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Photocatalytic degradation of methyl orange in a sparged tube reactor with TiO2-coated activated carbon composites
Catalysis Communications 6 (2005) 650–655 www.elsevier.com/locate/catcom
Photocatalytic degradation of methyl orange in a sparged tube reactor with TiO2-coated activated carbon composites Youji Li
a,b,*
, Xiaodong Li b, Junwen Li c, Jing Yin
c
a
c
College of Chemistry and Chemical Engineering, Jishou University, Jishou, Hunan 416000, PR China b CFC Key Laboratory, National University of Defence Technology, Changsha 410073, PR China Institute of Hygiene and Environmental Medicine, Academy of Military Medical Science, Tianjin 300050, PR China Received 1 March 2005; accepted 7 June 2005 Available online 9 August 2005
Abstract The photocatalytic oxidation of methyl orange has been performed in the presence of suspended TiO2-coated activated carbon (TiO2/AC) prepared by a properly controlled sol–gel method in an air sparged tube reactor. The key factors affecting the methyl orange oxidation efficiency were investigated, including the initial concentration of methyl orange, the pH value and the electric power of UV lamp. For photodegradation of methyl orange, it was observed that TiO2/AC has a higher decomposition efficiency than pure TiO2 particles, as well as a mixture of TiO2 powder with activated carbon. The optimal conditions were a concentration of 12 · 10 3 mol/l at pH 6 with 40 W of illumination for the fastest rate of methyl orange photodegradation. Diffuse reflectance UV– Vis analysis showed a shift in the threshold absorption value and a corresponding increase in bandgap energy for TiO2/AC, which is considered to be responsible for the decrease in TiO2/AC photoactivity after 20 trials. 2005 Elsevier B.V. All rights reserved. Keywords: TiO2-coated activated carbon; Sol–gel; Methyl orange; Photochemical technology
1. Introduction Industrialization and agricultural development, together with population growth, has drastically reduced clean water resources. Various kinds of contaminants enter water, most of them in industrial wastewater 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 high activity, chemical stability, robustness against photocorrosion, low toxicity, low pollution load, and availability at low cost [3,4]. However, the shortcomings of conventional powder catalysts *
Corresponding author. Tel.: +86 731 4533150; fax: +86 731 4533150/4574161. E-mail address: [email protected] (Y. Li). 1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.06.008
include low efficiency of light use, difficulty of stirring during reaction and separation after reaction, and low-concentration contamination near TiO2 [5]. These disadvantages of TiO2 result in low efficiency of photocatalytic activity in practical applications. In order to achieve rapid and efficient decomposition of organic pollutants and easy manipulation in a total photocatalytic process, it may be effective to load photocatalysts onto suitably fine adsorbents to concentrate the pollutants around the photocatalysts. Therefore, much recent work has focused on the preparation of composites such as TiO2/SiO2, TiO2/zeolite, TiO2/SnO2 and TiO2/ polymer [6–9]. In addition, composites such as TiO2/ carbon [10–12] have also been prepared. It has been reported that the combined roles of the activated carbon and TiO2 showed a synergistic effect on the efficient degradation of some organic compounds in the photocatalytic process [13]. During the photocatalytic process, organic pollutants present in water are converted to less
Y. Li et al. / Catalysis Communications 6 (2005) 650–655
harmful compounds or, most often, undergo complete mineralization via various intermediate products [14,15]. However, it has been reported that photocatalytic processes using TiO2 are greatly influenced by the reaction parameters (TiO2 concentration, light intensity, pH value, etc.) [16,17]. This work examined the photocatalytic activity of prepared TiO2/AC by a properly controlled sol–gel method for degradation of methyl orange in a sparged tube reactor. We also investigated the effects of parameters such as the initial methyl orange concentration, the electric power of UV lamp and the pH value on the degradation of methyl orange from aqueous solutions in an air-sparged reactor with suspended TiO2/AC.
UV lamp Entering
651
Water outflow
materials
exhaust Glass wall bleb
catalyst Reactant solution Air sparging
Air sparging
Cooling water inpour
Fig. 1. Experimental set-up for photocatalytic reaction.
2.3. Evaluation of the photocatalytic activity 2. Experimental 2.1. Preparation of TiO2/AC composites Precursor solutions for TiO2/AC 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 chemical composition of the starting alkoxide solution was Ti(OC4H9)4/C2H5OH/ H2O/NH(C2H4OH)2 in a 1:25.6:2:2 molar ratio. Activated carbon grains (30 g) used as substrate were immersed into the TiO2 sol when the viscosity of the TiO2 sol reached 5 mPa s; the mixture was subsequently stirred in an ultrasonic bath for 1 h. When the TiO2 sol coating the activated carbon changed to a TiO2 gel, the TiO2 gel-coated activated carbon was vacuum dried. Finally, the grains obtained were first heat-treated at 250 C for 2 h in air and then at 500 C in nitrogen for 2 h. In addition, naked TiO2 powders were prepared as a reference using the same hydrolysis procedure for tetrabutyl orthotitanate. Suitable ratios of TiO2 sol to active carbon, the right viscosity of the TiO2 sol before mixing and an exact ultrasonic stir time are key steps for preparing TiO2/AC with high photoactivity. 2.2. Characterization of the structure The structure of the TiO2-coated activated carbon composites was investigated by Raman spectroscopy (super labran, France). The chemical states of titanium and oxygen on the surface and near surface of the deposited active carbon were studied by XPS measurements, using a VG Scientific ESCALAB Mark II spectrometer (England). The morphology and size of the particles in samples were examined by scanning electron microscopy (JSM-5600LV, Japan).
Methyl orange was chosen as a model organic compound to evaluate the photoactivity of the prepared TiO2/AC. Fig. 1 shows a scheme of the experimental set-up (280 ml volume). The main component of the system was the reactor, with internal stirring by air sparging with a flow capacity of 56 ml/s. A 40 W ultraviolet lamp (Institute of Electric Light Source, Beijing) was positioned inside a Pyrex cell at irradiance of 135 mW/ cm2. The wavelength range and peak wavelength of the UV lamp were 320–400 nm and 365 nm, respectively. A 0.5 g sample of the photocatalyst (TiO2/AC, naked TiO2 or mixture of TiO2 with activated carbon) was suspended in 250 ml of air-sparged aqueous solution containing methyl orange in the above Pyrex reaction vessel. The temperature of photocatalytic reaction was maintained at 25 C by water circulation. To determine the change in methyl orange concentration in solution during the process, a few milliliters of the solution was taken from the reaction mixture, subsequently centrifuged, filtered through a Millipore filter (pore size 0.22 lm) to separate the TiO2/AC, and loaded in a UV–Vis spectrometer (JascoV-500, Japan). The methyl orange concentration was calculated from the absorbance at 480 nm using a calibration curve. The pH value was controlled by the addition of a weak acid or weak alkali. The bandgap energy of the TiO2/ AC photocatalyst, both fresh and after photocatalytic reaction, was determined using a UV–Vis spectrometer (Specord M40, Carl-Zeiss) equipped with an integrating sphere accessory for diffuse reflectance.
3. Results and discussion 3.1. Characterization of the samples Raman spectroscopy is available for clarifying structure of composites. Fig. 2 shows four typical raman spectra measurements for the composites prepared with
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Scatter intensity (a.u)
2500 152.86
2000
1500
1000 195.82
424.45 611.15
500
0 100
200
300
400 500 600 -1 Raman shift/cm
700
800
Fig. 2. Raman spectra of TiO2/AC prepared with heat-temperature of 500 C.
sol–gel method. The peaks located at 152.86, 195.82 and 424.45 cm 1 were attributed to anatase phase. However, the peak that appeared at 611.15 cm 1 was ascribed to the rutile phase of TiO2. It was obvious that TiO2 on activated carbon was consisted of anatase and rutile. It can be seen from Fig. 3(a) that pure TiO2 contains Ti, O and C elements. The photoelectron peak for Ti2p appeared clearly at a binding energy, Eb, of 458 eV, O1s at Eb = 531 eV and C1s at Eb = 284 eV. The element C in the pure TiO2 was attributed to the residual carbon.
a
O1s
C1s
Relative intensity (kc·s-1)
Ti2p
Ti3p Ti3s
As can be seen in Fig. 3(b), TiO2-coated activated carbon contained Ti, O and C elements, but the amount of C element increased. It can be concluded that the carbon in activated carbon was probably to occur to ascend. The binding energies of Ti3p, Ti3s and Ti2p of TiO2-coated activated carbon were same as those of pure TiO2, indicating the integrity of the TiO2 structure, which was not modified by activated carbon support. Fig. 4 shows the scanning electron micrograph of the surface of TiO2/AC. It is observed that the TiO2/AC has spherical microstructure and dispersing texture and fine TiO2 particles of approximately 30 nm are distributed uniformly over the activated carbon surface. In addition, prepared at 500 C for 2 hours, the TiO2/AC, in which the TiO2 content was 2.63%, were black grains with a crystallographic structure of anatase (65%) and rutile (35%) as determined by X-ray diffraction (HZG4 diffractometer, Zeiss, Germany), with a surface area of approximately 526 m2/g. 3.2. Photocatalytic activity of TiO2/AC Under identical experimental conditions, it was found that only 2% of methyl orange in solution absorbed on naked TiO2 in the dark after 1 h, while the amount of methyl orange removed by the TiO2/AC was 24.1%. This suggests that adsorption of methyl orange is mainly on the surface of the activated carbon carrier. The results of methyl orange removal by the photocatalysts are presented in Fig. 5. The remnant rate of the methyl orange decomposed by TiO2 is 30% after 200 min under UV radiation. This is due to the low adsorbability of TiO2. However, TiO2/AC achieved almost 100% methyl orange removal. This seems to suggest that the synergistic effect of the activated carbon carriers is remarkable. To demonstrate further the utility of the activated carbon carrier for TiO2 loading, photodecomposition of methyl orange was studied using naked TiO2 in the presence of dispersed activated carbon (mixture of
C1s
b
O1s Ti2p
Ti3p Ti3s 0
100
200
300
400
500
Binding energy/eV Fig. 3. XPS spectra of samples at heat-treatment of 500 C (a: TiO2, b: TiO2/AC).
Fig. 4. SEM images of activited carbon and TiO2/AC prepared at 500 C.
Y. Li et al. / Catalysis Communications 6 (2005) 650–655
100 degree of decomposition/ %
100
80 Remnant rate/ %
653
60
40
80 60 40 20 0 0
20
0 0
40
120 80 Time (min)
160
200
Fig. 5. Photocatalytic degradation properties of TiO2/AC, TiO2, and a mixture of TiO2 powder with AC for 5 mM Methyl orange under UV radiation ((d) 2.63 wt% load TiO2 on AC; (j) mixture of 2.63 wt% TiO2 with AC; (m) TiO2).
2.63 wt% TiO2 with activated carbon). As shown in Fig. 5, the reaction rate for naked TiO2 was increased by introducing activated carbon into the TiO2 suspension. The remnant rate for methyl orange decolorized by the mixture of TiO2 with activated carbon was 25%, but the enhancement was not as great as that obtained for 2.63 wt%-loaded TiO2/AC. This may be attributed to the high surface area of the activated carbon, which worked well as an effective adsorbant to concentrate methyl orange around the loaded TiO2. The adsorbed methyl orange seemed to be supplied to the loaded TiO2 mostly by surface diffusion. In addition, activated carbon can possibly indirectly prevent recombination of electron–hole pairs. 3.3. Effect of initial concentration of methyl orange The experiments on photocatalytic oxidation of methyl orange were conducted for solutions with various methyl orange concentrations: 3, 6, 7, 12 and 15 · 10 3 mol/l. Fig. 6 shows the changes in methyl orange concentration during the photocatalytic reaction. It is apparent from the results that the reaction time lengthened with increasing methyl orange concentration. In addition, the photodegradation velocity for 12 mM methyl orange was highest among the different initial concentrations. For example, after 4 h of illumination, for a concentration of 3 mM, the degree of methyl orange decomposition was 100%; for a concentration of 6 mM, after the same length of time, methyl orange loss was 100%. With further increases in concentration to 7, 12 and 15 mM, the degree of methyl orange decomposition was 95.4%, 68.7%, and 34.1%, respectively. These results indicate that when the concentra-
1
2
3
4
5 6 Time (h)
7
8
9
10
Fig. 6. Degree of methyl orange decomposition for different concentrations (TiO2/AC: (d) 15 mM, (e) 12 mM, (m) 7 mM, (j) 6 mM, (*) 3 mM;TiO2: (s) 15 mM).
tion of methyl orange was 12 · 10 3 mol/l, TiO2/AC was the most effective for methyl orange decomposition. This can be explained as follows. For a certain TiO2/ AC, the amount of active centers on the photocatalysts is finite, so photodegradation rate increases with initial concentration of methyl orange increase before reaching the concentration of about 12 · 10 3mol l 1. The photodegradation rate decreases when the concentration of methyl orange is more than 12 · 10 3mol l 1, possibly because the molecules of methyl orange is excessive in comparison with the amount of active centers on the photocatalysts to reduce UV light adsorption of catalysts. Thus, the photocatalytic process was influenced by the initial concentration of methyl orange. For photocatalytic degradation of 15 mM methyl orange, the degradation rate on TiO2 was faster than that on TiO2/AC in the beginning, while after 4 h the degradation rate on TiO2 was slower than that on TiO2/AC. This was possibly because the methyl orange concentration near TiO2 was high during the early photocatalytic reaction, so the synergistic effect of the activated carbon carriers was not apparent. In order to verify the activity of used TiO2/AC and determine the activated carbon carrier lifetime, decomposition of 5 mM methyl orange was followed. Results are presented in Fig. 7. The amount of methyl orange absorbed by activated carbon in the first and 2nd trials was 22.7% and 0%, respectively. However, for TiO2/AC photocatalyst after 4 h of illumination, in the third trial, decomposition of methyl orange was 100%, while in the fifth trial decomposition was 97.4%. This value remained at approximately 86.8% during the 20th trial after 10 h. This suggests that the lifetime of the activated carbon carrier in TiO2/AC was greatly lengthened and the used catalyst retained high photocatalytic activity. This was mainly due to the fact that the activated carbons were not blocked for TiO2 decomposition of methyl orange on them, when the activated carbons pro-
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Y. Li et al. / Catalysis Communications 6 (2005) 650–655
3.5. Effect of electric power of the UV lamp
degree of decomposition/ %
100 80 60 40 20 0 0
1
2
3
4
5 6 Time (h)
7
8
9
10
Fig. 7. Degree of methyl orange decomposition for initial methyl orange concentration of 5 mM (TiO2/AC: (d) 1st trial, (j) 5th trial, (m) 10th trial, (r) 15th trial, (*) 20th trial; activated carbon: (s) 1st trial, (h) 5th trial; activated carbon concentration 2 g dm 1).
Light intensity is a major factor in photocatalytic reactions, because electron–hole pairs are produced by light energy [19]. The relation between electric power of the UV lamp and its light intensity fits direct ratio, when positions of UV lamp and reactor are invariable in Fig. 1, so the electric power of the UV lamp also influences photodegradation of methyl orange during photocatalytic process. Fig. 9 shows an almost linear relation between the electric power and the apparent constant k for methyl orange degradation, which was 0.0231 min 1 for the 40 W UV lamp. The concentration of methyl orange decreased with radiation time increase. With increasing electric power of the UV lamp, the decomposition rate of methyl orange increased. For example, the decomposition rate at 40 W was faster than at 25, 15 and 10 W. This was because higher electric power provides
5
3.4. Effect of solution pH
4
The pH is an important factor in influencing the photocatalytic process. It is clearly observed that pH 6 is an advantage for the photocatalytic reaction of methyl orange (Fig. 8). The degradation degree of methyl orange for different pH is increasing with reaction time length. Strong acid or alkali is not available for decomposing methyl orange, it is due to the fact that the amount of hydroxy absorption on TiO2 is influenced by pH in solution [18]. The low and high pH values are not available for methyl orange absorption and hydroxy produce on TiO2/AC, respectively. So there is optimum pH in the photocatalytic process of methyl orange because high concentration hydroxy and plentiful methyl orange which are absorbed on TiO2/AC are available for the photocatalytic reaction.
Concentration/mol·L-1
vided a high concentration of the organic compound for TiO2 distributed on them by the absorption process.
1 0
3
1
0
25
50
75
100 125 Time (min)
150
175
200
Fig. 9. Effect of electric power of the UV lamp on the photodegradation of methyl orange ((j) 10 W, (r) 15 W, (d) 25 W, (m) 40 W).
0.215
Absorbance units
3
20 40
2
a
0.210
4
0
0
5 Concentration/ mmol·L-1
2
b
0.205
c
0.200 d
0.105
2
0.100 1
0.095 0
200 0
25
50
75
100 125 Time (min)
150
175
200
Fig. 8. Effect of pH on the photodegradation of methyl orange (pH (*) 2, (r) 5, (m) 6, (d) 10).
250
300
350
400
450
500
λ (nm) Fig. 10. Diffuse reflectance UV–Vis spectra of photocatalyst sample: (a) fresh TiO2/AC, used TiO2/AC; (b) 8th trial used; (c) 16th trial used; and (d) 20th trial used.
Y. Li et al. / Catalysis Communications 6 (2005) 650–655
higher energy for more TiO2 in the composite to produce electron–hole pairs. 3.6. Catalyst characterization An important feature of materials used in photocatalytic processes is the bandgap energy. The lower the value of this energy, the broader the range of light that can be absorbed to activate the photocatalyst. Fig. 10 shows the derivatives determined from the diffuse reflectance UV–Vis spectra of TiO2/AC samples. Shift in the threshold absorption value between the fresh and used photocatalyst was observed and the corresponding bandgap energy had changed from 3.15 for fresh TiO2/AC to 3.24 for 20-used photocatalyst. It indicated that with used degree increasing, bandgap energy of TiO2/AC increased and their photoactivity decreased.
4. Conclusions Due to the reciprocity of loading TiO2 and activated carbon, the TiO2/AC prepared showed high photoactivity for the photooxidation of methyl orange. In addition, the lifetime of the activated carbon carrier was considerably lengthened. The initial concentration of methyl orange influenced the photoactivity of TiO2/ AC. With an increase in the initial methyl orange concentrations from 3 to 15 mM, the photocatalytic degradation time lengthened. However, for a methyl orange concentration of 12 · 10 3 mol/l, TiO2/AC showed the highest decomposition velocity. In addition, the pH values and electric power of UV lamp also influenced the photoactivity of TiO2/AC. Values of pH 6 and 40 W
655
were suitable for methyl orange degradation. The photoactivity of used TiO2/AC composites in methyl orange photodegradation decreased due to bandgap energy increase of TiO2.
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) 1276. [5] K.B. Kyoko, S. Kazuhiro, K. Hitoshi, O. Kiyomi, A. Hironori, Appl. Catal. A-Gen. 165 (1997) 394. [6] Y. Xu, W. Zheng, W. Liu, J. Photochem. Photobiol. A: Chem. 122 (1999) 57. [7] S. Fukahori, H. Ichiura, T. Kitaoka, H. Tanaka, Appl. Catal. B: Environ. 46 (2003) 458. [8] H. Tada, A. Hattori, Y. Tokihisa, J. Phys. Chem. 104 (2000) 4586. [9] Y. Liu, A. Wang, R. Claus, J. Phys. Chem. 101 (1997) 1385. [10] B. Tryba, A.W. Morawaki, M. Inagaki, Appl. Catal. B: Environ. 46 (2003) 203. [11] H. Uchida, S. Itoh, H. Yoneyama, Chem. Lett. 22 (1993) 1996. [12] D.K. Lee, S.C. Kim, I.C. Cho, S.J. Kim, S.W. Kim, Separ. Purif. Technol. 34 (2004) 62. [13] D.K. Lee, S.C. Kim, S.J. Kim, I.S. Chung, S.W. Kim, Chem. Eng. J. 102 (2004) 93. [14] A. Fujishima, K. Honda, Nature 238 (1972) 37. [15] C.M. Jian, F.O. David, H.R. Wim, B. Harry, Water Res. 33 (1999) 1177. [16] B.G. Miray, O. Lhan, Water Sci. Technol. 33 (1996) 189. [17] M.R. Hoffman, S.T. Martin, W. Choi, Chem. Rev. 95 (1995) 70. [18] M.S. Kim, K.M. Hong, J.G. Chung, Water Res. 37 (2003) 3524. [19] T. Zhang, T. Oyama, A. Aoshima, J. Photohem. Photobiol. A: Chem. 140 (2001) 163.
Photocatalytic degradation of methyl orange in a sparged tube reactor with TiO2-coated activated carbon composites Youji Li
a,b,*
, Xiaodong Li b, Junwen Li c, Jing Yin
c
a
c
College of Chemistry and Chemical Engineering, Jishou University, Jishou, Hunan 416000, PR China b CFC Key Laboratory, National University of Defence Technology, Changsha 410073, PR China Institute of Hygiene and Environmental Medicine, Academy of Military Medical Science, Tianjin 300050, PR China Received 1 March 2005; accepted 7 June 2005 Available online 9 August 2005
Abstract The photocatalytic oxidation of methyl orange has been performed in the presence of suspended TiO2-coated activated carbon (TiO2/AC) prepared by a properly controlled sol–gel method in an air sparged tube reactor. The key factors affecting the methyl orange oxidation efficiency were investigated, including the initial concentration of methyl orange, the pH value and the electric power of UV lamp. For photodegradation of methyl orange, it was observed that TiO2/AC has a higher decomposition efficiency than pure TiO2 particles, as well as a mixture of TiO2 powder with activated carbon. The optimal conditions were a concentration of 12 · 10 3 mol/l at pH 6 with 40 W of illumination for the fastest rate of methyl orange photodegradation. Diffuse reflectance UV– Vis analysis showed a shift in the threshold absorption value and a corresponding increase in bandgap energy for TiO2/AC, which is considered to be responsible for the decrease in TiO2/AC photoactivity after 20 trials. 2005 Elsevier B.V. All rights reserved. Keywords: TiO2-coated activated carbon; Sol–gel; Methyl orange; Photochemical technology
1. Introduction Industrialization and agricultural development, together with population growth, has drastically reduced clean water resources. Various kinds of contaminants enter water, most of them in industrial wastewater 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 high activity, chemical stability, robustness against photocorrosion, low toxicity, low pollution load, and availability at low cost [3,4]. However, the shortcomings of conventional powder catalysts *
Corresponding author. Tel.: +86 731 4533150; fax: +86 731 4533150/4574161. E-mail address: [email protected] (Y. Li). 1566-7367/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2005.06.008
include low efficiency of light use, difficulty of stirring during reaction and separation after reaction, and low-concentration contamination near TiO2 [5]. These disadvantages of TiO2 result in low efficiency of photocatalytic activity in practical applications. In order to achieve rapid and efficient decomposition of organic pollutants and easy manipulation in a total photocatalytic process, it may be effective to load photocatalysts onto suitably fine adsorbents to concentrate the pollutants around the photocatalysts. Therefore, much recent work has focused on the preparation of composites such as TiO2/SiO2, TiO2/zeolite, TiO2/SnO2 and TiO2/ polymer [6–9]. In addition, composites such as TiO2/ carbon [10–12] have also been prepared. It has been reported that the combined roles of the activated carbon and TiO2 showed a synergistic effect on the efficient degradation of some organic compounds in the photocatalytic process [13]. During the photocatalytic process, organic pollutants present in water are converted to less
Y. Li et al. / Catalysis Communications 6 (2005) 650–655
harmful compounds or, most often, undergo complete mineralization via various intermediate products [14,15]. However, it has been reported that photocatalytic processes using TiO2 are greatly influenced by the reaction parameters (TiO2 concentration, light intensity, pH value, etc.) [16,17]. This work examined the photocatalytic activity of prepared TiO2/AC by a properly controlled sol–gel method for degradation of methyl orange in a sparged tube reactor. We also investigated the effects of parameters such as the initial methyl orange concentration, the electric power of UV lamp and the pH value on the degradation of methyl orange from aqueous solutions in an air-sparged reactor with suspended TiO2/AC.
UV lamp Entering
651
Water outflow
materials
exhaust Glass wall bleb
catalyst Reactant solution Air sparging
Air sparging
Cooling water inpour
Fig. 1. Experimental set-up for photocatalytic reaction.
2.3. Evaluation of the photocatalytic activity 2. Experimental 2.1. Preparation of TiO2/AC composites Precursor solutions for TiO2/AC 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 chemical composition of the starting alkoxide solution was Ti(OC4H9)4/C2H5OH/ H2O/NH(C2H4OH)2 in a 1:25.6:2:2 molar ratio. Activated carbon grains (30 g) used as substrate were immersed into the TiO2 sol when the viscosity of the TiO2 sol reached 5 mPa s; the mixture was subsequently stirred in an ultrasonic bath for 1 h. When the TiO2 sol coating the activated carbon changed to a TiO2 gel, the TiO2 gel-coated activated carbon was vacuum dried. Finally, the grains obtained were first heat-treated at 250 C for 2 h in air and then at 500 C in nitrogen for 2 h. In addition, naked TiO2 powders were prepared as a reference using the same hydrolysis procedure for tetrabutyl orthotitanate. Suitable ratios of TiO2 sol to active carbon, the right viscosity of the TiO2 sol before mixing and an exact ultrasonic stir time are key steps for preparing TiO2/AC with high photoactivity. 2.2. Characterization of the structure The structure of the TiO2-coated activated carbon composites was investigated by Raman spectroscopy (super labran, France). The chemical states of titanium and oxygen on the surface and near surface of the deposited active carbon were studied by XPS measurements, using a VG Scientific ESCALAB Mark II spectrometer (England). The morphology and size of the particles in samples were examined by scanning electron microscopy (JSM-5600LV, Japan).
Methyl orange was chosen as a model organic compound to evaluate the photoactivity of the prepared TiO2/AC. Fig. 1 shows a scheme of the experimental set-up (280 ml volume). The main component of the system was the reactor, with internal stirring by air sparging with a flow capacity of 56 ml/s. A 40 W ultraviolet lamp (Institute of Electric Light Source, Beijing) was positioned inside a Pyrex cell at irradiance of 135 mW/ cm2. The wavelength range and peak wavelength of the UV lamp were 320–400 nm and 365 nm, respectively. A 0.5 g sample of the photocatalyst (TiO2/AC, naked TiO2 or mixture of TiO2 with activated carbon) was suspended in 250 ml of air-sparged aqueous solution containing methyl orange in the above Pyrex reaction vessel. The temperature of photocatalytic reaction was maintained at 25 C by water circulation. To determine the change in methyl orange concentration in solution during the process, a few milliliters of the solution was taken from the reaction mixture, subsequently centrifuged, filtered through a Millipore filter (pore size 0.22 lm) to separate the TiO2/AC, and loaded in a UV–Vis spectrometer (JascoV-500, Japan). The methyl orange concentration was calculated from the absorbance at 480 nm using a calibration curve. The pH value was controlled by the addition of a weak acid or weak alkali. The bandgap energy of the TiO2/ AC photocatalyst, both fresh and after photocatalytic reaction, was determined using a UV–Vis spectrometer (Specord M40, Carl-Zeiss) equipped with an integrating sphere accessory for diffuse reflectance.
3. Results and discussion 3.1. Characterization of the samples Raman spectroscopy is available for clarifying structure of composites. Fig. 2 shows four typical raman spectra measurements for the composites prepared with
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Y. Li et al. / Catalysis Communications 6 (2005) 650–655
Scatter intensity (a.u)
2500 152.86
2000
1500
1000 195.82
424.45 611.15
500
0 100
200
300
400 500 600 -1 Raman shift/cm
700
800
Fig. 2. Raman spectra of TiO2/AC prepared with heat-temperature of 500 C.
sol–gel method. The peaks located at 152.86, 195.82 and 424.45 cm 1 were attributed to anatase phase. However, the peak that appeared at 611.15 cm 1 was ascribed to the rutile phase of TiO2. It was obvious that TiO2 on activated carbon was consisted of anatase and rutile. It can be seen from Fig. 3(a) that pure TiO2 contains Ti, O and C elements. The photoelectron peak for Ti2p appeared clearly at a binding energy, Eb, of 458 eV, O1s at Eb = 531 eV and C1s at Eb = 284 eV. The element C in the pure TiO2 was attributed to the residual carbon.
a
O1s
C1s
Relative intensity (kc·s-1)
Ti2p
Ti3p Ti3s
As can be seen in Fig. 3(b), TiO2-coated activated carbon contained Ti, O and C elements, but the amount of C element increased. It can be concluded that the carbon in activated carbon was probably to occur to ascend. The binding energies of Ti3p, Ti3s and Ti2p of TiO2-coated activated carbon were same as those of pure TiO2, indicating the integrity of the TiO2 structure, which was not modified by activated carbon support. Fig. 4 shows the scanning electron micrograph of the surface of TiO2/AC. It is observed that the TiO2/AC has spherical microstructure and dispersing texture and fine TiO2 particles of approximately 30 nm are distributed uniformly over the activated carbon surface. In addition, prepared at 500 C for 2 hours, the TiO2/AC, in which the TiO2 content was 2.63%, were black grains with a crystallographic structure of anatase (65%) and rutile (35%) as determined by X-ray diffraction (HZG4 diffractometer, Zeiss, Germany), with a surface area of approximately 526 m2/g. 3.2. Photocatalytic activity of TiO2/AC Under identical experimental conditions, it was found that only 2% of methyl orange in solution absorbed on naked TiO2 in the dark after 1 h, while the amount of methyl orange removed by the TiO2/AC was 24.1%. This suggests that adsorption of methyl orange is mainly on the surface of the activated carbon carrier. The results of methyl orange removal by the photocatalysts are presented in Fig. 5. The remnant rate of the methyl orange decomposed by TiO2 is 30% after 200 min under UV radiation. This is due to the low adsorbability of TiO2. However, TiO2/AC achieved almost 100% methyl orange removal. This seems to suggest that the synergistic effect of the activated carbon carriers is remarkable. To demonstrate further the utility of the activated carbon carrier for TiO2 loading, photodecomposition of methyl orange was studied using naked TiO2 in the presence of dispersed activated carbon (mixture of
C1s
b
O1s Ti2p
Ti3p Ti3s 0
100
200
300
400
500
Binding energy/eV Fig. 3. XPS spectra of samples at heat-treatment of 500 C (a: TiO2, b: TiO2/AC).
Fig. 4. SEM images of activited carbon and TiO2/AC prepared at 500 C.
Y. Li et al. / Catalysis Communications 6 (2005) 650–655
100 degree of decomposition/ %
100
80 Remnant rate/ %
653
60
40
80 60 40 20 0 0
20
0 0
40
120 80 Time (min)
160
200
Fig. 5. Photocatalytic degradation properties of TiO2/AC, TiO2, and a mixture of TiO2 powder with AC for 5 mM Methyl orange under UV radiation ((d) 2.63 wt% load TiO2 on AC; (j) mixture of 2.63 wt% TiO2 with AC; (m) TiO2).
2.63 wt% TiO2 with activated carbon). As shown in Fig. 5, the reaction rate for naked TiO2 was increased by introducing activated carbon into the TiO2 suspension. The remnant rate for methyl orange decolorized by the mixture of TiO2 with activated carbon was 25%, but the enhancement was not as great as that obtained for 2.63 wt%-loaded TiO2/AC. This may be attributed to the high surface area of the activated carbon, which worked well as an effective adsorbant to concentrate methyl orange around the loaded TiO2. The adsorbed methyl orange seemed to be supplied to the loaded TiO2 mostly by surface diffusion. In addition, activated carbon can possibly indirectly prevent recombination of electron–hole pairs. 3.3. Effect of initial concentration of methyl orange The experiments on photocatalytic oxidation of methyl orange were conducted for solutions with various methyl orange concentrations: 3, 6, 7, 12 and 15 · 10 3 mol/l. Fig. 6 shows the changes in methyl orange concentration during the photocatalytic reaction. It is apparent from the results that the reaction time lengthened with increasing methyl orange concentration. In addition, the photodegradation velocity for 12 mM methyl orange was highest among the different initial concentrations. For example, after 4 h of illumination, for a concentration of 3 mM, the degree of methyl orange decomposition was 100%; for a concentration of 6 mM, after the same length of time, methyl orange loss was 100%. With further increases in concentration to 7, 12 and 15 mM, the degree of methyl orange decomposition was 95.4%, 68.7%, and 34.1%, respectively. These results indicate that when the concentra-
1
2
3
4
5 6 Time (h)
7
8
9
10
Fig. 6. Degree of methyl orange decomposition for different concentrations (TiO2/AC: (d) 15 mM, (e) 12 mM, (m) 7 mM, (j) 6 mM, (*) 3 mM;TiO2: (s) 15 mM).
tion of methyl orange was 12 · 10 3 mol/l, TiO2/AC was the most effective for methyl orange decomposition. This can be explained as follows. For a certain TiO2/ AC, the amount of active centers on the photocatalysts is finite, so photodegradation rate increases with initial concentration of methyl orange increase before reaching the concentration of about 12 · 10 3mol l 1. The photodegradation rate decreases when the concentration of methyl orange is more than 12 · 10 3mol l 1, possibly because the molecules of methyl orange is excessive in comparison with the amount of active centers on the photocatalysts to reduce UV light adsorption of catalysts. Thus, the photocatalytic process was influenced by the initial concentration of methyl orange. For photocatalytic degradation of 15 mM methyl orange, the degradation rate on TiO2 was faster than that on TiO2/AC in the beginning, while after 4 h the degradation rate on TiO2 was slower than that on TiO2/AC. This was possibly because the methyl orange concentration near TiO2 was high during the early photocatalytic reaction, so the synergistic effect of the activated carbon carriers was not apparent. In order to verify the activity of used TiO2/AC and determine the activated carbon carrier lifetime, decomposition of 5 mM methyl orange was followed. Results are presented in Fig. 7. The amount of methyl orange absorbed by activated carbon in the first and 2nd trials was 22.7% and 0%, respectively. However, for TiO2/AC photocatalyst after 4 h of illumination, in the third trial, decomposition of methyl orange was 100%, while in the fifth trial decomposition was 97.4%. This value remained at approximately 86.8% during the 20th trial after 10 h. This suggests that the lifetime of the activated carbon carrier in TiO2/AC was greatly lengthened and the used catalyst retained high photocatalytic activity. This was mainly due to the fact that the activated carbons were not blocked for TiO2 decomposition of methyl orange on them, when the activated carbons pro-
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Y. Li et al. / Catalysis Communications 6 (2005) 650–655
3.5. Effect of electric power of the UV lamp
degree of decomposition/ %
100 80 60 40 20 0 0
1
2
3
4
5 6 Time (h)
7
8
9
10
Fig. 7. Degree of methyl orange decomposition for initial methyl orange concentration of 5 mM (TiO2/AC: (d) 1st trial, (j) 5th trial, (m) 10th trial, (r) 15th trial, (*) 20th trial; activated carbon: (s) 1st trial, (h) 5th trial; activated carbon concentration 2 g dm 1).
Light intensity is a major factor in photocatalytic reactions, because electron–hole pairs are produced by light energy [19]. The relation between electric power of the UV lamp and its light intensity fits direct ratio, when positions of UV lamp and reactor are invariable in Fig. 1, so the electric power of the UV lamp also influences photodegradation of methyl orange during photocatalytic process. Fig. 9 shows an almost linear relation between the electric power and the apparent constant k for methyl orange degradation, which was 0.0231 min 1 for the 40 W UV lamp. The concentration of methyl orange decreased with radiation time increase. With increasing electric power of the UV lamp, the decomposition rate of methyl orange increased. For example, the decomposition rate at 40 W was faster than at 25, 15 and 10 W. This was because higher electric power provides
5
3.4. Effect of solution pH
4
The pH is an important factor in influencing the photocatalytic process. It is clearly observed that pH 6 is an advantage for the photocatalytic reaction of methyl orange (Fig. 8). The degradation degree of methyl orange for different pH is increasing with reaction time length. Strong acid or alkali is not available for decomposing methyl orange, it is due to the fact that the amount of hydroxy absorption on TiO2 is influenced by pH in solution [18]. The low and high pH values are not available for methyl orange absorption and hydroxy produce on TiO2/AC, respectively. So there is optimum pH in the photocatalytic process of methyl orange because high concentration hydroxy and plentiful methyl orange which are absorbed on TiO2/AC are available for the photocatalytic reaction.
Concentration/mol·L-1
vided a high concentration of the organic compound for TiO2 distributed on them by the absorption process.
1 0
3
1
0
25
50
75
100 125 Time (min)
150
175
200
Fig. 9. Effect of electric power of the UV lamp on the photodegradation of methyl orange ((j) 10 W, (r) 15 W, (d) 25 W, (m) 40 W).
0.215
Absorbance units
3
20 40
2
a
0.210
4
0
0
5 Concentration/ mmol·L-1
2
b
0.205
c
0.200 d
0.105
2
0.100 1
0.095 0
200 0
25
50
75
100 125 Time (min)
150
175
200
Fig. 8. Effect of pH on the photodegradation of methyl orange (pH (*) 2, (r) 5, (m) 6, (d) 10).
250
300
350
400
450
500
λ (nm) Fig. 10. Diffuse reflectance UV–Vis spectra of photocatalyst sample: (a) fresh TiO2/AC, used TiO2/AC; (b) 8th trial used; (c) 16th trial used; and (d) 20th trial used.
Y. Li et al. / Catalysis Communications 6 (2005) 650–655
higher energy for more TiO2 in the composite to produce electron–hole pairs. 3.6. Catalyst characterization An important feature of materials used in photocatalytic processes is the bandgap energy. The lower the value of this energy, the broader the range of light that can be absorbed to activate the photocatalyst. Fig. 10 shows the derivatives determined from the diffuse reflectance UV–Vis spectra of TiO2/AC samples. Shift in the threshold absorption value between the fresh and used photocatalyst was observed and the corresponding bandgap energy had changed from 3.15 for fresh TiO2/AC to 3.24 for 20-used photocatalyst. It indicated that with used degree increasing, bandgap energy of TiO2/AC increased and their photoactivity decreased.
4. Conclusions Due to the reciprocity of loading TiO2 and activated carbon, the TiO2/AC prepared showed high photoactivity for the photooxidation of methyl orange. In addition, the lifetime of the activated carbon carrier was considerably lengthened. The initial concentration of methyl orange influenced the photoactivity of TiO2/ AC. With an increase in the initial methyl orange concentrations from 3 to 15 mM, the photocatalytic degradation time lengthened. However, for a methyl orange concentration of 12 · 10 3 mol/l, TiO2/AC showed the highest decomposition velocity. In addition, the pH values and electric power of UV lamp also influenced the photoactivity of TiO2/AC. Values of pH 6 and 40 W
655
were suitable for methyl orange degradation. The photoactivity of used TiO2/AC composites in methyl orange photodegradation decreased due to bandgap energy increase of TiO2.
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