Catalysed substrate monolith

An exhaust system for a lean-burn internal combustion engine, which system comprising a first catalysed substrate monolith and a second catalysed substrate monolith disposed downstream from the first catalysed substrate monolith, wherein:

• the first catalyst substrate monolith is for use in treating exhaust gas emitted from a lean-burn internal combustion engine and comprises a first washcoat coating and a second washcoat coating, wherein the first washcoat coating 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 at least one PGM in the first washcoat coating is liable to volatilise when the first washcoat coating is exposed to relatively extreme conditions including relatively high temperatures, wherein the second washcoat coating comprises at least one metal oxide for trapping volatilised PGM and wherein the second washcoat coating is oriented to contact exhaust gas that has contacted the first washcoat coating;
• and
• the second catalysed substrate monolith comprises a selective catalytic reduction (SCR) catalyst.

A catalysed substrate monolith for use in treating exhaust gas emitted from a lean-burn internal combustion engine, which catalysed substrate monolith comprising a first washcoat coating and a second washcoat coating, wherein the first washcoat coating 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 at least one PGM in the first washcoat coating is liable to volatilise when the first washcoat coating is exposed to relatively extreme conditions including relatively high temperatures, wherein the second washcoat coating comprises at least one metal oxide for trapping volatilised PGM and wherein the second washcoat coating is oriented to contact exhaust gas that has contacted the first washcoat coating, and wherein:

• the second washcoat coating comprises platinum and palladium, and wherein the weight ratio of Pt:Pd in the second washcoat coating is lower than the weight ratio of Pt:Pd in the first washcoat coating, and wherein the at least one metal oxide of the second washcoat coating comprises a 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.