Structural and Photocatalytic Activity of Lanthanide (Ce, Pr, and Nd

Apr 11, 2007 - Dipankar Saha , Sudarshan Mahapatra , T. N. Guru Row and ... Prangya Parimita Sahoo , Sumithra S. , Giridhar Madras and T. N. Guru Row...
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J. Phys. Chem. C 2007, 111, 6505-6511

6505

Structural and Photocatalytic Activity of Lanthanide (Ce, Pr, and Nd) Molybdovanadates 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 ReceiVed: December 29, 2006; In Final Form: February 23, 2007

Lanthanide (Ce, Pr, and Nd) molybdovanadates were synthesized by the solid-state method. These compounds crystallize in the tetragonal space group I41/amd with a ) b ) 7.3750 (4) and c ) 6.4867 (4) Å and Z ) 4. The crystal structure has been analyzed by FTIR, UV-visible spectroscopy, and powder X-ray diffraction. Particle sizes are in the range of 300-400 nm as observed by scanning electron microscopy. Energy-dispersive X-ray analysis suggests the formation of Ln0.95Mo0.15V0.85O4 (Ln ) Ce, Pr, and Nd) and the Rietveld refinements of the powder X-ray data substantiate this observation. Thermal analysis of these solids shows stability of the phase up to 800 °C. These materials were investigated for photocatalytic activity by degrading different dyes such as methylene blue(MB), orange G (OG), Rhodamine B (RB), Rhodamine Blue (RBL) Alizarine Red S (ARS), and Remazol brilliant blue (RBBR) under solar and UV irradiation since they exhibited lower band gaps. The degradation rates for all the dyes show enhancement as compared to the commercial titania catalyst, Degussa P-25, both in UV and sunlight. These compounds degrade chlorinated phenols much faster than titania, which indicates selectivity toward chloro substitution.

Introduction Lanthanide orthovanadates are major compounds among the binary system Ln2O3-V2O5. Orthovanadates crystallize in two types, tetragonal zircon1 (ZrSiO4) and monoclinic monazite2 (CePO4). Generally the larger Ln+3 ions prefer the monazite type compared to the zircon type due to the higher oxygen coordination number. Thus, LnVO4 crystallizes always in the monazite type in an equilibrium state under pressure, while other orthovanadates, including Sc and Y, crystallize in the zircon type.3 It is interesting to note that CeVO4 located in the boundary of zircon and monazite types in the phase diagram, exhibits three polymorphic modification, zircon, monazite, and scheelite, depending on the pressure employed.4 For example, Zircon type CeVO4 at ambient pressure could 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.5 A wide variety of chemistry for compounds with the zircon structure has so far been explored. LnVO4 compounds exhibit a variety of interesting physical properties, such as rare earth-activated luminescence, magnetic and JahnTeller phase transitions, which 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.6 Molybdovanadates are known to be catalysts for selective oxidation, ammoxidation, and oxidative dehydrogenation of olefins to the corresponding unsaturated aldehydes, nitriles, and diolefins.7-10 The high catalytic activity of the above compounds has been correlated to the simultaneous presence of Mo+6 in the crystal lattice and cation vacancies sites.11,12 In principle, all selective (ammo) oxidation catalysts must possess redox properties, which are dependent on the chemistry and the * Corresponding author. Tel: +91-80-2292796. Fax: +91-80-3601310. E-mail: [email protected]. † Solid State and Structural Chemistry Unit. ‡ Department of Chemical Engineering.

structure of both the surface and the bulk. In particular, they must be capable not only of undergoing reduction themselves giving the oxygen necessary for the olefin oxidation process, but also of being easily reoxidized by gaseous oxygen. Oxides in the Bi-Mo-V-O family have been examined extensively before in this context. A narrow stability region for the solid solution series Bi1-x/3MoxV1-xO4, where 0 e x e 0.55, was reported with the partial absence of Bi at the A site of the ABO4 type oxides.13 After examining the effect of various cations doping on the crystal structure of scheelite BiVO4, it was pointed out that higher-valent cations doping at B sites can stabilize the BO4 tetrahedra, whereas higher-valent cation doping at A sites may destabilize the rigid BO4 coordination and reduce the strength of the B-O bonds.14 Recent attempts to incorporate Na and Mo into the A and B sites of the ABO4 type oxides BiVO4 paved the way to the formation of a continuous solid solution for the series Nax/2MoxV1-xO4 (0 e x e 1).15 The application of semiconductor photocatalysts in wastewater treatment, water and air purification, cleanup of oil spills, photo splitting of water to produce hydrogen gas, and other problems of environmental interest has grown immensely.16 Simple oxide and sulfide semiconductors have band gap energies sufficient for promoting or catalyzing these chemical reactions of environmental interest. However, among all the semiconductors, TiO2 has proven to be the most suitable for widespread environmental applications such as remediation of hazardous wastes and contaminated groundwater, control of toxic air contaminants, removal of toxic dyes from the industrial effluents, photo splitting of water, and cleanup of oil spills.16 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.16 In this article, we report novel active UV/visible-light-driven photocatalytic properties along with the synthesis and structural features of Ln0.95Mo0.15V0.85O4 (Ln ) Ce, Pr, and Nd).

10.1021/jp069007e CCC: $37.00 © 2007 American Chemical Society Published on Web 04/11/2007

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Figure 1. Powder X-ray diffraction pattern for Cerium molybdovandates.

Experimental Section Materials. CeO2 (Fluka, 99.9%), Pr6O11 (Sigma Aldrich, 99.9%), Nd2O3 (Sigma Aldrich, 99.9%), MoO3 (Sigma Aldrich, 99.9%), and V2O5 (Merck, 99.99%) were used as such. Alizarin Red S, methylene blue, orange G, rhodamin B, rhodamin blue, Remazol brilliant blue, phenol, 2-chlorophenol, 4-cholorophenol, 2,4-dichlorophenol, 4-nitrophenol, 2,4-dinitrophenol, 4-chloro2-nitrophenol, and 2-chloro-4-nitrophenol (all from S. D. fineChem Ltd, India) were used without treating further for purification. Water was double distilled and filtered through a Millipore membrane filter prior to use. Preparation. Lanthanide (Ce, Pr, and Nd) oxides (CeO2, Pr6O11, and Nd2O3, respectively), MoO3, and V2O5 were taken as the starting materials for the synthesis of lanthanide molybdovanadate with variable molar ratios of Ln:Mo:V. These materials were ground and mixed well in an agate mortar and were made into pellets. These pellets were fired at 700 °C with a heating rate of 20 °C/min held for 2 h followed by an increase to 800 °C with a heating rate of 5 °C/min. Further, the resulting compounds were held at 800 °C for 6 h and cooled to room temperature at the rate of 20 °C/min. Of the several molar ratios tried like, for example, 1:0.5:1, 0.95:0.15:0.85, 0.90:0.3:0.7 and 0.85:0.45:0.55, only the first combination appeared to be monophasic with the rest of the combinations showing unreacted components as impurities (Figure 1). It is noteworthy that excess MoO3 is required to synthesize the monophasic compound and this may be explained to be due to the sublimation behavior at 800 °C of MoO3. Further, it is also found that either an increase or a decrease in the molar ratios other than the ratio 1:0.5:1 lead to a mixed phase. Characterization. Compositional analysis was done for all samples with the help of energy-dispersive X-ray (EDX) microanalysis on a JEOL JSM-840 SEM/EDAX machine. (Figure S1) X-ray Powder Diffraction. X-ray powder diffraction data were collected on a Philips X’pert pro diffractometer with a copper source. In order to get a Rietveld quality data a scan of 0.02 steps size and 10 s exposure per step was employed. After successful collection of quality data set, Rietveld refinements were performed with the help of GSAS.17 The starting model for the refinements the zircon type structure, 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.18

Photocatalytic Experiments: Photochemical Reactor (UV). The photochemical reactor employed in this study was 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 due to dissipative loss of UV energy. 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 the energy of 3.4 eV and photon flux is 5.8 × 10-6 photons/s. Further details of the experimental setup can be found elsewhere.19 Degradation Experiment (UV). The dyes/phenolic compounds were dissolved in double distilled Millipore filtered water and degraded in the photochemical reactor as described above. The effect of the initial concentration of these organics and the reaction rates were investigated. The compounds were further used for the selectivity test. The reactions were carried out at 40 °C, which was maintained by circulating water in the annulus of the jacketed quartz reactor. Samples were collected at regular intervals, filtered through Millipore membrane filters, and centrifuged to remove the catalyst particles prior to analysis. Degradation Experiment (Solar). The dyes/phenolic compounds were dissolved in double distilled Millipore filtered water and taken in 250 mL beaker covered with watch glass stirred constantly and exposed to solar radiation. The degradation of all dyes with the compounds synthesized in this study and the commercial catalyst (Degussa P-25) were carried out simultaneously to ensure similar solar radiation. Samples were collected at regular intervals from both. Then filtered through Millipore membrane filters and centrifuged prior to the analysis. Sample Analysis. Dyes were analyzed with a Lambda 32 Perkin-Elmer UV-visible spectrophotometer where as all the organic compounds analyzed by Water’s HPLC with a C18 reverse phase column. During analysis of organics 10% vol, methanol was taken as the eluent with a flow rate of 0.5 mL/ min. The calibration for MB, OG, RB, RBL, ARS, RBBR, phenol, 2-chlorophenol, 4-cholorophenol, 2,4-dichlorophenol, 4-nitrophenol, 2,4-dinitrophenol, 4-Cl-2-nitrophenol, and 2-Cl-

Lanthanide (Ce, Pr, and Nd) Molybdovanadates

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Figure 2. Powder X-ray diffraction pattern of Ln0.95Mo0.15V0.85O4 (Ln ) Ce, Pr, and Nd).

Figure 3. A representative experimental (crossed) and calculated (solid line) X-ray diffraction profiles of Cerium molybdovanadate.

4-nitrophenol was based on the Beer-Lambert law at its maximum absorption wavelength, λmax, of 664, 489, 554, 664, 424, 591, 270, 280, 275, 292, 320, 280, 270, and 320 nm, respectively. Results and Discussion Crystal Structure. The EDAX analysis suggests the composition for the pure phase obtained from the molar ratio of Ln:Mo:V ) 1:0.5:1 for all three compounds as Ln0.95Mo0.15V0.85O4. Figure 2 shows the powder X-ray diffraction patterns for these monophasic compounds, Ce0.95Mo0.15V0.85O4, Pr0.95Mo0.15V0.85O4 and Nd0.95Mo0.15V0.85O4, respectively. All three lanthanide molybdovanadates are isostructural and the starting model for the Rietveld refinement is based on the zircon structure.18 The positions of Molybdenum and Vanadium atoms were kept at the 4b site with the lanthanide atom situated at the 4a site while the Oxygen atom occupies the 16h site. All atoms were initially assigned full occupancy at their respective sites

and the electron density residues on the difference Fourier map suggests that the occupancy of the Lanthanide atom needs to be refined. The refinements on the Cerium compound converge to the composition Ce0.95Mo0.15V0.85O4 with reasonable residual factors of Rp ) 0.0593 and Rwp) 0.08569 (Figure 3) and the corresponding difference Fourier map is now flat (Figure S2) providing an unequivocal verification of the composition of the compound. Cerium molybdovanadate, whose refinement profiles are shown in Figure 3, represents 5% cation vacancy due to the incorporation of Mo at the Vanadium site. It appears this cation vacancy will play a significant role in the photodegradation, which is discussed in the subsequent section. The details of the refinement along with the coordinate, occupancy and thermal parameters are given in Table 1 with bond length and bond angle listed in Supporting Information Table S1. The bond valence sums have been calculated based on the method of Brown and Shannon20,21 (Table 2) and supports the

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TABLE 1: Crystallographic Parameter Obtained after Refinement crystallographic parameter r Ln3+(VIII)(Å) λ (Å) crystal system space Group a (Å) c (Å) V (Å3) Fcalc (gcm-3) Rp wRp DWd CHI**2

Ce0.95φ0.05Mo0.15V0.85O 4 1.143 1.5418 tetragonal I41/amd 7.3750(4) 6.4867(4) 352.816 4.701 0.0593 0.0856 0.742 7.613

atom

x

k

Uiso

Ce

0

0.75

0.125

0.95

0.01043

Mo

0

0.25

0.375

0.15

0.05536

V

0

0.25

0.375

0.85

0.05532

O

0

0.568 52(5)

0.801 25(5)

1

0.03625

TABLE 2: Bond Valence Sum for Cerium Molybdovanadate Ce0.95φ0.05Mo0.15V0. 85O4 bond type Ce-O V-O Mo-O

bond length

valence

bond valence sum

2.49054(9) 2.40052(7) 1.76139(5) 1.76139(2)

0.408 0.521 1.119 1.481

3.716 4.476 5.924

occurrence of the cation vacancy in the compound Ce0.95Mo0.15V0.85O4. The coordination polyhedra of the compound is shown in Figure S3. Ce+3 forms distorted dodecahedron by coordination with four corner shared oxygen from four V/MoO4 regular tetrahedron and four edge shared oxygen from two V/MoO4 tetrahedra. UV-Visible Spectra. The UV-visible diffuse reflectance spectra of CeVO4, Ce0.95Mo0.15V0.85O4, and Degussa P-25 titania are shown in Figure 4a. The band gaps were obtained from the absorption edges. Diffuse reflectance spectra of Pr and Nd

Figure 4. (a) UV diffuse reflectance of Cerium molybdovanadate, Cerium orthovanadate and Degussa P-25 titania. (b) Diffuse reflectance spectra of Pr and Nd molybdovanadates.

y

z

molybdovanadate are given in Figure 4b. The values of band gap thus obtained for lanthanide (Ce, Pr, and Nd) molybdovanadates and Degussa P-25 titania correspond to 2.33, 2.21, 2.14, and 3.1 eV, respectively. Photocatalysis. Photocatalytic degradation of five commonly used dyes covering a wide range of visible spectrum (λmax of dyes varying from 424 to 664 nm) and comprised of diverse functional groups such as sulfonate, amino, azo, hydroquinonic, and alkyl methyl and ethyl was investigated. In addition, degradation of phenolic compounds (phenol, 2-chlorophenol, 4-cholorophenol, 2,4-dichlorophenol, 4-nitrophenol, 2,4-dinitrophenol, 4-Cl-2-nitrophenol, 2-Cl-4-nitrophenol) was also investigated. There was negligible reduction in the concentration of all organic compounds of interest when a solution of 100 ppm was stirred with the catalyst for 12 h in the dark. Therefore, the actual concentration of the solution was taken to be the initial concentration for degradation analysis. All the photocatalytic degradation reactions were carried out at “natural pH” (no adjustment of the pH either before or during the experiments) conditions. Heterogeneous photocatalytic reactions19 normally follow Langmuir-Hinshelwood (LH) kinetics: r0 ) k0C0/(1 + K0C0), where r0 is the initial rate, C0 is the initial concentration, k0 is the kinetic rate constant, and the parameter K0 represents the equivalent of the adsorption equilibrium coefficient. BET surface areas of 0.95, 0.32, and 0.53 m2/g for Ce0.95Mo0.15V0.85O4, Pr0.95Mo0.15V0.85O4, and Nd0.95Mo0.15V0.85O4 were significantly lower than that of conventional catalysts like TiO2 (50 m2/g). This is further confirmed from X-ray powder patterns of the catalysts after the degradation reactions that indicate that the structure remains the same and no adsorption is observed (see the Supporting Information, Figure S4). As the value of K0 is very small, K0C0 , 1 and the LH rate expression reduces to a firstorder rate expression: r0 ) k0C0. This indicates that a plot of the initial rates r0 versus the initial concentration C0 would be linear. The linear dependency of initial rates, r0, and initial concentrations, C0, for degradation of methylene blue under UV and solar exposure (Figure 5a,b) confirms the first-order kinetics of photocatalytic degradation of dyes with Ce0.95Mo0.15V0.85O4. Figure 5c shows the degradation profile of MB dye in presence of all synthesized molybdovandates along with Degussa P-25 under UV radiation. The order of degradation for Ce0.95Mo0.15V0.85O4 varies as MB > ARS > RBB > RB > OG under solar radiation (see the Supporting Information, Figure S5ae), whereas the degradation rate is comparable with that of commercial catalyst Degussa P-25 in UV irradiation (see the Supporting Information, Figure S6a-e). It is interesting to note that the rate of degradation of MB in presence of Ce0.95Mo0.15V0.85O4 is nearly double than that for the commercial catalyst Degussa P-25 in solar radiation (Figure 5a) though the

Lanthanide (Ce, Pr, and Nd) Molybdovanadates

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6509 TABLE 3: Rate Constant for Different Dye Degradation by Cerium Molybdovanadate

Figure 5. (a) Variation of the degradation rate of MB with initial concentration under solar radiation for Cerium molybdovanadate and Degussa P-25 titania. (b) Variation of the degradation rate of MB with initial concentration under UV radiation for Cerium molybdovanadate and Degussa P-25 titania. (c) Degradation profile of 40 ppm MB with all synthesized molybdovandates and Degussa P-25 titania under UV radiation.

rates are nearly equal under UV exposure (Figure 5b). The rate constant for different dye degradation with Ce0.95Mo0.15V0.85O4 is shown in Table 3. The photocatalytic activity of cerium orthovanadate, CeVO4, and that of cerium molybdovanadate are significantly different. It would be interesting to compare the activities of CeVO4, CaMoO4, BiVO4, Bi-Mo-V-O family with the compounds investigated in this study. Compounds like Mo metal oxides such as Bi2MoO6 etc have MoO6 octahedra and are not of interest here, even though they exhibit photocatalytic activity. CeVO4 exhibit tetragonal zircon type structure and shows photocatalytic activity. However, no evidence relating the band structure to the photoactivity has been reported. CaMoO4 and

catalyst

compound

Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania Ce0.95Mo0.15V0.85O4 titania

MB MB MB MB RBB RBB RBB RBB OG OG OG OG AR S AR S AR S AR S RB RB RB RB RBL RBL

concentration time (ppm) exposure (min) 40 40 40 40 40 40 40 40 40 40 40 40 15 15 15 15 10 10 10 10 25 25

sun sun UV UV sun sun UV UV sun sun UV UV sun sun UV UV sun sun UV UV UV UV

30 30 30 30 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20

k 0.0305 0.0176 0.0322 0.0302 0.0091 0.0029 0.0296 0.0123 0.0027 0.0010 0.0306 0.0303 0.0173 0.0076 0.0247 0.0192 0.0073 0.0313 0.0402 0.0446 0.0451 0.0477

CaWO4 are similar to each other and in these compounds, the valence band maxima and conduction band minima are located at the Γ point and are therefore known as direct-gap material.22 The HOMO of CaMoO4 comprises of O 2p and LUMO is of Mo 4d which is tetragonally coordinated. As metal is in tetragonal symmetry, the d orbital can be split into two parts, t2g and eg state, because of Crystal Field effect. It is also shown that the lower part of conduction band consist of eg of Mo 4d approximately 0.5 eV from upper part of conduction band where as upper part of same consist of t2g of Mo and 3d of Ca. A similar case is also found to happen in case of monoclinic BiVO4, where the HOMO comprises of Bi 6s and O 2p and contribution of Bi 6s to HOMO is 10% only.23 The lower part of conduction band consist mainly of V 3d orbital where contribution of Bi and O are very small. However, the upper part of conduction band lies 5-7.4 eV mainly comprises of Bi 6p and O 2p.22 Recently, it was reported that the valence band of Ca1-xBixVxMo1-xO4 (0 e x e 1) consist of O 2p, V 3d, and Mo 4d with a little contribution from Bi 6s, where the conduction band is rather broad and the lower part consist of V 3d and Mo 4d orbital, which is divided in to two part for t2g and eg.23 This is consistent with crystal field effect, as observed in CaMoO4. In addition to these there is an unoccupied Ca 3d orbital that appears at the higher energy side on the conduction band. Considering the above concepts, it can be concluded to be the reason of high photocatalytic activity. The wide density of state distribution in conduction band is helpful for the mobility of the excited electron, which indicates a better photocatalytic performance in this compound.23 A similar behavior is expected in case of the compound investigated in this study. The valence band of Ce0.95Mo0.15V0.85O4 consists of O 2p, V 3d, and Mo 4d, whereas the conduction band consist of V 3d and Mo 4d. 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.24 Similarly, in this study better photocatalytic activity in cerium molybdovanadate is observed due to the presence of regular VO4 and MoO4 tetrahedra, which is beneficial for the charge transfer to surface of material.

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Figure 6. Photocatalytic degradation of 2-chlorophenol under UV radiation for lanthanide (Ce, Pr, and Nd) molybdovanadates.

Chlorospecificity. Our earlier studies with BaBi2Mo4O16 and LiBi4Nb3O14 have shown chloro and nitrospecificity, respectively.25,26 BaBi2Mo4O16 showed similar catalytic activity as TiO2 for all organics except for chlorine containing aromatics but second shows nitrospecificity. Therefore it is of interest to determine the photocatalytic specificity of compounds. So the degradation rates of various chlorophenols and nitrophenols were determined. The rates followed the order, 2,4-dichloro phenol > 4-chlorophenol > 2-chlorophenol > 4-nitrophenol > phenol > 4-chloro-2-nitrophenol > 2,4,-dinitrophenol > 4-nitro-2chlorophenol under UV irradiation. The degradation rates of nitrophenol were lower than that of only chloro substituted phenols. This indicates that the chloro substitution on phenol increases the rate of degradation. The complete degradation of 2,4dichlorophenol was observed within 1 min for all the three lanthanide molybdovanadate. For all monochloro substituted phenol the degradation time is within 10-15 min (Figure 6). To verify the activity at different substitution position 2-chlorophenol ,4-chlorophenol, and 4-chloro-2-nitro- and 2-chloro4-nitrophenol were taken, and it was found that the degradation is slightly faster in case of the 4-chloro-substituted phenol (see the Supporting Information, Figure S7a,b). The chlorospecificity was further checked by taking different substituted phenol other than chlorine. For this purpose, 4-nitro- and 2,4-dinitrophenol were degraded. These compounds degrade lower and thus with a lower rate constant for nitro substituted phenol (see the Supporting Information, Figure S8). It is interesting to note that as the chloro-substitution on phenol ring increases, the degradation rate increases. This above statement is confirmed by taking the case of 2,4-dichloro phenol, where there is 100% degradation of the organic compound within 1 min occurred for all three lanthanide (Ce, Pr and Nd) molybdovanadates by giving small amount of catechol and organic acid as intermediate. It is possibly interesting to compare the photocatalytic activity of this material with that of BiVO4. This has been studied for the degradation of Rhodamine dye,27 phenol28 and evolution of oxygen.29 The photocatalytic activity of BiVO4 has been attributed to the larger atom density of the basal (0 1 0) lattice plane on the exposed surfaces and the possible distortion of the Bi-O polyhedron.27 However, Ln0.95Mo0.15V0.85O4 (Ln ) Ce, Pr and Nd), synthesized by the solid-state technique has negligible surface area. This indicates that the photocatalytic degradation in this case is not a surface phenomenon. In photocatalysis, a frequently discussed issue is the oxidative pathway, i.e., direct hole attack or/and OH radical oxidation.

Mahapatra et al. In case of BiVO4, no OH radicals are present in the phenol solution with BiVO4 indicating that BiVO4-mediated degradation of phenol occurs by the direct hole attack rather than by OH radical oxidation. Based on the observation of OH radical absence in the phenol and BiVO4 system,28 it was suggested that the photogenerated electrons of BiVO4 were captured by oxygen molecule and the degradation of phenol occurred by the direct hole attack rather than by OH radical oxidation. However, the degradation of phenols by Ce0.95Mo0.15V0.85O4 (see the Supporting Information, Figure S9) 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 OH radicals unlike that of BiVO4. We hypothesize that the specificity of the photocatalytic degradation of chlorophenols can be attributed to the possibility of the affinity between chlorophenol and Mo. Summary Three lanthanide (Ce, Pr, and Nd) molybdovanadates were synthesized by the solid- state reaction for the first time. It is of interest to note that even though the molar ratio of Ce:Mo:V )1:0.5:1 is in excess, the single phase compound (Ln0.95Mo0.15V0.85O4) for all the three lanthanides is obtained only in this case. The photocatalytic activity of these materials was investigated. A high selectivity toward chloro substituted phenol has been shown for the first time in Ln0.95Mo0.15V0.85O4. The degradation rate for chloro substituted phenol increase with increase in number of chlorine substitution in the ring. The reported catalysts show significant catalytic behavior under ultraviolet as well as solar radiation. The higher photocatalytic activity may be attributed to wide density of state distribution in conduction band, which is helpful for the mobility of the excited electron. All synthesized lanthanide molybdovandates show chlorospecificity as well as better photocatalytic degradation for dyes and organic compounds compared to that for commercial catalyst Degussa P-25. Acknowledgment. We acknowledge funding from DST, India, and financial support for the XRD machine from the DST-FIST program. Supporting Information Available: Bond length and bond angle for Cerium molybdovanadate Table S1; SEM photograph of Ce0.95Mo0.15V0.85O4 (Figure S1); three-dimensional difference Fourier map for Ce0.95Mo0.15V0.85O4 (Figure S2); a representative coordination polyhedra of lanthanide molybdovanadate (Figure S3); powder X-ray diffraction pattern of Ce0.95Mo0.15V0.85O4 before and after degradation of 2,4-dichlorophenol (Figure S4); degradation profile of (a) MB, (b) ARS, (c) RBBR, (d) RB, and (e) OG in the presence of Ce0.95Mo0.15V0.85O4 and Degussa P-25 titania under solar radiation (Figure S5); degradation profile of (a) MB, (b) ARS, (c) RBBR, (d) RB, and (e) OG in the presence of Ce0.95Mo0.15V0.85O4 and Degussa P-25 titania under UV radiation (Figure S6); degradation profile of (a) 4-chlorophenol and 2-chlorophenol and (b) 4-Cl-2-nitrophenol and 2-Cl-4-nitrophenol by Ce0.95Mo0.15V0.85O4 under UV radiation (Figure S7); degradation profile of 4-nitrophenol and 2,4dinitrophenol in presence of Ce0.95Mo0.15V0.85O4 under UV radiation (Figure S8); degradation profile of phenol in the presence of Ce0.95Mo0.15V0.85O4 and Degussa P-25 titania under UV radiation (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.

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