# Patent

## Title
Method for the simultaneous removal of nitrogen oxides and laughing gas from a gas stream containing nitrogen oxides and laughing gas

## Abstract

A method for the simultaneous removal of nitrogen oxides and laughing gas from a stream containing nitrogen oxides and laughing gas, wherein a first catalyst that contains cobalt and, as support, an inorganic metal oxide, and a second catalyst that contains iron and, as support, an inorganic metal oxide, are used. In particular the method is carried out in such a way that (a) the gas stream containing nitrogen oxides and laughing gas is reacted with a reducing gas, the reducing gas comprising a saturated hydrocarbon, in the presence of the first catalyst and (b) the gaseous product from step (a) is reacted with a reducing gas, the reducing gas comprising a saturated hydrocarbon, in presence of a second catalyst. The catalyst system can be used for the treatment of off-gases from a nitric acid plant.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This is the 35 USC 371 national stage of international application PCT/NL02/00075 filed on Jan. 31, 2002, which designated the United States of America.
FIELD OF THE INVENTION
The invention relates to a method for the simultaneous removal of nitrogen oxides (NOx) and laughing gas (N2O) from a gas stream containing nitrogen oxides and laughing gas. In this description nitrogen oxides (NOx) are understood to be NO, NO2 and a mixture of NO and NO2.
BACKGROUND OF THE INVENTION
Li and Armor [Appl. Catal. B3, 55–60, 1993] describe the Co-ZSM-5-catalysed simultaneous decomposition of NO and N2O in the presence of methane or propane and oxygen:

2NO+CH4+O2→N2+2H2O+CO2

2N2O→2N2+O2

It has been found that although NO is still converted with a high degree of conversion at low temperature, the degree of conversion of N2O decreases sharply at lower temperature. A temperature in the range of 450° C. to 500° C. is found to be necessary for a high degree of conversion of N2O (see Li and Armor: Table 1). Furthermore, it is found that the degree of conversion of NO is higher when pure oxygen is used instead of N2O as oxygen source (see Li and Armor: Table 2). On the other hand, the reduction of N2O proceeds less favourably when both methane and oxygen are present (see Li and Armor: Table 3). The highest degrees of conversion are achieved at 500° C.; at this temperature 97% of the N2O and only 30% of the NO are converted. This method thus has the disadvantage that the temperature range in which acceptable conversion of both NO and N2O takes place is relatively narrow. In addition, this temperature is relatively high (450–500° C.). Moreover, the supply of oxygen has an advantageous effect (higher degree of conversion of NO), but also an adverse effect (lower degree of conversion of N2O).
Kögel et al. [Catal. Lett. 51, 23–25, 1998] describe the Fe-MFI-catalysed simultaneous decomposition of NO and N2O in the presence of propane and oxygen. It has been found that although NO is still converted with a high degree of conversion at low temperature, the degree of conversion of N2O decreases sharply at lower temperature (see Kögel et al.: FIG. 2): the maximum degree of conversion of NO (40%) is reached at 300° C. and at this temperature the degree of conversion of N2O is only approximately 5%. Moreover, the undesired CO is formed as a by-product. However, this disadvantage can be eliminated if a Pt-promoted Fe-MFI is used (oxidation of CO to CO2). However, this has an adverse effect on the degrees of conversion of NO and N2O (see Kögel et al.: FIG. 3). A possible combination of this type of catalyst with the catalyst described by Li and Armor is not obvious because, inter alia, the optimum temperatures are very different (300° C. and 450–500° C., respectively).
Perez-Ramirez et al. [Appl. Catal. B, 25, 191–203, 2000] describe a dual-bed catalytic system intended for the removal of nitrogen oxides (NO and NO2) from a gas stream. In the first step NOx conversion takes place with the aid of a Pt/AC catalyst in the presence of propene as reducing agent. A major disadvantage of such a catalyst is that during the conversion of NO the formation of laughing gas takes place as the main reaction. Perez-Ramirez et al. therefore also studied whether this laughing gas formed can be effectively converted into nitrogen and oxygen in a second step in the presence of propene using Fe-ZSM-5. However, the use of these two catalysts has the disadvantage that two reactors have to be used because the optimum temperature ranges for the two catalysts are far apart (Pt/AC: optimum temperature is approximately 200° C.; Fe-ZSM-5: >430° C.; see also Perez-Ramirez et al.: FIG. 10). It is true that the required temperature for the Fe-ZSM-5 catalyst can be lowered to some extent by supplying additional propene to the second reactor (the product stream from the first reactor no longer contains any propene: see page 197, left-hand column, second paragraph; when propene is supplied to the reactor containing Fe-ZSM-5 a higher degree of conversion of N2O is found; see page 199, right-hand column, second paragraph), but not to a sufficiently low value.
The reduction of N2O with the aid of a saturated hydrocarbon and an Fe-zeolite catalyst is described in WO 99/49954. The preferred temperature at which this catalyst converts N2O is below 350° C. It is neither described nor suggested that this catalyst can be used together with another catalyst for the simultaneous removal of nitrogen oxides (NO and NO2) and N2O.
The conversion of NOx with the use of, inter alia, a Co-zeolite catalyst is described in U.S. Pat. No. 5,149,512. Here NOx is understood to be a mixture of at least two nitrogen oxides, including laughing gas (see, for example, column 3, lines 55–59 and column 4, lines 44–48). However, the fact that in this patent NOx is not used to refer to N2O is confirmed by the later publication by Li and Armor in Appl. Catal. B 3, 1993, on page 56. The experiments show only conversions of NO and not conversions of N2O. U.S. Pat. No. 5,149,512 thus does not explicitly describe whether such a catalyst is able to reduce laughing gas. The best degrees of conversion are obtained at 450° C. (see U.S. Pat. No. 5,149,512: Table 2). A combination of this cobalt catalyst for the conversion of NO with, for example, the abovementioned iron catalyst for the conversion of N2O would, on the basis of the data described, give a catalyst combination with completely different operating temperatures for the different conversions (<350° C. and 450° C., respectively) and would probably give degrees of conversion for NO that are too low. Incidentally, on the basis of this patent (U.S. Pat. No. 5,149,512) it would be best to choose a rhodium catalyst for the conversion of NO (see U.S. Pat. No. 5,149,512: Tables 1, 2, 5, 8 and 9, from which it can be seen that in the presence of CH4 and O2 the rhodium catalyst yields a degree of conversion of 55%, compared with 26, 34, 27 and 34%, respectively).
A combination of an iridium catalyst and a platinum catalyst is described in U.S. Pat. No. 5,997,830.
In U.S. Pat. No. 5,524,432 a catalytic reduction of nitrogen oxide (this term includes laughing gas; see U.S. Pat. No. 5,524,432: column 5, lines 12–15) is described where a reducing agent that has to contain methane is used (see U.S. Pat. No. 5,524,432: Claim 1 and column 4, lines 47–56). The advantage of this method is said to be that ammonia is not required (see Example 11, lines 19–21). The catalytic reduction is followed by a catalytic oxidation of residual methane into carbon dioxide and water. The catalyst that is used for the catalytic reduction comprises a crystalline zeolite with an Si/Al ratio of 2.5 or more and the zeolite contains cobalt, nickel, iron, chromium and/or manganese cations (see column 4, lines 37–46). Co-ZSM-5 is explicitly described in Example 5. However, U.S. Pat. No. 5,524,432 does not teach the person skilled in the art that this catalyst is suitable for the reduction of laughing gas.
SUMMARY OR THE INVENTION
The aim of the present invention is to eliminate the disadvantages described above. The invention therefore relates to a method for the simultaneous removal of nitrogen oxides and laughing gas from a gas stream containing nitrogen oxides and laughing gas, wherein a first catalyst that contains cobalt and, as support, an inorganic metal oxide, and a second catalyst that contains iron and, as support, an inorganic metal oxides are used.
It has been found that use of a specific sequence (1. cobalt-containing catalyst, 2. iron-containing catalyst) of the catalysts gives better results than, for example, a mixture of the two catalysts or a different sequence. The invention therefore also relates in particular to a method for the simultaneous removal of nitrogen oxides and laughing gas from a gas stream containing nitrogen oxides and laughing gas, wherein:

    
    
        (a) the gas stream containing nitrogen oxides and laughing gas is reacted with a reducing gas, the reducing gas comprising a saturated hydrocarbon, in the presence of a first catalyst that contains cobalt and, as support, an inorganic metal oxide,
        (b) the gaseous product from step (a) is reacted with a reducing gas, the reducing gas comprising a saturated hydrocarbon, in the presence of a second catalyst that contains iron and, as support, an inorganic metal oxide.
    
    


Advantage of the present invention are that the simultaneous removal of nitrogen oxides and laughing gas can effectively take place over a broad temperature range. Another advantage of the present invention is that high degrees of conversion can be achieved at low temperatures, even in the case of high GHSV (gas hourly space velocities) values. Moreover, it is not necessary to supply additional reducing gas to step (b). Furthermore, the two steps (a) and (b) can be carried out in one, it not being necessary to heat or to cool the gas between steps (a) and (b). The use in step (a) of the catalyst that contains cobalt and, as support, an inorganic metal oxide has the advantage that no additional N2O is produced.
The (first) catalyst used in step (a) is preferably the cobalt catalyst which is described in U.S. Pat. No. 5,149,512 and that is incorporated here by reference. That is to say this catalyst is preferably a crystalline zeolite having a silicon:aluminium ratio of 2.5 or above, the zeolite having been exchanged with an effective amount of a cobalt cation, preferably Co2+. In particular, the zeolite has been exchanged with 0.1 to 15% (m/m) cation, based on the total weight of the exchanged zeolite.
According to the invention, after exchanging with cobalt, the zeolite can then also be exchanged with approximately 0.01 to 2.0% (m/m) of an additional metal, based on the total weight of the exchanged zeolite, said additional metal being a metal of the third period of Groups 5 to 11 in the Periodic System of the Elements. The exchanged zeolite can furthermore be impregnated with 0.01 to 15% (m/m), in particular 0.1 to 8.0% (m/m), based on the total weight of the impregnated, exchanged zeolite, of anionic or neutral material that contains a metal from Group 5, 6, 7 or 11 of the Periodic System of the Elements.
The (second) catalysts used in step (b) are preferably those which are described in WO 99/49954 and NL A 1 012 983 and which are incorporated here by reference. That is to say this catalyst is preferably a crystalline zeolite having an SiO2:Al2O3 ratio of less than 100, preferably less than 65 and in particular less than or equal to 40, the zeolite having been exchanged with an effective amount of an iron cation, preferably Fe2+. The zeolite is preferably at least partially exchanged with (NH4)2Fe(SO4)2.6H2O. A preferred embodiment of the catalyst used in step (b) is Fe-ZSM-5. The catalyst used in step (b) can also be a promoted catalyst, the catalyst being promoted with one or more noble metals, the noble metal preferably being ruthenium, rhodium, palladium, gold or platinum. A palladium-promoted Fe-ZSM-5 catalyst is preferably used.
Preferred embodiments of the zeolite supports comprise the MOR and MFI types. The zeolites can be used in the sodium, potassium, ammonium or hydrogen form. The zeolite, the counter-ion and also the Si/Al ratio can be suitably chosen by those skilled in the art. This is described, inter alia, in standard works on zeolites and also in U.S. Pat. No. 5,149,512 and WO 99/49954 and a number of other cited documents.
According to the invention steps (a) and (b) are carried out in one reactor, the two catalysts being used in series in a stacked bed.
The reducing gas is preferably methane, propane, natural gas, n-butane or LPG (where LPG is a mixture containing propane and butane). The molar ratio of the added reducing agent with respect to the sum of the concentrations of laughing gas and nitrogen oxides is 0.1 to 100 and preferably 0.1 to 20.
The method according to the invention is preferably carried out at a temperature of 150° C. to 700° C. and in particular of 250° C. to 600° C. The pressure is preferably 1 to 50 bar absolute (bara) and in particular 1 to 15 bar absolute.
The invention also relates to a catalyst system for the simultaneous removal of nitrogen oxides and laughing gas from a gas stream containing nitrogen oxides and laughing gas by means of a reducing gas, which catalyst system comprises the first and second catalysts as described above.
The catalyst system can be used for the treatment of off-gases from a nitric acid plant.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1: NOx conversion by Catalyst I (Co-ZSM-5), Catalyst II (Fe-ZSM-5) and a serial combination of the two.
FIG. 2: N2O conversion by Catalyst I (Co-ZSM-5), Catalyst II (Fe-ZSM-5) and a serial combination of the two.
FIG. 3: NOx conversion by Catalyst I (Co-ZSM-5) and Catalyst III (Pd—Fe-ZSM-5) as a mixture and in serial combination.
FIG. 4: N2O conversion by Catalyst I (Co-ZSM-5) and Catalyst III (Pd—Fe-ZSM-5) as a mixture and in serial combination.
FIG. 5: NOx and N2O conversion by a stacked bed catalyst of Catalyst IV (III(Co—Pd-MOR) and III (Pd—Fe-ZSM-5) using CH4 as reducing gas.



EXAMPLES
Catalysts
Example 1
A zeolite exchanged with cobalt was prepared by exchanging 100 g Na-ZSM-5 (Alsi Penta SN 27, dried for 24 hours at 125° C.) with a solution of 29.2 g Co(NO3)2.6H2O (Merck) in 3 litres of water for 24 hours at 80° C. The catalyst was then washed with water, dried and calcined for 5 hours at 550° C. The catalyst has a loading of 2.7% (m/m) cobalt.
Example 2
A zeolite exchanged with iron was prepared by adding 100 g NH4-ZSM-5 powder (Alsi Penta SM 27, dried for 24 hours at 125° C.) to a solution of 38.9 g (NH4)2Fe(SO4)2.6H2O (Merck) in 3 litres of water. Ion exchange then takes place for 24 hours at 80° C. After washing with water, the catalyst was dried for 16 hours at 80° C. and calcined for 5 hours at 550° C. The catalyst has a loading of 2.3% (m/m) iron.
Example 3
A promoted catalyst was prepared by impregnating 100 g of a catalyst according to Example 2 with 3.1 g of a solution of Pd(NO3)2 (10% (m/m) in nitric acid, Aldrich) in demineralised water so that the pores of the base material were precisely filled (so-called “incipient wetness impregnation”). The concentration of the noble metal precursor was adjusted such that a percentage by weight of Pd of 0.15 based on the total weight of the catalyst was obtained. Finally the catalyst was dried for 16 hours at 80° C. and calcined for 5 hours at 550° C. The catalyst had a loading of 2.7% (m/m) iron and 0.15% (m/m) palladium.
Test Apparatus
The reaction of NO, NO2 and N2O with propane was studied in an automated flow set-up. The gases N2, air, NO, NO2, N2O and propane were introduced and the partial pressures (see Table 2) controlled by means of calibrated Mass Flow Controllers (Brooks). Water was added via a Liquiflow controller and a Controlled Evaporator Mixer (Bronkhorst). Analysis of the outflowing gases was carried out by means of a calibrated FTIR spectrophotometer (Elsag, Bailey, Hartmann & Braun, model MB 100). The catalysts were pressed to form a tablet, ground and sieved. The catalyst particles were in a stainless steel reactor. The gases were passed through a preheating section before they came into contact with the catalyst. The temperature was measured by means of thermocouples at the inlet and at the outlet of the catalyst bed. The temperature at the inlet of the catalyst bed is given in the test results. The pressure in the test set-up could be set to 1 to 5 bar absolute (bara). The carrier gas in the examples is N2.
Example 4
In these experiments the degrees of conversion of NO, NO2 and N2O by a stacked bed of the following catalysts were determined: step (a) Co-ZSM-5 according to Example 1 (Catalyst I) and step (b) Fe-ZSM-5 according to Example 2 (Catalyst II).
The conditions are given in Table 1, the gas compositions in Table 2 and the degrees of conversion in Table 3.











TABLE 1









Volume
10
ml



Flow
5
l/min



GHSV
30000
l/h



p
1
bara






















TABLE 2









N2O
1500 ppm
H2O
0.5%



NO
 500 ppm
O2
2.5%



NO2
  0 ppm
C3H8
1500 ppm










The starting material in all tests was the gas mixture in Table 2 (unless stated otherwise). Tests were also carried out using a mixture of NO and NO2 instead of NO as the starting material. Comparable results were obtained in these tests.









TABLE 3









Cat.











II
I
I + II









Cat.bed









10 + 10 ml











T
10 ml
10 ml

NOx













(° C.)
N2O (%)
NOx (%)
N2O (%)
NOx (%)
N2O (%)
(%)
















280
18
46
0
3
39
58


309
66
47
0
15
86
62


338
90
39
1
44
98
72


368
98
30
9
88
100
94


399
100
23
59
99
100
100


431
100
17
85
99
100
98


462
100
13
96
95
100
94


491
100
11
99
88
100
88









The results in Table 3 are shown as a graph in FIGS. 1 and 2, from which it is clear that simultaneous efficient conversion of NOx and N2O is possible only with the catalyst combination.
Example 5
It should be possible to use the cobalt and iron catalysts as a mixture. A comparative example is given below in which the degrees of conversion of NOx and N2O in a reactor in which the catalysts are present as a mixture are compared with the degrees of conversion by the stacked bed catalyst system of the present invention. The conditions are given in Table 4 and the test results are shown in Table 5. The gas mixture from Table 2 and the Catalysts I (Co-ZSM-5 according to Example 1) and III (Pd—Fe-ZSM-5 according to Example 3) were used in the experiment.











TABLE 4









Volume
12.5
ml



Flow
5
l/min



GHSV
24000
l/h



p
4
bara



















TABLE 5







I
2.5 ml



III
 10 ml












Configuration
Mixture
Stack











T (° C.)
N2O (%)
NOx (%)
N2O (%)
NOx (%)














210
0
34
0
34


235
0
53
1
55


265
36
80
57
67


297
78
71
82
56


325
93
63
95
59


353
99
56
99
81


382
100
50
100
91


410
100
43
100
93


440
100
38
100
93


469
100
36
100
92


500
100
37
100
90









The test results from Table 5 are shown in FIGS. 3 and 4. It can be seen from these results that the N2O conversion under these conditions is advantageous for the stack configuration, especially at lower temperatures. The NOx conversion shows a substantial difference. Up to approximately 250° C. the degree of NOx conversion for the mixture increases with increasing temperature; above this temperature the degree of NOx conversion decreases. A complete conversion is thus impossible using a mixture of the Co and Fe catalysts. Surprisingly, the stack configuration under these conditions yields virtually complete conversion above approximately 380° C.
Example 6
In this example the various sequences of Catalysts I and II (see Example 4) were compared. The conditions are given in Table 6 and the results in Table 7. The gas composition was as in Example 4. It can clearly be seen from Table 7 that much better results are obtained with the sequence I–II, which can be seen in particular from the degree of NOx conversion.











TABLE 6









Volume
0.3
ml



Flow
0.15
l/min



GHSV
30000
l/h



p
1
bara





















TABLE 7








I
0.15 ml
II
0.15 ml



II
0.15 ml
I
0.15 ml














T(° C.)
N2O (%)
NOx (%)
N2O (%)
NOx (%)














300
6
29
3
29


320
23
38
16
40


340
63
47
52
56


359
88
48
82
64


378
98
67
95
60


396
100
88
100
53


414
99
93
100
41


433
98
92
100
26


451
87
99
100
16


489
99
94
100
6









Example 7
In this example the effect of the addition of Catalyst III on the CO concentration was determined. Catalyst III is Catalyst II with 0.15% (m/m) Pd according to Example 3.
It can be seen that replacement of half of Catalyst II by Catalyst III leads to an appreciable reduction in the emission of CO (see Table 8 for the conditions and Table 9 for the results). The degree of NOx and N2O conversion is not adversely affected by the addition of Catalyst III. The gas composition was as in Example 4.











TABLE 8









Volume
20
ml



Flow
5
l/min



GHSV
15000
l/h



p
1
bara



















TABLE 9









Cat.











I 10 ml



I 10 ml
II 5 ml



II 10 ml
III 5 ml












Emission
Conversion
Emission
Conversion















T
CO
C3H8
N2O
NOx
CO
C3H8
N2O
NOx


(° C.)
(ppm)
(ppm)
(%)
(%)
(ppm)
(ppm)
(%)
(%)


















288
1588
561
39
58
111
494
48
54


316
1799
254
86
62
78
206
88
59


345
1816
159
98
72
61
106
99
73


375
1540
59
100
94
43
42
100
88


407
714
4
100
100
28
6
100
96


437
284
0
100
98
17
0
100
96


466
147
0
100
94
11
0
100
92


495
114
0
100
88
7
0
100
87









Example 8
In this example it is demonstrated that using the combination of Catalysts I and III high degrees of conversion of NOx and N2O are possible without the adverse formation of CO. It is also demonstrated in this example that high degrees of conversion of NOx and N2O are possible with a space velocity that is twice as high (30000 h−1 instead of 15000 h−1). The conditions and results are given in Tables 10 and 11, respectively. The gas composition was as in Example 4.











TABLE 10









Volume
10
ml



Flow
5
l/min



GHSV
30000
l/h



p
4
bara





















TABLE 11








I
  5 ml
I
5 ml



II
2.5 ml
III
5 ml


Cat.
III
2.5 ml





T(° C.)
N20 (%)
NOx (%)
N20 (%)
NOx (%)














288
54
67
57
57


316
82
72
85
77


345
94
82
86
97


375
97
88
87
99


407
99
90
95
99


437
100
90
100
98


466
100
88
100
95


495
100
86
100
91









Example 9
In this example the effect of the pressure was determined. It is found that more NOx and N2O is converted at higher pressure. The effect for NOx is most pronounced at low temperature. The CO emission is lower at higher pressure. The conditions are summarised in Table 12 and the results in Table 13. The gas composition was as in Example 4.













TABLE 12









I
10 ml 
Volume
20
ml



II
5 ml
Flow
5
l/min



III
5 ml
GHSV
15000
l/h





p
1 to 4
bara



















TABLE 13









Pressure











1 bara
2 bara
4 bara

















CO

NOx
CO
N2O
NOx
CO
N2O



T (° C.)
(ppm)
N2O (%)
(%)
(ppm)
(%)
(%)
(ppm)
(%)
NOx (%)



















288
111
48
54
69
72
68
22
81
82


316
78
88
59
23
95
67
13
98
89


345
61
99
73
17
100
78
10
100
97


375
43
100
88
12
100
90
8
100
99


407
28
100
96
7
100
96
7
100
99


437
17
100
96
4
100
96
5
100
98


466
11
100
92
4
100
94
4
100
96


495
7
100
87
4
100
90
3
100
93









Example 10
The reaction where for the Co catalyst a mordenite support was chosen instead of a ZSM-5 support (see Example 1), is described in this example. Methane was also selected instead of propane (see Table 2). For this reaction a Catalyst IV, Co-mordenite, was first prepared by impregnating 5 g MOR (mordenite from Zeolyst CBV 21A) with 1 g Co(NO3)2.6H2O in solution (density 1.34 g/ml). The catalyst was promoted with a small amount of Pd (0.37% (m/m) Pd). The final Co concentration is 2.2% (m/m).
As described above, a reactor containing a stacked bed was used, the first catalyst in this example being the Co—Pd-MOR catalyst (Catalyst IV) and the second catalyst the Pd—Fe-ZSM-5 catalyst from Example 3 (Catalyst III). The conditions are given in Table 14. The gas composition is shown in Table 15.










TABLE 14









Volume
  30 ml



Flow
  51/min



GHSV
10000 l/h



p
  4 bara






















TABLE 15









N2O
1500 ppm
H2O
0.5%



NO
 500 ppm
O2
2.5%



NO2
  0 ppm
CH4
2500 ppm










The test results are given in Table 16 and plotted as a graph in FIG. 5.








TABLE 16







IV
15 ml


III
15 ml












T (° C.)
N2O (%)
NOx (%)












290
20
1


317
27
8


345
45
33


374
71
76


404
91
97


433
96
96


463
92
100









It can be seen from these data that high degrees of conversion to virtually complete conversions can also be achieved with methane over a broad temperature range.

## Claims

1. Method for the simultaneous removal of nitrogen oxides and laughing gas from a gas stream containing nitrogen oxides and laughing gas, wherein a first catalyst that contains cobalt and, as a support, a first inorganic metal oxide, and a second catalyst that contains iron and, as a second support, a second inorganic metal oxide, are used, and wherein each of the first and second inorganic metal oxide is a zeolite.

2. Method according to claim 1, wherein:
(a) the gas stream containing nitrogen oxides and laughing gas is reacted with a first reducing gas comprising a saturated hydrocarbon, in the presence of the first catalyst to obtain a gaseous product; and
(b) the gaseous product from step (a) is reacted with a second reducing gas comprising a saturated hydrocarbon, in the presence of the second catalyst.

3. Method according to claim 2, wherein steps (a) and (b) are carried out in one reactor.

4. Method according to claim 3, wherein each of the first and second reducing gas is selected from the group consisting of propane, natural gas, n-butane or LPG.

5. Method according to claim 1, wherein the support for the first catalyst is a crystalline zeolite having a silicon:aluminum ratio of 2.5 or above.

6. Method according to claim 1, wherein the support for the second catalyst is a crystalline zeolite having a SiO2:Al2O3 ratio of less than 100.

7. Method according to claim 1, wherein the support for the second catalyst is a crystalline zeolite having a SiO2:Al2O3 ratio of less than 65.

8. Method according to claim 1, wherein the support for the second catalyst is a crystalline zeolite having a SiO2:Al2O3 ratio of less than or equal to 40.

9. Method according to claim 1, wherein the second catalyst comprises a promoted catalyst.

10. Method according to claim 9, wherein the promoted catalyst has been promoted with Pt, Pd, Ru, Au or Rh.

11. Catalyst system for the simultaneous removal of nitrogen oxides and laughing gas from a gas stream containing nitrogen oxides and laughing gas with a reducing gas, wherein the catalyst system contains a first catalyst that comprises cobalt and, as support, a first inorganic metal oxide, and contains a second catalyst that comprises iron and, as support, a second inorganic metal oxide, and wherein each of the first and second inorganic metal oxide is a zeolite.

12. Catalyst system according to claim 11, wherein the catalysts are present in series in one reactor.

13. Catalyst system according to claim 11, wherein the support for the first catalyst is a crystalline zeolite having a silicon:aluminum ratio of 2.5 or above.

14. Catalyst system according to claim 13, wherein the support for the second catalyst is a crystalline zeolite having a SiO2:Al2O3 ratio of less than 100.

15. Catalyst system according to claim 13, wherein the support for the second catalyst is a crystalline zeolite having a SiO2:Al2O3 ratio of less than 65.

16. Catalyst system according to claim 13, wherein the support for the second catalyst is a crystalline zeolite having a SiO2:Al2O3 ratio of less than or equal to 40.

17. Catalyst system according to claim 13, wherein the second catalyst comprises a promoted catalyst.

18. Catalyst system according to 17, wherein the promoted catalyst has been promoted with Pt, Pd, Ru, Au or Rh.