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Preparation, characterization, and photocatalytic activity of sulfate-modified titania for degradation of methyl orange under visible light
Journal of Colloid and Interface Science 318 (2008) 231–237 www.elsevier.com/locate/jcis
Preparation, characterization, and photocatalytic activity of sulfate-modified titania for degradation of methyl orange under visible light K.M. Parida ∗ , N. Sahu, N.R. Biswal, B. Naik, A.C. Pradhan Colloids and Materials Chemistry Department, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar 751 013, Orissa, India Received 30 August 2007; accepted 16 October 2007 Available online 26 November 2007
Abstract Hydrated titania was prepared by a sol–gel method, taking tetraisopropyl orthotitanate as starting material, and then promoted with different weight percentages of sulfate by an incipient wetness impregnation method. The materials were characterized by various advanced techniques such as PXRD, BET surface area, N2 adsorption–desorption measurements, FTIR, and SEM. Analytical results demonstrated that TiO2 is mesoporous in nature, and sulfate modification could inhibit the phase transformation and enhance the thermal stability of TiO2 . It was also found that sulfate modification could reduce the crystallite size and increase the specific surface area of the catalysts. The degradation of methyl orange under solar radiation was investigated to evaluate the photocatalytic activity of these materials. Effects of different parameters such as pH of the solution, amount of catalyst, additives, and kinetics were investigated. At 2.5 wt% sulfate loading, the average percentage of degradation of methyl orange was nearly two times than that of neat TiO2 . The photocatalytic degradation followed first-order kinetics. © 2007 Elsevier Inc. All rights reserved. Keywords: Sulfate-modified titania; Photocatalyst; Crystallite size; Methyl orange
1. Introduction Semiconductor photocatalysis to remedy the problem of chemical waste is a promising approach and has attracted much attention [1–3]. TiO2 is the most investigated semiconductor photocatalyst and has been widely studied during the past decade [4–14]. It has been extensively used in environmental applications due to its nontoxicity, photostability, low cost, and water insolubility under most conditions. The basic principle of semiconductor photocatalysis involves photogenerated electrons and holes migrating to the surface and serving as redox sources that react with adsorbed reactants, leading to the destruction of pollutants. However, the high rate of the electron/hole pair recombination process reduces the quantum yield of the TiO2. A good photocatalyst depends strongly on its efficiency of electron–hole pair separation and its optical absorption properties. To increase the activity of the photocatalyst, the e− –h+ recombination rate should be reduced. The introduction * Corresponding author. Fax: +91 674 2581637.
E-mail address: [email protected] (K.M. Parida). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.10.028
of defects through selective metal ion doping has been demonstrated to be an effective approach to separation of e− and h+ [15–28]. However, metal doping causes thermal instability and metal centers act as electron traps, which reduces the photocatalytic efficiency [29]. Band gap narrowing of TiO2 can be better achieved by using anionic dopant series [30–33]. Modifi3− cation of the titania with different anions such as SO2− 4 , PO4 , and WO2− 4 has been found to enhance the acidity and certain other physicochemical properties of the catalyst, such as thermal stability and mesoporosity [34–36]. In particular, sulfation of semiconductor photocatalysts is of great interest for scientists and engineers because it has an enormous potential for environmental pollutant remediation [37–40]. Out of the total world production of dyes, 15% is lost during the dyeing process and is released in textile effluents [41]. These effluents contain azo dyes and huge amounts of inorganic salts. Physical methods such as adsorption, biological methods, and chemical methods are the most frequently used for the treatment of these textile dyes, but these processes have high cost and limited applicability. Photocatalytic degradation of dyes has been proposed as an alternative method for the re-
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moval of dyes from the effluents of textile industries. Among the textile dyes, methyl orange (MO) is an orange-colored anionic dye with λmax at 464 nm. MO has various harmful effects on human beings: it may cause eye or skin irritation, or inhalation may cause gastrointestinal irritation with nausea, vomiting, and diarrhea. From our earlier studies we have found that sulfated TiO2 is an effective photocatalyst for degradation of 4-nitrophenol and decolorization of methylene blue under solar radiation [42–45]. The present study reports the photocatalytic activity of sulfatemodified TiO2 for the degradation of methyl orange in aqueous solution.
3. Results and discussions
2. Experimental
3.1. Physicochemical characterization
2.1. Material preparation
The X-ray diffraction patterns of the pure TiO2 and SO2− 4 modified TiO2 samples are shown in Fig. 1. The crystallite sizes (D) of samples were determined by employing the Debye– Scherrer formula and are listed in Table 1,
Hydrated titania was prepared by a sol–gel method, taking tetraisopropyl orthotitanate (Fluka Chemica, 98%) as the starting material. In a typical preparation procedure, 50 ml of tetraisopropyl orthotitanate was dissolved in 400 ml of isopropanol (Qualigens, 99.7%), and to this solution 12.7 ml of distilled water at pH 3.0 (1 M H2 SO4 ) was added dropwise under vigorous stirring. The resulting colloidal suspension was stirred for 3 h and aged at 80 ◦ C for 10 h. The gel obtained was filtered, washed, and dried at 100 ◦ C for 12 h. One series of sulfated TiO2 samples with varying weight percentages of SO2− 4 were prepared by an aqueous wetness impregnation method using H2 SO4 . The suspended mass was evaporated to dryness on a hot plate with constant stirring. The samples were dried in an air oven at 100 ◦ C and subsequently activated at 400 ◦ C at a heating rate of 10 ◦ C/min in a muffle furnace for 4 h.
days), from 10:00 a.m. to 2:00 p.m., when the average solar intensity was 0.80 kW/m2 and the intensity fluctuations were minimal. Reactions were performed in the dark in order to determine the adsorption behavior of the catalysts under similar conditions. A blank experiment was performed taking MO solution without catalyst to know the extent of degradation of MO due to solar radiation. After irradiation, the suspension was centrifuged and the MO content was analyzed quantitatively at 464 nm using a Cary-1E (Varian, Australia) spectrophotometer. All the catalytic results were reproducible with ±4% variation.
D = Kλ/β cos θ, where λ is the wavelength of the CoKα used, β is the full width at half maximum of the diffraction angle considered, K is a shape factor (0.94), and θ is the angle of diffraction. The peaks (101) for anatase are used. The PXRD pattern shows that both anatase and rutile phases are present in pure TiO2 , whereas only the anatase phase is present in the case of SO2− 4 /TiO2 . From this result, we bemodification probably stabilizes the anatase lieved that SO2− 4
2.2. Physicochemical characterization To determine the crystal phase composition of the catalyst, powder X-ray diffraction (PXRD) was carried out on a Philips X-ray diffractometer (PW 3710, Co anode). The specific surface area (BET) of the catalysts was measured by N2 adsorption–desorption studies at liquid nitrogen temperature (−196 ◦ C) using Quantasorb (Quantachrome, USA). Prior to the analysis, samples were degassed at 200 ◦ C. Surface morphology and particle size were studied by scanning electron microscopy (Hitachi S-3400N). The specimen for SEM analysis was prepared by a gold sputtering process. FTIR spectra of the samples were recorded in a Varian FTIR spectrophotometer (FTS-800) in the range of 400–4000 cm−1 , taking KBr as the reference.
Fig. 1. PXRD pattern of (a) neat TiO2 , (b) 2.0 wt% SO2− 4 /TiO2 , (c) 2.5 wt% 2− 2− SO2− /TiO , (d) 3.0 wt% SO /TiO , and (e) 3.5 wt% SO 2 2 4 4 4 /TiO2 . Table 1 Crystallite size and surface properties of TiO2 and SO2− 4 /TiO2
2.3. Photocatalytic reaction and dark adsorption
Photocatalyst
Crystallite size (nm)
BET surface area (m2 /g)
Pore volume (cm3 /g)
Pore size (nm)
The photocatalytic degradation of methyl orange (MO) was carried out by taking 20 ml of 150 mg/l MO solution in a 100ml closed Pyrex flask, over 1.0 g/l of catalyst. The solutions were exposed to sunlight with constant stirring. All the irradiation was performed in triplicate during March 2007 (sunny
TiO2 2.0 wt% SO2− 4 /TiO2 2.5 wt% SO2− 4 /TiO2 3.0 wt% SO2− 4 /TiO2 3.5 wt% SO2− 4 /TiO2
22.5 19.4 18.5 18.6 20.2
59.2 108.8 111.0 115.0 114.5
0.13 0.42 0.42 0.31 0.30
3.6 4.0 5.4 5.4 5.8
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Fig. 3. FTIR spectra of neat TiO2 and 2.5 wt% SO2− 4 /TiO2 . (a)
(b) Fig. 2. N2 adsorption–desorption isotherm and pore size distribution of (a) neat TiO2 and (b) 2.5 wt% SO2− 4 /TiO2 .
phase and inhibits the phase transformation. It is observed that the crystallite size of titania decreases with SO2− 4 loading. In addition to stabilizing anatase TiO2 crystallites, sulfate species inhibit TiO2 crystallite sintering, leading to smaller crystallites than in pure TiO2 . The crystallite size decreases in the presence of sulfate ions, as SO2− 4 species could possibly interact with the TiO2 network, and thus hinder the growth of the crystal. Even a very small amount of SO2− 4 species is sufficient for this effect. Therefore, the change in sulfate concentration did not change crystallite size further. Therefore, it is assumed that a small amount of sulfate species is responsible for the lowering of crystallite size.
The N2 adsorption–desorption isotherms and the pore size distribution of neat TiO2 and 2.5 wt% SO2− 4 –TiO2 samples are shown in Figs. 2a and 2b, respectively. A type IV isotherm and an H1 hysteresis loop was observed, which clearly indicates the mesoporous nature of TiO2 . Pore volume (single point total pore volume of pores at P /P0 = 0.99), pore size (determined from BJH desorption isotherm), and BET specific surface area of the samples are given in Table 1. Sulfate modification leads to increased surface area, pore size, and pore volume of the catalyst. The relatively high surface areas of sulfate-modified samples as compare to neat samples confirms that the frameworks of mesoporous TiO2 have better thermal stability due to the stabilizing effect of sulfate ions. The increase in pore size and pore volume after sulfate modification is due to the progressive coalescence of small pores to form large pores. This implies that the presence of sulfate plays a role in making the material porous. The FTIR spectra of pure TiO2 and SO2− 4 /TiO2 are shown in Fig. 3. The peak corresponding to 1375 cm−1 is the stretching frequency of S=O, and the peaks corresponding to 1131 and 1044 cm−1 are the characteristic frequencies of SO2− 4 . −1 The peaks at 436 and 495 cm for pure TiO2 and 476 cm−1 for SO2− 4 /TiO2 are contributions from the vibration modes of anatase skeletal O–Ti–O bonds. The broad absorption band in the region of 3200–3400 cm−1 is characteristic of the OHstretching vibration of surface hydroxyl groups and the peak corresponding to 1630 cm−1 has been assigned to H–O–H bending of physically adsorbed water. SEM micrographs of neat TiO2 and SO2− 4 /TiO2 are shown in Fig. 4. The particles were found to be in spherical shape. Neat TiO2 particles were found to be in the form of aggre2− gates, unlike SO2− 4 /TiO2 . SO4 ion modification preferably retarded the aggregation and growth of well-dispersed particles. The average particle sizes of the samples were found to be in the range of 0.2–0.25 µm for neat TiO2 and 0.7–0.9 µm for SO2− 4 /TiO2 .
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2− 2− Fig. 4. SEM micrographs of neat TiO2 , 2.0 wt% SO2− 4 /TiO2 , 2.5 wt% SO4 /TiO2 , and 3.0 wt% SO4 /TiO2 .
3.2. Photocatalytic activity 3.2.1. Photocatalytic degradation and dark adsorption It was observed that photodegradation of MO is very negligible (only 3.0%) under solar radiation in absence of catalyst. With increase in reaction time up to 4 h, the percentage of photoreaction increases and then it remains constant. It was found that 61.0% of MO degradation takes place by taking 150 mg/l of MO solution over 1.0 g/l of 2.5 wt% SO2− 4 /TiO2 catalyst under solar radiation, whereas only 12% of MO adsorption takes place under similar conditions in dark. From this it can be concluded that equilibrium is achieved only after 4 h of illumination.
Fig. 5. Effect of catalyst amount on photodegradation of MO, [MO] = 150 mg/l, time = 4 h.
3.2.2. Effect of catalyst dose Fig. 5 shows the effect of catalyst concentration on the percentage of degradation of MO. It is observed that as we increase the catalyst concentration from 0.5 to 2.5 g/l, the percentage of degradation increases. This may be due to increase in the number of photons absorbed by TiO2 particles and the number of reacting molecules adsorbed on the TiO2 surface with increase
in catalyst concentration. But after a certain concentration no further reacting molecules are available for adsorption; hence additional catalysts are not involved in the catalytic activity. From this observation it can also be presumed that with increase in catalyst loading there is an increase in the surface area of the catalyst available for adsorption and hence photodegradation.
K.M. Parida et al. / Journal of Colloid and Interface Science 318 (2008) 231–237
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Fig. 6. Effect of sulfate on photodegradation of MO: catalyst dose = 1.0 g/l; [MO] = 150 mg/l; time = 4 h. Fig. 8. Kinetics of MO degradation: catalyst dose = 1.0 g/l; [MO] = 50, 100, and 150 mg/l.
Fig. 7. Effect of initial concentration of MO: catalyst dose = 1.0 g/l; time = 4 h.
3.2.3. Effect of sulfate loading Fig. 6 shows the effect of wt% of sulfate loading in TiO2 on the photocatalytic degradation of MO. It was demonstrated that all the TiO2 catalysts modified with SO2− 4 increase the percentage of degradation (in the case of pure TiO2 , 33% MO degradation, whereas after SO2− 4 modification, 61% MO degradation was observed). This may be due to SO2− 4 loading, which increases the BET surface area and decreases the crystallite size, thus increasing the number of reaction sites. Modification of the surface state of the catalyst might be causing effective separation of electron–hole pairs (e− –h+ ). Samples loaded with 2.5 wt% of sulfate show a higher percentage of degradation. However, further addition of higher sulfate content does not much affect the percentage of reduction. This may be due to the addition of a small amount of sulfate that decreases the crystallite size of titania, which facilitates a higher percentage of degradation. The reason may be the so-called particle size quantization effect [46]. 3.2.4. Effect of initial concentration and kinetics The pollutant concentration is an important factor. Fig. 7 shows the effect of MO decolorization over the concentration range of 50–150 mg/l. There is complete decolorization of MO under solar radiation for initial concentrations up to 100 mg/l and thereafter it decreases. With increasing concentration, the percentage of decolorization decreases. The concentration of MO has a significant effect on the degradation rates and the rate of degradation is higher when the ini-
Fig. 9. Effect of pH on MO degradation: catalyst dose = 1.0 g/l; [MO] = 150 mg/l; time = 4 h.
tial concentration is less. The variation of initial concentration of MO in the range of 50–150 mg/l was studied under constant pH and catalyst amount. Photocatalytic decolorization of MO followed first-order kinetics. A linear relationship was observed between MO concentration and irradiation time, as shown in Fig. 8 (log C0 /C versus time, where C0 is the initial concentration of MO and C is the concentration at time t ). The calculated data for first-order rate constants k at 50, 100, and 150 mg/l of MO concentration were found to be 0.664, 0.455, and 0.245 h−1 . The rate constant values were found to decrease with increased MO concentration. 3.2.5. Effect of pH The pH of the solution is one of the most important controlling parameters in the degradation of MO on semiconductor metal oxides. The wastewater from textile industries usually has a wide range of pH values. With increasing pH the percentage of degradation decreases (Fig. 9). The pH effect can be explained on the basis of point of zero charge of sulfate–TiO2 . The pHpzc of sulfate-modified TiO2 is 4.5–5.0. At pH values lower than pHpzc , the surface becomes positively charged, and the opposite occurs for pH values greater than pHpzc . Since MO is an anionic dye, it is conceivable that at low pH adsorption is favored on a positively charged surface and hence the reaction is faster at acidic pH.
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Fig. 10. Effect of H2 O2 : catalyst dose = 1.0 g/l; [MO] = 150 mg/l; time = 4 h.
Scheme 1.
radicals (Step 3). Oxygen present in the system acts as an efficient electron scavenger (Step 4), or any other oxidants such as OH− can also trapped the electron (Step 5) [26]:
Fig. 11. Effect of NaCl: catalyst dose = 1.0 g/l; [MO] = 150 mg/l; time = 4 h.
3.2.6. Effects of additives The photocatalytic degradation of organic substrates is significantly improved by the presence of H2 O2 , as the reactive OH radicals are easily generated by the breakdown of H2 O2 . Fig. 10 represents the effect of H2 O2 on the percentage of degradation. With increasing H2 O2 amount the percentage of degradation increases. As reported earlier, the addition of a small amount of H2 O2 greatly enhances the oxidation of organic pollutants [47]. This may be due to excess H2 O2 scavenging the OH radicals in the solution by forming H2 O and HO2 . Sodium chloride usually comes out in the effluent along with sectional wastes of textile mills. Fig. 11 reports the effect of sodium chloride on the photocatalytic degradation of MO. With increasing Cl− the degradation percentage decreases, possibly due to the Cl− ion reacting with Cl radicals formed at the vb to form Cl2− , for which the OH radical formation is decreased and the degradation process is stopped [48]. 3.2.7. Mechanism The mechanisms occurring on TiO2 surfaces exposed to light for the photodegradation of organic pollutants are summarized in Scheme 1 and also presented below stepwise. Absorption of light by TiO2 at wavelengths <385 nm is followed by electron (e− )–hole (h+ ) pair generation (Step 1). These charge carriers can migrate rapidly to the surfaces of catalyst particles, where they are ultimately trapped and undergo redox chemistry with suitable substrates. Thus, the trapped hole can react with chemisorbed OH− or H2 O to produce OH radical species (Step 2) or trapped by the organic substrate to produce organic
+ TiO2 + hv → TiO2 + e− CB + hVB ,
(1)
− • h+ VB + OH (surface) → OH ,
(2)
OH•
(3)
e− CB
+ dye → degradation products,
+ O2 →
O− 2,
dye + h+ VB → degradation products.
(4) (5)
4. Conclusion Sulfate modification can prevent the phase transformation, increase the BET surface area and thermal stability, and decrease the crystallite size of the catalysts. The optimum amount of sulfate loading was 2.5 wt%, at which the average percentage of degradation was nearly twice that of neat TiO2 . This sample possesses the lowest crystallite size (18.5 nm) and exhibits the highest photocatalytic activity. This material shows photocatalytic activity of 61% of MO degradation under solar radiation against 12% of adsorption in the dark. It was observed that degradation of MO is favored under acidic conditions. Addition of H2 O2 to the MO solution enhances the photocatalytic activity, whereas the presence of NaCl reduces it. The degradation rate of MO follows first-order kinetics. Acknowledgments The support and permission of Professor B.K. Mishra, Director, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar, are gratefully acknowledged. The authors are very much thankful to DST for financial support. References [1] S.N. Frank, A.J. Bard, J. Phys. Chem. 81 (1977) 1484. [2] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C Photochem. Rev. 1 (2000) 1.
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Preparation, characterization, and photocatalytic activity of sulfate-modified titania for degradation of methyl orange under visible light K.M. Parida ∗ , N. Sahu, N.R. Biswal, B. Naik, A.C. Pradhan Colloids and Materials Chemistry Department, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar 751 013, Orissa, India Received 30 August 2007; accepted 16 October 2007 Available online 26 November 2007
Abstract Hydrated titania was prepared by a sol–gel method, taking tetraisopropyl orthotitanate as starting material, and then promoted with different weight percentages of sulfate by an incipient wetness impregnation method. The materials were characterized by various advanced techniques such as PXRD, BET surface area, N2 adsorption–desorption measurements, FTIR, and SEM. Analytical results demonstrated that TiO2 is mesoporous in nature, and sulfate modification could inhibit the phase transformation and enhance the thermal stability of TiO2 . It was also found that sulfate modification could reduce the crystallite size and increase the specific surface area of the catalysts. The degradation of methyl orange under solar radiation was investigated to evaluate the photocatalytic activity of these materials. Effects of different parameters such as pH of the solution, amount of catalyst, additives, and kinetics were investigated. At 2.5 wt% sulfate loading, the average percentage of degradation of methyl orange was nearly two times than that of neat TiO2 . The photocatalytic degradation followed first-order kinetics. © 2007 Elsevier Inc. All rights reserved. Keywords: Sulfate-modified titania; Photocatalyst; Crystallite size; Methyl orange
1. Introduction Semiconductor photocatalysis to remedy the problem of chemical waste is a promising approach and has attracted much attention [1–3]. TiO2 is the most investigated semiconductor photocatalyst and has been widely studied during the past decade [4–14]. It has been extensively used in environmental applications due to its nontoxicity, photostability, low cost, and water insolubility under most conditions. The basic principle of semiconductor photocatalysis involves photogenerated electrons and holes migrating to the surface and serving as redox sources that react with adsorbed reactants, leading to the destruction of pollutants. However, the high rate of the electron/hole pair recombination process reduces the quantum yield of the TiO2. A good photocatalyst depends strongly on its efficiency of electron–hole pair separation and its optical absorption properties. To increase the activity of the photocatalyst, the e− –h+ recombination rate should be reduced. The introduction * Corresponding author. Fax: +91 674 2581637.
E-mail address: [email protected] (K.M. Parida). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.10.028
of defects through selective metal ion doping has been demonstrated to be an effective approach to separation of e− and h+ [15–28]. However, metal doping causes thermal instability and metal centers act as electron traps, which reduces the photocatalytic efficiency [29]. Band gap narrowing of TiO2 can be better achieved by using anionic dopant series [30–33]. Modifi3− cation of the titania with different anions such as SO2− 4 , PO4 , and WO2− 4 has been found to enhance the acidity and certain other physicochemical properties of the catalyst, such as thermal stability and mesoporosity [34–36]. In particular, sulfation of semiconductor photocatalysts is of great interest for scientists and engineers because it has an enormous potential for environmental pollutant remediation [37–40]. Out of the total world production of dyes, 15% is lost during the dyeing process and is released in textile effluents [41]. These effluents contain azo dyes and huge amounts of inorganic salts. Physical methods such as adsorption, biological methods, and chemical methods are the most frequently used for the treatment of these textile dyes, but these processes have high cost and limited applicability. Photocatalytic degradation of dyes has been proposed as an alternative method for the re-
232
K.M. Parida et al. / Journal of Colloid and Interface Science 318 (2008) 231–237
moval of dyes from the effluents of textile industries. Among the textile dyes, methyl orange (MO) is an orange-colored anionic dye with λmax at 464 nm. MO has various harmful effects on human beings: it may cause eye or skin irritation, or inhalation may cause gastrointestinal irritation with nausea, vomiting, and diarrhea. From our earlier studies we have found that sulfated TiO2 is an effective photocatalyst for degradation of 4-nitrophenol and decolorization of methylene blue under solar radiation [42–45]. The present study reports the photocatalytic activity of sulfatemodified TiO2 for the degradation of methyl orange in aqueous solution.
3. Results and discussions
2. Experimental
3.1. Physicochemical characterization
2.1. Material preparation
The X-ray diffraction patterns of the pure TiO2 and SO2− 4 modified TiO2 samples are shown in Fig. 1. The crystallite sizes (D) of samples were determined by employing the Debye– Scherrer formula and are listed in Table 1,
Hydrated titania was prepared by a sol–gel method, taking tetraisopropyl orthotitanate (Fluka Chemica, 98%) as the starting material. In a typical preparation procedure, 50 ml of tetraisopropyl orthotitanate was dissolved in 400 ml of isopropanol (Qualigens, 99.7%), and to this solution 12.7 ml of distilled water at pH 3.0 (1 M H2 SO4 ) was added dropwise under vigorous stirring. The resulting colloidal suspension was stirred for 3 h and aged at 80 ◦ C for 10 h. The gel obtained was filtered, washed, and dried at 100 ◦ C for 12 h. One series of sulfated TiO2 samples with varying weight percentages of SO2− 4 were prepared by an aqueous wetness impregnation method using H2 SO4 . The suspended mass was evaporated to dryness on a hot plate with constant stirring. The samples were dried in an air oven at 100 ◦ C and subsequently activated at 400 ◦ C at a heating rate of 10 ◦ C/min in a muffle furnace for 4 h.
days), from 10:00 a.m. to 2:00 p.m., when the average solar intensity was 0.80 kW/m2 and the intensity fluctuations were minimal. Reactions were performed in the dark in order to determine the adsorption behavior of the catalysts under similar conditions. A blank experiment was performed taking MO solution without catalyst to know the extent of degradation of MO due to solar radiation. After irradiation, the suspension was centrifuged and the MO content was analyzed quantitatively at 464 nm using a Cary-1E (Varian, Australia) spectrophotometer. All the catalytic results were reproducible with ±4% variation.
D = Kλ/β cos θ, where λ is the wavelength of the CoKα used, β is the full width at half maximum of the diffraction angle considered, K is a shape factor (0.94), and θ is the angle of diffraction. The peaks (101) for anatase are used. The PXRD pattern shows that both anatase and rutile phases are present in pure TiO2 , whereas only the anatase phase is present in the case of SO2− 4 /TiO2 . From this result, we bemodification probably stabilizes the anatase lieved that SO2− 4
2.2. Physicochemical characterization To determine the crystal phase composition of the catalyst, powder X-ray diffraction (PXRD) was carried out on a Philips X-ray diffractometer (PW 3710, Co anode). The specific surface area (BET) of the catalysts was measured by N2 adsorption–desorption studies at liquid nitrogen temperature (−196 ◦ C) using Quantasorb (Quantachrome, USA). Prior to the analysis, samples were degassed at 200 ◦ C. Surface morphology and particle size were studied by scanning electron microscopy (Hitachi S-3400N). The specimen for SEM analysis was prepared by a gold sputtering process. FTIR spectra of the samples were recorded in a Varian FTIR spectrophotometer (FTS-800) in the range of 400–4000 cm−1 , taking KBr as the reference.
Fig. 1. PXRD pattern of (a) neat TiO2 , (b) 2.0 wt% SO2− 4 /TiO2 , (c) 2.5 wt% 2− 2− SO2− /TiO , (d) 3.0 wt% SO /TiO , and (e) 3.5 wt% SO 2 2 4 4 4 /TiO2 . Table 1 Crystallite size and surface properties of TiO2 and SO2− 4 /TiO2
2.3. Photocatalytic reaction and dark adsorption
Photocatalyst
Crystallite size (nm)
BET surface area (m2 /g)
Pore volume (cm3 /g)
Pore size (nm)
The photocatalytic degradation of methyl orange (MO) was carried out by taking 20 ml of 150 mg/l MO solution in a 100ml closed Pyrex flask, over 1.0 g/l of catalyst. The solutions were exposed to sunlight with constant stirring. All the irradiation was performed in triplicate during March 2007 (sunny
TiO2 2.0 wt% SO2− 4 /TiO2 2.5 wt% SO2− 4 /TiO2 3.0 wt% SO2− 4 /TiO2 3.5 wt% SO2− 4 /TiO2
22.5 19.4 18.5 18.6 20.2
59.2 108.8 111.0 115.0 114.5
0.13 0.42 0.42 0.31 0.30
3.6 4.0 5.4 5.4 5.8
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Fig. 3. FTIR spectra of neat TiO2 and 2.5 wt% SO2− 4 /TiO2 . (a)
(b) Fig. 2. N2 adsorption–desorption isotherm and pore size distribution of (a) neat TiO2 and (b) 2.5 wt% SO2− 4 /TiO2 .
phase and inhibits the phase transformation. It is observed that the crystallite size of titania decreases with SO2− 4 loading. In addition to stabilizing anatase TiO2 crystallites, sulfate species inhibit TiO2 crystallite sintering, leading to smaller crystallites than in pure TiO2 . The crystallite size decreases in the presence of sulfate ions, as SO2− 4 species could possibly interact with the TiO2 network, and thus hinder the growth of the crystal. Even a very small amount of SO2− 4 species is sufficient for this effect. Therefore, the change in sulfate concentration did not change crystallite size further. Therefore, it is assumed that a small amount of sulfate species is responsible for the lowering of crystallite size.
The N2 adsorption–desorption isotherms and the pore size distribution of neat TiO2 and 2.5 wt% SO2− 4 –TiO2 samples are shown in Figs. 2a and 2b, respectively. A type IV isotherm and an H1 hysteresis loop was observed, which clearly indicates the mesoporous nature of TiO2 . Pore volume (single point total pore volume of pores at P /P0 = 0.99), pore size (determined from BJH desorption isotherm), and BET specific surface area of the samples are given in Table 1. Sulfate modification leads to increased surface area, pore size, and pore volume of the catalyst. The relatively high surface areas of sulfate-modified samples as compare to neat samples confirms that the frameworks of mesoporous TiO2 have better thermal stability due to the stabilizing effect of sulfate ions. The increase in pore size and pore volume after sulfate modification is due to the progressive coalescence of small pores to form large pores. This implies that the presence of sulfate plays a role in making the material porous. The FTIR spectra of pure TiO2 and SO2− 4 /TiO2 are shown in Fig. 3. The peak corresponding to 1375 cm−1 is the stretching frequency of S=O, and the peaks corresponding to 1131 and 1044 cm−1 are the characteristic frequencies of SO2− 4 . −1 The peaks at 436 and 495 cm for pure TiO2 and 476 cm−1 for SO2− 4 /TiO2 are contributions from the vibration modes of anatase skeletal O–Ti–O bonds. The broad absorption band in the region of 3200–3400 cm−1 is characteristic of the OHstretching vibration of surface hydroxyl groups and the peak corresponding to 1630 cm−1 has been assigned to H–O–H bending of physically adsorbed water. SEM micrographs of neat TiO2 and SO2− 4 /TiO2 are shown in Fig. 4. The particles were found to be in spherical shape. Neat TiO2 particles were found to be in the form of aggre2− gates, unlike SO2− 4 /TiO2 . SO4 ion modification preferably retarded the aggregation and growth of well-dispersed particles. The average particle sizes of the samples were found to be in the range of 0.2–0.25 µm for neat TiO2 and 0.7–0.9 µm for SO2− 4 /TiO2 .
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2− 2− Fig. 4. SEM micrographs of neat TiO2 , 2.0 wt% SO2− 4 /TiO2 , 2.5 wt% SO4 /TiO2 , and 3.0 wt% SO4 /TiO2 .
3.2. Photocatalytic activity 3.2.1. Photocatalytic degradation and dark adsorption It was observed that photodegradation of MO is very negligible (only 3.0%) under solar radiation in absence of catalyst. With increase in reaction time up to 4 h, the percentage of photoreaction increases and then it remains constant. It was found that 61.0% of MO degradation takes place by taking 150 mg/l of MO solution over 1.0 g/l of 2.5 wt% SO2− 4 /TiO2 catalyst under solar radiation, whereas only 12% of MO adsorption takes place under similar conditions in dark. From this it can be concluded that equilibrium is achieved only after 4 h of illumination.
Fig. 5. Effect of catalyst amount on photodegradation of MO, [MO] = 150 mg/l, time = 4 h.
3.2.2. Effect of catalyst dose Fig. 5 shows the effect of catalyst concentration on the percentage of degradation of MO. It is observed that as we increase the catalyst concentration from 0.5 to 2.5 g/l, the percentage of degradation increases. This may be due to increase in the number of photons absorbed by TiO2 particles and the number of reacting molecules adsorbed on the TiO2 surface with increase
in catalyst concentration. But after a certain concentration no further reacting molecules are available for adsorption; hence additional catalysts are not involved in the catalytic activity. From this observation it can also be presumed that with increase in catalyst loading there is an increase in the surface area of the catalyst available for adsorption and hence photodegradation.
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Fig. 6. Effect of sulfate on photodegradation of MO: catalyst dose = 1.0 g/l; [MO] = 150 mg/l; time = 4 h. Fig. 8. Kinetics of MO degradation: catalyst dose = 1.0 g/l; [MO] = 50, 100, and 150 mg/l.
Fig. 7. Effect of initial concentration of MO: catalyst dose = 1.0 g/l; time = 4 h.
3.2.3. Effect of sulfate loading Fig. 6 shows the effect of wt% of sulfate loading in TiO2 on the photocatalytic degradation of MO. It was demonstrated that all the TiO2 catalysts modified with SO2− 4 increase the percentage of degradation (in the case of pure TiO2 , 33% MO degradation, whereas after SO2− 4 modification, 61% MO degradation was observed). This may be due to SO2− 4 loading, which increases the BET surface area and decreases the crystallite size, thus increasing the number of reaction sites. Modification of the surface state of the catalyst might be causing effective separation of electron–hole pairs (e− –h+ ). Samples loaded with 2.5 wt% of sulfate show a higher percentage of degradation. However, further addition of higher sulfate content does not much affect the percentage of reduction. This may be due to the addition of a small amount of sulfate that decreases the crystallite size of titania, which facilitates a higher percentage of degradation. The reason may be the so-called particle size quantization effect [46]. 3.2.4. Effect of initial concentration and kinetics The pollutant concentration is an important factor. Fig. 7 shows the effect of MO decolorization over the concentration range of 50–150 mg/l. There is complete decolorization of MO under solar radiation for initial concentrations up to 100 mg/l and thereafter it decreases. With increasing concentration, the percentage of decolorization decreases. The concentration of MO has a significant effect on the degradation rates and the rate of degradation is higher when the ini-
Fig. 9. Effect of pH on MO degradation: catalyst dose = 1.0 g/l; [MO] = 150 mg/l; time = 4 h.
tial concentration is less. The variation of initial concentration of MO in the range of 50–150 mg/l was studied under constant pH and catalyst amount. Photocatalytic decolorization of MO followed first-order kinetics. A linear relationship was observed between MO concentration and irradiation time, as shown in Fig. 8 (log C0 /C versus time, where C0 is the initial concentration of MO and C is the concentration at time t ). The calculated data for first-order rate constants k at 50, 100, and 150 mg/l of MO concentration were found to be 0.664, 0.455, and 0.245 h−1 . The rate constant values were found to decrease with increased MO concentration. 3.2.5. Effect of pH The pH of the solution is one of the most important controlling parameters in the degradation of MO on semiconductor metal oxides. The wastewater from textile industries usually has a wide range of pH values. With increasing pH the percentage of degradation decreases (Fig. 9). The pH effect can be explained on the basis of point of zero charge of sulfate–TiO2 . The pHpzc of sulfate-modified TiO2 is 4.5–5.0. At pH values lower than pHpzc , the surface becomes positively charged, and the opposite occurs for pH values greater than pHpzc . Since MO is an anionic dye, it is conceivable that at low pH adsorption is favored on a positively charged surface and hence the reaction is faster at acidic pH.
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Fig. 10. Effect of H2 O2 : catalyst dose = 1.0 g/l; [MO] = 150 mg/l; time = 4 h.
Scheme 1.
radicals (Step 3). Oxygen present in the system acts as an efficient electron scavenger (Step 4), or any other oxidants such as OH− can also trapped the electron (Step 5) [26]:
Fig. 11. Effect of NaCl: catalyst dose = 1.0 g/l; [MO] = 150 mg/l; time = 4 h.
3.2.6. Effects of additives The photocatalytic degradation of organic substrates is significantly improved by the presence of H2 O2 , as the reactive OH radicals are easily generated by the breakdown of H2 O2 . Fig. 10 represents the effect of H2 O2 on the percentage of degradation. With increasing H2 O2 amount the percentage of degradation increases. As reported earlier, the addition of a small amount of H2 O2 greatly enhances the oxidation of organic pollutants [47]. This may be due to excess H2 O2 scavenging the OH radicals in the solution by forming H2 O and HO2 . Sodium chloride usually comes out in the effluent along with sectional wastes of textile mills. Fig. 11 reports the effect of sodium chloride on the photocatalytic degradation of MO. With increasing Cl− the degradation percentage decreases, possibly due to the Cl− ion reacting with Cl radicals formed at the vb to form Cl2− , for which the OH radical formation is decreased and the degradation process is stopped [48]. 3.2.7. Mechanism The mechanisms occurring on TiO2 surfaces exposed to light for the photodegradation of organic pollutants are summarized in Scheme 1 and also presented below stepwise. Absorption of light by TiO2 at wavelengths <385 nm is followed by electron (e− )–hole (h+ ) pair generation (Step 1). These charge carriers can migrate rapidly to the surfaces of catalyst particles, where they are ultimately trapped and undergo redox chemistry with suitable substrates. Thus, the trapped hole can react with chemisorbed OH− or H2 O to produce OH radical species (Step 2) or trapped by the organic substrate to produce organic
+ TiO2 + hv → TiO2 + e− CB + hVB ,
(1)
− • h+ VB + OH (surface) → OH ,
(2)
OH•
(3)
e− CB
+ dye → degradation products,
+ O2 →
O− 2,
dye + h+ VB → degradation products.
(4) (5)
4. Conclusion Sulfate modification can prevent the phase transformation, increase the BET surface area and thermal stability, and decrease the crystallite size of the catalysts. The optimum amount of sulfate loading was 2.5 wt%, at which the average percentage of degradation was nearly twice that of neat TiO2 . This sample possesses the lowest crystallite size (18.5 nm) and exhibits the highest photocatalytic activity. This material shows photocatalytic activity of 61% of MO degradation under solar radiation against 12% of adsorption in the dark. It was observed that degradation of MO is favored under acidic conditions. Addition of H2 O2 to the MO solution enhances the photocatalytic activity, whereas the presence of NaCl reduces it. The degradation rate of MO follows first-order kinetics. Acknowledgments The support and permission of Professor B.K. Mishra, Director, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar, are gratefully acknowledged. The authors are very much thankful to DST for financial support. References [1] S.N. Frank, A.J. Bard, J. Phys. Chem. 81 (1977) 1484. [2] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C Photochem. Rev. 1 (2000) 1.
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