Reoxidation dynamics of highly dispersed VOx species supported on γ-alumina

The VOx/γ-Al2O3 catalyst VA-200, which was introduced in a previous article, is further characterized by XPS and visible Raman spectroscopy. The reoxidation of highly dispersed VOx species with gas phase oxygen is investigated in detail and is described by an empirical kinetic model. It is observed that the reoxidation of reduced VOx/γ-Al2O3 catalyst is strongly affected by the presence of water. The proposed kinetic model includes a distinct coverage of VOx species with water or hydroxyl groups in the investigated temperature range of 479–712 K. Hydrated surface species are oxidized under release of water. Best fits of the experimental data can be achieved with first order rate laws with respect to oxygen concentration. Experiments are performed in an ideally mixed Berty-type reactor using oxygen step-marking over the reduced catalyst. The evolution of oxygen concentration using the kinetic model is in agreement with the experimentally observed behaviour. The signal of water released during the reaction can be modeled only qualitatively, which (presumably) stems from its sorption behaviour on the acidic alumina support. The oxidation of VOx species furthermore depends on the reaction temperature. The oxidation of V+III to V+V cannot be completely achieved at temperatures below 673 K. However, the activation energy of this reaction is low, as suggested by the absence of strong variations of the response shapes with respect to temperature.

The reoxidation of highly dispersed VOx species with gas phase oxygen as part of MvK redox cycles in ODH reactions is investigated in detail. This reaction is strongly affected by the presence of water and obeys a two-step mechanism. A reaction pathway is proposed. The oxidation has a low activation energy and average oxidation degree of +V cannot be obtained below 673 K. Figure optionsDownload as PowerPoint slide