Effects of four-mode hydrothermal aging on three-way catalysts for passive selective catalytic reduction to control emissions from lean-burn gasoline engine

Passive selective catalytic reduction (SCR) is a promising approach for the control of NOx emissions in lean burn gasoline exhausts. It requires ammonia (NH3) to be produced over a three-way catalyst (TWC) during periods of fuel-rich operation for the reduction of NOx during periods of fuel-lean operation. Previous research has shown the viability of this system but has not examined the effects of hydrothermal degradation. This work is focused on evaluating the effects of hydrothermal aging on the TWC in a passive SCR system. Two catalysts were studied: a Pd-TWC, and a NOx storage and reduction (NSR) TWC. Samples were aged at 920 degrees C for 100 h using a four-mode hydrothermal aging procedure. This causes the catalyst to be oxidized and reduced, as it would in a real system. The effects of aging were evaluated using simulated exhaust under both steady state and lean-rich cycling conditions. Hydrothermal aging caused significant changes in catalyst activity, leading to a decrease in low temperature conversion of carbon monoxide (CO) and propane (C3H8) on both catalysts, and degradation of oxygen storage and NOx storage components. However, the catalysts maintained their activity for NOx conversion and NH3 production, showing sufficient activity for the operation of a passive SCR with an optimum projected fuel consumption of 92-98% compared to stoichiometric operation.