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.
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