Zoned diesel oxidation catalyst

1. An oxidation catalyst for treating an exhaust gas from a diesel engine, which oxidation catalyst comprises an upstream washcoat layer, a downstream washcoat layer and a substrate;

wherein a first washcoat region comprising a part of the upstream washcoat layer is disposed on the substrate at an inlet end of the substrate, wherein the first washcoat region comprises a first platinum group metal (PGM) and a first support material, wherein the only PGM in the first washcoat region is the first PGM, and the first PGM is a combination of platinum and palladium, wherein the first washcoat region has a length of 10 to 50% of the length of the substrate;

wherein a second washcoat region is adjacent to the first washcoat region and comprises a second platinum group metal (PGM), a second support material, a rear part of the upstream washcoat layer and a front part of the downstream washcoat layer, wherein the second PGM is a combination of platinum and palladium, the second washcoat region has a total loading of the second PGM of 5 to 300 g ft −3 , and wherein either (i) the rear part of the upstream washcoat layer is disposed on the front part of the downstream washcoat layer or (ii) the front part of the downstream washcoat layer is disposed on the rear part of the upstream washcoat layer, and wherein the second washcoat region has a length of 10 to 40% of the length of the substrate;

wherein a third washcoat region comprising a part of the downstream washcoat layer is disposed on the substrate at an outlet end of the substrate, wherein the third washcoat region comprises a third platinum group metal (PGM) and a third support material, the only PGM in the third washcoat region is the third PGM, and the third PGM is a combination of platinum and palladium, and the third washcoat region has a total loading of the third PGM of 5 to 25 g ft −3 ; and wherein the third washcoat region is adjacent to the second washcoat region, and

the total loading of the second PGM is greater than the total loading of the first PGM, and the total loading of the second PGM is greater than the total loading of the third PGM, and the total loading of first PGM is greater than the total loading of the third PGM.

2. An oxidation catalyst according to claim 1 , wherein in the first washcoat region the ratio of the total mass of platinum to the total mass of palladium is from 3.5:1 to 1:3.5.

3. An oxidation catalyst according to claim 1 , wherein the first washcoat region comprises a ratio of the total mass of platinum to the total mass of palladium of 2.5:1 to 1:2.5.

4. An oxidation catalyst according to claim 1 , wherein the first washcoat region has a total loading of the first PGM of 5 to 300 g ft −3 .

5. An oxidation catalyst according to claim 1 , wherein the first washcoat region has a total loading of the first PGM of 15 to 100 g ft −3 .

6. An oxidation catalyst according to claim 1 , wherein the first washcoat region has a total loading of the first PGM of 20 to 75 g ft −3 .

7. An oxidation catalyst according to claim 1 , wherein the first washcoat region has a length of 15 to 45% of the length of the substrate.

8. An oxidation catalyst according to claim 1 , wherein the first washcoat region has a length of 20 to 40% of the length of the substrate.

9. An oxidation catalyst according to claim 1 , wherein the second washcoat region has a total loading of the second PGM of 10 to 250 g ft −3 .

10. An oxidation catalyst according to claim 9 , wherein the second washcoat region has a total loading of the second PGM of 15 to 200 g ft −3 .

11. An oxidation catalyst according to claim 9 , wherein the second washcoat region has a total loading of the second PGM of 20 to 150 g ft −3 .

12. An oxidation catalyst according to claim 9 , wherein the second washcoat region has a total loading of the second PGM of 25 to 100 g ft −3 .

13. An oxidation catalyst according to claim 1 , wherein the second washcoat region comprises a ratio of the total mass of platinum to the total mass of palladium of 10:1 to 1:10.

14. An oxidation catalyst according to claim 13 , wherein the second washcoat region comprises a ratio of the total mass of platinum to the total mass of palladium of 5:1 to 1:5.

15. An oxidation catalyst according to claim 1 , wherein in the third washcoat region the ratio of the total mass of platinum to the total mass of palladium is from 10:1 to 1:10.

16. An oxidation catalyst according to claim 1 , wherein the first support material comprises a refractory metal oxide selected from the group consisting of alumina, silica, titania, zirconia, ceria and mixed or composite oxides of two or more thereof.

17. An oxidation catalyst according to claim 1 , wherein the second support material comprises a refractory metal oxide selected from the group consisting of alumina, silica, titania, zirconia, ceria and mixed or composite oxides of two or more thereof.

18. An oxidation catalyst according to claim 1 , wherein the third support material comprises a refractory metal oxide selected from the group consisting of alumina, silica, titania, zirconia, ceria and mixed or composite oxides of two or more thereof.

19. An oxidation catalyst according to claim 1 , wherein the first washcoat region comprises an alkaline earth metal.

20. An oxidation catalyst according to claim 1 , wherein the second washcoat region comprises an alkaline earth metal.

21. An exhaust system comprising an oxidation catalyst according to claim 1 , and an emissions control device.

22. An exhaust system according to claim 21 , wherein the emissions control device is separate to the oxidation catalyst, and wherein the oxidation catalyst is upstream of the emissions control device.

23. An exhaust system according to claim 21 , wherein the emissions control device is selected from a diesel particulate filter (DPF), a NO x adsorber catalyst (NAC), a lean NO x catalyst (LNC), a selective catalytic reduction (SCR) catalyst, a diesel oxidation catalyst (DOC), a catalyzed soot filter (CSF), a selective catalytic reduction filter catalyst, and combinations of two or more thereof.

24. An exhaust system according to claim 21 , wherein the emissions control device is either a diesel particulate filter (DPF) or a catalyzed soot filter (CSF).

25. An exhaust system according to claim 21 , wherein the emissions control device is a selective catalytic reduction (SCR) catalyst or a selective catalytic reduction filter catalyst.