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1 In this paper results obtained with different particulate matter (PM) reduction technologies are presented. Diesel oxidation catalysts (DOC) are well known as a reliable PM reduction technology which can efficiently remove the soluble organic fraction (SOF) but which has no effect on the solid particles in PM. A drawback is that in combination with high sulfur fuel, oxidation of SO2 to SO3 by the DOC can occur, resulting in an increase of PM emissions. An alternative technology that is proven to significantly reduce soot emissions comprises diesel particulate wall-flow filters. High filtration efficiencies of up to 90% and beyond are feasible. The main obstacle is the combustion of the trapped soot. As shown in this paper, the application of a catalyst coating to the filter aids the filter regeneration by lowering the balance-point temperature. The main disadvantages of wall-flow filters are an increase in back-pressure and possible plugging caused by oil-ash accumulations.

The trend towards lower exhaust gas temperatures related to the introduction of modern, highly efficient diesel engines for passenger cars in conjunction with new legislative emission regulations will require the development of amended catalyst formulations. Not only excellent performance for carbon monoxide(CO), gaseous hydrocarbons (HC) and diesel particulates is desired but also the capability to additionally reduce nitrogen oxide (NOx) under lean conditions. Generally, as for the latter a passive system, i.e. without addition of secondary fuel, is most wanted but also an active system, i.e. with hydrocarbon enrichment before catalyst, could be successful provided the penalties in fuel consumption can be kept low. The present paper illustrates further progress in the area of diesel catalysts for passenger cars and introduces a novel washcoat formulation comprising zeolites as hydrocarbon adsorption components.