Microwave Synthesis and Photocatalytic Activity of Nano Lanthanide

Aug 6, 2008 - Lanthanide orthovanadates, CeVO4, PrVO4, and NdVO4, of nanodimensions were synthesized under microwave exposure, and the ...
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Ind. Eng. Chem. Res. 2008, 47, 6509–6516

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Microwave Synthesis and Photocatalytic Activity of Nano Lanthanide (Ce, Pr, and Nd) Orthovanadates Sudarshan Mahapatra,† Susanta K. Nayak,† Giridhar Madras,‡ and T. N. Guru Row*,† Solid State and Structural Chemistry Unit and Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 012, India

Lanthanide orthovanadates, CeVO4, PrVO4, and NdVO4, of nanodimensions were synthesized under microwave exposure, and the photocatalytic activities of these compounds were investigated. These compounds exhibit a tetragonal zircon structure, crystallizing in the space group I41/amd with a ) 7.3733(4) and c ) 6.4909(4) Å with Z ) 4. The crystal structure was analyzed by powder X-ray diffraction, and the band gap was analyzed by UV-visible spectroscopy. Particle sizes were in the range of 25-30 nm, as observed by transmission electron microscopy. While the energy dispersive X-ray analysis indicates the formation of LnVO4, thermal analysis of these solids shows stability of the phase up to 1000 °C. The photocatalytic activity of all these orthovanadates has been investigated by degrading various dyes like methylene blue (MB), Orange G (OG), Rhodamine B (RB), Rhodamine Blue (RBL), and Remazol brilliant blue (RBBR) under UV irradiation. Further, these compounds were also used to degrade organic pollutants like phenol, 2,4-dinitrophenol and 2,4-dichlorophenol. The degradation rates of dyes and organics in presence of microwave-synthesized orthovanadates are higher compared to that observed with orthovanadates synthesized by the solid state technique. The microwave-synthesized orthovandates also show chloro-specificity, with the rate of degradation of 2,4-dichlorophenol significantly higher than that observed in presence of Degussa P-25 titania. Introduction Lanthanide orthovanadates belong to ABO4 type structures. These compounds exhibit unique optical, catalytic, electrical, magnetic, luminescent properties1-7 and are also used as sensors and electrode in electrochromic devices.8-10 Lanthanide orthovandates also exhibit selective oxidative dehydrogenation of propane to propene.1 Recently, they have been examined as potential photocatalysts for the degradation of dyes and organics.11,12 Several techniques have been used for the synthesis of lanthanide orthovanadates like solid state reaction,2 hydrothermal,13,14 and solution-based techniques,11 and recently, CeVO4 has been synthesized by microwave.15 The microwave-assisted route is a novel methodology of synthesis16-20 and is a rapidly developing area of research. Clark and Sutton have reviewed various aspects of microwave applications reported in the literature.21 Microwaves have been in use for accelerating organic reactions,17,19,22,23 since they are generally faster, simpler, and energy efficient. The energy transfer from microwaves to the material is believed to occur either through resonance or relaxation, which results in rapid heating.20 Semiconductor photocatalysts has been applied in many fields like wastewater treatment, air purification, photo splitting of water to produce hydrogen gas, and other problems of environmental interest.24 Among all the semiconductors, TiO2 has proven to be the most suitable for various environmental applications.24 The primary criteria for good semiconductor photocatalysts for the degradation of an organic compound are that the redox potential of the H2O/OH- (E0 ) -2.8 eV) couple lies within the band gap domain of the material and that they are stable over prolonged periods of time.24 Surface area also plays an important role in photocatalytic degradation.25 As the * To whom correspondence should be addressed. Tel.: +91-802292796. Fax: +91-80-3601310. E-mail: [email protected]. † Solid State and Structural Chemistry Unit. ‡ Department of Chemical Engineering.

microwave synthesized orthovanadates exhibit a high surface area, a comparative photocatalytic activity of solid state synthesized orthovanadate with microwave synthesized orthovanadate was investigated. In our previous work,11 we have reported the synthesis of LnVO4 (Ln) Ce, Pr, and Nd) by a solution based method. This yielded a compound with low surface area and high photocatalytic activity comparable to that obtained in the presence of commercial titania, P-25. The objective of the current study was to synthesize nanoparticles of various orthovanadates LnVO4 (Ln ) Ce, Pr, and Nd) by a novel route using microwaves and compare its photocatalytic properties with commercial titania and orthovanadates synthesized by the solid state technique. Experimental Section Materials. Ce(NO3)3 · 6H2O (Sigma Aldrich, 99.9%), Pr(NO3)3 · 6H2O (Sigma Aldrich, 99.9%), Nd(NO3)3 · 6H2O (Sigma Aldrich, 99.9%), NaVO3 (Merck, 99.99%), and polyethyleneglycol (S. D. Fine Chem Ltd., India) were used as such. Methylene blue (MB), Orange G (OG), Rhodamine Blue (RBL), Remazol brilliant blue (RBBR), phenol, 2,4-dichlorophenol, and 2,4-dinitrophenol (all from S. D. Fine Chem Ltd., India) were used without treating further for purification. Water was double distilled and filtered through a Millipore membrane filter prior to use. Preparation. LnVO4 (Ln ) Ce, Pr, and Nd) were synthesized via microwave irradiation. Thus in a typical synthesis of CeVO4, 1 mmol NH4VO3 was taken in a 250 mL beaker and stirred for half an hour. One mmol Ce(NO3)3 · 6H2O was added to the resulting solution which resulted in a yellow precipitate. After 5 min of stirring 100 mg of polyethylene glycol (6000) was added and further stirred for 10 min. The product was then subjected to microwave irradiation for 15 min in domestic microwave (model MS2342AE4 of LG). Microwave was irradiated at different power from 320 to 800 W, but in all cases, it led to formation of orthovanadate. A single-phase compound

10.1021/ie8003094 CCC: $40.75  2008 American Chemical Society Published on Web 08/06/2008

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Figure 1. UV-vis absorption spectra of microwave synthesized LnVO4 (Ln ) Ce, Pr, and Nd).

was obtained where initial and final pH were in the range of 6-7. The synthesized compounds were washed thoroughly by water and acetone and further heated to 300 °C before using them as catalysts for photodegradation. Solid state synthesis of CeVO4 was carried out by taking CeO2 and V2O5 as the starting material with Ce:V ) 1:1 molar ratio. The starting materials were ground and mixed well with an agate mortar and were made into pellets. These pellets were fired at 700 °C with a heating rate of 10 °C/min held for 6 h followed by an increase to 800 °C with a heating rate of 10 °C/min. The compounds were held at 800 °C for 6 h and cooled to room temperature at the rate of 10 °C/min resulting in CeVO4. Lanthanide vanadates show different colors, CeVO4 is dark brown, PrVO4 is yellowish green, and NdVO4 is gray. Characterization. Morphology and composition analysis of samples were done with the help of FEI Sirion scanning electron microscope with an EDAX attachment. Transmission electron microscope (TEM) image has been taken from JEOL 2000 FX 11 transmission electron microscope and high-resolution TEM (HRTEM) images have been collected in Tecnai S30 machine. UV-vis absorption spectra (Figure 1) of CeVO4, PrVO4, and NdVO4 show broad absorption band starting from 800 to 300 nm. Surface Area Analysis. BET analysis of microwave synthesized CeVO4, PrVO4, NdVO4, and solid state synthesized CeVO4 shows surface area of 44.5, 64.4, 59.9, and 0.3 m2/g, respectively. X-ray Powder Diffraction. X-ray diffraction data for samples calcined at 200 °C were collected on a Philips X’pert Pro X-ray powder diffractometer in the range 2θ from 3 to 80° with a step size of 0.02° and 8 s exposure time/step. Photocatalytic Experiments: Photochemical Reactor. The photochemical reactor employed in this study comprised of a jacketed quartz tube of 3.4 cm id, 4 cm od, and 21 cm length and an outer Pyrex glass reactor of 5.7 cm id and 16 cm length. A high-pressure mercury vapor lamp (HPML) of 125 W (Philips, India) was placed inside the jacketed quartz tube after removal of the outer shell. The ballast and capacitor were connected in series with the lamp to avoid the fluctuations in the input supply. Water was circulated through the annulus of the quartz tube to avoid heating of the solution. A 100 mL portion of the solution was taken into the outer reactor and continuously stirred to ensure that the suspension of the catalyst was uniform. The lamp radiated predominantly at 365 nm corresponding to the energy of 3.4 eV and photon flux of 5.8 × 10-6 mol of photons/s. A further detail of the experimental setup has been provided elsewhere.26

Degradation Experiments. A certain initial concentration of the dye was taken and the photocatalyst (1 g/L) was added to the reactor. This solution was stirred for 12 h in dark. This resulting concentration was taken as the initial concentration after adsorption. After exposure to UV, the concentration further decreased and the variation of concentration with time was used for kinetic analysis. During the degradation experiment samples were collected at regular intervals, filtered through Millipore membrane filters, and centrifuged to remove the catalyst particles prior to analysis. Sample Analysis. All dyes were analyzed with a UV-visible spectrophotometer (Lambda 32 Perkin-Elmer). The calibration for MB, OG, RB, RBL, and RBBR was based on the BeerLambert law at its maximum absorption wavelength, λmax, of 664, 489, 554, 664, and 591 nm, respectively. All the organic compounds were analyzed by HPLC (Waters Inc.) with a C18 reverse phase column. During analysis of phenol, 10% vol methanol was taken as the eluent with a flow rate of 0.5 mL/ min. The analysis of 2, 4-dichlorophenol and 2,4-dinitrophenol was carried out by taking acetic acid:methanol:water in 1:39: 60 ratio as the eluent with a flow rate of 0.9 mL/min. The calibration for phenol, 2,4-dichlorophenol and 2,4-dinitrophenol was based on the Beer-Lambert law at its maximum absorption wavelength, λmax, of 270, 292, and 280 nm, respectively. Results and Discussion Crystal Structure. Powder diffraction data show the formation of single-phase orthovanadates of Ce, Pr, and Nd matching with the JCPDS, database nos. 00-012-0757, 00-017-0879, and 00-015-0769, respectively (see Supporting Information Figure S1). The data was subjected to Rietveld refinement to ascertain if the microwave synthesis gives high quality crystalline powders, a prerequisite for photo degradation studies. A full profile Rietveld treatment of the XRD data has been carried out. Zircon structure, the space group I41/amd with a ) b ) 7.3750 and c ) 6.4867 Å was used along with the known coordinates of the phase from literature.27 16h, 4a, and 4b Wyckoff sites were assigned for O, Ln (Ce, Pr, and Nd), and V, respectively. During the course of the refinement, full occupancy was assigned to all the atoms and profile refinements were done using GSAS.28 The experimental and calculated profiles for all synthesized orthovanadate are shown in (Figure 2 and Figure S2a-c) and refined parameters are listed in Table S1 (see the Supporting Information). Atomic coordinates, isotropic temperature factors, and bond lengths and angles are given in Tables S2 and S3 (see the Supporting Information), respectively. Ln-O bond distances clearly signify that Ln3+ is coordinated with two types of oxygen. All synthesized LnVO4 have zircon-type structure where Ln is coordinated with eight oxygen out of which four are from edge sharing and the remaining are from the corner sharing VO4 tetrahedra (see Supporting Information Figure S3). Ln3+ forms a distorted dodecahedron with eight coordinating oxygen atoms. Morphology and Composition Analysis. SEM photographs (Figure 3a-d) show a particle size of 25-30 nm in case of microwave synthesized compounds and 2-3 µm in case of solid state synthesized cerium orthovanadate. EDAX analysis signifies the presence of Ln, V, and O only. The composition derived by this indicates an empirical formula as LnVO4. TEM images (Figure 4a-c) were taken at 200 KV and 110 µA at different magnification showing nano particle of microwave synthesized compounds. Crystallinity of a representative microwave synthesized compound (CM) has been checked by taking the HRTEM image and selected area diffraction (Figure 4d and e).

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Figure 2. Representative experimental (crossed) and calculated (solid line) X-ray diffraction profiles of microwave synthesized CeVO4 (CM). The difference profile is located at the bottom of the figure.

Figure 3. Scanning electron micrographs of microwave synthesized (a) CeVO4 (CM), (b) PrVO4, (c) NdVO4, and (d) solid state synthesized CeVO4 (CS).

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Figure 4. Transmission electron micrographs of microwave synthesized (a) CeVO4 (CM), (b) PrVO4 (PM), and (c) NdVO4 (NM). (d) HRTEM image of microwave synthesized CeVO4 (CM). (e) Selected area diffraction of microwave synthesized CeVO4 (CM).

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Figure 5. Degradation of profile of methylene blue by microwave synthesized CeVO4 (CM), PrVO4 (PM), NdVO4 (NM), and solid state synthesized CeVO4 (CS) along with Degussa P-25 (TiO2) under UV radiation.

Photocatalysis. The photocatalytic degradation of different dyes and organic compounds were investigated in presence of these compounds. The solution of desired dye/organic concentration was stirred with LnVO4 (Ln ) Ce, Pr, Nd) at a loading of 1 kg/m3 for 12 h in the dark. Higher surface area of microwave-synthesized compounds leads to high adsorption, which causes significant decrease in concentration of dyes/ organics. So the photocatalytic experiments were done only after adsorption (stirring for 12 h in dark). All the photocatalytic degradation reactions were carried out at “natural pH” (there was no adjustment of the pH either before or during the experiments). The initial degradation rate increases with an increase in catalyst loading initially but the rates do not increase significantly beyond a loading of 1 kg/m3, which is consistent with previous studies.29 Therefore, this catalyst loading was used to investigate the degradation of dyes. The effect of initial concentration on the rate of photocatalytic degradation of methylene blue was investigated over the concentration range of 10 to 40 ppm with 1 kg/m3 of LnVO4. The degradation profiles show a continuous decrease in dye (methylene blue) concentration and the concentration decreases nearly to 0 in 10 min in the presence of microwave synthesized compounds CeVO4 (CM) (see Supporting Information Figure S4), PrVO4 (PM), and NdVO4 (NM). However, it takes about one hour in case of Degussa P-25 titania and solid state synthesized CeVO4 (CS) (Figure 5). To quantify the reaction rates, a higher concentration of 100, 150, and 200 ppm were taken for CM and compared with that for Degussa P-25 (Figure 6). For the kinetic study, the model based on LangmuirHinshelwood kineticssro ) kCo/(1 + KoCo)swas used, where ro is the initial rate, Co is the initial concentration of the dye, k ) k0K0 is the first order kinetic rate constant, and Ko represents the adsorption equilibrium coefficient. Adsorption is negligible in case of CS due to lower surface area (0.3 m2/g, as determined by BET), but the adsorption is quite significant in the case of CM, PM, and NM due to their high surface area (44, 64, and 59 m2/g, respectively, based on BET). The initial rates were calculated using degradation up to 10 min for all compounds. The rate constant k was calculated by plotting the inverse of the initial rate and the inverse of the initial concentration (Figure 7). The overall first order kinetic rate constants, k, for the degradation of methylene blue in presence of CM, CS, and Degussa are 0.196, 0.018, and 0.037 1/min, respectively (Figures 7 and 8). This indicates a very high degradation rate in case of microwave-synthesized nanocrystaline CeVO4 (CM) compared

Figure 6. Photocatalytic degradation of methylene blue by microwave synthesized CeVO4 (CM) and Degussa P-25 (TiO2) at higher concentration.

Figure 7. Variation of the degradation rate with the initial dye concentration for microwave synthesized CeVO4 (CM) and Degussa P-25 (TiO2).

Figure 8. Variation of the degradation rate with the initial dye concentration for solid state synthesized CeVO4 (CS).

to that for the solid state synthesized CeVO4 (CS) and commercial TiO2 (Degussa P-25). All three microwave synthesized orthovanadates have similar activity for the degradation of 40 ppm MB with the initial rate constant, k, being 0.186, 0.182, and 0.198 1/min for CM, PM, and NM, respectively (Figure 5).

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Figure 9. Degradation profile of (a) RBL, (b) OG, and (c) RBBR by all synthesized orthovanadate and Degussa P-25 (TiO2).

In addition to methylene blue (MB), other classes of dyes like Azoic (OG), Xanthane (RBL), and sulfonated (RBBR) were also degraded in presence of these microwave synthesized compounds (Figure 9a-c). The initial rates of degradation of all these dyes were highest in presence of microwave synthesized compounds and lowest in presence of solid state synthesized (CS) compound. For investigation of specificity in the synthesized compounds, the degradation of different organic compounds was investigated. In this context, phenol, 2,4dinitrophenol and 2,4-dichlorophenol were chosen for degradation. The concentration after one hour of degradation of phenol and 2,4-dinitrophenol follows the trend of NM ≈ CM > PM > TiO2 > CS and NM > TiO2 > CM > PM > CS indicating that microwave synthesized compounds have better catalytic activity compared to that of solid state synthesized orthovanadate (CS) (Figure 10a and b). All orthovanadates show high chlorospecificity and degrade 2,4-dichlorophenol within 30-40 min. Microwave synthesized compounds adsorb 2,4-dichlorophenol

Figure 10. Degradation profile of (a) phenol, (b) 2,4-dinitrophenol, (c) 2,4dichlorophenol by all synthesized orthovanadates and Degussa P-25 (TiO2).

significantly, and the concentration reduces from 100 ppm to less than 50 ppm. Subsequently, UV exposure reduces the concentration of 2,4-dichlorophenol to nearly 0 in 10-15 min (Figure 10c). One of the reasons for better photocatalytic activity in orthovanadates is observed to be due to the presence of regular VO4 tetrahedra, which is beneficial for the charge transfer to surface of material (see Supporting Information Tables S2 and S3). For example, the photocatalytic activity of CaIn2O4, SrIn2O4, and BaIn2O4 has been compared and the higher photocatalytic activity in the first two compounds compared to BaIn2O4 has been attributed to the presence of regular InO6 octahedra.30

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There can be two oxidative pathways namely direct hole attack or/and OH radical oxidation during photocatalysis. In the case of degradation of phenol in presence of BiVO4, no OH radicals are observed indicating that BiVO4-mediated degradation of phenol occurs by the direct hole attack rather than by OH radical oxidation. On the basis of the observation of OH radical absence in the phenol and BiVO4 system,31 it was hypothesized that the photogenerated electrons of BiVO4 were captured by oxygen molecules. However, the degradation of phenols by LnVO4 (Figure 10a) shows the evolution of small amounts of intermediates, which were identified to be hydroxylated phenolic compounds by HPLC analysis. This indicates that the degradation of phenols occurs primarily through the formation of OH radicals unlike that of BiVO4. All synthesized lanthanide orthovanadates show chlorospecificity as well as better photocatalytic degradation for dyes and organic compounds compared to that for commercial catalyst Degussa P-25 titania. Higher surface area promotes better adsorption and reaction, whereas lower particle size ensures higher photon absorption felicitating higher probability of charge recombination.24 However, though the particle size and surface area of CM and Degussa P-25 are similar, the former shows a higher activity of both for adsorption and photocatalysis. This has been demonstrated by stirring CM and Degussa P-25 with methylene blue in the dark for 24 h, and the result shows a significant adsorption in first case (see Supporting Information Figure S5). The increased photocatalytic activity may be attributed to the lower band gap of the orthovanadates compared to that of Degussa P-25. We hypothesize that the specificity of the photocatalytic degradation toward chlorophenols can be attributed to the possibility of the formation of a stable ligand between chlorophenol and Vanadium. This is similar to the chlorospecificity observed for the degradation of chlorophenols in presence of BaBi2Mo4-xWxO16, wherein the specificity was attributed to the ligand formation between chlorophenol and Mo.29,32 Summary Nanosized lanthanide orthovandates with high surface area were synthesized using microwaves. The compounds were characterized by X-ray powder diffractometer, UV-vis, and TEM. The photocatalytic activity of these compounds was investigated by degrading various dyes and organics. The activity was higher than that of the orthovandates synthesized by solid state method and Degussa P-25, TiO2. The microwave synthesized orthovandates also showed specificity to chlorocompounds, with the rate of degradation of 2,4-dichlorophenol in presence of orthovandates being much higher compared to that of Degussa P-25. Acknowledgment We acknowledge funding from DST, India, financial support for the XRD machine under the DST-FIST program, and Mr. K. C. Suresh for surface area analysis. The authors would like to thank Prof. M. S. Hegde for useful scientific discussion. Supporting Information Available: Table S1 crystallographic parameters of LnVO4; Table S2 atomic coordinates and isotropic temperature factors for LnVO4; Table S3 bond distance and bond angle in LnVO4; Figure S1 powder X-ray diffraction pattern of all synthesized orthovanadates; Figure S2 experimental and calculated profiles for all synthesized orthovanadates (a) CeVO4 (CS), (b) PrVO4 (PM), and (c) NdVO4 (NM); Figure

S3 representative tetragonal zircon type structure of LnVO4; Figure S4 degradation profile of methylene blue by CM over a range of 10-40 ppm; Figure S5 adsorption of methylene blue by stirring in the dark for 24 h. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Au, C. T.; Zhang, W. D. Oxidative Dehydrogenation of Propane over Rare-Earth Orthovandates. J. Chem. Soc. Faraday Trans. 1997, 93, 1195. (2) Guo, M. D.; Aldred, A. T.; Chan, S. K. J. Phys. Chem. Solids. 1987, 48 (3), 229. (3) Fuess, H.; Kallel, A. Refinement of the Crystal Structure of Some Rare Earth Vanadates RVO4 (R ) Dy, Tb, Ho, Yb). J. Solid State Chem. 1972, 5, 11. (4) Rice, C. E.; Robinson, W. R. Lanthanum Orthovanadate. Acta Crystalogr. B. 1976, 32, 2232. (5) Chakoumakos, B. C.; Abraham, M. M.; Boatner, L. A. Crystal Structure Refinements of Zircon-Type MVO4 (M ) Sc, Y, Ce, Pr, Nd, Tb, Ho, Er, Tm, Yb, Lu). J. Solid State Chem. 1994, 109, 197. (6) Range, K. J.; Meister, H.; Klement, U. High-Pressure Transformations of Cerium(III) Orthovanadate(V), CeVO4. Naturforschung 1990, 45b, 598. (7) Oka, Y.; Yao, T.; Yamamoto, N. Hydrothermal Synthesis of Lanthanum Vanadates: Synthesis and Crystal Structures of Zircon-Type LaVO4 and a New Compound LaV3O9. J. Solid State Chem. 2000, 152, 486. (8) Picardi, G.; Varsano, F.; Decker, F.; Opara-Krasovec, U.; Surca, A.; Orel, B. Electrochim. Acta 1999, 44, 3157. (9) Matta, J.; Courcot, D.; Abi-Aad, E.; Aboukais, A. Chem. Mater. 2002, 14, 4118. (10) Tsipis, E. V.; Patrakeev, M. V.; Kharton, V. V.; Vyshatko, N. P.; Frade, J. R. J. Mater. Chem. 2002, 12, 3738. (11) Mahapatra, S.; Madras, G.; Guru Row, T. N. Ind. Eng. Chem. Res. 2007, 46, 1013. (12) Mahapatra, S.; Madras, G.; Guru Row, T. N. J. Phys. Chem. C 2007, 111, 6505. (13) Luo, F.; Jia, C. J.; Song, W.; You, L. P.; Yan, C. H. Cryst. Growth Des. 2005, 5 (1), 137. (14) Mahapatra, S.; Ramanan, A. J. Alloys Compd. 2005, 395, 149. (15) Wang, H.; Meng, Y. Q.; Yan, H. Rapid synthesis of nanocrystalline CeVO4 by microwave irradiation. Inorg. Chem. Commun. 2004, 7, 553. (16) Sheppard, L. M. Manufacturing ceramics with microwaves: The potential for economical production. Am. Ceram. Soc. Bull. 1988, 67, 1656. (17) Michael, D.; Mingos, P.; Baghurst, D. R. Applications of microwave dielectirc heating effects to synthetic problems in chemistry. Chem. Soc. ReV. 1991, 20, 1. (18) Mingos, D. M. P.; Baghurst, D. R. The Application of Microwaves to the Processing of Inorganic Materials. Br. Ceram. Trans. J. 1992, 91, 124. (19) Mingos, D. M. P. The applications of microwaves in chemical syntheses. Chem. Ind. 1994, 596. (20) Rao, K. J.; Ramesh, P. D. Use of microwaves for the synthesis and processing of materials. Bull. Mater. Sci. 1995, 18, 447. (21) Clark, D. E.; Sutton, W. H. Microwave Processing of Materials. Annu. ReV. Mater. Sci. 1996, 26, 299. (22) Dagani, R. Nanostructured Materials. Chem. Eng. News 1997, 26. (23) Sridar, V. Microwave radiation as a catalyst for chemical reactions. Curr. Sci. 1998, 74, 446. (24) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. ReV. 1995, 95, 69. (25) Stafford, U.; Gray, K. A.; Kamat, P. V. Photocatalytic Degradation of Organic Contaminants: Halophenols and Related Model Compounds. Heterogeneous Chem. ReV. 1996, 3, 77. (26) Sivalingam, G.; Nagaveni, K.; Hegde, M. S.; Madras, G. Photocatalytic degradation of various dyes by combustion synthesized nano anatase TiO2. Appl. Catal., B 2003, 45, 23. (27) Mullica, D. F.; Sappenfield, E. L.; Abraham, M. M.; Chakoumakos, B. C.; Boatner, L. A. Structural investigations of several LnVO4 compounds. Inor. Chem. Acta 1996, 248, 85. (28) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); LAUR 86-748,Los Alamos National Laboratory Report, Los Alamos, NM, 2004. (29) Muktha, B.; Madras, G.; Guru Row, T. N. A novel scheelite-like structure of BaBi2Mo4O16: Photocatalysis and investigation of the solid

6516 Ind. Eng. Chem. Res., Vol. 47, No. 17, 2008 solution, BaBi2Mo4-xWxO16 (0.25 e x e 1). J. Photochem. Photobio. A: Chem. 2006, 187, 177. (30) Junwang, T.; Zhigang, Z.; Masahiko, K.; Tetsuya, K.; Jinhua, Ye. Photocatalytic degradation of MB on MIn2O4 (M)alkali earth metal) under visible light: effects of crystal and electronic structure on the photocatalytic activity. Catal. Today 2004, 93-95, 885. (31) Xie, B.; Zhang, H.; Cai, P.; Qiu, R.; Xiong, Y. Simultaneous photocatalytic reduction of Cr(VI) and oxidation of phenol over monoclinic BiVO4 under visible light irradiation. Chemosphere. 2006, 63, 956.

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ReceiVed for reView February 23, 2008 ReVised manuscript receiVed May 2, 2008 Accepted June 20, 2008 IE8003094