Chapter 11. Combined LNT–SCR Catalysts for NOx Reduction from Lean Exhaust Gas

Abstract

This chapter gives an overview on the combined lean NOx trap (LNT) and selective catalytic reduction (SCR) catalysts. In particular, the focus is on the reaction pathways involved in such systems when different reducing agents are being used to accomplish the rich phase in the LNT system. In combined LNT–SCR systems, NH3 emitted from the LNT catalyst during the rich phase is captured onto a SCR catalyst placed downstream the LNT brick. The stored ammonia is then used as reactant in the SCR catalyst upon reaction with NO slipped from the LNT catalyst during the lean phase. This increases the NOx removal efficiency and reduces the emissions of unwanted ammonia. When using hydrocarbons as reducing agents, a non-NH3 SCR reaction pathway is also operating: in fact, HCs slipped through the LNT catalyst react with NO over the SCR catalyst, thus contributing to the overall NOx abatement. The overall system efficiency depends on several factors including the catalyst formulation (LNT and SCR), the operating conditions and the reducing agents used during the rich phase. In addition, the system architecture (dual layer vs dual bed) plays a role in determining the system efficiency.