(180d) TWC formulation effects on NH3 generation for passive SCR applications in lean gasoline engine exhaust
Due to the dominance of gasoline vehicles in the U.S. light duty fleet, improvements in gasoline engine fuel efficiency would significantly reduce U.S. petroleum consumption and greenhouse gas emissions. Operating gasoline engines with more air than is required to burn the fuel (known as lean conditions) can improve fuel efficiency by 10% or more. However, lean gasoline vehicles have been unable to meet U.S. emissions regulations for oxides of nitrogen (NOx). The two lean NOx reduction technologies that have been successfully deployed on diesel engines, LNTs (lean NOx traps) and urea SCR (selective catalytic reduction), are both challenged by lean gasoline applications. The high NOx concentrations and exhaust temperatures from gasoline engines result in excessive NOx slip during LNT regeneration and a high fuel penalty due to frequent regeneration events. With SCR systems, high NOx concentrations require increased urea dosing rates, necessitating a large onboard storage tank and/or easily accessible refueling infrastructure. The cost of the urea storage and dosing system may also be prohibitive in gasoline applications. An alternate strategy, termed passive SCR, generates NH3 from engine-out NOx over a close-coupled catalyst during periodic excursions to slightly fuel-rich conditions. The NH3 is stored on an underfloor SCR catalyst, where it is used to reduce NOx upon returning to lean conditions. By generating NH3 over the three-way catalyst (TWC) already present on gasoline vehicles, the passive SCR approach enables use of proven commercial SCR catalysts while avoiding the cost and packaging issues associated with a urea dosing system. Oak Ridge National Laboratory, in collaboration with partners at General Motors, Umicore Autocat USA, and the University of South Carolina, is currently working to identify passive SCR catalyst formulations, system architectures, and operating strategies that would enable lean gasoline vehicles to meet emissions regulations while minimizing the associated fuel penalty and costs. This presentation will describe the results of flow reactor and engine experiments that evaluated the effects of TWC formulation on the NH3 formation process under both steady state and dynamic operating conditions. The TWC formulations used in the experiments included variations in precious metal composition, oxygen storage capacity, and NOx storage capacity. Correlations between TWC formulation and expected passive SCR fuel penalty will be discussed.