Role of Storage Material in NOX Storage and Reduction (NSR) and Three-Way Catalysts (TWC) During Cycling Operation
NOx storage and reduction (NSR) is a cyclic catalytic process that eliminates NOx from lean burn vehicles. There is an active debate about the NOx reduction mechanism, particularly at a high temperature and cycle frequency. Three-way catalysts (TWC) are widely applied in stoichiometric combustion vehicles. The invention of novel oxygen storage materials (OSM), such as spinels, requires more advanced understanding of their promotion impacts. NOx reduction under net lean and near-stoichiometric conditions was carried out on CeO2, Pt/Al2O3, Pt/CeO2/Al2O3 and Pt/BaO/Al2O3 washcoated monoliths to compare performance features and identify reaction pathways. The NOx storage functionality is essential for NOx reduction under net lean conditions while the oxygen storage functionality promotes NOx reduction for near-stoichiometric conditions. The improved NOx storage utilization is mainly responsible for NOx conversion enhancement under fast cycling. A ceria redox pathway has only a secondary effect on NOx conversion under excess O2. Simultaneous conversion of NO, CO and C3H6 under stoichiometric conditions was carried out on catalysts containing OSM-promoted mixed precious group metal (PGM: Pt(95%)/Pd(5%)). The studied OSMs include ceria-zirconia (Ce0.3Zr0.7O2; CZO) and mixed metal oxide spinel (Mn0.5Fe2.5O4; MFO). PGM supported directly on the OSM was found to be the best design for CZO while a dual-layer architecture (top PGM layer, bottom MFO layer) outperformed the MFO-supported PGM catalyst. Close coupling between the PGM and CZO is responsible for the activity promotion. Enhancement obtained with the dual-layer PGM-MFO architecture relies on a direct catalytic contribution from the spinel. A combined experimental and modeling study of dynamic oxygen storage capacity (DOSC) of CZO and MFO spinel using CO and H2 was conducted. During reduction (or oxidation), either oxide exhibits a transition in rate-controlling regime, from a fast, reaction-controlled process to a much slower diffusion-controlled process. For CZO the classical shrinking-core model is successfully applied to describe the transient DOSC performance. 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 for MFO that is capable of capturing the DOSC performance under both long cycle (60s) and short cycle (1~2s).
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