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 behavior. The signal of water released during the reaction can be modeled only qualitatively, which (presumably) stems from its sorption behavior on the acidic alumina support. The oxidation of VOx species furthermore depends on the reaction temperature. The oxidation of V to 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.