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NOx reduction activity in an oxidizing exhaust gas was significantly improved by discharging non-thermal plasma and catalysts (plasma assisted catalysis). We investigated effective catalyst for plasma assisted catalysis in view of hydrocarbon-selective catalytic reduction(HC-SCR). Plasma assist was effective for γ-alumina and alkali or alkaline earth metals loaded zeolite and γ-alumina showed the highest NOx conversion among these catalysts. On the other hand, Plasma assist was not effective for Cu-ZSM-5 and Pt loaded catalyst. The NOx conversion for the plasma assisted γ-alumina decreased by formation of a deposit on the catalyst below 400°C. It is shown that indium loading on γ-alumina improved the NOx reduction activity and suppressed the degradation of the NOx reduction activity at 300°C with plasma assist.

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.