The influence of the support material of vanadia catalysts on the reaction rate, activation energies, and defect formation enthalpies was investigated for the oxidative dehydrogenation of ethanol and propane. Characterization by infrared absorption–reflection spectroscopy (IRAS), Raman and UV–vis spectroscopy verifies a high dispersion of vanadia for powder and thin-film model catalysts. The support effect of ceria, alumina, titania, and zirconia is reflected in activation energy, oxidative dehydrogenation (ODH) rate, and temperature-programmed reductions (TPR) for both catalyst systems, ethanol and propane. Impendence spectroscopy and density functional theory (DFT) calculations were used to determine the defect formation enthalpy of the vanadyl oxygen double bond, providing the scaling parameter for a Bell–Evans–Polanyi relationship. On the basis of a Mars–van-Krevelen mechanism, an energy profile for the oxidative dehydrogenation is proposed
Electrochemical methods have been applied in the catalytic system V2O5 in order to investigate the redox properties and their correlation with catalytic properties. Temperature programmed conductivity measurements using electrochemical impedance spectroscopy enabled us to determine the onset of a thermally induced reduction at about 380°C. Rutherford backscattering analysis provides evidence for a reduction from V+5 to V+4. Experiments under different oxygen partial pressures showed that the vanadyl oxygen is involved in the reduction process and it was possible to determine the energy of formation for an oxygen vacancy as 1.23 ± 0.03 eV. The removability of the vanadyl oxygen is assumed to be a key factor for the catalytic activity so that it can be characterized by macroscopic transport properties.
The phase composition and defect structure of the system Li2O–MgO was investigated in terms of the long term stability of Li/MgO catalysts. The Li content was varied from 0 to 7 mol.%. Pure Li · MgO solid solutions were prepared via a special washing procedure. Li contents below 0.04 wt.% were stabilized within the MgO host lattice, whereas higher Li contents were found to segregate as Li2O and Li2CO3 phases. The catalytic activity in the oxidative coupling of methane was found to decay for all catalysts over a period of 19 h on stream, accompanied by a loss of Li as LiOH. Li in the Li · MgO solid solution was found to be more stable in the lattice than in the surface region of the solid. However, impedance measurements on transition metal stabilized Li/MgO catalysts indicated that even the Li ions within the Li · MgO solid solution are not sufficiently stabilized. Thus, neither the Li compounds nor the dissolved Li ions within the Li/MgO solution seem to be truly stable at 750°C under catalytic conditions.
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