Vibrational modes and structure of vanadium(V) complexes in M₂SO₄–V₂O₅ (M=K or Cs) molten salt mixtures

Abstract

Raman spectra of the M₂SO₄–V₂O₅ (M=K or Cs) binary molten salt systems have been recorded at temperatures up to 570°C under an oxygen atmosphere and at different compositions in the range 0.25⩽X(V₂O₅)⩽0.50. 33 mol% V₂O₅ mixtures contain V^V polymeric complexes consisted of VO₃^- and VO₂(SO₄)₂^3- units participating in chain-like or network-like configurations, while VO₂SO₄^- units were also detected in the K₂SO₄–V₂O₅ system. The spectral changes occurring upon addition of V₂O₅ up to X(V₂O₅)=0.50 are interpreted to indicate: (i) a gradual transformation of VO₂(SO₄)₂^3- units, where vanadium is six-coordinated, to VO₂SO₄^-, where vanadium is four-coordinated and (ii) extensive linking of polymeric chains giving rise to large and complex three-dimensional networks. The most characteristic bands observed for the various units comprising the polymeric complexes in the K₂SO₄–V₂O₅ system are assigned as follows: (i) for VO₂(SO₄)₂^3- at 1042 (terminal V=O stretches of six-coordinated vanadium), 940 (terminal S–O stretches of sulfate), 880 (bridging S–O), 668, 408 and 227 cm^-1; (ii) for VO₃^- units at 950 (terminal V=O stretches of four-coordinated vanadium), 486 and 365 cm^-1 and (iii) for VO₂SO₄^- units at 983 (terminal V=O stretches of four-coordinated vanadium) and 862 cm^-1 (bridging S–O). Similar values have been found for the band wavenumbers in the Cs₂SO₄–V₂O₅ system. The spectral data are discussed in terms of possible structural models. The spectral changes upon freezing the Cs₂SO₄–V₂O₅ molten mixtures with 0.33⩽X(V₂O₅)⩽0.50 indicate formation of the 1:1 V₂O₅·Cs₂SO₄ solid. This conclusion is confirmed also by powder XRD spectroscopy. For the first time, high-temperature vibrational spectroscopy has been used to establish the structural and vibrational properties of M₂SO₄–V₂O₅ (M=K or Cs) molten salt mixtures.

References

1. O. B. Lapina, B. Bal'zhinimaev, S. Boghosian, K. M. Eriksen and R. Fehrmann, Catal. Today, 1998, in press.
2. O. B. Lapina, V. M. Mastikhin, A. A. Shubin, K. M. Eriksen and R. Fehrmann, J. Mol. Catal. A, 1995, 99, 123.
3. S. Boghosian, F. Borup and A. Chrissanthopoulos, Catal. Lett., 1997, 48, 145.
4. G. E. Folkmann, K. M. Eriksen, R. Fehrmann, M. Gaune-Escard, G. Hatem, O. B. Lapina and V. Terskikh, J. Phys. Chem. B, 1998, 102, 24.
5. R. Fehrmann, M. Gaune-Escard and N. J. Bjerrum, Inorg. Chem., 1986, 25, 1132.
6. G. Hatem, R. Fehrmann, M. Gaune-Escard and N. J. Bjerrum, J. Phys. Chem., 1987, 91, 1987.
7. G. E. Folkmann, G. Hatem, R. Fehrmann, M. Gaune-Escard and N. J. Bjerrum, Inorg. Chem., 1993, 32, 1559.
8. H. F. A. Topsøe and A. Nielsen, Trans. Dan. Acad. Tech. Sci., 1947, 1, 18.
9. D. S. Schmidt, PhD Thesis, Georgia Institute of Technology, Atlanta, 1996.
10. D. J. McHenry and J. Winnick, AIChE J., 1994, 40, 143.
11. G. K. Boreskov, V. V. Ilarionov, R. P. Ozerov and E. V. Kildisheva, Zh Obshch. Khim., 1956, 24, 23.
12. S. Hähle and A. Meisel, Kinet. Katal., 1971, 12, 1132.
13. R. I. Dearnaley and D. H. Kerridge, Thermochimica Acta, 1983, 63, 219.
14. S. Boghosian and G. N. Papatheodorou, J. Phys. Chem., 1989, 93, 415.
15. N. H. Hansen, R. Fehrmann and N. J. Bjerrum, Inorg. Chem., 1986, 25, 1132.
16. Z. X. Shen, C. W. Ong, S. H. Tang and M. H. Kuok, Phys. Rev. B, 1994, 49, 1433.
17. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley-Interscience, New York, 4th edn., 1986, p. 248.
18. I. N. Belyaev, T. G. Lupeiko and G. P. Kirlii, Russ. J. Inorg. Chem., 1973, 18, 711.
19. I. N. Belyaev, T. G. Lupeiko and V. P. Braitsev, Russ. J. Inorg. Chem., 1973, 18, 1331.
20. O. B. Lapina, V. Terskikh, A. A. Shubin, V. M. Mastikhin, K. M. Eriksen and R. Fehrmann, J. Phys. Chem. B, 1997, 101, 9188.
21. F. D. Hardcastle and I. E. Wachs, J. Phys. Chem., 1991, 95, 5031.
22. R. K. Sharma, K. N. Rai and R. D. Srivastava, J. Catal., 1980, 63, 271.