Enhancement of Photocatalytic Efficiency of TiO 2 by Supporting on Clinoptilolite in the Decolorization of Azo Dye Direct Yellow 12 Aqueous Solutions

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1 Journal of the Chinese Chemical Society, 2007, 54, Enhancement of Photocatalytic Efficiency of TiO 2 by Supporting on Clinoptilolite in the Decolorization of Azo Dye Direct Yellow 12 Aqueous Solutions Manouchehr Nikazar, a Khodayar Gholivand b and Kazem Mahanpoor c * a Department of Chemical Engineering, Amirkabir University of Technology, Tehran, Iran b Faculty of Science, Department of Chemistry, University of Tarbiat Modares, Tehran, Iran c Department of Applied Chemistry, Faculty of Chemistry, Islamic Azad University, North Tehran Branch, Tehran, Iran Structure and physico-chemical properties of a photocatalyst, especially surface area and absorption ability, were correlated to catalytic activities in photodegradation of dye pollutants in water. In this investigation photocatalytic degradation of azo dye Direct yellow 12 (Chrysophenine G) in water was studied. Titanium(IV) oxide was supported on Clinoptilolite (CP) (Iranian Natural Zeolite) using solid-state dispersion (SSD) method. The results show that the TiO 2 /Clinoptilolite (SSD) is an active photocatalyst. The maximum effect of photo degradation was observed at 10 wt. TiO 2,90wt. Clinoptilolite. A first order reaction withk= min -1 was observed. The effects of some parameters such as ph, amount of photocatalyst, initial concentration of dye were examined. Keywords: Photodegradation; Photocatalysts; TiO 2 /zeolites; Direct yellow 12; Clinoptilolite (CP). INTRODUCTION Organic dyes constitute one of the larger groups of pollutants in wastewater released from textile industries. Heterogeneous photocatalysis has been considered as a cost effective alternative for the purification of dye-containing wastewater. As a result, photocatalytic decomposition of organic compounds in wastewater has attracted a great deal of attention. 1-9 TiO 2 is one of the most effective photocatalysts because it is biologically and chemically inert and photostable with near-uv band gap energy. TiO 2 can be used in the form of a fine powder or crystals dispersed in wastewater treatment applications. However, the need to filter TiO 2 after reaction makes such a process troublesome and costly. Thus, in order to solve this problem, many researchers have examined methods for fixing TiO 2 on supporting materials including glass beads, fiber glass, silica, 16,17 and zeolite. 18 When using zeolite as a support for TiO 2, care should be taken that TiO 2 does not lose its photo activity and that the adsorption properties of zeolite are not affected. Mattews showed the photoefficiency of TiO 2 is suppressed when Ti interacts with the zeolite. 11 In this work TiO 2 was supported on zeolite without losing photoefficiency and affecting the adsorption properties of zeolite. This mixture was used for degradation of aqueous Direct yellow 12. EXPERIMENTAL Materials The titanium dioxide used was Degussa P-25 with a crystallographic mode of 80% anatase and 20% rutile, a BET surface area of 50 m 2 g -1 and an average particle size of 30 nm, (according to the manufacturer s specifications) and the raw zeolite material was an Iranian commercial clinoptilolite (CP) (Afrand Tuska Company, Iran) from deposits in the region of Semnan. According to the supplier s specification, it contains about 90 wt.% CP (based on XRD internal standard quantitative analysis) and the Si/Al molar ratio is The concentration of Fe 2 O 3,TiO 2, MnO and P 2 O 5 impurities have been reported to be 1.30, 0.30, 0.04 and 0.01 wt.%. * Corresponding author. kazem_mahanpoor@yahoo.com

2 1262 J. Chin. Chem. Soc., Vol. 54, No. 5, 2007 Nikazar et al. The azo dye, Direct yellow 12 (DY12), was obtained from the Rang Azar company (Iran) and used without further purification. The molecular structure of DY12 is shown in Scheme I. Scheme I Molecular structure of direct yellow 12 (Chrysophenine G) Preparation of TiO 2 -Supported CP Catalysts The solid state dispersion (SSD) method was used for preparing the Zeolite-based photocatalyst. In this method, TiO 2 was mixed with zeolite using ethanol as a solvent using an agate pestle and mortar; the solvent was then removed by evaporation. Samples prepared by this method were dried at 110 C and calcined in air at 450 Cfor5hto obtain TiO 2 -supported zeolite catalysts. Apparatus For the UV/photocatalyst process, irradiation was performed in a batch photoreactor of 2 liters in volume with four mercury lamps, Philips 8W (UV-C). A magnetic stirrer was used to ensure complete mixing in the tank (stirring speed = 250 rpm) and air was dispersed at a flow rate of 1 lit. min -1 by using an air diffuser to supply oxygen for the enhancement of photo-oxidation (Fig. 1). UV/VIS Spectrophotometer, (Shimadzu 160A) was employed for absorbance measurements using silica cells of path length 1 cm. XRD analysis of the samples was done using a D-500 diffractometer (Siemens). BET surface areas of CP and TiO 2 /CP were measured in an all-glass high vacuum system by N 2 adsorption at 77 K. The morphology of the resultant was pure, and loaded TiO 2 was obtained with a transmission electron microscope (TEM; JEM-2000FX, Japan). Procedures For the photodegradation of DY12, a solution containing a known concentration of dye and photocatalyst was prepared and was allowed to equilibrate for 30 min in the darkness. The suspension ph values were adjusted at the desired level using dilute NaOH and H 2 SO 4 (the ph values were measured with a Horiba M12 ph meter). The photo degradation reaction took place under the radiation of a mercury lamp while agitation was maintained to keep the suspension homogeneous. The concentration of the samples was determined using a spectrophotometer (UV/ VIS Spectrophotometer, Shimadzu 160A) at max = 395 nm. The samples were filtered before the UV-vis spectroscopy to remove the photocatalyst. The degree of photodegradation (X) as a function of time is given by: Fig. 1. Schematic diagram of the batch photocatalytic reactor (cross-section view) (a), and UV lamp immersed directly in aqueous solution (b).

3 Enhancement of Photocatalytic Efficiency of TiO 2 J. Chin. Chem. Soc., Vol. 54, No. 5, X = C C o (1) C o where C o and C are the concentration of dye at t=0andt, respectively. RESULTS AND DISCUSSION UV-Vis Spectra The absorbance of DY12 solutions during the photocatalytic process at initial and after 2 h irradiation time verses are shown in Fig. 2. The spectrum of DY12 in the visible region exhibits a main band with a maximum at 395 nm. The decrease of absorption peaks of DY12 at max = 395 nm in this figure indicates a rapid degradation of the azo dye. Complete discoloration of dye was observed after 3 h under optimum conditions. Effect of UV Irradiation and Photocatalyst Employed Particles The effects of UV irradiation, TiO 2 and CP presence on the photodegradation of DY12 are shown in Fig. 3. This figure indicates that in the presence of mixed photocatalyst (10 wt. TiO 2 /CP) and UV irradiation 97.4 of dye was degraded at the irradiation time of 2 h while it was 63.2 for TiO 2 (without clinoptilolite) and UV irradiation. This was contrasted with 8 degradation for the same experiment performed in the absence of TiO 2, and 10.5 when the UV lamp had been switched on and the reaction was allowed to occur in the presence of CP. This is due to the fact that when TiO 2 is illuminated with the light of 390 nm, electrons are promoted from the valence band to the conduction band of the semi conducting oxide to give electron-hole pairs. The valence band (h VB ) potential is positive enough to generate hydroxyl radicals at the surface, and the conduction band (e CB ) potential is negative enough to reduce molecular oxygen. The hydroxyl radical is a powerful oxidizing agent and attacks organic pollutants present at or near the surface of TiO 2.It causes photo-oxidation of dye according to the following reactions: TiO 2 + h TiO 2 (e CB + h VB ) (2) +H 2 O (ads) H + + OH (ads) (3) h VB + OH ( ads ) OH (ads) (4) + O 2(ads) O 2( ads ) (5) H 2 O H + +OH (6) O 2( ads ) +H + HO 2 (7) 2 HO 2 H 2 O 2 +O 2 (8) H 2 O 2 +TiO 2 (e CB ) OH+OH +TiO 2 (9) OH (ads) + dye degradation of the dye (10) + dye dye oxidation of the dye (11) h VB e CB h VB The mechanism is summarized in Fig. 4. Effect of the Composition of the Supported Photocatalyst The effect of the ratio of TiO 2 /CP on the DY12 removal is shown in Fig. 5(A). The effective decomposition of DY12 after 120 min irradiation time was observed when the photocatalyst contained 10 TiO 2 and 90 CP, prepared by using the solid state dispersion (SSD) method. Fig. 2. UV-Vis spectra of DY12 (50 ppm) in aqueous photocatalyst (10 wt.% TiO 2 /CP) dispersion with concentration of 40 ppm, irradiated with a mercury lamp light at ph = 7, T = 298 K, at: t = 0, 0.5, 1.0, 1.5, 2.0. Fig. 3. Effect of UV light and different photocatalysts on photocatalytic degradation of DY12. [DY12] 0 = 50 ppm, TiO 2 and photocatalyst (10 wt.% TiO 2 /CP) concentration = 40 ppm, T = 298 K, ph = 7.

4 1264 J. Chin. Chem. Soc., Vol. 54, No. 5, 2007 Nikazar et al. BET surface area variations versus %TiO 2 content, shown in Fig. 5(B), confirmed catalytic activity. Effect of Catalyst Concentration The photodegradation efficiency increases with an increase in the amount of photocatalyst (10 wt. TiO 2 /CP), up to a value of 40 ppm and then decreases when the catalyst concentration is increased. This trend can be explained by the fact that when all dye molecules are adsorbed on the photocatalyst, the addition of higher quantities of photocatalyst would have no effect on the degradation efficiency. Another reason for this is supposedly an increased opacity of the suspension, brought about as a result of an excess of photocatalyst particles. 14 To comment on this result, we propose that the OH (hydroxyl radicals), on the surface of TiO 2, are easily transferred onto the surface of zeolite. That means the organic pollutants, which have already been adsorbed on the nonphotoactive zeolite, have chances to be degraded due to the appearance of OH, resulting in the enhancement of the photodegradation performance of TiO 2 -zeolite. Experimental results show that about 10 wt. of TiO 2 with respect to zeolite is the best condition to achieve the synergism between TiO 2 and CP. This synergic effect may be due to the fact that the presence of zeolite maintains the molecules of dye near the photocatalyst (local concentration effect) as depicted in Fig. 6. The enhanced photocatalytic activity over the composite TiO 2 +CP reflects the beneficial adsorption properties of CP. If the amount of TiO 2 in the composition of the photocatalyst decreases (less than 10 wt.), the rate of the OH production will not be enough. In this condition a few of the dye molecules that have been absorbed on the surface of zeolite can react with OH. If the amount of TiO 2 increases in the composition of the photocatalyst (more than 10 wt.), the rate of dye absorption will not be enough. In this condition a few of the OH can Fig. 5. (A) Effect of composition of photocatalyst (wt.% TiO 2 in mixture of TiO 2 and CP) on photocatalytic degradation of DY12 after 120 min. [DY12] 0 = 50 ppm, concentration of photocatalyst = 40 ppm, T = 298 K, ph = 7. (B) Effect of composition of photocatalyst (wt. TiO 2 in mixture of TiO 2 and CP) on BET surface area. Fig. 4. General mechanism of the photocatalysis. Fig. 6. Photocatalytic activity of TiO 2 + CP.

5 Enhancement of Photocatalytic Efficiency of TiO 2 J. Chin. Chem. Soc., Vol. 54, No. 5, react with dye molecules. Characterization of Photocatalysts As is known, the pore size of clinoptilolite is nm. 19 Fig. 7 shows the TEM images of pure TiO 2 nanoparticles and TiO 2 particles have loaded on the surface of CP. As seen, the particle size of pure TiO 2 is 30 nm or so, which is much larger than that of the loaded ones. Thus, TiO 2 particles are not able to enter into the pores. Therefore, we suggest that the protrusions are TiO 2 and nearly all of the TiO 2 particles have been loaded on the surface of zeolite, instead of into the pores and cavities. It means that the photocatalytic degradation of Direct yellow 12 mostly takes place at the zeolite surface, but not in the pores. In addition, as shown in Fig. 1b, the small TiO 2 particles do not distribute compactly but reserve lacunas on the zeolite surface, which enables the support to show its adsorption ability. The surface area of the CP (480 m 2 g -1 ) decreased in general on supporting TiO 2 (BET surface area of TiO 2 /CP with different TiO 2 wt. is between m 2 g -1, and the effect of the ratio of TiO 2 /CP on the surface area is shown in Fig. 5 (B)). The surface area of the TiO 2 -supported zeolite may be taken as an index of the available porosity. Considering the particle sizes of TiO 2 and the sizes of CP zeolite medium pores ( 1 nm) we conclude that there is no possibility of TiO 2 particles entering the zeolite pores in the SSD method of preparation. To reveal the interaction between the TiO 2 and the zeolite, the crystal structures of the raw zeolite and the TiO 2 - zeolite calcined at 450 C after 5 h were measured, as shown in Fig. 8. It is clear that the XRD patterns of TiO 2 -zeolite consist with the raw zeolite very well as calcined at 450 C for 5 h, and no diffraction peaks corresponded to typical TiO 2, including anatase and rutile, can be observed. And similar results have also been reported by other researchers. 21,22 It implies that the frame structure of zeolite after TiO 2 loading has not been destroyed. The infrared analysis confirms that the TiO 2 particles combine with the active sites of natural zeolite, clinoptilolite, and Ti-O-Al and Ti-O-Si which make the load of TiO 2 ; the IR spectra were measured, shown in Fig. 9. The stretching vibration of O-H (3610 cm -1 ), mostly corresponding to the surface -OH, shows a little weaker after TiO 2 loading, indicating that a certain amount of -OH has been demolished by the load of TiO 2. In addition, a new absorption band is found in the IR spectra of TiO 2 -zeolite compared with the raw one. The band covers a range from 945 to 905 cm -1 corresponding to the stretching vibration of Ti-O-Si and Ti-O-Al, which is similar to the results from Liu et al. 23 In this work and on the basis of the above analysis, Fig. 8. XRD pattern of 10 wt.% TiO 2 /CP prepared by the (SSD) method after calcination in air at 450 C for 5 h (1) raw CP after calcination in air at 450 C for 5 h (2). Fig. 7. TEM images of (a) pure TiO 2 nanoparticles and (b) TiO 2 particles loaded on the surface of CP (10 wt.% TiO 2 /CP). Fig. 9. IR spectra of raw CP (a) and TiO 2 supported on clinoptilolite by SSD method (10 wt.% TiO 2 /CP) (b).

6 1266 J. Chin. Chem. Soc., Vol. 54, No. 5, 2007 Nikazar et al. Scheme II Proposed incorporation of TiO 2 on CP when the TiO 2 solution and CP are mixed together and calcined, we think, TiO 2 colloid particles react with the active site of zeolite. It is notable that the O at the surface, which is combined with Al, is much more active than that combined with Si, due to the different electronic capability of Al and Si. This model is shown in Scheme II. Before loading, the clinoptilolite surface is covered with an oxide thin layer. And small cations, such as H,Na and K, are attracted close to the aluminum by the static force due to the replacement of Si with Al. To prove and further realize the above model, we studied ion-exchange capacity of zeolite. On one hand, before synthesis the adsorption capacity for NH 4 of raw zeolite by ion-exchange is mmol/100 g. While after the TiO 2 loading the value drops sharply to 62.7 mmol/100 g. The result indicates that the defects, which are caused by the replacement of Si with Al, are mostly eliminated by the combination of Ti and Al. Effect of Initial DY12 Concentration The effect of initial concentration of DY12 on photodegradation efficiency is shown in Fig. 10. The photodegradation conversion of DY12 decreases with an increase in the initial concentration of DY12. The presumed reason is that when the initial concentration of dye is increased, more and more dye molecules are adsorbed on the surface of the photocatalyst. The large amount of adsorbed dye is thought to have an inhibitive effect on the reaction of dye molecules with photogenerated sites or hydroxyl radicals, which is due to the lack of any direct contact between them. Once the concentration of dye is increased, it also causes the dye molecules to adsorb light and the photons never reach the photocatalyst surface, thus the photodegradation efficiency decreases. Effect of ph ph is one of the main factors influencing the rate of degradation of some organic compounds in the photocatalytic process. 15,16 It is also an important operational variable in actual wastewater treatment. Fig. 11 shows the photodegradation of DY12 at different ph from 2 to 12, which clearly shows that the best results were obtained in acidic solution, (ph = 2, X = 100%). The TiO 2 surface is presumably positively charged in acidic solution and nega- Fig. 10. Effect of initial DY12 concentration on photodegradation efficiency in different times, concentration of photocatalyst = 40 ppm, T = 298 K, ph = 7. Fig. 11. Effect of ph on photodegradation efficiency of DY12. Irradiation times = 1.0 h, [DY12] 0 = 50 ppm, concentration of photocatalyst (10 wt.% TiO 2 /CP) = 40 ppm, T = 298 K.

7 Enhancement of Photocatalytic Efficiency of TiO 2 J. Chin. Chem. Soc., Vol. 54, No. 5, tively charged in alkaline solution. 17,18 For the above reasons, for dyes that have a sulfuric group in their structure, which is negatively charged, the acidic solution favors adsorption of dye onto the photocatalyst surface, thus the photodegradation efficiency increases. There is also the photocatalytic degradation of DY12 in acidic solutions, which is probably due to the formation of OH as it can be inferred from the reactions (12)-(15) O 2(ads) O 2( ads ) (12) +H + HO 2 (13) 2HO 2 O 2 +H 2 O 2 (14) H 2 O 2 + O 2( ads ) OH+OH +O 2 (15) e CB O 2( ads ) Kinetics of Photocatalytic Degradation of DY12 Several experimental results indicated that the degradation rates of photocatalytic oxidation of various dyes over illuminated TiO 2 fitted by the first-order kinetic model Fig. 12 shows the plot of Ln ([Dye] o /[Dye]) versus irradiation time for DY12. The linear plot suggests that the photodegradation reaction approximately follows the pseudo-first order kinetics with rate coefficient K = min -1. CONCLUSIONS Fig. 12. Plot of reciprocal of pseudo-first order rate constant against initial concentration of DY12 = 50 ppm, concentration of photocatalyst (10% TiO % CP) = 40 ppm, T = 298 K, ph=7. 1- The SSD method is an effective method for supporting TiO 2 on CP. 2- Photocatalyst containing 10 TiO 2 and 90 CP has the maximum on photodegradation efficiency of DY The photodegradation conversion of DY12 decreases with an increase in the initial concentration of DY ph is one of the main factors and the optimum ph was obtained about Kinetics of photocatalytic degradation of DY12 is pseudo-first order with K = min -1. ACKNOWLEDGEMENT The authors would like to thank Dr. B. Bonakdarpour for editing the manuscript. Received May 15, REFERENCES 1. Ollis, D. F. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993; pp Serpone, N.; Perizzetti, E. Photocatalysis-Fundamentals and Aplications; Wiley: New York, 1989; pp Tsai, S.-J.; Cheng, S. J. Chin. Chem. Soc. 2001, 48, Greem, K. J.; Rudham, R. J. Chem. Soc. 1993, 89, Conningham, J.; Sl-Sayyed, G.; Stijarani, S. Aquatic and Surface Photochemistry; Helz, G. R.; Zepp; R. G.; Crosby, D. G., Ed.; Boca Raton: Lewis Publishers, 1994, p Aldullahm, M.; Low, G. C.; Mattews, R. W. J. Phys. Chem. 1990, 94, Mattews, R. W. Water Res. 1990, 24, Mattews, R. W. J. Catal. 1988, 113, Fox, M. A.; Doan, K. E.; Dulay, M. T. Res. Chem. Intermed. 1994, 20, Xu, Y.; Chen, X. Chem. Ind. (London) 1990, 6, Mattews, R. W. J. Phys. Chem. 1988,92, Sahate, J.; Anderson, M. A.; Kikkawa, H.; Edwards, M.; Hill, G. G. J. Catal. 1991, 127, Xu, Y.; Menassa, P. C.; Langford, C. H. Chemosphere 1988, 17, Mattews, R. W. Solar Energy. 1987, 38, Hofstandler, K.; Kikkawa, K.; Bauer, R.; Novalic, C.; Heisier, G. Environ. Sci. Technol. 1994, 28, Anpo, M.; Nakaya, H.; Kodama, S.; Kubokawal, Y.; Domen, K.; Onishi, T. J. Phys. Chem. 1986, 90, Sato, S. Langmuir. 1988, 4, Yoneyama, H.; Hag, S.; Yamanaka, S. J. Phys. Chem. 1989, 93, Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry and Use; John Wiley and Sons: New York, 1974, pp Kim, Y. I.; Keller, S. W.; Krueger, J. S.; Yonemoto, E. H.;

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