Development of the NOx Adsorber Catalyst for Use with High-Temperature Condition 2001-01-1298
NOx adsorber has already been used for the after-treatment system of series production vehicle installed with a lean burn or direct injection engine [1,2,3]. In order to improve NOx adsorbability at high temperatures, many researchers have recently been trying an addition of potassium (K) as well as other conventional NOx adsorbents. Potassium, however, reacts easily with the cordierite honeycomb substrate at high temperatures, and not only causes a loss in NOx adsorbability but also damages the substrate.
Three new technologies have been proposed in consideration of the above circumstances. First, a new concept of K-capture is applied in washcoat design, mixed with zeolite, to improve thermal stability of K and to keep high NOx conversion efficiency, under high temperatures, of NOx adsorber catalyst. Second, another new technology, pre-coating silica over the boundary of a substrate and washcoat, is proposed to prevent the reaction between potassium and cordierite. The NOx adsorber catalyst, adopting these technologies, has demonstrated less crack formation and high NOx adsorbability at high temperatures even after severe accelerated aging test on the engine bench. The last one of the three technologies is the mixing of K retention compound and titanium compound in the washcoat with the aim of suppressing sulfur poisoning. The compound mixing in the washcoat has proven its effectiveness since K adsorbent has the advantage of easier sulfur desorption as compared with barium (Ba).
A newly developed NOx adsorber system is considered to be one of the potential technologies to deal with increasingly stringent NOx regulations for even high exhaust temperature applications.