NOx Reduction Using a Dual-Stage Catalyst System with Intercooling in Vehicle Gasoline Engines under Real Driving Conditions

Selective catalytic reduction (SCR) of nitrogen oxides (NOx) is used in diesel-fueled mobile applications where urea is an added reducing agent. We show that the Ultera® dual-stage catalyst, with intercooling aftertreatment system, intrinsically performs the function of the SCR method in nominally stoichiometric gasoline vehicle engines without the need for an added reductant. We present that NOx is reduced during the low-temperature operation of the dual-stage system, benefiting from the typically periodic transient operation (acceleration and decelerations) with the associated swing in the air/fuel ratio (AFR) inherent in mobile applications, as commonly expected and observed in real driving. The primary objective of the dual-stage aftertreatment system is to remove non-methane organic gases (NMOG) and carbon monoxide (CO) slip from the vehicle’s three-way catalyst (TWC) by oxidizing these constituents in the second stage catalyst. The system includes an interstage exhaust gas cooler and air injection to reduce the exhaust temperature to 175-230 °C (347 °F-446 °F) in order to avoid the reformation of NOx. However, measurements show that a secondary benefit of this system is realized in mobile applications, that is, an unexpected reduction of NOx as well. During accelerations, the AFR control of many vehicles operates slightly rich. There is evidence that this leads to the production of ammonia in the TWC. Moreover, research has demonstrated that ammonia storage is optimal at lower temperatures, and hence at approximately 200 °C (392 °F), the second stage catalyst ammonia storage capacity could be many times more than typical catalysts operating at higher temperatures. Consequently, during deceleration when the exhaust is oxygen-rich and NOx is high, the ammonia stored in the second stage could act as the reductant for NOx. Test results over standard US06 drive cycles have demonstrated that additional NOx removal of up to 30% is realized. In this article, we show evidence that this mechanism is responsible for this reduction. Optimization of this system to improve the simultaneous CO/hydrocarbon and NOx removal includes alternate temperature and airflow setpoints during accelerations and decelerations and alternative catalyst formulations to improve storage and the SCR-like operation.