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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.

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. 1) Improvement of thermal resistance for the NO oxidation activity by addition of a different precious metal element. 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.

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.