Surface distribution state and storage performance of the potassium species in the lean-burn NO x trap catalyst Pt/K/Al 2O 3–TiO 2–ZrO 2

Abstract

A series of lean-burn NO x trap (LNT) catalysts Pt/K/Al 2O 3–TiO 2–ZrO 2 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 K 2CO 3, possibly forming –OK groups. The in situ DRIFTS results indicate that the dominant NO x 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 Al 2O 3–TiO 2–ZrO 2. In this case, most potassium exists in the form of K 2CO 3 on Al 2O 3–TiO 2–ZrO 2, which can transform to free ionic nitrates during NO x storage. High K loading and high calcination temperature facilitate the formation of K 2CO 3, which possesses higher NO x storage efficiency but lower sulfur resisting and regeneration ability than –OK groups.