Molecular sieve catalyst compositions, catalytic composites, systems, and methods
Described is a selective catalytic reduction catalyst comprising a zeolitic framework material of silicon and aluminum atoms, wherein a fraction of the silicon atoms are isomorphously substituted with a tetravalent metal. The catalyst can include a promoter metal such that the catalyst effectively promotes the reaction of ammonia with nitrogen oxides to form nitrogen and H 2 O selectively over a temperature range of 150 to 650° C. In another aspect, described is a selective catalytic reduction composite comprising an SCR catalyst material and an ammonia storage material comprising a transition metal having an oxidation state of IV. The SCR catalyst material promotes the reaction of ammonia with nitrogen oxides to form nitrogen and H 2 O selectively over a temperature range of 150° C. to 600° C., and the SCR catalyst material is effective to store ammonia at temperatures of 400° C. and above. A method for selectively reducing nitrogen oxides, and a method for simultaneously selectively reducing nitrogen oxide and storing ammonia are also described. Additionally, an exhaust gas treatment system is also described.
1. An SCR catalyst comprising a zeolite with a framework consisting of tetravalent metal, silicon, oxygen, and aluminum atoms, wherein the tetravalent metal is present in place of a fraction of the silicon atoms and the zeolite is promoted only with Cu, Fe, or a combination thereof, wherein the SCR catalyst is disposed on a honeycomb substrate, and wherein the catalyst is effective to promote the formation of NO + .
2. The catalyst of claim 1 , wherein the tetravalent metal comprises a tetravalent transition metal.
3. The catalyst of claim 2 , wherein the tetravalent transition metal is selected from the group consisting of Ti, Zr, Hf, Ge, and combinations thereof.
4. The catalyst of claim 3 , wherein the tetravalent transition metal comprises Ti.
5. The catalyst of claim 1 , having a silica to alumina ratio in a range of 1 to 300.
6. The catalyst of claim 5 , wherein the silica to alumina ratio is in the range of 1 to 50.
7. The catalyst of claim 1 , having a tetravalent metal to alumina ratio in a range of 0.0001 to 1000.
8. The catalyst of claim 7 , wherein the tetravalent metal to alumina ratio is in the range of 0.01 to 10.
9. The catalyst of claim 8 , wherein the tetravalent metal to alumina ratio is in the range of 0.01 to 2.
10. The catalyst of claim 1 , having a silica to tetravalent metal ratio in a range of 1 to 100.
11. The catalyst of claim 10 , wherein the silica to tetravalent metal ratio is in the range of 5 to 20.
12. The catalyst of claim 1 , wherein the framework comprises ring sizes no larger than 12.
13. The catalyst of claim 12 , wherein the framework comprises a d6r unit.
14. The catalyst of claim 13 , wherein the framework is selected from AEI, AFT, AFX, CHA, EAB, EMT, ERI, FAU, GME, JSR, KFI, LEV, LTL, LTN, MOZ, MSO, MWW, OFF, SAS, SAT, SAV, SBS, SBT, SFW, SSF, SZR, TSC, WEN, and combinations thereof.
15. The catalyst of claim 14 , wherein the framework is selected from AEI, CHA, AFX, ERI, KFI, LEV, and combinations thereof.
16. The catalyst of claim 1 , wherein the framework is CHA.
17. The catalyst of claim 1 , wherein the framework consists of MO 4 /SiO 4 /AlO 4 tetrahedra linked by oxygen atoms, wherein M is the tetravalent metal.
18. The catalyst of claim 1 , wherein the metal selected from Cu, Fe, and combinations thereof is ion-exchanged into the zeolite.
19. The catalyst of claim 1 , wherein the honeycomb substrate is a wall flow substrate.
20. A method for selectively reducing nitrogen oxides (NO x ), the method comprising contacting an exhaust gas stream containing NO x with the catalyst of claim 1 .
21. An exhaust gas treatment system comprising an exhaust gas stream containing ammonia and a catalyst in accordance with claim 1 effective for destroying at least a portion of the ammonia in the exhaust gas stream.