NOx removal over a double bed NSR-SCR reactor configuration

Introduction NOx Storage-Reduction (NSR) catalytic systems and urea-SCR are both considered viable solutions for the NOx removal under lean conditions. In NSR systems (e.g. PtBa/Al2O3) the removal of NOx is realized under cyclic conditions, by alternating long lean periods during which NOx are adsorbed on the catalyst with short excursions under rich conditions during which the stored NOx are reduced to N2. On the other hand in the urea-SCR technique the NOx reduction is accomplished by injecting urea (a precursor of NH3) in the exhaust gases. Although urea-SCR is preferred for heavy trucks and mini-van, NSR catalysts are cheaper for small engines. A drawback related to NSR systems is the emission of undesired NH3 during the rich phase, especially at low temperatures. A possible solution is to place a SCR catalyst downstream the NSR system, so that NH3 formed over NSR during the rich phase can be stored on the SCR catalyst, and consumed in the subsequent lean phase. This might lead in principle to both a decrease of the NH3 slip, and a simultaneous increase of NOx removal. The potentiality of combined NSR-SCR systems, in which NH3 formation over NSR is exploited on a SCR catalyst, is considered in this work by using H2 as a reductant.