Copper ion-exchanged ZSM-5 zeolite catalyst, which reduces nitrogen oxides (NOx) in the presence of oxygen and hydrocarbons, was applied to actual diesel engine exhaust. Copper ion-exchanged ZSM-5 zeolite effectively reduced NOx by 25% in normal engine operation, and by 80% when hydrocarbons in the exhaust were increased. Water in the exhaust gas decreased the NOx reduction efficiency, but oxygen and sulfur appeared to have only a small effect. Maximum NOx reduction was observed at 400°C irrespective of hydrocarbon species, and did not decrease with space velocity up to values of 20,000 1/h.THE PURPOSE of this paper is to evaluate the possibilities and problems in catalytic reduction of NOx in actual diesel engine exhaust. Here, a copper ion-exchanged ZSM-5 zeolite (Cu-Z) catalyst was applied to diesel engine exhaust to examine the dependency of the NOx reduction efficiency on temperature and space velocity. The effects of oxygen, water and hydrocarbons were also examined.The reduction of NOx from diesel engine exhaust gas has become an important research topic in recent years. One of the obstacles to reducing NOx is that there is a tradeoff between NOx and particulates in diesel combustion. Although the overall level of these pollutants have been reduced through improvements in the combustion process, further reductions will be difficult to obtain because of this tradeoff effect.Recently, Iwamoto, et al. - and Held, et al.  have independently reported that copper ion-exchanged ZSM-5 zeolite (Cu-Z) is able to reduce NOx in the presence of oxygen, with hydrocarbons (HC) as the reducing agent. This catalyst has been shown to have a large reduction effect on synthesized gas which was modeled after diesel exhaust. Other catalysts such as proton-exchanged zeolite [51 and alumina [61J71 have been discovered to also be effective for reduction of NOx. These are all expected to be applicable as NOx reducing catalysts for diesel engines.