КИНЕТИКА И КАТАЛИЗ, 2010, том 51, № 4, с. 554-557
УДК 541.145:546.821-316546.284'623
SYNTHESIS AND CHARACTERIZATION OF TiO2—MONTMORILLONITE NANOCOMPOSITES AND THEIR PHOTOCATALYTIC ACTIVITY
© 2010 Aiqin Zhang13, Rongbin Zhang1*, Ning Zhang1, Sanguo Hong1, Ming Zhang2
1 Department of Chemistry, Nanchang University, PR China 2 College of Chemistry, Jiangxi Normal University, Nanchang, Jiangxi, PR China 3 College of Environmental and Chemical Engineering, Nanchang Hangkong University, Jiangxi, PR China
*E-mail: rbzhang@ncu.edu.cn Received 02.05.2009
TiO2 nanoparticles have been synthesized on the surface of exfoliated montmorillonite at a low temperature in benzyl alcohol medium. According to X-ray diffraction(XRD), N2 adsorption-desorption isotherm and transmisson electron microscopy (TEM), it was found that the intercalation of TiO2 nanoparticles destroyed the ordered structure of montmorillonite to some extent, and the crystallites of the nanocomposites are assembled to form a house-of-cards structure. The size of the nanoparticles in the interlamellar space is about 4 nm. The nanocomposites exhibited excellent photocatalytic activity in methylene blue degradation due to the synergetic effect of the adsorptive ability to organic compound of cetyl trimethylammonium bromide — montmorillonite and the catalytic ability to TiO2 nanoparticles in it.
Montmorillonite (MMT) is a layered aluminosili-cate of 2 : 1 type structure: one octahedral layer takes place between two tetrahedral layers, resulting in weak van der Waals interaction between the adjacent lamellae (Si—O-"Si). This structure causes MMT to be easily disaggregated, and it has a high cationic exchange capacity ensuring favorable conditions for intercalation reactions. Up to date, MMT has been found many applications in heterogeneous catalysis as catalyst support [1—3]. It has been reported that metal oxides nanoparticles have been deposited on MMT with some nanoparticles intercalated into its interlayers. These MMT-based catalysts combines the functions of the nanoparticles and MMT together, and exhibited synergetic effects [4—6]. Now several transition metal oxide-pillared clay systems, such as TiO2 [7], ZnO [8], Fe2O3 [9], ZrO2 [10] and CoO on SiO2-aluminosili-cate [11], have been reported. Generally, most of these pillared clays are prepared by exchanging charge-compensating cations between the clay layers with larger inorganic hydroxyl cations, which are polymeric or oligomeric hydroxyl metal cations formed by the hydrolysis of metal salts. The calcination process is usually necessary to yield metal oxide "pillars". This article introduces a novel one-step route for the synthesis of TiO2—MMT nanohybrid. The simple approach involves the in situ growing of TiO2 nanoparticles with anatase phase immobilized on the exfoliated MMT at low temperature.
EXPERIMENTAL
Preparation of the Nanocomposites
TiCl4, NaCl, CTAB (cetyl trimethylammonium bromide) were analytical grade and were used without further purification. The chemical formula of the Ca— MMT can be expressed as Ca019Mg0 06Na0 01 •
• (Si3.96Al0.04)(Al1.44Fe0.09Mg0.47)O10(OH)2 • nH2O. ' The
cation exchange capacity of the clay was 66.5 mmol/100g.
The Ca—MMT was subjected to cation-exchange reaction to replace the interlayer ion with a cationic surfactant, cetyl trimethylammonium (CTA). After ion exchange reaction, excess surfactants were thoroughly removed by washing with hot distilled water and then dried under an ambient atmosphere. The CTA-exchanged MMT( CTAB—MMT ) was dispersed in benzyl alcohol under stirring to obtain 1 wt % colloidal suspension of delaminated clay particles. A certain amount of TiCl4 was slowly added to anhydrous benzyl alcohol (benzyl alcohol : TiCl4 = 20 : 1, v/v) under vigorous stirring at room temperature to obtain a solution. The colloidal suspension and the solution of TiCl4 obtained above were mixed to reach a Ti : MMT ratio of 10 mmol/g, stirred for 30 min and kept at 50 or 70°C for 24 h. The suspension was separated by cen-trifuging and precipitate was washed with ethanol and THF and dried at room temperature. All the samples obtained were milled into powder and marked as TiO2-CTAB-MMT-50 and TiO2-CTAB-MMT-70 respectively. The pure TiO2 was also prepared by the same method at 70°C without adding of the colloidal suspension of delaminated clay particles.
Relative intensity
0 10 20 30 40 50 60 70 80 90
29, deg
Fig. 1. XRD patterns of nanocomposites and MMT: 1 -pristine MMT, 2 - CTAB-MMT, 3 - Ti02-CTAB-MMT-50, 4 - Ti02-CTAB-MMT-70, 5 - Ti02-MMT-70 calcinated at 450°C.
Absorbance, rel. units 1.0
01-L
200 300 400 500 600 700 800
Wavelenght, nm
Fig. 2. UV-Vis absorption spectra: 1 - Ca-MMT, 2 -Ti02, 3 - Ti02-MMT-70.
Characterization
The crystallinity of the samples was evaluated by powder X-ray diffraction (XRD) on an X-ray diffracto-meter (Bruker D8 Advance) with CuZ"a radiation under operation conditions of 40 kV and 40 mA in the 29 range from 1.5 to 80°. Transmission electron microscopy (TEM) images were taken using JEM-2010(HR). The absorption edge of the samples was measured using a UV-Vis spectrophotometer (U-3310) and BaS04 was the reflectance standard in a UV-Vis diffuse reflectance experiments. Surface areas and porosities were determined from the adsorption-desorption isotherms of N2 at 77 K after degassing the samples at 393 K with a Micromeritics ASAP 2020 apparatus.
Photocatalytic Activity Measurement
The reactor was illuminated by a high pressure Hg lamps (500 W) which predominantly emit light at 365 nm. In a typical reaction, sample of Ti02-MMT-70 nanohybrid (50 mg) or Ti02 (16 mg) was first immersed in an aqueous methylene blue (MB) solution (20 mg/l, 100 ml) in the dark for 24 h to establish the adsorption equilibrium, then centrifuged and immersed in another MB solution (20 mg/l, 50 ml). The photocatalytic activities of the samples were assessed by monitoring the adsorbance of MB at 660 nm at given irradiation time intervals.
RESULTS AND DISCUSI0N
XRD Patterns
The powder XRD patterns of pristine MMT, CTAB-MMT and Ti02-MMT nanocomposites are
shown in Fig. 1. The XRD patterns of MMT generally show basal 001 reflection and two dimensional diffraction hk only, and other hkl diffractions are usually not observed [12]. Ti02-CTAB-MMT-70 nanocomposite do not exhibit 001 reflection, indicating the delamina-tion of the aluminosilicate layers [13]. The peak at 25.3° in all nanohycomposite is the (101) plane of the anatase phase of titania. No rutile phase of titania was found in the samples. The peak at 25.3° became more intense and sharper after Ti02-CTAB-MMT-70 was calcined at 450°C for 2 h, but the peak is still relatively broad due to the nanosized Ti02 particles. It revealed that the presence of MMT hindered the agglomeration of the Ti02 nanoparticles.
UV-Vis Diffuse Reflectance Spectra
Fig. 2 illustrates UV-Vis spectra for the Ti02-MMT-70, in comparison with those for MMT and Ti02. Pure MMT was almost transparent in the wavelength range longer than 350 nm [14], the absorption edge of the nanocomposite has a blue-shift in comparison to that of pure Ti02. Some authors [15, 16] have also reported that the absorption edge of Ti02-mont-morillonite nanocomposite had a blue-shift that was due to quantum size effects arising from Ti02.
Transmission Electron Microscopy
According to TEM micrographs in Fig. 3, the nanohybrid exhibited two distinct images, sheets and spherical particles. The former are the exfoliated sheets of montmorillonite whereas the latter are the Ti02 nanoparticles. The mean size of spherical particles is around 4 nm estimated from the scale bar, which is in good agreement with that calculated from XRD
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Fig. 3. TEM photographs of Ti02-MMT-70.
Pore volume, ml/g
240 220 200 180 160 140 120 100 80 60 40 20 0
0.4 0.6 0.8 1.0 Relative pressure (p/p0)
Fig. 4. N2 adsorption-desorption isotherms: 1 — pristine MMT, 2 - TiO2-CTAB-MMT-70, 3 MMT-70 calcinated at 450°C.
Ti02-CTAB-
peak broadening according to Scherrer equation. In Fig. 3, two HR-TEM images were also presented. They clearly showed that the lattice fringe spacing corresponding to the (101) planes of the anatase phase is about 0.35 nm.
N2 Adsorption—Desorption
Fig. 4 showed the adsorption-desorption isotherms of nitrogen for pristine MMT, TiO2-MMT-70 and TiO2-MMT-70 calcinated at 450°C. All these isotherms can be classified as the BDDT-type IV shape, implying bimodal pore size distributions in the meso-porous and macroporous regions. The nitrogen uptakes of the TiO2-MMT-70 was substantially lower, compared with that of the pristine MMT. The small hysteresis of the nanocomposite indicated less meso-porosity. But the nitrogen uptakes of the nanocompos-ite increased after it was calcinated at 450°C for 2 h, implying that the elimination of the organic moiety giving rise to mesopores in the interlayers. According to adsorption data, the specific surface area is reduced from 80 m2/g for the pristine MMT to 10 m2/g for the
Desorption, cm3/g 0.600.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0
-0.05
J 3
~ i
- 1J1-
; i i 2 1 1 1 1 1
200
400
600
800 1000 10 ,
Pore diameter, 10 10 m
Fig. 5. Pore size distribution in samples: 1 — pristine MMT, 2 - Ti02-CTAB-MMT-70, 3 - Ti02-CTAB-MMT-70 calcinated at 450°C.
Relative absorbance 1.0
0.8
0.6
0.4
0.2
0
10 15 20 25
Irradiation time, min
Fig. 6. Degradation of methylene blue on commercial TiO2 (1) and Ti02-CTAB-MMT-70 (2).
2
5
Ti02-MMT-70 due to the modification of CTAB and Ti02 nanoparticles and enlarged to 180 m2/g for Ti02-MMT after calcination. The bimodal size distributions were further confirmed by the corresponding pore size distribution curves in Fig. 5.
Photocatalytic Activity
According to elemental analysis, the mass content of Ti02 in Ti02-CTAB-MMT-70 was 33.5%, so the weight of Ti02 in nanocomposite was the same as commercial Ti02 powder in photocatal
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