New Insights into the Role of the SCR Catalyst in Coupled LNT-SCR Systems

Mark  Crocker, Center for Applied Energy Research, U. of Kentucky

NOx storage and reduction was studied using a coupled LNT-SCR system consisting of a low-precious metal loaded Pt/Rh LNT catalyst and a commercial Cu-zeolite SCR catalyst. Under cycling conditions, NOx conversion over the SCR catalyst exceeded the corresponding conversion of NH3 when a CO+H2+C3H6 mixture or C3H6 by itself was used as the reductant. This is explained by the presence of propene, which slipped through the LNT catalyst and reacted with the LNT NOx slip. Separate experiments, incorporating the use of SpaciMS, confirmed the ability of propene to function as a NOx reductant over the SCR catalyst and revealed that the SCR catalyst was able to store propene, such that NOx reduction by the stored olefin continued into the lean phase (after the switch from rich conditions). According to adsorption experiments, significant co-adsorption of NH3 and propene occurred in the SCR catalyst, while under lean-rich cycling conditions the contributions of NH3 and C3H6 to NOx conversion were found to be essentially additive. The SCR catalyst was also found to mitigate (in part) N2O emissions from the LNT catalyst during lean-rich cycling.

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