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A laboratory study was performed to assess the potential capability of TWC+LNT/SCR systems to satisfy the Tier 2, Bin 2 emission standards for lean-burn gasoline applications. It was assumed that the exhaust system would need a close-coupled (CC) TWC, an underbody (U/B) TWC, and a third U/B LNT/SCR converter to satisfy the emission standards on the FTP and US06 tests while allowing lean operation for improved fuel economy during select driving conditions. Target levels for HC, CO, and NOx during lean/rich cycling were established. Sizing studies were performed to determine the minimum LNT/SCR volume needed to satisfy the NOx target. The ability of the TWC to oxidize the HC during rich operation through steam reforming was crucial for satisfying the HC target.

A laboratory study was performed to assess the potential capability of passive TWC+SCR systems to satisfy the Tier 2, Bin 2 emission standards for lean-burn gasoline applications. In this system, the TWC generates the NH3 for the SCR catalyst from the feedgas NOx during rich operation. Therefore, this approach benefits from high feedgas NOx during rich operation to generate high levels of NH3 quickly and low feedgas NOx during lean operation for a low rate of NH3 consumption. It was assumed that the exhaust system needed to include a close-coupled (CC) TWC, an underbody (U/B) TWC, and an U/B SCR converter to satisfy the emission standards during the FTP and US06 tests while allowing lean operation for improved fuel economy during select driving conditions. Target levels for HC, CO, and NOx during lean/rich cycling were established. With a 30 s lean/10 s rich cycle and 200 ppm NO lean, 1500 ppm NO rich and the equivalent of 3.3 L of SCR volume were required to satisfy the NOx target.

Desulfation characteristics of several model and fully-formulated monolithic lean NOx trap materials were studied in a laboratory flow reactor employing a chemical ionization mass spectrometer. For all samples, desulfation at elevated temperatures under reducing conditions resulted in appearance of sulfur dioxide (SO2) followed by carbonyl sulfide (COS) and hydrogen sulfide (H2S). The data appear consistent with a desulfation mechanism involving elimination of SO2 from stored sulfates under reducing conditions, followed by reaction of the SO2 with CO and H2 to produce COS and H2S, respectively. Based on these observations, several cyclic and multistage desulfation strategies were devised which greatly decreased H2S emissions while achieving relatively rapid and complete sulfur removal.

Selective catalytic reduction (SCR) with NH3 provides an attractive alternative to lean NOx traps for controlling the NOx emissions from lean-burn gasoline engines. This paper summarizes a laboratory study to assess the effects of temperature, space velocity, and the concentrations of NO, NH3, and O2 on the NOx conversion of an iron/zeolite SCR catalyst. A fresh sample was evaluated on slow temperature ramps with 5% O2 and 250, 500, or 1000 ppm of NO and NH3. The NOx conversion at low temperatures decreased with increasing NO and NH3 concentrations due to kinetic limitations. Conversely, the conversion at high temperatures increased with increasing NO and NH3 concentrations because the portion of NH3 oxidized by O2 decreased with increasing NO concentration.

In anticipation that future gasoline engines will have improved fuel efficiency and therefore lower exhaust temperatures during low load operation, a project was initiated in 2014 to develop three-way catalysts (TWC) with improved activity at lower temperatures while maintaining the durability of current TWCs. This project is a collaboration between Ford Motor Company, Oak Ridge National Laboratory, and the University of Michigan and is funded by the U.S. Department of Energy. The ultimate goal is to show progress towards the USDRIVE goal of 90% conversion of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) at 150°C after high mileage aging. A reactor was set up at Ford to follow the catalyst testing protocols established by the USDRIVE ACEC tech team for evaluating catalysts for stoichiometric gasoline direct-injection (S-GDI) engines; this protocol specifies a stoichiometric blend of CO/H2, NO, C3H6, C2H4, C3H8, O2, H2O, and CO2 for the evaluations.