Lean NOx Reduction with H2 and CO in Dual-Layer LNT–SCR Monolithic Catalysts: Impact of Ceria Loading

Abstract

A series of monolithic catalysts consisting of a layer of selective catalytic reduction (SCR) catalyst deposited on top of lean NOx trap (LNT) catalyst were synthesized for lean reduction of NOx (NO&NO2) with H2 and CO. The LNT catalyst exhibited a rather low NOx conversion below 250 °C due to CO inhibition. The top SCR layer comprising Cu/ZSM5 significantly increased the NOx conversion at low temperature by its reaction with NH3 formed during the regeneration phase. The addition of CeO2 to the LNT layer promoted the water gas shift reaction (CO + H2O ↔ H2 + CO2). The WGS reaction mitigated the CO inhibition and the generated H2 enhanced the low-temperature catalyst regeneration. The ceria addition decreased the performance at high temperatures due to increased oxidation of NH3. The ceria loading was optimized by applying a non-uniform axial profile. A dual-layer catalyst with an increasing ceria loading axial profile improved the performance over a wide (low and high) temperature range.