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Low-Temperature Synthesis and Photocatalytic Activity of TiO2 Pillared Montmorillonite Gao K. Zhang,* Xin M. Ding, Fang S. He, Xin Y. Yu, J. Zhou, Yan J. Hu, and Jun W. Xie School of Resources and EnVironmental Engineering, Wuhan UniVersity of Technology, Luoshi Road 122, Wuhan 430070, People’s Republic of China ReceiVed August 28, 2007. In Final Form: October 15, 2007 TiO2 pillared montmorillonite (PILM) was synthesized by hydrolyzing TiO2 sol into the interlayers of montmorillonite (MMT) at low temperatures. It is novel that all the as-prepared catalysts without calcination show a high photocatalytic activity for the degradation of acid red G (ARG) under UV light irradiation. The as-prepared powders were characterized by XRD, TEM, UV-vis diffuse reflectance spectroscopy, BET, and FT-IR. The X-ray diffraction analysis indicated that the (001) plane of MMT in the composites disappeared and that the layered structure became disordered, which also was confirmed by the TEM photographs. The UV absorption edge of the composites shows a red-shift in comparison to that of pure TiO2 particles. The obtained catalyst has the highest photocatalytic activity when the composite temperature is 70 °C, which could be attributed to the synergetic effects of the adsorbability of MMT and the photocatalytic property of TiO2 in it.
1. Introduction Organic dyes are an important source of environmental contamination, as they are toxic and mostly nonbiodegradable. Conventional treatment methods, such as coagulating sedimentation, electro-coagulation, and adsorption by activated carbon, are ineffective for removing the dyes from industry wastewater. As such, it is important to find new effective approaches to treat textile effluents and to decrease environmental pollution. TiO2 as a semiconductor has been widely used for the decomposition of a great variety of organic pollutants because of its particular advantages, such as high chemical stability, strong oxidizing power, low cost, nontoxicity,1-3 and so on. However, the pollutants in wastewater are usually very diluted, and those such as endocrine disruptors are thought to be harmful even if they are present in an extremely low concentration in the environment. The TiO2 photocatalyst has a small surface area and low adsorbability, and its photocatalytic effect is low in very dilute solutions. Hence, enrichment of reactants by adsorption is required for a highly efficient photocatalytic performance.4 Clays, such as montmorillonite (MMT), rectorite, and kaolinite, have attracted much attention in recent years. These natural materials possess layered structures, large surface areas, and a high cation exchange capacity and can adsorb organic substances either on their external surfaces or within their interlaminar spaces by interaction or substitution.5 TiO2 pillared clays (PILCs) have a mesoporous structure, high adsorption ability, stabile photocatalytic activity, and large specific surface area due to their nanosized TiO2 particles,6,7 which are located as pillars between the silicate layers. * Corresponding author. E-mail:
[email protected]. (1) Li, H. X.; Li, J. X.; Huo, Y. N. J. Phys. Chem. B 2006, 110, 1559-1565. (2) Yu, J. G.; Zhou, M. H.; Cheng, B.; Zhao, X. J. J. Mol. Catal. A: Chem. 2006, 246, 176-184. (3) Wong, M. S.; Chou, H. P.; Yang, T. S. Thin Solid Films 2006, 494, 244249. (4) Ooka, C.; Yoshida, H.; Suzuki, K.; Hattori, T. Microporous Mesoporous Mater. 2004, 67, 143-150. (5) Miao, S. D.; Liu, Z. M.; Han, B. X.; Zhang, J. L.; Yu, X.; Du, J. M.; Sun, Z. Y. J. Mater. Chem. 2006, 16, 579-584. (6) Sun, S. M.; Jiang, Y. S.; Yu, L. X.; Li, F. F.; Yang, Z. W.; Hou, T. Y.; Hu, D. Q.; Xia, M. S. Mater. Chem. Phys. 2006, 98, 377-381. (7) Ooka, C.; Yoshida, H.; Suzuki, K.; Hattori, T. Appl. Catal., A 2004, 260, 47-53.
The photocatalytic activity of TiO2 PILCs is greatly affected by the concentration of the metal ion, the temperature, the preparation method, the crystalline and phase type of the nanosized TiO2 pillars,8 and the different dry methods,9 etc. Generally, TiO2 PILCs are prepared by exchanging charge-compensating cations between the clay layers with larger inorganic hydroxy cations, which are polymeric or oligomeric hydroxy metal cations formed by the hydrolysis of metal salts.10,11 There are two traditional methods for the synthesis of the precursors of TiO2: hydrolysis of TiCl412 or TiOSO410 or the sol-gel method.4 Miao et al. prepared a TiO2 pillared montmorillonite (PILM) via impregnating titanium tetrabutyloxide into the interlayers of MMT in supercritical ethanol, which showed an excellent ability to catalytically degrade methylene blue.5 Ooka et al. synthesized TiO2 PILM by hydrothermal treatment at different temperatures for different times and investigated the dependence of the crystallization of TiO2 pillars on the photocatalytic activity.13 TiO2 PILM was synthesized by microwave irradiation, which exhibited a good photocatalytic performance.6 The calcination process is usually necessary for the preparation of TiO2 PILCs. In this work, we obtained the TiO2 PILM with the anatase phase of TiO2 without calcination via hydrolyzing TiCl4 into an HCl aqueous solution and then impregnating Tipolycations into the interlayer of MMT through the exchange of the cations. The characterization and photocatalytic activities of the catalysts were studied. 2. Experimental Procedures 2.1. Preparation of the Catalysts. TiCl4, HCl, Na2CO3, and other chemicals were analytical grade and were used without further purification. Ca-MMT clay (Xinjiang Province) was purified by (8) Valverde, J. L.; Sanchez, P.; Dorado, F.; Molina, C. B.; Romero, A. Microporous Mesoporous Mater. 2002, 54, 155-165. (9) Ding, Z.; Zhu, H. Y.; Lu, G. Q.; Greenfield, P. F. J. Colloid Interface Sci. 1999, 209, 193-199. (10) Binitha, N. N.; Sugunan, S. Microporous Mesoporous Mater. 2006, 93, 82-89. (11) Hutson, N. D.; Hoekstra, M. J.; Yang, R. T. Microporous Mesoporous Mater. 1999, 28, 447-459. (12) Long, R. Q.; Yang, R. T. J. Catal. 1999, 186, 254-268. (13) Ooka, C.; Akita, S.; Ohashi, Y.; Horiuchi, T.; Suzuki, K.; Komai, S.; Yoshida, H.; Hattori, T. J. Mater. Chem. 1999, 9, 2943-2952.
10.1021/la702649v CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008
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the method of second-centrifugal sedimentation and then treated with a 5 wt % Na2CO3 aqueous solution. The obtained clay was Na-MMT. The TiO2 sol was prepared by dropwise adding TiCl4 into a 6 M HCl aqueous solution, followed by successively stirring for 0.5 h, and a flaxen TiO2 sol was obtained. The TiO2 sol was diluted by slowly adding deionized water to reach a concentration of 0.83 M Ti and 1.0 M HCl14 and then was constantly stirred for 1 h and aged for 6 h; solution A was obtained. Na-MMT powders were dispersed in deionized water under stirring for 3 h to obtain a 0.4 wt % suspension. Solution A was then dropwise added to the vigorously stirred Na-MMT suspension until the ratio of Ti/clay increased to 30 mmol of Ti/g of clay. After adding the pillaring solution, the reaction mixture was stirred for an additional 3 h and aged for 13 h.8 The reaction temperatures in this process were kept at 30, 40, 50, 60, 70, and 80 °C, respectively. The mixture was centrifuged and washed by centrifugation with deionized water until the pH was 2.0-2.5. The obtained wet cakes were dried in air at 80 °C. All the samples obtained at different temperatures were ground into powders in an agate mortar, termed as samples A-F, respectively. 2.2. Characterization. The crystallinity of the samples was evaluated by powder X-ray diffraction (XRD) patterns obtained on an X-ray diffractometer (Rigaku D/MAX-RB) with Cu KR radiation under operation conditions of 40 kV and 50 mA. Crystallite sizes, phases, and shapes were observed using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) (H-600 STEM/EDX PV9100). The absorption edge of the samples was measured using a UV-vis spectrophotometer (UV2550), and BaSO4 was used as the reflectance standard in a UV-vis diffuse reflectance experiment. The specific surface area of the products was measured by BET methods in a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The chemical bonds of the composites were detected by Fourier transform infrared spectroscopy (FT-IR) (Nexus, Thermo Nicolet). 2.3. Photocatalytic Activity. The photocatalytic activities of the as-prepared catalysts were measured by degradation of acid red G (ARG) aqueous solution under UV light irradiation.15,16 A total of 100 mL of the ARG aqueous solution (30 mg/L) and 0.10 g of the as-prepared catalysts was mixed in a 500 mL beaker. Prior to UV illumination, the suspension was magnetically stirred in the dark for 10 min to reach adsorption equilibrium with the photocatalysts. The irradiation was performed with a 20 W UV light lamp (λ ) 253.7 nm) 8 cm above the liquid surface. The light intensity was approximately 0.524 mW/cm2 measured with an UV radiometer (UV-B). At a defined time interval, the absorbance of the ARG solution was analyzed using a UV-vis spectrophotometer (UV751GD) at 505 nm. The relation between the absorbance and the concentration of the ARG solution follows the equation:17 A ≈ 0.01C
(1)
3. Results and Discussion 3.1. XRD Patterns. X-ray powder diffraction patterns of the purified MMT and TiO2 PILM are shown in Figure 1. The XRD patterns of MMT generally show basal (001) reflection and twodimensional diffraction hk only, and other hkl diffractions are usually not observed.12 The TiO2 PILM does not exhibit (001) reflections, which have been referred to the delaminated phenomena during the process of fabricating the composites but not destroyed completely.5 The pillared montmorillonite samples did not have a sufficiently ordered and oriented silicate layer (14) Yuan, P.; Yin, X. L.; He, H. P.; Yang, D.; Wang, L. J.; Zhu, J. X. Microporous Mesoporous Mater. 2006, 93, 240-247. (15) Zhang, G. K.; Zou, X.; Gong, J.; He, F. S.; Zhang, H.; Zhang, Q.; Liu, Y.; Yang, X.; Hu, B. J. Alloys Compd. 2006, 425, 76-80. (16) Zhang, G. K.; He, F. S.; Zou, X.; Gong, J.; Tu, H. B.; Zhang, H.; Zhang, Q.; Liu, Y. J. Alloys Compd. 2007, 427, 82-86. (17) Zhang, G. K.; Gong, J.; Zou, X.; He, F. S.; Zhang, H.; Zhang, Q.; Liu, Y.; Yang, X.; Hu, B. Chem. Eng. J. 2006, 123, 159-164.
Figure 1. XRD patterns of the samples (a) and acidic MMT (b).
structure to show the (001) peak.6 The diffuse XRD patterns of TiO2 PILM may be attributed to the hydrolysis property of the Ti4+ cations, as the presence of a wide range of hydrolyzed species of Ti with different sizes (such as monomeric TiO2+/ Ti(OH)22+ and polymeric species) in the pillaring solution causes the nonuniform pillaring of the clay layers.14 Figure 1b shows the XRD of MMT treated by 1.0 M HCl according to the same procedures described previously, and it can be seen that the HCl solution has no influence on the structure of MMT. The peak at 25.3° in all TiO2 PILM samples is the (101) plane of the anatase phase of TiO2, its intensity increases, and the shape becomes sharper with increasing reaction temperature. No rutile phase of TiO2 was formed in the samples. The intercalation of the precursor into the interlayers of MMT destroyed the ordered structure of MMT to some extent, resulting in some exfoliated one-layer and multilayer sheets, as shown in Figure 2a,b. The precursor molecules could be adsorbed on the outer surface of the exfoliated silicate layers by the hydrogen bond and van der Waals force. Therefore, TiO2 nanoparticles were formed in the interlayers of MMT and on the surface of MMT during the hydrolysis process of the precursor.5 The image in Figure 2c clearly shows that the lattice fringe spacing of the TiO2 (anatase) nanoparticles is about 0.33 nm, that the size of the TiO2 particles is less than 5 nm, and that some of them are marked by arrows in Figure 2c. The structure of the TiO2 powders is further confirmed to be the anatase phase by selective area electron diffraction (SAED) as shown in Figure 2d. 3.2. UV-vis Diffuse Reflectance Spectra. Figure 3 shows the UV-vis diffuse reflectance spectra of TiO2 PILM composites, the purified MMT, and TiO2, in which TiO2 was obtained by adding ammonia into the TiO2 sol slowly with vigorous stirring
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Figure 2. TEM photographs and SAED pattern of sample E.
Figure 3. UV-vis DRS of the composites and pristine MMT.
for several minutes and then was separated and dried at 80 °C in air. The purified MMT was almost transparent in the wavelength range longer than 350 nm.4 Some articles5,18 have reported that the absorption edge of TiO2 PILM has a blue-shift that is due to quantum size effects. In this work, the absorption edge of sample E has a red-shift in comparison to that of pure TiO2. This may be attributed to the effects of MMT or to some elements of MMT, which may be doped into TiO2 in TiO2 PILM. 3.3. N2 Adsorption-Desorption. The nitrogen isotherm on sample E is shown in Figure 4. The shape of the isotherm seems to be nearly a type IV isotherm (BDDT classification)19 with two capillary condensation steps, implying bimodal pore size distributions in the mesoporous and macroporous regions. Yu et al. considered that the bimodal pore size distribution results from two different aggregates in the powders. The smaller pore is related to finer intraaggregated pores formed between intraag(18) Ooka, C.; Yoshida, H.; Horio, M.; Suzuki, K.; Hattori, T. Appl. Catal., B 2003, 41, 313-321. (19) Jose´, L. V.; Antonio, de L.; Fernando, D.; Amaya, R.; Prado, B. G. Ind. Eng. Chem. Res. 2005, 44, 2955-2965.
Figure 4. N2 adsorption-desorption isotherm and pore size distribution inset of sample E.
glomerated primary particles, and the larger pore is associated with larger interaggregated pores produced by interaggregated secondary particles.20 As mentioned previously, the prepared TiO2 PILM was considered to be the mixture of pillared and partially delaminated structures, which resulted in the bimodal pore size distribution in the TiO2 PILM. On the other hand, the presence of a hysteresis loop would indicate some degree of mesoporosity.11 The mesoporosity results from stacking defects that are inherent in the clay itself. These stacking defects are the result of the attraction between negatively charged basal surfaces and positively charged crystal edges to form an internal “house of cards” disordered structure,21 which can combine many polymerized titanium oxide particles. This bimodal mesopore size distribution is further confirmed by the corresponding pore size distributions shown in the inset (20) Yu, J. G.; Liu, S. W.; Yu, H. G. J. Catal. 2007, 249, 59-66. (21) Olphen, H. V. Introduction to Clay Colloid Chemistry, 2nd ed.; Wiley: New York, 1963.
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Figure 5. FT-IR absorption spectra of pristine MMT, the original sample E, and recovered sample E.
in Figure 4. The specific surface area determined by the BET equation was 276.6 m2/g, and the pore volume was 0.335 cm3/g at a relative pressure of 0.976. 3.4. FT-IR Absorption Spectra. Figure 5 shows the FT-IR spectra of pristine MMT and sample E. On pillaring, the disappearance of the peak at 3623 cm-1, which belongs to the Al2OH octahedral layer group stretching vibrations,22 results from the exchange of titanium hydroxy cations and the cations in MMT. The absorption peaks of H-OH also have a shift to 3352 and 1630 cm-1 that compares to pristine MMT, which is produced by the interaction of the interbedded water and the hydroxyl of the titanium hydroxy cations. The band broadens at 3352 cm-1 due to the introduction of more -OH groups of the pillar, which is interpreted as an effect of pillaring.23 The peak at 1034 cm-1 is due to asymmetric stretching vibrations of SiO2 tetrahedra, which becomes weak because of the Si-O-Ti band in sample E. The disappearance of the absorption bands between 914 and 520 cm-1 may be caused by the exchange effects of the cations. The retention of the peaks at 520 and 467 cm-1 as in pristine MMT clearly shows that the basic structure of the clay remains on pillaring. This is co-incident with the XRD patterns and TEM images. As compared to the original sample, the FTIR spectrum of the recovered sample has no change, which indicates that the composite structure of the TiO2 PILM is stable. 3.5. Degradation ARG. The as-prepared samples may combine the adsorbility of MMT and the catalytic ability of TiO2 together to remove ARG from its aqueous solutions efficiently. As shown in Figure 6a, the absorption spectrum of the original solution shows three distinctive peaks at 215, 331, and 505 nm, which corresponds to the structure of the benzene ring, the naphthalene ring, and the nitrogen to nitrogen double bond -(sNdNs EnDash), respectively. The absorption peak at λ ) 505 nm decreased rapidly with increasing the photocatalytic time, and no new adsorption peaks appeared. This confirmed the photodegradation of ARG (i.e., the breakup of the chromophore responsible for the characteristic color of the azo dyes, rather than its discoloration or bleaching).24 It also indicated that the sEnDashNdNsEnDash bonds of the dyes in this study were the most active sites for oxidative attack. The specific peaks of the naphthalene ring and benzene ring become smoother gradually during the degradation processes, which illuminated that the
Figure 6. UV-vis absorption spectra of sample E (a) and reaction rate constants (b).
catalyst not only destroyed the chromophore of ARG but also decomposed the naphthalene ring and benzene ring partly.17 During the experiments, the as-prepared samples show a strong adsorbility, which is very important for photocatalytic reactions and can be confirmed by the UV-vis absorption spectra of sample E at 0 min. Sample E obtained at 70 °C shows the highest degradation efficiency, and no residual ARG was detected on the recovered sample, which is supported by the FT-IR spectra in Figure 5. The photocatalytic degradation reactions of ARG on the catalyst are a pseudo-first-order reaction, and their kinetics follows the Langmuir-Hinshelwood rule and may be expressed as25
ln(C0/Ct) ) k(min-1)t + a
(2)
where k is the apparent reaction rate constant, C0 is the initial concentration of aqueous ARG, and Ct is the concentration of aqueous ARG at the reaction time t. Figure 6b shows that the reaction rate constants of samples increased with the composite temperature increasing except for sample F. The reasons may be attributed to the fact that the aggregate reaction occurred between the hydroxyl chloride and the SiO32- on the surface of MMT, which can increase the particle size and decrease the surface area and the photocatalytic activity of the catalysts. As such, an appropriate composite temperature is a very important factor in the cross-linking process.
4. Conclusion (22) Bodoardo, S.; Figueras, F.; Garrone, E. J. Catal. 1994, 147, 223-230. (23) Kurian, M.; Sugunan, S. Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem. 2003, 42, 2480-2486. (24) Sun, Z. S.; Chen, Y. X.; Ke, Q.; Yang, Y.; Yuan, J. J. Photochem. Photobiol., A 2002, 149, 169-174.
TiO2 PILM was successfully synthesized without calcination by impregnating a TiO2 sol into the interlayers of MMT using (25) Ollis, D. F. EnViron. Sci. Technol. 1985, 19, 480-484.
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TiCl4 as the precursor of TiO2. The layered structure of MMT in the as-prepared samples was destroyed to some extent. The particle sizes of TiO2 in the composites are less than 5 nm. The UV absorption edge of the composites shows a red-shift as compared to that of pure TiO2 particles. The sample obtained at 70 °C was with two pore size distributions and a large specific surface area and shows a high photocatalytic activity, and no residual ARG was detected on the recovered sample, which could
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be attributed to a combination of the adsorptive ability of MMT and the catalytic degradation ability of TiO2 in it. Acknowledgment. This work was supported by NCET050662 and the National Basic Research Program of China (973 Program) 2007CB613302. LA702649V
