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

The NOx 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 FTIR spectroscopy. The study is accomplished using H2 as the reductant, in the presence and absence of CO2 and H2O and in a wide temperature interval (150–350 °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 N2. The following main points have been clarified: (1) the storage of NOx 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 Pt–Ba/Al2O3 particles to give N2 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 N2, 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 °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.

In the LNT/SCR dual bed during the rich phase the reduction of NOx stored onto LNT catalyst gives NH3 (I step) and then N2 (II step). NH3 slipped from LNT catalyst is adsorbed onto SCR catalyst. In the lean phase NOx are stored onto LNT catalyst (I step) and react with NH3 adsorbed onto SCR catalyst via SCR reaction (II step). This lowers the ammonia slip and increases the NOx removal efficiency. The LNT/SCR physical mixture also shows a NOx removal efficiency higher than the single LNT catalyst.Figure optionsDownload high-quality image (122 K)Download as PowerPoint slideHighlights
► We study two hybrid LNT/SCR catalytic systems compared with single catalysts.
► The catalytic behaviour is systematically studied by transient experiments and FTIR.
► Hybrid systems and single catalysts, CO2/H2O presence and temperature are analysed.
► The hybrid systems are always better than the single LNT catalyst.