Catalytic behaviour of hybrid LNT/SCR systems: Reactivity and in situ FTIR study

The NO storage-reduction over the single Pt-Ba/Al2O3 and Fe-ZSM-5 systems and over the hybrid Pt-Ba/Al2O3 + Fe-ZSM-5 systems, both physically mixed and in dual bed configuration with the SCR catalyst placed-downstream of the LNT catalyst, is systematically investigated by Isothermal Step Concentration (ISC) experiments and FUR spectroscopy. The study is accomplished using H-2 as the reductant, in the presence and absence of CO2 and H2O and in a wide temperature interval (150-350 degrees C). The results are explained at the quantitative level by applying a scheme of the reduction of stored NOx, which implies at first the reduction of nitrates to give NH3, followed by the reaction of ammonia with NOx stored downstream in the reactor to give N-2. The following main points have been clarified: (1) the storage of NO occurs primarily onto Pt-Ba/Al2O3 and is lower in the presence of CO2 and H2O because BaCO3 is formed which retards NOx adsorption; (2) NH3 formed as intermediate in the reduction of stored NOx is trapped onto the Fe-ZSM-5 catalyst. The trapping of NH3 is most favoured when the particles of Pt-Ba/Al2O3 and of Fe-ZSM-5 are extensively in contact, as in the LNT/SCR physical mixture, and at low temperature. NH3 trapped onto Fe-ZSM-5 reacts during the subsequent lean phase with gaseous NOx slipped from the PtBa/Al2O3 particles to give N-2 via SCR; (3) the NOx removal efficiency is always higher for both LNT/SCR dual bed and physical mixture compared to single LNT, due to the additional contribution of SCR over Fe-ZSM-5 during the lean phase; (4) in the case of the LNT/SCR physical mixture, NH3 trapped onto the Fe-ZSM-5 particles can be oxidised over the LNT particles upon admission of oxygen to give N-2, N2O and NO. The selectivity to nitrogen is almost complete at any temperature in the presence of CO2 and H2O. while in the absence of CO2 and H2O, it increases significantly with temperature and is almost complete at 350 degrees C; (5) the ammonia slip is low to the best over the LNT/SCR dual bed at any temperature in the absence of CO2 + H2O. In the presence of CO2 and H2O, the ammonia slip is nil at any temperature over the LNT/SCR dual bed. (C) 2011 Elsevier Inc. All rights reserved.