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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.

a b s t r a c tPeriodical regeneration of NO x storage catalyst (also known as lean NO x trap) by short rich pulses of CO, H 2 and hydrocarbons is necessary for the reduction of nitrogen oxides adsorbed on the catalyst surface. Ideally, the stored NO x is converted into N 2 , but N 2 O and NH 3 by-products can be formed as well, particularly at low-intermediate temperatures. The N 2 and N 2 O products are formed concurrently in two peaks. The primary peaks appear immediately after the rich-phase inception, and tail off with the breakthrough of the reductant front accompanied by NH 3 product. The secondary N 2 and N 2 O peaks then appear at the rich-to-lean transition as a result of reactions between surface-deposited reductants/intermediates (CO, HC, NH 3 , NCO) and residual stored NO x under increasingly lean conditions. Based on these mechanistic insights, we propose and demonstrate a novel strategy for driving the selectivity of the secondary peaks towards desired products. It is based on a transition phase of neutral or slightly lean (nearly stoichiometric) character inserted between the rich and the fully lean phase. This strategy allows more complete regeneration of the catalyst with higher N 2 yield and without the undesired formation of a secondary N 2 O peak. Furthermore, NH 3 can be formed during this slightly lean transition phase without any CO or hydrocarbons breakthrough. Such ammonia formation is desirable in the exhaust gas aftertreatment systems combining LNT with passive SCR technology.

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