Exhaust gas purification system for the treatment of engine exhaust gases by means of SCR catalyst

1. A method of treatment of diesel engine exhaust gases in an exhaust system containing nitrogen oxides and hydrocarbons comprising:

(a) addition of ammonia or of a compound decomposable to ammonia from a source that does not form part of the exhaust system into the exhaust gas stream containing nitrogen oxides and hydrocarbons;

(b) passing the exhaust gas stream containing nitrogen oxides, hydrocarbons and ammonia or a compound decomposable to ammonia over a first, upstream SCR catalyst, which catalyzes the comproportionation of nitrogen oxides with ammonia in a temperature range between 300° C. and 500° C., while at least partially oxidizing the hydrocarbons contained in the exhaust gas; and

(c) passing the exhaust gas resulting from (b) over a second, downstream SCR catalyst, which catalyzes the comproportionation of nitrogen oxides with ammonia in a temperature range between 150° C. and 400° C. and at the same time stores excess ammonia.

2. The method as claimed in claim 1 , wherein

the exhaust gas stream containing nitrogen oxides and hydrocarbons is led, before the addition of ammonia or of a compound decomposable to ammonia in (a), over an oxidation catalyst that oxidizes at least a portion of the NO contained in the nitrogen oxides to NO 2 .

3. The method as claimed in claim 2 , wherein

the NO 2 /NO x ratio in the exhaust gas is altered so that the exhaust gas stream, before being led over the first, upstream SCR catalyst in (b), has an NO 2 /NO x ratio from 0.3 to 0.7.

4. The method as claimed in claim 1 , wherein

the amount of ammonia or of a compound decomposable to ammonia to be added to the exhaust gas stream in (a) is selected so that the exhaust gas stream, before being led over the first, upstream SCR catalyst in (b), has an NH 3 /NO x ratio from 0.8 to 1.2.

5. The method as claimed in claim 4 , wherein

the exhaust gas stream, after being passed through the second, downstream SCR catalyst in (c), is led over an oxidation catalyst, which selectively catalyzes the oxidation of ammonia to nitrogen.

6. The method as claimed in claim 5 , wherein

the oxidation catalyst is arranged on a soot particle-filtering support.

7. The method as claimed in claim 1 , wherein

the first, upstream SCR catalyst, which the exhaust gas stream is passed over in (b), does not contain zeolite compounds.

8. The method as claimed in claim 7 , wherein

the first, upstream SCR catalyst, which the exhaust gas stream is passed over in (b), contains neither vanadium compounds nor zeolite compounds.

9. The method as claimed in claim 1 , wherein

the first, upstream SCR catalyst, which the exhaust gas stream is passed over in (b), contains tungsten oxide WO 3 and a homogeneous cerium-zirconium mixed oxide (Ce,Zr)O 2 .

10. An exhaust gas purification system for lowering the content of nitrogen oxides in the lean exhaust gas of an internal combustion engine containing, in combination and in this order:

(1) a feeding device that feeds ammonia or a compound decomposable to ammonia into the exhaust gas stream containing nitrogen oxides from a source that does not form part of the exhaust system;

(2) a first, upstream SCR catalyst, which catalyzes the comproportionation of nitrogen oxides with ammonia in a temperature range between 300° C. and 500° C. and contains neither vanadium compounds nor zeolite compounds; and

(3) a second, downstream and vanadium-free SCR catalyst, which catalyzes the comproportionation of nitrogen oxides with ammonia in a temperature range between 150° C. and 400° C. and contains a copper- (Cu-)exchanged zeolite compound.

11. The exhaust gas purification system as claimed in claim 10 , wherein

each of the two SCR catalysts is in form of a coating on an inert support.

12. The exhaust gas purification system as claimed in claim 10 , wherein

the first and the second SCR catalyst are arranged as coatings on the same support.

13. The exhaust gas purification system as claimed in claim 12 , wherein

the first, upstream SCR catalyst is in the form of a coating zone arranged on the inlet side on the support and the second, downstream SCR catalyst forms the coating zone on the outlet side on the support.

14. The exhaust gas purification system as claimed in claim 13 , wherein

the ratio of the lengths of coating zone arranged on the inlet side to coating zone on the outlet side is between 0.1 and 3.

15. The exhaust gas purification system as claimed in claim 10 , wherein

the first SCR catalyst contains tungsten oxide WO 3 and a homogeneous cerium-zirconium mixed oxide (Ce,Zr)O 2 .

16. The exhaust gas purification system as claimed in claim 15 , wherein

the content of tungsten oxide WO 3 in the first SCR catalyst is between 5 and 25 wt. %, relative to the total amount of the first SCR catalyst without consideration of the mass of the inert support.

17. The exhaust gas purification system as claimed in claim 15 , wherein

the content of homogeneous cerium-zirconium mixed oxide (Ce,Zr)O 2 in the first SCR catalyst is between 50 and 95 wt. %, relative to the total amount of the first SCR catalyst without consideration of the mass of the inert support.

18. The exhaust gas purification system as claimed in claim 17 , wherein

the weight ratio of cerium oxide CeO 2 and zirconium oxide ZrO 2 in the homogeneous cerium-zirconium mixed oxide (Ce,Zr)O 2 is between 0.43 and 2.33.

19. The exhaust gas purification system as claimed in claim 10 , wherein

the second SCR catalyst contains a copper- (Cu-)exchanged zeolite compound with a content of 2 to 10 wt. % Cu relative to the total weight of the zeolite compound.

20. The exhaust gas purification system as claimed in claim 18 , wherein

the zeolite compound is selected from the group consisting of beta-(β-)zeolite, Y-zeolite, ZSM-5, ZSM-20, ferrierite and mordenite.