Oxidation catalyst for internal combustion engine exhaust gas treatment

1. A catalysed substrate monolith comprising an oxidising catalyst on a 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 second washcoat coating is disposed in a layer above the first washcoat coating, wherein the first washcoat coating comprises a catalyst composition comprising platinum and at least one support material for the platinum, wherein the second washcoat coating comprises a catalyst composition comprising both platinum and palladium and at least one support material for the platinum and the palladium and wherein a weight ratio of platinum to palladium in the second washcoat coating is ≤2, wherein there is no hydrocarbon adsorbent in the one or more of the first washcoat coating and the second washcoat coating.

2. The catalysed substrate monolith according to claim 1 , wherein the substrate monolith is a flow-through substrate monolith.

3. The catalysed substrate monolith according to claim 1 , wherein the second washcoat coating comprises both platinum and palladium and wherein the first washcoat coating comprises both platinum and palladium at a higher Pt:Pd weight ratio than in the second washcoat coating.

4. The catalysed substrate monolith according to claim 1 , wherein a Pt:Pd weight ratio of both the first washcoat coating and the second washcoat coating combined is ≥1:1.

5. The catalysed substrate monolith according to claim 4 , wherein a Pt:Pd weight ratio of both the first washcoat coating and the second washcoat coating combined is ≥2:1.

6. The catalysed substrate monolith according to claim 1 , wherein a Pt:Pd weight ratio of both the first washcoat coating and the second washcoat coating combined is ≤10:1.

7. An exhaust system for a lean-burn internal combustion engine, which system comprises a first catalysed substrate monolith according to claim 1 .

8. The exhaust system according to claim 7 , comprising a second catalysed substrate monolith comprising a selective catalytic reduction (SCR) catalyst, which second catalysed substrate monolith being disposed downstream from the first catalysed substrate monolith.

9. The exhaust system according to claim 8 comprising a third catalysed substrate monolith, wherein the substrate monolith of the first catalysed substrate monolith is a flow-through substrate monolith, wherein the third catalysed substrate monolith is a filtering substrate monolith having inlet surfaces and outlet surfaces and wherein the inlet surfaces are separated from the outlet surfaces by a porous structure, which third catalysed substrate monolith comprises an oxidation catalyst and is disposed between the first catalysed substrate monolith and the injector for injecting nitrogenous reductant into exhaust gas between the first catalysed substrate monolith and the second catalysed substrate monolith.

10. The exhaust system according to claim 9 , wherein the filtering substrate monolith is a wall flow filter.

11. The exhaust system according to claim 8 , wherein the second catalysed substrate monolith in the exhaust system of the invention is a filtering substrate monolith having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure.

12. The exhaust system according to claim 11 , wherein the wherein the filtering substrate monolith is a wall flow filter.

13. The exhaust system according to claim 8 , comprising a third substrate monolith, wherein the third substrate monolith is a filtering substrate monolith having inlet surfaces and outlet surfaces, wherein the inlet surfaces are separated from the outlet surfaces by a porous structure, which third substrate monolith is disposed downstream from the second catalysed substrate monolith.

14. The exhaust system according to claim 13 , wherein the third substrate monolith comprises an oxidation catalyst.

15. The exhaust system according to claim 13 , wherein the filtering substrate monolith is a wall flow filter.

16. A lean-burn internal combustion engine, particularly for a vehicle, comprising an exhaust system according to claim 1 .

17. The catalysed substrate monolith according to claim 1 , wherein the first washcoat coating extends for substantially an entire length of channels in the substrate monolith.

18. The catalysed substrate monolith according to claim 17 , wherein the second washcoat coating substantially covers the first washcoat coating.

19. The catalysed substrate monolith according to claim 17 , wherein the second washcoat coating is arranged in a zone of substantially uniform length at a downstream end of the substrate monolith, which zone being defined at a downstream end by the outlet end of the substrate monolith itself and at an upstream end by a point less than the entire length of the first washcoat coating.

20. A method of reducing or preventing a selective catalytic reduction (SCR) catalyst in an exhaust system of a lean-burn internal combustion engine from becoming poisoned with platinum which may volatilise from a first washcoat coating comprising a catalyst composition comprising platinum and at least one support material for the platinum disposed on a substrate monolith upstream of the SCR catalyst when the catalyst composition comprising platinum is exposed to relatively extreme conditions including relatively high temperatures, which method comprising trapping volatilised platinum in a second washcoat coating disposed in a layer above the first washcoat coating, which second washcoat coating comprising a catalyst composition comprising both platinum and palladium and at least one support material for the platinum and the palladium and wherein a weight ratio of platinum to palladium in the second washcoat coating is ≤2, wherein there is no hydrocarbon adsorbent in the one or more of the first washcoat coating and the second washcoat coating.