Redox behavior and reduction mechanism of Fe₂O₃–CeZrO₂ as oxygen storage material

Fe₂O₃–CeZrO₂ is a suitable oxygen storage material for the production of pure hydrogen by a cyclic water gas shift (CWGS) process which is based on the reduction of the material by syngas followed by the re-oxidation of the reduced material with water vapor. For identification of the reduction kinetics H₂-temperature programmed reduction experiments were performed. Several kinetic models were tested and the activation energy of reduction was calculated by the Kissinger method, by model-based curve fitting and by the isoconversional analysis method. The reduction of Fe₂O₃–CeZrO₂was found to occur in a four-step process including the reduction of Fe₂O₃,Fe₃O₄, and CeZrO₂. The overall process can be interpreted as phase-boundary controlled reduction of Fe₂O₃ to Fe₃O₄, and two-dimensional nucleation controlled reduction of Fe₃O₄ to Fe and of CeO₂ to Ce₂O₃. At higher oxygen conversion, the reduction of Fe₃O₄ and CeO₂ are significantly influenced by volume-diffusion in the solid bulk.