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When the alkali metal-supported catalyst was treated at 830°C, the NOx conversion decreased because the alkali metals in the catalyst layer gradually moved to the cordierite honeycomb layer and reacted with the cordierite elements. This phenomena decreased to be added the basic metal oxide (α) in the catalyst layer. The improved catalyst with α 2 showed higher performance than the conventional catalyst in the model gas test. Moreover, the emission values of NOx, HC, and CO were 50% or less than Japanese domestic regulation values even after 830°C×60h heat treatment in a vehicle test.

Sulfur oxides endurance and heat resistance of the conventional catalyst were improved from the studies using synthetic model gas and vehicle test. The NOx purification performance was improved for lean NOx catalyst deteriorated due to SOx poising. However, the trapped SOx could be removed with CO in rich gas at 600°C. Moreover, the SOx removal could be promoted with a SOx release material (γ1). On the other hand, heat resistance of the catalyst has been improved by using new mixed metal oxides (C) as a NO2 adsorbent. The improved catalyst which contained γ1 and C showed superior heat resistance at 800°C and SOx durability compared to the conventional lean NOx catalyst in the model gas test. Also, in the vehicle test, the emission values of NOx, HC, and CO were 50% or less than J-ULEV regulation standards. These results show that the improved catalyst has a good purification performance of exhaust gas from a lean burn vehicle.

Development is proceeding on catalysts which separate the NOx in lean exhaust gas by adsorption and then reduce the adsorbed NOx in combustion exhaust gas with the stoichiometric or a slightly richer air fuel ratio, as well as exhaust conversion technology that uses these catalysts. Amidst this research it has been found that catalysts containing mixed metal oxides exhibit superior NOx adsorption performance, so the authors prepared a mixed metal oxide catalyst by adding precious metals and promoters, etc. The resulting catalyst has high heat resistance and also offers excellent SOx durability. These properties were presumed to be due to an adsorbent including the mixed metal oxide, and the relation between the physical properties and NOx conversion properties of the catalyst was investigated.