Filtering particulate matter from exhaust gas

摘要

A filter for filtering particulate matter (PM) from exhaust gas emitted from a positive ignition engine comprises a porous substrate 10 having inlet and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure containing pores 12 of a first mean pore size. The porous structure is coated with a catalytic washcoat 14 comprising a plurality of solid particles, wherein the porous structure of the washcoated porous substrate contains pores 16 of a second mean pore size. The second mean pore size is less than the first mean pore size. The catalytic washcoat is a hydrocarbon trap comprising at least one molecular sieve which is un-metallised. Alternatively, the molecular sieve is catalysed with a platinum group metal. The washcoat may substantially cover surface pores of the porous structure or may sit substantially within the porous structure of the porous substrate. An exhaust system comprising the filter, a positive ignition engine comprising the exhaust system and a method of trapping particulate matter is also claimed.