NOx Storage and Reduction Coupled with Selective Catalytic Reduction for NOx Removal in Light‐Duty Vehicles

There are two alternative, main technologies for removal of nitrogen oxides (NOx) from vehicle exhaust gases, namely, selective catalytic reduction (SCR) and NOx storage and reduction (NSR). The SCR technology consists of using ammonia (NH3), produced by hydrolysis of urea that is stored into an on‐board tank in the vehicle, to reduce NOx selectively to nitrogen (N2). In the NSR strategy, the conversion of NOx into N2 occurs through a two‐step cyclic operation. During the fuel‐lean stage, the NOx is trapped on the catalyst; then, the engine is switched to a fuel‐rich condition, under which NOx is released and reduced preferentially to N2, although some NH3 can also be produced. The concept of coupling NSR with SCR is based on tuning the operation of the NSR catalyst to produce a controlled amount of NH3, which is stored on the SCR catalyst during the fuel‐rich stage and used to reduce the remaining NOx on the SCR catalyst, placed downstream of the NSR catalyst in a sequential NSR+SCR configuration. Alternatively, the same concept applies to the dual‐layer architecture, which comprises a SCR layer deposited on top of the NSR layer in a single monolith. The internal generation of NH3 in the NSR catalyst avoids the need of external supply from the on‐board tank, but also achieves impressive NOx‐to‐N2 efficiency, apparently allowing zero‐level pollutant emission. Combined efforts: A comprehensive overview of the coupling of NOx storage and reduction (NSR) and NH3‐selective catalytic reduction (SCR) catalysts in the architecture of two sequential NSR+SCR monoliths, or alternatively as a dual‐layer NSR–SCR single monolith, is presented. The combination takes advantage of the benefits of individual NSR and SCR technologies, improving the global NOx elimination and maximizing the N2 selectivity, together with the prevention of the ammonia slip.