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Preparation and photocatalytic performance of anatase-mounted natural porous silica, pumice, by hydrolysis under hydrothermal conditions
Applied Catalysis B: Environmental 51 (2004) 255–260
Preparation and photocatalytic performance of anatase-mounted natural porous silica, pumice, by hydrolysis under hydrothermal conditions Xiu-Yun Chuan a,b,∗ , Masanori Hirano a , Michio Inagaki a a
Faculty of Engineering, Aichi Institute of Technology, Yakusa, Toyota 470-0392, Japan b School of Earth and Space Science, Peking University, Beijing 100871, PR China Received 22 February 2004; accepted 11 March 2004 Available online 24 April 2004
Abstract A natural porous silica, pumice, was used as a substrate for photocatalytic TiO2 . Mounting of anatase-type TiO2 was successfully performed through hydrolysis of TiOSO4 under hydrothermal conditions. Anatase-mounted pumice particles thus prepared were floating and rotating on the surface of water with stirring. Adsorption and photocatalytic performances were examined in methylene blue aqueous solution in the dark and under ultraviolet ray irradiation, respectively. Methylene blue in water could be decomposed rapidly by anatase-mounted on pumice particles. Their photoactivity was evaluated by subtracting adsorbed amount of methylene blue. High efficiency of usage of UV rays for photodecomposition of organic pollutants in water was expected. © 2004 Elsevier B.V. All rights reserved. Keywords: Photocatalyst; Anatase; Porous silica; Pumice; Hydrothermal treatment
1. Introduction Titania TiO2 possesses various interesting properties, optical, dielectric and catalytic, which lead to its industrial applications. Recently anatase-type TiO2 attracted great attention in relation to various environmental problems, of which photocatalytic activity gives promising possibilities for the decomposition of various organic pollutants [1,2]. Many authors have investigated the degradation of organic molecules on the surface of TiO2 particles for environment cleanup, e.g. decomposition of acetaldehyde in air [1], oxidation of NO followed by removal as HNO3 [3], decomposition of methylene blue and phenol in water [2,4], degradation of chlorinated organic compounds in water [5], etc. It was also applied to photocatalytic purification of soil contaminated with spilled heavy oils [6]. Because of its lower surface area, however, pure anatase powders might have difficulties to contact with pollutants efficiently and in some cases be not practical for rapid treatment for water and air purification. ∗ Corresponding author. Tel.: +86-10-62767965; fax: +86-10-62751159. E-mail addresses: [email protected], [email protected] (X.-Y. Chuan).
0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.03.004
An efficient way is that anatase was mounted on some inorganic substrates, where harmful organic molecules were adsorbed and close to active TiO2 particles. It leads to a broad range of degradation reaction rate of harmful organic molecules [7]. The adsorption by substrate is an important parameter in determining photocatalytic degradation rates [8]. Activated carbon was used as supporting materials for TiO2 [3,9–12]. Its high adsorptive activity was reported to make propyzamide, a typical herbicide, concentrated around the loaded TiO2 , resulting in its rapid photocatalytic degradation [13]. But it is not strong enough to be used in a large scale and the wild cases, such as waved lake, river and sea, because of its lower hardness and it is also expensive even though it has good properties. Silica is a potential substrate of anatase. Silica beads mounted with TiO2 and mixed with calcium phosphate were studied and found that they could treat well gaseous odor compounds containing sulphur, such as methylmercaptan, dimethyl sulphide and dimethyldissulfide in the air emitted by fermentation treatment of animal feces in livestock industry and from sewage disposal plant [7,14]. Silica gel impregnated with Ti(OC4 H9 ), which formed the surface conjugated TiO2 /SiO2 photocatalyst [8], offered higher photoactivity for eliminating azodyes than pure TiO2 in the photocatalytic
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treatment of colored waste water [15] because of greater surface areas and higher capacity for adsorption. Although photocatalytic treatment of these organic molecules by using TiO2 has been reported, either activated carbons or silica particles are expensive and not strong enough in the wild case. Therefore, more rapid, efficient and economic removal methods with stable substrate have to be established. In the present paper, a natural porous silica rock, pumice, particles were used as a substrate for photocatalytic anatase-type TiO2 . Particles of TiO2 -mounted pumice could float on the surface of water, which might lead to effective and economical usages of ultraviolet radiation for photoreaction of TiO2 . The photocatalytic performance of anatase-mounted pumice was studied by using methylene blue and by separating its adsorption into catalyst particles.
pumice (about 1.6 g, in which about 0.1 g of TiO2 was estimated to be present) were added and then irradiated by ultraviolet rays with an intensity of 1.6 mW/cm2 under stirring condition. Photocatalytic performance of anatase-mounted pumice particles was compared to the control sample anatase, which was obtained as a by-products during the hydrothermal treatment of pumice particles for anatase-mounting. Adsorption of MB into TiO2 -mounted pumice particles and the control sample was also evaluated in the dark, without UV irradiation. During adsorptivity and photoactivity measurements, MB solution was stirred by using a magnet at the bottom of the beaker. Change in concentration of MB in solution either due to adsorption and decomposition was determined from the relative absorbance at a wavelength of 665 nm in UV-Vis spectrum by using a calibration curve.
2. Experimental 3. Results and discussion 2.1. Sample preparation Natural pumice particles obtained in China had a round shape and a size of less than 10 mm. The particles, which were floating on water, were selected, roughly more than a half of the particles being usable. Mounting of anatase-type TiO2 particles onto these pumice particles was carried out under hydrothermal condition [2,16]. Reagent grade TiOSO4 was dissolved into distilled water in a concentration of 1 mol/l. This solution of 80 ml with pumice particles of about 1.5 g was heated at 200 ◦ C for 15 h in a Telflon container with a volume of 100 ml which was backed up by a stainless steel vessel, with a constant rotation of 1.0 rpm. After hydryothermal treatment pumice particles were washed by distilled water until the pH value of the rinsed water became 7, separated from the solution by centrifuging and dried in air at 65 ◦ C. By this hydrothermal treatment, fine particles of anatase were mounted on the pumice particle, but at the same time anatase-type fine particles were precipitated separately. Fine anatase particles thus formed were used as a control sample. The crystallinity of anatase and pumice was examined by X-ray diffraction (XRD) using Cu K␣ radiation. The surface morphology of the particles, particularly after TiO2 -mounting, was observed by FE-SEM. The amount of the mounted TiO2 was estimated by the change of weight of pumice before and after TiO2 -mounted and also examined by X-ray fluorescence spectrometry (S4-Explorer, Rh bar).
3.1. Structure and morphology of TiO2 -mounted pumice In Fig. 1, XRD pattern is compared among original pumice, TiO2 -mounted pumice and TiO2 with anatase structure, which was formed as a byproduct of TiO2 mounting on pumice particles under hydrothermal condition. Original pumice composes of amorphous phase, which gives a broad band at around 24◦ in 2q, and crystalline feldspar, showing sharp diffraction peaks (Fig. 1a), which is one of impurities in natural mineral pumice. The distribution of impurity minerals among pumice particles seemed to be inhomogeneous, because they could not be detected on some pumice particles. In Fig. 1b on TiO2 -mounted pumice only small peaks of feldspar were observed together with some small peaks of impurities which could not be identified and not to be found in Fig. 1a. On the particles after hydrothermal treatment (Fig. 1b), diffraction peaks corresponding to anatase
2.2. Adsorption and photocatalytic decomposition of methylene blue Photocatalytic activity was evaluated from the decomposition of methylene blue (guaranteed reagent grade, C16 H18 N3 S, MB) in its aqueous solution with 1.0 × 10−5 mol/l concentration. Into aqueous MB solution of 40 ml in a 50 ml beaker, 10 particles of TiO2 -mounted
Fig. 1. XRD patterns of original pumice (a), TiO2 -mounted pumice (b), and pure anatase phase (c).
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phase were clearly observed, overlapping on the broad band due to pumice, which is the same as the reference anatase [8,17] (Fig. 1c). The results of XRD show that TiO2 particles mounted on porous pumice were anatase-phase with relatively high crystallinity.
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In Fig. 2, a series of SEM images are shown on original pumice without mounting of anatase (Fig. 2a–c) and anatase-mounted pumice (Fig. 2d–h). Under low magnifications, appearance of the particle surfaces of anatase-mounted pumice (Fig. 2d and e) could not be differentiated from the original pumice (Fig. 2a and b). Under high magnifi-
Fig. 2. SEM images of pure pumice and TiO2 -mounted pumice. (a–c) Surface of pure pumice in the different magnifications; (d) and (e) surface of the particle in low magnifications of TiO2 -mounted pumice; (f) anatase particles on the surface of TiO2 -mounted pumice; (g) spherical morphology of anatase particles of TiO2 -mounted pumice; and (h) aggregated anatase particles occasionally observed on the surface of TiO2 -mounted pumice.
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Fig. 2. (Continued ).
time. The concentration increased in the solution at around 10 h, which means re-dissolution of adsorbed MB into solution, was always observed not only TiO2 -mounted pumice but also pumice particles without TiO2 mounting. 3 Anatase-mounted pumice Control anatase
2 Abs
cations, higher than 15,000 magnification, however, all surfaces of the particles of anatase-mounted pumice (Fig. 2f) were much rough than original pumice (Fig. 2c). The particles of anatase-mounted pumice were covered by minute particles, which can be reasonably supposed to be anatase (Fig. 2e and f). Primary particles of anatase look rather spherical and are supposed to be 20–30 nm in diameter (Fig. 2g). In most parts of surface of pumice, anatase particles deposited along the substrate surface with apparently homogeneous thickness and had spherical morphology (Fig. 2f and g). Occasionally, however, anatase particles make pretty large aggregates, as shown in Fig. 2h.
1
3.2. Adsorption and photocatalytic decomposition of methylene blue
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Under UV irradiation, the decomposition of methylene blue by the control sample and anatase-mounted pumice particles was evaluated by change of MB concentration with irradiation time and shown in Fig 3a. The decomposition by anatase-mounted pumice particles is much faster than that by the control anatase. After 24 h under UV irradiation, the decomposition rate induced by anatase-mounted pumice particles is about 60% but only 30% by the control anatase. After 48 h, all of MB was almost decomposed by anatase-mounted pumice particles but about 70% MB still existed in the solution in the existence of control anatase. It is obvious that the photocatalytic performance of anatase-mounted pumice particles is much more higher than that of anatase which was synthesized under the same hydrothermal condition. In the dark, change of MB concentration with time was measured, which was due to the adsorption of MB into anatase-mounted pumice particles. The result is shown in Fig. 3b. MB concentration decreases rapidly during the first 30 min and continues a gradual decrease with time, but it recovers after 10 h and then becomes almost constant. During this adsorption process in the dark, pumice particles changed their color to blue, becoming deeper blue with increasing
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Fig. 3. Changes in concentration of methylene blue with irradiation time in the existence of anatase-mounted pumice particles and control anatase: (a) control anatase and TiO2 (anatase)-mounted pumice under UV irradiation; (b) anatase-mounted pumice particles in the dark and under UV irradiation.
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The reason for this phenomenon was not clear yet, but it was supposed that MB adsorbed quickly at the beginning and dissolved out into the solution because of the reduction of MB concentration in the solution. Two steps adsorption of MB was observed on fine anatase powders and carbon-coated anatase [17,18], the first step being very fast and supposed to correspond to its adsorption into pores of carbon and anatase, and the second being very slow due to condensation on their surface. Change of MB concentration with time was measured under UV irradiation, which was reasonably supposed to be due to both adsorption of MB into particles and photodecomposition of MB by anatase on pumice particles. For comparison with that in the dark, the result is plotted in Fig. 3. Abrupt decrease of MB concentration is also observed at the beginning up to around 10 h. After 10 h irradiation, MB concentration decreases gradually, but more rapidly than that in the dark. The particles after 24 h irradiation of UV rays showed light blue, in contrast to deep blue color of the particles after the same period in the dark. After 48 h the blue color of MB solution was disappeared completely. In other words, most of MB was decomposed by anatase on pumice particles. The increase in concentration of MB at around 10 h irradiation, like the case in the dark, was not observed under UV irradiation, probably because its decrease occurred rapidly. In order to evaluate the amount of MB decomposed, the difference in relative concentration between the concentration changes measured in the dark and under UV irradiation, [(c/c0 )dark − (c/c0 )UV ], (two curves in Fig. 3) was calculated, which was reasonably supposed to be the concentration decrease of MB due to photodecomposition. In Fig. 4, therefore, the value of {1 − [(c/c0 )dark − (c/c0 )UV ]}, which is the relative concentration of MB remained without decomposition, is plotted in logarithmic scale against irradiation time. The relation can be approximated to be linear, at least up to about 10 h irradiation. After 10 h, the decrease
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in relative concentration of MB remained seems to be slow. On pure anatase samples synthesized under hydrothermal condition and annealed at different temperatures, and also carbon-coated anatase samples prepared with different carbon contents and heat-treatment temperatures, the linear relations between logarithm of relative concentration of MB remained in the solution were observed and the rate constants determined from these linear relations were discussed in the relation with BET surface area and crystallinity of the photocatalysts [16,18]. In Fig. 4, the change in 1 − [(c/c0 )dark − (c/c0 )UV ]} on the same particles in second run under UV irradiation is also shown. In the beginning of irradiation, decomposition of MB is supposed to proceed almost the same rate as the first run. Changes in relative concentration of MB remained, 1 − [(c/c0 )UV − (c/c0 )dark ]}, with irradiation time were assumed to be almost the same in two repeated runs and composed of two steps, the first with relatively fast rate and the second with slow rate. The reason why the decomposition rate slowed down in the second step is not clear yet, but it might be closely related to the phenomenon of dissolution of adsorbed MB from the particles into the solution because it occurred at around 10 h of irradiation. During these measurements of MB concentration changes either in the dark or under UV irradiation and also in the second run under UV irradiation, anatase-mounted pumice particles were always floating on the MB solution and rotating by stirring using magnet at the bottom of beaker. Although quantitative comparison of photocatalytic activity with other composite catalysts, such as TiO2 -mounted silica beads, and anatase powders under suspension in the solution, effective usage of UV rays was reasonably expected because these particles are directly irradiated by UV on the surface of the solution and constant exposure of fresh MB solution by rotating the particles.
4. Conclusion
Fig. 4. Plots of {1 − [(c/c0 )UV − (c/c0 )dark ]} against irradiation time on the same TiO2 -mounted pumice particles in first and second runs.
Photocatalyst anatase-type TiO2 particles were successfully mounted on the round particles of natural porous silica, pumice, under hydrothermal treatment at 200 ◦ C for 15 h. TiO2 -mounted pumice particles prepared showed both adsorptivity and photocatalytic activity, which were evaluated through the measurements of concentration change of methylene blue in the water in the dark and under irradiation of UV rays, respectively. Methylene blue could be decomposed by anatase-mounted pumice particles through their photoactivity. The present anatase-mounted pumice particles were confirmed to be floating and rotating on the surface of methylene blue solution during the measurements, which were expected to be advantageous for efficient usage of UV rays. Because natural porous silica particles, pumice, can be floated and rotated on the surface of water, it could be a potential substrate of high efficiency of usage of UV rays
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for photodecomposition of organic pollutants in water with both adsortivity and photoactivity. Acknowledgements The author (X.Y. Chuan) would like to express sincere thanks to Daikou Foundation for their financial support to work in Aichi Institute of Technology. The present work was supported partly by the grants of Frontier Research Project from Ministry of Education, Culture, Sports, Science and Technology and partly by National Natural Science Foundation of China (No. 50272005) and Analysis Foundation of Peking University. References [1] D.F. Ollis, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993, p. 89. [2] M. Inagaki, Y. Nakazawa, M. Hirano, Y. Kobayashi, M. Toyoda, J. Inorg. Mater. 3 (2001) 809–811. [3] T. Ibusuki, S. Kutsuna, K. Takeuchi, K. Shinkai, T. Sasamoto, M. Miyamoto, Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993, p. 375–377.
[4] B. Tryba, A.W. Morawski, M. Inagaki, Appl. Catal. B: Environ. 41 (2003) 427–433. [5] S.T. Martin, J.M. Kesseiman, D.S. Park, N.S. Park, N.S. Lewis, M.R. Hoffmann, Environ. Sci. Technol. 30 (1996) 2535. [6] J. Grzechulska, M. Hamerski, A.W. Morawski, Water Res. 34 (5) (2000) 1638–1644. [7] H. Nishikawa, Y. Takahara, J. Mol. Catal. A: Chem. 172 (2001) 247–251. [8] X.R. Yan, X.H. Li, K.X. Song, M. Huo, J.P. Wang, Shuichuli Jishu 26 (1) (2000) 42–47. [9] C. Hu, Y.Z. Wang, H.X. Tang, Appl. Catal. B: Environ. 35 (2001) 95–105. [10] N. Takeda, T. Torimoto, S. Sampath, S. Kuwabara, H. Yoneyama, J. Phys. Chem. 99 (1995) 9986. [11] T. Torimoto, S. Itoh, S. Kuwabara, H. Yoneyama, Environ. Sci. Technol. 30 (1996) 1275. [12] J. Matos, J. Laine, J.M. Herrmann, J. Catal. 200 (2001) 10–13. [13] M. Toyoda, Y. Nanbu, T. Kito, M. Hirano, M. Inagaki, Desalination 159 (2003) 273–282. [14] H. Uchida, S. Itoh, H. Yoneyama, Chem. Lett. 1993 (1993) 1995– 1998. [15] H. Nishikawa, C. Shirai, Phosph. Res. Bull. 12 (2001) 45–52. [16] M. Toyoda, Y. Nanbu, Y. Nakazawa, M. Hirano, M. Inagaki, Appl. Catal. B: Environ. (in press). [17] M. Hirano, C. Nakahara, K. Ota, O. Tanaike, M. Inagaki, J. Solid State Chem. 170 (2003) 39–47. [18] T. Tsumura, N. Kojitan, I. Izumi, N. Iwashita, M. Toyoda, M. Inagaki, J. Mater. Chem. 12 (2002) 1391–1396.
Preparation and photocatalytic performance of anatase-mounted natural porous silica, pumice, by hydrolysis under hydrothermal conditions Xiu-Yun Chuan a,b,∗ , Masanori Hirano a , Michio Inagaki a a
Faculty of Engineering, Aichi Institute of Technology, Yakusa, Toyota 470-0392, Japan b School of Earth and Space Science, Peking University, Beijing 100871, PR China Received 22 February 2004; accepted 11 March 2004 Available online 24 April 2004
Abstract A natural porous silica, pumice, was used as a substrate for photocatalytic TiO2 . Mounting of anatase-type TiO2 was successfully performed through hydrolysis of TiOSO4 under hydrothermal conditions. Anatase-mounted pumice particles thus prepared were floating and rotating on the surface of water with stirring. Adsorption and photocatalytic performances were examined in methylene blue aqueous solution in the dark and under ultraviolet ray irradiation, respectively. Methylene blue in water could be decomposed rapidly by anatase-mounted on pumice particles. Their photoactivity was evaluated by subtracting adsorbed amount of methylene blue. High efficiency of usage of UV rays for photodecomposition of organic pollutants in water was expected. © 2004 Elsevier B.V. All rights reserved. Keywords: Photocatalyst; Anatase; Porous silica; Pumice; Hydrothermal treatment
1. Introduction Titania TiO2 possesses various interesting properties, optical, dielectric and catalytic, which lead to its industrial applications. Recently anatase-type TiO2 attracted great attention in relation to various environmental problems, of which photocatalytic activity gives promising possibilities for the decomposition of various organic pollutants [1,2]. Many authors have investigated the degradation of organic molecules on the surface of TiO2 particles for environment cleanup, e.g. decomposition of acetaldehyde in air [1], oxidation of NO followed by removal as HNO3 [3], decomposition of methylene blue and phenol in water [2,4], degradation of chlorinated organic compounds in water [5], etc. It was also applied to photocatalytic purification of soil contaminated with spilled heavy oils [6]. Because of its lower surface area, however, pure anatase powders might have difficulties to contact with pollutants efficiently and in some cases be not practical for rapid treatment for water and air purification. ∗ Corresponding author. Tel.: +86-10-62767965; fax: +86-10-62751159. E-mail addresses: [email protected], [email protected] (X.-Y. Chuan).
0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.03.004
An efficient way is that anatase was mounted on some inorganic substrates, where harmful organic molecules were adsorbed and close to active TiO2 particles. It leads to a broad range of degradation reaction rate of harmful organic molecules [7]. The adsorption by substrate is an important parameter in determining photocatalytic degradation rates [8]. Activated carbon was used as supporting materials for TiO2 [3,9–12]. Its high adsorptive activity was reported to make propyzamide, a typical herbicide, concentrated around the loaded TiO2 , resulting in its rapid photocatalytic degradation [13]. But it is not strong enough to be used in a large scale and the wild cases, such as waved lake, river and sea, because of its lower hardness and it is also expensive even though it has good properties. Silica is a potential substrate of anatase. Silica beads mounted with TiO2 and mixed with calcium phosphate were studied and found that they could treat well gaseous odor compounds containing sulphur, such as methylmercaptan, dimethyl sulphide and dimethyldissulfide in the air emitted by fermentation treatment of animal feces in livestock industry and from sewage disposal plant [7,14]. Silica gel impregnated with Ti(OC4 H9 ), which formed the surface conjugated TiO2 /SiO2 photocatalyst [8], offered higher photoactivity for eliminating azodyes than pure TiO2 in the photocatalytic
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treatment of colored waste water [15] because of greater surface areas and higher capacity for adsorption. Although photocatalytic treatment of these organic molecules by using TiO2 has been reported, either activated carbons or silica particles are expensive and not strong enough in the wild case. Therefore, more rapid, efficient and economic removal methods with stable substrate have to be established. In the present paper, a natural porous silica rock, pumice, particles were used as a substrate for photocatalytic anatase-type TiO2 . Particles of TiO2 -mounted pumice could float on the surface of water, which might lead to effective and economical usages of ultraviolet radiation for photoreaction of TiO2 . The photocatalytic performance of anatase-mounted pumice was studied by using methylene blue and by separating its adsorption into catalyst particles.
pumice (about 1.6 g, in which about 0.1 g of TiO2 was estimated to be present) were added and then irradiated by ultraviolet rays with an intensity of 1.6 mW/cm2 under stirring condition. Photocatalytic performance of anatase-mounted pumice particles was compared to the control sample anatase, which was obtained as a by-products during the hydrothermal treatment of pumice particles for anatase-mounting. Adsorption of MB into TiO2 -mounted pumice particles and the control sample was also evaluated in the dark, without UV irradiation. During adsorptivity and photoactivity measurements, MB solution was stirred by using a magnet at the bottom of the beaker. Change in concentration of MB in solution either due to adsorption and decomposition was determined from the relative absorbance at a wavelength of 665 nm in UV-Vis spectrum by using a calibration curve.
2. Experimental 3. Results and discussion 2.1. Sample preparation Natural pumice particles obtained in China had a round shape and a size of less than 10 mm. The particles, which were floating on water, were selected, roughly more than a half of the particles being usable. Mounting of anatase-type TiO2 particles onto these pumice particles was carried out under hydrothermal condition [2,16]. Reagent grade TiOSO4 was dissolved into distilled water in a concentration of 1 mol/l. This solution of 80 ml with pumice particles of about 1.5 g was heated at 200 ◦ C for 15 h in a Telflon container with a volume of 100 ml which was backed up by a stainless steel vessel, with a constant rotation of 1.0 rpm. After hydryothermal treatment pumice particles were washed by distilled water until the pH value of the rinsed water became 7, separated from the solution by centrifuging and dried in air at 65 ◦ C. By this hydrothermal treatment, fine particles of anatase were mounted on the pumice particle, but at the same time anatase-type fine particles were precipitated separately. Fine anatase particles thus formed were used as a control sample. The crystallinity of anatase and pumice was examined by X-ray diffraction (XRD) using Cu K␣ radiation. The surface morphology of the particles, particularly after TiO2 -mounting, was observed by FE-SEM. The amount of the mounted TiO2 was estimated by the change of weight of pumice before and after TiO2 -mounted and also examined by X-ray fluorescence spectrometry (S4-Explorer, Rh bar).
3.1. Structure and morphology of TiO2 -mounted pumice In Fig. 1, XRD pattern is compared among original pumice, TiO2 -mounted pumice and TiO2 with anatase structure, which was formed as a byproduct of TiO2 mounting on pumice particles under hydrothermal condition. Original pumice composes of amorphous phase, which gives a broad band at around 24◦ in 2q, and crystalline feldspar, showing sharp diffraction peaks (Fig. 1a), which is one of impurities in natural mineral pumice. The distribution of impurity minerals among pumice particles seemed to be inhomogeneous, because they could not be detected on some pumice particles. In Fig. 1b on TiO2 -mounted pumice only small peaks of feldspar were observed together with some small peaks of impurities which could not be identified and not to be found in Fig. 1a. On the particles after hydrothermal treatment (Fig. 1b), diffraction peaks corresponding to anatase
2.2. Adsorption and photocatalytic decomposition of methylene blue Photocatalytic activity was evaluated from the decomposition of methylene blue (guaranteed reagent grade, C16 H18 N3 S, MB) in its aqueous solution with 1.0 × 10−5 mol/l concentration. Into aqueous MB solution of 40 ml in a 50 ml beaker, 10 particles of TiO2 -mounted
Fig. 1. XRD patterns of original pumice (a), TiO2 -mounted pumice (b), and pure anatase phase (c).
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phase were clearly observed, overlapping on the broad band due to pumice, which is the same as the reference anatase [8,17] (Fig. 1c). The results of XRD show that TiO2 particles mounted on porous pumice were anatase-phase with relatively high crystallinity.
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In Fig. 2, a series of SEM images are shown on original pumice without mounting of anatase (Fig. 2a–c) and anatase-mounted pumice (Fig. 2d–h). Under low magnifications, appearance of the particle surfaces of anatase-mounted pumice (Fig. 2d and e) could not be differentiated from the original pumice (Fig. 2a and b). Under high magnifi-
Fig. 2. SEM images of pure pumice and TiO2 -mounted pumice. (a–c) Surface of pure pumice in the different magnifications; (d) and (e) surface of the particle in low magnifications of TiO2 -mounted pumice; (f) anatase particles on the surface of TiO2 -mounted pumice; (g) spherical morphology of anatase particles of TiO2 -mounted pumice; and (h) aggregated anatase particles occasionally observed on the surface of TiO2 -mounted pumice.
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Fig. 2. (Continued ).
time. The concentration increased in the solution at around 10 h, which means re-dissolution of adsorbed MB into solution, was always observed not only TiO2 -mounted pumice but also pumice particles without TiO2 mounting. 3 Anatase-mounted pumice Control anatase
2 Abs
cations, higher than 15,000 magnification, however, all surfaces of the particles of anatase-mounted pumice (Fig. 2f) were much rough than original pumice (Fig. 2c). The particles of anatase-mounted pumice were covered by minute particles, which can be reasonably supposed to be anatase (Fig. 2e and f). Primary particles of anatase look rather spherical and are supposed to be 20–30 nm in diameter (Fig. 2g). In most parts of surface of pumice, anatase particles deposited along the substrate surface with apparently homogeneous thickness and had spherical morphology (Fig. 2f and g). Occasionally, however, anatase particles make pretty large aggregates, as shown in Fig. 2h.
1
3.2. Adsorption and photocatalytic decomposition of methylene blue
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Under UV irradiation, the decomposition of methylene blue by the control sample and anatase-mounted pumice particles was evaluated by change of MB concentration with irradiation time and shown in Fig 3a. The decomposition by anatase-mounted pumice particles is much faster than that by the control anatase. After 24 h under UV irradiation, the decomposition rate induced by anatase-mounted pumice particles is about 60% but only 30% by the control anatase. After 48 h, all of MB was almost decomposed by anatase-mounted pumice particles but about 70% MB still existed in the solution in the existence of control anatase. It is obvious that the photocatalytic performance of anatase-mounted pumice particles is much more higher than that of anatase which was synthesized under the same hydrothermal condition. In the dark, change of MB concentration with time was measured, which was due to the adsorption of MB into anatase-mounted pumice particles. The result is shown in Fig. 3b. MB concentration decreases rapidly during the first 30 min and continues a gradual decrease with time, but it recovers after 10 h and then becomes almost constant. During this adsorption process in the dark, pumice particles changed their color to blue, becoming deeper blue with increasing
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Fig. 3. Changes in concentration of methylene blue with irradiation time in the existence of anatase-mounted pumice particles and control anatase: (a) control anatase and TiO2 (anatase)-mounted pumice under UV irradiation; (b) anatase-mounted pumice particles in the dark and under UV irradiation.
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The reason for this phenomenon was not clear yet, but it was supposed that MB adsorbed quickly at the beginning and dissolved out into the solution because of the reduction of MB concentration in the solution. Two steps adsorption of MB was observed on fine anatase powders and carbon-coated anatase [17,18], the first step being very fast and supposed to correspond to its adsorption into pores of carbon and anatase, and the second being very slow due to condensation on their surface. Change of MB concentration with time was measured under UV irradiation, which was reasonably supposed to be due to both adsorption of MB into particles and photodecomposition of MB by anatase on pumice particles. For comparison with that in the dark, the result is plotted in Fig. 3. Abrupt decrease of MB concentration is also observed at the beginning up to around 10 h. After 10 h irradiation, MB concentration decreases gradually, but more rapidly than that in the dark. The particles after 24 h irradiation of UV rays showed light blue, in contrast to deep blue color of the particles after the same period in the dark. After 48 h the blue color of MB solution was disappeared completely. In other words, most of MB was decomposed by anatase on pumice particles. The increase in concentration of MB at around 10 h irradiation, like the case in the dark, was not observed under UV irradiation, probably because its decrease occurred rapidly. In order to evaluate the amount of MB decomposed, the difference in relative concentration between the concentration changes measured in the dark and under UV irradiation, [(c/c0 )dark − (c/c0 )UV ], (two curves in Fig. 3) was calculated, which was reasonably supposed to be the concentration decrease of MB due to photodecomposition. In Fig. 4, therefore, the value of {1 − [(c/c0 )dark − (c/c0 )UV ]}, which is the relative concentration of MB remained without decomposition, is plotted in logarithmic scale against irradiation time. The relation can be approximated to be linear, at least up to about 10 h irradiation. After 10 h, the decrease
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in relative concentration of MB remained seems to be slow. On pure anatase samples synthesized under hydrothermal condition and annealed at different temperatures, and also carbon-coated anatase samples prepared with different carbon contents and heat-treatment temperatures, the linear relations between logarithm of relative concentration of MB remained in the solution were observed and the rate constants determined from these linear relations were discussed in the relation with BET surface area and crystallinity of the photocatalysts [16,18]. In Fig. 4, the change in 1 − [(c/c0 )dark − (c/c0 )UV ]} on the same particles in second run under UV irradiation is also shown. In the beginning of irradiation, decomposition of MB is supposed to proceed almost the same rate as the first run. Changes in relative concentration of MB remained, 1 − [(c/c0 )UV − (c/c0 )dark ]}, with irradiation time were assumed to be almost the same in two repeated runs and composed of two steps, the first with relatively fast rate and the second with slow rate. The reason why the decomposition rate slowed down in the second step is not clear yet, but it might be closely related to the phenomenon of dissolution of adsorbed MB from the particles into the solution because it occurred at around 10 h of irradiation. During these measurements of MB concentration changes either in the dark or under UV irradiation and also in the second run under UV irradiation, anatase-mounted pumice particles were always floating on the MB solution and rotating by stirring using magnet at the bottom of beaker. Although quantitative comparison of photocatalytic activity with other composite catalysts, such as TiO2 -mounted silica beads, and anatase powders under suspension in the solution, effective usage of UV rays was reasonably expected because these particles are directly irradiated by UV on the surface of the solution and constant exposure of fresh MB solution by rotating the particles.
4. Conclusion
Fig. 4. Plots of {1 − [(c/c0 )UV − (c/c0 )dark ]} against irradiation time on the same TiO2 -mounted pumice particles in first and second runs.
Photocatalyst anatase-type TiO2 particles were successfully mounted on the round particles of natural porous silica, pumice, under hydrothermal treatment at 200 ◦ C for 15 h. TiO2 -mounted pumice particles prepared showed both adsorptivity and photocatalytic activity, which were evaluated through the measurements of concentration change of methylene blue in the water in the dark and under irradiation of UV rays, respectively. Methylene blue could be decomposed by anatase-mounted pumice particles through their photoactivity. The present anatase-mounted pumice particles were confirmed to be floating and rotating on the surface of methylene blue solution during the measurements, which were expected to be advantageous for efficient usage of UV rays. Because natural porous silica particles, pumice, can be floated and rotated on the surface of water, it could be a potential substrate of high efficiency of usage of UV rays
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for photodecomposition of organic pollutants in water with both adsortivity and photoactivity. Acknowledgements The author (X.Y. Chuan) would like to express sincere thanks to Daikou Foundation for their financial support to work in Aichi Institute of Technology. The present work was supported partly by the grants of Frontier Research Project from Ministry of Education, Culture, Sports, Science and Technology and partly by National Natural Science Foundation of China (No. 50272005) and Analysis Foundation of Peking University. References [1] D.F. Ollis, H. Al-Ekabi (Eds.), Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993, p. 89. [2] M. Inagaki, Y. Nakazawa, M. Hirano, Y. Kobayashi, M. Toyoda, J. Inorg. Mater. 3 (2001) 809–811. [3] T. Ibusuki, S. Kutsuna, K. Takeuchi, K. Shinkai, T. Sasamoto, M. Miyamoto, Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993, p. 375–377.
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