Evaluation of sol‐gel processed BaO· n Al 2 O 3 materials as NO x traps

Among various strategies to treat NO x , trapping is considered an economical, consumer transparent approach for gasoline engines. The conventional NO x traps are based on alkaline‐earth metal impregnated alumina. These traps work well, but cannot sustain their NO x trapping efficiency after repeated exposure to high temperatures during regeneration. In search of thermally stable materials, a series of sol‐gel processed BaO·nAl 2 O 3 (n = 1, 4, 6) materials including BaO·6Al 2 O 3 molecular sieves were synthesized and evaluated as NO x 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·nAl 2 O 3 materials significantly affect the NO x trapping efficiency of these materials. Among these materials, sol‐gel processed and precious metal impregnated BaO·6Al 2 O 3 powder offers the optimum combination of thermal stability and NO x trapping efficiency. We deposited this powder on a honeycomb substrate, impregnated it with 2% Pt and 0.4% Rh, and reevaluated it. The NO x trapping efficiency of this catalyst is 95% (cf. 75% for the powder) at 310°C. Our data on Pt/BaO·6Al 2 O 3 also suggest that the optimum lean cycle length for this class of NO x traps is 1 min. The efficiency of NO x traps decreases rapidly and falls in 10–20% range with an alternating 5 min lean cycle and 1 min rich cycle.