High-temperature spectrophotometric and electron spin resonance spectroscopic investigations of vanadium complexes in the molten salt–gas system V2O5–K2S2O7/SO2–SO3–N2

Abstract

Electronic absorption (UV/VIS) and ESR spectra have been obtained in the temperature ranges 430–480 °C and 355–478 °C from V₂O₅–K₂S₂O₇ molten mixtures in the composition range 0.1–0.5 mol dm^–3 V₂O₅ in contact with SO₂ and SO₃(P_SO2/P_SO3= 0.1–5, P_SO2+P_SO3= 0.1 atm, P_N2≈ 0.9 atm). The data obtained by both Spectrophotometric and ESR methods indicate that the interaction of SO₂ and SO₃ with the melts results in a redox equilibrium between monomeric or dimeric V^V and monomeric V^IV complexes. An absorption band centred at 730 nm due to the V^IV complexes was observed and the molar absorption coefficients at 730 nm due to the V^IV and V^V complexes (tail of V^V charge transfer band) were found to be ε_V^IV,730= 18.91 ± 0.08 and ε_V^V,730=–2.7 + 9.48 × 10^–3T dm³ mol^–1 cm^–1 in the temperature range 430 < T < 480 °C. The characteristic eight-line feature of the ESR spectra with g= 1.979 ± 0.005 and Ā= 111 ± 5 G confirms the presence of V^IV in monomeric complexes, presumably [VO(SO₄)₂]^2–, in the melts. From the temperature-dependence studies of the electronic absorption spectra the equilibrium constants and enthalpies for a number of possible equilibria have been calculated.