Abstract
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► Pt–K2O/MgAl2O4 catalysts with different K2O loading were prepared and tested. ► Temperature where maximum NOx uptake occurs was shifted from 300°C to 450°C with increasing K2O loading from 5wt% to 10wt%. ► Durability issues such as sulfur poisoning and removal remains to be overcome.
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 5wt% on MgAl2O4, the temperature of maximum NOx uptake (Tmax) is 300°C. Increasing the potassium loading from 5wt% to 10wt% 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 10wt%, 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.