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

In this study the pathways involved in N 2 O formation over a commercial LNT catalyst are addressed, when using H 2 as a reductant. For this purpose, flow microreactor experiments coupled with FT-IR under operando conditions are used. The results indicate that N 2 O formation occurs both at the lean-to-rich (primary N 2 O) and rich-to-lean (secondary N 2 O) transitions. Primary N 2 O 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 N 2 O. At variance, secondary N 2 O originates upon oxidation with NO/O 2 of reducing species left adsorbed on the catalyst surface (adsorbed CO, isocyanates and possibly NH 3 ) 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 N 2 O is much smaller than that of primary N 2 O, when using H 2 as reducing agent. The emissions of N 2 O reduce upon increasing the temperature, and above 250 °C N 2 O formation is negligible. Finally the reactivity of N 2 O with adsorbed NO x species (nitrites) and with the actual reductants (H 2 and NH 3 ) is also investigated, to provide further indications concerning the pathways leading to N 2 O emissions. It is found that N 2 O does not react with NO x species stored downstream the reduction front; at variance both H 2 and NH 3 may reduce N 2 O to N 2 and water at rather low temperatures. The role of this reaction on N 2 O emission is herein discussed.