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Characteristics of the Hydration Layer Structure in Porous Titania-Silica Obtained by the Chemical Vapor Deposition Method R. Leboda,*,† V. V. Turov,‡ M. Marciniak,† A. A. Malygin,§ and A. A. Malkov§ Department of Chemical Physics, Faculty of Chemistry, Maria Curie-Skłodowska University, 20-031 Lublin, Poland, Institute of Surface Chemistry of the National Academy of Sciences of Ukraine, 252022 Kiev, Ukraine, and St-Petersburg State Institute of Technology, Moskovskiy pr. 26, 198013 St-Petersburg, Russia Received September 17, 1998. In Final Form: August 10, 1999 The titania-silica samples prepared by means of the chemical vapor deposition technique on the basis of mesoporous silica with different contents of grafted TiO2 were investigated. For these samples, the chemical shifts of adsorbed water, the thickness of the hydrate shell, and the free surface energy of aqueous medium were measured using the 1H NMR method. It was estimated that the chemical shift (δ) of the adsorbed water is between 2.6 and 3.8 ppm. For the sample with 7.7 wt % TiO2, the signal of strongly bonded water with δ ) 1.3 ppm is present in the spectra of adsorbed water. Probably this signal is caused by the water molecules, which form the electron-donor complexes with Ti atoms. With increasing TiO2 concentration, the free surface energy (∆GΣ) at first decreases from 160 mJ/m2, for the initial silica gel, to 90 mJ/m2 for the sample containing 4.5 wt % TiO2 and then increases up to 130 mJ/m2 for the sample with 7.7 wt % TiO2 concentration. The values of ∆GΣ for porous and nonporous materials were compared.
Introduction Titanium dioxide is widely used in industry not only as a support or a pigment of paints but also as an adsorbent of the high surface area and high catalytical activity.1,2 The properties of this material are determined to a great extent by the structure of its active surface sites. The most important sites are surface hydroxyl groups, Levis acid sites (Ti3+), and Brønsted acid sites Ti-O(H)-Ti. The powders of titanium dioxide characterized by the high specific surface area may be obtained using two basic methods, such as combustion of TiCl4 in the hydrogen/ oxygen flame or the sol-gel method.3,6 To improve the TiO2 catalytic properties and to reduce the costs, the layer of titanium dioxide may be formed on the surface of amorphous or porous silica particles using the chemical vapor deposition (CVD) method.7,8 Such a mixture maintains all features related to the structure for particles and porous structure typical for initial silica and the surface properties and catalytic activity similar to those of TiO2. At the same time, on the titania-silica (TS) surface, new surface sites such asTi-O-Si(OH), Si-O-Ti(OH), or SiO(H)-Ti appear. The surface concentration of such groups and their accessibility for the adsorbed water molecules may influence significantly the properties of TS. Either initial silicas or TS interact strongly with water molecules, which can form thick adsorbed water layers on †
Maria Curie-Skłodowska University. Institute of Surface Chemistry of the National Academy of Science of Ukraine. § St-Petersburg State Institute of Technology. ‡
(1) Fernandez, A.; Gonzalez-Elipe, A. R.; Real, C.; Caballero, A.; Munuera, G. Langmuir 1993, 9, 121. (2) Shellej, S.; Fouhy, K.; Moore, S. Chem. Eng. 1994, no. 3, 69. (3) Gete, A. V.; Zarko, V. I.; Kozub, G. M.; Chuiko, A. A. Ukr. Khim. Zh. 1988, 54, 653. (4) Spanos, N.; Slavov, S.; Kordulis, Ch.; Lycourghiotis, A. Colloid Surf. 1995, A97, 109. (5) Ko, E. I.; Chen, J. P.; Weissman, J. Q. J. Catal. 1987, 105, 511. (6) Reichman, M. G.; Bell, A. T. Appl. Catal. 1988, 32, 315. (7) Liu, Z.; Tabora, J.; and Pavis, R. J. J. Catal. 1994, 149, 117. (8) Hoffmann, M. R.; Martin, S. T.; Choi, W.; and Bahnemann, D. W. Chem. Rev. 1995, 95, 69.
the surface of such oxides. In aqueous suspensions the thickness of these layers estimated using the 1HNMR spectroscopy method ranges between 6 and 30 water molecular diameters.11-13 At the same time, the interactions with water molecules are determined mainly by formation of the hydrogen bond network between water molecules and surface hydroxyl groups. Moreover, except of the primary adsorption sites, the sites of TiIV-O(H)TiIV type forming during dissociative adsorption of water molecules can form.14 Proton chemical shifts for the water molecules bonded to different types of surface sites differ significantly. Nevertheless, for the most systems, only one averaged signal corresponding to the adsorbed water molecules bonded to different active surface sites can be observed. It is caused by rapid molecular exchange, but in some cases, when the rate of exchange is slow, the signals corresponding to the water molecules bonded to different types of active surface sites may be recognized separately. For amorphous TS, the separate signals corresponding to the water molecules adsorbed on the SiO(H)-Ti sites and entering the composition of greater clusters of water molecules were registered.11,12 The aim of this paper is to determine the effect of heterogeneity of a titania-silica adsorbent surface on the properties of hydration layers. This heterogeneity was modeled by bonding different amounts of titania to the surface of mesoporous silica gel. In addition, comparative studies of nonporous silica were carried out. (9) Contescu, C.; Popa, V.T.; Miller, B.; Ko, E. I., Schwarz, J. A. J. Catal. 1995, 157, 244. (10) Zarko, V. I.; Sivalov, E. G.; Kozub, G. M.; Chuiko, A. A. Dokl. Akad. Nauk. Ukr. SSR, Ser. Khim. 1985, N9, 37. (11) Gun’ko, V. M.; Zarko, V. I.; Turov, V. V.; Voronin, E. F.; Tischenko, V. A. and Chuiko, A. A. Langmuir 1995, 11, 2115. (12) Turov, V. V.; Zarko, V. I. and Chuiko, A. A. Zh. Fiz. Khim. 1995, 69, 677. (13) Gun’ko, V. M.; Zarko, V. I.; Turov, V. V.; Leboda, R.; Chibowski, E.; Holysz, L.; Pachlov, E. M.; Voronin E. F.; Dudnik, V. V.; Gornikov, Yu. I. J. Colloid. Interface Sci. 1998, 198, 141. (14) Yatzyuk, S. P.; Brei, V. V.; Chuiko, A. A. Ukr. Khim. Zh. 1988, 54, 653.
10.1021/la981279o CCC: $18.00 © 1999 American Chemical Society Published on Web 10/01/1999
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Figure 1. Apparatus scheme for the prepared TS adsorbents: (1) microcompressor; (2) barboter with water; (3) quartz reactor with resistance heating and porous diaphragm; (4) potentiometer; (5) thermocouple; (6) glass absorber; (7) valves; (8) container with chlorides; (9) rotameter; (10) vent valve; (11) gas drying block; (12) manometer; (13) carrier gas source.
Experimental Section Preparation of Adsorbents. Industrial mesoporous silica gel (Init. SG) (Russia) was used as the initial material to prepare titania-silica adsorbents. Commercial silica gel was purified from surface impurities by treating with 0.2 mol/dm3 HCl, washing with distilled water until chloride ions disappearance, and then drying at 200 °C. The synthesis of titanium oxide coverage on the SiO2 surface was carried out in a horizontal flowing-type reactor (Figure 1). The zone of a constant temperature was 100 mm long. Dried nitrogen and helium were used as carrier gases. The dew point of nitrogen was -65 °C (content of residual moisture ∼5H2O molecules per m3), and the dew point of helium was -100 °C (content of residual moisture, ∼1 × 10-3 H2O molecules per m3). The synthesis of titanium oxide was performed in the following stages: (a) preliminary heating of the substrate (SiO2) in air at 873 K (air was cleaned in a column with zeolite); (b) cooling of the substrate to 473 K in a dry helium flow; (c) chemisorption of TiCl4 on the surface of the substrate at 473 K in the dry helium flow (prepared samples contained 4.5 and 7.7 wt % of titanium dioxide) and at 673 K for the samples containing 3.2 wt % of this oxide; (d) removal of TiCl4 and HCl excess from the reactor by a helium flow at 473 K; (e) hydrolysis of TiCl4 fixed on the surface of the substrate with water vapors in the nitrogen flow between 473 and 673 K. Removal of the moisture by helium flow at 400 K. The subsequent cycles of synthesis include the above-mentioned stages, except the first one. Adsorption of Nitrogen. The nitrogen adsorption isotherms at 77 K (low-temperature adsorption method) were obtained using a Micromeritics model ASAP 2405 N (V 1.01) adsorption analyzer. The specific surface area values of the investigated adsorbents were calculated using the BET method.15 Differential distributions of the mesopores based on their radii were calculated from the desorption branch of the isotherms using the BJH method.16 The obtained data for adsorption and desorption and differential distributions of the mesopores (smaller figure) for the investigated adsorbents are shown in Figure 2. 1H NMR Investigation. 1H NMR spectra were obtained by means of a high-resolution Bruker WP-100 SY spectrophotometer (15) Kamegawa, K.; Yoshida, H. J. Colloid Interface Sci. 1993, 159, 324. (16) Barret, E. B.; Yoyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373.
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Figure 2. Isotherms of low-temperature adsorption/desorption of nitrogen and mesopore radius distribution for the prepared adsorbents: open symbols, adsorption; closed symbols, desorption; R [Å]. (operating frequency of 100 MHz, transmission band of 50 kHz). The sensor temperature was regulated by means of a Bruker VT-1000 thermostat with the accuracy of (1 K. Signal intensities were determined with an accuracy of (10% using an electron integrator. The adsorbed water amounts in the hydrated powders and frozen suspensions (CH2O) were determined by comparing signal intensities, which corresponded to unfrozen water (I) and the water adsorbed on powders, based on the calibration graph I ) f(CH2O). This graph was plotted from the intensity measurements for the adsorbed water signals after addition of a given amount of water into the ampule containing the weighted sample of adsorbent. In the case of the aqueous suspensions, when the concentration of solid phase was compared to its amount in the powder, the corresponding scale coefficient was used. To prevent overcooling of the suspension, the measurements of the concentration of unfrozen water were performed using heating of the suspensions cooled previously to 210 K. The changes in the free energy of the adsorbed water (∆G) were determined from the temperature dependence of the ice free energy (Gi). At the same time, one could assume that water at the interface undergoes freezing at G ) Gi,17-19 and the difference ∆G ) G0 - G was determined from a decrease in the free energy of water molecules caused by adsorption. One could assume also that the G0 value is equal to the free energy of ice at 273 K. Thermodynamic functions of ice are known over a wide range of temperatures. In this connection, for each value of water, the freezing temperature definite changes in the free energy of ice may be assigned. In aqueous media, the capillary phenomena are absent and, in such a case, the ∆G ) f(CH2O) function describes the radial dependence of the adsorption free energy. The magnitude of the surface free energy for the adsorbents in the aqueous medium (∆GΣ) may be determined by measuring the area, which is limited by the curve ∆G ) f(CH2O) extrapolated to the coordinate axes, according to the equation
∆GΣ ) K
∫
Cmax
0
∆Gd dCH2O
(1)
(17) Turov, V. V.; Bogillo, V. I. and Leboda, R. Extended Abstracts of EUROFILLERS′95, Sept. 11-14, Mulhouse, France 1995, 131-134. (18) Turov, V. V.; Barvinchenko, V. N. Colloids Surf., B: Biointerfaces 1997, 8, 166. (19) Gun’ko, V. M.; Turov, V. V.; Zarko, V. I.; Voronin E. F.; Tischenko, V. A.; Dudnik, V. V.; Pakhlov, E. M.; Chuiko A. A. Langmuir 1997, 13, 1529.
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Figure 4. XRD patterns of the investigated adsorbents. Table 1. Structural Characteristics of the Investigated Adsorbents
Figure 3. IR spectra of the investigated adsorbents. where K is the scale coefficient and Cmax is the thickness of the unfrozen water layer at ∆G f 0. If the CH2O value is expressed in milligrams of adsorbed water per 1 gram of adsorbent and ∆G in kJ/mol, then to calculate the free surface energy (expressed in mJ/m2), K ) 55.6/S is used, where S is the specific surface area of the adsorbent. The accuracy of the ∆GΣ calculation was about (20%. Chemical shifts were measured with respect to tetramethylsilane (TMS) after placing the hydrated adsorbent powder in deuteriochloroform containing a small amount of TMS. Such a procedure did not lead to the changes in the intensity and the shape of the adsorbed water signal, which allowed us to conclude that the chemical shifts of the adsorbed water in air and CDCl3 are the same. The deuteriochloroform medium allows us to obtain relatively narrow signals for the adsorbed water molecules.11,12 This is caused by a decrease in the rate of molecular exchange of water molecules bonded to different sites on the surface. Infrared Spectroscopy: The IR spectra were recorded using a Specord-M80 (Carl Zeiss) spectrophotometer. The pressed samples (28 × 2 mm) of oxides were utilized.for spectra recording. The obtained data are presented in Figure 3. X-ray Diffractometry. An X-ray diffraction (XRD) study was performed using a DRON-3 diffractometer (Lomo, St.Petersburg) equipped with a copper anticathode (Cu HR) and Ni filtered. Cracks of 2 mm × 8 mm × 0.1 mm were used during measurements. The rotation rate of the counter was 0.015° with the counting time of 2 s. The quantitative phase analysis was based on identification of roentgenogram maxima according to the ASTM database. Mass fractions of the anatase and rutile phases of titania were determined on the basis of intensity ratios of the reflections corresponding to anatase {101} and rutile {110}. The roentgenograms of the tested adsorbents are shown in Figure 4.
Results and Discussion Structure of CVD, Titania/Silica. Figure 2 represents the correlation between the isotherms of nitrogen adsorption/desorption of the investigated adsorbents. The parameters of the porous structure (Table 1) and differential distributions mesopores from their radii shown in Figure 2 were determined. The specific surface area values of the TS samples and their total pore volumes do not differ significantly from the data for the initial silica gel (Table 1). Also in the case of mesopores, from differential distributions based on their radii, very small differences
sample
content of TiO2, wt %
surface area, m2/g
total pore volume, cm3/g
av pore radius, Å
Init SG TS1 TS2 TS3
3.2 4.5 7.7
278 291 286 274
0.91 0.93 0.90 0.87
78 63 63 63
were observed between the adsorbents under investigation. Therefore we may conclude that modification of the adsorbent surface is reflected poorly in the characteristics of the porous structure. The OH groups (3000-3880 cm-1), siloxane groups, and Ti-O-Ti bonds (400-1000 cm-1) were studied using spectroscopic IR methods in the valency vibrations area (Figure 3). As follows from the data in Figure 3, the reaction with TiCl4 causes a complete disappearance of absorption of single SiOH groups (3750 cm-1) (spectra 1 and 2 in Figure 3). Hydrolysis of chlorotitanium groups involves partial attenuation of band intensity in the area from 800 to 1000 cm-1. When TiCl4 chemisorption is over, absorption bands appear on the initial silica gel and the maxima are observed at 910-920 cm-1, which are connected with formation of Si-O-Ti bands.20,21 The subsequent hydrolysis occurring in the vapor phase causes an insignificant decrease in the band intensity due to a partial decomposition of Si-O-Ti during hydrolysis of mainly monofunctional groups SiOTiCl3.21,22 The absorption increase at 550-570 cm-1 is most probably associated with an increase in the number of Ti-O bonds. However, due to a significant overlapping of adsorption bands, separation of bands corresponding to different bonds is not possible. Greater changes in absorption were observed at 570 cm-1 during the synthesis of the TS3 sample compared with other adsorbents, which depend on a large amount of titanium oxide chemically sorbed onto this sample (the results refer to a chemical composition of the sample) and thereby on a larger number of Ti-O bonds responsible for absorption in this range. The results of X-ray analysis indicate the presence of a small number of titanium oxide structures in the crystalline state, whereby depending on the synthetic temperature only anatase phase (200 °C, TS1, TS3) or simultaneously anatase and rutile phase (400 °C, TS2) are formed. As follows from the above data, the adsorbents studied possess neither uniform nor homogeneous titanium dioxide on the silica surface. They are energetically and (20) Habibova, S. W.; Tolstoy, W. P.; Malkov, A. A.; Malygin, A. A. Zh. Prikl. Chim. 1989, 62 (2), 341-344. (21) Osipenkova, O. W.; Malkov, A. A.; Malygin, A. A. Zh. Obszcziej Chim. 1994, 64 (4), 549-553. (22) Malkov, A. A.; Sosnov, E. A.; Osipenkova O. W.; Malygin, A. A. Appl. Surf. Sci. 1997, 108, 133-139.
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Figure 5. 1H NMR spectra of the water adsorbed on the surface of initial silica gel (1): SG with 3.2 wt % TiO2 (TS1); (2) 4.5 wt % TiO2 (TS2) (3).
Figure 6. Influence of adsorbed water concentration and temperature on the shape of 1H NMR spectra of adsorbed water on the TS3 (7.7 wt % TiO2) Ti surface: (a) (1) CH2O ) 150 mg/g, (2) CH2O ) 250 mg/g, and (3) CH2O ) 350 mg/g; (b) (1) T ) 287 K; (2) T ) 360 K; (3) T ) 380 K; (4) T ) 400 K.
structurally heterogeneous. It is estimated13 that a minimal amount of TiO2 necessary to form a monolayer on the surface comparable with the silica surface used in the studies (Table 1) is about 17 wt %. 1H NMR Investigations. Figure 5 represents the 1H NMR spectra of hydrated powders of the initial silica gel and TS materials synthesized on its basis. The contents of water in the samples correspond to a maximal amount of water adsorbed on these samples at 293 K and 20% relative humidity. The spectra were obtained in the deuteriochloroform medium, and for this reason, apart from the water signal, narrow signals corresponding to the CH groups of chloroform and CH3 groups of TMS in them appeared. The signals corresponding to the surface hydroxyl groups of silica and TS are not observed in the spectra because of large differences between the width of the signals corresponding to the mobile adsorbed and solid phases. For initial silica gel and TS containing 3.2 and 4.5% TiO2 (TS1 and TS2, respectively), the spectra consist of one single signal, and its width decreases with increasing water content in the sample. For the TS adsorbent containing 7.7% TiO2 (TS3) on its surface apart from basic signals, an additional signal of lower intensity appears, which is compared to the basic signal and shifted toward the strong magnetic fields. Figure 6 illustrates the changes in the 1H NMR spectra of water contained in the TS3 sample in air due to an increase in the adsorbed water concentration at a constant temperature (Figure 6a). Free
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evaporation of water from the ampule on heating of the sample over the 287-400 K temperature range is shown in Figure 6b. As can be seen from this figure, an increase in the water concentration in the sample leads to an increase in the basic intensity signal at the unchanged intensity of the signal in the strong magnetic field. The intensity of that signal remains practically unchanged even in the case of heating to 400 °C when the main part of sorbed water is evaporated from the surface. The observed regularities allow the conclusion that water molecules responsible for the signal characterized by the chemical shift δ ) 1.3 ppm are bonded to the TS3 surface significantly strongly than the main portion of the water is localized in the adsorbent pores. The structure of water complexes formed in the pores of adsorbents may be studied using the 1H NMR spectroscopy method to determine the magnitude of proton chemical shift for the adsorbed water molecules. During the formation of hydrogen-bonded associates, water molecules may act either as proton or electron donors. From this reason, water may form polyassociates, in which the number of the hydrogen bonds per water molecule ranges from two for water dimers to three to four for icelike clusters being on the basis of liquid water. The chemical shift of water molecules is determined by the strength of hydrogen bonds23,24 and the average number of hydrogen bonds per a molecule (m). Sensitivity of the chemical shift to the changes in the “m” value is higher than that to the strength of hydrogen bonded complexes. For example, with increasing the electron-donor ability of a solvent, the chemical shift of water molecules increases from 1.7 ppm in chloroform or benzene11 to 3.5 ppm in dimethyl sulfoxide.25 At the same time, the chemical shift for ice is 7 ppm,26 and that for liquid water is 5 ppm.25 For the water adsorbed on the silica (Aerosil) surface, the magnitude of chemical shift ranges from 3 to 4 ppm, when the water concentration is significantly smaller than the concentration of the surface hydroxyl groups,26,27 to δ ) 4.5-4.7 ppm for the strongly hydrated surfaces.11,12 In the first case, there are primary complexes of water molecules bonded to the surface hydroxyl groups, and in the second, water polyassociates form. In this way, from the chemical shift magnitude of the adsorbed water, the average number of water molecules in the water associates formed on the surface can be estimated. The values in Figures 5 and 6 (δ ) 2.6-3.8 ppm) are significantly lower than those obtained for the water adsorbed on the highly dispersed silica and TS.11,12 These differences result probably from the fact that in the case of relatively lowly hydrated surfaces, formation of large polyassociates does not occur in the mesopores of adsorbents and water forms a thin layer on the internal surface on the pores. For initial silica gel, the chemical shift value of the adsorbed water molecules is close to the values obtained for the water bonded to the hydroxyl groups of silica. Assuming that apart from such molecules there is a great number of water molecules not bonded directly to the surface but entering the composition on the surface of polyassociates, one can conclude that the average coordination number of the adsorbed water molecules does not exceed 3. (23) Davis, J. C.; Deb, K. K. Adv. Magn. Reson. 1969, 4, 1. (24) Pimentel, G. C.; McClellan, A. L. Annu. Rev. Phys. Chem. 1971, 22, 347. (25) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; A Wiley International Publication: New York, 1972. (26) Kinney, D. R.; Chuang, I. S.; Maciel, G. E. J. Am. Chem. Soc. 1993, 115, 6786. (27) Gorlov, Y. I.; Brei, V. V.; Samoson, A. V.; Chuiko, A. A. Teor. Eksp. Khim. 1988, 24, 235.
Characteristics of the Hydration Layer
Figure 7. Dependence of adsorbed water free energy vs water concentration: (1) SG; (2) TS1; (3) TS2; (4) TS3.
The analysis of the structure of water complex localized on the TS surface, which has spot characteristics and contains the fragments of initial silica, TiO2, and the interface regions with Si-O-Ti or Si-O(H)-Ti groups, is much more difficult. The presence of the Brønsted acid sites, with which water molecules form more stable hydrogen bonded associates than with SiOH groups, allow one to expect a greater chemical shift of the adsorbed water than that of the silica proton. Small values of chemical shifts of water molecules observed for TS may be explained by the mesoporous TS obtained using the CVD technique, this type of association is not predominate. This fact can also be explained by the surface of such adsorbents; the charged sites influencing the magnitude of chemical shift of the adsorbed water molecules are present. An especially surprising fact is the appearance in the1H NMR spectrum of the water adsorbed on the TS3 surface, the signal corresponding to the water molecules bonded most strongly to the surface with the chemical shifts in the region typical of monomeric water molecules. These molecules probably do not participate in both formation of surface polyassociates and molecular exchange with other water molecules. This may be confirmed by a small width of the signal corresponding to this type of bound water (broadening of the signal caused by the molecular exchange is not observed) (Figure 5). Probably these molecules form strong donor-acceptor complexes with surface sites of the Levis type, in which no protons of water molecules participate in formation of hydrogen bonds with other water molecules or surface hydroxyl groups. Figure 7 represents the dependence of the changes in the free energy of adsorbed water molecules on the concentration of the unfrozen water contained in the frozen aqueous suspensions of initial and modified silicas. Extrapolation of these dependencies to intercept with abscissa permits determination of a maximal concentration of bound water. For all the studied systems, the Cmax value is significantly lower than the total volume of pores. In consequence, a main portion of water filling pores is not bonded to the surface and its properties are close to those of the bulk water. ∆G ) f(CH2O) dependences, contrary to the analogous dependences obtained for
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nonporous adsorbents,11,28 are characterized by a significant curvature resulting from sharply defined structural and energetic heterogenites of the surface. This makes it difficult to determine the characteristics of the water layers. The values of the free surface energies of adsorbents may be calculated on the basis of eq 1. Figure 8a represents the dependence of the ∆GΣ values on the concentration of TiO2 in the samples. For the initial silica gel, the ∆GΣ values are close to those obtained earlier for another type of silica gel.29 With increasing titania concentration, the thickness of the layer of adsorbed water and the free surface energy decrease for the TS1 and TS2 samples and increase for TS3. The surface concentration of the hydroxyl groups is higher for TiO2 than that of silica.30 For this reason, a decrease of bonded water deteriorates the conditions of formation of structurized water polyassociates, which results from the concentration observed for the TS. At the same time, the main role in formation of the bonded water layer may be played not exactly by the concentration of the surface hydroxyl groups but by their mutual localization as well as the presence of charged groups creating the electrostatic field, in which the layers of bonded water are formed. Then an increase in the hydrophilic properties of the TS3 sample may be caused by an increase in the concentration of the Levis acid sites, responsible for the presence of the water signal characterized by the chemical shift δ ) 1.3 ppm. The dependence of the free surface energy vs the TiO2 concentration for TS synthesized on the basis of silica gel differs significantly from that obtained for highly dispersed TS synthesized by using the same method on the surface of Aerosil particles. In Figure 8b, there is presented the diagram of the corresponding dependence made from the literature data.13 Comparing parts a and b of Figure 8, one can conclude, that for nonporous materials, significantly higher values of the free surface energy than that of porous ones are observed. Especially great values of ∆GΣ were registered for the samples containing 0.5 wt % TiO2. For these samples, the ∆GΣ values reached up to 1100 mJ/m2. Such great values of ∆GΣ are caused by polarization of the particle surfaces. For the polarization component of the surface forces, the extent of their action is proportional to the distance between oppositely charged fragments of the surface. In suspension formed using nonporous materials, the size of particle agglomerates may attain several micrometers. At the same time, there are no limitations related to the distance between the charged patches of the surface and, thus, the polar component of the surface forces may attain high values. In porous adsorbents, it is possible to form the charged patches mainly on the internal surfaces of pores whose linear dimensions are limited to several nanometers. In this case, a portion of the polar component in formation of the layer of bonded water is relatively small. The concentration and mutual localization of hydrophilic surface sites play a significant role. Conclusions The characteristics of TS adsorbents with titania deposited on the surface of mesoporous silica gel and synthesized using the CVD method were determined. Adsorbed water forms the layers on its surface, whose (28) Turov, V. V.; Leboda, R.; Bogillo, V. I.; Skubiszewska-Zieˆba J. J. Chem. Soc., Faraday Trans. 1997, 93, 4047. (29) Turov, V. V.; Leboda, R.; Bogillo, V. I.; Skubiszewska-Zieˆba, J. Langmuir 1997, 13, 1237. (30) Yatzuk, S. P.; Brei, V. V.; Chuiko A. A. Zh. Fiz. Khim. 1988, 62, 1940.
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Figure 8. Influence of titania concentration on the free surface energy value for porous adsorbents (a) and for amorphous adsorbents (b).
thickness does not exceed half of the average pore radius. So we may conclude that the main portion of the water molecules localized in the pores is not bonded to the surface. For the water molecules adsorbed onto the surface, the values of chemical shifts are in the 2.6 < δ < 3.8 ppm range. These low values of chemical shift may be explained by relatively low values (∼3.0) of the coordination number of water molecules localized in the adsorption layer. For the samples containing 7.7% TiO2, the water adsorbed on their surface is represented in the NMR spectrum by two signals, among which one is characterized by the chemical shift δ ) 1.3 ppm. This type of water molecule corresponds to strongly bonded complexes on the surface and may be removed only at T > 400 °C.
With increasing titanium concentration on the silica surface, the decrease in the thickness of the bonded water layer is observed. At the same time, the free surface energy decreases initially from 160 mJ/m2 for the initial silica gel to 90 mJ/m2 for the TS2 sample and then it increases again to 130 mJ/m2 for the TS3 sample. Considering a high value of the surface concentration of the hydroxyl groups in TS, one can conclude that not their concentration but their mutual localization as well as the presence of Lewis acid sites may play a predominant role in formation of the adsorption water layers on the TS surface. Acknowledgment. This work was supported by the State Committee for Scientific Research. Research (Warsaw, KBN), Grant No. 3 TO9A 03611. LA981279O
