Improving high-temperature NOx conversion in the combined NSR-SCR system with an SCR catalyst mixed with an NH3 adsorbent

Abstract

The growing worldwide interest in environmental protection and the strong demand to decrease the emission of pollutants from automotive exhaust require the development of catalysts and post-cleaning systems that are very efficient and stable. To that end, we investigated ways of improving the high-temperature NH3 selective catalytic reduction (SCR) activity in the NOx storage reduction (NSR) and SCR combined system, and found that NH3 oxidation is promoted under high-temperature NSR-SCR reaction conditions. To suppress NH3 oxidation and improve the NH3-SCR reaction in the NSR-SCR system, we examined mixing an NH3 adsorbent with the SCR catalyst (NSR-SCR + NH3 adsorbent system). The H-type mordenite zeolite (H-MOR) adsorbent demonstrated superior performance for retaining NH3 without oxidation under high-temperature transient conditions than the other NH3 adsorbents examined. NOx conversion under high-temperature transient conditions was improved in going from the single NSR system (71% at 450 °C) to NSR-SCR (75% at 450 °C), and NSR-SCR+H-MOR (85% at 450 °C). We confirmed that N2 formed over long times, from the commencement to the end of the lean phase, in the NSR-SCR+H-MOR system. This phenomenon, in which the NH3 adsorbed on H-MOR is gradually released to the SCR up to the end of the lean phase, enhances NOx reduction in the NSR-SCR system.