Exhaust system for a lean-burn IC engine comprising a PGM component and a SCR catalyst

1. An exhaust system for a vehicular lean-burn internal combustion engine, which system comprising:

(a) a first substrate monolith comprising a SCR catalyst;

(b) at least one second substrate monolith comprising a catalytic washcoat coating comprising at least one platinum group metal (PGM), which at least one second substrate monolith is disposed upstream of the first substrate monolith; and

(c) a third substrate monolith disposed between the first substrate monolith and the or each second substrate monolith,

wherein at least one PGM on the or each second substrate monolith is liable to volatilise when the or each second substrate monolith is exposed to relatively extreme conditions including relatively high temperatures, and wherein the third substrate monolith comprises at least one material for trapping volatilised PGM, and wherein:

(i) the first 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; or

(ii) the second substrate monolith or each second 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.

2. The exhaust system according to claim 1 , wherein the at least one material for trapping volatilised PGM comprises a metal oxide selected from the group consisting of optionally stabilised alumina, optionally stabilised zirconia, an optionally stabilised cerin-zirconia mixed oxide and mixtures of any two or more thereof.

3. The exhaust system according to claim 2 , wherein the at least one material for trapping volatilised PGM is a component of a washcoat coating applied to the third substrate monolith.

4. The exhaust system according to claim 1 , wherein at least one second substrate monolith comprises platinum.

5. The exhaust system according to claim 1 , wherein the at least one material for trapping volatilised PGM comprises a catalyst composition comprising at least one metal selected from the group consisting of palladium, copper, silver, gold and combinations of any two or more thereof, supported on at least one support material.

6. The exhaust system according to claim 5 , wherein the third substrate monolith comprises a washcoat coating comprising both platinum and palladium at a lower Pt:Pd weight ratio than the or each washcoat of the or each second substrate monolith.

7. The exhaust system according to claim 6 , wherein a weight ratio of Pt:Pd in the washcoat coating of the third substrate is ≦2.

8. An exhaust system for a vehicular lean-burn internal combustion engine, which system comprising:

(a) a first substrate monolith comprising a SCR catalyst;

(b) at least one second substrate monolith comprising a catalytic washcoat coating comprising at least one platinum group metal (PGM), which at least one second substrate monolith is disposed upstream of the first substrate monolith; and

(c) a third substrate monolith comprising a washcoat coating comprising both platinum and palladium at a lower Pt:Pd weight ratio than the or each washcoat of the or each second substrate monolith, wherein the third substrate monolith is disposed between the first substrate monolith and the or each second substrate monolith,

wherein at least one PGM on the or each second substrate monolith is liable to volatilise when the or each second substrate monolith is exposed to relatively extreme conditions including relatively high temperatures, and wherein the third substrate monolith comprises at least one material for trapping volatilised PGM, which at least one material comprises a catalyst composition comprising at least one metal selected from the group consisting of palladium, copper, silver, gold and combinations of any two or more thereof, supported on at least one support material.

9. The exhaust system according to claim 8 , wherein at least one second substrate monolith comprises platinum.

10. An exhaust system for a vehicular lean-burn internal combustion engine, which system comprising:

(a) a first substrate monolith comprising a SCR catalyst;

(b) at least one second substrate monolith comprising a catalytic washcoat coating comprising at least one platinum group metal (PGM), wherein the at least one platinum group metal (PGM) comprises both platinum and palladium and a weight ratio of Pt:Pd is ≦2, and wherein at least one second substrate monolith is disposed upstream of the first substrate monolith; and

(c) a third substrate monolith disposed between the first substrate monolith and the or each second substrate monolith,

wherein at least one PGM on the or each second substrate monolith is liable to volatilise when the or each second substrate monolith is exposed to relatively extreme conditions including relatively high temperatures, and wherein the third substrate monolith comprises at least one material for trapping volatilised PGM.

11. The exhaust system according to claim 10 , wherein the at least one material for trapping volatilised PGM comprises a metal oxide selected from the group consisting of optionally stabilised alumina, optionally stabilised zirconia, an optionally stabilised cerin-zirconia mixed oxide and mixtures of any two or more thereof.

12. The exhaust system according to claim 11 , wherein the at least one material for trapping volatilised PGM is a component of a washcoat coating applied to the third substrate monolith.