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 18 of a second mean pore size. The second mean pore size is less than the first mean pore size. The catalytic washcoat is a three-way catalyst comprising: platinum and rhodium; palladium and rhodium; or platinum, palladium and rhodium as active components supported on a high surface area oxide; and an oxygen storage component. The pores at a surface of the porous structure comprise a pore opening and the washcoat causes a narrowing of substantially all the surface pore openings. Alternatively, the washcoat sits substantially within the porous structure. An exhaust system comprising the filter, a positive ignition engine comprising the exhaust system and a method of trapping particulate matter is also claimed.