Datensatz

Zusammenfassung

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 (A_g) vibration, also contains contributions of an ¹⁶O1–V–¹⁶O2 stretching vibration (B_2g). If only the O1 position is exchanged, the B_2g 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 A_g 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.