Surface distribution state and storage performance of the potassium species in the lean-burn NOx trap catalyst Pt/K/Al2O3–TiO2–ZrO2

A series of lean-burn NOx trap (LNT) catalysts Pt/K/Al2O3–TiO2–ZrO2 were prepared by sequential impregnation. By altering support calcination temperature and K loading, the distribution state of the storage medium K on different catalysts is carefully investigated. Pyridine-IR results show that a lot of hydroxyl groups exist on the supports calcined at low temperatures (<800 °C), which can react with the supported K2CO3, possibly forming –OK groups. The in situ DRIFTS results indicate that the dominant NOx storage species are monodentate or/and bidentate nitrates. As the calcination temperature is increased to 800 °C or higher, much less hydroxyl groups are detected on the surface of Al2O3–TiO2–ZrO2. In this case, most potassium exists in the form of K2CO3 on Al2O3–TiO2–ZrO2, which can transform to free ionic nitrates during NOx storage. High K loading and high calcination temperature facilitate the formation of K2CO3, which possesses higher NOx storage efficiency but lower sulfur resisting and regeneration ability than –OK groups.