Fundamental study on durability of Photocatalyst-Plasma-Honeycomb (PPH) converter

To cope with increasingly stringent emission regulations in the future, the exhaust gas aftertreatment system of a vehicle is required to be highly efficient and more durable as well. To obtain the overall higher conversion efficiency of an aftertreatment system, the light-off time of the catalyst has to be reduced in particular, since more than 85% of the total hydrocarbon emissions is coming out during the cold start period, i.e. before the catalyst reaches its light-off temperature or chemically activated temperature for conversion. A new system called Photocatalyst-Plasma-Honeycomb (PPH) to cut light-off time of a catalyst effectively to zero has been developed and it has successfully demonstrated a very high potential for practical application in a car through extensive bench and vehicle tests. Generally, an automotive aftertreatment system is likely to be exposed to a variety of severe conditions in the field that are known to deteriorate the activity of a catalytic converter system. Some examples of severe conditions are the exposure to very high temperatures and the presence of poisoning species like zinc, phosphorous and sulfur. In this paper, some of the fundamental durability tests are performed to demonstrate the practical application potential of a PPH system as an automotive aftertreatment system. Hydrothermal aging in an electric furnace is done to check the thermal durability of a PPH system from the viewpoint of a phase transformation of the photocatalyst material itself, because its phase transformation at high temperature leads to a large drop in photocatalytic activity and, thus, overall conversion efficiency. In addition, a high energy density of arc discharge is sometimes observed from the non-thermal plasma generating system of a PPH when operating conditions are inappropriate and this arc discharge is known to adversely affect the electrical characteristics of a photocatalyst. Therefore, a PPH reactor is tested by simulated rapid aging cycle tests on an arc discharge bench system that is designed to intentionally generate large quantities of small arc discharges. Also, the poisoning effects of sulfur and particulate matter (PM) on photocatalytic activities are examined using a light-oil fuelling small-sized boiler system that is designed to intentionally generate PM containing sulfur compounds.