Passive NOx Reduction Activity of a Silver Catalyst under Real Diesel-Engine Exhaust Conditions

The hydrocarbon-SCR activity of a silver catalyst has been examined using actual exhaust gas from a diesel engine, without any fuel being added to the reactor inlet. This work is a further step in the development of an active lean-NO_x catalyst for aftertreatment of exhaust streams that contain an excess of hydrocarbon relative to NO_x. The engine tests follow on from laboratory studies, in which the activity was related to the composition and formulation of the catalyst, the concentration and speciation of the hydrocarbon reductants, and the composition and temperature of simulated exhaust gas. In all the tests described here, the exhaust gas has been provided by an engine operating on ultra-low sulphur diesel fuel. NO_x-reduction has been measured as a function of engine load, engine speed, in-cylinder fuel injection timing, exhaust gas temperature, and exhaust gas recirculation. Over 60% conversion to N₂ has been achieved at exhaust gas temperatures around 290°C. At lower and higher temperatures, the activity is suppressed due to catalyst coking and hydrocarbon oxidation, respectively, as predicted from the laboratory tests.