Surface and Textural Properties of Network-Modified Silica as a

J. Phys. Chem. B 2001, 105, 9093-9100

9093

Surface and Textural Properties of Network-Modified Silica as a Function of Transition Metal Dopant Zirconium S. H. Teo and H. C. Zeng* Department of Chemical and EnVironmental Engineering, Faculty of Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed: March 5, 2001; In Final Form: July 20, 2001

Surface and textural properties of Zr-doped silica [xZrO2-(100 - x)SiO2] have been investigated as a function of zirconium content in the range of x ) 5 to 50 (mol % Zr) via heating their respective xerogels (prepared with a novel sol-gel route developed recently) in air at 500 °C for 4 h. Nitrogen adsorption-desorption investigations (BET/BJH) on the 500 °C-heated samples show a maximum specific surface area of 484 m2g-1 at x ) 20, and a minimum of 105 m2g-1 at x ) 50. With increase of zirconium content, porosity of the Zr-doped silica can be tuned conveniently from the mesoporous (100 Å) to the microporous (e 20 Å) region with unimodal (or nearly unimodal) pore size distributions. Material issues such as thermal reactions, oxide mixing level and formation mechanisms have also been investigated with FTIR, XRD, DTA/TGA, and XPS methods. It is found that silica networks have been significantly modified with the introduction of zirconium, and crystallization of ZrO2 in the doped silica occurs only at 903-953 °C. Moreover, surface analysis shows that there is no appreciable element enrichment in the surface region, whereas significant changes in binding energies of Zr 3d, Si 2p, and O 1s have been detected. The above observations are indications that a good mixing has been attained.

Introduction In the past two decades, mesoporous and microporous oxide materials have attracted great research attention owing to their many important technological applications.1-14 In particular, the synthesis of porous materials has entered an entirely new era since the discovery in 1992 of periodic mesoporous materials.8 These investigations have led to a significant advance in tailormaking nanostructured materials and hybrid nanocomposites and in understanding molecular templating and imprinting, functionalization, and inclusion chemistry of this class of materials.9-14 Concerning the modification of physicochemical properties, introduction of one or more secondary metals to the host oxide framework has been a fundamental process in the synthesis of porous materials,11,14,15 including isomorphical incorporation of transition metals into the periodic mesoporous materials, and/ or post-synthesis treatment of porous host materials.11,14 Among various synthesis routes,1-7,11,14-17 the sol-gel method offers many advantages for preparation of porous materials at relatively low temperature.1-7,15 Such technological merits include controlling relative precursor reactivity, chemical composition, and textural properties of oxide materials with a high degree of mixing of different metal elements.1-7,15 Incorporation of transition metal oxides into the silica framework represents an important area in porous oxide research.15 Regarding the fabrication of doped silica via organometallic routes, there have existed five general approaches to match relative activities among alkoxide precursors, which are commonly used in sol-gel synthesis: (i) selection of the “right” precursor of secondary metals to match that of silicon; (ii) chemical modification (normally with a chelating agent) of * To whom correspondence should be addressed.Tel: +65 874 2896. Telefax: +65 779 1936. Email: [email protected]

metal alkoxide to reduce the precursor reactivity; (iii) prehydrolysis of metal precursor; (iv) selection of the “right” reaction temperature to bring the chemical reactivities closer among the precursors; and (v) use of an acid or base catalyst to control the hydrolysis rate and condensation rate.4,15 Recently, a significant progress in this area has been made, aiming at a high degree of mixing at molecular level.4,5,15,18,19 For example, excellent single-source molecular precursors such as metal tris(tert-butoxy)siloxy complexes, M[OSi(OtBu)3]4, for synthesis of oxides of MO2-4SiO2 (M ) Zr and Hf) had been tailorprepared via conversion of M(NEt2)4: M(NEt2)4 + 4HOSi(OtBu)3 f M[OSi(OtBu)3]4 + 4HNEt2.5 In the search for other possible alternatives, very recently, we have developed a synthetic method for ZrO2-SiO2 without using an acid or base catalyst.20 Moreover, the self-catalytic role of the zirconium precursor had been recognized with respect to the synthetic parameters.21 It has been found that gelation time can be shortened with a high concentration of tetraethoxysilane in the presence of a small amount of zirconium npropoxide. Other important synthetic parameters, such as water content and chelating agent concentration, had also been investigated in detail to understand gelation processes and precursor chemistry.21 In the present work, with a smaller increment of zirconium content in this synthetic method, we will focus on an investigation of physicochemical properties of this material system by examining thermal processes for the Zr-doped silica formation, bulk and surface chemical compositions, and textual properties of the resultant oxides. Our new experimental results indicate that the Zr-doped silica xZrO2(100 - x)SiO2 possess high specific surface areas in the range of 105-484 m2 g-1. A uniform porosity ranging from the mesoporous to microporous regime (about 100 Å to < 20 Å) can be selected by control of zirconium (x) concentration in

10.1021/jp010828n CCC: $20.00 © 2001 American Chemical Society Published on Web 09/05/2001

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Teo and Zeng

these oxides. Our present investigation indicates that this new approach may offer certain advantages for controlling surface and textural properties. Experimental Section Sample Preparation. Xerogel precursors for xZrO2-(100 x)SiO2 oxides with x between 5 and 50 mol % were prepared using the sol-gel technique. The chemicals used were tetraethoxysilane (TEOS, purity > 98%), zirconium n-propoxide (ZP, 70 wt % in n-propanol solvent), acetylacetone (acac, 99.5%), and 2-propanol (99.7%), which were obtained from Merck. The basic procedure for obtaining xZrO2-(100 - x)SiO2 gels was similar to our previous work,20,21 in which neither an acid nor a base catalyst was used. The main steps of the preparation can be described as follows: as-received ZP-n-propanol solution with a desired molar concentration was diluted in a premixed 2-propanolacac solvent under vigorous stirring in a glovebox (flowed with nitrogen at normal atmosphere), where acac was used as a chelating agent to stabilize ZP. The molar ratio of acac to ZP was kept at 1 in all sample preparations. A desired amount of TEOS was then added to the above solution, and the entire solution was stirred for 10 min. Cohydrolysis of xZrO2-(100 x)SiO2 occurred when water was added drop-by-drop to the mixture, and this marked the start of the hydrolysis step. The molar ratios of H2O to Zr + Si and 2-propanol to Zr + Si were fixed at 5 and 12, respectively. After 15 min of hydrolysis with stirring, the mixed sol was kept still in order to form a rigid gel. The preparation beaker was also covered with a plastic film to prevent chemical evaporation. The gelation time was normally a few hours (measured form the moment water was added until the sol finally lost its fluidity).20,21 The obtained wet gels were aged for one week at room temperature, then dried at 80 °C for 48 h. The dried gels were crushed into fine powders and stored in sample bottles for further thermal treatments and characterization. Calcination of the binary gels was carried out in an electric furnace (Carbolite) under the static air. The gels were heated at a rate of 4° min-1 (as was used for cooling) to a constant temperature of 500 °C for 4 h before cooling. Characterization Techniques. FTIR Measurement. Chemical bonding of the oxides and functional groups was investigated with Fourier transform infrared spectroscopy (FTIR, Shimadzu FTIR 8108) using the potassium bromide (KBr) pellet technique. The sample to KBr ratio was according to the general approach that the weight of KBr was usually 100 times of that of the sample. The FTIR spectrum background was corrected using a freshly prepared pure KBr pellet. Each spectrum was collected after 100 scans at a resolution of 4 cm-1. XRD Measurement. Crystallographic information on the samples was investigated by powder X-ray diffraction (XRD). Diffraction patterns of intensity versus two-theta (2θ) were recorded with a Shimadzu XRD-6000 X-ray diffractometer using Cu KR radiation (λ ) 1.5406 Å) from 10° to 80° at a scanning rate of 4° min-1. X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. The choice of this scanning range was to check whether there are tetragonal and monoclinic phases of ZrO2 or other crystalline phase(s) in the oxides of xZrO2(100 - x)SiO2 after heat-treatment. DTA/TGA Measurement. The study with differential thermal analysis (DTA, Shimadzu DTA-50) was carried out to understand the thermal behaviors of the as-prepared xerogels. Approximately 10-15 mg of powdered dried gel was used in each experiment. Samples for DTA measurement were heated from room temperature to 1100 °C at a rate of 10 °C min-1 in

Figure 1. FTIR spectra for xZrO2-(100 - x)SiO2 oxide samples prepared at 500 °C for 4 h.

air atmosphere (or in nitrogen) with a gas flow rate of 60 mL min-1; the heating was started after the powder sample of dry gel was purged with the same gas flow at room temperature for 20 min. Thermogravimetric analysis (TGA, Shimadzu TGA50) was also conducted to understand thermal decomposition processes of the oxide gels. The TGA measurements were carried out under the same conditions as used for DTA, except that the final temperature was limited to below 950 °C. XPS Measurement. XPS investigation was conducted on an AXIS-Hsi 165 Ultra (Kratos Analysis) spectrometer using Al KR X-ray source (1486.8 eV, 400 W) at the constant analyzer pass energy of 20.0 eV. Curve-fitting was carried out using a nonlinear (Shirley type) least-squares fitting software (XPSPEAK 41) to separate the overlapping peaks. Due to the charging problem of the insulating samples, the binding energy values were referenced to the C 1s line at 284.5 eV (arising from the inadvertent carbon contamination). BET/BJH Measurements. Full adsorption-desorption isotherms of nitrogen at -195.8 °C on all calcined samples were measured at various partial pressures in a Quantachrome NOVA3000 apparatus. Specific surface areas (SBET) and pore-size distributions (PSD) were determined with Brunauer-EmmettTeller (BET) method and Barret-Joyner-Hallenda (BJH) method, respectively. BET surface areas were obtained from the linear adsorption data points over relative pressure (P/Po), whereas the PSDs were derived from the desorption isotherms. Prior to the BET/BJH measurements, the samples were degassed for 3 h at 300 °C in an outgassing station of the adsorption apparatus. Results and Discussion Insertion of ZrO2 into SiO2. Figure 1 shows FTIR spectra of the oxides formed at 500 °C. The bands at 3464 to 3365 and

Surface and Textural Properties of Zr-Doped Silica 1635 to 1628 cm-1 are due to OH stretching and bending vibrations of water in the samples. As indicated, water is easily adsorbed into the pores of the oxide materials, because they are mesoporous in nature (see later discussion). A number of bands are observed in the low wavenumber regions of these spectra, owing to the presence of silica-like structure. The large absorption bands at 1090-1017 cm-1 with a shoulder at ca. 1200 cm-1 are assigned to asymmetric stretching vibrations of Si-O-Si bond,14,22-24 while another shoulder at ca. 980 cm-1 is assigned to Si-OH (silanol group) vibration.23,25,26 The latter shoulder component becomes less clear when the Zr content is increased and it is clearly proportional to the Si content in the gels. The gradual disappearance of the silanol groups had been recently recorded after initial hydrolysis using 29Si and 17O NMR methods.27 Nonetheless, the silanol groups observed in our calcined samples may be due to the rehydrolysis of silica surface with ambient water. The peaks at 800 and 454-439 cm-1 can be attributed to the presence of the ring structure formed by Si-O bonds in the silica network.24,25 In these heated gels, as the Zr content increases, the band at 1090 cm-1 (5 mol % Zr) gradually shifts toward a lower wavenumber 1017 cm-1 (50 mol % Zr). At the same time, the peaks at 800 and 454-439 cm-1 decrease gradually in their intensities and become negligible when the Zr content exceeds 35 mol %. The shift of the Si-O (or initial Si-O-Si homo-linkage, at 1090-1017 cm-1) bands has also been observed in other studies by introduction of metallic oxides, such as ZrO2, TiO2, Al2O3, and CaO, into the SiO2 network.4,14,15,22,25,26,28 The formation of Si-O-M (where M is a second metal atom) heterolinkages is responsible for this band shift because the Si-O bonds now become less symmetrical due to more different atoms that are incorporated into the new linkages. The bands at 1090-1017 and 980 cm-1 have been utilized to measure the relative heterogeneity of mixing in this material system.20,21 Due to the possible hydrolysis with ambient water, the Si-OH groups (980 cm-1) are expected to be located on the surface, while the bulk silicon atoms are present in the forms of both Si-O-Si and Zr-O-Si linkages. When more Zr atoms insert into the Si-O-Si, i.e., more Zr-O-Si linkages are formed, the homogeneity of the oxides increases, which corresponds to the shifts of the above two bands toward lower wavenumber regions, as shown in Figure 1. In addition to the band shifts, intensity reduction in the ring-structural Si-O bands at 800 and 454-439 cm-1 also reflects a gradual increase of Zr in the SiO2 network in forming the heterolinkages of ZrO-Si. With more increment steps of Zr mol % investigated in the present work, Figure 1 indicates that the ring-structure of SiO2 has largely gone when the Zr content is increased to 35 mol %, which means the original silica structure (highly crosslinked SiO4 tetrahedra) has been severely modified at this composition. It should be mentioned that although the above FTIR analysis does not give a quantitative result (unlike the 29Si and 17O NMR methods that will be employed in our future investigation), a mixing level or modification of the silica matrices can be compared at least in a relative sense from the above evolution pattern. Thermal Reactions and Material States. The DTA/TGA thermographs are shown in Figure 2 for samples heated in air atmosphere. There are a number of thermal events that can be observed. The small endothermic bands around 100 °C are attributed to the evaporation of water, organic solvents, and hydrolysis products. The first exothermic peaks can be assigned

J. Phys. Chem. B, Vol. 105, No. 38, 2001 9095

Figure 2. DTA scans (in air atmosphere) for all xerogel samples of xZrO2-(100 - x)SiO2.

to the combustion of unhydrolyzed ligands (-OR) of the precursor alkoxides, especially of TEOS, as the position of this peak is quite independent of the ZP content in the precursor solutions. In fact, this peak shifts slightly to the lower temperature when the TEOS content is predominant (see the samples with 5-10 mol % Zr). The second exothermic peaks, on the other hand, can be attributed to the combustion of chelating agent acac, noting that the position of this peak depends strongly on the content of ZP (and thus of acac; molar ratio of acac/ ZP ) 1:1). After these exothermic events, our XRD investigation indicates that all xZrO2-(100 - x)SiO2 oxides (heated at 500 °C) are still in amorphous state. Crystallization of ZrO2 from these amorphous oxides in air occurs only at 903-953 °C (the third exothermic peaks, Figure 2; when the same samples were heated in nitrogen, a further delay in crystallization of 2 to 15 °C can be observed) as XRD patterns show broad diffraction peaks of tetragonal phase of ZrO2. For the low mol % Zr samples, this crystallization is apparently delayed to a higher temperature, as long-range diffusion of Zr will be required when its concentration is low. Nonetheless, when the concentration of Zr becomes too low (e 20 mol %), no crystallization can be observed. Note that this does not mean that crystallization does not occur (perhaps due to the sensitivity of our DTA equipment). It has been found that, in general, component crystallization is delayed in well-mixed oxides.15 Our observations in Figure 2 in fact suggest that the dispersion of ZrO2 in the SiO2 (i.e., the formation of heterolinkages of Zr-O-Si) for the low Zr content samples (e 20 mol %) is reasonably good, which agrees well with the mixing level analysis in the FTIR study and will be further addressed shortly.

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Teo and Zeng TABLE 1: FWHM Data of Zr 3d and Si 2p Spectra, Peak Area Ratio of Two O 1s Components (Figure 4), and Textural Properties of xZrO2-(100 - x)SiO2 Oxides Calcined at 500 °C for 4 h

x 5 10 15 20 25 30 35 40 45 50

Figure 3. TGA/DTA data (in air atmosphere) for three representative xerogel samples of xZrO2-(100 - x)SiO2 (x ) 5, 20 and 50).

To further confirm above results, TGA study is reported in Figure 3. As can be seen, each thermal event in the DTA curves (Figure 2) corresponds to a weight loss here. There is a major weight loss (5 to 8%) over room temperature to 140 °C, which is assigned to the water, organic solvents, and hydrolysis products trapped in the gel matrices. The weight losses due to the first and second exothermic events are approximately 5 and 7%, depending on composition of the oxides. The total weight losses for 5, 20, and 50 mol % Zr oxides are 15, 23, and 27%, respectively. The increase in percentage weight loss can be attributed to the increase in the amount of acetylacetone in the xerogels. It should be mentioned that the crystallization of the ZrO2 phase is preparation method dependent. For example, using other precursor systems, early crystallization of tetragonal and monoclinic phases of ZrO2 in the same binary oxides has been reported, which shows the formation of mixed crystalline phases of t-ZrO2 and m-ZrO2 in the samples of 28.6 to 37.5 mol % Zr calcined at the same temperature (500 °C).29 On the other hand, it has been reported that, using the single-source precursor Zr[OSi(OtBu)3]4 for synthesis of 20 mol % Zr xerogel, the crystallization to t-ZrO2 can be delayed to as high as 1200 °C.5

total pore av. pore fwhm fwhm A1/A2 SBET volume diameter (Zr 3d; eV) (Si 2p; eV) ratio (m2 g-1) (cm3 g-1) (Å) 1.6 1.6 1.6 1.5 1.5 1.5 1.5 1.4 1.4 1.4

1.9 1.9 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.7

11.894 4.868 4.908 3.434 2.895 2.087 2.413 1.670 1.546 1.114

375 400 458 484 463 396 339 258 235 105

0.99 0.99 0.98 0.49 0.30 0.21 0.19 0.15 0.11 0.02

100 99 86 41 33 27 25 23 23 20

For the same oxide with 50 mol % Zr, high crystallization temperature greater than 1150 °C has been achieved with a prehydrolysis technique, in which TEOS is first reacted with water.15 The above comparison of crystallization temperature seems to indicate the mixing level in the samples prepared with our simple approach is indeed reasonably high (note that the crystallization is not observable with DTA for x e 20, Figure 2), although they are not as high as those in the latter two cases.5,7 Surface Chemical States. Three representative XPS Zr 3d spectra for xZrO2-(100 - x)SiO2 samples are displayed in Figure 4a. The intensity of the peaks in the Zr 3d5/2 and 3d3/2 becomes stronger as the Zr content increases, and the binding energy gradually approaches that of pure ZrO2 (Zr 3d5/2 ) 182.4 eV).30 The enhancement in resolution of XPS signals is also reflected in their full-width-at-half-maximum data (fwhm, Table 1) that show a decreasing trend. In all these spectra, the Zr 3d5/2 and 3d3/2 can be well resolved into singular peaks respectively, giving an area ratio strictly at 3:2. This result indicates that there is no sign of a secondary component (i.e., a new peak) of Zr species on the surfaces, although overall peak movement shows a shift of ca. -0.5 eV to the low binding energy side, as detailed in Figure 5a. It is thus indicative that our oxides are chemically homogeneous, noting that multicomponents of Zr 3d5/2 and 3d3/2 are observed in other studies for samples derived from inorganic zirconium salts and TEOS.29,31 Similar to the trends observed for metal zirconium, XPS spectra of Si 2p displayed in Figure 4b show gradual decreases in both binding energy and fwhm (detailed in Table 1). The binding energy of Si 2p in pure silica is 103.3-103.7 eV.32 With increase in Zr content, this binding energy is gradually shifted from 103.4 to 102.1 eV in our samples with 5 and 50 mol % Zr, respectively. Compared to those in Zr 3d spectra, peak-shifts of Si 2p are more significant (about -1.2 eV, Figure 5b). From Figures 5a and 5b, we note that the differences in binding energies of Zr 3d and Si 2p versus the Zr content increases linearly. Similar increase has been reported for the low Zr content (