Evaluation of NOx Storage Catalysts for Lean Burn Gasoline Fueled Passenger Cars

Engine and laboratory tests were carried out to examine the performance of NO_x adsorption catalysts for gasoline lean burn engines in fresh and aged condition. The results show that fresh NO_x adsorption catalysts have the potential to meet EURO III emission standards. However, to accomplish these the fuel must contain a low sulfur concentration and the engine must be tuned to optimize the efficiency of the catalyst. After engine or furnace aging upto 750°C the catalyst shows some loss of NO_x adsorption efficiency. This deterioration can be offset somewhat by increasing the frequency of lean/rich switching of the engine. Temperatures higher than 750°C may cause an irreversible destruction of the NO_x, storage features while the three-way activity of the catalyst remains intact or even may improve. With reference to several physicochemical investigations it is believed that the detrimental effect of catalyst aging is attributed to two different deactivation modes. The first can be traced back to the reaction of the adsorbents with other washcoat ingredients, i.e. alumina (Al₂O₃), zirconia (ZrO₂) or titania (TiO₂) resulting in the formation of the corresponding aluminates, zirconates and titanates. The second deactivation mode is attributed to a significant particle growth of both the precious metal and the adsorbents leading to a substantial loss of interface between these two components. This loss of interface reduces the rate of spill-over Of NO₂ from the precious metal active sites onto the adsorbents, and hence reduces the NO_x adsorption efficiency.