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For fuel economy improvement by lean-burn gasoline engines, extension of their lean operation range to higher loads is desirable as more fuel is consumed during acceleration. Urgently needed therefore is development of emission control systems having as high NOx conversion efficiency as three-way catalysts (TWC) even with more frequent lean operation. The authors conducted a study using catalysts loaded with potassium (K) as the only NOx trapping agent in an emission control system of a lean-burn gasoline engine.

Practical application of leanburn gasoline engines for passenger cars has been increasing in recent years. The leanburn gasoline engine is operated under high air fuel (A/F) ratio to improve fuel economy. However, reduction of NOx from the exhaust under oxygen-rich condition is very difficult to achieve. The Three Way Catalyst (TWC) designed for conventional gasoline engines cannot be applied to the leanburn gasoline engine because of low NOx conversion under lean conditions. To achieve better fuel economy and lower emissions, a breakthrough in NOx control technology for oxygen-rich conditions is needed. To address this need, a new type of catalyst technology using Iridium was developed. This new catalyst can constantly reduce NOx from exhaust under lean conditions using hydrocarbons as reducing agents. This is accomplished without formation of N2O. In addition, the newly developed catalyst has a tolerance for sulfur poisoning and exhibits reasonable heat stability.