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Preparation and photocatalytic activity of TiO2–carbon surface composites by supercritical pretreatment and sol–gel process
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Catalysis Communications 9 (2008) 1583–1587 www.elsevier.com/locate/catcom
Preparation and photocatalytic activity of TiO2–carbon surface composites by supercritical pretreatment and sol–gel process Youji Li *, Mingyuan Ma, Shuguo Sun, Xiaohong Wang, Wenbin Yan, Yuzhu Ouyang College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, Hunan, PR China Received 30 October 2007; received in revised form 1 December 2007; accepted 4 January 2008 Available online 12 January 2008
Abstract TiO2–carbon surface composites (TCS) were prepared by supercritical pretreatment and then sol–gel process. The key factors affecting the methylene blue (MB) oxidation efficiency were investigated, including the initial concentration of MB, the pH value and the catalysts concentration. The results show that TCS have higher degradation efficiency than pure TiO2 and TiO2–carbon composites (TC) prepared by sol–gel process. The optimal conditions were a MB concentration of 20 mg/l at pH 6 with TCS concentration of 2.5 g/l for the fastest rate of MB degradation. Total organic carbon (TOC) analysis indicates complete mineralisation of MB. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Titania; Carbon surface; Supercritical; Sol–gel process; Methylene blue; Degradation
1. Introduction Industrialization and agricultural development, together with population growth, has drastically reduced clean water resources with various kinds of contaminants entering water. Developments in the field of chemical water purification have led to an improvement in oxidative degradation processes applying catalytic and photochemical methods [1,2]. Recently, environmental purification using TiO2 as a photocatalyst has attracted a great deal of attention because of its chemical stability, robustness against photocorrosion, low toxicity, low pollution load, and availability at low cost [3,4]. However, the photoactivity of TiO2 should be improved in practical use for rapid degradation of organic contaminants. The photocatalytic efficiency of TiO2 is greatly influenced by crystal structure, particle size, surface area and porosity. One of the strategies to improve the photocatalytic efficiency is to increase the surface area of the catalyst. Thus, fixing TiO2 particles of high surface area on inert support will simplify the recovery of TiO2 from the treated effluent. The fixation of TiO2 onto glass *
Corresponding author. Tel./fax: +86 7438563911. E-mail address: [email protected] (Y. Li).
1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.01.006
matrix, optical fibers and stainless steel plate were studied extensively [5,6]. Unfortunately, photocatalytic efficiency of immobilized TiO2 is often less than the suspended TiO2 particles. Alternative route for supporting fine TiO2 on porous materials of larger particle size has been investigated using silica gel, carbon, sand, clay and zeolite [7]. The composites like TiO2/carbon [8,9] or TiO2–mounted exfoliated graphite [10,11] were also prepared. It was confirmed that the introduction of carbon to titania slurry could increase decomposition of some organic compounds in the photocatalytic process [12]. However it was found that only TiO2 impregnated onto the carbon surface showed photoactivity in MB degradation [13]. Furthermore, TiO2 in carbon holes reduce photoactivity of composite due to its surface area decrease with micropores jamming. In the present work a new preparation of TiO2–carbon surface composites (TCS) is presented. Porous carbon is jammed in supercritical carbon dioxide using paraffine as plugging agent, subsequently pretreatment carbon is acted as sealing substrate for nano-TiO2 particles by a sol–gel process using tetrabutyl orthotitanate as precursor. Finally, composites that TiO2 nanoparticles are only coated on carbon surface (TCS) are succeed after removal of paraffine in carbon by heat treatment in air. Meanwhile, the effects of
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Y. Li et al. / Catalysis Communications 9 (2008) 1583–1587
photocatalytic conditions on TCS photoactivity are studied, including MB concentration, TCS concentration and pH values.
3. Results and discussion
2. Experimental
The XRD patterns of TC and TCS are shown in Fig. 1. The XRD peak of crystal plane 101 for anatase appeared at 25.4° (2h) and crystal plane 110 for rutile appeared at 27.5° (2h), which is in agreement with the literature report [15]. It was obvious that TiO2 on carbon was consisted of anatase and rutile. By supercritical pretreatment, the crystalline size decreases from 31.8 nm to 11.8 nm. As shown in Fig. 2, the surface morphology of TCS, TC and pure TiO2 was investigated using scanning electronic microscopy. It is observed that the surface of TCS has a large amount of micropores, spherical microstructures and dispersing texture, composed of about 18 nm sphere particles (Fig. 2a) in coordination with XRD results. However, the surface of TC has no micropores except for a few mesopore gaps with 30– 40 nm TiO2 particles (Fig. 2b). Without a carbon substrate, the size of pure TiO2 particles with a reuniting microstructure is larger than that of both (Fig. 2c). This is attributed to the carbon substrate interfacial energy effects, which control the growth of TiO2 nanoparticles and baffle the agglomeration of TiO2 particles. It can be assumed that carbon shows a high ‘‘barrier” effect so that during the calcination step the sintering between the dispersed TiO2 particles is greatly suppressed, resulting in a smaller particle and the higher energy of the band gap. If the amount of carbon on the composite surface is too few, such as in TC, most of the TiO2 particles were connected to each other to form larger particles, and this is a probable reason why the size of the TiO2 particles of TCS is smaller than that of TC.
2.1. Preparation of samples Porous carbon (PC, Hainan, China) was used for substrate support TiO2 nanoparticles. The plugging agent of paraffine was dissolved in supercritical CO2, and then impregnated into porous carbon in the desired supercritical condition to form sealing substrates. Precursor solutions for TiO2 sol were prepared as follows. Tetrabutyl orthotitanate (Aldrich, 99.9%; 8.51 ml) and diethanolamine (2.6 ml) were dissolved in 64.82 ml of ethanol. The solution was stirred vigorously for 2 h at 20 °C, followed by the addition of a mixture of distilled water (0.9 ml) and ethanol (10 ml). The resulting alkoxide solution was left at 20 °C to hydrolyze to a TiO2 sol. The desired amount of sealing substrates were immersed into the TiO2 sol with a certain viscosity, subsequently the mixture was stirred in an ultrasonic bath. When the TiO2 sol coating the sealing substrates changed to a TiO2 gel, the TiO2 gel-coated sealing substrates was vacuum dried and subsequently repeating process from immersing to dryness. Finally, the grains obtained were first heat-treated at 250 °C for 3 h in air and then at 500 °C in nitrogen for 2 h, which resulted into preparation of TiO2-carbon surface (TCS) composites. In addition, pure TiO2 were prepared as a reference using the same hydrolysis procedure for tetrabutyl orthotitanate. According to the narrated process, porous carbon replaced the sealing substrate to the TiO2-carbon (TC) composites, only prepared by the sol–gel process.
3.1. Characterization of the samples
3.2. Photocatalytic activity of samples 2.2. Characterization of the structure
2.3. Evaluation of the photocatalytic activity Methylene blue (MB) was chosen as a model organic compound to evaluate the photoactivity of the prepared samples and key factors affecting degradation. The particular photocatalytic course and setup was the same as previously described [14]. The MB concentration was calculated from the absorbance at 660 nm using a calibration curve by a UV–Vis spectrometer (JascoV-500, Japan). The extent of mineralisation was determined using total organic carbon (TOC) analyser (Euroglas TOC 1200).
Under identical experimental conditions, it was found that only 2.3% of MB in solution absorbed on pure TiO2
TCS
Intensity (arb.u)
The samples obtained were characterized by measuring BET surface area by nitrogen absorption method (ASAP2010 of Micromeritics Company, USA) at 77 K. Single point surface area was determined at P/P0 = 0.2. The crystalline phase was investigated by X-ray diffraction technique (Diffractometer HZG-4, Zeiss, Germany). The morphology and particle size of TiO2 in the prepared samples were observed by SEM (JSM-5600LV, Japan).
TC
24
26 28 2 theta (CuKα ), deg.
30
Fig. 1. XRD patterns of TCS and TC with heat treatment at 500 °C for 2 h.
Y. Li et al. / Catalysis Communications 9 (2008) 1583–1587
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Fig. 2. SEM images of TCS (a), TC (b) and pure TiO2 (c).
in the dark after 1 h, while the amount of MB removed by the TC and TCS was 29.5% and 36.2%, respectively. This suggests that adsorption of MB is mainly on carbon carrier, and the greater adsorption of MB on TCS compared to TC is attributed to the higher surface area of TCS (BET surface area of the catalysts: TCS = 486.9 m2/g; TC = 378.3 m2/g). The results of MB removal by the photocatalysts are presented in Fig. 3, the pure TiO2 has low decomposition rate of MB under UV irradiation, however TCS achieve almost 100% MB removal for 160 min. The photoactivity of TiO2 has been enhanced by carbon support of supercritical pretreatment. It is attributed that a synergetic effect of adsorption and photocatalytic decomposition of MB, additional the electron–hole pairs of excited particles are separated over carbon to give high degradation rate. To demonstrate effect of supercritical pretreatment of carbon substrate on photoactivity, photodecomposition of MB was studied using TC composites without supercritical pretreatment. As shown in Fig. 3, TC achieved only 85% MB removal for 160 min. It is regarded that the decomposition of MB mainly occurred on TiO2 particles and must be related to the structure of composites. The higher photoactivity of the TCS than the TC and bare TiO2 is related to two factors: smaller particle
size and a higher adsorptivity toward the organic substrate due its high surface area, both of which could be cooperative in making the photocatalytic reaction of organic molecules more accessible to the active sites on the TiO2 surface. In Fig. 3, MB photocatalytic degradation is a pseudo-first-order reaction by pure TiO2, TC as well as TCS. 3.3. Effect of initial MB concentration The effect of initial MB concentration on degradation rate was studied by varying the initial concentration from 5 to 55 mg l/l, and the results are depicted in Fig. 4. It is clear that the degradation rate increases with an increase in initial concentration of the MB from 5 to 55 mg l/l and then decreases. The degradation rate for 20 mg/l MB was highest among the different initial concentrations. This can be explained as follows. The rate of degradation is related to the formation of OH radical, which is the critical species in the degradation process [16]. For a certain TCS, the amount of active centers on the photocatalysts is finite, so degradation rate increases with initial concentration of MB increase before reaching the concentration of about 20 mg/l. The degradation rate decreases when the concentration of MB is more than 20 mg/l, possibly because the molecules of MB is excessive in comparison with the
100
5
20
4
18
Rate constant K×103 (min-1)
Remnant rate/ %
80 60
3 40
2
20 0
1 0
40
80
120
160
200
Time (min) Fig. 3. Photocatalytic degradations for MB solution by 1, TCS; 2, TC; 3, pure TiO2; 4, same as curve 1 but without UV; 5, AC. For all, TiO2 was heat-treated at 500 °C. pH 6; MB concentration = 20 mg l/l; samples content = 2.5 g/l
16 14 12 10 8 6
5
15 25 35 45 MB concentration (mg/l)
55
Fig. 4. Effect of initial concentration on degradation rate of MB concentration; TCS content = 2.5 g/l; pH 6.
Y. Li et al. / Catalysis Communications 9 (2008) 1583–1587
amount of active centers on the photocatalysts to reduce UV light adsorption of catalysts. This is accordance with literature report [17]. It also depends on the type of catalyst, reactor geomentry and irradiation source. 3.4. Effect of the TCS concentration The reaction rate as a function of catalyst concentration is important [15–18]. Hence a series of experiments were carried out to find the optimum catalyst concentration by varying TiO2 from 1 to 6 g/l (Fig. 5). The turbidity of the solution above 2.5 g/l reduced the light transmission through the solution, while below this level the adsorption on TiO2 surface and the absorption of light by TiO2 were the limiting factors. It is reported that the catalyst concentration has both positive and negative impact on the photodecomposition rate. The increased concentration of catalyst increases the quantity of photons absorbed and consequently the degradation rate. Further increase in catalyst concentration beyond 2.5 g/l may result in the deactivation of activated molecules due to collision with the ground state molecules [19]. At concentrations higher than 2.5 g/l, TCS aggregation (particle–particle interactions) may commence and lower the effective surface area of the catalyst, and adsorption of the reactant. 3.5. Effect of pH The amphoteric behaviour of titania influences the surface charge of the photocatalyst. The role of pH on the degradation rate was studied in the pH range 3–11. The results are shown in Fig. 6. It is observed that the rate of degradation increases with increase in pH exhibiting a maximum at pH 6 and then decreases. Strong acid or alkali is not available for decomposing MB, it is due to the fact that the amount of hydroxy absorption on TiO2 is influenced by pH in solution [20]. The low and high pH values are not available for MB absorption and hydroxy produce on TCS, respectively. So there is optimum pH in the photocatalytic process of MB because high concentration
Rate constant K×103 (min-1)
20 18 16 14
20
Rate constant K×103 (min-1)
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18 16 14 12 10 8 6
3
5
7
9
11
pH Fig. 6. Effect of pH on degradation rate of MB concentration = 20 mg/l; TCS concentration = 2.5 g/l.
hydroxy and plentiful MB which are absorbed on TCS are available for the photocatalytic reaction. 3.6. Mineralisation studies The extent of mineralisation of MB were followed by total organic carbon (TOC) analyser. As irradiation time increases, MB molecules degrade into fragments and consequently mineralized completely. The decrease in degradation rate after 1 h is due to the formation of less polar intermediates and their poor adsorption on the surface of titania. The TOC results show that pure TiO2 and TC respectively require 580 min and 460 min for complete mineralisation, whereas TCS requires only 320 min. 4. Conclusion TCS shows higher relative photocatalytic efficiency for both degradation and mineralisation due to the greater adsorption of MB and capability to separate the electronhole pairs with the small-nanosize TiO2 particles without agglomerate, besides a synergetic effect of adsorption and photocatalytic degradation. The initial concentration of MB influenced the photoactivity of TCS. With an increase in the initial MB concentrations from 5 to 55 mg/l, the photocatalytic degradation time lengthened. However, for a MB concentration of 20 mg/l, TCS showed the highest decomposition velocity. In addition, the pH values and catalysts concentration also influenced the photoactivity of TCS. Values of pH 6 and 2.5 g/l of TCS concentration were suitable for MB degradation.
12
Acknowledgements
10 8 6
1
2
3
4
5
6
TCS concentration (g/l) Fig. 5. Effect of TCS concentration on degradation rate of MB concentration = 20 mg/l; pH 6.
The authors would like to express their thanks for financial supported by Hunan Provincial Natural Science Foundation of China (No: 06JJ50150), Scientific Research Fund of Hunan Provincial science and technology Department (No: 2007GK3060) and to Jishou University for their financial support of this project (No: JSDXKYZZ200648).
Y. Li et al. / Catalysis Communications 9 (2008) 1583–1587
References [1] V. Vamathevan, R. Amal, D. Beydoun, G. Low, J. Photochem. Photobiol. A: Chem. 148 (2002) 237. [2] P.J. Senogles, J.A. Scott, G. Shaw, H. Stratton, Water Res. 35 (2001) 1246. [3] J. Yu, X. Zhao, Mater. Res. Bull. 36 (2001) 99. [4] C. Trapalis, A.D. Modestov, O. Lev, J. Mater. Sci. 28 (1993) 276. [5] C. Minero, F. Catozzo, E. Pelizzetti, Langmuir 8 (1992) 489. [6] J. Matos, J. Laine, J.M. Herrmann, Appl. Catal. B: Environ. 18 (1998) 281. [7] Y. Xu, W. Zheng, W. Liu, J. Photochem. Photobiol. A: Chem. 122 (1999) 57. [8] T. Tsumura, N. Kojitani, I. Izumi, N. Iwashita, M. Toyoda, M. Inagaki, J. Mater. Chem. 12 (2002) 1391. [9] C. Lettmann, K. Hildenbrand, H. Kisch, W. Macyk, W.F. Maier, Appl. Catal. B. Environ. 32 (2001) 215. [10] T. Tsumura, N. Kojitani, H. Umemura, M. Toyoda, M. Inagaki, Appl. Surf. Sci. 196 (2002) 429.
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[11] A.D. Modestov, O. Lev, J. Photochem. Photobiol., A. Chem. 112 (1998) 261. [12] J. Matos, J. Laine, J.M. Hermann, J. Catal. 200 (2001) 10. [13] A.K. Subramani, K. Byrappa, S. Ananda, K.M. Lokanatha Rai, C. Ranganathaiah, M. Yoshimura, Bull. Mater. Sci. 30 (1) (2007) 39. [14] Y.J. Li, X.D. Li, J.W. Li, J. Yin, Catal. Commun. 6 (2005) 651. [15] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Sol. Energy Mater. Sol. Cells 77 (2003) 65. [16] M.V. Shankar, B. Neppolian, S. Sakthivel, M. Banumathi Arabindoo, V. Palanichamy, Murugesan, Ind. J. Eng. Mater. Sci. 8 (2001) 104. [17] Z. Mengyue, C. Shifu, T. Yaowu, J. Chem. Technol. Biotechnol. 64 (1995) 339. [18] L. Zhang, C.Y. Liu, X.M. Ren, J. Photochem. Photobiol. A: Chem. 85 (1995) 239. [19] L.C. Chen, T.C. Chou, J. Mol. Catal. 85 (1993) 201. [20] M.S. Kim, K.M. Hong, J.G. Chung, Water Res. 37 (2003) 3524.
Catalysis Communications 9 (2008) 1583–1587 www.elsevier.com/locate/catcom
Preparation and photocatalytic activity of TiO2–carbon surface composites by supercritical pretreatment and sol–gel process Youji Li *, Mingyuan Ma, Shuguo Sun, Xiaohong Wang, Wenbin Yan, Yuzhu Ouyang College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, Hunan, PR China Received 30 October 2007; received in revised form 1 December 2007; accepted 4 January 2008 Available online 12 January 2008
Abstract TiO2–carbon surface composites (TCS) were prepared by supercritical pretreatment and then sol–gel process. The key factors affecting the methylene blue (MB) oxidation efficiency were investigated, including the initial concentration of MB, the pH value and the catalysts concentration. The results show that TCS have higher degradation efficiency than pure TiO2 and TiO2–carbon composites (TC) prepared by sol–gel process. The optimal conditions were a MB concentration of 20 mg/l at pH 6 with TCS concentration of 2.5 g/l for the fastest rate of MB degradation. Total organic carbon (TOC) analysis indicates complete mineralisation of MB. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Titania; Carbon surface; Supercritical; Sol–gel process; Methylene blue; Degradation
1. Introduction Industrialization and agricultural development, together with population growth, has drastically reduced clean water resources with various kinds of contaminants entering water. Developments in the field of chemical water purification have led to an improvement in oxidative degradation processes applying catalytic and photochemical methods [1,2]. Recently, environmental purification using TiO2 as a photocatalyst has attracted a great deal of attention because of its chemical stability, robustness against photocorrosion, low toxicity, low pollution load, and availability at low cost [3,4]. However, the photoactivity of TiO2 should be improved in practical use for rapid degradation of organic contaminants. The photocatalytic efficiency of TiO2 is greatly influenced by crystal structure, particle size, surface area and porosity. One of the strategies to improve the photocatalytic efficiency is to increase the surface area of the catalyst. Thus, fixing TiO2 particles of high surface area on inert support will simplify the recovery of TiO2 from the treated effluent. The fixation of TiO2 onto glass *
Corresponding author. Tel./fax: +86 7438563911. E-mail address: [email protected] (Y. Li).
1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.01.006
matrix, optical fibers and stainless steel plate were studied extensively [5,6]. Unfortunately, photocatalytic efficiency of immobilized TiO2 is often less than the suspended TiO2 particles. Alternative route for supporting fine TiO2 on porous materials of larger particle size has been investigated using silica gel, carbon, sand, clay and zeolite [7]. The composites like TiO2/carbon [8,9] or TiO2–mounted exfoliated graphite [10,11] were also prepared. It was confirmed that the introduction of carbon to titania slurry could increase decomposition of some organic compounds in the photocatalytic process [12]. However it was found that only TiO2 impregnated onto the carbon surface showed photoactivity in MB degradation [13]. Furthermore, TiO2 in carbon holes reduce photoactivity of composite due to its surface area decrease with micropores jamming. In the present work a new preparation of TiO2–carbon surface composites (TCS) is presented. Porous carbon is jammed in supercritical carbon dioxide using paraffine as plugging agent, subsequently pretreatment carbon is acted as sealing substrate for nano-TiO2 particles by a sol–gel process using tetrabutyl orthotitanate as precursor. Finally, composites that TiO2 nanoparticles are only coated on carbon surface (TCS) are succeed after removal of paraffine in carbon by heat treatment in air. Meanwhile, the effects of
1584
Y. Li et al. / Catalysis Communications 9 (2008) 1583–1587
photocatalytic conditions on TCS photoactivity are studied, including MB concentration, TCS concentration and pH values.
3. Results and discussion
2. Experimental
The XRD patterns of TC and TCS are shown in Fig. 1. The XRD peak of crystal plane 101 for anatase appeared at 25.4° (2h) and crystal plane 110 for rutile appeared at 27.5° (2h), which is in agreement with the literature report [15]. It was obvious that TiO2 on carbon was consisted of anatase and rutile. By supercritical pretreatment, the crystalline size decreases from 31.8 nm to 11.8 nm. As shown in Fig. 2, the surface morphology of TCS, TC and pure TiO2 was investigated using scanning electronic microscopy. It is observed that the surface of TCS has a large amount of micropores, spherical microstructures and dispersing texture, composed of about 18 nm sphere particles (Fig. 2a) in coordination with XRD results. However, the surface of TC has no micropores except for a few mesopore gaps with 30– 40 nm TiO2 particles (Fig. 2b). Without a carbon substrate, the size of pure TiO2 particles with a reuniting microstructure is larger than that of both (Fig. 2c). This is attributed to the carbon substrate interfacial energy effects, which control the growth of TiO2 nanoparticles and baffle the agglomeration of TiO2 particles. It can be assumed that carbon shows a high ‘‘barrier” effect so that during the calcination step the sintering between the dispersed TiO2 particles is greatly suppressed, resulting in a smaller particle and the higher energy of the band gap. If the amount of carbon on the composite surface is too few, such as in TC, most of the TiO2 particles were connected to each other to form larger particles, and this is a probable reason why the size of the TiO2 particles of TCS is smaller than that of TC.
2.1. Preparation of samples Porous carbon (PC, Hainan, China) was used for substrate support TiO2 nanoparticles. The plugging agent of paraffine was dissolved in supercritical CO2, and then impregnated into porous carbon in the desired supercritical condition to form sealing substrates. Precursor solutions for TiO2 sol were prepared as follows. Tetrabutyl orthotitanate (Aldrich, 99.9%; 8.51 ml) and diethanolamine (2.6 ml) were dissolved in 64.82 ml of ethanol. The solution was stirred vigorously for 2 h at 20 °C, followed by the addition of a mixture of distilled water (0.9 ml) and ethanol (10 ml). The resulting alkoxide solution was left at 20 °C to hydrolyze to a TiO2 sol. The desired amount of sealing substrates were immersed into the TiO2 sol with a certain viscosity, subsequently the mixture was stirred in an ultrasonic bath. When the TiO2 sol coating the sealing substrates changed to a TiO2 gel, the TiO2 gel-coated sealing substrates was vacuum dried and subsequently repeating process from immersing to dryness. Finally, the grains obtained were first heat-treated at 250 °C for 3 h in air and then at 500 °C in nitrogen for 2 h, which resulted into preparation of TiO2-carbon surface (TCS) composites. In addition, pure TiO2 were prepared as a reference using the same hydrolysis procedure for tetrabutyl orthotitanate. According to the narrated process, porous carbon replaced the sealing substrate to the TiO2-carbon (TC) composites, only prepared by the sol–gel process.
3.1. Characterization of the samples
3.2. Photocatalytic activity of samples 2.2. Characterization of the structure
2.3. Evaluation of the photocatalytic activity Methylene blue (MB) was chosen as a model organic compound to evaluate the photoactivity of the prepared samples and key factors affecting degradation. The particular photocatalytic course and setup was the same as previously described [14]. The MB concentration was calculated from the absorbance at 660 nm using a calibration curve by a UV–Vis spectrometer (JascoV-500, Japan). The extent of mineralisation was determined using total organic carbon (TOC) analyser (Euroglas TOC 1200).
Under identical experimental conditions, it was found that only 2.3% of MB in solution absorbed on pure TiO2
TCS
Intensity (arb.u)
The samples obtained were characterized by measuring BET surface area by nitrogen absorption method (ASAP2010 of Micromeritics Company, USA) at 77 K. Single point surface area was determined at P/P0 = 0.2. The crystalline phase was investigated by X-ray diffraction technique (Diffractometer HZG-4, Zeiss, Germany). The morphology and particle size of TiO2 in the prepared samples were observed by SEM (JSM-5600LV, Japan).
TC
24
26 28 2 theta (CuKα ), deg.
30
Fig. 1. XRD patterns of TCS and TC with heat treatment at 500 °C for 2 h.
Y. Li et al. / Catalysis Communications 9 (2008) 1583–1587
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Fig. 2. SEM images of TCS (a), TC (b) and pure TiO2 (c).
in the dark after 1 h, while the amount of MB removed by the TC and TCS was 29.5% and 36.2%, respectively. This suggests that adsorption of MB is mainly on carbon carrier, and the greater adsorption of MB on TCS compared to TC is attributed to the higher surface area of TCS (BET surface area of the catalysts: TCS = 486.9 m2/g; TC = 378.3 m2/g). The results of MB removal by the photocatalysts are presented in Fig. 3, the pure TiO2 has low decomposition rate of MB under UV irradiation, however TCS achieve almost 100% MB removal for 160 min. The photoactivity of TiO2 has been enhanced by carbon support of supercritical pretreatment. It is attributed that a synergetic effect of adsorption and photocatalytic decomposition of MB, additional the electron–hole pairs of excited particles are separated over carbon to give high degradation rate. To demonstrate effect of supercritical pretreatment of carbon substrate on photoactivity, photodecomposition of MB was studied using TC composites without supercritical pretreatment. As shown in Fig. 3, TC achieved only 85% MB removal for 160 min. It is regarded that the decomposition of MB mainly occurred on TiO2 particles and must be related to the structure of composites. The higher photoactivity of the TCS than the TC and bare TiO2 is related to two factors: smaller particle
size and a higher adsorptivity toward the organic substrate due its high surface area, both of which could be cooperative in making the photocatalytic reaction of organic molecules more accessible to the active sites on the TiO2 surface. In Fig. 3, MB photocatalytic degradation is a pseudo-first-order reaction by pure TiO2, TC as well as TCS. 3.3. Effect of initial MB concentration The effect of initial MB concentration on degradation rate was studied by varying the initial concentration from 5 to 55 mg l/l, and the results are depicted in Fig. 4. It is clear that the degradation rate increases with an increase in initial concentration of the MB from 5 to 55 mg l/l and then decreases. The degradation rate for 20 mg/l MB was highest among the different initial concentrations. This can be explained as follows. The rate of degradation is related to the formation of OH radical, which is the critical species in the degradation process [16]. For a certain TCS, the amount of active centers on the photocatalysts is finite, so degradation rate increases with initial concentration of MB increase before reaching the concentration of about 20 mg/l. The degradation rate decreases when the concentration of MB is more than 20 mg/l, possibly because the molecules of MB is excessive in comparison with the
100
5
20
4
18
Rate constant K×103 (min-1)
Remnant rate/ %
80 60
3 40
2
20 0
1 0
40
80
120
160
200
Time (min) Fig. 3. Photocatalytic degradations for MB solution by 1, TCS; 2, TC; 3, pure TiO2; 4, same as curve 1 but without UV; 5, AC. For all, TiO2 was heat-treated at 500 °C. pH 6; MB concentration = 20 mg l/l; samples content = 2.5 g/l
16 14 12 10 8 6
5
15 25 35 45 MB concentration (mg/l)
55
Fig. 4. Effect of initial concentration on degradation rate of MB concentration; TCS content = 2.5 g/l; pH 6.
Y. Li et al. / Catalysis Communications 9 (2008) 1583–1587
amount of active centers on the photocatalysts to reduce UV light adsorption of catalysts. This is accordance with literature report [17]. It also depends on the type of catalyst, reactor geomentry and irradiation source. 3.4. Effect of the TCS concentration The reaction rate as a function of catalyst concentration is important [15–18]. Hence a series of experiments were carried out to find the optimum catalyst concentration by varying TiO2 from 1 to 6 g/l (Fig. 5). The turbidity of the solution above 2.5 g/l reduced the light transmission through the solution, while below this level the adsorption on TiO2 surface and the absorption of light by TiO2 were the limiting factors. It is reported that the catalyst concentration has both positive and negative impact on the photodecomposition rate. The increased concentration of catalyst increases the quantity of photons absorbed and consequently the degradation rate. Further increase in catalyst concentration beyond 2.5 g/l may result in the deactivation of activated molecules due to collision with the ground state molecules [19]. At concentrations higher than 2.5 g/l, TCS aggregation (particle–particle interactions) may commence and lower the effective surface area of the catalyst, and adsorption of the reactant. 3.5. Effect of pH The amphoteric behaviour of titania influences the surface charge of the photocatalyst. The role of pH on the degradation rate was studied in the pH range 3–11. The results are shown in Fig. 6. It is observed that the rate of degradation increases with increase in pH exhibiting a maximum at pH 6 and then decreases. Strong acid or alkali is not available for decomposing MB, it is due to the fact that the amount of hydroxy absorption on TiO2 is influenced by pH in solution [20]. The low and high pH values are not available for MB absorption and hydroxy produce on TCS, respectively. So there is optimum pH in the photocatalytic process of MB because high concentration
Rate constant K×103 (min-1)
20 18 16 14
20
Rate constant K×103 (min-1)
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18 16 14 12 10 8 6
3
5
7
9
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pH Fig. 6. Effect of pH on degradation rate of MB concentration = 20 mg/l; TCS concentration = 2.5 g/l.
hydroxy and plentiful MB which are absorbed on TCS are available for the photocatalytic reaction. 3.6. Mineralisation studies The extent of mineralisation of MB were followed by total organic carbon (TOC) analyser. As irradiation time increases, MB molecules degrade into fragments and consequently mineralized completely. The decrease in degradation rate after 1 h is due to the formation of less polar intermediates and their poor adsorption on the surface of titania. The TOC results show that pure TiO2 and TC respectively require 580 min and 460 min for complete mineralisation, whereas TCS requires only 320 min. 4. Conclusion TCS shows higher relative photocatalytic efficiency for both degradation and mineralisation due to the greater adsorption of MB and capability to separate the electronhole pairs with the small-nanosize TiO2 particles without agglomerate, besides a synergetic effect of adsorption and photocatalytic degradation. The initial concentration of MB influenced the photoactivity of TCS. With an increase in the initial MB concentrations from 5 to 55 mg/l, the photocatalytic degradation time lengthened. However, for a MB concentration of 20 mg/l, TCS showed the highest decomposition velocity. In addition, the pH values and catalysts concentration also influenced the photoactivity of TCS. Values of pH 6 and 2.5 g/l of TCS concentration were suitable for MB degradation.
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Acknowledgements
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TCS concentration (g/l) Fig. 5. Effect of TCS concentration on degradation rate of MB concentration = 20 mg/l; pH 6.
The authors would like to express their thanks for financial supported by Hunan Provincial Natural Science Foundation of China (No: 06JJ50150), Scientific Research Fund of Hunan Provincial science and technology Department (No: 2007GK3060) and to Jishou University for their financial support of this project (No: JSDXKYZZ200648).
Y. Li et al. / Catalysis Communications 9 (2008) 1583–1587
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