Zone coated catalyst to simultaneously reduce NOx and unreacted ammonia
Provided is an emissions treatment system and method for reducing NOx emissions in the exhaust stream produced from an internal combustion engine. The system has an injector for periodically metering ammonia or an ammonia precursor into an exhaust stream; and a first substrate with a first SCR catalyst composition, downstream of the injector. The first substrate has an inlet end, an outlet end, a length extending between the inlet end to the outlet end, and wall elements. The first SCR catalyst composition is disposed on the wall elements from the inlet end toward the outlet end to form an inlet zone. The first substrate also has an NH3 destruction catalyst composition with a platinum group metal component dispersed on a refractory metal oxide. The NH3 destruction catalyst is disposed on the wall elements from the outlet end to form an outlet zone.
1. A method for reducing NOx emissions in the exhaust stream produced from an internal combustion engine, the method comprising:
(a) metering at periodic intervals ammonia or an ammonia precursor into the exhaust stream;
(b) passing the exhaust stream through a first substrate comprising a SCR catalyst composition;
wherein the first substrate has an inlet end, an outlet end, a length extending between the inlet end to the outlet end, wall elements and a plurality of passages defined by the wall elements;
wherein said SCR catalyst composition is disposed on the wall elements from the inlet end toward the outlet end to a length that is less than the substrate's axial length to form an inlet zone;
wherein the NH 3 destruction catalyst composition comprises a platinum group metal component dispersed on a refractory metal oxide, wherein the NH 3 destruction catalyst composition is disposed on the wall elements from the outlet end toward the inlet end to a length that is less than the substrate's axial length to form an outlet zone; and
wherein there is from 0.1 to 10 g/ft 3 of platinum group metal component in the outlet zone.
2. The method of claim 1 wherein the platinum group metal component is a platinum component.
3. The method of claim 1 wherein the amount of ammonia or ammonia precursor metered into the exhaust stream provides a normalized stoichiometric ratio of between 0.2 to 2.0.
4. The method of claim 1 wherein the exhaust stream in the first substrate has a space velocity of from 30,000 to 90,000 hr −1 at rated power.
5. The method of claim 1 further comprising (a1) passing the exhaust stream through a second substrate after (a) and prior to (b).
6. The method of claim 5 wherein the second substrate of (a1) is selected from the group consisting of a honeycomb flow-through substrate, an open-cell foam substrate and a honeycomb wall flow substrate.
7. The method of claim 5 wherein the second substrate of (a1) is a honeycomb flow-through substrate comprising a second SCR catalyst composition.
8. The method of claim 7 wherein said SCR catalyst composition comprises V 2 O 5 , WO 3 , and TiO 2 .
9. The method of claim 7 wherein said SCR catalyst composition comprises a zeolite.
10. The method of claim 9 wherein said zeolite comprises a copper-exchanged or iron-exchanged zeolite.
11. The method of claim 5 wherein the first and second SCR catalyst compositions are the same.
12. The method of claim 1 wherein said SCR catalyst composition comprises V 2 O 5 , WO 3 , and TiO 2 .
13. The method of claim 1 wherein said SCR catalyst composition comprises a zeolite.
14. The method of claim 13 wherein said zeolite comprises a copper-exchanged or iron-exchanged zeolite.