Impact of Oxygen Storage Material and Catalyst Architecture on Three-Way Catalyst Activity
Zhiyu Zhou, University of Houston
The Three-Way Catalyst (TWC) is used in stoichiometric gasoline combustion vehicles to simultaneously eliminate CO, NOx and non-methane hydrocarbons (NMHCs). The multi-component TWC contains a mixture of precious group metal (PGM) and oxygen storage material (OSM) supported on a high surface area substrate to enable the three-way conversion. This is accomplished under the transient conditions by buffering the lean-rich swings of the exhaust gas composition around a neutral stoichiometric point. We study the effects of OSM composition (ceria-zirconia (CZO); mixed metal oxide spinel, Fe0.5Mn2.5O4), OSM and PGM loading, washcoat architecture (dual layer, mixed layer), mode of operation (stationary or modulated feed), and feed temperature on CO, NOx, and C3H6 conversion under near-stoichiometric condition. The findings provides insight into the beneficial impact of oxygen storage material and provides guidance for improvement in the TWC catalyst formulation.
The washcoated monolith catalysts studied are the following:
- PA: single-layer PGM-Al2O3 with PGM deposited on Al2O3
- PC: single-layer PGM-CZO with PGM deposited on CZO (ceria-zirconia)
- PS: single-layer PGM-Spinel with PGM deposited on spinel
- PC_D: dual-layer architecture with PGM-Al2O3 top layer and CZO-Al2O3 as bottom layer
- PS_D: dual-layer architecture with PGM-Al2O3 top layer and spinel- Al2O3 as bottom layer
The typical experiment involves measurement of reactant conversions as a function of feed temperature spanning 50 oC to 500 oC for both stationary and modulated feeds. A comparison of the light-off curves for the three catalyst types enables an assessment of the impact of proximity of the PGM and OSC functions on catalytic activity. With CZO supported on the OSC (type PC) the light-off temperature (T80; temperature giving 80% conversion) is ~100 °C lower than that for the PA sample. In contrast, the dual-layer architecture of PS_D, comprising a top PGM layer and a bottom spinel layer gives better performance than PS. Compared to the results with PA, PS_D gives ~ 70 °C in T80 for NO, CO and C3H6 simultaneously. The close coupling between the PGM and CZO is responsible for the promotion effect. The enhancement by the dual-layer PGM-spinel design relies on the direct contribution from the spinel layer. At temperatures exceeding the light-off temperature, modulation decreases the CO and C3H6 conversions for PC and PA, but has negligible impact on the performance of PS_D, as spinel has a rather high (dynamic) oxygen storage capacity (OSC). Compared to CZO, spinel exhibits higher theoretical, complete and dynamic oxygen storage capacity (OSC), last of which helps to stabilize reductant (CO and C3H6) conversion during modulation at high temperatures.