Evaluation of sol-gel processed BaO • nAl2O3 material as NOx traps

Among 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 . nAl(2)O(3) (n=1,4,6) materials including BaO . 6Al(2)O(3) 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 . nAl(2)O(3) materials significantly affect the NO, 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, trapping efficiency. We deposited this powder on a honeycomb substrate, impregnated it with 2% Pt and 0.4% Rh, and reevaluated it. The NO, trapping efficiency of this catalyst is 95% (cf: 75% for the powder) at 310 degreesC. Our data on Pt/BaO . 6Al(2)O(3) also suggest that the optimum lean cycle length for this class of NOx traps is I min. The efficiency of NOx traps decreases rapidly and falls in 10-20% range with an alternating 5 min ban cycle and I min rich cycle.