Spectroscopic Characterization of Sol−Gel-Derived Mixed Oxides

J. Phys. Chem. B 1998, 102, 6465-6470

6465

Spectroscopic Characterization of Sol-Gel-Derived Mixed Oxides Jack M. Miller* and L. Jhansi Lakshmi Department of Chemistry, Brock UniVersity, St. Catharines, Ontario, Canada L2S 3A1 ReceiVed: February 11, 1998; In Final Form: June 2, 1998

Al2O3-, TiO2-, SiO2-, and ZrO2-based mixed-oxide supports were synthesized by sol-gel techniques using 2,4-pentanedione as the complexing/templating agent. These materials have been investigated by employing nitrogen adsorption, 27Al, 29Si, and 1H solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), and Fourier transform laser Raman spectroscopy (FT-Raman). Nitrogen adsorption isotherms of the mixed oxides Al2O3-SiO2 and TiO2SiO2 were of type IV, with hysteresis loops of type I, indicating well-defined pore structure, while other mixed oxides Al2O3-TiO2, Al2O3-ZrO2, TiO2-ZrO2, and ZrO2-SiO2 exhibited type II hysteresis loops, indicating pores of undefined nature. 27Al MAS NMR studies on the alumina-based mixed oxides showed the presence of four-, five-, and six-coordinated aluminum species, the number of four-coordinated framework aluminum sites being highest in the Al2O3-SiO2 mixed oxide. 29Si CP MAS NMR spectra of the oxides Al2O3-SiO2, TiO2-SiO2, and ZrO2-SiO2 indicated the substitutional insertion of the cations (Al, Ti, and Zr) into the silica framework. DRIFTS spectra of these mixed oxides, where the vibrations corresponding to Si-O-M′ bonds were observed, are in conformity with 29Si CP MAS results. FT-Raman studies indicated the amorphous nature of the oxides, except in the case of the TiO2-SiO2 sample where the peaks corresponding to titania microcrystallites were noticed.

Introduction Mixed oxides such as alumina-silica, alumina-titania, titania-silica, zirconia-titania, etc., have been studied by various investigators as they find extensive application as advanced materials in the optical industry and ceramics and as catalysts and catalyst support materials.1,2 These materials were found to exhibit higher thermal stability, surface acidity, porosity, surface area, and catalytic activity. Different techniques such as mechanical mixing of the component oxides, coprecipitation of alkoxide precursors, sol-gel synthesis, etc., were used for the preparation of the mixed oxides. Sol-gel techniques are promising for synthesizing catalytic materials with a homogeneous distribution of components. The textural properties of the mixed oxides, such as pore size distribution, surface area, etc., are strongly dependent upon synthesis conditions, such as the nature and composition of the alkoxide precursors, solvent, complexing/templating agent, hydrolysis, and gelation conditions. Sol-gel synthesis of mixed oxides generally involves the acid or base hydrolysis of the component alkoxide precursors either in the presence or absence of a complexing or templating agent, gelation by condensation, polymerization and drying, followed by calcination at higher temperatures to burn off the organics. Drying of the gels at 110 °C to remove the solvent results in xerogels, and supercritical extraction of the solvent in flowing CO2 from the gel produces aerogels. The preparation, properties, and catalysis of aerogels is discussed in detail in a recent review of the topic by Schneider and Baiker.3 When the two alkoxides are hydrolyzed prior to mixing, large clusters of individual components will be formed. Therefore, it is preferable to hydrolyze the alkoxides after dissolving in a solvent. Mixing the precursor * To whom the correspondence should be addressed. Fax: 905-6829020. E-mail: [email protected].

alkoxides prior to hydrolysis in a solvent results in chemical modification, resulting in M-O-M′ bond formation. In some cases, one component may be more reactive than the other, leading to segregation of one of the oxides. To overcome this difficulty, the less reactive alkoxide is partially hydrolyzed prior to mixing with another alkoxide and then the mixture is hydrolyzed together, which is often referred to as step hydrolysis/two-stage hydrolysis or a prehydrolysis technique. The rapid hydrolysis of the precursor alkoxides may result in inhomogeneitysthis can be prevented by use of a complexing/ modifying agent which reacts with the precursors and reduces the rate of hydrolysis, thus allowing the components to react, to obtain homogeneity at atomic level. Sol-gel-derived mixed oxides such as alumina-silica,4-6 alumina-titania,7 titaniasilica,8-13 and zirconia-silica14-16 were subjected to investigation by various researchers. Spectroscopic techniques such as laser Raman spectroscopy (LRS), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS),27Al and 29Si solidstate nuclear magnetic resonance (MAS NMR), etc., were commonly employed for the characterization of the mixed oxides. The 27Al MAS NMR studies on Al2O3-SiO24,6,26 and Al2O3-TiO27 materials exhibited the peaks corresponding to four-, five-, and six-coordinated, i.e., framework and nonframework, Al species, the intensity of four-coordinated being more intense in the sol-gel-synthesized materials in comparison to other preparative techniques. 29Si CP NMR studies on silicabased mixed-oxide sol-gel materials such as Al2O3-SiO2,4,6,26 TiO2-SiO2,9,11-13 and ZrO2-SiO215 were found to provide information regarding the homogeneity of the materials. Qn notation is commonly used to indicate the extent of substitution of the silica framework with hydroxyl groups or cations.17 The observance of the resonances corresponding to Q1-Q3 are an indication of the substitution of the silica framework with either hydroxyl groups or cations. These studies were further sup-

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TABLE 1: Amounts of Precursor, Solvent, Surface-Modifying Agent, and Water Used in the Preparation of the Mixed Oxides mixed oxide

precursor

solvent

2,4-pentanedione/ alkoxide (mol/mol)

water/alkoxide (mol/mol)

Al2O3-SiO2 Al2O3-TiO2 Al2O3-ZrO2 TiO2-SiO2 TiO2-ZrO2 ZrO2-SiO2

aluminum tri(sec-butoxide), tetraethyl orthosilicate aluminum tri(sec-butoxide), titanium tetrabutoxide aluminum tri(sec-butoxide), zirconium n-propoxide titanium tetrabutoxide, tetraethyl orthosilicate titanium n-propoxide, zirconium n-propoxide zirconium n-propoxide, tetraethyl orthosilicate

n-butanol n-butanol n-propanol n-butanol n-propanol n-propanol

0.3 0.5 0.5 0.5 0.5 0.5

13.0 4.2 6.9 11.0 6.9 11.0

ported by the application of DRIFTS; the vibrations corresponding to Si-O-M′ bonds were noticed in TiO2-SiO210 and ZrO2-SiO214 mixed oxides. 1H MAS NMR was employed as a valuable characterization tool to differentiate various types of hydroxyl groups on supports and catalysts.18-25 The oxides such as alumina,18-21 silica, titania-modified alumina and silica,23,24 and mixed oxides such as TiO2-ZrO225 synthesized by coprecipitation methods were studied earlier by employing this technique. However, proton NMR studies on sol-gelderived mixed oxides have not been reported, so far, to our knowledge. Our earlier investigations on 2,4-pentanedione and other diketonates as surface-modifying agents during the sol-gel synthesis resulted in highly porous materials.26,27 The addition of 2,4-pentanedione (H-acac) during the sol-gel synthesis of aluminosilicates, alumina, and silica was found to result in transparent gels, preventing the formation of crystalline precipitates. The complexing agent controls the rate of hydrolysis of the precursor alkoxides, resulting in homogeneous mixedmetal oxides. In the present investigation, we report the synthesis of various mixed oxides such as Al2O3-SiO2, Al2O3TiO2, Al2O3-ZrO2, TiO2-SiO2, TiO2-ZrO2, and ZrO2-SiO2 using 2,4-pentanedione as the complexing agent. These solgel materials were characterized by 27Al, 29Si, and 1H MAS NMR, DRIFTS, FT-Raman spectroscopy, and nitrogen adsorption to evaluate BET surface area and pore size distribution. Experimental Section Various mixed oxides were synthesized with a 1:1 molar ratio using 2,4--pentanedione as the complexing agent. The amounts of alkoxide precursor, solvent, and water for hydrolysis are shown in Table 1. The solvents from Caledon Laboratory and chemicals from Aldrich were used as received. The typical procedure for the synthesis of the mixed oxide is as follows: required amounts of the alkoxide precursors were dissolved in 200 mL of solvent followed by heating at 70 °C to get a clear solution. The complexing agent, 2,4-pentanedione, was added to this clear solution prior to hydrolysis. Transparent gels were obtained in all cases and were aged at ambient temperature for 12 h. The solvents were removed at 110 °C, and after drying, the materials were finely powdered and then calcined at 500 °C to remove the organic residues. BET surface areas were determined using a Coulter SA 3100 instrument and an automated gas volumetric method employing nitrogen as the adsorbate at -196 °C. Samples were outgassed under vacuum at 200 °C for 1 h immediately prior to analysis. Pore size distributions and pore volumes were measured by using the BJH (Barrett, Joyner, and Halenda) method. The mixed oxides exhibited type IV isotherms. For the materials with type I hysteresis loops, the BJH calculations were made using the desorption branch of the isotherm. For the oxides with type II hysteresis loops, the calculations were made by using the adsorption branch of the isotherm.28 MAS NMR experiments were carried out on a Bruker Avance DPX 300 multinuclear FT NMR instrument. A standard bore Bruker

MAS/CPMAS probe with 4 mm zirconia rotors was used. 27Al MAS NMR spectra were obtained at 78.21 MHZ with a 30° pulse width of 2 µs and a 250 ms delay between pulses and a spectral range of 50 kHz. All the spectra were referenced to external aqueous aluminum nitrate (δ ) 0 ppm). The samples are spun at 10 kHz in air, and typically 540 FID’s were collected for each sample. 29Si spectra were obtained at 59.62 MHZ using cross polarization with proton decoupling during acquisition and referenced to tetramethylsilane (δ ) 0 ppm). Samples were spun at 4 kHz. A 5 ms cross-polarization contact time was used with a 5 s delay between the pulses, and between 1000 and 2000 FID’s were accumulated with a spectral window of 18 kHz. 1H MAS NMR spectra were recorded at 300 MHZ with a 30° pulse length of 3 µs with a 1 s delay between the pulses over a spectral window of 120 kHz. The chemical shifts (in ppm) were referenced to external TMS using neat p-dioxane as a secondary reference. The samples were spun at 10 kHz, and 124 FID’s were accumulated for each sample. Prior to recording the spectra, the samples were outgassed at 350 °C for 30 min in a flow of He and immediately transferred to zirconia rotors. GRAMS/32 software was used for the deconvolution of the NMR spectra. DRIFTS spectra were acquired using a SPECTRA TECH DRIFT accessory “THE COLLECTOR” in an ATI Mattson Research Series FT-IR spectrometer equipped with Michelson interferometer, heliumneon laser, KBr beamsplittter, DTGS detector (deuterated triglycerol sulfate) with a spectral range of 6,000-400 cm-1, and standard high-intensity source. The diffuse reflectance FTIR spectra were recorded after evacuating the samples at 200 °C and under reduced pressures in a controlled environmental chamber which was designed to be used on the collector DRIFTS accessory. FT-Raman spectra were obtained on a Bruker FRA 106 FT-Raman module interfaced to a Bruker IFS66 FT-IR bench. The spectra were recorded at ambient conditions using a 125 mW power setting for the incident radiation of 934.4 nm from a Nd:YAG laser. The highsensitivity Raman detector (D 418-S) was cooled with liquid nitrogen for optimum sensitivity. Typically 400 scans were collected for each sample. Results and Discussion The specific surface areas of the mixed oxides, pore volume, and pore size distribution data derived from nitrogen adsorption are shown in Table 2. The pore volume and pore diameter data of Al2O3-SiO2 and TiO2-SiO2 indicate the mesoporous nature of these materials. Figure 1 shows the nitrogen adsorption isotherms of the oxides Al2O3-SiO2 and TiO2-SiO2. It can be seen that both of the isotherms are of type IV, with hysteresis loops of type I, showing broad distributions of pore size according to IUPAC classification.28 Our earlier investigations26,27 on the sol-gel alumina, silica, and aluminosilicates in the presence of 2,4-pentanedione (H-acac) as a complexing agent showed type IV isotherms and type II isotherms for the unmodified material, indicating the nonporous/macroporous

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TABLE 2: BET Surface Areas, Pore Volumes, and Pore Diameters of Mixed Oxide Supports mixed oxide

surface area (m2/g)

pore volume (cc/g)

pore diameter (Å)

Al2O3-SiO2 Al2O3-TiO2 Al2O3-ZrO2 TiO2-SiO2 TiO2-ZrO2 ZrO2-SiO2

442 308 218 427 169 292

0.81 0.26 0.18 0.73 0.13 0.17

100-200 33-56 33-56 60-160 32-46 33-68

Figure 3. Pore size distribution of the oxides Al2O3-SiO2 and TiO2SiO2.

Figure 1. Nitrogen adsorption-desorption isotherms of the mixed oxides Al2O3-SiO2 and TiO2-SiO2.

Figure 4. Pore size distribution of the oxides Al2O3-TiO2, Al2O3ZrO2, TiO2-ZrO2, and ZrO2-SiO2.

Figure 2. Nitrogen adsorption-desorption isotherms of the mixed oxides Al2O3-TiO2, Al2O3-ZrO2, TiO2-ZrO2, and ZrO2-SiO2.

nature of the oxides. Figure 2 shows the nitrogen adsorption isotherms of the mixed oxides Al2O3-TiO2, Al2O3-ZrO2, TiO2-ZrO2, and ZrO2-SiO2 which exhibit type II hysteresis loop, indicating “ink bottle” type pores, having narrow necks and wide bodies.28 Pore size distributions calculated according to the BJH method are shown in Figure 3. In the case of mesoporous supports Al2O3-SiO2 and TiO2-SiO2, broad pore size distributions can be noticed. Other supports shown in Figure 4 exhibited only micropores of less than 6 nm, and the plot shows that the pore size distributions of the samples are identical. The 27Al MAS NMR spectra of the alumina-based mixedoxide supports, Al2O3-SiO2, Al2O3-TiO2, and Al2O3-ZrO2,

with peak deconvolution and curve fitting are shown in Figure 5. The chemical shifts and the relative amounts of three Gaussian peaks corresponding to the three different Al sites are given in Table 3. The peak at about ∼6 ppm indicates octahedral Al species and, therefore, nonframework aluminum. The peaks at ∼60 and 30 ppm correspond to lattice aluminum in four- and five-coordinate environments. However, there are disagreements in the literature for the assignment of the latter peak to five-coordinated aluminum.30 The increasing relative intensity of the signal at ∼60 ppm in the order Al2O3-SiO2 > Al2O3-ZrO2 > Al2O3-TiO2 indicates more framework aluminum sites and, thereby, higher homogeneity in the aluminasilica mixed oxide. Figure 6 shows 29Si CP MAS NMR spectra of the mixed oxides Al2O3-SiO2, TiO2-SiO2, and ZrO2-SiO2. Usually broad resonances due to an overlap of signals from different environments in the second coordination sphere of silica were observed in 29Si CP MAS NMR studies.4,6,9,11-13,15 In the case of Al2O3-SiO2 and ZrO2-SiO2 mixed oxides, an acceptable fit was obtained with a single-component Gaussian peak at -97.6 and -96.8 ppm, which can be assigned to Q3 Si(OSi)3 (OAl) and Si(OSi)3 (OZr) sites.17 The deconvolution of the 29Si MAS resonance signal of TiO2-SiO2 showed peaks at -92, -100, and 108 ppm with relative peak areas of 11.4%, 31.6%, and 57%. These peaks are associated with Q2 Si(OSi)2 (OTi)2, Q3 Si(OSi)3 (OTi), and Q4 Si(OSi)4 sites. Liu and co-workers11

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Figure 5. 27Al MAS NMR spectra of the mixed oxides (a) Al2O3-SiO2, (b) Al2O3-TiO2, and (c) Al2O3-ZrO2 with peak deconvolution and curve fitting: experimental spectrum (s), curve-fitted spectrum (- - -).

Figure 6. 29Si CP MAS NMR spectra of the mixed oxides (a) Al2O3-SiO2, (b) TiO2-SiO2, and (c) ZrO2-SiO2 with peak deconvolution and curve fitting: experimental spectrum (s), curve-fitted spectrum (- - -).

TABLE 3: Results from Deconvolution of 27Al MAS NMR Spectra five-coordinated

six-coordinated

mixed oxide

four-coordinated δ ppm

%

δ ppm

%

δ ppm

%

Al2O3-SiO2 Al2O3-TiO2 Al2O3-ZrO2

4.0 1.4 5.0

18.4 54.6 38.6

30 31 34

46.1 20.1 44.0

60 63 58

35.5 25.3 17.4

in their 29Si SP and CP MAS studies have shown that contributions to the signals Q2 and Q3 are mainly due to the presence of titanium in the second coordination sphere of silica, not the hydroxyl groups. The higher percentage of Q4 sites in TiO2-SiO2 indicate the presence of SiO2 agglomerates in the mixed oxide. 29Si CP MAS NMR results indicate the formation of M-O-M′ bonds in the sol-gel mixed oxides. The results are confirmed by FT-Raman studies (vide infra). 1H MAS NMR spectra of various mixed-oxide materials deconvoluted with Voigt line shapes are shown in Figure 7. The spectra were decomposed into a minimum number of peaks to give an acceptable fit. The chemical shifts and relative areas of various peak are given in Table 4. It can be seen from the table that the percentage areas of the upfield peaks are higher, indicating more bridged hydroxyl groups in comparison to the terminal hydroxyl groups. In the experimental Al2O3-SiO2 1H MAS NMR spectrum, resonances were observed at -0.5, 1.5, and 3.5 ppm. The peak at -0.5 ppm corresponds the basic hydroxyl groups coordinated to aluminum.18-21 The signal at 1.5 ppm can be assigned to Si-OH groups. The peak at 3.5 ppm can be attributed to acidic hydroxyl groups of alumina. Deconvolution of the 1H MAS NMR signals of deuterated η-Al2O3 and η-Al2O3 by DeCanio et al.19 and Mastkhin et al.21 revealed five types of hydroxyl groups in alumina with the chemical shifts ranging from -0.3, corresponding to basic or

terminal hydroxyl groups and upfield resonances, to ∼5 ppm, corresponding to acidic or bridged hydroxyl groups. One common feature among these investigations is the observance of a broad signal at 4-5 ppm corresponding to the physisorbed water. In the present investigation, we also noticed a broad signal around 4 ppm in the samples calcined at 500 °C. Upon outgassing the samples at 350 °C for 30 min, this broad feature disappeared and signal resolution was improved. Deconvolution of the 1H MAS resonance signal of Al2O3-TiO2 showed five components. The peaks at 7.6, 5.5, and 3.3 ppm can be assigned to hydroxyl groups coordinated to titania.22 The peaks at -0.3 and 1.9 ppm can be assigned to Al-OH groups. The hydroxyl groups on alumina with chemical shifts around 2-3 ppm may also contribute to the signal at 3.3 ppm. Mastikhin et al.21 in their 1H MAS NMR studies on vanadia catalysts supported on titania-modified alumina noticed peaks at -0.1 to -0.3, 2-3, 5-7 ppm. The signals at -0.1 and -0.3 ppm were assigned to basic hydroxyl groups on alumina, the peak at 1.4 ppm was assigned to an overlap of signals from isolated H2O molecules and terminal Al-OH groups, and the peak at 7.6 ppm was assigned to bridging hydroxyls Ti-OH-Ti/Ti-OH-Al. In the Al2O3-ZrO2 sample, the peaks are noticed at 1.9, 3.3, 4.5, and 7.6 ppm. These resonances can be due to an overlap of signals from hydroxyl groups coordinated to zirconia or bridged hydroxyl groups between Al and Zr, i.e., Al-OH-Zr groups. The line at -0.3 ppm can be attributed to Al-OH groups. In TiO2-SiO2, a weak resonance at 6.2 ppm may correspond to bridged hydroxyl groups in titania, i.e., Ti-OH-Ti groups, and the line at 1.6 ppm to terminal Si-OH groups. The curve fitting of this spectrum indicated a peak at 2.3 ppm which may due to overlap of resonances of acidic hydroxyl groups of silica or hydroxyl groups of titania. Similarly, the resonance at 7.6 ppm

Sol-Gel-Derived Mixed Oxides

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Figure 8. DRIFTS spectra of the mixed oxides obtained at 200 °C under reduced pressure.

Figure 7. 1H MAS NMR spectra of the mixed oxides (a) Al2O3SiO2, (b) Al2O3-TiO2, (c) Al2O3-ZrO2, (d) TiO2-SiO2, (e) TiO2ZrO2, and (f) ZrO2-SiO2 with peak deconvolution and curve fitting: experimental spectrum (s), curve-fitted spectrum (- - -).

TABLE 4: Results from Deconvolution of 1H MAS NMR Spectraa mixed oxide

1H

chemical shift (ppm)

Al2O3-SiO2 -0.5 (22), 1.5 (24.8), 3.6 (31.8) Al2O3-TiO2 -0.3 (25.6), 1.9 (13.6), 3.3 (18.3), 5.5 (24.0), 7.6 (18.5) Al2O3-ZrO2 -0.3 (17.4), 1.9 (14.8), 3.3 (18.7), 4.5 (31.8), 7.6 (17.3) TiO2-SiO2 1.7 (66.8), 2.3 (21.3), 6.8 (11.9) TiO2-ZrO2 0.03 (7.3), 1.95 (9.3), 4.5 (23.1), 5.6 (27.7), 7.6 (34.6) ZrO2-SiO2 1.6 (24.7), 2.3 (5.9), 5.1 (69.3) a The number in parentheses is the percentage area of the peak with respect to the total area of the component peaks.

in the TiO2-ZrO2 sample can be assigned to bridged Ti-OHTi/Ti-OH-Zr groups. The signals observed at 5.6, 4.1, and 2.0 ppm may be due to an overlap of resonances from Ti-OH and Zr-OH groups. In TiO2-ZrO2 mixed oxides synthesized by the coprecipitaion method,25 the peaks at 3.0 and 4.8 ppm were assigned to Zr-OH groups and the signal at 7.1 ppm to Ti-OH groups. In the ZrO2-SiO2 sample, the peaks were observed at 1.6, 2.3, and 5.1 ppm. The signal at 5.1 ppm can be assigned to Zr-OH groups, the signal at 1.6 ppm can be attributed to terminal Si-OH groups, and the upfield peak at 2.3 ppm can be assigned to acidic hydroxyl groups in silica.

Figure 8 shows the diffuse reflectance FT-IR spectra of the mixed-oxide supports Al2O3-SiO2, TiO2-SiO2, and ZrO2-SiO2 in the 2000-400 cm-1 region, the lower frequency limit being imposed by the DTGS detector. The oxides Al2O3-TiO2, Al2O3-ZrO2, and TiO2-ZrO2 exhibited broad features without any structural information. SiO2 was known to exhibit the peaks at 1180 and 1108 cm-1 due to the asymmetric stretching vibrations νasy of Si-O- and the peak at 803 cm-1 corresponding to a symmetric stretching vibration νsy. The peak at 970 cm-1, characteristic of Si-OH vibrations, was also observed. The other vibrations at 565 and 461 cm-1 correspond to the bending vibrations of the Si-O-Si groups. The substitutional insertion of the cations into the silica framework resulting in the weakening of the framework can easily be detected by the shift in the νasy and νsy frequencies to lower wavenumbers. Similar observations were made by Jones et al.5 and SchramlMarth et al.10 in their FT-IR and diffuse reflectance FT-IR studies on Al2O3-SiO2 and TiO2-SiO2 mixed oxides synthesized by sol-gel methods. In the present Al2O3-SiO2 sample, the major peak could be seen at 1168 cm-1 with a shoulder at 1047 cm-1. The shift in the asymmetric stretching vibrations of SiO2 from 1180 to 1168 cm-1 and 1108 to 1047 cm-1 in this sample indicates weakening of the silica framework by incorporation of aluminum. The TiO2-SiO2 sample is characterized by asymmetric stretching vibrations at 1194 and 1061 cm-1. A weak resonance could be seen at 804 cm-1 corresponding to the symmetric stretching vibration of Si-O- groups. The shift of νasy to lower wavenumber in comparison to the silica vibration indicates the weakening of the silica framework, as observed in the Al2O3-SiO2 sample. In the TiO2-SiO2 sample, another vibration could be seen at 929 cm-1, which is as intense as asymmetric vibrations. This peak is associated with vibrations of Si-O-Ti bridges. Similar observations were made earlier10 in their studies on TiO2-SiO2 mixed oxides. A band in the 940-960 cm-1 region is typical for TiO2-SiO2 glasses or crystalline titanium silicates. The relative intensity of this peak was considered as a measure of Si-O-Ti bridges and incorporation of TiIV cations into the silica framework. No vibrations due to crystalline TiO2 (at 440 and 750 cm-1) could be seen in the DRIFTS spectra. However, FT-Raman studies

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Miller and Lakshmi Acknowledgment. We thank NSERC, Canada, for financial support. Thanks are due to Tim Jones for his help during the NMR investigations. References and Notes

Figure 9. FT-Raman spectra of the titania-based oxides recorded at ambient temperature.

indicated the presence of crystalline TiO2. In the sample ZrO2SiO2, the major vibrations could be seen at 1115, 952, and 638 cm-1. The band at 1115 cm-1 could be assigned to silica vibrations, the peak at 952 cm-1 to Zr-O-Si vibrations, and that of 638 cm-1 to vibrations from zirconia as no vibrations in this region could be detected in the silica sample. Miller and co-workers14 in their diffuse reflectance studies on ZrO2-SiO2 attributed the peak at 950 cm-1 to both Si-OH and Zr-O-Si vibrations. The FT-Raman spectra of the titania-based mixed-oxide samples are shown in Figure 9; it can be seen that the lines corresponding to titania microcrystallites are more prominent in the TiO2-SiO2 sample, though corresponding vibrations were not detected in diffuse reflectance FT-IR. The Raman spectra of Al2O3-SiO2 and TiO2-ZrO2 are also shown in Figure 9; it can be seen that among the three samples the TiO2 crystallites are clearly visible in TiO2-SiO2, which may be due to the faster hydrolysis rate of titanium alkoxide than silicon alkoxide. Conclusions The nitrogen adsorption studies on the sol-gel mixed oxides indicated well-defined pore structures in Al2O3-SiO2 and TiO2SiO2 mixed oxides. 27Al MAS NMR studies showed the presence of a higher percentage of framework Al sites in the Al2O3-SiO2 mixed oxide.29 29Si CP MAS NMR and DRIFTS studies indicated the mixing of the oxides at the atomic level. FT-Raman and 29Si CP MAS NMR studies indicated the presence of titania and silica aggregates in the TiO2-SiO2 mixed oxide.

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