Dynamics of N-2 and N2O peaks during and after the regeneration of lean NOx trap

摘要

The dynamics and selectivity of N-2 and N2O formation during and after the regeneration of a commercial NOx storage catalyst containing Pt, Pd, Rh, Ba on Ce/Zr, Mg/AI and Al oxides was studied with high-speed FTIR and SpaciMS analyzers. The lean/rich cycling experiments (60s/5 s and 60 s/3 s) were performed in the temperature range 200-400 degrees C, using Hy, CO, and C3H6 individually for the reduction of adsorbed NOx. Isotopically labeled (NO)-N-15 was employed in combination with Ar carrier gas in order to quantify the N-2 product by mass spectrometry. N-2 and N2O products were formed concurrently. The primary peaks appeared immediately after the rich-phase inception, and tailed off with breakthrough of the reductant front (accompanied by NH3 product). Secondary N-2 and N2O peaks appeared at the rich-to-lean transition as a result of reactions between surface-deposited reductants/intermediates (CO, HC, NH3, -NCO) and residual stored NOx. At 200-300 degrees C, up to 30% of N-2 and 50% of N2O products originated from the secondary peaks. The N2O/N-2 selectivity ratio as well as the magnitude of secondary peaks decreased with temperature and duration of the rich phase. Among the three reductants, propene generated secondary N-2 peak up to the highest temperature. The primary N-2 peak exhibited a broadened shoulder aligned with movement of reduction front from the zone where both NOx, and oxygen were stored to the NOx-free zone where only oxygen storage capacity was saturated. N-2 formed in the NOx-free zone originated from reaction of NH3 with stored oxygen, while N2O formation in this zone was very low. (C) 2014 Elsevier B.V. All rights reserved.