Thermodynamic and kinetic study of the synthesis of mixed-valence V4O7 via oxidation in designed quench system with atmospheric control

The phase stability diagram of the V–O Magnéli system was established based on a previous work, and a designed quench system was applied to the experimental investigation to examine the presented thermodynamic data. This equipment can bring a sample to a set temperature in several minutes and quench it to room temperature under a controlled atmosphere. The atmosphere is controlled by setting the volume ratio of CO2/H2 to 300, which was evaluated by thermodynamic calculations. By tuning the temperature, the oxygen partial pressure in the system can be adjusted across regions of different stable phases. V2O3 was used as the raw material for oxidation in the gas mixture at equilibrium temperatures of 1323, 1373, 1423, 1473 and 1523 K. The XRD results of the quenched samples from different equilibriums at a series of temperatures verified the thermodynamic considerations. When the quenched system was connected to a balance, conversion curves were obtained during oxidation in the mixture gas for the last 300 min at 1373, 1423, and 1473 K. The complete oxidation from V2O3 to V4O7 proceeds via two reactions: one from V2O3 to V3O5 and the other from V3O5 to V4O7. Fitting results showed that both reactions obey the first-order Avrami–Erofeev equation but with different parameters.