Stoichiometry, Vibrational Modes, and Structure of Niobium(V

Jun 24, 2010 - Visiting scientist at FORTH/ICE-HT. ... The spectral data are discussed in terms of the most plausible structural models, for which con...
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J. Phys. Chem. A 2010, 114, 7485–7493

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Stoichiometry, Vibrational Modes, and Structure of Niobium(V) Oxosulfato Complexes in the Molten Nb2O5-K2S2O7-K2SO4 System Studied by Raman Spectroscopy Andreas L. Paulsen,†,§ Flemming Borup,†,§ Rolf W. Berg,‡ and Soghomon Boghosian*,† Department of Chemical Engineering, UniVersity of Patras and Institute of Chemical Engineering and High Temperature Chemical Processes (FORTH/ICE-HT), GR-26500 Patras, Greece, and Chemistry Department, The Technical UniVersity of Denmark, DK-2800 Kgs. Lyngby, Denmark ReceiVed: April 23, 2010; ReVised Manuscript ReceiVed: June 4, 2010

The structural and vibrational properties of NbV oxosulfato complexes formed in Nb2O5-K2S2O7 and 0 Nb2O5-K2S2O7-K2SO4 molten mixtures with 0 e XNb e 0.25 have been studied by high-temperature 2O5 Raman spectroscopy under static equilibtrium at temperatures up to 700 °C. The spectral features for the binary Nb2O5-K2S2O7 molten system indicate that the dissolution of Nb2O5 proceeds with consumption of S2O72- leading to the formation of a NbV oxosulfato complex according to Nb2O5 + nS2O72- f C2n-; a simple formalism exploiting the relative Raman band intensities is used for determining the stoichiometric coefficient, n, pointing to n ) 3 and to the following reaction: Nb2O5 + 3S2O72- f 2NbO(SO4)33-, which is consistent with the Raman spectra of the molten mixtures. Nb2O5 could be dissolved much easier when K2SO4 was present in an equimolar (1:1) SO42-/Nb ratio; the incremental presence of K2SO4 in Nb2O5-K2S2O7 melts induces composition effects in the Raman spectra that terminate when n(SO42-)/n(Nb) ) 1. The composition effects and the temperature-dependent features of the Raman spectra obtained for Nb2O5-K2S2O7-K2SO4 molten mixtures together with the spectral changes occurring upon freezing are accounted for by a Nb2O5 · 3K2S2O7 · 2K2SO4 stoichiometry for the complete reaction taking place: Nb2O5 + 3S2O72- + 2SO42- f NbO(SO4)4S2O77- + NbO2(SO4)23-. The spectral data are discussed in terms of the most plausible structural models, for which consistent band assignments are made. The most characteristic Raman bands for the NbV oxosulfato complexes pertain to NbdO modes: (i) at 937 cm-1 for the mono-oxo NbdO mode of NbO(SO4)33-; (ii) at 958 cm-1 for the mono-oxo NbdO mode of NbO(SO4)4S2O77-; and (iii) at 926 cm-1 for the symmetric dioxo Nb(dO)2 mode of NbO2(SO4)23-. Introduction Previously we have studied the dissolution of a number of metal oxides in molten alkali pyrosulfates, alkali sulfates, and mixtures thereof and investigated the structural properties of the formed complexes by high-temperature Raman spectroscopy. The original interest for these studies had arisen from the prospect of developing a pyrosulfate melting process for metal ore extraction. The molten systems V2O5-M2S2O7-M2SO4 (M ) K, Cs)1,2 and V2O5-M2SO4 (M ) K, Cs)3 have received much more attention owing to their importance as constituents of the homogeneous liquid catalytic phase used in the process of catalytic oxidation of sufur dioxide.4 Furthermore, we have derived a formalism for determining the stoichiometry of solute complexes in ionic liquid solvents based on Raman intensity correlations and applied it for studying the dissolution reactions of V2O5 with molten Cs2S2O7 and Nb2O5 and MoO3 with molten K2S2O7.5 To date we have also studied the dissolution reactions of ZnO in K2S2O7 or Na2S2O76 and of WO3 in K2S2O7-K2SO4 melts.7 The type of complexes formed during the dissolution of niobium pentoxide in pure molten alkali pyrosulfate is not known; NbV is expected to form anionic sulfato and/or oxosulfato complexes in molten pyrosulfates in analogy to VV.1-3,8-12 The crystal structure of certain compounds of this type has been determined.13 Moreover, a comprehensive review on the coordination chemistry of niobium * Corresponding author, [email protected]. † University of Patras and FORTH/ICE-HT. ‡ Technical University of Denmark. § Visiting scientist at FORTH/ICE-HT.

is available.14 Much more information is available on the chemistry and structure of crystalline vanadium sulfato and oxosulfato complexes15-22 that are recognized as deactivation products of the sulfuric acid catalyst.4,23 Crystalline oxosulfato complexes of WVI and MoVI have also been synthesixed by precipitation from molten mixtures of WO3 and MoO3 dissolved in M2S2O7-M2SO4 (M ) alkali) melts.7,24-26 The present work is concerned with the characterization of the structural and vibrational properties of the NbV oxosulfato complexes formed in the molten binary Nb2O5-K2S2O7 as well as in the molten ternary Nb2O5-K2S2O7-K2SO4 systems at temperatures of 450-700 °C under static equilibrium conditions. 0 e The study is extended in the mole fraction range 0 e XNb 2 O5 0 0.25 (XNb2O5 denotes the initial Nb2O5 mole fraction in the Nb2O5-K2S2O7 binary mixture); incremental amounts of K2SO4 are added to the binary mixtures (0 < n(SO42-)/n(Nb) < 2.5), where n(SO42-)/n(Nb) (hereinafter denoted Y) is the number of added moles of K2SO4 per Nb atom). During the dissolution of niobium pentoxide in molten potassium pyrosulfate, the following general reaction takes place

Nb2O5 + nS2O72- f C2n-

(1)

The stoichiometry of the complex formed according to eq 1 is determined based on a formalism that correlates the relative Raman band intensities with the stoichiometric coefficient, n. Furthermore, the temperature and composition effects on the

10.1021/jp103667h  2010 American Chemical Society Published on Web 06/24/2010

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Raman spectra of the binary (Nb2O5-K2S2O7) and ternary (Nb2O5-K2S2O7-K2SO4) systems are examined together with the spectral changes occurring upon freezing of certain molten mixtures of special composition. The data are adequate to suggest the presence of certain structural units in the melt mixtures and to discuss possible structural models and consistent band assignments. Experimental Section Materials and Sample Preparation. The samples were prepared by mixing Nb2O5 (Cerac/Pure) with K2SO4 (Fluka) and K2S2O7 that was made by thermal decomposition of K2S2O8 (Fluka), as described previously.8 All handling of chemicals and filling of the Raman optical cells (made of cylindrical fused silica tubing (6 ( 0.1 mm o.d., 4 ( 0.1 mm i.d., and ∼3 cm long for the part containing the molten salts)) took place in a dry nitrogen-filled glovebox. The total amount of salt mixture added into each cell was 350-500 mg. Proper mixing of the components was necessary, as the melting point of Nb2O5 (1510 °C) is very high compared to the fusion temperatures of the Nb2O5-K2S2O7 and Nb2O5-K2S2O7-K2SO4 mixtures. Thus, the optical cells were filled either by transferring Nb2O5 and K2SO4 with approximately half the K2S2O7 into the cell and then adding the remaining K2S2O7 on top or by grinding all components intimately in an agate mortar before transferring into the optical cell. The samples were sealed under a low pressure (ca. 0.2 bar) of O2 (L’Air Liquide, 99.99%) in order to prevent self-reduction. Afterward they were equilibrated at 500-850 °C for several days (up to 4 weeks) before recording the Raman spectra. The long equilibration time was necessary due to the slow dissolution of Nb2O5. Upon dissolution of niobium oxide in potassium pyrosulfate, the resulting melts became transparent (colorless to pale yellow) and were extremely viscous. Often, it was necessary to remove bubbles and/ or accelerate the dissolution of solids by torching the samples. 0 Slow cooling of the samples with XNb g 0.17 leads to 2O5 formation of glasses. With further lowering of the temperature, several of the glassy samples exploded (tensions). The symbol X0i is used to denote the mole fractions of nonreacted components of the Nb2O5-K2S2O7 binary mixture (weighed-in amounts) before any reaction had started. The composition of the ternary mixture is defined by combining X0Nb2O5 (neglecting K2SO4) with the ratio Y (Y ) n(SO42-)/n(Nb)) of the number of sulfate groups added per niobium atom, and this ratio was varied between 0 and 2.5. It was not possible to dissolve more than 22.2 mol % Nb2O5 in K2S2O7. A very steep increase of the melting point was observed on going from X0Nb2O5 0 ) 0.20 (mp ∼490 °C) to XNb ) 0.222 (mp ∼700 °C). The 2O5 dissolution of Nb2O5 is facilitated in ternary mixtures where sulfate is also present. Table 1 summarizes the compositions of the cells made during the course of the present work. Raman Spectra. The Raman setup, the furnace for the optical cells, and the procedures for obtaining Raman spectra from molten salts and vapors at high temperatures have been described in detail elsewhere.2,3,5,27,28 Raman spectra were excited with the 488.0 nm line of a Spectra Physics model 164 argon ion laser operated at a power of 80 and 50 mW (at the sample) for recording the spectra from the Nb2O5-K2S2O7 and Nb2O5-K2S2O7-K2SO4 molten mixtures, respectively. The scattered light was collected at an angle of 90° (horizontal scattering plane), rotated with a 90° image rotator, analyzed with a 0.85 m Spex 1403 double monochromator, and detected with a -20 °C cooled RCA PMT equipped with EG&G/ ORTEC photon counting electronics. Spectra were recorded by

Paulsen et al. 0 TABLE 1: Relative Molar Compositions, XNb ,a and 2O5 Indicators of Incremental Sulfate Content, n(SO42-)/n(Nb), of Nb2O5-K2S2O7-K2SO4 Samples

cell no.

0 a XNb 2O5

Y ) n(SO42-)/n(Nb)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

0 0.089 0.126 0.166 0.199 0.212 0.220 0.100 0.150 0.166 0.199 0.199 0.202 0.200 0.200 0.199 0.250 0.249 0.251 0.250 0.249 0.250

1.00 1.00 1.00 0.204 0.257 0.494 1.00 1.50 2.02 0.248 0.500 0.748 1.00 1.01 1.50

a 0 XNb2O5 denotes the mole fractions of nonreacted components of the Nb2O5-K2S2O7 binary mixture (weighed-in amounts) before any reaction had started and any K2SO4 was added.

using two polarizations of the incident scattered light to the scatteringplane:thevertical-vertical(VV)andthehorizontal-vertical (HV). During the experiments, the optical geometry, the spectral slit width, and the laser power measured before and after the entrance and exit windows of the furnace were kept constant in each series of spectra (i.e., spectra of binary or ternary molten salts). The Raman cells were placed inside the mechanically stable metal core of the furnace and were always in a fixed position relative to the collecting lens and entrance slit. The intensity of the scattered light was maximized by positioning the focusing and collecting lenses with two x,y,z micropositioners. After the spectra were obtained, the cell could be removed and cooled to room temperature and then reintroduced into the optical furnace, yielding with no further micropositioner adjustments essentially the same Raman intensities. Thus, by use of the same experimental conditions, the Raman intensities could be reproduced to within 2-5%. Results and Discussion Raman Spectra of Nb2O5-K2S2O7 Mixtures. Several 0 ) 0-0.22 Nb2O5-K2S2O7 mixtures with compositions XNb 2O5 were placed in cells, sealed under oxygen atmosphere (PO2) 0.2 bar), and heated until equilibrium was attained as described in the Experimental Section. Upon dissolution of Nb2O5 in K2S2O7, transparent colorless or pale-yellowish viscous melts were obtained which commonly formed glasses upon cooling. Most of the samples that formed glasses exploded while reaching room temperature (most probably due to volume expansion). Representative Raman spectra of melts are shown in Figure 1. 0 ) 0-0.17 melt readily at 450 °C, while Samples with XNb 2O5 higher temperatures are required for melting the mixtures with 0 ) 0.20-0.22 (see Figure 1). The Raman spectra of pure XNb 2O5 molten potassium pyrosulfate are well-known2,29 and were also recorded and included in Figure 1 for comparison. The most characteristic Raman bands of the S2O72- ion in molten K2S2O7

Niobium(V) Oxosulfato Complexes

J. Phys. Chem. A, Vol. 114, No. 28, 2010 7487 Therefore, it appears that the complex species formed according to eq 1 consists of NbO3+ and SO42- units. The four sulfate fundamentals (ν1-ν4) for a tetrahedral Td configuration span the following representation:

Γvib ) A1(ν1) + E(ν2) + 2F2(ν3 + ν4)

Figure 1. Raman spectra obtained for molten Nb2O5-K2S2O7 mixtures (0 < X0Nb2O5 < 0.22) in oxygen atmosphere (pO2 ) 0.2 bar) at temperatures 0 denotes the mole fraction of as indicated by each spectrum: XNb 2O5 Nb2O5; laser wavelength, λ0 ) 488.0 nm; laser power, w ) 80 mW; resolution, 6 cm-1.

TABLE 2: NbdO Stretching Raman Wavenumbers for Different Nb Compounds compound

NbdO stretching, cm-1

ref

NbOCl3(g) PPh3Me[NbOCl4(CH3CN)] [As(C6H5)4]2NbOCl5 NbOFn(n-3)- (in LiF-NaF-KF melt) CsNbOCl4 (l, T ) 560 °C) Cs2NbOCl5 (l, T ) 650 °C)

997 960 923 921 970 932

30 31 32 33 34 34

at 450 °C occur at 1085 (terminal stretching), 730 (bridging S-O-S stretching), and 318 cm-1 (S-O-S deformation). Addition of Nb2O5 gives rise to the appearance of several new bands. The new features due to the complex(es) formed occur at 1275 (dp), 1193 (p), 1050 (p), 937 (p), 693 (p), 665 (p), 618 (p), 591 (dp), 540 (p), 463 (p), 414 (dp), 263 (p), and 190 cm-1. Notably, the band positions are slightly red-shifted for spectra recorded at higher temperatures. The intensities of the above new bands increase relative to the bands of the S2O72- ion with 0 and dominate the spectra of the samples with increasing XNb 2O5 0 XNb2O5 ) 0.21 and 0.22. This is indicating that the reaction taking place leads to a niobium(V) complex at the expense of the S2O72- ion, most likely according to a general reaction scheme represented by eq 1

Nb2O5 + nS2O72- f C2n-

All modes are Raman active, and only F2 is IR allowed; in the usual approximation of weak couplings, modes labeled as ν1 and ν3 are stretchings and ν2 and ν4 are angle bendings. The Raman wavenumbers of the four fundamentals are well-known from Raman work on aqueous solutions: ν1(A1) ≈ 980 cm-1, ν2(E) ≈ 450 cm-1, ν3(F2) ≈ 1100 cm-1, and ν4(F2) ≈ 615 cm-1.30 However, coordination of the sulfate groups is expected to shift the bands, reduce the symmetry, and lift the degeneracies of the ν2, ν3, and ν4 modes. This behavior has already been observed and demonstrated from Raman studies of molten V2O5-M2S2O7-M2SO4 and V2O5-M2SO4 (M ) K, Cs) mixtures.2,3 Table 3 provides a summary of the observed Raman wavenumbers of the studied Nb2O5-K2S2O7 and Nb2O5K2S2O7-K2SO4 molten mixtures. More detailed assignments for the Nb2O5-K2S2O7 system will follow the determination of the stoichiometry of eq 1. A final notable composition effect (apart the gradual increase of the bands due to the NbV complex with 0 ) pertains to the gradual intensity reversal increasing XNb 2O5 among the 693 and 665 cm-1 ν4 sulfate components (marked by dashed vertical lines in Figure 1, see discussion below). Stoichiometry of the NbV Complex in the Binary Nb2O5-K2S2O7 System. For determining the stoichiometry of reaction 1, we applied a procedure,1,5 which correlates the relative Raman band intensities with the stoichiometric coefficient, n. Reaction 1 is assumed to be complete and its equilibrium mixture consists of the complex species, C2n-, and S2O72-. The basic concept of the method originates from the theory of vibrational Raman scattering from an assembly of randomly oriented molecules, which is well-established, and detailed formulas and derivations can be found, e.g., in ref 35. For brevity, it is sufficient to state that the measured integrated Raman intensity due to a vibrational fundamental ν(i) of species j, Ij,ν(i), is related to the number of moles of species j contained in the scattering volume, Nj, according to

Ij,ν(i) ) A

(2)

where f(ν(i),T) equals 1 - exp(-hcν(i)/kT) and A embodies a number of factors such as the molecular scattering properties, excitation laser wavelength, spectrometric (instrumental) factors, scattering volume, etc. Now, the determination of the stoichiometry of eq 1 is based on the assertion that the ratio of the scattering power per ion of S2O72- divided by the scattering power per ion of C2n- represented by

(1a) Io )

The most characteristic and well-defined bands due to the NbV complex (C2n-) are the 937 and 1050 cm-1 polarized bands (marked by dashed lines in Figure 1). The 937 cm-1 lies in the NbdO stretching region (see Table 2 for a pertinent summary of NbdO Raman wavenumbers for compounds of interest to the present work). The rest of the bands due to the complex are reminiscent of sulfate modes in environments of moderately reduced symmetry resulting from coordination and bridging.

1 N f(ν(i), T) j

(IS2O72-,ν(S2O72-))f(νS2O72-, T)/Neq,S2O72(IC2n-,ν(C))f(νC2n-, T)/Neq,C2n-

(3)

is constant independent of composition as well as on total amounts contained in a given scattering volume and on temperature. Neq,S2O72- and Neq,C2n- are the number of moles of the components S2O72- and C2n- in the final equilibrium mixture. The inclusion of the Boltzmann thermal population factor, f(ν(i),T), in the numerator and denominator of eq 3 disentangles

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TABLE 3: Raman Band Wavenumbers (cm-1) and Assignments for Molten Nb2O5-K2S2O7/O2(g) and Nb2O5-K2S2O7-K2SO4/ O2(g) Mixturesa Nb2O5-K2S2O7(l) binary mixtures band wavenumber

Nb2O5-K2S2O7-K2SO4 (l) ternary mixtures

tentative assignment for NbO(SO4)33-(l)b

1275 w, dp 1193 m, p

ν3(SO4) ν3(SO4)

1050 s, p

ν1(SO4)

937 vs, p

ν(NbdO)

693 m, p 665 m, p 618 w, p 591 w, dp 540 m, p 463 w, dp 448 w, dp 414 w, dp

ν4(SO4) ν4(SO4), [NbO(SO4)3m]3m- (?) ν4(SO4) ν4(SO4), ν(Nb-O) ν(Nb-O) ν2(SO4) ν2(SO4), [NbO(SO4)3m]3m- (?) ν2 (SO4)

263 vs, p 190 w, p

band wavenumber

tentative assignment

c

1210 w, dp 1175 m, p 1079 m, p 1049 vs, p 958 s, p

ν3(SO4) ν3(SO4) ν1(S2O7) ν1(SO4) ν(NbdO), NbO(SO4)4S2O77-

926 vs, p (∼910)d m, dp 727 w, p

νs[Nb(dO)2], NbO2(SO4)23νas[Nb(dO)2], NbO2(SO4)23ν(S2O7)

662 s, p 604 m, dp

ν4(SO4) ν4(SO4)

528 m, p 458 m, dp (?) (446)d w, dp 414 w, dp 315 m, p 274 vs, p 180 w, dp

ν(NbsO), νbending[Nb(dO)2] ν2(SO4) ν2(SO4) ν2(SO4) ν(S2O7)

a Abbreviations: s ) strong; m ) medium; w ) weak; br ) broad; v ) very; p ) polarized; dp ) depolarized. b With increasing 0 concentration (i.e., XNb > 0.17) formation of polymeric [NbO(SO4)3m]3m- becomes evident. c Assignments for NbO(SO4)4S2O77- and 2O5 NbO2(SO4)23-. d Obscured band.

the experimentally measured Raman band intensities from temperature effects. Indeed, I° should be a universal constant apart from a combination of factors (including inter alia the excitation wavelength and the instrumental response) which cancel in eq 3. On account of the stoichiometry of reaction 1, Neq,S2O72- and Neq,C2n- may be expressed in terms of n as follows 0 Neq,S2O72- ) NS0 2O72- - nNNb 2O5

(4)

0 Neq,C2n- ) NNb 2O5

(5)

and the total number of moles as 0 Neq,total ) NS0 2O72- - (n - 1)NNb 2O5

(6)

Thus, for a correct choice of n, I° should be constant independent of cell composition. Obviously, the procedure can be applied only under the assumption that there is one single independent stoichiometric process taking place in the considered system. Although it is immaterial which particular band is representing a certain species, it is reasonably preferable that the choice pertains to bands due to totally symmetric vibrations, not overlapping with other bands. In general, sharp, symmetric, and polarized bands are preferred over broad depolarized bands. Below, the I° ratio is used for determining the stoichiometric coefficient, n. Six binary Nb2O5-K2S2O7 mixtures were prepared (see Table 1) and heated until equilibrium was attained, the Raman spectra were recorded (see Figure 1), and the integrated intensities (peak areas) of bands representative for each species were measured (see Table S1 in Supporting Information). Two bands were chosen to represent the NbV complex (C2n-), namely, the 937 cm-1 NbdO and the 1050 cm-1 sulfate bands, while S2O72- is represented by its strongest 1085 cm-1 band. I° was then computed (for both choices of bands representing C2n-)

for five choices of n (n ) 1, 2, 3, 3.5, and 4) for all mixtures and the partial results are listed in Table S2 (Supporting Information). The correct choice of n is the one that results in a constant I° value for all samples. Figure 2 shows the plots of 0 (i.e., for all six cells made for the binary the I° ratio vs XNb 2O5 Nb2O5-K2S2O7 mixtures) for both choices of bands representing the C2n- complex (vide ante). The correct stoichiometry (giving rise to constant I° independent of cell composition) is reflected by n ) 3, as shown by the horizontal lines in Figure 2 (see also Table S2 in Supporting Information). Significant deviations from this line are observed for n ) 1, 2, 3.5, and 4. Thus the product formula for reaction 1 is Nb2S6O266- and by taking into account the spectral features discussed in the context of Figure 1, i.e., the occurrence of a NbdO unit and of coordinated sulfate groups, the most plausible form for reaction 1 is

Nb2O5 + 3S2O72- f 2NbO(SO4)33-

(7)

The above reaction is the only possible one, if we take into account the consumption of pyrosulafte ions and the appearance of bands which can be assigned to NbdO stretching (937 cm-1) and to coordinated sulfate groups (terminal S-O stretching at 1050 cm-1 and several split components of the ν2, ν3, and ν4 fundamental sulfate modes). Furthermore, finding the same stoichiometry (n ) 3) for both choices of the most characteristic bands due to the NbV complex(es) formed provides an independent confirmation for the occurrence of one single stoichiometric process in the studied system. Reaction 7 should be rather regarded as a scheme accounting for the formation of the NbO(SO4)33- unit which can occur as a monomer in dilute melts, whereas polymeric units and/or three-dimensional [NbO(SO4)3]m3m- networks may be formed with increasing X0Nb2O5 as judged from the increasing viscosity and glass-forming 0 g 0.17. ability of melts with XNb 2O5 Structural Model for NbO(SO4)33-. Figure 3 shows a simple and “balanced” plausible structure for the NbO(SO4)33- complex with the Nb atom in a seven-coordinated arrangement of a distorted

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Figure 2. Plots of the intensity ratio Io for five assumed values of the stoichiometric coefficient, n. (A) Io is computed based on IC,1050; (B) Io is computed based on IC,937.

Figure 3. Plausible structural model for the NbO(SO4)33- molten complex.

pentagonal bipyramid. The terminal oxygen occupies an apical position, along the direction of which the niobium is displaced above the equatorial plane of the bipyramid. Two bidentate chelating sulfates are positioned in the equatorial plane, while the third one occupies the remaining axial and equatorial positions of the pentagonal bipyramid. Such an arrangement (which conforms to the coordination chemistry of Nb14,36) would result to relatively similar symmetries for the sulfate groups, thereby accounting for the moderate splitting of the degenerate ν2, ν3, and ν4 modes observed in the Raman spectra of Figure 1. A summary of assignments together with intensity and polarization characteristics which are consistent with the proposed structural model for the NbO(SO4)33- molten complex is compiled in Table 3. 0 , formation of It is evident that with increasing XNb 2O5 polymeric and/or three-dimensional [NbO(SO4)3]m3m- networks may take place (vide ante). Such a polymerization may be achieved if, e.g., a chelating bidentate sulfate “opens up” and transforms to bridging bidentate sulfate, thereby resulting in a substitution of a chelating sulfate with two bridging sulfates shared between two neighboring NbO(SO4)3 units. Such a transformation would not affect significantly (with increasing 0 ) the remaining structural and vibrational features of the XNb 2 O5 complex, in agreement with the Raman spectra shown in Figure 1. The observed gradual intensity reversal among the 693 and 665 cm-1 ν4 sulfate components (marked by dashed vertical

lines in Figure 1) and among the 463 and 448 cm-1 ν2 sulfate components may thus be accounted for by assigning the 665 and 448 cm-1 bands as due to the respective ν4 and ν2 sulfate modes of the polymeric [NbO(SO4)3]m3m- chains. Raman Spectra of Nb2O5-K2S2O7-K2SO4 Molten Mixtures. The presence of K2SO4 in the binary Nb2O5-K2S2O7 molten mixtures results in easier dissolution of larger Nb2O5 quantities. 0 ) 0.25 and ratio of added For example, a mixture with XNb 2O5 sulfate per Nb, Y, equal to 0.5 melts readily at 700 °C. Figures 4 and 5 show the dependence of the Raman spectra for mixtures 0 ) 0.20 and with formal mole fractions of Nb2O5 equal with XNb 2O5 0.25, respectively from Y, i.e., the amount of added sulfate in the initial mixture (weighed-in amounts). The Raman spectra of molten K2S2O7 are included in both figures for comparison. It is evident 0 ) 0.20 contain excess amounts of the that all mixtures with XNb 2O5 solvent, S2O72-, as indicated by the prominent presence of the sharp 1080 cm-1 S2O72- band in Figure 4. The composition effects on the Raman spectra show that the incremental addition of sulfate results in gradual changes that appear to terminate for a value of Y ) n(SO42-)/n(Nb) equal to 1. In particular, the following observations are made: (i) The 937 cm-1 NbdO band broadens and gradually transforms to a doublet (main peak at 926 cm-1 with a shoulder at ∼958 cm-1). The occurrence of two bands in the NbdO stretching region indicates the presence of Nb in two distinctly different coordination environments (vide infra). (ii) The intensity of the ∼1050 cm-1 sulfate (terminal S-O stretching) band progressively increases (i.e., relative to the NbdO band) with increasing n(SO42-)/n(Nb) up to approximately Y ) 1. (iii) The ∼693 cm-1 ν4 component (chelating sulfate bending) vanishes with increasing Y and the neighboring 664 cm-1 ν4 component becomes more prominent. (iv) The 1079 cm-1 S2O72- band is present in the Raman spectra of all melts. This was (at a first view) surprising 0 ) 0.25 (i.e., Figure for the molten mixtures with XNb 2O5 5), in view of the arbitrary expectation (valid for the

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Paulsen et al. (v) Presence of K2SO4 in larger amounts (i.e., for Y > 1) results in precipitation of K2SO4(s), judged from the appearance of a white solid in the melts and the observation of the characteristic ν1(SO42-) at 965 cm-1.

Figure 4. Titration-like series of Raman spectra obtained in oxygen atmosphere (pO2 ) 0.2 bar) at 700 °C for molten 0 Nb2O5-K2S2O7-K2SO4 mixtures with XNb ) 0.20 and incremental 2O5 0 presence of K2SO4 (0 < Y < 2) as indicated by each spectrum. XNb 2O5 denotes the formal mole fraction of Nb2O5 in the binary Nb2O5-K2S2O7 mixture. Recording parameters: λ0 ) 488.0 nm; w ) 50 mW; resolution, 6 cm-1.

Figure 5. Titration-like series of Raman spectra obtained in oxygen atmosphere (pO2 ) 0.2 bar) at 700 °C for molten 0 Nb2O5-K2S2O7-K2SO4 mixtures with XNb ) 0.25 and incremental 2O5 0 presence of K2SO4 (0 < Y < 1.5) as indicated by each spectrum. XNb 2O5 denotes the formal mole fraction of Nb2O5 in the binary Nb2O5-K2S2O7 mixture. Recording parameters: see caption of Figure 4.

binary system) that Nb2O5 would react stoichiometrically with molten K2S2O7 at a 1:3 ratio (vide ante).

A first hypothetical interpretation was to propose a reaction “following up” the one occurring in the binary system (eq 7) with addition of sulfate at a ratio Y )n(SO42-)/n(Nb) ) 1, e.g., Nb2O5 + 3S2O72- + 2SO42- f 2NbO(SO4)45-. If this was the 0 ) 0.25 and case, then the spectrum (e) in Figure 5 (i.e., XNb 2O5 Y ) 1) should not show the presence of S2O72- and should exhibit only one NbdO mode. However, the above observations (i) and (iv) provide direct evidence that such an interpretation would be dubious. The simplest plausible reaction scheme that would be consistent with the spectral data should give rise to formation of at least two oxoniobium species with the inclusion of S2O72- as ligand to one of them, thereby accounting for the above observations (i) and (iv), respectively. The two bands at 926 and 958 cm-1 (Figures 4e and 5e) assigned to NbdO cannot be due to a single, i.e., a dioxo Nb(dO)2 unit; one could probably envisage such a proposal in view of the expectation that a dioxo OdNbdO unit would possess two stretching modes (a symmetric and an antisymmetric mode). However, in the case of a transition metal dioxo unit, the Raman band due to the symmetric stretching is much stronger in intensity and its wavenumber is expected to be 10-40 cm-1 higher compared to the respective characteristics of the antisymmetric mode,30 whichsin additionsis partly depolarized. However, both bands at 926 and 958 cm-1 are polarized and the strong one (at 926 cm-1) is at lower wavenumber, thereby justifying their assignment to the respective NbdO modes of two different NbV oxosulfato units (cores). Furthermore, Figure 5 shows that the incremental presence of sulfate is not “titrating out” the S2O72-, of which the strongest band at 1079 cm-1 (marked by a dotted line in Figure 5) is still present for Y ) 1 (spectrum 5e). Notably, temperature variations in the range 550-700 °C do not affect the relative intensities of the bands (see spectra b and c in Figure 6), indicating that the presence of S2O72- is not due to equilibrium shifts in dissociation-like reactions and that S2O72- is present as a ligand. Table 3 provides a summary of the observed Raman wavenumbers of the Nb2O5-K2S2O7-K2SO4 molten mixtures. More detailed assignments will follow the presentation of the plausible structural models for the formed complex(es). Spectral Changes upon Freezing. Further insight comes from a study of spectral changes occurring upon freezing the molten mixtures. Figure 6 shows the Raman spectra obtained 0 ) 0.25 and Y ) for the Nb2O5:3K2S2O7:2K2SO4 (i.e., XNb 2O5 2n(SO4 )/n(Nb) )1) mixture in the molten state (spectra b and c in Figure 6 obtained at 700 and 570 °C, respectively) and after gradual cooling, freezing, and reheating (spectra d, e, and f in Figure 6). Figure 6 includes reference spectra of molten K2S2O7 (spectrum a) and of the crystalline reference compounds K7Nb(SO4)6 (cr), K2SO4 (cr), and K2S2O7 (cr) (spectra g, h, and i, respectively). The formation of the crystalline K7Nb(SO4)6 compound is known to occur by dissolving Nb2O5 in molten K2S2O7 at 450 °C followed by slow cooling.13 The structure consists of hexasulfatoniobate(V) Nb(SO4)67- ions with unidentate coordination of sulfate to the Nb atom, resulting in a distorted octahedral NbO6 arrangement around Nb, as shown in Figure 7.13 Notably, a complete reaction between Nb2O5, K2S2O7, and K2SO4 in the molten state could lead to the

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Figure 8. Plausible structural models for the NbO(SO4)4S2O77- molten complex: (A) configuration with Nb in 7-fold coordination of a pentagonal bypiramid; (B) arrangement with Nb in 6-fold coordination with a unidentate instead of a bidentate S2O72-. (Notes: NbO(SO4)4S2O77- is isomeric with Nb(SO4)67-, shown in Figure 7).

Figure 6. Raman spectra obtained in an oxygen atmosphere (pO2 ) 0 0.2 bar) at 700 °C for Nb2O5-K2S2O7-K2SO4 mixtures with XNb ) 2O5 0.25 and Y ) 1 and for reference compounds at physical states and 0 temperatures as indicated by each spectrum. XNb denotes the formal 2O5 mole fraction of Nb2O5 in the binary Nb2O5-K2S2O7 mixture. Recording parameters: see caption of Figure 4.

Figure 7. Qualitative structural depiction of the Nb(SO4)67- ion in the K7Nb(SO4)6 crystalline complex.13

formation of the same compound after gradual cooling as follows

Nb2O5 + 5K2S2O7 + dissolution at 700°C followed by gradual cooling

2K2SO4 98 2K7Nb(SO4)6(s) (8)

Indeed, a cell with the above composition (i.e., Nb2O5:5K2S2O7: 2K2SO4; cell 10, see Table 1) was made and the Raman spectrum of the solid phase obtained after gradual cooling is shown in Figure 6g. This cell was then cut open in the glovebox and a powder XRD analysis confirmed its identity as K7Nb(SO4)6. We note the consistent absence of bands in the NbdO stretching region (900-1000 cm-1) of the spectrum in Figure 6g, as all oxygen atoms of the NbO6 octahedron are bonding to sulfur atoms (along Nb-O-S bridges). Now, a comparison between the spectra g and f in Figure 6 shows that all bands due to the K7Nb(SO4)6 crystalline compound are present in spectrum f of the solid phase obtained

after slow cooling of the ternary Nb2O5:3K2S2O7:2K2SO4 molten mixture. Furthermore, by comparing spectrum f with the spectra h and i, it turns out that this solid phase does not contain crystalline K2S2O7 and/or K2SO4. Thus, slow cooling of the Nb2O5:3K2S2O7:2K2SO4 molten mixture results in formation of K7Nb(SO4)6 plus a second crystalline species, of which the presence is evidenced by the rest of the bands in Figure 6f. Structural Models and Band Assignments for Nb2O5-K2S2O7-K2SO4 Molten Mixtures. After examining the spectral changes occurring upon freezing the Nb2O5K2S2O7-K2SO4 molten mixtures, we proceed with the structural interpretation and band assignments pertaining to the molten mixtures, for which we have already presented and preliminarily discussed the Raman spectra in the context of Figures 4 and 5. We have also shown that cooling and freezing of the Nb2O5-K2S2O7-K2SO4 molten mixture with X0Nb2O5 ) 0.25 and Y ) n(SO42-)/n(Nb) ) 1 does not result in formation of crystalline K2S2O7, thereby indicating that the presence of S2O72(see Figure 5e) in the ternary molten mixtures is not in free form but in the form of a coordinated ligand. We have, moreover, shown that there exist two NbV oxosulfato complexes in solution, of which the one is most probably an isomeric precursor of the sulfato species Nb(SO4)67-, judged from the formation of the respective K7Nb(SO4)6 crystalline compound upon freezing. A simple and plausible formula for a NbV oxosulfato complex isomeric to Nb(SO4)67- that contains a NbdO unit and a S2O72- ion as ligand is NbO(SO4)4S2O77-. Therefore, our proposal for the complete reaction that takes place in the ternary system is

Nb2O5 + 3S2O72- + 2SO42- f NbO(SO4)4S2O77-(l) + NbO2(SO4)23-(l) (9) in full consistency with the spectral features observed in Figures 4-6. The proposed formula for the second product species, NbO2(SO4)23-, is deduced by a simple mass balance consideration in reaction 9. Notably, the vanadium(V) analogue VO2(SO4)23- is known and its existence has been established in V2O5-M2S2O7-M2SO4 (M ) K, Cs) and V2O5-M2SO4 (M ) K, Cs) molten mixtures.1-3 From a structural point of view, there are more than one possibilities for the NbO(SO4)4S2O77- molten complex. The first one is shown in Figure 8A and exhibits a 7-fold coordination around the Nb atom, which is found in the center of a distorted pentagonal bipyramid. The Nb atom is shown displaced above

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Nb2O5 + 3S2O72- f 2NbO(SO4)33-

Figure 9. Plausible structural model for the NbO2(SO4)23- molten complex.

the equatorial plane of coordination along the direction of the double NbdO bond. All sulfate groups are assumed unidentate and the pyrosulfate group is shown in bidentate coordination. Alternatively, the structural model shown in Figure 8B is equaly plausible and pertains to an isomeric configuration with Nb in a 6-fold coordination, in which the pyrosulfate group is unidentate and, in addition, exhibits a coordinative saturation (Nb connected to a terminal O atom with a double bond and coordinated to four unidentate sulfate groups and one unidentate pyrosulfate group). Figure 9 shows a plausible configuration for the NbO2(SO4)23complex, which possesses a dioxo OdNbdO unit. The proposed configuration in Figure 9 is identical to the one of the VO2(SO4)23-.2,3 At this point, it is of special interest to discuss the assignments of the Raman bands seen in the NbdO stretching region. The NbO2(SO4)23- is expected to exhibit two Raman bands due to NbdO stretching modes: (i) a strong polarized band due to the symmetric stretching, νs(OdNbdO) and (ii) a weak depolarized band due to the antisymmetric stretching, νas(OdNbdO) expected 10-40 cm-1 lower compared to the band due to the symmetric stetching.30 Furthermore, the symmetric stretching of a dioxo OdNbdO unit is expected at a lower wavenumber compared to the symmetric stretching of mono-oxo NbdO unit. Conversely, the NbsO distance is expected to be longer within a dioxo unit as compared to the respective distance within a mono-oxo unit. Therefore, we assign the 958 cm-1 polarized band as due to the NbdO stretching of the NbO(SO4)4S2O77molten complex and the 926 cm-1 polarized band as due to the symmetric stretch νs(OdNbdO) of the NbO2(SO4)23- complex. Now, a careful inspection of the (HV) spectrum (e) in Figure 5 reveals that there are few depolarized components in the 850-920 cm-1 region, one of which is located at ∼910 cm-1 and could be assigned to the depolarized antisymmetric stretching νas(OdNbdO) of the dioxo complex. The band envelope of the 926/958 cm-1 doublet obscures a number of weak bands; notably, mixing of group vibrations is expected to take place. A summary of assignments together with intensity and polarization characteristics which are consistent with the proposed structural model for the NbO(SO4)4S2O77- and NbO2(SO4)23molten complexes is compiled in Table 3. Conclusions The composition and temperature effects in the Raman spectra obtained for molten Nb2O5-K2S2O7 and Nb2O5-K2S2O7K2SO4 mixtures under O2 atmosphere are used for establishing the structural and vibrational properties of NbV oxosulfato complexes formed in the studied systems. The spectral features and a quantitative exploitation of the Raman band intensities for the binary Nb2O5-K2S2O7 molten system in the presence of O2 gas point to NbO(SO4)33- as the NbV oxosulfato complex formed according to the reaction

An arrangement with Nb in 7-fold coordination of a distorted pentagonal bipyramid is proposed for NbO(SO4)33-, involving a terminal NbdO bond and three bidentate chelating sulfates, thereby accounting for the observed 937 cm-1 NbdO band and for the 1050 cm-1 ν1(SO4) band as well as for the observed moderate splitting of the degenerate ν2, ν3, and ν4 sulfate modes. The spectral features along with the pertinent composition and temperature-dependent effects in the Raman spectra obtained for the Nb2O5-K2S2O7-K2SO4 molten mixtures can be accounted for by the formation of two NbV oxosulfato complexes. The changes induced by the incremental presence of sulfate terminate at an equimolar (1:1) SO42-/Nb ratio and the stoichiometric representation for the complete dissolution reaction is

Nb2O5 + 3S2O72- + 2SO42- f NbO(SO4)4S2O77- + NbO2(SO4)23Structural models and consistent band assignments are proposed for the NbV complexes. The Raman wavenumbers of the NbdO stretching modes are found at 958 and 926 cm-1 for the monooxo NbO(SO4)4S2O77- and for the symmetric stretching of the dioxo NbO2(SO4)23- complex, respectively. Acknowledgment. Anastasia Spyraki is thanked for assisting some of the sample preparation experiments. The authors are furthermore indebted to Professor Kurt Nielsen for the X-ray analysis of crystalline phases. Supporting Information Available: Tables of relative integrated Raman intensities (peak areas in arbitrary umits) of representative bands of the C2n--NbV complex (at 937 and 1050 cm-1) and of the S2O72- solvent (at 1085 cm-1) measured from the Raman spectra of molten Nb2O5-K2S2O7 mixtures and calculated equilibrium mole fractions of NbV complex (C2n-) and S2O72- and values of Io (based on both main complex bands at 937 and 1050 cm-1) for various assumed integral values for the stoichiometric coefficient, n, of eq 1. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Boghosian, S.; Borup, F.; Chrissanthopoulos, A. Catal. Lett. 1997, 48, 145. (2) Boghosian, S.; Chrissanthopoulos, A.; Fehrmann, R. J. Phys. Chem. B 2002, 106, 49. (3) Boghosian, S. J. Chem. Soc., Faraday Trans. 1998, 94, 3463. (4) Lapina, O. B.; Bal’zhinimaev, B.; Boghosian, S.; Eriksen, K. M.; Fehrmann, R. Catal. Today 1999, 51, 469. (5) Boghosian, S.; Berg, R. W. Appl. Spectrosc. 1999, 53, 565. (6) Berg, R. W.; Thorup, N. Inorg. Chem. 2005, 44, 3485. (7) Berg, R. W.; Ferre´, I. M.; Scha¨ffer, S. J. C. Vib. Spectrosc. 2006, 42, 346. (8) Hansen, N. H.; Fehrmann, R.; Bjerrum, N. J. Inorg. Chem. 1982, 21, 744. (9) Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. Inorg. Chem. 1986, 25, 1132. (10) Hatem, G.; Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. J. Phys. Chem. 1987, 91, 195. (11) Folkmann, G.; Hatem, G.; Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. Inorg. Chem. 1993, 32, 1559. (12) Karydis, D. A.; Erisken, K. M.; Fehrmann, R.; Boghosian, S. J. Chem. Soc., Dalton Trans. 1994, 2151. (13) Borup, F.; Berg, R. W.; Nielsen, K. Acta Chem. Scand. 1990, 44, 328, and references therein. (14) Berg, R. W. Coord. Chem. ReV. 1992, 113, 1.

Niobium(V) Oxosulfato Complexes (15) Fehrmann, R.; Boghosian, S.; Berg, R. W.; Papatheodorou, G. N.; Nielsen, K.; Bjerrum, N. J. Inorg. Chem. 1989, 28, 1847. (16) Fehrmann, R.; Boghosian, S.; Berg, R. W.; Papatheodorou, G. N.; Nielsen, K.; Bjerrum, N. J. Inorg. Chem. 1990, 29, 3294. (17) Berg, R. W.; Boghosian, S.; Bjerrum, N. J.; Fehrmann, R.; Krebs, B., N. Strater; Mortensen, O. S.; Papatheodorou, G. N. Inorg. Chem. 1993, 32, 4714. (18) Boghosian, S.; Fehrmann, R.; Nielsen, K. Acta Chem. Scand. 1994, 48, 724. (19) Boghosian, S.; Eriksen, K. M.; Fehrmann, R.; Nielsen, N. Acta Chem. Scand. 1995, 49, 703. (20) Nielsen, K.; Boghosian, S.; Fehrmann, R.; Berg, R. W. Acta Chem. Scand. 1999, 53, 15. (21) Karydis, D. A.; Boghosian, S.; Nielsen, K.; Eriksen, K. M.; Fehrmann, R. Inorg. Chem. 2002, 41, 2417. (22) Rasmussen, S. B.; Boghosian, S.; Nielsen, K.; Eriksen, K. M.; Fehrmann, R. Inorg. Chem. 2004, 43, 3697. (23) Christodoulakis, A.; Boghosian, S. J. Catal. 2003, 215, 139. (24) Cline Scha¨ffer, S. J.; Berg, R. W. Acta Crystallogr., Sect. E: Struct. Rep. Online 2008, E64, i20. (25) Cline Scha¨ffer, S. J.; Berg, R. W. Acta Crystallogr., Sect. E: Struct. Rep. Online 2008, E64, i73.

J. Phys. Chem. A, Vol. 114, No. 28, 2010 7493 (26) Ståhl, K.; Berg, R. W. Acta Crystallogr., Sect. E: Struct. Rep. Online 2009, E64, i88. (27) Boghosian, S.; Papatheodorou, G. N. J. Phys. Chem. 1989, 93, 415. (28) Knudsen, C.; Kalampounias, A. G.; Fehrmann, R.; Boghosian, S. J. Phys. Chem. B 2008, 112, 11996. (29) Dyekjaer, J. D.; Berg, R. W.; Johansen, H. J. Phys. Chem. A. 2003, 107, 5826. (30) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 6th ed.; Wiley-Interscience: New York, 2009. (31) Hiller, W.; Stra¨hle, J.; Prinz, H.; Dehnicke, K. Z. Naturforsch. 1984, 39B, 107. (32) Mu¨ller, U.; Lorentz, I. Z. Anorg. Allg. Chem. 1980, 463, 110. (33) von Barner, J. H.; Christensen, E.; Bjerrum, N. J.; Gilbert, B. Inorg. Chem. 1991, 30, 561. (34) Rosenkilde, C.; Voyiatzis, G. A.; Jensen, V. R.; Ystenes, M.; Østvold, T. Inorg. Chem. 1995, 34, 4360. (35) Long, D. A. Raman Spectroscopy; McGraw-Hill: London, 1977; Chapter 4. (36) Serafim, M. J. S.; Bessler, K. E.; Lemos, S. S.; Sales, J. A. Transition Met. Chem. 2007, 32, 112.

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