New insights on N₂O formation pathways during lean/rich cycling of a commercial lean NO_x trap catalyst

Pathways for N₂O formation during lean/rich cycling of a commercial LNT catalyst (containing Pt, Pd, Rh as well as Ba, CeZr, MgAl and Al oxides) were investigated based on bench-reactor experiments with spatiotemporally resolved gas measurements (SpaciMS) and in situ surface analysis (DRIFTS). The inlet gas temperature, composition of the rich gas mixture and regeneration length were varied in order to reveal the underlying mechanisms. Particular attention was given to low temperatures where the regeneration was kinetically limited and N₂O was emitted in two peaks. The primary N₂O peak appeared immediately after rich-phase inception when platinum-group-metal (PGM) sites over which NO_x reduction took place were only partially reduced, and tailed off with the breakthrough of the reductant front. The secondary N₂O peak appeared at the rich-to-lean transition as a result of reactions between surface-deposited reductive species (NH₃, CO and/or isocyanates) and residual stored NO_x. Therefore, more thorough regeneration (longer rich phase, more efficient reductant, or higher temperature) led to a reduced secondary N₂O peak. In contrast, CO either produced from reverse water gas shift or injected as a feed could poison PGM sites resulting in self-inhibition of the regeneration and a subsequent significant secondary N₂O peak.