Photocatalytic Degradation of Nitrobenzene in an Aqueous System by

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Photocatalytic Degradation of Nitrobenzene in an Aqueous System by Transition-Metal-Exchanged ETS-10 Zeolites Praveen K. Surolia, Rajesh J. Tayade, and Raksh V. Jasra*,† Discipline of Inorganic Materials and Catalysis, Central Salt & Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR), G. B. Marg, BhaVnagar-364021, India

Engelhard titanosilicate ETS-10 was synthesized using Degussa P25 as the titanium source. Metal-exchanged microporous titanium silicate M-ETS-10 (M ) Fe, Co, Ni, Cu, and Ag) samples were prepared by ion exchange with respective metal salt solutions. The synthesized samples were characterized by powder X-ray diffraction, BET surface area, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and UV-vis diffuse reflectance measurements (DRS). The photocatalytic activity of these samples was investigated for the decomposition of nitrobenzene (NB). The pristine ETS-10 and transition-metal-ion-exchanged ETS10 samples were found to be active photocatalysts that can decompose nitrobenzene under UV irradiation. Results demonstrated that silver-ion-exchanged ETS-10 shows the highest photocatalytic activity for degradation and mineralization of nitrobenzene. 1. Introduction The photocatalytic degradation of organic compounds present in aqueous media has attracted the attention of many researchers,1-9 and TiO2 both in powder and supported form is one of the most widely reported photocatalysts for this purpose. TiO2 has been supported on silica,10,11 zeolites,12-14 fiber glass,15,16 and electrodes,17 promising to avoid catalyst loss during use. However, the recovery of the catalysts after use is still a challenge even in case of supported photocatalysts. The titanosilicate ETS-10 has recently attracted attention as a photocatalyst as it has built-in photocatalytically active TiO2 units. This structural feature makes it an attractive material for photocatalysis, as it will not have the leaching problem encountered with supported TiO2. In ETS-10, each Ti atom is octahedrally connected to four Si and two Ti atoms through oxygen bridges. This arrangement of Ti and Si atoms generates a threedimensional structure which generally contains a considerable degree of disorder and has small 7-membered and 12-membered rings.18 ETS-10 contains chains of active -O-Ti(IV)-O- that exhibit quantum confinement effects and behave as onedimensional semiconductor nanowires in its framework.19-21 The TiO6 octahedra form linear chains running in two perpendicular directions of the crystal and are isolated from one another by the siliceous matrix. Thus generated one dimensionally confined atomic Ti-O-Ti-O-Ti wires give rise to the peculiar optical and electronic properties of the material.22 This structural feature can help to overcome the problem of leaching encountered in supported photocatalysts. Consequently, research efforts are in progress to enhance photocatalytic activity of ETS-10. ETS-10 and its ion-exchanged isomorphs have shown applications in adsorption,17 photochemistry,23,24 acylation of alcohols,25 and oxidative dehydrogenation of alcohols.26 Furthermore, ETS-10 has been studied as a catalyst for photodegradation of alcohols1 and also for shape selective photocatalytic transformation of phenols in an aqueous medium.27 As ETS-10 has high cation exchange capacity, transition and alkali-metal-ionexchanged ETS-10 has been studied for various catalytic * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]. Tel.: +91 265 6693935. Fax: +91 265 6693934. † Present address: Reliance Technology Group, Vadodara-391346, Reliance Industries Limited, Gujarat, India.

applications. Co-ETS-10 and Ag-ETS-10 has been reported to show the photocatalytic activity to decompose acetaldehyde.28 The apparent semiconducting properties of ETS-10 and its ability to bring reactant molecules in close proximity to the semiconductor chains in ETS-10 allow efficient interception of photoproduced holes and electrons by the adsorbed species. These interesting attributes of ETS-10 prompted us to investigate its potential as photocatalyst. It was further shown that the ion exchange may cause the changes in the electronic properties of the pristine ETS-10 material.29 The addition of transition metal ions offers a way to trap the charge carrier and extend the lifetime of one or both of the charge carriers. It is known that the ions to be exchanged must have a work function higher than that of the semiconductor providing a Schottky barrier.14 In view of this, we have investigated the effect of ion exchange on the photoreactivity of ETS-10 zeolite. We chose the low percentage (0.5 wt %) of transition metal ion (Fe, Co, Ni, Cu and Ag) exchange, as this has shown promise for the improvement in photocatalytic activity.30,31 Photocatalytic activity of these photocatalysts for nitrobenzene degradation under the UV-light irradiation was investigated. The mineralization of nitrobenzene in aqueous solution was confirmed by chemical oxygen demand (COD) analysis. 2. Experimental Section 2.1. Chemicals and Materials. Titanium dioxide (P25) was purchased from Degussa Corporation (Degussa AG, Frankfurt, Germany). Sodium silicate was procured from Aquagel, India. NaCl, KCl, nitrobenzene (NB), and metal nitrates (AR grade) were purchased from S.D. Fine-Chem Ltd., India. Deionized distilled water was used to make up the reaction mixture. 2.2. Synthesis of the Catalysts. The synthesis of ETS-10 samples was as per the procedure reported by Yang et al.32 Typically, 20 g of sodium silicate solution (24 wt % SiO2, 8 wt % Na2O) was diluted with 70 mL of water and 13.8 g of NaCl; 2.6 g of KCl was dissolved in this solution. In the resultant translucent thick gel, 2.6 g of Degussa P25 TiO2 was added under vigorous stirring. The slurry was stirred for 30 min at room temperature and transferred to a Teflon-lined autoclave and heated statically at 573 K for 42 h. The starting gel had the pH around 10.83. The solid product was isolated from mother

10.1021/ie901603k  2010 American Chemical Society Published on Web 03/19/2010

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Table 1. Structural and Electronic Properties of the Catalysts catalyst

BET surface area (m2/g)

pore volume (cm3/g)

(Na+K)/Ti

% metal (EDX)

% metal (ICP)

band gap (eV)

ETS-10 Fe-ETS-10 Co-ETS-10 Ni-ETS-10 Cu-ETS-10 Ag-ETS-10

232 191 203 201 216 205

0.134 0.125 0.125 0.119 0.124 0.116

1.49 1.30 1.39 1.35 1.30 1.37

0.53 0.63 0.53 0.61 0.47

0.55 0.68 0.59 0.65 0.41

3.15 3.13 3.12 3.14 3.15 3.16

liquor by filtration, washed with distilled water, dried overnight at 333 K, and then calcined at 723 K for 5 h in air. The thus obtained ETS-10 had the stoichiometry of (Na,K)2TiSi5O13. For preparation of the transition-metal-ion-exchanged ETS10 samples, the weighed amount of ETS-10 (2 g) was stirred for 24 h with the metal salt solution in 100 mL of water of the concentration calculated for 0.5% (w/w) metal ion exchange at room temperature. During the ion exchange, the alkali metal ions are replaced by transition metal ion from ETS-10 giving rise to composition (Na,K)2sxMyTiSi5O13 (M ) transition metal ion). After the ion exchange, the samples were washed with distilled water and dried at 393 K for 12 h. The thus obtained samples were named as M-ETS-10, where M represents the transition metal. Some of the catalyst samples were observed to develop color after ion exchange as mentioned in Table 1. 2.3. UV Irradiation Experiments. The photocatalytic activity of the catalysts was determined for nitrobenzene degradation under UV irradiation using a reactor consisting of two parts as described earlier.33 The reactor consists of inner quartz double wall jacket with inlet and outlet facility for water circulation to maintain the temperature of reaction mixture throughout the reaction time. The center of the jacket has an empty chamber for immersion of a mercury vapor lamp. Photocatalytic reaction takes place in the outer borosilicate glass container having a volume of 250 mL after insertion of the quartz inner part. The mercury vapor lamp of 125 W was used for UV irradiation (Crompton Greaves Ltd. India). The magnetic stirrer was kept below the reactor for continuous stirring. A 5 mL portion of the reaction mixture was withdrawn from the port by syringe at different time intervals. Photocatalytic activity was measured at the pH of the initial reaction mixture, and temperature was maintained at 293 K for all the reactions. The photocatalytic activity of the catalysts was evaluated by measuring the decrease in concentration of nitrobenzene in the reaction solution using UV-vis spectroscopy. Standard samples of nitrobenzene of 5, 10, 20, 30, 40, and 50 ppm were prepared, and the calibration curve was plotted for nitrobenzene solutions by measuring absorption of these samples. The slope of the curve was calculated and used to determine the nitrobenzene concentration from absorbance values employing the Beer-Lambert’s law from the absorption data. Prior to commencing irradiation, a suspension containing 50 mg of the catalyst and a 250 mL aqueous solution of ca. 50 ppm of nitrobenzene was stirred continuously for 30 min in the dark. An aliquot was then taken for analysis. Following this, the solution was irradiated, and samples were withdrawn for analysis by syringe at intervals of 10 min for the first hour and then every hour thereafter for 4 h. The catalyst was separated from the samples by centrifugation prior to analysis. The absorbance was measured at λmax ) 267 nm for nitrobenzene. The amount of TiO2 in the ETS-10 catalyst calculated as per its stoichiometry was 17%. Thus the amount of photocatalytic active part (-O-Ti(IV)-O-) of the prepared catalyst in 50 mg is 8.5 mg. Adsorption studies in the dark were performed separately using an aqueous 50 ppm solution of nitrobenzene for all the

catalysts for 1 h at the reaction conditions used for measuring photocatalysis, to find out the decrease in the concentration due to the adsorption. The decrease in concentration was observed by using UV-vis spectroscopy. The mineralization of nitrobenzene in aqueous solution was confirmed by COD analysis of the samples taken after different reaction time intervals by using a HACH DR 2800 photometer. 2.4. Catalyst Characterization. The X-ray diffraction data were collected on a Phillips X’pert MPD system using Cu KR1 (λ ) 0.15405 nm) radiation at 295 K. Diffraction patterns were taken in 10°- 80° 2θ range at the scan speed of 0.1 deg sec-1. The FT-IR spectroscopic measurements were carried out using Perkin-Elmer GX spectrophotometer in the range 400-4000 cm-1 with a resolution of 4 cm-1 as KBr pellets. The Brunauer-Emmett-Teller (BET) surface area of the catalysts was obtained from nitrogen adsorption data at 77.4 K in the relative pressure range of 0.05-0.25 using Micrometrics ASAP 2010 volumetric adsorption apparatus. The band gap energy of the catalysts was determined using diffuse reflectance spectroscopy (DRS) at room temperature in the wavelength range of 250-700 nm. The spectrophotometer (Shimadzu UV-3101PC) was equipped with an integrating sphere and BaSO4 was used as a reference. The band gap energy of the catalysts was calculated according to the equation, band gap (EG) ) hc/λ

(1)

where EG is the band gap energy (eV), h is Planck’s constant, c is the light velocity (m/s), and λ is the wavelength (nm). Scanning electron microscope (Leo series 1430 VP) equipped with INCA, energy dispersive system (EDX), Oxford Instruments, was used to confirm the presence of metal ions in microporous ETS-10 samples as well as to determine the morphology of catalysts. The sample powder was supported on aluminum stubs prior to measurement. An inductively coupled plasma-optical emission spectrophotometer (Optima 2000 DV, Perkin-Elmer, Eden Prarie, MN) was used to determine the percentage of metal ion present in the catalyst. Thermogravimetry and differential thermal analysis were performed on a TGA/ SDTA851e, Mettler Toledo system in a temperature range of 50-700 °C, with a heating rate of 5 °C min-1 under an air stream. 3. Results and Discussion 3.1. Structural, Textural, and Electronic Properties. The powder XRD patterns of pristine and ion-exchanged ETS-10 samples are shown in Figure 1. The pattern of ion-exchanged samples is similar to the parent ETS-10. This implies that the structure of ETS-10 remains intact after metal ion exchange. The absence of any peak due to exchanged metal ion could be because of a very small amount of ion exchange. The XRD pattern exhibits a high degree of disorder in the ETS-10 structure.34 The broad absorbance peak having values at around 3590 and 3350 cm-1 seen in FT-IR spectra are due to stretching vibrations, whereas absorbance at 1650 cm-1 is due to the

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Figure 1. The peak for TiO2 at 2θ ) 25.3 XRD patterns of ETS-10 and metal-ion-exchanged ETS-10 catalysts.

Figure 2. FT-IR spectra of ETS-10 and metal-ion-exchanged ETS-10 catalysts.

bending vibration of water molecules and OH groups present at the surface of the prepared catalysts.35,36 The absorbance at 1136 cm-1 is attributed to asymmetric stretching vibrations of Si-O-Si bridges, while the peak at 1020 cm-1 is assigned to Si-O-Ti stretching vibrations. The stretching and bending vibrations of Ti-O-Ti give rise to the absorption peaks at 750 and 661 cm-1, respectively. The comparison of the IR spectra of pristine ETS-10 and transition-metal-exchanged ETS-10 samples showed that there is an additional peak at around 1386 cm-1 in the IR spectra of transition-metal-exchanged ETS-10 which is because of vibrations of H+ ion.37 The IR absorbance spectra (Figure 2) of the samples demonstrate that the exchange of Na ions by different transition metal cations does not affect the framework and the presence of water molecules at the surface, probably due to the small amount, only 0.5 wt % of metal ion exchange, taken for the study. The presence of transition metal ions in different M-ETS10 samples could be seen from the UV-vis diffuse reflectance spectra (Figure 3). The comparison of diffuse reflectance spectra of pristine and ion-exchanged catalysts showed a change in the band gap due to ion exchange (Table 1). From the data, it is evident that all the ion-exchanged

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Figure 3. Diffuse reflectance UV-vis spectra of ETS-10 and metal-ionexchanged ETS-10 catalysts.

catalysts have extended a red shift, except the silver-doped ETS-10 sample, where slight blue shift of 2 nm was observed. The highest red shift of 3.5 nm was observed with the cobalt metal ion exchange. The surface areas of ion-exchanged catalysts were found to decrease (Table 1) compared to pristine ETS-10. The decrease was observed to vary from 16 m2/g for the Cu-ETS-10 catalyst to a high of 41 m2/g for Fe-ETS-10. This could be due to the pore plugging of the starting material by ion exchange. The SEM micrographs (Supporting Information) of the catalysts show identical morphology of a truncated bipyramidal as reported earlier.18,32 The largest dimension of the crystal is along the 4-fold axis. The EDAX as well as ICP analysis confirms the presence of transition metal ions in these samples. The ratio of Na and K to Ti was observed to decrease after metal ion exchange (Table 1), confirming the metal ion exchange of the Na and K ions. The weight loss observed by TGA analyses varied from 5-13%, with a high of 13% for Fe-ETS-10 and 5% for Ag-ETS-10. Other samples showed similar weight loss values of 7-9%. This weight loss is attributed to loss of moisture present in the samples.18 3.2. Photocatalytic Activity. The spectral changes in absorption spectra of nitrobenzene in Ag-ETS-10 under UV-light irradiation were observed at 267 nm wavelength at which nitrobenzene shows maximum absorbance. The absorbance for different samples at this λmax was observed, and the concentration of the nitrobenzene in each respective sample was calculated from the absorbance using the Beer-Lambert’s law. The percentage degradation of nitrobenzene after 4 h reaction is shown in Figure 4. Comparison of the results from this figure shows that transition-metal-ion-exchanged ETS-10 samples exhibit higher activity than pristine ETS-10. The percentage degradation after 4 h reaction was 33, 41, 38, 39, 47, and 57% for pristine, Fe, Co, Ni, Cu, and Ag ion-exchanged ETS-10, respectively. Moreover, Ag-ETS-10 catalyst showed the highest photocatalytic photodegradation (57%) compared to all other transition-metal-impregnated ETS-10 catalysts. The photocatalytic active sites are titanols located on the external surfaces where the -O-Ti-O-Ti-O- chain emerges, exposing a surface Ti-OH titanol group.38

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Figure 6. Reduction in COD value with different synthesized ETS-10 catalysts.

Figure 4. Percentage degradation of nitrobenzene after 4 h irradiation with different ETS-10 catalysts.

centration due to adsorption in the dark was found in the range of 1.9-7.3%. This decrease was observed to be 7.3, 2.9, 5.8, 1.9, 5.6 and 6.5% for pristine ETS-10, Fe, Co, Ni, Cu, and Ag ion-exchanged ETS-10 samples, respectively. The mineralization of nitrobenzene was determined from COD values of the reaction mixture samples at different time intervals (Figure 6). The COD values also confirm that silver-metal-ionexchanged ETS-10 shows the highest photocatalytic activity. The transition metal incorporated in the pores of ETS-10 could be reduced to M0 by photogenerated electrons in the -O-Ti(IV)-O- semiconductor nanowires under UV-irradiation. The reduction of transition metals by trapping photogenerated e- over TiO2-based photocatalysts is reported earlier. Thus, the transition metal acts as a sink for photogenerated electrons. This trapping of e- by the transition metal ion minimizes the e-/h+ recombination and assists the photocatalytic oxidation of nitrobenzene with the oxidizing species being free h+ or OH• radicals. The contact of a metal with a semiconductor creates a heterojunction at the interface, with the difference in electrochemical potential of both the surfaces set by their position of work function (φm) and Fermi level (EF), respectively. The contact between these two phases leads to the electron flow from the phase with more negative electrochemical potential to the other, until the electrochemical potentials of both phases are in equilibrium, in this case from the semiconductor to the metal ion. As a result, a barrier height (φb) is formed at the interface which stops the back transfer of electron, and the barrier height energy (qφb) is the difference between the equilibrium Fermi level and the energy of the conduction band edge:

Figure 5. First-order kinetic plot for the synthesized catalysts.

Figure 5 shows the ln Co/Ct vs time dependence. The straight line in Figure 5 confirms the pseudo-first-order kinetics which follows the equation ln(Co/Ct) ) kappt

(2)

Here Co is the initial concentration of nitrobenzene, while Ct is the concentration at time “t”. The apparent first rate constant kapp was calculated by the linear regression of the slope of ln Co/ Ct vs time plot up to first 60 min. Thus the photocatalytic degradation process follows the Langmuir-Hinshelwood (L-H) law. The initial rates and apparent rate constants of photodegradation calculated for all catalyzed reactions are shown in Table 2. From initial rate, it can be concluded that M-ETS-10 are better catalysts than ETS-10 as such, and Ag-ETS-10 is observed to be the best catalyst among studied metal-ionexchanged ETS-10 samples. The percentage decrease in con-

φb ) (φm - χS)/q

(3)

where φm is the work function of the metal (in eV) and χS is the electron affinity of the semiconductor.

Table 2. Kinetics of the Nitrobenzene Degradation from Its Aqueous Media during Photocatalysis with ETS-10-Based Catalysts catalyst

initial rate × 106 (mole liter-1 sec-1)

rate constant (k × 103)

R2 value

standard redox potential E0 (V) of metal

ETS-10 Fe-ETS-10 Co-ETS-10 Ni-ETS-10 Cu-ETS-10 Ag-ETS-10

2.0 2.7 2.4 2.3 3.3 3.7

4.9 5.3 3.6 4.8 5.5 4.7

0.98 0.96 0.93 0.97 0.99 0.92

-2.71 +0.33 -0.28 -0.25 +0.68 +0.80

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The conduction band energy level of TiO2 (Ec) is approximately -0.3 eV,39 and thus the -O-Ti(IV)-O- semiconductor unit of ETS-10 is higher than that of the reduction potential of the transition metal as shown in Table 2. This makes it thermodynamically feasible to transfer an e- from the conduction band to metallic particles. As the e-/h+ pair generation occurs in the semiconductor particle by the absorption of UV light, the electron in the conduction band can be trapped by the exchanged transition metal particles due to their lower reduction potential. This trapping of the electron frees the oxidizing species for the photocatalytic oxidation. The Ag-ETS10 catalyst shows the blue shift in band gap energy. This is helpful in our experimental condition, where we used the UV light for e-/h+ pair generation. On the other hand, Na+ and K+ ions in the original ETS-10 are not reduced to Na and K metal by photogenerated e- because the reduction potentials of these ions are much higher (Table 2); this causes the lower photoactivity of ETS-10 toward nitrobenzene degradation. Thus the presence of transition metal ions enhances the photocatalytic activity of the ETS-10, which is a special kind of photocatalyst having photocatalytically active -O-Ti(IV)-O- nanowires in the structure. The photocatalytically active -O-Ti(IV)-O- unit of this material is only 17%. Considering this, the observed results are satisfactory, and further research for the modification in this existing material is needed to enhance its photocatalytic activity. 4. Conclusions Microporous ETS-10 exhibits photocatalytic activity under UV light irradiation due to presence of the photocatalytically active -O-Ti(IV)-O- nanowires in the structure. Such material could be an alternative to the conventional photocatalysts to overcome the problems of TiO2 leaching from their support. The presence of transition metal ions as exchanged cations enhances ETS-10 photocatalytic activity. This could be due to the trapping of photogenerated e- in -O-Ti(IV)-O- nanowires and the transition metal working as a sink for photogenerated electrons. Ag-ETS10 catalyst shows the highest photocatalytic activity among the transition-metal-ion-exchanged ETS-10 catalysts. Acknowledgment The authors are thankful to the Council of Scientific and Industrial Research, New Delhi, for the financial assistance and analytical science discipline of the institute for analytical support. Supporting Information Available: SEM micrographs; TGA results; absorbance spectra; concentration decrease curve. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Fox, M. A.; Doan, K. E.; Dulay, M. T. The effect of the “inert” support on relative photocatalytic activity in the oxidative decomposition of alcohols on irradiated titanium dioxide composites. Res. Chem. Intermed. 1994, 20, 711. (2) Greem, K. J.; Rudham, R. Photocatalytic oxidation of propan-2-ol by semiconductorszeolite composites. Faraday Trans., J. Chem. Soc. 1993, 89, 1867. (3) Mattews, R. W. An adsorption water purifier with in situ photocatalytic regeneration. J. Catal. 1988, 113, 549. (4) Kim, Y.; Yoon, M. TiO2/Y-Zeolite encapsulating intramolecular charge transfer molecules: A new photocatalyst for photoreduction of methyl orange in aqueous medium. J. Mol. Catal., A 2001, 168, 257. (5) Lee, S.-K; Mills, A. Detoxification of water by semiconductor photocatalysis. J. Ind. Eng. Chem. 2004, 2, 173.

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ReceiVed for reView October 14, 2009 ReVised manuscript receiVed February 19, 2010 Accepted March 6, 2010 IE901603K