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APPLIED CHEMISTRY Synthesis, Characterization and Photocatalytic Activity of Lanthanide (Ce, Pr and Nd) Orthovanadates Sudarshan Mahapatra,† 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
A new approach to the synthesis of the lanthanide orthovanadates CeVO4, PrVO4, and NdVO4 is proposed. These compounds exhibit a tetragonal zircon structure, crystallizing in the space group I41/amd with a ) 7.3733(4) Å, c ) 6.4909(4) Å, and Z ) 4. The crystal structure has been analyzed by powder X-ray diffraction and FTIR and UV-visible spectroscopies. Particle sizes were found to be in the range of 200-300 nm as seen by scanning electron microscopy, and energy-dispersive X-ray analysis indicated the formation of LnVO4. Thermal analysis of these solids indicated stability of the phase to 1000 °C. The photocatalytic activity of these compounds was investigated for the first time by the degradation of methylene blue. The degradation rates were similar for all compounds and comparable to that of commercial Degussa titania. Introduction LnVO4 compounds exhibit a variety of interesting physical properties, such as rare-earth-activated luminescence and magnetic and Jahn-Teller phase transitions, that have been the subject of a number of experimental and theoretical investigations.1-6 These solids are also potential catalysts for the oxidative dehydrogenation of propane.1 Lanthanide orthovanadates are major compounds among the binary system Ln2O3-V2O5, which have been characterized via crystallography and in terms of their physical properties. Orthovanadates crystallize in two types, tetragonal zircon2 (ZrSiO4) and monoclinic monazite3 (CePO4). Generally, the larger Ln3+ ion prefers the monazite type over the zircon type because of the higher oxygen coordination number. Thus, LnVO4 crystallizes solely in the monazite type in an equilibrium state under pressure, whereas other orthovanadates including Sc and Y crystallize in the zircon type.4 It is interesting to note that CeVO4, located at the boundary between the zircon and monazite types in the phase diagram, exhibits three polymorphic forms, zircon, monazite and scheelite, depending on the pressure employed.5 For example, zircon-type CeVO4 at ambient pressure can be transformed to a metastable monazite-type CeVO4 by application of pressure. However, the reverse transition from a stable monazite to a metastable zircon is not observed.6 All LnVO4 systems have been prepared by either solid-state reactions, precursor routes, or chelating-ligandmediated pathways at elevated temperatures. The synthesis of noncrystalline LaVO4 was first reported by Ropp and Carroll,7 where La3+ and VO2+ were reacted in the presence of NH4OH and H2O2. However, the formation of high-quality crystalline material of lanthanide orthovanadates is of extreme importance for potential applications in catalysis. A suggested solution* To whom correspondence should be addressed. Address: Professor T. N. Guru Row, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560 012, India. Tel.: +91-80-2292796. Fax: +91-80-3601310. E-mail:
[email protected]. † Solid State and Structural Chemistry Unit. ‡ Department of Chemical Engineering.
based method,8 which involves conversion of the lanthanide oxide to the corresponding nitrate using 70% nitric acid solution followed by the conversion of the nitrate to the corresponding orthovanadate, works for all lanthanides except cerium and promethium. In this article, we report a generalized procedure for the synthesis of crystalline zircon-type LnVO4. It is noteworthy that these materials show low band gaps; therefore, we investigated the photocatalytic degradation of a dye, methylene blue. Thus, the objective of this study was to devise a new method for synthesizing LnVO4 compounds and testing their photocatalytic activity. Experimental Section CeVO4 was synthesized from ammonium metavanadate and cerium(III) nitrate hexahydrate as the cerium source. During the synthesis, vanadium was reduced by sodium borohydride before the reaction with cerium. The reduction of vanadium prior to reaction with cerium was checked by cerimetric titration. It was found that CeVO4 is formed irrespective of whether the ratio of Ce(NO3)3‚6H2O to NH4VO3 is 1:2 or 1:1 because the extra unreacted vanadium salt is washed away by water. Synthesis of CeVO4 taking the reactants in a 1:1 ratio along with reduction of vanadium results in a pH of 9-10. However, after addition of Ce(NO3)3‚6H2O, the pH is reduced to 6-7. Thus, in a typical synthesis of CeVO4, 0.25 mmol of NaBH4 was dissolved in 10 mL of water and stirred with 0.25 mmol of NaOH for 5 min. To the resultant solution was added 1 mmol of NH4VO3, and the mixture was stirred for 15 min, after which 100 mL of doubly distilled water was added. To this solution was added 1 mmol of Ce(NO3)3‚6H2O, which resulted in a yellow precipitate. After the solution had been stirred for 24 h, a dark brown product was obtained that was filtered, washed with water, and dried with acetone. The same product could also be obtained by refluxing at 100 °C while stirring for 12 h. Initially, the products were less crystalline. However, upon heating of the product to 400 °C for 1 h, highly crystalline orthovanadate could be obtained. The same procedure was used
10.1021/ie060823i CCC: $37.00 © 2007 American Chemical Society Published on Web 01/12/2007
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Figure 2. Scanning electron micrographs of (a) CeVO4, (b) PrVO4, and (c) NdVO4 before heating and corresponding images of samples after being heated for 1 h at 400 °C (d-f, respectively).
Figure 1. (a) FTIR and (b) UV-vis diffuse reflectance spectra of LnVO4.
for the syntheses of orthovanadates PrVO4 and NdVO4. All of the orthovanadates synthesized in this study were insoluble in water and stable against photocorrosion. A Perkin-Elmer Spectrum 1000 Fourier transform infrared spectrometer was used for the analysis of the above compounds in the wave number range from 200 to 4000 cm-1. Morphology and composition analyses of the samples were done using a JEOL JSM-5600 LV scanning electron microscope with an EDAX attachment. X-ray diffraction data for samples calcined at 400 °C were collected on a Philips Xpert Pro X-ray powder diffractometer in the 2θ range from 5° to 75° with a step size of 0.02° and an 8-s exposure time per step. The photochemical reactor employed in this study consisted of a jacketed quartz tube of 3.4-cm i.d., 4-cm o.d., and 21-cm length and an outer Pyrex glass reactor of 5.7-cm i.d. and 16cm length. A high-pressure mercury vapor lamp (HPML) of 125 W (Philips, Mumbai, 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 fluctuations in the input supply. Water was circulated through the annulus of the quartz tube to avoid heating of the solution due to dissipative loss of UV energy. One hundred milliliters of the solution was taken in 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 an energy of 3.4 eV and a photon flux of 5.8 × 10-6 mol of photons/s. Further details of the experimental setup can be found elsewhere.9 The dye was dissolved in doubly distilled Millipore filtered water and degraded in the photochemical reactor as described above. Samples were collected at regular intervals,
filtered through Millipore membrane filters, and centrifuged to remove the catalyst particles prior to analysis. All samples were analyzed with a Lambda 32 Perkin-Elmer UV-visible spetrophotometer. The calibration for methylene blue was based on the Beer-Lambert law at its maximum absorption wavelength, λmax, of 664 nm. Results and Discussion Powder diffraction data show the formation of single-phase orthovanadates of Ce, Pr, and Nd matching results reported in the JCPDS database, nos. 00-012-0757, 00-017-0879, and 00015-0769, respectively. The powder XRD patterns of all compounds suggest crystallization (Figure S1; Supporting Information) in a tetragonal cell with a ) 7.3733(4) Å and c ) 6.4909(4) Å, space group I41/amd. The nitrate of lanthanide gives a uniphasic orthovandate at room temperature, whereas the corresponding oxide gives additional unreacted phases. We obtained a single-phase compound where the initial and final pH values were in the ranges of 9-10 and 6-7, respectively. The lanthanide vanadates exhibit different colors: CeVO4 is dark brown, whereas PrVO4 and NdVO4 are yellowish green and gray, respectively. The FTIR spectra of all three orthovandates show broad bands around 795-820 and 3400 cm-1 due to V-O stretching and O-H stretching of surface water, respectively (Figure 1a). The UV-vis diffuse reflectance spectra of CeVO4, PrVO4, and NdVO4 show distinctive absorption edges around 452.2, 432.7, and 414.7 nm, respectively, with corresponding band gaps of 2.74, 2.86, and 2.99 eV (Figure 1b); in comparison, the band gap of Degussa P-25 is 3.1 eV. SEM images (Figure 2a-c) indicate a particle size of ∼200 nm, and the crystallinity increases on heating to 400 °C for 1 h, as shown clearly in Figure 2d-f, respectively. EDAX analysis
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Figure 3. Experimental (crossed) and calculated (solid line) X-ray diffraction profiles of CeVO4 as a representative of results obtained for LnVO4 (Ln ) Ce, Pr, and Nd). The difference profile is located at the bottom of the profile.
Table 1. Crystallographic Parameters of LnVO4 parameter
CeVO4
PrVO4
NdVO4
r[Ln3+(VIII)] (Å) λ (Å) crystal system space group a (Å) c (Å) V (Å3) Fcalc (g cm-3) Rp Rwp DWd χ2
1.143 1.5418 tetragonal I41/amd 7.3668(5) 6.4852(5) 351.957(2) 4.801 0.0632 0.0843 1.027 2.022
1.126 1.5418 tetragonal I41/amd 7.3365(5) 6.4410(5) 346.689(2) 4.966 0.0742 0.0982 1.250 1.580
1.109 1.5418 tetragonal I41/amd 7.3607(5) 6.4631(5) 350.177(2) 4.853 0.0798 0.1034 1.002 2.021
verifies the presence of Ln, V, and O only. The composition derived from these results indicates an empirical formula of LnVO4. A full-profile Rietveld treatment of the XRD data was carried out. During the refinement, zircon-type LnVO4 was taken as a starting model, with the space group I41/amd where 16h, 4a, and 4b Wyckoff sites were assigned for O, Ce, and V, respectively. During the course of the refinement, full occupancy was assigned to all atoms, and profile refinements were done using GSAS.10 The experimental and calculated profiles for CeVO4 are shown in Figure 3, and refined parameters are listed in Table 1. The bond length and angles are given in Tables S1 and S2 (Supporting Information), respectively. Ln-O bond distances clearly indicate that Ln3+ is coordinated to two types of oxygen. All synthesized LnVO4 compounds have zircon-
type structures in which Ce is coordinated with eight oxygen atoms, of which four are from edge-sharing VO4 tetrahedra and the remaining are from corner-sharing VO4 tetrahedra (Figure S2, Supporting Information). Ln3+ forms a distorted dodecahedron with eight coordinating oxygen atoms. Photocatalytic degradation of methylene blue was investigated in the presence of each of these compounds. The solution of desired dye concentration was stirred with LnVO4 (Ln ) Ce, Pr, Nd) at a loading of 1 kg/m3 for 12 h in the dark. The concentration of the dye decreased, but the actual concentration of the solution was taken to be the initial concentration for kinetic analysis. All 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 increased with increasing catalyst loading, but the rate did not increase for loadings above 1 kg/m3. Therefore, this catalyst loading was used to investigate the degradation of the dye. The effect of initial concentration on the rate of the photocatalytic degradation of methylene blue was investigated over the dye concentration range of 10-40 ppm with 1 kg/m3 of LnVO4. The degradation profiles in the presence of CeVO4 (Figure 4a), PrVO4 (Figure 4b), and NdVO4 (Figure 4c) show a continuous decrease in dye concentration that almost reaches zero after 1 h. To quantify the reaction rates, a model based on LangmuirHinshelwood kinetics was used: r0 ) k0C0/(1 + K0C0), where r0 is the initial rate, C0 is the initial concentration of the dye, k0 is the kinetic rate constant, and K0 represents the adsorption equilibrium coefficient. Because of the low surface areas of these
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Figure 4. Photocatalytic degradation of methylene blue by (a) CeVO4, (b) PrVO4, (c) NdVO4, and (d) Degussa P-25.
Conclusions A new synthetic method involving borohydrate reduction before initiation of the reaction to form orthovanadates of lantanides has been proposed. The photocatalytic activity of LnVO4 (Ln ) Ce, Pr, and Nd) has been demonstrated for the first time by the degradation of methylene blue, and the rates were found to be comparable to that of commercial titania. Acknowledgment We acknowledge funding from DST, New Delhi, India, and acknowledge financial support for the XRD machine under the DST-FIST program. Figure 5. Variation of the degradation rate with the initial dye concentration.
materials (less than 1 m2/g, as determined by BET), adsorption is negligible, and K0C0 , 1. The initial rates were calculated using degradation up to 10 min. The linear dependency of the initial rates, r0, and initial concentrations, C0, for all of the dyes, shown in Figure 5, confirms the first-order kinetics of the photocatalytic degradation of methylene blue dye with these compounds. The overall kinetic rate constants, k0, for the degradation of methylene blue in presence of CeVO4, PrVO4, and NdVO4 were determined to be 0.064, 0.066, and 0.058 min-1, respectively. These values are comparable to the degradation rate constant (0.079 min-1) in the presence of commercial TiO2 (Degussa P-25). This indicates that, even though the band gaps of these materials are different, the photocatalytic properties are similar.
Supporting Information Available: Indexed powder pattern of CeVO4 (Figure S1), representative tetragonal zircon type (Figure S2), atomic coordinates and isotropic temperature factors for LnVO4 (Table S1), bond distances and bond angles in LnVO4 (Table S2). 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) 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. (3) Rice, C. E.; Robinson, W. R. Lanthanum Orthovanadate. Acta Crystallogr. B 1976, 32, 2232. (4) 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.
Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007 1017 (5) Range, K. J.; Meister, H.; Klement, U. High-Pressure Transformations of Cerium(III) Orthovanadate(V), CeVO4. Z. Naturforsch. 1990, 45b, 598. (6) 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. (7) Ropp, R. C.; Carroll, B. Dimorphic Lanthanum Orthovanadate. J. Inorg. Nucl. Chem. 1973, 35, 1153. (8) Krylov, V. S.; Popkov, I. N.; Magunov, R. L.; Puring, M. N.; Bagdasarow, K. S.; Sagina, R. F.; Popov, V. I.; Oblast, F. M. Method of Producing Orthovandates of Rare-Earth Metals. U.S. Patent 3,667,901, 1972.
(9) 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. (10) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS); Los Alamos National Laboratory Report LAUR 86-748; Los Alamos National Laboratory: Los Alamos, NM, 2004.
ReceiVed for reView June 28, 2006 ReVised manuscript receiVed December 5, 2006 Accepted December 5, 2006 IE060823I
