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Emission of carbon dioxide from mobile sources is receiving interest as part of a coordinated approach to limit greenhouse gases. Coupled with the relatively high price of gasoline in some countries this has resulted in the development of lean burn and direct injection gasoline engines. These engines will require conversion of nitrogen oxides (NOx) in excess of 70% in a net oxygen rich exhaust stream to meet future emission limits. This paper describes recent advances in the performance of NOx trap technology in terms of adsorption capacity, temperature of operation and thermal durability. The application of a new NOx trap together with a newly developed starter catalyst, to a direct injection gasoline vehicle shows that European stage IV limits can be reached for NOx and CO with a fresh system.

One possibility to improve the fuel economy of SI-engines is to run the engine with a lean air-fuel-ratio (AFR). Hydrocarbon and carbon monoxide after-treatment has been proven under lean operation, but NOx-control remains a challenge to catalyst and car manufacturers. One strategy that is being considered is to run the engine lean with occasional operation at stoichiometry. This would be in conjunction with a three-way-catalyst (TWC) to achieve stoichiometric conversion of the three main pollutants in the normal way and a NOx trap. The NOx trap stores NOx under lean operation to be released and reduced under rich conditions. The trap also functions as a TWC and has good HC and CO conversion at both lean and stoichiometric AFR's. Under lean conditions NO is oxidised to NO2 on Pt which is then adsorbed on an oxide surface. Typical adsorbent materials include oxides of potassium, calcium, zirconium, strontium, lanthanum, cerium and barium.