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To clarify the thermal deterioration mechanism of three-way catalysts quantitatively, we investigated the relationship between the catalytic performance and the catalyst characteristics for thermally aged Pt/Rh catalysts. 1. Experimentally, the HC oxidation reaction, which occurs on the surface of Pt/Rh particles on catalysts, is approximated by a first-order reaction of HC concentration with a constant activation energy. 2. The relationship between the Pt mean diameter D and the HC 50% conversion temperature T50 is described by the following equation; where E and kB are the activation energy of the reaction and the Boltzman constant, respectively. 3. The sintering rate of Pt particles on the three-way catalysts was found to depend on the aging temperature(T) of catalyst and the concentration of oxygen in the gas phase([O2]).

A new 3-way catalyst with NOx conversion performance for lean-burn engines has been developed. The catalyst oxidizes NOx and stores the resulting nitrate, which is then reduced by HC and CO during engine operation around the stoichiometric air/fuel ratio. Both the composition of the storage component and the particle sizes of the noble metal were optimized. In addition, a special air fuel mixture control has been developed to make the best of the NOx storage-reduction function. The present catalyst showed 90% conversion efficiency and improved fuel economy by 4% in the Japanese 10-15 mode test cycle. The efficiency remained at 60% or more after durability test.

The effects of “periodic operation (variations of both A/F modulation width and their frequency)” on the activities of fresh and aged Pt/Rh catalysts were systematically evaluated on engine dynamometer in terms of A/F and cold-start performance. A/F modulation width and its frequency were varied from ±2 to ±10% A/F and from 0.5 to 5 Hz, respectively. The periodic operation effects for both catalysts were the most preferable for diminishing THC emission and a little effect was obtained for CO emission. A remarkable improvement of NOx emission on the aged catalyst was confirmed, although a medium effect was appeared on the fresh catalyst.