A non-NH ₃ pathway for NO _x conversion in coupled LNT-SCR systems

NO _x 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+H ₂+C ₃H ₆ mixture or C ₃H ₆ by itself was used as the reductant, the NO _x conversion over the SCR catalyst exceeded the conversion of NH ₃ over the same catalyst. This is explained by the presence of propene, which slipped through the LNT catalyst and reacted with the LNT NO _x 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 NO _x reductant over the SCR catalyst. Cycling experiments also revealed that the SCR catalyst was able to store propene, such that NO _x reduction by stored propene continued into the lean phase (after the switch from rich conditions). According to adsorption experiments, significant co-adsorption of NH ₃ and propene occured in the SCR catalyst, while under lean-rich cycling conditions the contributions of NH ₃ and C ₃H ₆ to NO _x conversion were found to be essentially additive. These findings suggest that under actual driving conditions, NO _x reduction by non-NH ₃ reductants (olefins and possibly other hydrocarbons) in the SCR catalyst can contribute to the mitigation of lean and rich phase NO _x.