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A method of selectively catalysing the reduction of NOx in an exhaust gas flowing in an exhaust system of an internal combustion engine comprising a filter (40) for particulate matter comprising a catalyst capable of selectively catalysing the reduction of NOx with a reducing agent, which method com

A method of selectively catalysing the reduction of NOx in an exhaust gas flowing in an exhaust system of an internal combustion engine comprising a filter (40) for particulate matter comprising a catalyst capable of selectively catalysing the reduction of NOx with a reducing agent, which method comprising introducing a reducing agent, or a precursor thereof, into the exhaust gas and contacting the resulting gas with the filter (40).

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The invention claimed is: 1. A method of selectively catalysing the reduction of NOx in an exhaust gas flowing in an exhaust system of a vehicle comprising a lean-burn internal combustion engine, which exhaust system comprising a first SCR catalyst for selectively catalysing the reduction of NOx wi

The invention claimed is: 1. A method of selectively catalysing the reduction of NOx in an exhaust gas flowing in an exhaust system of a vehicle comprising a lean-burn internal combustion engine, which exhaust system comprising a first SCR catalyst for selectively catalysing the reduction of NOx with a nitrogenous reducing agent located at up to 1 metre downstream from an exhaust manifold or turbo of the engine, a filter for particulate matter located in an underfloor position, which filter comprising a second selective catalytic reduction (SCR) catalyst, which method comprising contacting the first SCR catalyst with the nitrogenous reducing agent in the presence of the exhaust gas. 2. A method according to claim 1, wherein the first SCR catalyst is located at up to 0.5 meters downstream from the exhaust manifold or turbo. 3. A method according to claim 1, wherein the nitrogenous reducing agent contacts the first SCR catalyst when the first SCR catalyst is below a first temperature which is in the range 175-600째 C. 4. A method according to claim 3, wherein the first temperature is in the range 250-450째 C. 5. A method according to claim 3, wherein the first temperature is in the range 300-400째 C. 6. A method according to claim 3 further comprising the steps of introducing the nitrogenous reducing agent into the exhaust gas between the first and second SCR catalysts and contacting the second SCR catalyst with the nitrogenous reducing agent, wherein the nitrogenous reducing agent contacts the second catalyst when the second SCR catalyst is above a second temperature in the range 175-600째 C. 7. The method of claim 6, wherein the first temperature is the same as the second temperature. 8. A method according to claim 3 further comprising the steps of generating NOx from nitrogenous reducing agent on the first SCR catalyst and combusting particulate matter trapped with the filter in NO2 in the NOx. 9. A method according to claim 8, comprising the step of oxidising NO in the NOx to NO2 and combusting PM trapped on the filter in the NO2. 10. A method according to claim 1 further comprising the step of discontinuing the contact of the nitrogenous reducing agent with the first SCR catalyst when the first SCR catalyst is below a third temperature which is in the range 75-200째 C. 11. A method according to claim 10, wherein the third temperature is in the range 100-175째 C. 12. A method according to claim 10, wherein the third temperature is in the range 125-160째 C. 13. A method according to claim 1 further comprising the steps of introducing the nitrogenous reducing agent into the exhaust gas between the first and the second SCR catalysts and contacting the second SCR catalyst with the nitrogenous reducing agent. 14. A method according to claim 13, wherein the nitrogenous reducing agent contacts the second SCR catalyst when the second SCR catalyst is above a second temperature which is in the range 175-600째 C. 15. A method according to claim 14, wherein the second temperature is in the range 250-450째 C. 16. A method according to claim 14, wherein the second temperature is in the range 300-400째 C. 17. A method according to claim 13 further comprising the step of discontinuing the contact of the nitrogenous reducing agent with the second SCR catalyst when the second SCR catalyst is above a fourth temperature which is at least 250째 C. 18. A method according to claim 17, wherein the fourth temperature is in the range 250-600째 C. 19. A method according to claim 17, wherein the fourth temperature is in the range 450-600째 C. 20. A method according to claim 17, wherein the fourth temperature is at least 600째 C. 21. A method according to claim 1 further comprising the step of oxidising NO in the exhaust gas to NO2 and combusting PM trapped on the filter with the NO2. 22. A method according to claim 1, comprising the step of contacting gas leaving the filter with a third SCR catalyst. 23. A method according to claim 1, wherein the nitrogenous reducing agent is a nitrogenous compound. 24. A method according to claim 23, wherein the nitrogenous compound is a nitrogen hydride. 25. A method according to claim 24, wherein the nitrogen hydride is ammonia or hydrazine. 26. A method according to claim 23 further comprising the step of reacting a hydrocarbon and lean exhaust gas over a reforming catalyst to form the nitrogenous compound. 27. A method according to claim 26, wherein the nitrogenous compound is selected from the group of ammonia, a nitrile, an amine, an oxime and an isocyanate. 28. A method according to claim 26, wherein the reforming catalyst comprises a platinum group metal (PGM) or Ni. 29. A method according to claim 28, wherein the PGM is Rh, Pt or Rh-Pt. 30. A method according to claim 26, wherein the reforming catalyst comprises a support selected from the group consisting of alumina, titania, ceria, zirconia, silica, silica-alumina, mixtures, and mixed oxides containing any two or more thereof. 31. A vehicle comprising a lean-burn internal combustion engine and an exhaust system therefor, which exhaust system comprising a first SCR catalyst for selectively catalysing the reduction of NOx with a nitrogenous reducing agent located at up to 1 metre from an exhaust manifold or turbo of the engine, a first means for introducing a nitrogenous reducing agent, or a precursor thereof, into an exhaust gas carried by the exhaust system upstream of the first selective catalytic reduction (SCR) catalyst and a filter for particulate matter located in an underfloor position of the vehicle, which filter comprising a second SCR catalyst. 32. A vehicle according to claim 31, wherein the first SCR catalyst is located at up to 0.5 meters downstream from the exhaust manifold or turbo. 33. A vehicle according to claim 31, comprising a second nitrogenous reducing agent introducing means for introducing a nitrogenous reducing agent into the exhaust gas between the first SCR catalyst and the second SCR catalyst. 34. A vehicle according to claim 33 further comprising means for controlling the introduction of the nitrogenous reducing agent, or a precursor thereof, into the exhaust gas through the second introducing means during a drive cycle, wherein the control means causes the introduction of the nitrogenous reducing agent into the exhaust gas through the second introducing means when the second catalyst is above a second temperature which is in the range 175-600째 C. 35. A vehicle according to claim 34, wherein the second temperature is in the range 250-450째 C. 36. A vehicle according to claim 34, wherein the second temperature is in the range 300-400째 C. 37. A vehicle according to claim 34 further comprising means for controlling the introduction of the nitrogenous reducing agent, or a precursor thereof, into the exhaust gas through the first introducing means during a drive cycle and for causing the introduction of the nitrogenous reducing agent, or a precursor thereof, into the exhaust gas through the first introducing means when the first catalyst is below a first temperature which is in the range 175-600째 C., wherein the first temperature is the same as the second temperature. 38. A vehicle according to claim 34, wherein the control means interrupts the introduction of the nitrogenous reducing agent into the exhaust gas through the second introducing means when the second catalyst is above a fourth temperature which is at least 250째 C. 39. A vehicle according to claim 38, wherein the fourth temperature is in the range 450-600째 C. 40. A vehicle according to claim 38, wherein the fourth temperature is at least 600째 C. 41. A vehicle according to claim 33 further comprising a catalyst for oxidising NO to NO2 disposed between the second means for introducing the nitrogenous reducing agent and the filter. 42. A vehicle according to claim 41, wherein the oxidation catalyst comprises a platinum group metal. 43. A vehicle according to claim 42, wherein the platinum group metal is platinum, not Pt/MgO. 44. A vehicle according to claim 31, comprising means for controlling the introduction of the nitrogenous reducing agent, or a precursor thereof, into the exhaust gas through the first introducing means during a drive cycle. 45. A vehicle according to claim 44, wherein the control means causes the introduction of the nitrogenous reducing agent, or a precursor thereof, into the exhaust gas through the first introducing means when the first catalyst is below a first temperature which is in the range 175-600째 C. 46. A vehicle according to claim 45, wherein the first temperature is in the range 250-450째 C. 47. A vehicle according to claim 45, wherein the first temperature is in the range 300-400째 C. 48. A vehicle according to claim 44, wherein the control means interrupts the introduction of the nitrogenous reducing agent through the first introducing means when the first catalyst is below a third temperature which is in the range 75-200째 C. 49. A vehicle according to claim 48, wherein the third temperature is in the range 100-175째 C. 50. A vehicle according to claim 48, wherein the third temperature is in the range 125-160째 C. 51. A vehicle according to claim 48, wherein the fourth temperature is in the range 250-600째 C. 52. A vehicle according to claim 31, comprising means for controlling the introduction of the nitrogenous reducing agent, or a precursor thereof, into the exhaust gas through the second introducing means during a drive cycle. 53. A vehicle according to claim 31, wherein the filter comprises an oxidation catalyst. 54. A vehicle according to claim 53, wherein the oxidation catalyst is selected from the group consisting of a platinum group metal, a cerium component and a zirconium component. 55. A vehicle according to claim 54, wherein the platinum group metal is platinum, not Pt/MgO. 56. A vehicle according to claim 31 further comprising a catalyst for oxidising NO to NO2 disposed between the first catalyst and the filter. 57. A vehicle according to claim 56, wherein the oxidation catalyst comprises a platinum group metal. 58. A vehicle according to claim 57, wherein the platinum group metal is platinum, not Pt/MgO. 59. A vehicle according to claim 31 further comprising a further catalyst for selectively catalysing the reduction of NOx with a reducing agent disposed downstream of the filter. 60. A vehicle according to claim 31 further comprising a reservoir of nitrogenous reducing agent or a precursor thereof. 61. A vehicle according to claim 60, wherein the nitrogenous reducing agent is a nitrogenous compound. 62. A vehicle according to claim 61, wherein the nitrogenous compound is a nitrogen hydride. 63. A vehicle according to claim 62, wherein the nitrogen hydride is ammonia or hydrazine. 64. A vehicle according to claim 31, wherein the nitrogenous reducing agent precursor is a precursor of a reducing agent for catalysing the reduction of NOx. 65. A vehicle according to claim 64, wherein the nitrogenous reducing agent is ammonia and the precursor is urea or ammonium carbamate. 66. A vehicle according to claim 65, wherein the urea in an aqueous solution. 67. A vehicle according to claim 66, wherein the aqueous urea solution is an eutectic mixture. 68. A vehicle according to claim 66, wherein the aqueous urea solution further comprises additives to reduce the aqueous urea solution crystallisation temperature. 69. A vehicle according to claim 65, wherein the urea is a solid. 70. A vehicle according to claim 31, wherein the first SCR catalyst comprises a platinum group metal. 71. A vehicle according to claim 70, wherein the platinum group metal is platinum, not Pt/MgO. 72. A vehicle according to claim 31, wherein the second SCR catalyst comprises a platinum group metal. 73. A vehicle according to claim 72, wherein the second SCR catalyst comprises platinum, not Pt/MgO. 74. A vehicle according to claim 31, wherein the first and second SCR catalysts comprise a base metal selected from the group consisting of iron, cerium, copper and vanadium. 75. A vehicle according to claim 74, wherein the first and second SCR catalysts comprise at least one of Mo and W. 76. A vehicle according to claim 74, wherein the base metal is supported. 77. A vehicle according to claim 76, wherein the base metal support is selected from the group consisting of alumina, titania, silica, silica-alumina, ceria, zirconia, zeolite, mixture, and mixed oxide of any two or more thereof. 78. A vehicle according to any of claim 76, wherein the base metal support is a zeolite. 79. A vehicle according to claim 78, wherein the zeolite is selected from the group consisting of ZSM-5, mordenite, gamma-zeolite and beta-zeolite. 80. A vehicle according to claim 78, wherein the zeolite comprises at least one metal or is metallised with at least one metal. 81. A vehicle according to claim 80, wherein the at least one metal is selected from the group consisting of Cu, Ce, Fe, Pt and any two or more thereof. 82. A vehicle according to claim 78, wherein the zeolite is ion-exchanged or impregnated. 83. A vehicle according to claim 31, wherein the lean-burn internal combustion engine is a diesel engine. 84. A vehicle according to claim 83, wherein the diesel engine is a heavy-duty diesel engine. 85. A vehicle according to claim 31, wherein the lean-burn internal combustion engine is a gasoline engine. 86. A vehicle according to claim 85 wherein the gasoline engine is a direct injection (GDI) engine.