EPR and UV/VIS spectroscopic investigations of VO2+ complexes and compounds formed in alkali pyrosulfates

Abstract

The catalytically important molten salt–gas system M₂S₂O₇–M₂SO₄–V₂O₅/SO₂(g) (M = Na, K, Rb, Cs) has been investigated by X- and Q-band EPR spectroscopy. In order to obtain information about the V(IV) complex formation in the melts, samples rather dilute in V₂O₅ were quenched from the molten state at 450–460 °C to 0 °C. EPR spectra of the quenched samples were recorded on samples with alkali to vanadium (M/V) ratios 40, 80 and 160. The spectra show that two V(IV) complexes dominate in the melt regardless of the type of alkali metal ion. In systems with low activity of sulfate a paramagnetic V(IV) complex with g_∥ = 1.915, g_⊥ = 1.978 and line widths 5–15 Gauss is observed. In systems saturated with M₂SO₄ the obtained EPR spectra show a paramagnetic complex with the g-tensors g_∥ = 1.930, g_⊥ = 1.980 and line widths 20–60 Gauss. These results fit very well with the assumption that the species VO(SO₄)₂²⁻ and SO₄²⁻ are in equilibrium with VO(SO₄)₃⁴⁻. It has also been shown for the system M₂S₂O₇–M₂SO₄(sat)–V₂O₅/SO₂(g) that the line widths in the system increase with higher cation radius, and depend linearly on the volume fraction of the sample occupied by the cation. This indicates that spin–spin relaxation effects are the major contribution to line broadening. Combining information from UV/VIS and EPR spectra shows that the VO²⁺ unit in the molten salt solvent exhibits electronic properties close to aqueous solutions of V(IV).