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Intrinsic photocatalytic oxidation of the dye adsorbed on TiO2 photocatalysts by diffuse reflectance infrared Fourier transform spectroscopy
Applied Catalysis B: Environmental 30 (2001) 293–301
Intrinsic photocatalytic oxidation of the dye adsorbed on TiO2 photocatalysts by diffuse reflectance infrared Fourier transform spectroscopy Thomas C.-K. Yang a,∗ , Sea-Fue Wang b , Stanley H.-Y. Tsai c , Shi-Yi Lin a a
Department of Chemical Engineering, National Taipei University of Technology, Taipei, Taiwan, ROC b Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan, ROC c Union Chemical Laboratories, Industrial Technology Research Institute, Hsin-Chu, Taiwan, ROC Received 28 June 2000; received in revised form 29 September 2000; accepted 29 September 2000
Abstract Photocatalytic oxidation of aqueous pollutants by semiconductor photocatalysts was found efficient. The overall process by which the heterogeneous photocatalysis proceeds includes: the sequence of the adsorption of reactants, surface reaction and the desorption of final products. As a result, factors such as, the presence of oxygen concentration, pH values of the aqueous solution, pore properties for photocatalyst particles all determine the rate of photodegradation. This study is to concentrate on the photocatalytic mechanisms of the intrinsic reaction occurred on the photocatalysts and the adsorbed dye in air atmosphere. Additionally, the photocatalytic activities of adsorbed dyes prepared under different pH values were also examined and the solid-state results were compared with the aqueous systems at the same pH conditions. Dark adsorption experiments at different pH conditions showed that the saturation amount of dyes adsorbed on the catalysts differs significantly. However, the solid-state photodegradation rates of adsorbed dyes on TiO2 at various pH values only showed slightly different, which is opposite to the results obtained from the aqueous systems. This evidence reveals that the external and internal mass transport processes are rate-controlling steps that restricted the photodegradation reaction of aqueous dyes at different pH conditions. Furthermore, this investigation supports a proposed direct photocatalytic mechanism for aqueous systems that the photocatalytic oxidation always begins with the adsorption process and the adsorbed dye will then be attacked by the excited hole–electron pairs and hydroxyl radicals from TiO2 surface to produce final products. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Photocatalytic oxidation; Aqueous pollutant; Photodegradation; Photocatalysts
1. Introduction The semiconductor photocatalysts such as ZnO and TiO2 , when illuminated with band gap light in aerated aqueous suspension could efficiently oxidize ∗ Corresponding author. E-mail address: [email protected] (T.C.-K. Yang).
organic solutes [1–12]. As a result, the applications of photocatalytic oxidation to waste water treatment have been extensively developed and many of organic compounds were found decomposed to mineral products including CO2 . Factors influencing the photodegradation rate of aqueous systems have been extensively studied in the subjects such as the effect of pH values, dissolved oxygen contents and amount of
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photocatalysts added in the aqueous solution [13–17]. In addition, porous properties of catalysts and the application of metal electrodes as well as the addition of transition metal oxide dopants in TiO2 powders are all concerned by several study groups [18–21]. To further explain the rate differentiation among these photocatalysis systems, two photocatalytic mechanisms were proposed. One suggests that the oxidation of organic compounds is first initiated by the free radicals (including OH radicals) in the aqueous solution, which are mainly induced by the hole–electron pairs at the photocatalyst surface [22]. The other proposal states that the organic compound has to be firstly adsorbed on the catalyst surface and then reacts with excited superficial hole–electron pairs or OH radicals from adsorbed water to form the final products [23]. Yet, the mechanism for aqueous photocatalysis is still not fully understood. In this paper, TiO2 powders were first mixed with the Acid Blue 9 (AB-9) dye aqueous solution of various pH values for overnight. After a complete dark adsorption, slurries were filtered and dried before the further experiment. In order to study the surface reaction occurred on photocatalysts during the UV illumination, a tool of diffuse reflectance infrared spectroscopy (DRIFTS) is applied to directly measure the emitted IR spectra from the catalyst surface [24]. By way of an appropriate data collection and interpretation of time resolved IR spectrum, characteristic peaks can be assigned and consequently, a reaction mechanism occurred on the catalyst surface could be revealed. The prior efforts were implemented to exclude the influences of the mass transport and diffusion limitation occurred in the aqueous photocatalytic process. As a result, photocatalytic surface reactions on excited TiO2 can be well described by the time-dependant DRIFTS-IR technique.
2. Experimental The dye used in this experiment is an analyticalreagent grade. Triphenylmethane compound Acid Blue 9 (AB-9), which is commonly used for textile dyeing and food industry. Its chemical structure is shown in Fig. 1. The nano-sized TiO2 (mainly anatase form) procured from R.D.H. Corporation was chosen as the photocatalyst.
Fig. 1. The chemical structure of the Acid Blue 9 (AB-9).
2.1. Photodegradation experiments of the AB-9 in TiO2 suspensions The aqueous suspensions were prepared by mixing the 10 ppm AB-9 solution with 2.7 g (1.5 g/l) TiO2 photocatalysts. The total volume of suspensions for the photocatalytic reaction was 1.8 l and the desired pH conditions of aqueous suspensions were adjusted by the titration of hydrochloric acid and sodium hydroxide. The photocatalytic reactions were carried out in a cylindrical batch reactor of 15 cm i.d. × 20 cm H. A 100 W high pressure mercury-vapor UV lamp (Hanovia 608A-0360) was placed in the center of the reactor and was jacketed by a quartz tube of 6 cm o.d. × 20 cm H. A constant-temperature water bath was connected to the quartz tube for maintaining a constant working temperature. The photoreactor was equipped with a pH meter and a D.O. meter (Orion model 1230) for a continuous monitoring of the proton and dissolved oxygen concentrations in the aqueous suspensions. During the UV illumination, a 3 ml aqueous suspension was taken out at different period of time and was filtered by a 0.22 m membrane filter. The filtrate was then analyzed by an UV-Vis spectrometer (UNICAM 500) to measure the concentration of dye in the suspension. 2.2. In situ diffuse reflectance FT-IR experiments AB-9 aqueous solutions were prepared at different pH conditions and then mixed with TiO2 powders. These suspensions were stirred overnight in the dark room for the saturated adsorption with dyes. After
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Fig. 2. The schematic diagram of an in situ micro-photoreactor.
the stage of dark adsorption, suspensions were then filtered by a 0.22 m membrane filter. The residue containing TiO2 and dye was dried in a vacuum oven and heated at 353 K for 4 h. After the dehydration, the sample was kept at a desiccant jar before the experiment. The success of a surface adsorption of the AB-9 dyes on TiO2 can be proved by carrying out a DRIFTS test on powders. The analysis tool consists of a diffusive reflectance accessory as well as a FT-IR spectrometer. The IR spectra were scanned in the region of 4000–580 cm−1 at the resolution of 4 cm−1 . For achieving a fast response as well as a higher sensitivity, an accumulation of 18 scans (0.5 min) as well as a MCT detector was used. Spectra were smoothed and presented in the Kubelka–Munk mode in order to obtain the peak area information. For the in situ measurement of photodegradation, adsorbed dye on TiO2 particles were carefully placed in a 0.04 cm3 micro-reactor. For the photoexcitation of the TiO2 , an UV light is illuminated toward the top of the reactor by an optical cable, where the light source is generated by a 400 W mercury lamp with the wavelength ranging from 254 to 365 nm (UVP Bio-Lite Model Spot Cure L1-2). Its photon intensity was 15 mW cm−2 at the distance of 50 mm away from the tip of the wand. The schematic diagram of an in situ micro-reactor and the light excitation apparatus
is shown in Fig. 2. Also shown in the figure, a diffuse reflectance accessory is used to reflect the time evolution of infrared spectrum of the dye under the UV excitation.
3. Results and discussion 3.1. Photodegradation kinetic analysis of aqueous suspensions — aqueous system Fig. 3 shows a typical time-dependent UV-Vis spectra of AB-9 dye solution (pH 6.5) during the photoirradiation. By an appropriate data reduction, the plot of residual concentration ratio C/C0 of dyes versus time under pH 6.5 was obtained and shown in Fig. 4. It is apparent that the photodegradation is more favored in an acid solution than the neutral and base solutions. Since TiO2 presents a positive-charged in surface at low pH, which may promote the adsorption of dye since it contains a negative sulfonate group. Consequently, the order of rate constants was pH 2 (0.063 min−1 ) > pH 3.5 (0.036 min−1 ) > pH 6.5 (0.014 min−1 ) ; pH 10 (0.014 min−1 ) by fitting a first-order kinetic model as can be seen in the Fig. 5. These apparent rate constants include the effects of dye adsorption as well as the intrinsic surface reactions.
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Fig. 3. Typical UV-Vis spectrum profiles of AB-9 during the photodegradation of the aqueous system at the pH value of 6.5.
The photocatalytic degradation mechanism at low pH could be induced by way of photogenerated holes through a direct electron transfer due to a strong bonding between dyes and TiO2 . However, the
decrease in photodecomposition rate at high pH could be due to the Coulombic repulsion between AB-9 anion and negatively charged oxide surface. For this reason, the increased distance between reactants and
Fig. 4. The time-dependent residual concentration ratio of the AB-9 in aqueous system (C 0 = 10 ppm, TiO2 = 1.5 g/l, air aerated, pH = 6.5).
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Fig. 5. The first-order photodegrading kinetics of the AB-9 aqueous systems at various pH conditions (C0 = 10, TiO2 = 1.5 g/l, air aerated, pH = 2.0, 3.5, 6.5 and 10.0).
photocatalysts made the decomposition process a diffusion-controlled mechanism where OH radicals generated at surface sites are hard to attack reactants. Therefore, a slower process than direct electron transfer was expected.
3.2. Intrinsic kinetic analysis by DRIFTS — solid-state system The IR spectra of an adsorbed AB-9 dye on TiO2 are shown in Fig. 6. The peak positions are noticed
Fig. 6. The diffuse reflectance IR spectra of AB-9: (a) mixing with the KBr; (b) adsorbed with TiO2 .
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at 3063, 2978, 2933, 2872, 1618, 1580, 1407, 1390 and 1340 cm−1 . These peaks were found identical to that of the dye mixed with a KBr. The coincidence for these peaks gives the proof of a successful adsorption of dye on the powder. The peak of an AB-9 dye absorbed strongly at 1580 cm−1 is due to the cyclic conjugated C=N stretching [25]. While the band at 1618 cm−1 is the C=C stretching from the conjugated aryl. Bands at 2978 and 2872 cm−1 were originated from the CH3 asymmetric and symmetric stretching vibrations and the band at 2933 cm−1 is the CH2 asymmetric stretching vibration. The aromatic C–H stretching at 3063 cm−1 is small yet detectable. The band at 1407 and 1390 cm−1 can be assigned to be a C–H bending [25,26]. However, an overlap band with the S=O vibrations can be shown in most sulfur-oxygen compounds in the area of 1340–1407 cm−1 [27]. Fig. 7 showed the evolution of peak intensities of an adsorbed dye while the photocatalysis reaction proceeds in the air atmosphere. A remarkable decrease at peaks of 1580, 1407 and 1390 cm−1 was assigned as the degradation of cyclic conjugated C=N stretching and C–H bending from the ethyl group. It indicates that the excited holes and OH radicals attack –CH2 – and –C2 H5 bonds that are attached on the nitrogen
atom. As a result, the color of AB-9 dye fades quickly since the conjugated structures such as C=N and C=C bonds were broken by the photoredox process. At intermediate time, these conjugate-broken intermediates were easily decomposed into small molecules such as gaseous products. For instance, the sulfonated compound without the benzene ring could be mineralized into CO2 . However, the sulfonate compound containing benzene rings acts as a strong electron-drawing molecule and is hard to be depleted further. Fig. 8 shows the plot of an optical concentration of the adsorbed dye as a function of the irradiation time. As mentioned in the sample preparation, AB-9 dyes were adsorbed onto the photocatalyst at three different pH conditions. The affinities between the dye and TiO2 powders are determined by the initial amount of IR optical concentrations. Their intensities are read as 0.33 (pH 2.3), 0.30 (pH 6.5) and 0.22 (pH 9.8) optical units. These values indicate the effects of adsorption capability of ad-species with catalysts as well as the surface properties of catalysts at different pH solutions. Apparently, different acidifies of aqueous solution determine the ionized forms of TiO2 catalysts and dye compounds as well as the interfacial interactions to each other. As reported in the literature, TiOH2 + ,
Fig. 7. The time-dependent IR spectra of AB-9 dye, which is adsorbed on TiO2 surface, showed the decrement of peak intensity at 1580 cm−1 in the solid-state system. The illuminated time was from 0 to 240 min.
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Fig. 8. The solid-state optical concentrations of the adsorbed AB-9 dye on TiO2 decrease as a function of the illuminated time at pH conditions of 2.3, 6.5 and 9.8.
Fig. 9. The plot of fractional conversion of dye adsorbed at various pH values during the photon illumination.
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TiOH and TiO− are thought as a significant type of surface hydroxyls in the conditions of pH < 4.2, 4.2 < pH < 8.1 and pH > 8.1, respectively, [28,29]. In the case of a proton-rich solution, the positively charged TiO2 favors the adsorption of AB-9 anion at a low pH solution. As pH values increase, the more negatively charged TiO2 would induce the electrostatic repulsion between the dye anion and the oxide surface, which is unfavorable to the dye adsorption. For further investigation of the dye degradation, the optical density of peak at 1580 cm−1 was chosen to reveal the concentration of dye on TiO2 surface and the rate curve for the disappearance of the bands corresponding to the dye is shown in Fig. 9, where the fractional conversion of dye (fdye ) is defined as a ratio of the change of peak intensity at time (t) to the change of intensity at infinite period of time under the same initial peak intensity basis, fdye = [I (t) − I (0)]/[I (∞) − I (0)]. It is interesting to note that fractional conversions of dye (fdye ) for all three pH cases are all nearly the same throughout the whole photoillumination experiments. And the decaying tendency of dye concentration shows no relationship to the initial amount of dye adsorption. Apparently, the slight difference in rate constants for the solid-state system is unlike the results obtained in the aqueous systems, which show that the photodegradation rate in the acid condition is faster than that of neutral and basic ones. This comparison verifies that the mass transport and diffusion processes are rate-controlling steps for aqueous systems that restricted the photodegradation of dyes in both neutral and base solutions. Furthermore, this investigation implicitly supports a direct photocatalytic mechanism for aqueous systems that the photocatalytic oxidation always begins with the adsorption process of TiO2 and reactants. The excited hole–electron pairs and hydroxyl radicals of photocatalysts will then decompose the adsorbed dye to smaller molecules.
4. Conclusions The DRIFTS technique can be used to investigate the solid-state photodegradation mechanisms of the AB-9 dye adsorbed on the TiO2 . This experiment restricted the possible diffusion limitation or any type of mass transfer resistance, whereas it normally
participates in a conventional aqueous photodegradation experiment. Consequently, analysis obtained from the solid-state experimental data shows us the intrinsic kinetic information. It is interesting to show that even though the amounts of initial adsorption are quite different for all three pH cases, fractional conversions of dye (fdye ) are all nearly the same throughout the whole photoillumination experiments. However, the aqueous photodegradation rate for acid solution is two times faster than that of neutral and base solutions. This evidence supports our proposed co-adsorption and reaction mechanism, where mass transport and diffusion processes are rate-controlling steps that restricted the photodegradation rate of dyes in aqueous solution. Furthermore, this investigation supports a proposed direct photocatalytic mechanism for aqueous systems that the photocatalytic oxidation always begins with the adsorption process and the dye adsorbed on TiO2 will then be attacked by the excited hole–electron pairs and hydroxyl radicals to produce final products.
Acknowledgements Financial support of this work by the National Science Council of the Republic of China (Grant NSC 88-2214-E-027-002) is gratefully acknowledged. We also would like to express our appreciation to Dr. W.-J. Guo of National Taipei University of Technology and Dr. Y.-S. Shen of Da-Yeh University for valuable suggestions in this paper. References [1] M.M. Halmann, Photodegradation of Water Pollutants, CRC Press, Boca Raton, FL, 1996. [2] A.L. Pruden, D.F. Ollis, J. Catal. 82 (1983) 404. [3] C.S. Turchi, D.F. Ollis, J. Catal. 122 (1990) 178. [4] A. Mills, R.H. Davies, D. Worsley, Chem. Soc. Rev. 22 (1993) 417. [5] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341. [6] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 93 (1993) 671. [7] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [8] A. Mills, S.L. Hunte, J. Photochem. Photobiol. A: Chem. 108 (1997) 1. [9] K.E. O’Shea, S. Beightol, I. Garcia, M. Aguilar, D.V. Kalen, W.J. Cooper, J. Photochem. Photobiol. A: Chem. 107 (1997) 221.
T.C.-K. Yang et al. / Applied Catalysis B: Environmental 30 (2001) 293–301 [10] M.A. Fox, Acc. Chem. Res. 16 (1983) 314. [11] K. Tanaka, K. Padermpole, T. Hisanaga, Water Res. 34 (2000) 327. [12] Y. Wang, Water Res. 34 (2000) 990. [13] J. Chen, D.F. Ollis, H. Rulkens, H. Bruning, Water Res. 33 (1999) 661. [14] Y. Xu, C.H. Langford, J. Photochem. Photobiol. A: Chem. 133 (2000) 67. [15] D.D. Dionysiou, M.T. Suidan, E. Bekou, I. Baudin, J.-M. Laine, Appl. Catal. B 26 (2000) 153. [16] E. Selli, D. Baglio, L. Montanarella, G. Bidoglio, Water Res. 33 (1999) 1827. [17] D. Chen, A.K. Ray, Water Res. 32 (1998) 3223. [18] F. Kiriakidou, D.I. Kondarides, X.E. Verykios, Catal. Today 54 (1999) 119. [19] K.T. Ranjit, B. Viswanathan, J. Photochem. Photobiol. A: Chem. 107 (1997) 215. [20] M.R. Dhananjeyan, V. Kandavelu, R. Renganathan, J. Mol. Catal. A 151 (2000) 217.
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[21] N.J. Peill, L. Bourne, M.R. Hoffmann, J. Photochem. Photobiol. A: Chem. 108 (1997) 221. [22] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, Environ. Sci. Technol. 25 (1991) 494. [23] D.F. Ollis, C.-Y. Hsiao, L. Budiman, C.-L. Lee, J. Catal. 88 (1984) 89. [24] C. Nasr, K. Vinodgopal, L. Fisher, S. Hotchandani, A.K. Chattopadhyay, P.V. Kamat, J. Phys. Chem. 100 (1996) 8436. [25] G. Socrates, Infrared Characteristic Group Frequencies, Wiley, New York, 1994. [26] D.V. Lin, C. Norman, J. Fateley, R. Grasselli, Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, New York, 1991. [27] D. Dolphin, A. Wick, Tabulation of Infrared Spectral Data, Wiley, New York, 1977. [28] C.-H. Weng, J.-H. Wang, C.-P. Huang, Water Sci. Tech. 35 (1997) 55. [29] I. Poulios, M. Kositzi, A. Kouras, J. Photochem. Photobiol. A: Chem. 115 (1998) 175.
Intrinsic photocatalytic oxidation of the dye adsorbed on TiO2 photocatalysts by diffuse reflectance infrared Fourier transform spectroscopy Thomas C.-K. Yang a,∗ , Sea-Fue Wang b , Stanley H.-Y. Tsai c , Shi-Yi Lin a a
Department of Chemical Engineering, National Taipei University of Technology, Taipei, Taiwan, ROC b Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan, ROC c Union Chemical Laboratories, Industrial Technology Research Institute, Hsin-Chu, Taiwan, ROC Received 28 June 2000; received in revised form 29 September 2000; accepted 29 September 2000
Abstract Photocatalytic oxidation of aqueous pollutants by semiconductor photocatalysts was found efficient. The overall process by which the heterogeneous photocatalysis proceeds includes: the sequence of the adsorption of reactants, surface reaction and the desorption of final products. As a result, factors such as, the presence of oxygen concentration, pH values of the aqueous solution, pore properties for photocatalyst particles all determine the rate of photodegradation. This study is to concentrate on the photocatalytic mechanisms of the intrinsic reaction occurred on the photocatalysts and the adsorbed dye in air atmosphere. Additionally, the photocatalytic activities of adsorbed dyes prepared under different pH values were also examined and the solid-state results were compared with the aqueous systems at the same pH conditions. Dark adsorption experiments at different pH conditions showed that the saturation amount of dyes adsorbed on the catalysts differs significantly. However, the solid-state photodegradation rates of adsorbed dyes on TiO2 at various pH values only showed slightly different, which is opposite to the results obtained from the aqueous systems. This evidence reveals that the external and internal mass transport processes are rate-controlling steps that restricted the photodegradation reaction of aqueous dyes at different pH conditions. Furthermore, this investigation supports a proposed direct photocatalytic mechanism for aqueous systems that the photocatalytic oxidation always begins with the adsorption process and the adsorbed dye will then be attacked by the excited hole–electron pairs and hydroxyl radicals from TiO2 surface to produce final products. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Photocatalytic oxidation; Aqueous pollutant; Photodegradation; Photocatalysts
1. Introduction The semiconductor photocatalysts such as ZnO and TiO2 , when illuminated with band gap light in aerated aqueous suspension could efficiently oxidize ∗ Corresponding author. E-mail address: [email protected] (T.C.-K. Yang).
organic solutes [1–12]. As a result, the applications of photocatalytic oxidation to waste water treatment have been extensively developed and many of organic compounds were found decomposed to mineral products including CO2 . Factors influencing the photodegradation rate of aqueous systems have been extensively studied in the subjects such as the effect of pH values, dissolved oxygen contents and amount of
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photocatalysts added in the aqueous solution [13–17]. In addition, porous properties of catalysts and the application of metal electrodes as well as the addition of transition metal oxide dopants in TiO2 powders are all concerned by several study groups [18–21]. To further explain the rate differentiation among these photocatalysis systems, two photocatalytic mechanisms were proposed. One suggests that the oxidation of organic compounds is first initiated by the free radicals (including OH radicals) in the aqueous solution, which are mainly induced by the hole–electron pairs at the photocatalyst surface [22]. The other proposal states that the organic compound has to be firstly adsorbed on the catalyst surface and then reacts with excited superficial hole–electron pairs or OH radicals from adsorbed water to form the final products [23]. Yet, the mechanism for aqueous photocatalysis is still not fully understood. In this paper, TiO2 powders were first mixed with the Acid Blue 9 (AB-9) dye aqueous solution of various pH values for overnight. After a complete dark adsorption, slurries were filtered and dried before the further experiment. In order to study the surface reaction occurred on photocatalysts during the UV illumination, a tool of diffuse reflectance infrared spectroscopy (DRIFTS) is applied to directly measure the emitted IR spectra from the catalyst surface [24]. By way of an appropriate data collection and interpretation of time resolved IR spectrum, characteristic peaks can be assigned and consequently, a reaction mechanism occurred on the catalyst surface could be revealed. The prior efforts were implemented to exclude the influences of the mass transport and diffusion limitation occurred in the aqueous photocatalytic process. As a result, photocatalytic surface reactions on excited TiO2 can be well described by the time-dependant DRIFTS-IR technique.
2. Experimental The dye used in this experiment is an analyticalreagent grade. Triphenylmethane compound Acid Blue 9 (AB-9), which is commonly used for textile dyeing and food industry. Its chemical structure is shown in Fig. 1. The nano-sized TiO2 (mainly anatase form) procured from R.D.H. Corporation was chosen as the photocatalyst.
Fig. 1. The chemical structure of the Acid Blue 9 (AB-9).
2.1. Photodegradation experiments of the AB-9 in TiO2 suspensions The aqueous suspensions were prepared by mixing the 10 ppm AB-9 solution with 2.7 g (1.5 g/l) TiO2 photocatalysts. The total volume of suspensions for the photocatalytic reaction was 1.8 l and the desired pH conditions of aqueous suspensions were adjusted by the titration of hydrochloric acid and sodium hydroxide. The photocatalytic reactions were carried out in a cylindrical batch reactor of 15 cm i.d. × 20 cm H. A 100 W high pressure mercury-vapor UV lamp (Hanovia 608A-0360) was placed in the center of the reactor and was jacketed by a quartz tube of 6 cm o.d. × 20 cm H. A constant-temperature water bath was connected to the quartz tube for maintaining a constant working temperature. The photoreactor was equipped with a pH meter and a D.O. meter (Orion model 1230) for a continuous monitoring of the proton and dissolved oxygen concentrations in the aqueous suspensions. During the UV illumination, a 3 ml aqueous suspension was taken out at different period of time and was filtered by a 0.22 m membrane filter. The filtrate was then analyzed by an UV-Vis spectrometer (UNICAM 500) to measure the concentration of dye in the suspension. 2.2. In situ diffuse reflectance FT-IR experiments AB-9 aqueous solutions were prepared at different pH conditions and then mixed with TiO2 powders. These suspensions were stirred overnight in the dark room for the saturated adsorption with dyes. After
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Fig. 2. The schematic diagram of an in situ micro-photoreactor.
the stage of dark adsorption, suspensions were then filtered by a 0.22 m membrane filter. The residue containing TiO2 and dye was dried in a vacuum oven and heated at 353 K for 4 h. After the dehydration, the sample was kept at a desiccant jar before the experiment. The success of a surface adsorption of the AB-9 dyes on TiO2 can be proved by carrying out a DRIFTS test on powders. The analysis tool consists of a diffusive reflectance accessory as well as a FT-IR spectrometer. The IR spectra were scanned in the region of 4000–580 cm−1 at the resolution of 4 cm−1 . For achieving a fast response as well as a higher sensitivity, an accumulation of 18 scans (0.5 min) as well as a MCT detector was used. Spectra were smoothed and presented in the Kubelka–Munk mode in order to obtain the peak area information. For the in situ measurement of photodegradation, adsorbed dye on TiO2 particles were carefully placed in a 0.04 cm3 micro-reactor. For the photoexcitation of the TiO2 , an UV light is illuminated toward the top of the reactor by an optical cable, where the light source is generated by a 400 W mercury lamp with the wavelength ranging from 254 to 365 nm (UVP Bio-Lite Model Spot Cure L1-2). Its photon intensity was 15 mW cm−2 at the distance of 50 mm away from the tip of the wand. The schematic diagram of an in situ micro-reactor and the light excitation apparatus
is shown in Fig. 2. Also shown in the figure, a diffuse reflectance accessory is used to reflect the time evolution of infrared spectrum of the dye under the UV excitation.
3. Results and discussion 3.1. Photodegradation kinetic analysis of aqueous suspensions — aqueous system Fig. 3 shows a typical time-dependent UV-Vis spectra of AB-9 dye solution (pH 6.5) during the photoirradiation. By an appropriate data reduction, the plot of residual concentration ratio C/C0 of dyes versus time under pH 6.5 was obtained and shown in Fig. 4. It is apparent that the photodegradation is more favored in an acid solution than the neutral and base solutions. Since TiO2 presents a positive-charged in surface at low pH, which may promote the adsorption of dye since it contains a negative sulfonate group. Consequently, the order of rate constants was pH 2 (0.063 min−1 ) > pH 3.5 (0.036 min−1 ) > pH 6.5 (0.014 min−1 ) ; pH 10 (0.014 min−1 ) by fitting a first-order kinetic model as can be seen in the Fig. 5. These apparent rate constants include the effects of dye adsorption as well as the intrinsic surface reactions.
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Fig. 3. Typical UV-Vis spectrum profiles of AB-9 during the photodegradation of the aqueous system at the pH value of 6.5.
The photocatalytic degradation mechanism at low pH could be induced by way of photogenerated holes through a direct electron transfer due to a strong bonding between dyes and TiO2 . However, the
decrease in photodecomposition rate at high pH could be due to the Coulombic repulsion between AB-9 anion and negatively charged oxide surface. For this reason, the increased distance between reactants and
Fig. 4. The time-dependent residual concentration ratio of the AB-9 in aqueous system (C 0 = 10 ppm, TiO2 = 1.5 g/l, air aerated, pH = 6.5).
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Fig. 5. The first-order photodegrading kinetics of the AB-9 aqueous systems at various pH conditions (C0 = 10, TiO2 = 1.5 g/l, air aerated, pH = 2.0, 3.5, 6.5 and 10.0).
photocatalysts made the decomposition process a diffusion-controlled mechanism where OH radicals generated at surface sites are hard to attack reactants. Therefore, a slower process than direct electron transfer was expected.
3.2. Intrinsic kinetic analysis by DRIFTS — solid-state system The IR spectra of an adsorbed AB-9 dye on TiO2 are shown in Fig. 6. The peak positions are noticed
Fig. 6. The diffuse reflectance IR spectra of AB-9: (a) mixing with the KBr; (b) adsorbed with TiO2 .
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at 3063, 2978, 2933, 2872, 1618, 1580, 1407, 1390 and 1340 cm−1 . These peaks were found identical to that of the dye mixed with a KBr. The coincidence for these peaks gives the proof of a successful adsorption of dye on the powder. The peak of an AB-9 dye absorbed strongly at 1580 cm−1 is due to the cyclic conjugated C=N stretching [25]. While the band at 1618 cm−1 is the C=C stretching from the conjugated aryl. Bands at 2978 and 2872 cm−1 were originated from the CH3 asymmetric and symmetric stretching vibrations and the band at 2933 cm−1 is the CH2 asymmetric stretching vibration. The aromatic C–H stretching at 3063 cm−1 is small yet detectable. The band at 1407 and 1390 cm−1 can be assigned to be a C–H bending [25,26]. However, an overlap band with the S=O vibrations can be shown in most sulfur-oxygen compounds in the area of 1340–1407 cm−1 [27]. Fig. 7 showed the evolution of peak intensities of an adsorbed dye while the photocatalysis reaction proceeds in the air atmosphere. A remarkable decrease at peaks of 1580, 1407 and 1390 cm−1 was assigned as the degradation of cyclic conjugated C=N stretching and C–H bending from the ethyl group. It indicates that the excited holes and OH radicals attack –CH2 – and –C2 H5 bonds that are attached on the nitrogen
atom. As a result, the color of AB-9 dye fades quickly since the conjugated structures such as C=N and C=C bonds were broken by the photoredox process. At intermediate time, these conjugate-broken intermediates were easily decomposed into small molecules such as gaseous products. For instance, the sulfonated compound without the benzene ring could be mineralized into CO2 . However, the sulfonate compound containing benzene rings acts as a strong electron-drawing molecule and is hard to be depleted further. Fig. 8 shows the plot of an optical concentration of the adsorbed dye as a function of the irradiation time. As mentioned in the sample preparation, AB-9 dyes were adsorbed onto the photocatalyst at three different pH conditions. The affinities between the dye and TiO2 powders are determined by the initial amount of IR optical concentrations. Their intensities are read as 0.33 (pH 2.3), 0.30 (pH 6.5) and 0.22 (pH 9.8) optical units. These values indicate the effects of adsorption capability of ad-species with catalysts as well as the surface properties of catalysts at different pH solutions. Apparently, different acidifies of aqueous solution determine the ionized forms of TiO2 catalysts and dye compounds as well as the interfacial interactions to each other. As reported in the literature, TiOH2 + ,
Fig. 7. The time-dependent IR spectra of AB-9 dye, which is adsorbed on TiO2 surface, showed the decrement of peak intensity at 1580 cm−1 in the solid-state system. The illuminated time was from 0 to 240 min.
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Fig. 8. The solid-state optical concentrations of the adsorbed AB-9 dye on TiO2 decrease as a function of the illuminated time at pH conditions of 2.3, 6.5 and 9.8.
Fig. 9. The plot of fractional conversion of dye adsorbed at various pH values during the photon illumination.
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TiOH and TiO− are thought as a significant type of surface hydroxyls in the conditions of pH < 4.2, 4.2 < pH < 8.1 and pH > 8.1, respectively, [28,29]. In the case of a proton-rich solution, the positively charged TiO2 favors the adsorption of AB-9 anion at a low pH solution. As pH values increase, the more negatively charged TiO2 would induce the electrostatic repulsion between the dye anion and the oxide surface, which is unfavorable to the dye adsorption. For further investigation of the dye degradation, the optical density of peak at 1580 cm−1 was chosen to reveal the concentration of dye on TiO2 surface and the rate curve for the disappearance of the bands corresponding to the dye is shown in Fig. 9, where the fractional conversion of dye (fdye ) is defined as a ratio of the change of peak intensity at time (t) to the change of intensity at infinite period of time under the same initial peak intensity basis, fdye = [I (t) − I (0)]/[I (∞) − I (0)]. It is interesting to note that fractional conversions of dye (fdye ) for all three pH cases are all nearly the same throughout the whole photoillumination experiments. And the decaying tendency of dye concentration shows no relationship to the initial amount of dye adsorption. Apparently, the slight difference in rate constants for the solid-state system is unlike the results obtained in the aqueous systems, which show that the photodegradation rate in the acid condition is faster than that of neutral and basic ones. This comparison verifies that the mass transport and diffusion processes are rate-controlling steps for aqueous systems that restricted the photodegradation of dyes in both neutral and base solutions. Furthermore, this investigation implicitly supports a direct photocatalytic mechanism for aqueous systems that the photocatalytic oxidation always begins with the adsorption process of TiO2 and reactants. The excited hole–electron pairs and hydroxyl radicals of photocatalysts will then decompose the adsorbed dye to smaller molecules.
4. Conclusions The DRIFTS technique can be used to investigate the solid-state photodegradation mechanisms of the AB-9 dye adsorbed on the TiO2 . This experiment restricted the possible diffusion limitation or any type of mass transfer resistance, whereas it normally
participates in a conventional aqueous photodegradation experiment. Consequently, analysis obtained from the solid-state experimental data shows us the intrinsic kinetic information. It is interesting to show that even though the amounts of initial adsorption are quite different for all three pH cases, fractional conversions of dye (fdye ) are all nearly the same throughout the whole photoillumination experiments. However, the aqueous photodegradation rate for acid solution is two times faster than that of neutral and base solutions. This evidence supports our proposed co-adsorption and reaction mechanism, where mass transport and diffusion processes are rate-controlling steps that restricted the photodegradation rate of dyes in aqueous solution. Furthermore, this investigation supports a proposed direct photocatalytic mechanism for aqueous systems that the photocatalytic oxidation always begins with the adsorption process and the dye adsorbed on TiO2 will then be attacked by the excited hole–electron pairs and hydroxyl radicals to produce final products.
Acknowledgements Financial support of this work by the National Science Council of the Republic of China (Grant NSC 88-2214-E-027-002) is gratefully acknowledged. We also would like to express our appreciation to Dr. W.-J. Guo of National Taipei University of Technology and Dr. Y.-S. Shen of Da-Yeh University for valuable suggestions in this paper. References [1] M.M. Halmann, Photodegradation of Water Pollutants, CRC Press, Boca Raton, FL, 1996. [2] A.L. Pruden, D.F. Ollis, J. Catal. 82 (1983) 404. [3] C.S. Turchi, D.F. Ollis, J. Catal. 122 (1990) 178. [4] A. Mills, R.H. Davies, D. Worsley, Chem. Soc. Rev. 22 (1993) 417. [5] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341. [6] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. 93 (1993) 671. [7] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [8] A. Mills, S.L. Hunte, J. Photochem. Photobiol. A: Chem. 108 (1997) 1. [9] K.E. O’Shea, S. Beightol, I. Garcia, M. Aguilar, D.V. Kalen, W.J. Cooper, J. Photochem. Photobiol. A: Chem. 107 (1997) 221.
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