NOx Trap Catalyst Technologies to Attain 99.5% NOx Reduction Efficiency for Lean Burn Gasoline Engine Application

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. The findings were as follows: 1) Addition of magnesia (MgO) to the washcoat enhances gas diffusivity by increasing macro-pores in the washcoat, and thus leads to a substantial improvement in the NOx purification performance, particularly of metal substrate NOx trap catalysts having inherently uneven washcoat thickness. 2) Addition of single nano-size ceria (CeO₂) to NOx trap catalysts enhances the NOx purification performance of platinum (Pt). 3) For NOx purification, there is an optimum loading amount ratio between palladium (Pd) and Pt having better performance at low temperature and high temperature operation, respectively.

The authors also found that adding heat-resistant titania (TiO₂) to the improved NOx trap catalyst drastically suppressed NOx increase even after a 10,000km urban driving test most prone to sulfur accumulation. The developed NOx trap catalyst exhibited a 99.5% NOx conversion efficiency after an 80,000km endurance test even with lean operation extended to the acceleration.