EXHAUST SYSTEM COMPRISING CATALYSED SUBSTRATE MONOLITH INCLUDING WASHCOAT FOR TRAPPING VOLATILISED PT UPSTREAM FROM SCR CATALYST

An exhaust system (20) for a lean-burn internal combustion engine comprises: a first A catalysed flow-through substrate monolith (12) for use in treating exhaust gas emitted from a the lean-burn internal combustion engine, which catalysed flow-through substrate monolith (12) comprising a first washcoat coating (16) and a second washcoat coating (18), wherein the first washcoat coating (16) comprises a catalyst composition comprising at least one platinum group metal (PGM) and at least one support material for the at least one PGM, wherein the at least one PGM in the first washcoat coating (16) comprises platinum; or both platinum and palladium at a weight ratio of greater than 2:1, which is liable to volatilise when the first washcoat coating is exposed to relatively extreme conditions including relatively high temperatures of ≥700°C, wherein the second washcoat coating comprises, for trapping volatilised PGM, at least one metal oxide selected from the group consisting of optionally stabilised alumina, amorphous silica-alumina, optionally stabilised zirconia, ceria, titania, an optionally stabilised ceria-zirconia mixed oxide and mixtures of any two or more thereof supporting at least one metal, which is palladium, for trapping volatilised PGM and wherein the second washcoat coating (18) is oriented to contact exhaust gas that has contacted the first washcoat coating; a second catalysed substrate monolith (14) comprising a selective catalytic reduction (SCR) catalyst, which second catalysed substrate monolith (14) is disposed downstream from the first catalysed flow-through substrate monolith (12); and an injector (13) for injecting nitrogenous reductant into exhaust gas between the first catalysed flow-through substrate monolith (12) and the second catalysed substrate monolith (14).