Solid-state kinetic investigations of non-isothermal reduction of VO2

The solid-state kinetics of the reduction of VO2 to corundum-type V2O3 were investigated by temperature-programmed measurements. Experimental parameters were chosen to be similar to those used in conventional testing of heterogeneous catalysts. In situ X-ray diffraction experiments showed that the oxidation of V2O3 to corundum-type VO2 during propene oxidation and subsequent reduction of VO2 to V2O3 with propene were single step processes with no intermediate vanadium oxides detectable. Here, H2 consumption profiles obtained for the non-isothermal reduction of VO2 to corundum-type V2O3 were transformed to reduction degree α traces. Extracted α traces were analyzed by the model-free Ozawa, Flynn and Wall (OFW) method, the Kissinger method, and the model depending Coats–Redfern analysis method, respectively. Relevant kinetic characteristics (i.e. apparent activation energy Ea, and solid-state reaction model g(α)) was determined for the reduction of VO2 to corundum-type V2O3. The Kissinger method yielded a value of Ea ~83 kJ/mol for the rate determining step of the reduction of VO2 to corundum-type V2O3. Evolution of apparent activation energy Eaα obtained from a model-free OFW analysis exhibited a constant value of Eaα ~104 kJ/mol in the reduction degree α range between 0.05 and 0.45. In the α range from 0.55–0.95 the apparent activation energy Eaα increased significantly to ~133 kJ/mol. A model dependent Coats–Redfern analysis yielded a nucleation model without growth restriction [Power law (P4)] as best candidate to describe a suitable reaction model for the reduction of VO2.