Durability Aspects of NOx Storage Catalysts for Direct Injection Gasoline Vehicles 1999-01-1285
The introduction of gasoline direct injection technology into the European market will depend mainly on the availability of an effective and durable aftertreatment system, in order to reach future stringent European emission standards. NOx storage technology provides a reasonable chance of fulfilling future emission goals, but durability problems such as thermal degradation and sulfur poisoning have yet to be overcome. The present paper is dedicated to these problems, and demonstrates the progress achieved so far. The influence of different aging modes and aging severity on the NOx conversion efficiency of an advanced generation of NOx storage catalysts, is described in detail. It was found that the severity of aging at comparable catalyst bed temperatures, increases in the following order: hydrothermal aging in N2/H2O < engine aging w/o fuel cut at λ-1 < furnace aging in air < engine aging with fuel cut at λ-1. Using a special low temperature sulfation / high temperature desulfation cycle at a stationary homogeneous lean burn engine fuelled with high sulfur fuel (189 ppm by weight of sulfur), almost full reversibility of sulfur poisoning could be attained with fresh and aged NOx storage catalysts. Furthermore, the selection of λ-values of 0.98 for the fresh catalyst and 0.95 to 0.98 for the aged catalyst, proved to be optimal for desulfation speed and avoidance of H2S formation. In spite of these promising findings, the need for reduced sulfur concentration in gasoline becomes obvious, as particularly the aged NOx storage catalyst loses its adsorption efficiency very quickly when high sulfur fuel (189 ppm by weight of sulfur) is applied. As a result, very frequent desulfation events have to be initiated, so that the fuel benefits of gasoline direct injection technology would otherwise be exchanged for costly desulfation events. The on-road driving of a Toyota gasoline direct injection vehicle equipped with two close coupled start catalysts and a NOx storage catalyst in an under-body position, which was operated in a mixed mode comprising mainly urban and aggressive highway driving, demonstrates an excellent performance of both start catalysts and NOx storage catalyst after a mileage equivalent to 9000 km. Commercial super-plus fuel was used, and the frequent highway operation of the vehicle, provided a sufficient number of naturally occurring desulfation processes.