Composite, zoned oxidation catalys for a compression ignition internal combustion engine

A method of making a composite oxidation catalyst for use in an exhaust system for treating an exhaust gas produced by a vehicular compression ignition internal combustion engine, wherein the composite oxidation catalyst comprises:

• a honeycomb flow-through substrate monolith having a total length L and a longitudinal axis and having a substrate surface extending axially between a first substrate end and a second substrate end;
• two catalyst washcoat zones arranged axially in series on and along the substrate surface, wherein a first catalyst washcoat zone having a length L1, wherein L1<L, is defined at one end by the first, substrate end and at a second end by a first end of a second catalyst washcoat zone having a length L2, wherein L2<L, wherein the second catalyst washcoat zone is defined at a second end thereof by the second substrate end, wherein the first catalyst washcoat zone comprises a first refractory metal oxide support material and two or more platinum group metal component supported thereon comprising both platinum and palladium at a weight ratio of platinum to palladium of ≥1;
• and the second catalyst washcoat zone comprises a second refractory metal oxide support material and one or more platinum group metal component supported thereon, and
• a washcoat overlayer extending axially from the first substrate end for protecting at least part of an underlying first catalyst washcoat zone from phosphorus and/or zinc poisoning when in use, which washcoat overlayer comprising a particulate metal oxide loading of >0.8 g/in3, wherein the particulate metal oxide has a mean pore diameter of ≥10 nm and/or the washcoat overlayer has a mean interparticle pore diameter of ≥10 nm,
• wherein a total platinum group metal loading in the first catalyst washcoat zone defined in grams of platinum group metal per cubic foot of substrate volume (g/ft3) is greater than a total platinum group metal loading in the second catalyst washcoat zone and wherein the first catalyst washcoat zone comprises a barium or strontium as a first alkaline-earth metal component supported on the first refractory metal oxide support material, which method comprising the steps of:
• (a) applying a catalyst washcoat layer to the substrate surface for a length extending from one end of the substrate to less than the total length of the substrate, which catalyst washcoat layer comprising a refractory metal oxide support material and one or more platinum group metal components;
• and
• (b) impregnating the catalyst washcoat layer in a zone of length Li defined at one end by the first substrate end with a solution containing one or more platinum group metal,
• which method further comprising a step (a′) before step (a) or after step (a) but in either case before step (b) of applying a second catalyst washcoat layer to the substrate for a length extending from the second substrate end to less than the total length of the substrate, such that the first catalyst washcoat layer in part overlaps the second catalyst washcoat layer or the second catalyst washcoat layer in part overlaps the first catalyst washcoat layer, wherein the second catalyst washcoat layer comprises a refractory metal oxide support material and one or more platinum group metal components, wherein the barium or strontium alkaline-earth metal component is present in the catalyst washcoat layer of step (a) and/or an impregnation solution used in step (b).