Evaluation of Sol-Gel Processed BaO·nAl₂O₃ 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·nAl₂O₃ (n = 1, 4, 6) materials including BaO·6Al₂O₃ 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·nAl₂O₃ materials significantly affect the NO_x 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 NO_x trapping efficiency. We deposited this powder on a honeycomb substrate, impregnated it with 2% Pt and 0.4% Rh, and re evaluated it. The NO_x trapping efficiency ofthis 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 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.