Dynamic Oxygen Storage Capacity of Ceria-Zirconia and Mn₀.₅Fe₂.₅O₄ Spinel: Experiments and Modeling
2021
Zhou, Zhiyu | Harold, Michael P. | Luss, Dan
A combined experimental and modeling study of dynamic oxygen storage capacity (DOSC) of ceria-zirconia (CZO) and Al₂O₃-supported Mn₀.₅Fe₂.₅O₄ (MFO) spinel using CO and H₂ as reductants is conducted to provide understanding of the redox performance of CZO and MFO under periodic reduction–oxidation cycling conditions. Particular attention is placed on fast cycling (cycle time of several seconds) encountered in emission control applications. Fixed bed reactor experiments with CZO show that during reduction (or oxidation) a transition in rate-controlling regimes occurs, from a reaction-controlled process during the first ∼20 s to a slower diffusion-controlled process. The classical shrinking-core model is successfully applied to describe the transient DOSC performance. In contrast, for MFO with its higher oxygen storage capacity (OSC), reduction is confined to oxygen within the first surface layer of the dispersed spinel crystallites. A progressive model is developed that is capable of capturing the DOSC performance under both a long cycle (60 s) and short cycle (1–2 s). The reaction steps of MFO spinel are determined by CO-TPR and involve two sequential steps: Mn₀.₅Fe₂.₅O₄ → Mn₀.₅Fe₂.₅O₃ and Mn₀.₅Fe₂.₅O₃ → Mn₀.₅Fe₂.₅O₀.₅. The kinetic parameters obtained from the fixed-bed reactor are applied to model the modulation performance in a Pt/Pd-spinel dual-layer monolith with CO as the reductant. The model-predicted results show that the periodic reduction–oxidation on MFO during fast cycling is limited to the first reaction step (Mn₀.₅Fe₂.₅O₄ → Mn₀.₅Fe₂.₅O₃). The DOSC modeling can be applied to the analysis and identification of other oxygen storage materials.
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