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An exhaust system for a compression ignition engine comprising an oxidation catalyst for treating carbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from the compression ignition engine, wherein the oxidation catalyst comprises: a platinum group metal (PGM) component selected from the group

An exhaust system for a compression ignition engine comprising an oxidation catalyst for treating carbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from the compression ignition engine, wherein the oxidation catalyst comprises: a platinum group metal (PGM) component selected from the group consisting of a platinum (Pt) component, a palladium (Pd) component and a combination thereof; an alkaline earth metal component; a support material comprising a modified alumina incorporating a heteroatom component; and a substrate, wherein the platinum group metal (PGM) component, the alkaline earth metal component and the support material are disposed on the substrate.

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1. An exhaust system for a compression ignition engine comprising an oxidation catalyst for treating carbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from the compression ignition engine, wherein the oxidation catalyst comprises one or more layers or zones comprising: a platinum group meta

1. An exhaust system for a compression ignition engine comprising an oxidation catalyst for treating carbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas from the compression ignition engine, wherein the oxidation catalyst comprises one or more layers or zones comprising: a platinum group metal (PGM) component selected from the group consisting of (i) a platinum (Pt) component and (ii) a combination of a platinum (Pt) component and a palladium component, wherein the ratio by mass of the combination of the platinum (Pt) component to the palladium (Pd) component is >2:3;an alkaline earth metal component;a support material comprising a modified alumina incorporating a heteroatom component; anda substrate, which is a flow-through monolith; andwherein the platinum group metal (PGM) component, the alkaline earth metal component and the support material are disposed on the substrate, wherein the alkaline earth metal component is disposed or supported on the support material and wherein the total amount of the alkaline earth metal component is 10 to 500 gft−3. 2. An exhaust system according to claim 1, wherein the modified alumina incorporating a heteroatom component is an alumina doped with a heteroatom component, an alkaline earth metal aluminate or a mixture thereof. 3. An exhaust system according to claim 2, wherein the heteroatom component comprises silicon, magnesium, barium, lanthanum, cerium, titanium, zirconium or a combination of two or more thereof. 4. An exhaust system according to claim 1, wherein the modified alumina incorporating a heteroatom component is alumina doped with silica, and optionally the alumina is doped with silica in an amount of 0.5 to 45% by weight. 5. An exhaust system according to claim 1, wherein the modified alumina incorporating a heteroatom component is an alkaline earth metal aluminate, and optionally wherein the alkaline earth metal aluminate is magnesium aluminate. 6. An exhaust system according to claim 1, wherein the alkaline earth metal component comprises magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) or a combination of two or more thereof. 7. An exhaust system according to claim 6, wherein the alkaline earth metal component comprises strontium (Sr) or barium (Ba), preferably barium (Ba). 8. An exhaust system according to claim 1, wherein the oxidation catalyst comprises a single layer disposed on the substrate, wherein the single layer comprises the platinum group metal (PGM) component; the alkaline earth metal component; and the support material. 9. An exhaust system according to claim 1, wherein the oxidation catalyst comprises two layers, wherein a first layer is disposed on the substrate and a second layer is disposed on the first layer. 10. An exhaust system according to claim 9, wherein the first layer comprises a platinum group metal (PGM) component consisting of a combination of a platinum (Pt) component and a palladium (Pd) component, and the second layer comprises a platinum group metal (PGM) component consisting of a combination of a palladium (Pd) component and a platinum (Pt) component, and wherein the ratio by mass of the platinum (Pt) component to the palladium (Pd) component in the first layer is different to the ratio by mass of the platinum (Pt) component to the palladium (Pd) component in the second layer. 11. An exhaust system according to claim 9, wherein the first layer comprises a platinum group metal (PGM) component selected from the group consisting of a Pd component and a combination of a palladium (Pd) component and a platinum (Pt) component, and wherein the second layer comprises a platinum group metal (PGM) component consisting of a Pt component. 12. An exhaust system according to claim 9, wherein the first layer comprises the platinum group metal (PGM) component, and the second layer comprises a platinum group (PGM) component consisting of a palladium (Pd) component. 13. An exhaust system according to claim 9, wherein the first layer comprises a platinum group (PGM) component selected from the group consisting of a platinum (Pt) component and a palladium (Pd) component, and the second layer comprises a platinum group metal (PGM) component consisting of a combination of a palladium (Pd) component and a platinum (Pt) component. 14. A vehicle comprising a compression ignition engine and the exhaust system according to claim 1. 15. A method of treating carbon monoxide (CO) and hydrocarbons (HCs) in an exhaust gas from a compression ignition engine, which method comprises contacting the exhaust gas with an oxidation catalyst, wherein the oxidation catalyst comprises one or more layers or zones comprising: a platinum group metal (PGM) component selected from the group consisting of (i) a platinum (Pt) component and (ii) a combination of a platinum (Pt) component and a palladium component, wherein the ratio by mass of the combination of the platinum (Pt) component to the palladium (Pd) component is >2:3;an alkaline earth metal component;a support material comprising a modified alumina incorporating a heteroatom component; anda substrate, which is a flow-through monolith; andwherein the platinum group metal (PGM) component, the alkaline earth metal component and the support material are disposed on the substrate, wherein the alkaline earth metal component is disposed or supported on the support material and wherein the total amount of the alkaline earth metal component is 10 to 500 gft−3. 16. A process of preparing an oxidation catalyst as defined in claim 1, which process comprises disposing a platinum group metal (PGM) component, an alkaline earth metal component, and a support material onto a substrate, which is a flow-through monolith, wherein the platinum group metal (PGM) component is selected from the group consisting of (i) a platinum (Pt) component and (ii) a combination of a platinum (Pt) component and a palladium component, wherein the ratio by mass of the combination of the platinum (Pt) component to the palladium (Pd) component is >2:3, and the support material comprises a modified alumina incorporating a heteroatom component.