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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.

Enhancements in the thermal stability of three-way catalysts have been achieved by: 1) developing improved methods for the incorporation of ceria into catalyst formulations and 2) identifying a proprietary stabilizer which reduces the rate of ceria sintering at high temperature. Improvements in thermal stability are demonstrated by comparing the FTP and engine dynamometer performance of new formulations with a standard formulation after aging on several high temperature engine dynamometer cycles.