Method for the reduction of nitrogen oxides

A process for the catalytic reduction of NO.sub.x, the reduction taking place in the presence of a catalyst which comprises (a) from 20 to 97 wt % of A.sub.2 O.sub.3, (b) from 1 to 40 wt % of CuO, (c) from 1 to 50 wt % of ZnO, (d) from 1 to 40 wt % of Ag, (e) from 0 to 2 wt % of Pt, (f) from 0 to 20 wt % of oxides of rare earth metals, elements of the 3rd subgroup of the Periodic Table of the Elements or mixtures thereof, based on the total weight of the components (a) to (e), which adds up to 100 wt %, wherein, in each case, up to half the weight of the component (a) may be replaced by Fe.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Ga.sub.2 O.sub.3 or mixtures thereof, of the component (b) by CoO, of the component (c) by MgO, of the component (d) by Au and of the component (e) by Pd, Ru, Os, Ir, Rh, Re or mixtures thereof, is used for reducing NO.sub.x, especially in combustion off-gases, the components (a), (b) and (c) forming a spinel which is doped with the components (d), (e) and (f).

The invention relates to the use of certain catalysts for reducing nitrogen 
oxides (NO.sub.x) and to a corresponding process. Nitrogen oxides 
primarily come from combustion off-gases, in particular from 
internal-combustion engines such as diesel engines. 
The combustion of hydrocarbons with air as an oxidant gives rise, 
particularly with excess air and at high temperatures, to nitrogen oxides 
via oxidation of the nitrogen present in the air. Examples of such 
nitrogen oxides are NO, NO.sub.2, NO.sub.3, N.sub.2 O.sub.3, N.sub.2 
O.sub.4 and N.sub.2 O.sub.5. Being pollutants, the nitrogen oxides are to 
be removed as completely as possible from the combustion off-gases, to 
avoid environmental pollution. While emissions from power stations and 
industry are progressively declining, owing to the use of off-gas 
treatment plants, abating the pollutant fraction in motor vehicle exhaust 
gases is becoming increasingly important, particularly given the increase 
in the number of motor vehicles. 
Many solutions have been proposed for abating NO.sub.x emissions of motor 
vehicle engines. Effective solutions for abating the amounts of NO.sub.x 
must meet numerous criteria, particularly if catalysts are used, for 
example: 
High conversion ratio, ie. extensive removal of NO.sub.x, even at high and 
low temperature and in the event of frequent load changes during operation 
Avoiding the use of auxiliary materials such as ammonia or urea 
Low production and operating costs 
Long on-stream time 
Low N.sub.2 O production 
High mechanical catalyst stability 
A number of catalysts for reducing nitrogen oxides have been proposed. 
EP-A1-0 687 499 describes spinel catalysts made of a spinel comprising 
copper, zinc and aluminum, for reducing nitrogen oxides. 
U.S. Pat. No. 3,974,255 describes a magnesium aluminate spinel catalyst 
coated with platinum. The catalyst is employed for reducing NO.sub.x in 
exhaust gases from internal-combustion engines. 
EP-B1-0 494 388 describes a process for removing nitrogen oxides from an 
off-gas in which the catalyst used is a polyvalent metal phosphate, a 
polyvalent metal sulfate or a spinel aluminate of a transition metal of 
the 4th period of the Periodic Table of the Elements. In particular, a 
cobalt aluminate catalyst is described which is prepared by 
coprecipitation of cobalt nitrate and aluminum nitrate, followed by drying 
and calcination. 
JP-A2-08024648 describes catalysts for removing nitrogen oxides from an 
off-gas. The catalyst, for example, has the composition Ag.sub.0.01 
P.sub.0.01 Pr.sub.0.01 Cu.sub.0.2 Zn.sub.0.5 Al.sub.2.0, and Fe.sub.0.02 
Co.sub.0.02 or 0.01% of MgO may also be present. The catalyst is prepared 
by the nitrates of Ag and Pr being mixed together with H.sub.3 PO.sub.4 
and this mixture being introduced into oxide mixtures of Cu, Zn and Al in 
the presence of aqueous solutions of ammonia or ammonium carbonate or 
ammonium sulfate. The calcination is carried out at 500.degree. C. and 
800.degree. C., respectively. The silver content of the catalysts is 0.67 
wt %. 
Also known are catalysts for the catalytic decomposition of dinitrogen 
monoxide (N.sub.2 O). DE-A1-42 24 881 describes a silver-containing 
aluminum oxide supported catalyst and a process for the catalytic 
decomposition of dinitrogen monoxide, either pure or present in gas 
mixtures, where dinitrogen monoxide is decomposed selectively without 
other nitrogen oxides being decomposed to a significant extent into the 
elements. The catalyst used may be in the form of copper/zinc/aluminum 
spinels which are doped with silver. This involves, for example, an 
aluminum oxide support being impregnated with a solution of copper nitrate 
and zinc nitrate, being dried, calcined and then being impregnated with a 
solution of silver nitrate, being dried and calcined. 
It is an object of the present invention to provide improved catalysts, 
compared with known catalysts, for reducing NO.sub.x. In addition, a 
process for reducing NO.sub.x, particularly in exhaust gases from 
internal-combustion engines, is to be provided. 
We have found that this object is achieved, according to the invention, by 
the use of a catalyst which comprises 
(a) from 20 to 97 wt % of Al.sub.2 O.sub.3, 
(b) from 1 to 40 wt % of CuO, 
(c) from 1 to 50 wt % of ZnO, 
(d) from 1 to 40 wt % of Ag, 
(e) from 0 to 2 wt % of Pt, 
(f) from 0 to 20 wt % of oxides of rare earth metals, elements of the 3rd 
subgroup of the Periodic Table of the Elements or mixtures thereof, 
based on the total weight of the components (a) to (e), which adds up to 
100 wt %, 
wherein, in each case, up to half the weight of the component (a) may be 
replaced by Fe.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Ga.sub.2 O.sub.3 or 
mixtures thereof, of the component (b) by CoO, of the component (c) by 
MgO, of the component (d) by Au and of the component (e) by Pd, Ru, Os, 
Ir, Rh, Re or mixtures thereof, 
for reducing NO.sub.x. 
At the same time it was found, according to the invention, that catalysts 
as described in DE-A1-42 24 881 can be employed for reducing NO.sub.x. 
In this context, the catalysts used according to the invention preferably 
comprise from 30 to 80, particularly preferably from 40 to 75, in 
particular from 45 to 65 wt % of Al.sub.2 O.sub.3, preferably from 3 to 
35, particularly preferably from 5 to 30, in particular from 8 to 25 wt % 
of CuO, preferably from 2 to 40, particularly preferably from 5 to 30, in 
particular from 10 to 26 wt % of ZnO, preferably from 2 to 35, 
particularly preferably from 3 to 30, in particular from 5 to 25 wt % of 
Ag and preferably from 0 to 1, particularly preferably from 0 to 0.5, in 
particular from 0 to 0.1 wt % of Pt. As described above, the specified 
weights are based on the total weight of the components (a) to (e), which 
adds up to 100 wt %. 
Preferably, in each case, at most 1/3 particularly preferably at most 1/5, 
in particular 1/10 of the weight of the components (a),(b),(c),(d) and/or 
(e) are replaced as described above. Preferably, none of the components 
(a),(b),(c),(d) and (e) is replaced. 
Eligible for component (f) are oxides of the rare earth metals and the 
elements of the 3rd subgroup of the Periodic Table of the Elements, 
preferably oxides of 3-valent rare earth metals, in particular La and/or 
Ce. Preferably, Pr is not present in the catalyst. 
Component (f) is preferably employed in amounts of from 0 to 15, 
particularly preferably from 0 to 10, in particular from 0 to 5 wt %, 
based on the total weight of the components (a) to (e). Preferably, no 
component (f) is present. 
Particularly preferably, the catalyst consists of the components (a) to 
(e), specifically only of Al.sub.2 O.sub.3, CuO, ZnO, Ag and possibly Pt. 
According to the invention a copper/zinc/aluminum oxide compound is used 
which can be generally represented as follows: Cu.sub.a Zn.sub.b Al.sub.2 
O.sub.3+a+b, where a&gt;0, b&gt;0, a+b.ltoreq.1. 
Preferably, the components (a), (b) and (c) form a spinel. Spinels are 
described, for example, in C. W. Correns, Einfuhrung in die Mineralogie 
[Introduction to mineralogy], Springer Verlag 1949, pp. 77-80, H. Remy, 
Lehrbuch der Anorganischen Chemie [Textbook of inorganic chemistry], 
Akademische Verlagsgesellschaft Geest & Portig K.-G. Leipzig 1950, pp. 
308-311, Romp, Chemielexikon, 9th edition 1995, p. 4245. Spinels formally 
derive from MgAl.sub.2 O.sub.4, where magnesium may be replaced by other 
divalent ions such as zinc, copper, iron. Aluminum may be replaced by 
other trivalent ions such as iron or chromium. In the spinel lattice the 
oxygen atoms form a cubic close-packed structure corresponding to a 
face-centered lattice. Half of the octahedral vacancies therein are 
occupied by aluminum, the other half of the vacancies are empty. One 
eighth of the tetrahedral vacancies are occupied by magnesium. 
What is preferably present is essentially a copper/zinc spinel. For a+b=1 
no vacant sites exist in the spinel lattice. Al.sub.2 O.sub.3 can act as a 
matrix, in which the other metal oxides are present. This is the case, in 
particular, for a+b.ltoreq.1. 
The novel catalysts may contain small amounts of SiO.sub.2, TiO.sub.2, 
ZrO.sub.2, talc and/or cements, as long as these do not significantly 
affect the properties of the catalysts. According to one embodiment the 
catalyst is phosphorus-free, particularly if Pr is present. 
The novel catalysts have a pore volume of from 0.01 to 1, preferably from 
0.01 to 0.8, particularly preferably from 0.1 to 0.7 ml/g, the pore size 
distribution being monomodal, bimodal or polymodal. Bimodal or polymodal 
catalysts in this context preferably have mesopores and macropores. 
Mesopores have a diameter of less than 50 nm, macropores have a diameter 
of from 50 to 10000 nm. The catalyst preferably has a bimodal or polymodal 
pore size distribution, from 40 to 99%, preferably from 50 to 98%, 
particularly preferably from 55 to 95% of the pore volume being present in 
mesopores and from 1 to 60%, preferably from 2 to 50%, particularly 
preferably from 5 to 45% of the pore volume being present in macropores. 
Particularly preferably the catalyst is bimodal. 
Oligomodal catalysts may also contain pores having a diameter of more than 
10000 nm, the fraction of these pores preferably being from 0.1 to 20, 
particularly preferably from 1 to 15% of the pore volume, where the 
above-specified values for the mesopores and macropores relate to the 
residual pore volume. 
In the case of catalysts having a bimodal or polymodal pore size 
distribution, the major fraction of the pore volume is preferably within a 
pore size range of from 10 to 1000 nm. 
The size of the silver particles present in the catalyst is preferably from 
0.1 to 200, particularly preferably from 5 to 50 nm. Above 300.degree. C. 
in this context, silver is present as a metal and below this temperature 
may also be present as an oxide. The weights specified above are based on 
the metal. 
The pore volume and the pore volume distribution are preferably determined 
by Hg porosimetry. The size of the silver particles is determined, for 
example, by means of measuring the line width in X-ray diffraction. 
The BET surface area is preferably from 1 to 200, particularly preferably 
from 20 to 150, in particular from 50 to 100 m.sup.2 /g. 
The catalysts employed according to the invention may be present in any 
form, for example as pellets, tablets, which may be hollow or solid; 
granules having a diameter of preferably from 0.5 to 3 mm, chips, 
honeycombs, etc. The novel catalysts may also be present on other support 
materials such as glass fiber mats, ceramic or metallic supports, and the 
supports may have various shapes, for example corrugated or rolled. The 
above-specified quantities in this context relate to the catalyst proper, 
without the additional support. Catalysts used according to the invention 
which are to be employed in the automotive sector preferably are 
honeycomb-shaped, the hole diameter preferably being from 0.1 to 10 mm, in 
particular from 0.5 to 5 mm, and the web width preferably being from 0.1 
to 5 mm, in particular from 0.3 to 3 mm. 
The catalysts used according to the invention can be prepared by any 
suitable process. Suitable processes are described, for example, in 
DE-A1-42 24 881. For example, AlOOH (boehmite), CuO, ZnO and any further 
metal oxides required may be kneaded with water in the presence of a 
binder, extruded to form extrudates, dried and calcined. The catalyst base 
bodies thus prepared can be impregnated with an aqueous solution of silver 
nitrate. The impregnated catalysts are then dried and calcined. 
Instead of metal oxides, the corresponding hydroxides, oxyhydrates, 
carbonates, salts of organic acids, nitrates, chlorides, sulfates or 
phosphates can be used. To prepare bimodal or polymodal catalysts it is 
possible to use, instead of AlOOH, a mixture of AlOOH and Al.sub.2 
O.sub.3, preferably .gamma.- or .delta.-Al.sub.2 O.sub.3, with the option 
of employing aluminum oxide (Al.sub.2 O.sub.3) of different pore size 
distributions. 
Drying preferably takes place at from 10 to 200.degree. C., particularly 
preferably from 20 to 150.degree. C., in particular from 30 to 120.degree. 
C. Calcination takes place at less than 1100.degree. C., preferably at 
from 600 to 900.degree. C. Calcination after impregnation with silver 
nitrate solution preferably takes place at from 200 to 800.degree. C. 
The catalysts used according to the invention are preferably employed for 
reducing NO.sub.x in combustion off-gases. The reduction of the NO is 
effected by reaction with a reducing agent. 
The invention also relates to a process for the catalytic reduction of 
NO.sub.x in mixtures containing NO.sub.x, O.sub.2 and hydrocarbon 
compounds, the reduction taking place in the presence of a catalyst as 
defined above, and the hydrocarbon compounds serving as reducing agents. 
The mixture is preferably a combustion off-gas, the combustion off-gas in 
particular coming from internal-combustion motors or internal-combustion 
engines. 
Such an off-gas inter alia contains nitrogen oxides (NO.sub.x), oxygen 
(O.sub.2), water vapor and possibly hydrocarbon compounds. Hydrocarbon 
compounds are, for example, oxygen-containing hydrocarbon compounds such 
as alcohols, ethers, aldehydes, ketones, epoxides etc. The term 
"hydrocarbon compounds" also subsumes hydrocarbons such as alkanes, 
alkenes, alkynes or aromatic compounds. Instead of the hydrocarbon 
compounds it is also possible to use CO or H.sub.2. Preference is given to 
the use of added hydrocarbon compounds. For example, short-chain 
hydrocarbons such as propene may be metered into the off-gas stream. 
Another preferred option is for a portion of the fuel, for example in the 
case of a motor vehicle, to be supplied to the off-gas stream, so that 
hydrocarbon compounds are present in the off-gas. An example of a reaction 
taking place if propene is used as the hydrocarbon compound is shown in 
the following reaction equation. 
EQU 4 NO.sub.2 +NO+CH.sub.3 --CH.dbd.CH.sub.2 .fwdarw.5/2 N.sub.2 +3 CO.sub.2 
+3 H.sub.2 O 
Diesel exhaust gases, in particular, additionally contain oxygen, since the 
combustion is carried out with excess air. This means that further 
reactions may take place in which organic oxygen-containing compounds are 
formed. 
Hydrocarbon compounds present can therefore, on the one hand, react with 
oxygen present and, on the other hand, with nitrogen oxides present, the 
catalyst used according to the invention preferentially catalyzing the 
reaction of hydrocarbons with NO.sub.x, compared with the reaction of 
hydrocarbons with oxygen. 
Exhaust gases, in particular of diesel engines, in addition to NO.sub.x and 
hydrocarbons also contain CO, possibly soot, SO.sub.2, and water vapor, 
oxygen, nitrogen (N.sub.2) and CO.sub.2. A diesel exhaust gas can have the 
following composition: 
NO.sub.x from 10 to 10000, on average 2000 ppm 
hydrocarbons from 10 to 2000, on average 200 ppm 
CO from 10 to 4000, on average 100 ppm 
soot from 0 to 1, on average 0.3 g/l 
sulfur dioxide from 0 to 200, on average 40 ppm 
water vapor from 1.5 to 8, on average 7 vol % 
oxygen from 3 to 18, on average 4 vol % 
CO.sub.2 from 2 to 15, on average 3 vol % 
Typical catalyst loadings are from 20,000 to 30,000, peak loadings up to 
100,000 m.sup.3 (s.t.p.) of gas per m.sup.3 of catalyst per hour. The 
invention is explained below in more detail with reference to examples. 
General preparation procedure for the catalyst 
The catalyst used according to the invention can be prepared in a manner 
similar to the procedure described in DE-A1-42 24 881. 
This, for example, involves 400 g of AlOOH (Boehmite, Pural.RTM. SB from 
Condea), 50 g of CuO, 154.5 g of ZnO and 25 g of methylcellulose 
(Walocel.RTM. from Wolff, Walsrode) being kneaded with 270 g of water for 
one hour, being extruded to produce solid extrudates having a diameter of 
3 mm and a length of 8 mm, dried and calcined for 4 hours at 800.degree. 
C. The material obtained after calcination has a surface area of 54 
m.sup.2 /g. The porosity is 0.32 ml/g. 
The inorganic constituents have the following composition: Cu: 0.63 mol 
(CuO: 10 wt %), Zn: 1.90 mol (ZnO: 28 wt %), Al: 6.67 mol (Al.sub.2 
O.sub.3 : 62 wt %). This corresponds to the empirical formula Cu.sub.0.2 
Zn.sub.0.6 Al.sub.2 O.sub.3.8. 
108 g of this material (spinel) are impregnated with 51 ml of an aqueous 
solution containing 30.2 g of silver nitrate and are left for one hour. 
The impregnated material is dried for one hour at 120.degree. C. to 
constant weight and then calcined at 600.degree. C. The catalyst pellets 
thus obtained contain 19.2 g of metallic silver, corresponding to 17.8 wt 
%. 
In the following examples percentages relate to weight unless otherwise 
stated.

EXAMPLE 1 
Monomodal catalyst having the composition 20% of CuO, 20% of ZnO, 45% of 
Al.sub.2 O.sub.3, 15% of Ag 
400 g of AlOOH (Boehmite, Pural SB from Condea), 151 g of CuO and 151 g of 
ZnO as well as 30 g of methylcellulose (Walocel from Wolff, Walsrode) were 
kneaded with 320 g of water for one hour, extruded to produce solid 
extrudates having a diameter of 3 mm and a length of 8 mm, dried and 
calcined for 4 hours at 800.degree. C. 
640 g of this spinel body were impregnated with 178 g of silver nitrate in 
the form of an aqueous 50% strength AgNO.sub.3 solution. This was followed 
by drying for one hour at 120.degree. C. and calcination for 4 hours at 
600.degree. C. 
The pellets thus obtained contained 15% of Ag, 20% of CuO, 20% of ZnO and 
45% of Al.sub.2 O.sub.3. They were monomodal and contained pores, 95% of 
which had a diameter of 50-50000 nm. The total porosity was 0.32 ml/g. 
EXAMPLE 2 
Bimodal catalyst with 5% of Ag, 15% of CuO, 19% of ZnO, 61% of Al.sub.2 
O.sub.3 
The bimodal catalyst was obtained by a mixture of AlOOH and Al.sub.2 
O.sub.3 being used instead of just AlOOH. 
611 g of Cu(NO.sub.3).sub.2 .times.3 H.sub.2 O, 581 g of Puralox.RTM. SCF-A 
230 (Al.sub.2 O.sub.3, produced by Condea) and 322 g of Pural SB (AlOOH, 
produced by Condea) were mixed well for 3 hours. The dry composition was 
then admixed with enough water to produce a plastic kneading composition. 
45 g of formic acid were then introduced. The composition was kneaded for 
70 minutes, formed into extrudates, dried for 16 hours at 120.degree. C. 
and calcined for 4 hours at 800.degree. C. 
800 g of the body thus obtained were impregnated with 373 g of 
Zn(NO.sub.3).sub.2 .times.6 H.sub.2 O, which had been dissolved in water 
and was made up to 400 l of total solution. 
After 1.5 hours' impregnation, drying took place for 16 hours at 
120.degree. C., followed by calcination for 4 hours at 600.degree. C. 
As described in Example 1, the catalyst was impregnated with a 50% strength 
AgNO.sub.3 solution, so that the finished catalyst contained 5% of silver. 
The catalyst had the composition 5% of Ag, 19% of ZnO, 15% of CuO, 61% of 
Al.sub.2 O.sub.3. The BET surface area was about 100 m.sup.2 /g. The water 
uptake was about 0.5 ml/g, which corresponded to an overall porosity of 
the same magnitude. 
EXAMPLE 3 
The catalyst was prepared in a manner similar to that of Example 2, but had 
the following composition: 15% of Ag, 17% of ZnO, 13.6% of CuO, 54.4% of 
Al.sub.2 O.sub.3. 
EXAMPLE 4 
The procedure from Example 2 was repeated, but the catalyst obtained had 
the following composition: 25% of Ag, 15% of ZnO, 12% of CuO, 46% of 
Al.sub.2 O.sub.3. 
EXAMPLE 5 
The procedure from Example 2 was repeated, but the following composition 
was obtained: 15% of Ag, 17% of ZnO, 13.6% of CuO, 54.3% of Al.sub.2 
O.sub.3, 0.1% of Pt. Doping with Pt was effected by impregnation with an 
aqueous platinum nitrate solution. This impregnation took place at the 
same time as the impregnation with silver nitrate, but can also be carried 
out independently. 
EXAMPLE 6 
The catalyst was prepared in a manner similar to the procedure described in 
Example 2, but had the following composition: 15% of Ag, 25.5% of ZnO, 
8.5% of CuO, 51% of Al.sub.2 O.sub.3. 
COMATIVE EXAMPLE 1 
For comparative purposes a catalyst was prepared in a manner similar to 
that of Example 2, except that no silver was employed. The catalyst had 
the following composition: 15% of ZnO, 21.3% of CuO, 63.7% of Al.sub.2 
O.sub.3. 
COMATIVE EXAMPLE 2 
The comparative catalyst was prepared in a manner similar to that of 
Example 2, except that instead of silver a very small amount of palladium 
was employed. The catalyst had the following composition: 
20% of ZnO, 16% of CuO, 64% of Al.sub.2 O.sub.3, 0.1 % of Pd. 
Study of the catalysts 
The catalysts obtained were studied as follow: 
Of the respective catalysts, 10 g of chippings of the fraction from 1.6 to 
20 mm were introduced into a vertically positioned quartz reactor 
(diameter 20 mm, height about 500 mm) in the center of which a gas- 
permeable frit was arranged for accommodating the catalyst. The bed height 
was about 15 mm. Around the quartz reactor a furnace was arranged which 
heated the central section of the reactor over a length of 100 mm, 
temperatures up to 550.degree. C. being achievable. 
A gas mixture was passed through the catalyst at a flow rate of about wt 
10000 (1 [s.t.p.] of gas)/(1 of cat x h). The gas mixture consisted of 
1000 ppm of NO, 1000 ppm of propene, 10 vol % of oxygen and argon 
(remainder) as the carrier gas. 
Downstream of the reactor the NO concentration was measured with a gas 
detector, any NO.sub.2 formed upstream of the detection being reduced in a 
converter to NO. The reaction was carried out at temperatures in the range 
of from 200.degree. C. to 400.degree. C. The study results obtained are 
listed in the following table, including the NO.sub.x concentration 
measured downstream of the reactor and the minimum NO.sub.x concentration 
and the temperature at which the minimum NO.sub.x concentration had been 
measured. In addition the maximum conversion ratio is listed, ie. the 
ratio (NO.sub.x upstream) minus (NO.sub.x downstream) to (NO.sub.x 
upstream). 
TABLE 
__________________________________________________________________________ 
NO.sub.x min. 
Catalyst from 
NO.sub.x upstream of 
NO.sub.x downstream of reactor (ppm) 
downstream of 
T (.degree. C.) 
Conversion 
Example No. 
reactor (ppm) 
200.degree. C. 
250.degree. C. 
300.degree. C. 
350.degree. C. 
400.degree. C. 
reactor (ppm) 
NO.sub.x min. 
ratio (%) 
__________________________________________________________________________ 
max. 
1 1000 620 400 305 390 705 300 295 70.0 
2 1000 810 300 280 600 840 235 280 76.5 
3 1000 945 480 195 365 540 190 295 81.0 
4 1000 980 520 345 820 910 345 300 65.5 
5 1000 885 575 495 725 860 485 285 51.5 
6 1000 610 385 220 415 595 215 290 78.5 
Comparative 
1000 800 605 580 615 685 575 280 42.5 
Example 1 
Comparative 
1000 935 660 635 815 895 605 285 39.5 
Example 2 
__________________________________________________________________________ 
The results of the table demonstrate that the catalysts used according to 
the invention according to Examples 1-6 lead to considerably better 
abatement of NO.sub.x compared with the comparative catalysts according to 
Comparative Examples 1 and 2.