STRUCTURAL AND REDOX PROPERTIES OF VANADIUM COMPLEXES IN MOLTEN SALTS OF INTEREST FOR THE CATALYTIC OXIDATION OF SULFUR DIOXIDE

摘要

Electronic absorption (UV/VIS) spectra have been obtained at 450℃ from V_2O_5-K_2S_2O_7, molten mixtures in SO_2 (P_(SI_2) = 0 - 1.2 atm) gas atmospheres. The data are in agreement with the V~Ⅴ→← V~(Ⅳ) equilibrium: (VO)_2O(SO_4)_4~(4-)(l) + SO_2(g) →← 2VO(SO_4)_2~(2-)(l) + SO_3(g). Sulfur dioxide does not coordinate to the V~Ⅴ complex but starts significantly to coordinate to V~(Ⅳ) for P_(SO_2) > 0.4 atm in accordance with the equilibrium: VO(SO_4)_2~(2-)(l) + SO_2(g) →← VO(SO_4)_2SO_2~(2-)(l). Furthermore, high temperature Raman spectroscopy has been used to establish the structural and vibrational properties of the vanadium complexes formed in the molten salt-gas system V_2O_5-M_2S_2O_7-M_2SO_4/SO_2-O_2 (M= K or Cs). The spectral features and the exploitation of the relative Raman intensities indicate that the (VO)_2O(SO_4)_4~(4-) dimeric complex unit which possesses a V-O-V bridge is formed in the V_2O_5-M_2S_2O_7 molten mixtures. Interaction with SO_2 or addition of sulfate leads to cleavage of the V-O-V bridge and formation of the V~(Ⅳ)O(SO_4)_2~(2-) or V~Ⅴ O_2(SO_4)_2~(3-) complex units respectively. The most prominent Raman bands due to the V~Ⅴ and V~(Ⅳ) complexes formed are assigned and the spectral data are discussed in terms of possible structural models.