Chapter 5: Mechanism and Kinetics of NO_x Storage

In this chapter, experimental studies and kinetic models are presented to elucidate the fundamental mechanisms for NO_x storage on the NO_x storage and reduction (NSR) catalysts. A well-accepted reaction pathway for NO_x storage includes, as an initial step, the oxidation of NO to NO₂ under lean conditions, and subsequently storage on the NO_x storage sites in the form of nitrites or nitrates. NO can also be stored directly in the presence of NO and oxygen, in the so-called nitrite route. However, the NO_x storage process is largely influenced by the primary gas components in the exhaust stream, such as CO₂, H₂O and SO₂. In particular, significant attention has been focused on sulphur poisoning and the interaction of SO₂ with different NSR components due to its large potential of harming NSR catalyst activity. Finally, a kinetic model describing in detail the sulphur poisoning over a commercial LNT catalyst is discussed. The model can well describe the gradual deactivation of the catalyst during sulphur exposure.