Coupled LNT–SCR catalysts for lean NOx abatement: mechanistic insights from operando FT-IR spectroscopy

In this work the removal of NOx over a combined LNT + SCR system was investigated under lean–rich cycling conditions by coupling operando FT-IR spectroscopy with complementary gas-phase analysis. A model Pt-Ba/Al2O3 LNT catalyst and a commercial Cu-zeolite SCR catalyst were arranged in a sequential dual-bed configuration, with the SCR catalyst placed downstream in an operando FT-IR reactor cell to enable its real-time characterization under realistic reaction conditions. Experiments were performed in the 150–250 °C temperature range with H2, H2/CO, and C3H6 as reductants, both in the absence and presence of H2O in the feed stream. The results provided direct spectroscopic evidence that the NH3 released from the LNT catalyst during the rich phase is stored on the SCR catalyst and then consumed in the subsequent lean phase due to the occurrence of SCR reactions, thereby enhancing overall NOx conversion and improving N2 selectivity compared to the single LNT catalyst. Increasing temperature (150–250 °C) enhances both NOx storage capacity and N2 selectivity, while decreasing the formation of undesired by-products such as N2O and the NH3 slip. The nature of the reductant is shown to play a crucial role: H2 and H2/CO promote the NH3–mediated pathways, whereas C3H6 activates additional hydrocarbon–SCR routes involving the formation of surface hydrocarbon-derived intermediates, but with possible formation of carbonaceous deposits. Water is found to positively affect performance by promoting NH3 formation and facilitating the hydrolysis of C3H6-derived intermediate species, ultimately increasing N2 yield. Overall, this study provides new insights into the interplay between LNT and SCR functionalities, offering a valuable basis for the optimization of integrated LNT + SCR aftertreatment systems aimed at meeting stringent NOx emission standards.