Evaluation of NOx Adsorber Catalysts Systems to Reduce Emissions of Lean Running Gasoline Engines

Most automobile manufacturers are looking for methods to increase fuel economy. It is possible to achieve this by operating under lean conditions. The problem remains, however, that while hydrocarbon and carbon monoxide oxidation is very efficient on a 3-way catalyst in an oxidising atmosphere, reduction of oxides of nitrogen (NOx) is poor. One method to improve the efficiency for NOx reduction under lean conditions is to use a 3-way catalyst in conjunction with a NOx trap. The NOx trap stores NOx under lean conditions which is then released and reduced under rich conditions.

Bench evaluations using a lean tune engine have been carried out on a number of 3-way catalysts containing a range of different NOx adsorbent components. Measurements of adsorption capacity, temperature window for lean NOx conversion and thermal durability have been made and a comparison of different adsorption materials determined. The effect of Pt loading on these parameters has also been ascertained. A lean NOx efficiency of close to 100 % has been observed over a wide temperature range. The effect of addition of a close coupled catalyst upstream of the NOx trap is also discussed.