Evaluation of sol‐gel processed BaO·nAl2O3 materials as NOx traps

Abstract

AbstractAmong various strategies to treat NOx, trapping is considered an economical, consumer transparent approach for gasoline engines. The conventional NOx traps are based on alkaline‐earth metal impregnated alumina. These traps work well, but cannot sustain their NOx trapping efficiency after repeated exposure to high temperatures during regeneration. In search of thermally stable materials, a series of sol‐gel processed BaO·nAl2O3 (n = 1, 4, 6) materials including BaO·6Al2O3 molecular sieves were synthesized and evaluated as NOx traps using simulated exhaust at a 25,000 h−1 space velocity. Changes in structure and surface properties occurring on thermal treatment of sol‐gel processed BaO·nAl2O3 materials significantly affect the NOx trapping efficiency of these materials. Among these materials, sol‐gel processed and precious metal impregnated BaO·6Al2O3 powder offers the optimum combination of thermal stability and NOx trapping efficiency. We deposited this powder on a honeycomb substrate, impregnated it with 2% Pt and 0.4% Rh, and reevaluated it. The NOx trapping efficiency of this catalyst is 95% (cf. 75% for the powder) at 310°C. Our data on Pt/BaO·6Al2O3 also suggest that the optimum lean cycle length for this class of NOx traps is 1 min. The efficiency of NOx traps decreases rapidly and falls in 10–20% range with an alternating 5 min lean cycle and 1 min rich cycle.