Characteristics of Pt–K/MgAl2O4 lean NOx trap catalysts

We report the various characteristics of Pt–K/MgAl2O4 lean NOx trap (LNT) catalysts including the effect of K loading on nitrate formation/decomposition, NOx storage activity, and durability. This work provides results aimed at developing a fundamental understanding of this new class of LNT catalysts, which could be good candidates for high temperature engine exhaust NOx removal applications. Upon the adsorption of NO2 on K/MgAl2O4 samples, potassium nitrates, formed at Mg-related sites on the MgAl2O4 support material surface, are observed in addition to the two (ionic and bidentate) potassium nitrate species observed to form on Al2O3-supported samples. Based on NO2 TPD and FTIR results, the Mg-bound KNO3 species thermally decompose at higher temperatures than Al-bound KNO3, implying its superior thermal stability. At a potassium loading of 5 wt% on MgAl2O4, the temperature of maximum NOx uptake (Tmax) is 300 °C. Increasing the potassium loading from 5 wt% to 10 wt% results in a monotonic shift in the Tmax to 450 °C, demonstrating an unexpectedly significant dependence of Tmax on the potassium loading. Further increases in potassium loading above 10 wt%, however, only give rise to a reduction in the overall NOx storage capacity. This work also highlights the obstacles to the implementation of these materials for practical applications associated with their durability and sulfur poisoning/removal.