Enhanced Photocatalytic Activity by Silver Metal Ion Exchanged NaY

Ind. Eng. Chem. Res. 2008, 47, 7545–7551

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Enhanced Photocatalytic Activity by Silver Metal Ion Exchanged NaY Zeolite Photocatalysts for the Degradation of Organic Contaminants and Dyes in Aqueous Medium Rajesh J. Tayade, Praveen K. Surolia, Manoj A. Lazar, and Raksh V. Jasra*,† Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, (Council of Scientific and Industrial Research), G. B. Marg, BhaVnagar-364002, India

Zeolite-based photocatalyst have been prepared using TiO2-coated NaY zeolite by post-synthesis modification with silver metal ion exchange. The characterization of the catalysts was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), Inductively coupled plasma-optical emission spectrophotometry (ICP), Diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FTIR), and N2 adsorption techniques. The presence of metal ions and TiO2 on zeolite surface was observed by UV-visible diffuse reflectance spectroscopy. It was found that the structure of the support materials is retained after the silver metal ion exchange. The photocatalytic activity of the catalysts was studied by degradation of aqueous nitrobenzene, acetophenone, methylene blue, and malachite green in the presence of UV light. The highest photocatalytic activity was obtained for silver metal ion exchanged photocatalysts coated with TiO2 (2%, w/w) as compared to other silver exchanged NaY zeolite catalysts. This study demonstrated that the Ag metal ion exchanged TiO2-coated NaY is a better catalyst as compared to the TiO2-coated NaY photocatalysts. 1. Introduction 1-3

have demonstrated their potential as unique and Zeolites versatile host materials for variety of chemical transformations.4-8 Semiconductor-loaded zeolite and mesoporous materials such as MCM-41 have drawn attention as potential photocatalysts due to their unique pore structure and adsorption properties. Titanium dioxide has proved to be a potential photocatalytic semiconductor as it allows for the complete degradation of pollutants under ultraviolet irradiation. The advantages using zeolite or mesoporous support for titania photocatalysis include formation of ultrafine titania particles during sol-gel deposition; increased adsorption in the pores; surface acidity which enhances electron-abstraction; and decreased UV-light scattering as the main component of zeolite is silica.9-13 Zeolites are microporous materials with framework anions (AlO2-) and exchangeable cations in its structure, and the anion-cation pairs generate strong electrostatic fields13,14 which strongly interact with polar adsorbates. Because of these attributes, zeolites are widely used as catalysts and adsorbents for organic synthesis and decomposition15 as well as in wastegas adsorption and wastewater treatment.16,17 The fascinating properties of zeolites involving transition metal ions within the zeolite frameworks or cavities have opened new possibilities for many applications in catalysis18 and photochemical processes.19,20 The transition metal ion loaded or impregnated photocatalysts using various methods were shown to possess higher photocatalytic activity as compared to the bare photocatalyst due to charge separation.21 These photocatalytic reactions were found to proceed with high efficiency and selectivity, displaying reaction mechanisms different from those observed on semiconducting photocatalysts, in which the photoelectrochemical reaction mechanism or charge separation plays an important role in determining the efficiency. Furthermore, the * To whom correspondence should be addressed. Tel.: +91 265 6693935. Fax: +91 265 6693934. E-mail: [email protected]. † Current address: R&D Centre, VMD, Reliance Industries Limited, Vadodara, 391 346, India.

counter-cations in zeolites can easily be exchanged by other cations by an ion exchange method. The exchangeable sites are separated from each other within the zeolite cavities under wellcontrolled conditions. Zeolites can be modified through the incorporation of active species in the pores by various methods, for instance, ion exchange and impregnation.22,23 A method of supporting TiO2 on zeolite, without losing the photosensitization of TiO2 and the adsorption properties of zeolite, is an important aspect while preparing zeolite-based photocatalysts. Our earlier work has demonstrated the enhanced photocatalytic activity of the TiO2-coated zeolites.24 There is great interest in silver because of its importance in photocatalytic processes and because of its unique catalytic properties.25,26 Therefore, to explore the possibility of further enhancement of the TiO2-coated photocatalysts, we have synthesized silverexchanged zeolite photocatalysts using the metal ion exchange method, and a photocatalytic study of the degradation of organic compounds like nitrobenzene and acetophenone and dyes like methylene blue and malachite was carried out in this work. 2. Experimental Section 2.1. Chemicals. Zeolite NaY with Si/Al ratio 5.5 was procured from Süd Chemie, Vadodara, India. Titanium tetraisopropoxide (97%), was procured from Aldrich. Silver nitrate, AR grade, was procured from Ranbaxy, India. Nitrobenzene (NB), AR grade (99.0%), Methylene blue (MB), and Malachite green (MG) were procured from s. d. Fine Chem. Limited, India. Acetophenone (AP) and COD standard chemical reagents from E. Merck India Ltd., India were used for the photocatalyst preparation. 2.2. Activation of Zeolite. Prior to the coating of TiO2, it is required to remove the water present in the zeolite samples as the presence of water in the zeolite cavities and on an external surface could hydrolyze titanium tetraisopropoxide outside the zeolite cavity which may significantly affect the coating of TiO2 inside the pores of the zeolite. Therefore, NaY zeolite was

10.1021/ie800441c CCC: $40.75  2008 American Chemical Society Published on Web 09/20/2008

7546 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 Table 1. Molecular Structure and Chemical Properties of Substrates

Figure 1. Scheme for preparation TiO2-coated catalysts.

activated by calcinations at 723 K for 4 h in a tubular furnace and then was cooled to room temperature under the flow of nitrogen. 2.3. Synthesis of TiO2-Coated Catalysts. The solution of dry ethanol (100 mL) and an appropriate amount of titanium tetra isopropoxide was taken in a 250 mL round-bottom flask to achieve loading of 1, 2, 4, 10 wt % TiO2 in NaY zeolites. Prior to 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 zeolites. The solvent was slowly removed from the mixture by using rotavapor (Buchi Rotavapor 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 25 mL water to the powder sample and the solution was dried in an oven at 393 K for 12 h. This sample was further calcined under air at 753 K for 11 h. The catalysts thus obtained were designated as NYT1, NYT2, NYT4, and NYT10 for 1, 2, 4, and 10% coating of TiO2 on NaY zeolite. TiO2 powder was also prepared using hydrolysis of titanium tetraisopropoxide with dry ethanol and was used as a reference material throughout this study. 2.4. Post-Modification of TiO2-Coated Catalysts Using Silver. Complete scheme for preparation of TiO2-coated catalysts with post-modification using silver is given in Figure 1. TiO2-coated catalysts were used for post-modification by ion exchange method. The TiO2-coated catalysts were treated with silver nitrate solution at 353 K for 4 h for ion exchange in dark.

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 353 K for 8 h and then calcined at 753 K for 6 h. The catalyst thus obtained was termed by applying suffix Ag to the TiO2coated catalysts. 2.5. UV Irradiation Experiments. The photocatalytic activity of coated catalyst was evaluated by measuring the decrease in concentration of substrates like nitrobenzene, acetophenone, methylene blue, and malachite green from the reaction solution separately. Prior to commencing illumination, a suspension containing 100 mg of the catalyst and 250 mL aqueous solution of ca. 50 ppm of substrate was stirred continuously for 30 min in the dark. Following this, at each interval of 10 min for first one hour and every one hour thereafter, 5 mL of sample was withdrawn by syringe from the irradiated suspension and analyzed for substrates. For analysis, the catalyst was separated by centrifuge from the aqueous solution prior to analysis. The absorption maxima (λmax) of 268, 244, 644, and 628 nm in aqueous solution for nitrobenzene, acetophenone, methylene blue, and malachite green, respectively, were observed. The molecular structure and chemical properties of substrates are given in Table 1. The photocatalytic degradation setup comprised a reactor which was designed and fabricated locally. It consists of a glass vessel where the solution to be degraded was placed. This glass vessel houses 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 125 W mercury vapor lamp. The wavelength of irradiation of mercury vapor lamp was in the range of 200-400 nm. The spectral distribution of subject UV source is described earlier.21,24 The irradiation source was indirectly cooled by circulating water to 293 K during experiments.

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The UV-visible absorbance of aqueous solution of substrate was measured by a UV-vis spectrophotometer Cary 500 (Varian) at their respective λmax (Table 1) equipped with a quartz cell having path length of 1 cm. The spectral absorbance was measured with baseline correction with scan rate of 600 nm min-1 and a data point interval of 1 nm. The concentration of substrates in the solution was determined by using calibration curve of respective substrates (concentration vs absorbance) prepared with their known concentrations. Oxygen equivalent of the organic matter of a sample, i.e., chemical oxygen demand (COD) was measured by using SPECTROQUANT NOVA 60 photometer. The reagents for COD analysis and 3 mL of sample taken at different time were mixed together in glass cells and digested in Spectroquant TR 320 thermodigester for two hours 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 interval. 2.6. Catalysts Characterization. X-ray powder diffraction studies were carried out at ambient temperature by Philips X’pert MPD system in the 2θ range 10-60 degrees using Cu KR1 (λ ) 1.54056Å). Specific surface area of calcined coated photocatalyst samples was determined from N2 adsorption-desorption isotherms at 77.4 K by ASAP 2010, Micromeritics, U.S.A. Surface area was determined using BET method.27 The presence of nanocrystalline TiO2 particles in zeolite was confirmed using the UV-visible diffuse reflectance spectroscopy (DRS) at room temperature in the range of 250-600 nm by Shimadzu UV-3101PC spectrophotometer with an integrating sphere.21,28,29 All spectra were recorded with respect to BaSO4 as reference. The FT-IR spectroscopic studies were carried out using Perkin-Elmer GX spectrophotometer to see the presence of adsorbed substrates on the catalysts surface before and after photocatalytic experiments. The spectra were recorded in the range 400-4000 cm-1 with a resolution of 4 cm-1 as KBr pellets. An inductively coupled plasma-optical emission spectrophotometer (Optima2000 DV, Perkin-Elmer, Eden Prarie, MN) was used to determine the percentage leaching of Ti and silver metal ion present in the degraded solution after performing photocatalytic experiments. The electron microscopic study has been done with scanning electron microscope (Leo series 1430 VP) to see the morphology of catalysts, Energy to see the morphology of catalysts. The sample powder was supported on aluminum stubs after dispersion of catalysts in acetone and then coated with gold by plasma prior to measurement. 3. Results and Discussions 3.1. X-ray Powder Diffraction. The diffraction patterns of the NaY zeolite showed that it is highly crystalline in the range 10-60 degrees typical of zeolites and crystallinity is observed to be retained on sodium ion exchange with silver.24 There was no peak obtained after ion exchange because of low amount of silver ion exchange. XRD pattern of the post-modified TiO2coated NaY catalysts are shown in Figure 2. 3.2. Surface Area Analysis (N2 Adsorption). The surface area for bare NaY zeolite was 737 m2g-1 while it was reduced to 680, 675, 660, and 647 m2g-1 for NYT1, NYT2, NYT4, and NYT10 catalysts, respectively, because of doping of TiO2. The surface area of NaY zeolite was observed reduced after

Figure 2. X-ray diffraction pattern of bare TiO2, NaY coated, and postmodified catalysts using silver metal ion exchange (A: anatase).

Figure 3. DRS spectra of Bare TiO2, NaY Zeolite, and post-modified catalysts using silver.

coating of TiO2, which shows the presence of TiO2 in the zeolite cage or at the surface of zeolite. The post-modified catalysts showed the reduced surface area due to the removal of Na ions with replacement of silver ion. The surface area attributed to the silver modified catalysts was observed as 668, 650, 627, and 620 m2g-1 for NYT1-Ag, NYT2-Ag, NYT4-Ag, and NYT10-Ag, respectively. 3.3. Diffuse Reflectance Spectroscopy. The diffuse reflectance spectra of bulk TiO2, NaY and post-modified TiO2-coated NaY are shown in Figure 3. 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.30,31 On other hand,

7548 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008

Figure 4. SEM images of the catalysts (a) NaY-5.5, (b) NYT2, (c) NYT2-Ag, (d) TiO2 (bare).

TiO2-coated NaY catalysts exhibit absorption bands in the wavelength region of 330-370 nm. It was observed that the absorption intensity in the range of 250-350 nm increases with increase in the content of TiO2 in both the TiO2-coated and post-modified NaY catalysts. This indicates the increase in the response of the prepared catalysts toward the UV light. This may help to enhance the photocatalytic activity of these catalysts in presence of UV light. The band edge position could not be determined in TiO2-coated photocatalyst from the spectra after taking the differential of the DRS spectra due to the presence of small amount of TiO2 in the catalyst. 3.4. Scanning Electron Microscopy. The morphology of the synthesized catalysts remained unaltered after post-modification as shown in Figure 4. The particle size of silver exchanged zeolite catalysts was in the range of ca. 0.3-0.6 µm and had a hexagonal shape. The particle size of the parallel synthesized bare TiO2 was ca. 300 nm. It was further observed that there is some agglomeration in the zeolite catalysts with increase in the TiO2 amount but no such observation was made after silver metal ion exchange. The results of the energy dispersive system (EDX) are tabulated in Table 2, and they show the presence of silver metal ions in the post-modified catalysts. 3.5. Inductively Coupled Plasma-Optical Emission Spectrophotometry and Energy Dispersive X-ray Analysis. To study the leaching of Ti and silver metal ion, the percentage amount of Ti and silver metal ion was determined in the degraded reaction mixture. The results demonstrated that there was no leaching of Ti or silver metal ion after performing the photocatalytic experiments for 4 h. 3.6. Adsorption of Substrate on Catalysts Surface. To study the adsorption of substrate on the catalyst surface, catalysts

Table 2. Elemental Analysis Results by EDX percentage of metal ion (weight %) catalysts

Na

Ag

Si

Ti

O

NYT1-Ag NYT2-Ag NYT4-Ag NYT10-Ag

5.02 4.94 5.13 5.20

1.12 1.10 1.15 1.06

29.44 28.74 28.68 27.41

0.56 1.25 2.83 5.84

54.74 55.06 56.54 69.74

were first dried at 398 K for overnight to remove the moisture. These dried catalysts were added to substrates solution and kept under dark for 30 min. The amount of zeolite and volume of substrate solutions were kept same as these were used in the irradiation experiments to determine the adsorption of substrates on zeolite and synthesized catalysts. 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. The adsorption on the surface of catalysts was in the range of 21-26% for nitrobenzene, 8-11% for acetophenone, 26-28% for methylene blue and 16-18% for malachite green in the case of silver-modified catalysts. There was not much difference observed in TiO2-coated and postmodified catalysts adsorption values. 3.7. Photocatalytic Activity. Silver loaded TiO2 catalysts studied by various methods have exhibited good photocatalytic activity, and the TiO2-coated or encapsulated zeolite also demonstrated higher photocatalytic activity.9,15,21,32 The UVvisible absorbance spectrum of reaction mixture containing substrate solution, and catalyst with respect to irradiation time was recorded after separating the catalyst by centrifugation to determine the concentration of substrate. The trends of UVvisible spectra of reaction mixture taken at different time interval

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Figure 5. Photocatalytic degradation of (A) nitrobenzene, (B) acetophenone, (C) methylene blue, (D) malachite green using NYT2-Ag.

were the same using all the TiO2-coated photocatalysts and postmodified catalysts. The decrease in absorbance at λmax of the respective substrates (Table 1) was observed with respect to irradiation time. There was no additional peak observed during the degradation of any substrate. Figure 5 shows UV-visible spectra of reaction mixture taken at different time interval of the degraded substrate solution using post-modified catalyst (NYT2-Ag). The initial rates of degradation of substrates are shown in Figure 6 which indicates that the initial rates using the silver metal exchanged catalysts are higher than those of the TiO2-coated NaY catalysts for all substrates. The highest initial rates of degradation were observed using the NYT2-Ag catalysts for all the substrates which could indicate that these catalysts have the sufficient silver metal ion interactions with the TiO2 present in the catalysts for enhancement of the photocatalytic activity. The higher activity of this catalyst as compared to other Ag exchanged catalyst is due to the optimum amount of TiO2. Higher loading of TiO2 may cause the pore blocking of zeolite, resulting the decrease in adsorption capacity. Again the activity of NYT2-Ag is higher than NYT-2, it is due to the presence of silver metal ions which can trap the electrons and leave the holes behind to oxidize the organic materials. Thus, the presence of silver ions prevents the recombination and enhances the photocatalytic activity. It was further observed that the initial rates of degradation of substrates using all the catalysts were in the order of AP > NB > MB > MG. There was no purging of oxygen employed during the photocatalytic degradation experiment.

Figure 6. Initial rate of degradation of substrates using TiO2-coated and post-modified catalysts.

The degradation of substrates was observed to be 55-60% for nitrobenzene and 88-90% for acetophenone, while complete degradation of dyes took place in 2-3 h for both the methylene blue and malachite green using the post-modified catalysts. The degradation using only TiO2-coated catalysts was found to

7550 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 Table 4. Reduction in COD Values Using Silver Modified TiO2-Coated NaY Catalysts

Table 3. Reduction in COD Values Using TiO2-Coated NaY Catalysts COD of reaction mixture with respect to irradiation time using TiO2-coated NaY catalysts (mg/L)

COD of reaction mixture with respect to irradiation time using silver modified TiO2-coated NaY catalysts (mg/L)

irradiation time (min)

irradiation time (min)

catalysts

substrate

0

30

60

120

180

240

catalysts

substrate

0

30

60

120

180

240

NYT1

NB AP MB MG NB AP MB MG NB AP MB MG NB AP MB MG

94 120 36 72 94 120 63 72 94 120 63 72 94 120 63 72

60 69 34 26 57 65 29 21 68 66 34 24 65 69 30 25

49 44 19 18 48 45 17 12 56 43 21 17 57 53 20 16

40 38 06 10 42 39 04 08 45 35 15 10 52 42 14 14

32 30 00 06 40 31 00 00 36 28 08 06 47 33 06 07

29 20 00 00 38 28 00 00 28 20 00 00 40 22 00 00

NYT1-Ag

NB AP MB MG NB AP MB MG NB AP MB MG NB AP MB MG

94 120 63 74 94 120 63 74 94 120 63 74 94 120 63 74

54 54 36 30 52 52 23 27 54 55 28 35 57 57 32 28

42 36 19 20 40 35 13 15 48 39 19 20 52 39 21 16

36 28 07 09 32 26 07 10 45 31 09 14 44 32 11 10

28 24 00 00 24 21 00 00 40 26 00 12 39 27 00 00

18 16 00 00 10 08 00 00 34 20 00 00 36 20 00 00

NYT2

NYT4

NYT10

be to 40-50% for nitrobenzene and 70-85% for acetophenone, whereas complete degradation of dyes took place in 2-3 h. However, the initial rate of degradation was higher with silverexchanged catalysts. Our previous results showed that the photocatalytic activity of the TiO2-coated zeolite catalysts was better than the bare TiO2 for the degradation of methylene blue. There was little decrease in concentration of MB due to the adsorption but no photocatalytic degradation using the bare zeolite.24 The initial rates of degradation of substrates are shown in Figure 6. It is evident from Figure 6 that the highest initial rate of degradation was obtained for the NYT2-Ag catalysts as compared to all the other catalysts. The FT-IR spectra of the catalyst after adsorption of substrates and after performing reaction for 4 h were recorded (not shown). The peaks corresponding to the substrates were not seen in the substrate-adsorbed catalysts except for methylene blue and malachite green. The absorption peaks corresponding to a wavelength of 1396 and 1339 cm-1 were observed in pure methylene blue as well as in methylene-blue-absorbed catalysts (30 min, in dark), while these were not seen in the used catalyst. In the case of malachite green, peak at 1368 cm-1 was observed in adsorbed catalyst, whereas the same peak disappeared in the used catalysts. A slight gray color was observed after the photocatalytic reaction in the case of nitrobenzene, acetophenone. In the case of methylene blue and malachite green, the color of used catalysts was light blue and light green, respectively. In our earlier studies, we could not detect the abovementioned peaks corresponding to the methylene blue dye after a photocatalytic degradation experiment of 4 h using TiO2coated NaY and HY zeolite catalysts.24 3.8. Chemical Oxygen Demand (COD). To confirm the complete mineralization of substrates, the chemical oxygen demand (COD) was determined and data are given in Tables 3 and 4. The reduction in COD values using TiO2-coated NaY catalysts (Table 3) were lower than that of silver-modified catalysts (Table 4) for all the substrates. The decrease in COD values confirms the results obtained from UV-visible analysis. In the case of TiO2-coated NaY catalysts, COD values after 4 hours of irradiation time for acetophenone and nitrobenzene was in the range of 20-28 and 28-40 mgL-1, respectively. Whereas in the case of silver-exchanged catalysts, COD values after 4 hours of irradiation time for acetophenone and nitrobenzene were in the range of 8-20 and 10-36 mgL-1, respectively. In

NYT2-Ag

NYT4-Ag

NYT10-Ag

the case of dyes, the complete mineralization was obtained in 3-4 hours in the case of TiO2-coated NaY catalysts, whereas in the case of silver-modified catalysts it was obtained in 3 h. Among all synthesized photocatalysts NYT2-Ag has exhibited higher decrease in COD values for all the substrates. 4. Conclusions Silver metal ion exchanged TiO2-coated NaY zeolite demonstrated the higher photocatalytic activity than simply TiO2coated zeolite catalysts. This is attributed to the changes in the electronic structure of the TiO2-coated catalysts due to the presence of silver ion. The results demonstrated that the zeolitebased photocatalysts can degrade the dyes as well as nitroaromatic compounds like nitrobenzene. The zeolites with highly dispersed TiO2 with silver metal ions in their structure or on the TiO2 surface are playing major role in the degradation of substrates. Further investigations like photoluminescence, XPS, NMR, EPR study are under consideration and will appear in separate publication. Acknowledgment We are thankful to Council of Scientific and Industrial Research, New Delhi and Dr. P. K. Ghosh, Director of CSMCRI for the financial assistance and support. We also thankful to Mr. Chandrakanth C. K., Dr. Pragnya Bhatt, Mr. Vinod Agrawal, and Mr. Sunil A.P. for analytical support. Literature Cited (1) Breck D. W. Zeolite Molecular SieVes: Structure, Chemistry and Use; John Wiley and Sons: New York, 1974. (2) Introduction to Zeolite Science and Practice; Bekkum H. V, Flanigen E. M., Jansen J. C., Eds.; Elsevier Science Ltd.: Amsterdam, 1991. (3) Dyer A. an Introduction to Zeolite Molecular SieVes; John Wiley and Sons: Bath, 1988. (4) Scaiano, J. C.; GarcïaH. Intra-Zeolite Photochemistry, Toward Supramolecular Control of Molecular Photochemistry. Acc. Chem. Res. 1999, 32, 783–793. (5) Turro, N. J. from Boiling Stones to Smart Crystals: Supramolecular and Magnetic Isotope Control of Radical-Radical Reactions in Zeolites. Acc. Chem. Res. 2000, 33, 637–646. (6) Yoon, K. B. Electron- And Charge-Transfer Reactions within Zeolites. Chem. ReV. 1993, 93, 321–339. (7) Thomas, J. K. Physical Aspects of Photochemistry and Radiation Chemistry of Molecules Adsorbed on Silica, Gamma-Alumina, Zeolites, and Clays. Chem. ReV. 1993, 93, 301–320.

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ReceiVed for reView March 18, 2008 ReVised manuscript receiVed July 4, 2008 Accepted July 20, 2008 IE800441C