Development of NOx Storage-Reduction Three-way Catalyst for D-4 Engines

It is an essential task for automobiles to reduce their fuel consumption. A direct injection gasoline engine (D-4 engine) is effective in reducing fuel consumption, but the reduction of NO_x in the lean combustion region is impossible with a conventional three-way catalyst. The NO_x storage-reduction three-way catalyst was put into practical use in 1994 for vehicles with lean-burn engines. This catalyst, however, is poisoned by SO₂ caused by fuel sulfur, thus its activity is reduced. The conversion efficiency of this sulfur poisoned catalyst was not sufficient for reducing NO_x in the exhaust gas of D-4 engine. We have, therefore, studied the mechanism of sulfur poisoning, and succeeded in improving the catalytic performance with the newly developed monolithic substrate and the newly developed additives.