Chapter 2: NSR Technology

The NO_x storage and reduction (NSR) technology is a demonstrated catalytic strategy for the removal of nitrogen oxides from the exhaust gases emitted by diesel and lean burn engines. The conversion of NO_x into N₂ occurs through a two-step process which requires a cyclic operation between oxidising and reducing environments. This chapter deals with a comprehensive review of the main aspects of this technology. Firstly, the NSR concept will be described and details on the complex mechanism of the process will be given over a model Pt–Ba/Al₂O₃ catalyst, thus paying special attention to the role of each catalytic function. Next, the effect of numerous engineering parameters involved in the technology, such as temperature, duration of the lean and rich period and the type and concentration of reductant, on both trapping efficiency and global NO_x-to-N₂ will be examined. The applicability of the NSR strategy mainly to light duty vehicles will be emphasised with respect to the selective catalytic reduction (SCR) of NO_x with NH₃. Furthermore, the viability of the integration of NO_x traps and oxidation catalysts for the simultaneous removal of NO_x, carbon monoxide, unburned hydrocarbons and particulate matter in a single monolithic catalyst will be discussed. Finally, the coupling of NSR and SCR, which enhances the NO_x removal efficiency with lower NH₃ emissions when compared to the single NSR process, will be analysed.