Oxidation catalyst for internal combustion engine exhaust gas treatment

A catalysed substrate monolith for the oxidative treatment of a hydrocarbon (HC) and carbon monoxide (CO) in an exhaust gas, the catalysed substrate monolith comprising a substrate monolith and a plurality of catalyst layers supported on the substrate monolith, wherein the plurality of catalyst layers comprise a washcoat material, an active metal and a hydrocarbon adsorbent, and wherein one catalyst layer lies on a catalyst surface layer side and one or more other catalyst layers lie on a side between the said one catalyst layer and the supporting substrate, wherein the said one catalyst layer is a first washcoat coating and comprises a catalyst composition comprising platinum and at least one support material for the platinum, and one of the said other catalyst layers is a second washcoat coating and comprises a catalyst composition comprising both platinum and palladium and at least one support material for the platinum and the palladium, wherein a weight ratio of platinum to palladium in the second washcoat coating is ≦2;

• and wherein:
• (a) the amount of hydrocarbon adsorbent in the said one catalyst layer is greater than the amount of hydrocarbon adsorbent in the said one or more other catalyst layers, and the concentration of active metal in the said one catalyst layer is the same as or less than the concentration of active metal in the said one or more other catalyst layers;
• or
• (b) the amount of hydrocarbon adsorbent in the said one catalyst layer is the same as the amount of hydrocarbon adsorbent in the said one or more other catalyst layers, and the concentration of active metal in the said one catalyst layer is less than the concentration of active metal in the said one or more other catalyst layers.