NO_<i>x</i> Storage and Reduction Coupled with Selective Catalytic Reduction for NO_<i>x</i> Removal in Light‐Duty Vehicles

<jats:title>Abstract</jats:title><jats:p>There are two alternative, main technologies for removal of nitrogen oxides (NO<jats:sub><jats:italic>x</jats:italic></jats:sub>) from vehicle exhaust gases, namely, selective catalytic reduction (SCR) and NO<jats:sub><jats:italic>x</jats:italic></jats:sub> storage and reduction (NSR). The SCR technology consists of using ammonia (NH<jats:sub>3</jats:sub>), produced by hydrolysis of urea that is stored into an on‐board tank in the vehicle, to reduce NO<jats:sub><jats:italic>x</jats:italic></jats:sub> selectively to nitrogen (N<jats:sub>2</jats:sub>). In the NSR strategy, the conversion of NO<jats:sub><jats:italic>x</jats:italic></jats:sub> into N<jats:sub>2</jats:sub> occurs through a two‐step cyclic operation. During the fuel‐lean stage, the NO<jats:sub><jats:italic>x</jats:italic></jats:sub> is trapped on the catalyst; then, the engine is switched to a fuel‐rich condition, under which NO<jats:sub><jats:italic>x</jats:italic></jats:sub> is released and reduced preferentially to N<jats:sub>2</jats:sub>, although some NH<jats:sub>3</jats:sub> 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 NH<jats:sub>3</jats:sub>, which is stored on the SCR catalyst during the fuel‐rich stage and used to reduce the remaining NO<jats:sub><jats:italic>x</jats:italic></jats:sub> 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 NH<jats:sub>3</jats:sub> in the NSR catalyst avoids the need of external supply from the on‐board tank, but also achieves impressive NO<jats:sub><jats:italic>x</jats:italic></jats:sub>‐to‐N<jats:sub>2</jats:sub> efficiency, apparently allowing zero‐level pollutant emission.</jats:p>