Catalysed soot filter

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A catalysed soot filter (CSF) 8, for a diesel engine, comprises a plurality of inlet and outlet channels defined by internal walls of a substrate. Surfaces of the internal walls of the inlet channels comprise a washcoat that extends from an open inlet end to a downstream inlet coating end. External surfaces of the internal walls of the plurality of outlet channels comprise an on-wall washcoat that extends from an upstream outlet end to an open outlet end. The outlet coating extends for 55-90% of the substrate length, and is longer than the inlet coating length. The outlet coating composition has a mean particle size (D50) of 4-15µm. Both the inlet and outlet coatings comprise at least one catalytically active metal from the group consisting of platinum, palladium, iridium, rhodium, silver and gold, and at least one refractory metal oxide or an optionally stabilised rare earth metal as a catalyst for increasing the percentage of NO2 of total NOx in exhaust gas passing through the filter.