Automotive catalytic aftertreatment system

1. A catalytic aftertreatment system for a diesel engine exhaust gas, said system comprising a diesel oxidation catalyst (DOC) and an aftertreatment device located downstream of the diesel oxidation catalyst (DOC), which aftertreatment device requires periodic heat treatment, and means to generate a temperature increase within the aftertreatment device, said diesel oxidation catalyst (DOC) having a diameter of from 4 to 15 inches and a length of from 2.5 to 10 inches, said diesel oxidation catalyst (DOC) comprising:

a catalyst composition supported on a substrate unit, wherein the catalyst composition comprises platinum, palladium and one or more support materials selected from the group consisting of alumina, silica, alumina, cerium, ceria-zirconia and titania and does not include a hydrocarbon adsorbent;

an upstream zone of the catalyst composition and a remainder;

wherein the upstream zone has a concentration of platinum (Pt) that is greater than the concentration of platinum (Pt) in the remainder; and

wherein the upstream zone has a length of 0.5 to 1.5 inches (12.7-38.1 mm), and is of higher oxidation activity for hydrocarbons (HC) than the remainder; and

the weight ratio of platinum (Pt) to palladium (Pd) in the remainder is 1:3 to 5:1.

2. The catalytic aftertreatment system according to claim 1 wherein the diesel oxidation catalyst comprises a mixture of platinum (Pt) and palladium (Pd), or an alloy of platinum (Pt) and palladium (Pd).

3. The catalytic aftertreatment system according to claim 1 , wherein the platinum group metal (PGM) concentration of the upstream zone is from 10 to 150 g ft −3 .

4. The catalytic aftertreatment system according to claim 1 , wherein the aftertreatment device is a soot filter.

5. The catalytic aftertreatment system according to claim 1 , wherein the aftertreatment device is a catalyst unit for selective catalytic reduction (SCR).

6. The catalytic aftertreatment system according to claim 1 , further comprising a heavy duty diesel engine.

7. The catalytic aftertreatment system according to claim 1 , wherein the concentration of palladium (Pd) in the upstream zone is less than the concentration of palladium (Pd) in the remainder.

8. The catalytic aftertreatment system according to claim 1 , wherein the remainder of the diesel oxidation catalyst (DOC) comprises a plurality of zones.

9. The catalytic aftertreatment system according to claim 1 , wherein the remainder of the diesel oxidation catalyst (DOC) comprises a plurality of layers.

10. A vehicle comprising a diesel engine and a catalytic aftertreatment system according to claim 1 .