Optimal Operating Conditions of Coupled Sequential NOx Storage/Reduction and Cu/CHA Selective Catalytic Reduction Monoliths

Abstract

The optimal control of the combined NSR–SCR system has been derived by looking for optimal conditions, namely temperature and H2 concentration during the rich period, under which the NSR catalyst has to be operated in order to generate the adequate amount of NH3 needed to be used as the reductant agent for the NOx reduction reaction on the downstream SCR monolith, with the final objective of producing selectively only-N2 with minimum ammonia slip at the exit of the system. This conceives the so expected “zero emission level”. The analysis has been carried out for a model Pt–BaO/Al2O3 monolith followed by the Cu/CHA monolith, which allows the storing of ammonia to react with NOx leaving the NSR during the subsequent lean phase, establishing the complete “operational map” for any combination of temperature (150–400 °C) and H2 concentration used during the rich period (1–4 %) to know the reached NOx efficiency. Maximum efficiency, with almost total selectivity to N2 and no NH3 slip is achieved at low-intermediate temperature (around 180–200 °C) and high H2 concentration (3–4 %), which generates about 30 % of ammonia in the NSR catalyst to be used in the SCR catalyst.