Enhanced Photocatalytic Activity of TiO2-Coated NaY and HY Zeolites

Dec 21, 2006 - Photocatalytic degradation of p -nitrotoluene (PNT) using TiO 2 -modified silver-exchanged NaY zeolite: kinetic study and identificatio...
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Ind. Eng. Chem. Res. 2007, 46, 369-376

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APPLIED CHEMISTRY Enhanced Photocatalytic Activity of TiO2-Coated NaY and HY Zeolites for the Degradation of Methylene Blue in Water Rajesh J. Tayade,† Ramchandra G. Kulkarni,‡ and Raksh V. Jasra*,† Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, Gijubhai Badheka Marg, BhaVnagar 364002, India, and Department of Physics, Saurashtra UniVersity, Rajkot 360005, India

The TiO2-coated zeolite photocatalysts were prepared by dispersing zeolite powders in dilute titanium tetraisopropoxide solution. The characterization of the catalysts was carried out by X-ray diffraction, scanning electron microscopy, and N2 adsorption. The presence of TiO2 on zeolite surface was confirmed by UVvisible diffuse reflectance spectroscopy. The photocatalytic activity of TiO2-coated NaY and HY zeolite was investigated by degradation of aqueous solution of methylene blue dye. The highest photocatalytic activity was obtained with 1% TiO2-coated zeolite catalysts. This study demonstrated that the photocatalytic activity of TiO2-coated catalyst is higher than that of bare TiO2 at a low amount of TiO2 coating on the zeolite surface. 1. Introduction Semiconductor TiO2 is one of the most widely studied photocatalysts because it is biologically and chemically inert and is photostable with near-ultraviolet band gap energy. The various factors influencing the activity of a photocatalyst include smaller band gap and electron-hole pair recombination of a photocatalyst. Both these factors are influenced by the particle size of the photocatalyst. For example, the recombination rate of the electron-hole pair is higher in bulk or large size semiconductor photocatalyst particles. Therefore, attempts are made to prepare the photocatalyst with finer particle size distribution. However, fine particle sized photocatalyst dispersed in water to be treated by irradiation poses difficulties as separation of the catalyst by post-reaction filtration is tedious and costly. As a result, many researchers have examined methods for dispersing TiO2 particles on solid support materials like glass,1,2 optical fiber,2,3 silica,4 electrode,5,6 clays,7,8 stainless steel mesh,9 and zeolites.11-13 Zeolites due to their large surface areas, internal pore volume, unique uniform pores, and channel size12-18 are interesting hosts to disperse semiconductor photocatalyst on their surfaces. Surface areas in the range of 400650 m2 g-1 with pore volumes of above 0.1 cm3 g-1 are common for conventional zeolites. Zeolites exhibit several other specific features14,19,20 that make them suitable for their use as hosts for photocatalysts, such as the following: (i) full photochemical stability and large thermal and chemical inertness; (ii) transparency to UV-visible radiation above 240 nm, thus allowing a certain penetration of the exciting light into the solid opaque powder to reach the substrate molecules located in intraparticle positions; (iii) zeolite’s high adsorption for organic compounds from solution concentrating the substrate molecule in the proximity of active sites of the photocatalyst; * To whom correspondence should be addressed. Tel.: +91 278 2471793. Fax: +91 278 2567562. E-mail: [email protected]. † Central Salt and Marine Chemicals Research Institute. ‡ Saurashtra University.

(iv) the polarizing strength inside the zeolite pores by varying the nature of internal charge balancing cations and the size of the channels; (v) the ability of the zeolite framework to participate actively in electron-transfer processes, either as electron acceptor or electron donor.14 For example, upon photoexcitation, an encapsulated molecule can eject an electron that will become delocalized through the framework or in clusters of the charge balancing cations. Also, there are reverse processes in which electron-rich sites of the zeolite can donate an electron to a photoexcited molecule.19-20 In the present paper, we report the simple route to synthesize TiO2-coated NaY and HY zeolite catalysts to develop a photocatalyst with enhanced photocatalytic activity and facile separation from the reaction products. The degradation of methylene blue dye was carried out with TiO2-coated catalysts and compared with bare TiO2 prepared using a similar synthetic procedure. 2. Experimental Section 2.1. Chemicals. Zeolite NaY with Si/Al ratio of 5.5 from United Catalysis India Ltd., Vadodara; titanium tetraisopropoxide (97%) from Sigma Aldrich India, Mumbai; methylene blue (MB) and chemical oxygen demand (COD) standard chemical reagents from E. Merck India Ltd., Mumbai; were used for the photocatalyst preparation and photocatalytic studies. Double distilled water was used in all the experiments. 2.2. Preparation of H-Zeolite. The sodium form of zeolite was converted to the hydrogen form by ammonium ion exchange method. The zeolite was treated with 1 M ammonium nitrate solution at 353 K for 4 h; the zeolite to ammonium nitrate solution ratio was 1:80. The residue was filtered and washed with hot distilled water. The above cycle was repeated three times to get complete exchange of sodium. The sample was dried at a temperature of 353 K for 8 h and then heated at 823 K for 6 h to decompose to H-zeolite and ammonia. 2.3. Activation of Zeolite. The presence of water in the zeolite prior to addition of titanium tetraisopropoxide signifi-

10.1021/ie060641o CCC: $37.00 © 2007 American Chemical Society Published on Web 12/21/2006

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Scheme 1 . Schematic Diagram of Preparation of TiO2-Coated Zeolites

from the characteristic peak of 2θ ) 25.3° (101) for the anatase phase using the Scherrer formula, with a shape factor22 (K) of 0.9:

crystallite size ) Kλ /W cos θ

cantly affects the coating of TiO2 as water may hydrolyze titanium tetraisopropoxide to give TiO2 precipitates during titanium tetraisopropoxide addition, resulting in nonuniform and coarse TiO2 coating. Therefore, prior to addition of titanium tetraisopropoxide, activation of the zeolite was carried out for removal of water from zeolite cavities. The NaY and HY zeolites were calcined at 723 K for 4 h in a tubular furnace and then were cooled to room temperature under the flow of nitrogen. 2.4. Preparation of TiO2-Coated Zeolite Catalysts. The procedure for preparation of TiO2-coated catalysts is given in Scheme 1. The solution of dry ethanol (100 mL) and an equivalent amount of titanium tetraisopropoxide was taken in a 250 mL round-bottom flask to achieve 1, 2, 4, and 10% TiO2 coating in both NaY and HY zeolites. Prior to the TiO2 coating, this mixture was continuously stirred for 30 min and then ultrasonicated for 5 min. The activated zeolites were added to the above solution and stirred for 2 h for penetration of solution inside the cavities of the zeolites. The solvent was slowly removed from the mixture by using rotavapour (Buchi Rotavapour R-205) at 343 K. The powder sample was then dried in an oven at 393 K for 12 h. The hydrolysis of TiO2 was done by adding 100 mL of water to the powder sample, and the solution was dried in an oven at 393 K for 12 h. Thus obtained samples were further calcined to remove isopropyl alcohol produced during hydrolysis as well as unused water present in the cavities under air at 753 K for 11 h. As we have observed in our earlier study21 that the bare TiO2 gets converted from amorphous to anatase phase at this temperature and it showed higher photocatalytic activity as compared to the standard P25 Degussa photocatalyst. The catalysts thus obtained were designated as NYT1, NYT2, NYT4, and NYT10 for 1, 2, 4, and 10% coating of TiO2, respectively, on NaY zeolite and HYT1, HYT2, HY4, and HYT10 for 1, 2, 4, and 10% coating of TiO2, respectively, on HY zeolite. TiO2 powder was also prepared from titanium tetraisopropoxide using a similar synthetic procedure without zeolite addition and was used as a reference sample referred to as bare TiO2. 2.5. Catalyst Characterization. X-ray powder diffraction (XRD) studies were carried out at ambient temperature using Philips X’pert MPD System in the 2θ range of 10-60° using Cu KR1 (λ ) 1.540 56 Å). The diffraction pattern was measured in 2θ ranging from 10 to 60° at a scan speed of 0.1° s-1 for all the catalysts. The crystallite size of bare TiO2 was determined

(1)

Here, W ) Wb - Ws, Wb is the broadened profile width of the experimental sample, Ws is the standard profile width of the reference silicon sample, and λ is the wavelength of X-ray radiation (Cu KR1). Specific surface area, pore volume, and pore size distributions of calcined TiO2-coated photocatalyst samples were determined from N2 adsorption-desorption isotherms at 77 K using volumetric adsorption equipment (ASAP 2010, Micromeritics, USA). The surface area was determined using the BET equation.23 The FT-IR spectroscopic studies were carried out using a Perkin-Elmer GX spectrophotometer. The spectra were recorded in the range of 400-4000 cm-1 with a resolution of 4 cm-1 as KBr pellets. The TiO2 coating of various amounts of the nanocrystalline TiO2 particles in zeolite were confirmed using the UV-vis diffuse reflectance spectroscopy (DRS) at room temperature in the range of 250-600 nm by Shimadzu UV-3101PC spectrophotometer with an integrating sphere.24-26 All spectra were recorded with respect to BaSO4 as reference. The morphological study was done with a scanning electron microscope (SEM; Leo series 1430 VP) equipped with EDX to see the morphology of catalysts. The sample powder was supported on aluminum stubs using silver paint and then coated with gold by plasma prior to measurement. 2.6. Photocatalytic Degradation. The photocatalytic activity of TiO2-coated catalyst was evaluated by measuring the decrease in concentration of methylene blue dye in the reaction solution. Prior to commencing illumination, a suspension containing 100 mg of the catalyst and 250 mL of an aqueous solution of ca. 50 ppm of methylene blue was stirred continuously for 30 min in the dark. After the irradiation, the catalyst was separated by centrifugation from the aqueous solution prior to analysis. A 5 mL aliquot of sample was withdrawn by syringe from the irradiated solution at an interval of 10 min for the first 1 h and every 1 h thereafter. The photocatalytic degradation setup comprised a reactor which was designed and fabricated locally. It consisted of a glass vessel wherein the solution to be degraded was placed. This glass vessel housed a double wall quartz vessel to hold the irradiation source. This double wall jacketed quartz vessel had two lines, one for inlet and another for outlet of water for cooling the irradiation source. The irradiation source was a 125 W mercury vapor lamp (high-pressure mercury vapor lamp, Crompton Greaves Ltd., India). The spectral response of the subject UV source is as shown in Figure 1. The irradiation source was indirectly cooled by circulating water to 303 K during experiments. 2.7. Chemical Analysis and COD Measurements. The concentration of methylene blue dye in the aqueous solution was measured at λmax ) 664 with a Carry-500 UV-visible spectrophotometer (Varian, Palo Alto, CA ) equipped with a quartz cell having a path length of 1 cm. The spectral absorbance was measured with baseline correction at a scan rate of 600 nm min-1 and a data-point interval of 1 nm. The concentration of methylene blue in the solution was determined using a calibration curve of methylene blue (concentration vs absorbance) obtained with known concentrations. The oxygen equivalent of the organic matter of a sample, i.e., chemical oxygen demand, was measured by using a

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Figure 1. Spectral distribution of the UV source.

Figure 3. X-ray diffraction patterns of NaY zeolite and various amounts of TiO2-coated NaY zeolite and bare TiO2.

Figure 2. X-ray diffraction pattern of synthesized bare TiO2 after calcination at 753 K.

SPECTROQUANT NOVA 60 Photometer. The reagents for COD analysis and 3 mL of samples taken at different time intervals during photocatalytic reaction mixed together in glass cells and digested in a Spectroquant TR 320 thermodigester for 2 h at 421 K. After digestion, the mixture was cooled to room temperature and the COD was measured using the photometer. The COD was measured for the original solution and the centrifuged sample taken out at different time intervals. 3. Results and Discussions 3.1. X-ray Powder Diffraction. The reference TiO2 prepared was amorphous in nature after drying and got converted to nanocrystalline form having predominantly anatase phase after calcining at 753 K, as seen from Figure 2. The XRD pattern also shows the synthesized bare TiO2 is highly crystalline in nature with a crystallite size of 14 nm. The diffraction patterns of the NaY and HY zeolite showed that it is highly crystalline, exhibiting the reflections at 2θ values of 10, 12, 15, 18, 20, 23, 31, and 34 typical to zeolite of zeolite Y. The TiO2-coated catalysts exhibited crystallinity similar to those of NaY and HY zeolites, and the structure of zeolite remains unaltered, showing that TiO2 coating does not affect the structure of the zeolite. X-ray diffraction of TiO2 samples did not show a peak corresponding to TiO2. This could be due to a low amount of TiO2 loading.12,27 Xu and Langford15 also could observe peak

Figure 4. X-ray diffraction patterns of HY zeolite and various amounts of TiO2-coated HY zeolite and bare TiO2.

broadening only above 14.6% TiO2 loading in ZSM5 and zeolite-A, whereas they did not observed any peak corresponding to anatase when they recorded X-ray diffraction of the

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Figure 5. DRS spectra of TiO2-coated NaY zeolite catalyts, NaY zeolite, and bare TiO2.

Figure 6. DRS spectra of TiO2-coated HY zeolite catalysts, HY zeolite, and bare TiO2.

mechanical mixture of 56% TiO2 P25 and zeolite. Chen et al. did not observe the TiO2 peaks by ion exchange method when the amount of TiO2 was below 83 mg in each gram of mixture of TiO2 and zeolite.28 This shows that the detection of the TiO2 peak may depend upon the amount of TiO2 in the zeolite. X-ray diffraction patterns for the bare TiO2, NaY zeolite, and TiO2coated NaY samples are shown in Figure 3. In NaY zeolite, the intensity of peak at 2θ ) 11.98 was found to slightly increase with the intensities of the peaks corresponding to 2θ values of 20.47, 23.66, 30.89, 34.25, 54.11, and 59.43, showing slight decrease (Figure 3) on TiO2 coating while there was no change observed in the rest of the peaks. In HY zeolites the peak intensities of all peaks showed small reduction with an increase in the amount of coated TiO2 (Figure 4). 3.2. Surface Area Analysis. The BET surface areas of the NaY zeolite and TiO2-coated samples NYT1, NYT2, NYT4, and NYT10 were observed to be 698, 678, 670, 665, and 661 m2/g, respectively, showing reduction with TiO2 coating. After sodium ion exchange with H+ ions the surface area was found to increase to 754 m2 g-1 due to replacement of sodium cations with small-sized protons. The surface area of synthesized bare TiO2 was observed to be 124 m2 g-1. Surface area values of

the TiO2-coated HY samples HYT1, HYT2, HYT4, and HYT10 were found to be 740, 735, 731, and 622, respectively, showing reduction in the surface area on TiO2 coating. 3.3. Diffuse Reflectance Spectroscopy. The diffuse reflectance spectra of synthesized pure TiO2, NaY, and TiO2-coated NaY catalysts are shown in Figure 5. A significant blue shift in the spectrum was observed for all catalysts as compared to bulk TiO2. The shift in the absorption bands of TiO2 can be attributed to the size quantization effects due to the presence of extremely small TiO2 particles and/or the presence of highly unsaturated TiO2 species having tetrahedral coordination.12,27 On the other hand TiO2-coated NaY and HY catalysts exhibit absorption bands in the wavelength region of 330-370 nm. It was observed that the absorption intensity increases with an increase in the amount of TiO2 in the catalysts. The DRS spectra of HY and TiO2-coated HY catalysts are shown in Figure 6. The band edge and band gap of the synthesized TiO2 were attributed to 400 nm and 3.04 eV, respectively. The band edge position could not be determined in TiO2-coated photocatalyst from the spectra after taking the differential of the DRS spectra. This may be due to the encapsulation, the presence of a small amount of TiO2 in the catalyst. 3.4. SEM. Micrographs of the starting zeolite and TiO2-coated zeolite samples do not show (Figure 7) significant change in the morphology of the catalysts before and after TiO2 coating. TiO2-coated zeolite catalyst particles were of ca. 0.5-0.8 µm size for all the samples with some agglomeration being observed with an increase in the TiO2 amount. 3.5. Adsorption of Methylene Blue on Catalysts Surface. The adsorption of methylene blue on TiO2, bare zeolite, and TiO2-coated photocatalysts was determined by spectrophotometer. The amount of zeolite and the volume of methylene blue solution were kept the same as were used in the irradiation experiments to determine the adsorption of methylene blue on zeolite and catalysts. The catalyst was dried overnight at 398 K to remove the moisture. This dried catalyst was added to the methylene blue dye solution and kept under dark for 30 min. After this 5 mL of solution was withdrawn by syringe and centrifuged to separate the catalyst, and the concentration of substrate was determined by spectrophotometer. On bare TiO2, adsorption of methylene blue was observed to be 4 wt %. However, for NaY and HY zeolites adsorption of methylene blue was found to be 26 and 40 wt %, respectively. The experimental results showed a slight reduction in methylene blue adsorption after coating with TiO2. For example, it was in the range of 23-25 wt % for TiO2-coated NaY and 32-39 wt % for TiO2-coated HY catalyst. Adsorption of organic compounds in zeolites occur through van der Waals interactions with lattice oxygens of zeolites, electrostatic interactions of organic compounds with extraframework cations present in the zeolites, and π-π interactions of unsaturated bonds of organic compounds with transition metal cations if present as extraframework cations. In the present study, we have only used zeolite having protons or sodium as cations; the possibility of π-π interactions is not there. However, a mechanism other than π-π interactions is responsible for the adsorption. It must be noted that adsorption of organic compounds does help in enhancing the photocatalytic activity of the coated TiO2 as it brings the organic molecules in proximity to phototactive sites. 3.6. Photocatalytic Activity. To study the photocatalytic activity of the TiO2-coated photocatalysts, degradation of methylene blue was carried out under irradiation of UV light. The reaction temperature was maintained at 303 K. The UVvisible absorbance spectrum of the reaction mixture containing

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Figure 8. UV-vis absorption spectra of degradation of methylene blue using NYT2 catalyst.

Figure 9. UV-vis absorption spectra of degradation of methylene blue using HYT2 catalyst. Figure 7. SEM images of TiO2-coated zeolite photocatalysts.

substrate solution and catalyst with respect to irradiation time was recorded to determine the concentration of the substrate after separating the catalyst by centrifugation. The trends of UV-visible spectra of the reaction mixture taken at different time intervals were the same using all the TiO2-coated photocatalysts and bare TiO2 catalysts. The decreases in the peaks at 664, 610, 293, and 246 nm were observed with respect to irradiation time. No additional peak was observed during the degradation of methylene blue. Figures 8 and 9 show UVvisible spectra of the reaction mixture taken at different time intervals of the degradation of methylene blue using TiO2-coated NaY and HY catalysts (NYT2, HYT2). There was no purging of oxygen during the photocatalytic degradation experiment. Figure 10 shows the degradation of methylene blue using bare TiO2 catalysts as well as without catalyst (Blank) under the irradiation of UV light. The degradation of methylene blue without catalyst was found to be 57%, while it was between 75 and 82% using synthesized bare TiO2 catalysts in 4 h irradiation of UV light. The highest degradation (82%) was obtained with 10 mg of the bare catalysts. Complete degradation of methylene blue was not obtained with bare TiO2 catalyst in 4 h irradiation time.

The photocatalytic activities of TiO2-coated catalysts are shown in Figures 11 and 12 using TiO2-coated NaY and HY catalysts, respectively. From the above experiments, it is evident that although adsorption of methylene blue occurs on bare zeolite, no appreciable degradation of methylene blue is observed. This indicates that the TiO2 coating in zeolites enhances the degradation of methylene blue probably due to generation of an electron-hole pair on absorption of ultraviolet light. The photocatalytic activity of the TiO2-coated HY catalyst was slightly higher than that of TiO2-coated NaY catalysts. This could be due to higher surface area and higher adsorption capacity of HY (32-39%) catalysts than NaY (23-25%) catalysts coated with TiO2. The complete degradation of the methylene blue dye was obtained using TiO2-coated catalyst within 2-3 h. Initially, the methylene blue solution having pH 4.7 that increased to pH 5.7 after addition of the catalyst and decreased to pH 4.9 for the reaction mixture after irradiation of UV light for 4 h in the case of TiO2-coated NaY photocatalyst. Similarly, after addition of TiO2-coated HY photocatalysts the pH increased to 6.7 and then reduced to pH 4.9 after irradiation of UV light for 4 h. The catalysts containing a lower amount of TiO2 showed higher photocatalytic activity. Particularly in case of TiO2-coated NaY and HY photocatalysts with 1% TiO2

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Figure 10. Degradation of methylene blue using different amounts of synthesized TiO2 under irradiation of UV light.

Figure 12. Degradation of methylene blue using HY and TiO2-coated HY catalysts under irradiation of UV light.

Table 1. Reduction in COD Values, with Respect to Irradiation Time, of an Aqueous Solution of Methylene Blue Using Different Amounts of Bare TiO2 Catalyst COD value (mg/L) at given TiO2 weight

Figure 11. Degradation of methylene blue using NaY and TiO2-coated NaY catalysts under irradiation of UV light.

methylene blue was completely degraded within 2 h. This might be due to fine dispersion of TiO2 in the zeolite cages as compared to a higher amount of TiO2 coating. Similar results were reported by Xu and Langford,15 wherein at low (