SiO2 may be caused by (75) Kiselev, A. V.; Kuznetsov, A. I.; Lygin, V. I. Kolloid. Zh. 1980, 42, 964. (76) Karakchiev, L. G. Kinet. Katal. 1965, 6, 904. (77) Bondar, L. A.; Kustov, L. M.; Beletskii, I. P.; Stakheev, A. Yu.; Chuiko, A. A.; Kazansky V. B. Izv. Akad. Nauk USSR, Ser. Khim. 1991, N10, 2217.
(8)
tSi-Cl + H2O f tSi-OH + HCl
(9)
Therefore, even for a small amount of TiCl4 these molecules have a chance to interact with H2O hitherto than SiCl4 and to form the TiO2 nuclei. Besides, discrepancy in the TiO2 (rutile or anatase) and SiO2 (amorphous) structures is higher than that for Al2O3 (amorphous) and SiO2. In eqs 8 and 9 one water molecule can substitute one Cl, but in the case of Al2Cl6 for substitution of the central Cl atom it is needed to interact with two water molecules as Cl has two bonds Cl Al
Al Cl
+ H2O
Al
OH ClH +H2O Al Cl Al
OH OH2 Al + HCl Cl
(10)
and the second H2O molecule is needed for breaking the donor-acceptor bond AlrClH. These peculiarities of AlCl3 lead to the lower reaction rate for formation of the Al2O3 phase than that for SiO2; therefore, for generation of the individual Al2O3 phase the required Al2O3 content is higher than that for TiO2. However, the energy of formation of the ClnMrOH2 bonds is equal to 51, 103, and 120 kJ/mol for M ) Si, Ti, and Al, respectively, according to the semiempirical calculation by the NDDO78 method. It should be noted that calculated TiO2/SiO2 polarizability is more than that for AS by a factor 2. Hydrolysis of the tM-O-Sit bond occurs with the participation at least of two water molecules. The first creates the bridging and terminal OH groups
tSi-O-Mt + H2O f tSi-O(Hδ+) f M(OHδ-)t (11) and the second attacks the Si atom and cleaves the bridge
tSi-O(H)-M(OH)t f tSiOH + H2O f M(OH)t (12) (78) Gun’ko, V. M. Mechanisms of Chemical Reactions on Highly Disperse Oxide Surfaces, Doctorial Thesis, Institute of Surface Chem, Kiev, 1995.
edge edge edge node node node
a Clusters were calculated using 6-31G(d), r III bond. Others were calculated using Al-O(H)Si ) 0.19339 nm and rAl-O-Si ) 0.16765 nm; ∆Et ) -96 kJ/mol upon water dissociation on the Si-O-Al LANLDZ.
rM2O(W), nm
0.3501 0.2008
rM1O(W), nm rH2OH, nm
0.171 64 0.167 83 0.164 79 0.169 11 0.171 24 0.168 58 0.178 61 0.170 82 0.180 66 0.163 44 0.161 44 0.162 99
rM1OH, nm
0.417 0.442 0.434 0.478 0.496 0.472
rM1OM2, nm qH(W)
0.495
qH(OH) qH(b)
0.458 0.477 0.529 0.526 0.529 0.890
-qO(W) -qO(b)
1.121 1.063 1.138 1.141 0.946 0.870 2.114 2.107 2.240 2.032 1.348 1.402
qM2 qM1
2.163 1.946 1.937 2.359 1.643 1.516 3.25 2.75 0.71 4.16 1.66 0.65 0.99 1.08 1.19 4.27 3.55 2.65 761.79825 533.73227 610.99461 534.10794 1059.3310 983.2844 TiVIO(H)AlVI TiIVO(H)AlIV TiVO(HO)SiIV SiIVO(H)AlIV SiIVO(HO)AlIV a SiIVOAlIII a
10.38 12.06 12.48 11.63 12.42 13.10
µ, D ELUMO, eV -EHOMO, eV -EHF, a.u. structure
Table 5. Parameters of Molecules and Complexes (ab Initio)
0.200 77 0.181 08 0.244 86 0.170 25 0.170 45 0.160 23
common
Active Site Nature of Pyrogenic Alumina/Silica
Langmuir, Vol. 13, No. 6, 1997 1541
An analogous process with the participation of OH-*nH2O*H+ instead H2O can lead to dealumination of zeolites and other alumina/silicas in electrolytes. The question arises about the energy of dissociative adsorption of the water molecules on different sites as tSi-O-Sit, tM-O-Sit, and dM(OH)-O-Si(OH)d. We used direct calculations of the potential energy surface (PES) sections along the reaction pathway for dissociative adsorption of a water molecule, according to the search method for a saddle on PES.79 A vector of length 3N 6 (N is number of atoms in the system) defining the distance between two geometries of the prereaction and postreaction states is calculated as a generalized reaction coordinate and used for determination of the system motion to the transition state (TS).79 Calculation of PES gives important information for a deeper understanding of the reaction mechanism.80 The obtained PES sections (Figure 16) demonstrate that the process, corresponding to eq 11, is more probable for water adsorption on the tSi-O-Alt bridge than that for tSi-O-Sit (rO-H as the X-axis was set off from the integral reaction coordinate), but in all instances reaction 11 is exothermic. Typically in the TS of analogous reactions the polarity of the active bonds is higher than that in a prereaction complex (hydrogen bonded or donor acceptor);78,81,82 therefore, clustering water molecules near this bridge can reduce the activation energy and the probability of dissociative adsorption of the water molecules increases (Figure 17). This process can be written for two water molecules as follows OH Al O Si + HOH
Al O Si
+HOH
Al O Si H O H
Al O Si
OHH
O H
H H O
H
H
The PES fragment for this reaction is shown in Figure 16 where dashed lines are rO-H between H from the first water molecule and O from the second. The PES sections along the reaction pathway using the generalized reaction coordinate are shown in Figure 17. It should be noted that transfers of two protons from two water molecules (the first from a molecule having the donor-acceptor bond MrOH2 to the second H2O and next H+ from the second H2O to O from tSi-O-Mt) are not synchronous; some displacement is observed in a phase of their motion and in TS rOH1 ) 0.1239 nm and rOH2 ) 0.1113 nm. It seems likely that the likelihood of dissociative adsorption of one molecule without formation of the water cluster is little if any takes place especially for the tSiO-Sit bridge as the activation energy Eq ≈ 200 kJ/mol for eq 11 for M ) Si (Figure 16), but for M ) Al the second water molecule gives a slight change of Eq. For a pictorial rendition the bond length was used between the bridging oxygen atom and H from adsorbed H2O (Figure 16, solid lines) or between H from the first water molecule and O from the second (Figure 16, dashed lines), but the system motion corresponds to change of all atom coordinates between the prereaction state and postreaction complex; therefore it was also used for the generalized reaction coordinate (Figure 17). Interaction of the second water molecule with adsorbed (molecularly or dissociatively) H2O (79) Dewar, M. J. S.; Healy, E. F.; Stewart, J. J. P. J. Chem. Soc., Faraday Trans. 2 1984, 3, 227. (80) Garrison, B. J.; Srivastava, D. Annu. Rev. Phys. Chem. 1995, 46, 373. (81) Gun’ko, V. M. Kinet. Katal. 1991, 32, 576. (82) Gun’ko, V. M.; Chuiko, A. A. Kinet. Katal. 1991, 32, 322.
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Table 6. Parameters of Clusters and Complexes (AM1)a structure R3Si(OH)rOH2 R3SiO‚‚‚CH2 R3SiO/H.HOH tSi(rOH2)OSit tSi(rOH-)O(H+)Sit tAlIVO(H)Sit tAlVIO(H)Sit tAlIVO(H)OSit tAlIVO(H)AlIVtb tAlIVr(OH2)O(H)Sit tAl(rOH2)OSit tAl(rOH-)O(H+)Sit dHOAlrOH-(OH+)Sit Al (
OHH) OSi H2O
Al (
∆Et, -EHOMO, ELUMO, kJ/mol eV eV µ, D
-43 -77 -105 -91
10.61 10.86 11.01 10.42 10.65 9.84 9.59 9.63 9.99 9.29 10.03 9.90 9.74 9.92
0.82 0.57 0.36 0.52 0.38 -0.47 -0.09 0.16 0.58 -0.05 0.45 -0.20 0.15 0.25
1.29 0.58 3.02 2.44 0.30 6.90 3.07 3.57 7.43 5.98 2.69 4.44 4.75 1.02
-111
9.90
-0.05
-37 -25 -28 -21 -82
qM
-qC
1.897 1.926 1.925 1.941 1.937 0.926 0.841 0.901 1.509 0.872 0.906 0.99 0.854 0.982
0.715 0.678 0.689 0.966 0.683 0.565 0.693 0.672 0.764 0.666 0.887 0.583 0.547 0.846
qH
rMO, nm
0.233 0.174 88 0.252 0.172 93 0.237 0.178 30 0.171 05 0.185 49 0.273 0.182 47 0.264 0.360 18 0.265 0.244 55 0.309 0.175 49 0.244 0.372 29 0.241 0.183 03 0.280 0.181 59 0.278 0.181 61 0.283 0.181 55
rMrO, nm 0.214 82
-qO(W) qH(W) rOH(W), nm common
0.207 80 0.189 24
0.351 0.403 0.406 0.354 0.662
0.247 0.190 0.217 0.254 0.292
0.096 92 0.095 93 0.096 38 0.097 04
0.248 17 0.339 31 0.282 13 0.238 96 0.187 67 0.181 36 0.179 78 0.250 66
0.340 0.275 0.565 0.570 0.430
0.224 0.273 0.278 0.294 0.241
0.098 30 0.098 02
3.70 0.904 0.587 0.272 0.181 57 0.180 88
0.570
0.292
0.096 52
node node node node edge node node node node node node
OH2 OH–) O(H+) Si
node
H2O H2O a
R ) O3*SiO. b Calculation by NDDO method.
Figure 16. Dependence of ∆Et on bond length (rOH) for dissociative adsorption of a water molecule on the tSi-O-Sit (1, 2) and tAl-O-Sit (3, 4) bridges for only one H2O (1, 3) and with the participation of two water molecules (2, 4).
Figure 17. PES sections along the reaction coordinate for dissociative adsorption of a water molecule on the tSi-O-Sit (1) and tAl-O-Sit (2) bridges with the participation of the second water molecule.
gives additional stabilization of postreaction complexes (Table 6, ∆Et) as well as of TS (Figure 17). In the case of dissociation of a water molecule in the
adsorbed cluster a proton may not form the bridging OH group but adds to other H2O and H3+O or H5+O2 are formed. This process is more probable for the boundary with liquid water as for tSi-O(H)-Alt f tSi-O--Alt + H+ ∆Et ) 1122 kJ/mol, and for tSi-O(H)-Alt*5H2O f tSiO--Alt*4H2O*H3+O, ∆Et ) 143 kJ/mol, according to the calculations by the MNDO/H method (improved version of MNDO83 for study of the hydrogen bonds84). Consequently, stabilization of the ion pair is not reached upon dissociation of the B-site with 5H2O adsorbed, but ∆Et is significantly lower than that without adsorbed water cluster. With an increase of this cluster to 10H2O or above, it is possible to stabilize the separated ion pair.15,43,78 That leads a charge on the solid surface and the beginning of formation of the electrical double layer observed at the boundary of oxide/liquid water. Calculation of the solvation energy (Es) by the AM1 method, according to previous works,85,86 shows that the difference in the Es values upon solvation of the clusters (O3*SiO)3AlO(H)Si(OSiO3*)3 and (O3*SiO)3SiOSi(OSiO3*)3 is small (-6 kJ/mol), but upon dissociation of the B-site, Es equals -214 kJ/mol and a contribution of O- equals -179 kJ/mol. Notice that the Es value for solvation of the charged cluster is small as a result from the positive magnitudes of the contributions to Es for the boundary groups SiO3* (70-80 kJ/mol). Consequently, the main effect of solvation of the interfaces of AS is caused by interaction of water with the B-sites. Process of dissociative adsorption of water molecules can promote oxide dissolution.18,37 According to the AM1 calculations, in the case of Al2O3/SiO2, hydrolysis of the tAl-O-Sit bond is more probable for the bridge, where the Al atom already has one OH group (Table 6); hence, upon dealumination of AS a limiting stage is hydrolysis of the first tAl-O-Sit bond. Dissociative adsorption of the water molecules should first of all occur with the participation of the strained structures, e.g., the strained siloxane rings in silica, dislocations, etc. Elongation of the tSi-O-Sit bonds (up to 4%) in the dislocation region of quartz leads to easily hydrolysis of them.36 The availability of dissociatively adsorbed water on silica and mixed oxides makes possible the explanation of some peculiarities of the active sites and phenomena as particle swelling in water. Some of (83) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899. (84) Burshtein, K. Ya.; Isaev, A. N. Zh. Struct. Khim. 1984, 25, 25. (85) Still, W. C.; Tempczyk, A. J. Am. Chem. Soc. 1990, 112, 6127. (86) Cramer, C. J.; Truhlar, D. G. J. Am. Chem. Soc. 1991, 113, 8305.
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Langmuir, Vol. 13, No. 6, 1997 1543
Figure 19. TPD of water (m/z at 17) from alumina/silica with 3 wt % of Al2O3.
Figure 18. Structures of the AS cluster (O*3SiO)3Si-O(H)Al(OH)(O(H)SiO*3)(OSiO*3)2 (O* is oxygen pseudoatom) with dissociative chemisorption of a water molecule used in semiempirical calculations and a complex of a water molecule with Si(OH)4 calculated by an ab initio method using the 6-31G(d,p) basis set.
them have been explained due to the availability of the water molecules forming donor-acceptor complexes SirOH2.31 But the ab initio calculations show that even in the case of interaction of H2O with Si(OH)4 the bond length SirOH2 is high (Table 4, rMrO) and the main part of the energy of complex formation is caused by two hydrogen bonds (Figure 18). Explanation of some phenomena based on formation of the SirOH2 bonds in the interior of the particles for Eq ≈ 80 kJ/mol for their linking87 is inadequate as such energy is higher than the energy of formation of such complexes (Tables 4 and 6). However, this energy may be close to the effective activation energy of dissociative adsorption of a water molecule in the cluster adsorbed on the oxide surface when
Eeffective ≈ Eq - Q
(13)
where Q is heat of adsorbed cluster formation (e.g., the Es value for Si(OH)4 equals 45 kJ/mol). The increase in the passivity of heated silica for adsorption of water31,87 may be explained by the defect annealing with relieving a stress of the tSi-O-Sit bonds, on which the dissociative adsorption of water occurs with lower Eq, and reduction of heat of adsorption and increase of Eq for eq 11. The IR bands of the tAlO-H and tTiO-H groups are not observed for pyrogenic Al2O3/SiO2 (Figure 2) and TiO2/ (87) Brei, V. V.; Gorlov, Yu. I.; Chuiko, A. A. Teor. Eksperim. Khim. 1991, 27, 94.
SiO2 (after heating),15,68,88 which may be explained by easy desorption of the water molecules from titania/silica, observed according to temperature-programmed desorption (TPD) data.89 The difference in the TPD spectra for water desorption from the pyrogenic and CVD TiO2/SiO2 samples is a result of deeper hydrolysis of the boundary between two phases of CVD TiO2/SiO2. However, for CA ) 3 wt %, TPD of water (Figure 19) occurs in wide temperature region in contrast to the desorption from TiO2/ SiO289 (i.e., effect of the availability of alumina in AS is lower than that for TiO2 in titania/silica). Without pretreatment of AS the IR bands of tMO-H are shifted due to formation of the hydrogen bonds with the adsorbed water molecules. Although the B-sites of TiO2/SiO245,47,90 and on pyrogenic Al2O3/SiO2 (Figures 5, 7, and 8) are observed by a more sensitive method. In the same time, electrophoretic study of pyrogenic SiO2 and Al2O3/SiO246 shows that zeta potential dependence on pH of Al2O3/SiO2 suspension in water has a small distinction in comparison with suspension of parent SiO2. These phenomena may be explained as a mosaic covering the Al2O3 phase by the SiO2 patches and different influence of acidic and basic solution on the silica and alumina/silica surfaces and the contribution of processes
tAl-O(H)-Mt + nH2O f tAl-O--Mt*(n/2)H2O + H3+O*(n - 1)/2*H2O (14) tAl-OH + nH2O f tAl+*n/2*H2O + OH-*n/2*H2O (15) and dissolution of oxides and adsorption of the dissolved fragments on another phase. According to the 1H NMR study of the water/oxide system, for the boundary of air/ water/AS and water/AS the essential difference is observed in the free energy of the interfacial water for AS and SiO2. Action of the oxide surface on the water layers increases at the boundary with liquid water that can be a result from growth of the surface charge, which is responsible to the long-range component of the surface potential (Figure 13, Tables 2 and 3). At the boundary with air, the water molecules are adsorbed mainly in the molecular shape and dissociation of the surface OH groups is limited; for formation of the stable ion pair a distance between (88) Platonov, V. V.; Filimonov, V. I. Uspekh. Fotoniki 1971, N2, 92. (89) Zarko, V. I.; Kozub, G. M.; Pokrovskiy, V. A.; Chuiko, A. A. Teor. Eksper. Khim. 1991, 27, 735. (90) Zarko, V. I.; Sivalov, E. G.; Kozub, G. M.; Chuiko, A. A. Dokl. Akad. Nauk UkrSSR, Ser. Khim. 1985, N9, 37.
1544 Langmuir, Vol. 13, No. 6, 1997
ions near 1 nm is needed, corresponding to 3-4 molecular layers of water. As a consequence, formation of higher charge on the oxide surface is possible for the boundary with liquid water that is in good agreement with a tendency found by the quantum chemical calculations of the interface clusters and solvation energy. Conclusion The alumina distribution in the silica matrix of pyrogenic Al2O3/SiO2 is more uniform than that for pyrogenic TiO2/SiO2 as the individual alumina phase forms only when the alumina content is equal to 20 wt % or above, but for TiO2/SiO2 the individual titania phase forms for Ctitania g 5 wt %. The alumina content in pyrogenic AS has a weak influence on the IR spectra independently of pretreatment temperatures. However, other experiments as 1H NMR, DRS, TSD, optical spectroscopy of adsorbed Hammet indicators, and water adsorption show essential influence of CA on the AS surface characteristics, e.g., appearance of the Brønsted and Lewis acid sites, the change of free energy of the interfacial water layers, structure of adsorbed water clusters on oxide in air, etc., but this influence is nonlinear. This effect may be caused by change of the
Gun’ko et al.
coordination number of Al (AlVI appears for CA g 20 wt %) and the nature of the surface sites for different distribution of alumina in AS as the CA value changes. Adsorbed water can be removed via the associative and molecular desorption mechanisms from the AS surface more easily than that from parent silica, which makes observation of the IR spectra of thermally unstable surface OH groups difficult. Adsorbed water can promote its dissociative adsorption and associative desorption especially at the phase boundary or near AlOn structures embedded in the silica matrix, where the Brønsted acid sites are located. The polarizability of AS is lower in comparison with titania/silica; therefore, the AS surface action on water adsorbed on oxides at the boundary with air or liquid water is smaller than that for TiO2/SiO2. Nevertheless, the AS surface can strongly change near ten or above molecular layers of interfacial liquid water which is unfrozen at T < 273 K. Acknowledgment. We are grateful to Professor V. A. Pokrovskiy for measurement of temperature-programmed desorption of water from alumina/silica. LA960441P
