Selective Catalytic Reduction for Treating the NOx Emissions from Lean-Burn Gasoline Engines: Durability Assessment 2008-01-0811

A laboratory study was performed to assess the potential of using selective catalytic reduction (SCR) with NH₃ to treat the NO_x emissions from lean-burn gasoline engines. A primary concern was the potential for hot rich exhaust conditions on the vehicle, as such conditions could degrade the zeolite-based SCR catalysts being developed for automotive applications. Samples of an iron/zeolite formulation were aged for 34 hours behind samples of a three-way catalyst (TWC) on a pulse-flame combustion reactor using different A/F ratio schedules that exposed the catalysts to either continuously lean operation, mostly stoichiometric operation, or mostly rich operation. For each A/F ratio schedule, separate SCR samples were aged with inlet temperatures of 750°C, 800°C, or 850°C. The aged SCR samples were evaluated for NO_x conversion at 25K hr^-1 during lean temperature ramps with 500 ppm NO and NH₃. After aging under mostly rich conditions at 750°C inlet, the Fe SCR catalyst provided a NO_x conversion of 94% at 500°C. After aging under mostly stoichiometric or continuously lean conditions at 800°C inlet, which is a reasonable maximum temperature for underfloor converters on some vehicles, the NO_x conversions at 500°C were 93% and 97%, respectively.

Samples of a copper/zeolite formulation were similarly aged with the different A/F ratio schedules at an inlet temperature of 800°C. When evaluated at 350°C, the Cu SCR samples provided peak NO_x conversions of approximately 95%, 100%, and 100% after aging under mostly rich conditions, mostly stoichiometric conditions, and continuously lean conditions, respectively. These results suggested that both SCR formulations have sufficient durability to be considered for potential use in controlling the NO_x emissions from lean-burn gasoline engines.