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

• N2O formation investigated during regeneration of a commercial LNT catalyst.
• Spatiotemporally resolved gas measurements (SpaciMS), surface analysis (DRIFTS).
• Secondary N2O peak indicates incomplete reduction of the catalyst.
• Adsorbed NH3, CO and/or isocyanates contribute to secondary N2O formation.
• Presence of CO2 inhibits regeneration by H2, forming CO on Pt sites.

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 NOx 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 NOx. 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.

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