TiO2 Nanosol Particles in Two-Dimensional

SiO2/TiO2 Nanosol Particles in Two-Dimensional Aluminosilicate Catalyst−Support ... It is therefore proposed that the products are intercalation...
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J. Phys. Chem. B 1998, 102, 5991-5995

5991

Multilayered SiO2/TiO2 Nanosol Particles in Two-Dimensional Aluminosilicate Catalyst-Support Jin-Ho Choy,* Joo-Hyoung Park, and Joo-Byoung Yoon Department of Chemistry, Center for Molecular Catalysis, College of Natural Sciences, Seoul National UniVersity, Seoul 151-742, Korea ReceiVed: March 23, 1998; In Final Form: May 12, 1998

A new layered nanocomposite, which is one-to-one interstratified with a montmorillonite layer and a mixed SiO2/TiO2 sol particle one, has been prepared by ion exchange reaction of the Na+ ion in montmorillonite with the positively charged SiO2/TiO2 sol particles. The ion exchange reaction was performed at three different temperatures of 45, 60, and 75 °C by mixing an aqueous suspension of 1 wt % Na+ montmorillonite with SiO2/TiO2 sol solution where the molar ratio of Si/Ti was selected as 20/2. According to the powder X-ray diffraction analysis, the basal spacings of layered nanocomposites calcined at 400 °C were found to increase from 35.4 Å, to 47.3 Å, and to 60.0 Å as the ion exchange reaction temperature was raised from 40 °C, to 60 °C, and to 75 °C. Their BET and Langmuir specific surface areas and porosities, estimated from nitrogen adsorption-desorption isotherms, become larger with the increment of basal spacing, and the highest BET specific surface area and the largest porosity are found to be 683 m2/g and 0.50 mL/g, respectively. Despite the large increment of the basal spacing, the porous properties such as specific surface areas, porosities, and pore sizes, those which are calculated from t-plots and chemical shift of 129Xe NMR, respectively, are determined to be almost constant. From the UV/vis spectra, the blue shift of the absorption edge was observed, indicating that the TiO2 sol particles in the interlayer are quantum sized. It is therefore proposed that the products are intercalation-type nanocomposites with the multistacked structure of the SiO2/TiO2 nanoparticles in the interlayer space of montmorillonite.

Introduction It is well-known that a number of nanosized particles such as TiO2, Al2O3, ZrO2, SiO2, CdS, and Fe2O3 have high activity in oxidation-reduction catalysis or acid-base catalysis. In particular the semiconductors such as TiO2, ZrO2, and CdS are widely studied due to their photocatalytic activities on the decomposition of environmental contaminants such as halogenated hydrocarbons, pesticides, surfactants, and organophosphorus compounds.1-4 Since the semiconductor particles below ca. 10 nm in size show higher activities than the bulk in photocatalysis, it has become important to stabilize quantumsized particles in various porous nanocomposites.5-8 Nanocomposites refers to composites of more than one Gibbsian solid phase where at least one dimension is in the range of nanometer size and typically all solid phases are in the 1-20 nm range.9 Such typical materials are pillared clays, gels and xerogels with nanosized particles, zeolite-inorganic nanocomposites, etc. Among them, the pillared clays with inorganic oxides such as Al2O3, ZrO2, Fe2O3, TiO2, and SiO2 as pillaring agent are of interest due to their high thermal stability and porosity,10-14 those which have been considered as useful catalysts, molecular sieves, selective adsorbents, and membrane and sensor materials.15-20 However, there is little information on the best synthetic conditions to obtain highly porous pillared clay, on the interlayer structure, on the nanocrystalline structure of pillars, and on the mechanisms during intercalation and pillaring reactions. * To whom all correspondence should be addressed. Tel: 82-2-880-6658. Fax: 82-2-872-9864. E-mail: [email protected].

In the present study, taking advantage of the formation of aggregate of nanometer-sized SiO2/TiO2 sols in highly acidic solution, we attempted to synthesize new nanocomposites with multistacked SiO2/TiO2 sol particles (double, triple, and quadruple) between montmorillonite layers and to understand interlayer structure of SiO2/TiO2 particles using powder X-ray diffraction (XRD), nitrogen adsorption-desorption isotherms, diffuse reflectance ultraviolet-visible (UV/vis) spectroscopy, and 129Xe nuclear magnetic resonance (129Xe NMR) spectroscopy. Experiments Materials. The montmorillonite (Kunimine Co.) with the structural formula Na0.35K0.01Ca0.02(Si3.89Al0.11)(Al1.60Mg0.32Fe0.08)O10 (OH)2‚nH2O was used as a starting material without any purification, where layer charge was determined to be 0.31e-/(Si,Al)4O10 by an n-alkylammonium method.21 The Na+-activated montmorillonite was prepared by adding the colloidal fractions into the 1 N sodium chloride solution, and then it was washed thoroughly with distilled water. A 1 wt % aqueous suspension of montmorillonite was preswelled for 1 day before ion exchange reaction. Preparation of Sol Solutions. A silica sol solution was prepared by mixing Si(OC2H5)4, 2 N HCl, and ethanol with a ratio of 13.6 mL/3.5 mL/3.0 mL at room temperature. Titanium tetrachloride (TiCl4) was also hydrolyzed by adding it to 1 N HCl solution with a molar ratio of HCl/TiCl4 ) 6, and the resulting translucent slurry was aged to a clear sol solution by continuous stirring for 1 h at room temperature. Finally the silica sol solution and the titania solution were intermixed with

S1089-5647(98)01586-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/11/1998

5992 J. Phys. Chem. B, Vol. 102, No. 31, 1998 a ratio of Si/Ti ) 10/1, and the mixture was stirred further for 1 h at room temperature and used for pillaring reaction. Pillaring Reaction. The ion exchange reaction was carried out by titrating the mixed solution into the clay suspension in a ratio of Si/Ti/clay ) 20 mmol/2 mmol/1 g. The mixture was allowed to stand for 40 h at three different temperatures (45, 60, and 75 °C) to exchange the Na+ ion in the clay with the mixed SiO2/TiO2 sol particles. Each reaction product was separated by centrifugation, washed thoroughly with deionized water (abbreviated hereafter as NMs-wet; NM2-wet, NM3-wet, and NM4-wet prepared at three different temperatures of 45, 60, and 75 °C, respectively, for ion exchange reaction), and dried in an oven at 80 °C for 24 h (abbreviated hereafter as NMs-80; NM2-80, NM3-80, and NM4-80). Finally all NMs80 were calcined at 400 °C for 2 h to obtain a stable nanocomposite (abbreviated hereafter as NMs-400; NM2-400, NM3-400, and NM4-400). Sample Characterization. Powder XRD patterns for the orientated samples were taken using a Philips PW1830 diffractometer with Ni-filtered Cu KR radiation (λ ) 1.54056 Å). To observe the thermal behavior of NMs-80, thermogravimetry-differential thermal analysis (TG-DTA) carried out with a Rigaku TAS-100 under an ambient atmosphere where the heating rate was fixed at 10 °C/min. Chemical analyses for the samples, fused with lithium metaborate at 900 °C and then dissolved in a 3% HNO3 solution, were carried out with the inductively coupled plasma (ICP: Shimazu ICPS-5000) method. Diffuse reflectance UV/vis spectra were recorded on a PerkinElmer Lambda 12 spectrometer equipped with an integrating sphere of 60 mm in diameter using BaSO4 as a standard. The UV/vis spectra obtained in diffuse reflectance mode [R∞] were transformed to a magnitude proportional to the extinction coefficient (κ) through the Kubelka-Munk function [F(R∞)].22 129Xe NMR spectroscopic analyses were performed using a Bruker AMX-400 Fourier transform NMR spectrometer at a frequency of 110.7 MHz at 25 °C, typically under conditions of a 6 µs pulse and a pulse delay of 2 s. The reference signal of 129Xe was taken as that of 304 kPa of xenon gas, which was extrapolated to zero pressure, using the Jameson equation.23 The NMR samples were prepared by drying them at 300 °C in a vacuum and sealed at very low pressure (20 Torr) of xenon gas. Nitrogen adsorption-desorption isotherms were measured volumetrically at the liquid nitrogen temperature with a homemade computer-controlled measurement system.24 The calcined samples were degassed at 300 °C for 3 h under vacuum prior to the adsorption measurement. Results and Discussion Powder X-ray Diffraction Analyses. Figure 1 shows a series of typical powder XRD patterns for the NMs-80 and NMs400, of which the (00l) diffraction peaks gradually shift toward lower angle in two theta (2θ), as the ion exchange reaction temperature was raised from 45 °C, to 60 °C, and to 75 °C. The basal spacings of NM2-400, NM3-400, and NM4-400 are determined to be 35.4, 47.3, and 60.0 Å with their gallery heights of 25.8, 37.7, and 50.4 Å, respectively, by subtracting the thickness of the aluminosilicate layer (9.6 Å) from their basal spacings. It is noticeable that the increment of the gallery height remains almost constant with a value of ca. 12 Å, resulting from the multistacked aggregation of homogeneous nanosol particles in the interlayer space of montmorillonite.

Choy et al.

Figure 1. Powder X-ray diffraction patterns for (a) NM2-80 and NM2400, (b) NM3-80 and NM3-400, and (c) NM4-80 and NM4-400.

Figure 2. Thermal analysis for NMs-80: (a) thermogravimetric analysis and (b) differential thermal analysis.

Thermal Analysis. All NMs-80 show two resembling thermal behaviors though they are different from one another in terms of weight changes as shown in TG-DTA curves (Figure 2). The first dramatic weight loss with a strong endothermic peak below 250 °C is mostly attributed to the desorption of water on internal and external surfaces of montmorillonite and pillar, and the second gradual weight loss with a feeble endothermic peak above 600 °C corresponds to dehydroxylation mainly due to the decomposition of the lattice hydroxyl group in octahedral sheets of aluminosilicate. From the thermal analysis, the pillaring temperature of 300-500 °C is desirable, and the resulting NMs-80 become stable at least up to 500 °C. Chemical Analysis. Na+ montmorillonite and NMs-400 have been quantitatively analyzed by ICP to estimate pillar contents of SiO2 and TiO2. Assuming that the chemical composition of montmorillonite based on the Al2O3 content is unchanged during pillaring reaction, those of pillared product and pillar itself could be determined as shown in Table 1. It is

Multilayered SiO2/TiO2 Nanosol Particles

J. Phys. Chem. B, Vol. 102, No. 31, 1998 5993

TABLE 1: Chemical Analysis Data for NMs-400 in Comparison with Those of Na+ Montmorillonite and Pillar Itself mol compositiona

analytical results (%)

pillar compositionb

sample

SiO2

TiO2

Al2O3

Si

Ti

Al

Si

Ti

Na+-mont. NC-2MST NC-3MST NC-4MST

63.0 65.7 63.0 60.8

0.0 19.9 23.5 26.9

23.5 10.6 9.9 9.1

3.89 9.04 9.27 9.73

0.00 2.06 2.60 3.23

1.71 1.71 1.71 1.71

0.00 5.15 5.38 5.84

0.00 2.06 2.60 3.23

a The compositions given in moles for O10(OH)2 anion basis of silicate layer. b The compositions of pillars in the interlayer of montmorillonite are given in moles for O10(OH)2 anion basis of silicate layer; these values are calculated by subtracting the nanocomposites from Na+ montmorillonite.

Figure 4. Relationship between chemical shift of Xe peak and mean free path.

Figure 3. 129Xe NMR spectra for (a) NM2-400, (b) NM3-400, (c) NM4-400, and (d) Xe reference.

found that the pillar content such as SiO2 and TiO2 becomes larger linearly with an increase of gallery height of NMs-400, although the pillar composition of TiO2 increases more significantly than that of SiO2. This indicates that SiO2 and TiO2 sol particles are intercalated into the interlayer space of montmorillonite with an increase of gallery height in NMs-400. 129Xe Nuclear Magnetic Resonance Spectroscopy. 129Xe NMR spectra of xenon adsorbed in NMs-400 are shown in Figure 3 together with that of 304 kPa xenon gas as reference for comparison. The chemical shift δ of xenon adsorbed in a cavity corresponds to the following equation:25,26

δ ) FWδW‚Xe + FXeδXe‚Xe + δE where δW‚Xe denotes the influence of collisions between xenon atoms and the internal wall of the cavity, and δXe‚Xe corresponds to the effect of collision between xenon atoms. FW and FXe depend on the density of xenon adsorbed, i.e., on the number of xenon atoms per cavity. δE expresses the effect of mean electric field within the cavity. In the present work, the second and third terms (δE and FXeδXe‚Xe) are negligible since the samples, which do not contain any polarizing cations in the cavity of NMs-400, are dried at 300 °C in a vacuum and sealed at very low pressure (20 Torr) of xenon gas. In this case, the pore diameter could be calculated from the correlation curve between chemical shift and mean free path (t) of xenon26 as shown in Figure 4. For the case of spherical pores, the pore diameter (D) can be estimated by the following equation;

D ) 2(t + rXe) where rXe is van der Waals radius of xenon (2.2 Å).

Figure 5. Nitrogen adsorption-desorption isotherms for (a) NM2400, (b) NM3-400, and (c) NM4-400.

All 129Xe NMR spectra of NMs-400 exhibited similar chemical shifts of ca. 0 and 98 ppm, irrespective to their basal spacings. The former chemical shift results from xenon adsorbed in the macropore or free xenon, and the latter corresponds to one adsorbed in a micropore with a size of ca. 10 Å, whose value was calculated from the above relationship. Nitrogen Adsorption-Desorption Isotherms. Figure 5 shows the nitrogen adsorption-desorption isotherms of NMs400. All isotherms, characterized as type IV according to the BDDT (Brunauer, Deming, Deming, and Teller) classification, exhibit relatively large hysteresis, whose loops are of type B in Boer’s five types,27,28 indicating the presence of open slit-shaped capillaries with fairly wide bodies and narrow short necks. Such a finding allows us to conclude that all NMs-400 are analogous in microstructure.

5994 J. Phys. Chem. B, Vol. 102, No. 31, 1998

Choy et al. TABLE 2: Adsorption Properties of NMs-400 in Comparison with Those of Na+ Montmorillonite sample

BSa (Å)

BET (m2/g)

SWb (Å)

TPc (mL/g)

MPd (mL/g)

PS (Å)

mont. NC-2MST NC-3MST NC-4MST

9.6 35.4 47.3 60.0

30 469 591 683

10.2 10.2 11.4

0.35 0.43 0.50

0.25 0.30 0.37

10.4 10.4 10.4

a BS ) Basal spacing calculated from powder X-ray diffraction. b SW ) slit width. c TP ) total porosity. d MP ) microporosity. e PS ) pore size evaluated form chemical shift of 129Xe NMR.

Figure 6. t-plots calculated from nitrogen adsorption isotherms for (a) NM2-400, (b) NM3-400, and (c) NM4-400.

Their BET specific surface area and porosities, estimated from the nitrogen adsorption-desorption isotherms, become larger as the basal spacing increases. The largest BET specific surface area and the largest porosity are found to be 683 m2/g and 0.50 mL/g, respectively, for NM4-400. To evaluate the micropore size of NMs-400, t-plots28 are calculated from the isotherms as shown in Figure 6, in which the amount of nitrogen adsorbed is plotted against the statistical thickness (t) obtained from the t-curve. The t-curve28 is the plot of the statistical thickness (t) of adsorbed species on the surface of nonporous adsorbents with regard to relative vapor pressure (P/P0). The thickness t is given by σ(V/Vm), where σ is the average thickness of the molecular single layer depending upon the way successive layers are stacked, and V/Vm is the number of statistical molecular layers in the film; V is the total volume of adsorbed species, and Vm is the monolayer capacity of a nonporous reference solid. In this work, the Harkins-Jura equation28 was used as the standard t-curve as follows:

log(P/P0) ) 0.034 -13.99/t2

Figure 7. Relationships between basal spacing and (a) pillar composition, (b) specific surface area, (c) porosity, and (d) pore size.

Figure 8. Proposed model of the interlayer microstructure for NMs-400.

The micropore volumes and the slit widths for NM2-400, NM3-400, and NM4-400 can be calculated from the t-plots, since the microporosity is equivalent to the intercept value given by the line extrapolated from the high-pressure branch to the adsorption axis, and the slit width is twice the t-value of inflection point, giving the real pore size in the interlayer of montmorillonite. The micropore volumes for NM2-400, NM3400, and NM4-400 are estimated as 0.25, 0.30, and 0.37 mL/g, respectively, which are presented in Table 2. From these values, we found that the major part of their total pores are mostly composed of micropores with slit widths of 10.2, 10.2, and 11.4 Å, respectively. It is worthy to note here that the microporosity

Multilayered SiO2/TiO2 Nanosol Particles

J. Phys. Chem. B, Vol. 102, No. 31, 1998 5995 layer space of montmorillonite. According to the XRD, nitrogen adsorption-desorption isotherms, and 129Xe NMR analyses, it is found that all the nanocomposites are highly porous with a supergallery height of NM2-400 ) 25.8 Å, NM3-400 ) 37.7 Å, and NM4-400 ) 50.4 Å, and their porous properties are stable up to at least 500 °C. The highest BET specific surface area obtained is 683 m2/g in NM4-400, whose value is highest in the same kind of pillared clay so far reported. From UV/vis spectra, it is concluded that the quantum-sized TiO2 of ca. 20 Å are stabilized in the interlayer space. We are going to characterize the local symmetry and the electronic structure of the TiO2 particles using X-ray absorption fine structure spectroscopy, since they are very important in photocatalysis of halogenated hydrocarbon, phenol, etc. Acknowledgment. This work was in part supported by the Korea Science and Engineering Foundation through the Center for Molecular Catalysis and the Research Institute of Molecular Science in Seoul National University, and by the Ministry of Education (BSRI-97-3413). References and Notes

Figure 9. Ultraviolet-visible spectra for (a) NM2-wet, NM2-80, and NM2-400, (b) references of TiO2 (rutile and antanse) and montmorillonite used in this work.

increases as the gallery height becomes larger, but the pore size remains unchanged with a value of 11 Å, which is in agreement with the 129Xe NMR study. According to the above results, a linear correlation could be deduced among their gallery heights and porous properties such as specific surface area, total porosity, slit width, and microporosity, as shown in Figure 7. It is now clearly concluded that NMs-400 are the intercalationtype nanocomposites with multistacked structure of nanoparticles in the interlayer space of montmorillonite, as shown in Figure 8. Ultraviolet-Visible Spectroscopy. Figure 9 shows the UV/ vis spectra of NMs-wet, NMs-80, and NMs-400 together with those of anatase, and Na+ montmorillonite as reference compounds for comparison. There are two important findings to be underlined in the spectra of NMs-wet, NMs-80, and NMs400; the first is a large blue shift in the absorption threshold and the λmax compared to the bulk anatase due to the quantum size effect5,29,30 of nanosized TiO2 particles of ca. 20 Å in the interlayer space of montmorillonite. The absorption threshold is the extrapolated energy position where the peak starts to increase steeply, and λmax is the maximum position of absorption. The second is a red shift of absorption threshold in UV/ vis spectra for NMs-400 due to the coagulation of TiO2 particles in the interlayer space and/or the formation of Si-O-Ti covalent bonding among intercalated TiO2, SiO2 sol particles, and silicate layers. Conclusion We were successful in preparing the intercalation-type nanocomposite with multistacked structure (double, triple, and quadruple) of intermixed SiO2/TiO2 nanoparticles in the inter-

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