Exhaust Aftertreatment for Diesel Vehicles*

Compared to gasoline engines, diesel catalytic exhaust aftertreatment is complicated by two additional problems: the emission of soot and the fact that NO x reduction is difficult in an environment with excess oxygen. The application of catalyst of diesel engines has to be adapted to their lower exhaust gas temperature compared to that of stoichiometric gasoline engines. The diesel oxidation catalyst (DOC) is a standard equipment for diesel vehicles since the 1990s. Oxidation catalysts can reduce the mass of particulate emissions. Besides solid carbon, diesel particulate contains liquid hydrocarbons, so-called soluble organic fraction. Special application of the DOC is its use as a heatup catalyst for particulate filter regeneration. In contrast to the catalytic conversion of the gaseous pollutants, removal of particulate matter is a physical separation process. The conventional three-way catalysts cannot remove NO x from lean exhaust gas because the reduction of NO by CO, H 2 , or hydrocarbons is inhibited by excess concentration of oxygen.