Hydrothermally stable metal oxide-zeolite composite catalysts for low-temperature NOx reduction with improved N2 selectivity

Abstract

Development of hydrothermally stable, low-temperature catalysts for controlling nitrogen oxides emissions from mobile sources remains an urgent challenge. We have prepared a metal oxide-zeolite composite catalyst by depositing Mn active species on a mixture support of CeO2/Al2O3 and ZSM-5. This composite catalyst is hydrothermally stable and shows improved low-temperature SCR activity and significantly reduced N2O formation than the corresponding metal oxide catalyst. Comparing with a Cu-CHA catalyst, the composite catalyst has a faster response to NH3 injection and less NH3 slip. Our characterization results reveal that such an oxide-zeolite composite catalyst contains more acidic sites and Mn3+ species as a result of oxide-zeolite interaction, and this interaction leads to the generation of more NH4+ species bound to the Brønsted acid sites and more reactive NOx species absorbed on the Mn sites. Herein, we report our mechanistic understanding of the oxide-zeolite composite catalyst and its molecular pathway for improving the low-temperature activity and N2 selectivity for NH3-SCR reaction. Practically, this work may provide an alternative methodology for low-temperature NOx control from diesel vehicles.