A new type of lean NOx catalyst has been developed with improved thermal resistance. This lean NOx catalyst contains precious metals and NO2 adsorbents. The precious metals are used mainly for the oxidation reaction of NO, and the NO2 adsorbents are for the adsorption removal of generated NO2.
The thermal resistance of the catalyst was raised by paying attention to the following points.
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1)
Improvement of thermal resistance for the NO oxidation activity by addition of a different precious metal element.
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2)
Prevention of thermal degradation of the NO2 adsorbent by addition of a new metal oxide.
For item 1, Pd was added to the catalyst which had already included Pt. By having Pd coexist with Pt, the precious metal dispersion was kept high even after heat treatment, so the NO oxidizing ability was improved. For item 2, thermal degradation of NO2 adsorbent was prevented by addition of the new metal oxide (B) to the adsorbent. Furthermore, the absorption of sulfur oxide by catalyst could also be lessened, and it was found that the (B) addition contributed to the improvement of both thermal resistance and SOx durability.
The NOx conversion of the improved catalyst about above-mentioned two points was measured by a model gas test. The catalyst was confirmed to have higher performance than the conventional catalyst for both thermal resistance and SOx durability, especially the former. Also, in the vehicle test which used exhaust gas from a commercial lean burn vehicle, even after thermal treatment, the emission values of NOx, HC, CO were 50% or less of the J-LEV regulation standard, and the catalyst had excellent exhaust gas purification performance.