Pathways for N₂O Formation/Reduction During Operation of Commercial LNT Catalysts

In this study the pathways involved in N₂O formation over a commercial LNT catalyst are addressed, when using H₂ as a reductant. For this purpose, flow microreactor experiments coupled with FT-IR under operando conditions are used. The results indicate that N₂O formation occurs both at the lean-to-rich (primary N₂O) and rich-to-lean (secondary N₂O) transitions. Primary N₂O originates at the reduction front due to the presence of partially reduced PGM sites that do not readily dissociates NO released from the stored NO_x. Undissociated NO couples with N-adatoms leading to the formation of N₂O. At variance, secondary N₂O originates upon oxidation with NO/O₂ of reducing species left adsorbed on the catalyst surface (adsorbed CO, isocyanates and possibly NH₃) during the rich phase, as pointed out by FT-IR spectroscopy. The concentration of such adsorbed species is however limited and hence the formation of secondary N₂O is much smaller than that of primary N₂O, when using H₂ as reducing agent. The emissions of N₂O reduce upon increasing the temperature, and above 250 °C N₂O formation is negligible. Finally the reactivity of N₂O with adsorbed NO_x species (nitrites) and with the actual reductants (H₂ and NH₃) is also investigated, to provide further indications concerning the pathways leading to N₂O emissions. It is found that N₂O does not react with NO_x species stored downstream the reduction front; at variance both H₂ and NH₃ may reduce N₂O to N₂ and water at rather low temperatures. The role of this reaction on N₂O emission is herein discussed.