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Abstract:
The isotopic exchange of 18O2 on polycrystalline V216O5 was studied by Raman spectroscopy at different temperatures between 300 and 580 °C and in the presence of different mixtures of oxygen with ethane, propane, or n-butane in the gas phase. Supported by DFT calculations, a method was developed to determine which of the three differently coordinated oxygen atoms in the crystal structure of V2O5 (vanadyl oxygen O1, 2-fold-coordinated oxygen O2, and three-coordinated oxygen O3) are involved in the exchange with 18O2 from the gas phase. Thus, it was found that the band at 994 cm–1, which is commonly exclusively assigned to a V═16O1 stretching (Ag) vibration, also contains contributions of an 16O1–V–16O2 stretching vibration (B2g). If only the O1 position is exchanged, the B2g component shifts to 964.2 cm–1, while if both O1 and O2 are exchanged, a shift to 953.4 cm–1 is expected. In contrast, the Ag component shifts only to 955 cm–1, regardless of whether only the O1 position or all three oxygen atoms are exchanged. On this basis, it was found that oxygen exchange at 573 °C in absence of an alkane involves O1 and O3 atoms, whereas in the presence of propane all three oxygen atoms are exchanged. In the latter case, the overall exchange rate appears to be limited by bulk diffusion. At typical reaction temperatures for the oxidative dehydrogenation of propane between 320 and 430 °C, no exchange occurs in pure oxygen. In presence of ethane or propane, only O1 is partly exchanged possibly at the surface and/or in a near-surface region. Under the typical reaction conditions of oxidative dehydrogenation of propane at 400 °C, there is hardly any variation in the spectra, and the small changes observed after long times on stream only affect O1, which, considering the sensitivity of the measurement method, leaves open whether the Mars–van Krevelen mechanism is indeed the predominant reaction mechanism under the conditions of oxidative dehydrogenation of alkanes on V2O5.
