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Lean NOx Traps (LNTs) are one type of lean NOx reduction technology typically used in smaller diesel passenger cars where urea-based Selective Catalytic Reduction (SCR) systems may be difficult to package . However, the performance of lean NOx traps (LNT) at temperatures above 400 C needs to be improved. The use of Rapidly Pulsed Reductants (RPR) is a process in which hydrocarbons are injected in rapid pulses ahead of a LNT in order to expand its operating window to higher temperatures and space velocities. This approach has also been called Di-Air (diesel NOx aftertreatment by adsorbed intermediate reductants) by Toyota. There is a vast parameter space which could be explored to maximize RPR performance and reduce the fuel penalty associated with injecting hydrocarbons. In this study, the mixing uniformity of the injected pulses, the type of reductant, and the concentration of pulsed reductant in the main flow were investigated.

A study was performed on a lean NOx trap in which the loading of a ceria-containing mixed oxide in the washcoat was varied. After a mild stabilization of the traps, the time required to purge the NOx trap generally increased with increasing amount of mixed oxide. The purge NOx release also increased with increasing mixed oxide level but was greatly diminished after thermal aging. The sulfur tolerance of the NOx trap improved as the mixed oxide content was increased from 0% to 37%. The sample with 0% mixed oxide was more difficult to desulfate than the other samples due to poor water-gas-shift capability. After thermal aging, the NOx reduction efficiency on a 60 second lean/5 second rich cycle was highest for the samples with 0% to 37% mixed oxide at evaluation temperatures of 400°C to 500°C.

An aftertreatment system involving a LNT followed by a SCR catalyst is proposed for treating the NOx emissions from a diesel engine. NH3 (or urea) is injected between the LNT and the SCR. The SCR is used exclusively below 400°C due to its high NOx activity at low temperatures and due to its ability to store and release NH3 below 400°C, which helps to minimize NH3 and NOx slip. Above 400°C, where the NH3 storage capacity of the SCR falls to low levels, the LNT is used to store the NOx. A potassium-based LNT is utilized due to its high temperature NOx storage capability. Periodically, hydrocarbons are oxidized on the LNT under net lean conditions to promote the thermal release of the NOx. NH3 is injected simultaneously to reduce the released NOx over the SCR. The majority of the hydrocarbons are oxidized on the front portion of the LNT, resulting in the rapid release of stored NOx from that portion of the LNT.

The mechanism for the purging of a lean NOx trap has been investigated. For realistic purge times (e.g., 2 to 5 seconds), the stored NOx species do not decompose simply from equilibrium considerations (i.e., from the drop in O2 and NO concentrations during the rich purge). Instead, the decomposition of stored NOx is promoted by the reductants in the exhaust by a process referred to as reductive elimination. H2 is far more effective than CO or C3H6 for promoting this reductive elimination, particularly at low temperatures (e.g., 250°C). As long as H2 is available in the feedgas, H2O does not participate in the reductive elimination. However, if CO is the only reductant, H2O is needed to convert some of the CO to H2 through the water-gas-shift reaction. H2O is also important for the efficient storage of NOx during lean operation, possibly by enhancing the spillover of NO2 from a precious metal site to a NOx storage site.

The effects of PGM zoning and washcoat staging have been investigated as a means to lower the cost and simultaneously improve the performance of a lean NOx trap system. It is shown that reverse PGM zoning can be used to reduce the cost of the LNT while essentially maintaining the NOx performance of a similarly-sized trap with a uniformly high PGM loading. In addition, the effective temperature window of the trap can be expanded by staging different NOx trap formulations that are optimized for different temperature ranges. Alternatively, LNT washcoat staging can be used to improve the hydrocarbon conversion of the trap while maintaining good NOx performance. Laboratory data and vehicle data are presented for several NOx trap system combinations that demonstrate the improved performance that can be obtained from a combination of reverse PGM zoning and washcoat staging.

Lean NOx Traps are used to treat the NOx emissions from lean-burn engines by storing the NOx under lean conditions and reducing the NOx during periodic rich excursions. However, sulfur poisons the adsorption sites of the traps. The sulfur can be removed from the NOx trap by operating rich at high temperatures for several minutes. This results in the release of some SO2 but also large quantities of H2S, which is a source of customer dissatisfaction that must be reduced or eliminated. This paper describes the use of a nickel-containing catalyst and air/fuel control to maximize the release of SO2 and minimize the emissions of H2S during the desulfation of a lean NOx trap. We present laboratory and vehicle data with a nickel-containing catalyst located downstream of a lean NOx trap during desulfations of the trap. The nickel effectively reduced the emissions of H2S during the desulfation while improving the robustness to fluctuations in the air/fuel control.