Raman Spectroscopic Study of Tungsten(VI) Oxosulfato Complexes in WO₃−K₂S₂O₇−K₂SO₄ Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structure

Abstract

The dissolution reaction of WO3 in pure molten K2S2O7 and in molten K2S2O7-K2SO4 mixtures is studied under static equilibrium conditions in the XWO3(0) = 0-0.33 mol fraction range at temperatures up to 860 °C. High temperature Raman spectroscopy shows that the dissolution leads to formation of W(VI) oxosulfato complexes, and the spectral features are adequate for inferring the structural and vibrational properties of the complexes formed. The band characteristics observed in the W=O stretching region (band wavenumbers, intensities, and polarization characteristics) are consistent with a dioxo W(=O)2 configuration as a core unit within the oxosulfato complexes formed. A quantitative exploitation of the relative Raman intensities in the binary WO3-K2S2O7 system allows the determination of the stoichiometric coefficient, n, of the complex formation reaction WO3 + nS2O7(2-) --> C(2n-). It is found that n = 1; therefore, the reaction WO3 + S2O7(2-) > WO2(SO4)2(2-) with six-fold W coordination is proposed as fully consistent with the observed Raman features. The effects of the incremental dissolution and presence of K2SO4 in WO3-K2S2O7 melts point to a WO3 · K2S2O7 · K2SO4 stoichiometry and a corresponding complex formation reaction in the ternary molten WO3-K2S2O7-K2SO4 system according to WO3 + S2O7(2-) + SO4(2-) --> WO2(SO4)3(4-). The coordination sphere of W in WO2(SO4)2(2-) (binary system) is completed with two oxide ligands and two chelating sulfate groups. A dimeric [{WO2(SO4)2}2(μ-SO4)2](8-) configuration is proposed for the W oxosulfato complex in the ternary system, generated from inversion symmetry of aWO2(SO4)3(4-) moiety resulting in two bridging sulfates. The most characteristic Raman bands for the W(VI) oxosulfato complexes pertain to W(=O)2 stretching modes (i) at 972 (polarized) and 937 (depolarized) cm(-1) for the ν(s) and ν(as) W(=O)2 modes of WO2(SO4)2(2-), and (ii) at 933 (polarized) and 909 (depolarized) cm(-1) for the respective modes of [{WO2(SO4)2}2(μ-SO4)2](8-).

Photocorrosion of WO3 Photoanodes in Different Electrolytes

“…We speculate that the higher stability of WO 3 electrodes in more stable electrolytes can be due to the fast detachment of intermediates (ClO 4 · and possibly NO 3 ). Sulfuric acid and methanesulfonic acid, electrolytes that form stable persulfate complexes, might, on the other hand, complex dissolved W cations, enhancing dissolution . A similar effect of electrolytes on photoanode dissolution was recently shown for BiVO 4 .…”

“…Sulfuric acid and methanesulfonic acid, electrolytes that form stable persulfate complexes, might, on the other hand, complex dissolved W cations, enhancing dissolution. 48 A similar effect of electrolytes on photoanode dissolution was recently shown for BiVO 4 . 28 Such an effect is well-known in classical electrocatalysis, for example, for Pt that becomes significantly less stable in the presence of Cl − anions.…”

Chromate‐Mediated One‐Step Quantitative Transformation of PW12 into P2W20 Polyoxometalates

“…In the case of tungstate, the condensation reactions were also influenced by the presence of sulfate,35 although its effect was less pronounced. The existence of cis ‐dioxosulfatotungsten(VI) species was later demonstrated by Raman spectroscopy upon dissolution of WO 3 in K 2 S 2 O 7 /K 2 SO 4 melts 36…”

Temperature-Dependent Evolution of the Molecular Configuration of Oxo-Tungsten(VI) Species Deposited on the Surface of Titania

“…Dioxo (O) 2 W moieties are expected to exhibit a symmetric stretch at 930−975 cm −1 and an antisymmetric stretch at 910− 940 cm −1 . 28,52 Indeed, broad asymmetric (i.e., consisting of partially overlapping bands) features are exhibited in the respective spectral ranges of the Raman "snapshots" obtained at 150−250 °C (Figures 8 −10). The final (at T = 430 °C) position of the WO terminal stretching mode is observed at 1005 cm −1 for the (WO x ) n /TiO 2 samples with surface densities of 1.7 and 1.9 W/nm 2 (Figures 8 and 9), whereas for the 3.7 W/nm 2 (WO x ) n /TiO 2 sample the respective band is seen at 1008 cm −1 (Figure 10), suggesting a slightly larger domain size (see explanation below) for the surface (poly)oxotungstate species.…”