New operation strategy for driving the selectivity of NO_x reduction to N₂, NH₃ or N₂O during lean/rich cycling of a lean NO_x trap catalyst

Periodical regeneration of NO_x storage catalyst (also known as lean NO_x trap) by short rich pulses of CO, H₂ and hydrocarbons is necessary for the reduction of nitrogen oxides adsorbed on the catalyst surface. Ideally, the stored NO_x is converted into N₂, but N₂O and NH₃ by-products can be formed as well, particularly at low-intermediate temperatures. The N₂ and N₂O 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 NH₃ product. The secondary N₂ and N₂O peaks then appear at the rich-to-lean transition as a result of reactions between surface-deposited reductants/intermediates (CO, HC, NH₃, 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 N₂ yield and without the undesired formation of a secondary N₂O peak. Furthermore, NH₃ 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.