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

Periodical regeneration of NOx storage catalyst (also known as lean NOx 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 NOx is converted into N-2, but N2O and NH3 by-products can be formed as well, particularly at low-intermediate temperatures. The N-2 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 N-2 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 NOx 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 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. (C) 2015 Elsevier B.V. All rights reserved.