A non-NH3 pathway for NOx conversion in coupled LNT-SCR systems

NOx storage-reduction experiments were performed using a coupled LNT-SCR system consisting of a low-precious metal loaded Pt/Rh LNT catalyst and a commercial Cu–zeolite SCR catalyst. Cycling experiments revealed that when a CO + H2 + C3H6 mixture or C3H6 by itself was used as the reductant, the NOx conversion over the SCR catalyst exceeded the conversion of NH3 over the same catalyst. This is explained by the presence of propene, which slipped through the LNT catalyst and reacted with the LNT NOx slip. Separate experiments, conducted under continuous flow and lean-rich cycling conditions, confirmed the ability of propene, as well as ethene, to function as a NOx reductant over the SCR catalyst. Cycling experiments also revealed that the SCR catalyst was able to store propene, such that NOx reduction by stored propene continued into the lean phase (after the switch from rich conditions). According to adsorption experiments, significant co-adsorption of NH3 and propene occured 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. These findings suggest that under actual driving conditions, NOx reduction by non-NH3 reductants (olefins and possibly other hydrocarbons) in the SCR catalyst can contribute to the mitigation of lean and rich phase NOx.

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► NOx reduction was studied in a combined LNT-SCR system.
► Propene and ethene function as NOx reductants over the SCR catalyst.
► NH3 and propene can co-adsorb in the SCR catalyst.
► Contributions of NH3 and C3H6 to NOx conversion are additive under lean-rich cycling.