Dynamics of N2 and N2O peaks during and after the regeneration of lean NO trap

Abstract

a b s t r a c tThe dynamics and selectivity of N 2 and N 2 O formation during and after the regeneration of a commercial NO x storage catalyst containing Pt, Pd, Rh, Ba on Ce/Zr, Mg/Al and Al oxides was studied with high-speed FTIR and SpaciMS analyzers. The lean/rich cycling experiments (60 s/5 s and 60 s/3 s) were performed in the temperature range 200-400 • C, using H 2 , CO, and C 3 H 6 individually for the reduction of adsorbed NO x . Isotopically labeled 15 NO was employed in combination with Ar carrier gas in order to quantify the N 2 product by mass spectrometry. N 2 and N 2 O 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 NH 3 product). Secondary N 2 and N 2 O peaks appeared at the rich-tolean transition as a result of reactions between surface-deposited reductants/intermediates (CO, HC, NH 3 , -NCO) and residual stored NO x . At 200-300 • C, up to 30% of N 2 and 50% of N 2 O products originated from the secondary peaks. The N 2 O/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 NO x and oxygen were stored to the NO x -free zone where only oxygen storage capacity was saturated. N 2 formed in the NO x -free zone originated from reaction of NH 3 with stored oxygen, while N 2 O formation in this zone was very low.

New operation strategy for driving the selectivity of NO reduction to N2, NH3 or N2O during lean/rich cycling of a lean NO trap catalyst

“…We also proved experimentally that the secondary peaks originate from the reactions between residual stored NO x and adsorbed reduction intermediates remaining on the catalyst surface when switching back to lean phase, and their selectivity is insensitive to the presence of NO x in the gas phase [14]. Finally, we demonstrated that the secondary peaks diminish with a longer rich phase and more complete regeneration that leave less residual NO x and/or reduction intermediates on the catalyst surface [6,14].…”

“…Synthetic gases were used to perform a set of cycling experiments. A rapid switching 4-way valve was used to alternate between individual gas mixtures that were prepared independently in two lines [6,9,14]. The gas hourly space velocity (GHSV) was 30,000 h −1 and the experiments focused on the temperature range 200-250 • C where the contribution of secondary N 2 O and N 2 peaks is most significant.…”

“…During and after catalyst regeneration, N 2 and N 2 O products are formed concurrently in two peaks. While the primary peaks are associated with rich pulse inception and appear at the catalyst outlet during regeneration, the secondary peaks can be observed immediately after the rich pulse as a result of subsequent reactions [6][7][8][9][10][11][12][13]. The secondary peak diminishes with a longer and more complete regeneration that does not leave enough residual NO x and reduction intermediates on the catalyst surface [14].…”

“…After switching back to lean conditions, the adsorbed reductants and reduction intermediates can further react with the residual NO x remaining on the surface due to an incomplete regeneration and form additional N-products [11,14]. These secondary products can significantly contribute to the overall N 2 and N 2 O yield [6,7].…”

“…Although up to 50% of the total N 2 O product can originate from the secondary peak [6], no suggestions have yet been published for specifically addressing this issue. Our previous research [6,9,11,14] enabled better understanding of the general principle of NO x reduction selectivity depending on the actual oxidation state of PGM sites. Particularly, we have shown that N 2 O formation is minimized over well reduced PGM sites, regardless of the specific reductant used [11].…”