The reduction of NOx stored on LNT and combined LNT-SCR systems

The adsorption–reduction by H2 of NOx stored on a model LNT Pt–Ba/Al2O3 catalyst sample and on a combined LNT + Fe-ZSM5 SCR catalyst is investigated in this work. The storage of NOx over the LNT Pt– Ba/Al2O3 catalyst leads to nitrite formation at low temperatures, and to nitrate at high temperatures (above 300 8C). The stored NOx are reduced mainly to NH3 at low temperatures, and to N2 at high temperatures. In line with previous data and according to experiments carried out with H2 and NH3, a two-step in-series pathway for nitrogen formation is proposed involving at first the fast formation of ammonia upon reaction of NOx with H2, followed by the slower reaction of the so-formed ammonia with the stored NOx leading to N2. This two-step in-series pathway together with the integral behaviour of the reactor accounts for the temporal evolution of the products which is observed during regeneration of LNT catalysts. A Fe-ZSM5 SCR placed downstream the LNT catalyst stores NH3 released during the rich phase from the LNT sample. The stored ammonia is effectively converted to N2 during the subsequent lean phase, due to the occurrence of the SCR reaction over the Fe-ZSM5 catalyst bed. Hence the presence of a SCR catalyst layer placed downstreama LNT sample leads to the twofold benefit of reducing the NH3 slip and to increase the NOx removal efficiency. These advantages are most important at low temperatures, and tend to vanish at high temperatures where the NH3 slip is less important.