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The successful commercialization of lean burn gasoline engines is dependent upon development of an effective emission aftertreatment system which can provide HC, CO, and NOx control not only under lean operating conditions, but also when the engine operates at the stoichiometric point under conditions of high engine speed and/or load. NOx adsorber catalysts (NOx traps) are capable of storing NOx under lean condition, and subsequently releasing and catalyzing its reduction under conditions rich of the stoichiometric point. Aftertreatment systems based on these types of catalysts show great potential for reaching current and future emission standards. Key to the successful application of NOx adsorber catalysts is the development of engine control strategies which maximize NOx conversion while minimizing the fuel economy penalty associated with adsorber regeneration. In this paper limitations associated with NOx trap adsorption and regeneration strategies are discussed.

Although improvements in NOx adsorber formulations are increasing the sulfur resistance of these materials, and legislation continues to further restrict sulfur levels in fuels, sulfur poisoning remains as one of the key issues associated with successful commercialization of NOx adsorber technology throughout the world. Because of the stability of the sulfate poisons, high temperatures which stress the thermal stability of some of the most efficient NOx adsorbents are required for desulfation. Additionally, the rich condition which favors sulfur release simultaneously increases the H2S content of the emission. Sulfur traps offer the potential for reducing the formation of poisoning sulfates on downstream NOx adsorbents. Results characterizing the sulfur scavenging efficiency of these materials, as well as the conditions required for their regeneration will be presented. Strategies for their successful application on motor vehicles will be discussed.