Catalyst for purifying exhaust gas

A catalyst for purifying an exhaust gas to remove nitrogen oxides, carbon monoxide and hydrocarbons from an oxygen-rich exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons, comprising (i) a zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of at least 15 and (ii) (a) cobalt, (b) an alkaline earth metal and (c) silver, or nickel and/or zinc, or platinum and/or manganese, or copper and/or rhodium, incorporated thereinto.

BACKGROUND OF THE INVENTION 1. Field of the Invention 
The present invention relates to a catalyst for purifying an exhaust gas to 
remove nitrogen oxides, carbon monoxide and hydrocarbons contained in an 
exhaust gas discharged, for example, from internal combustion engines of 
automobiles and the like, and in particular, to a catalyst for removing 
nitrogen oxides contained in an oxygen-rich exhaust gas. 
The term "oxygen-rich exhaust gas" used in the present invention is 
intended to mean an exhaust gas containing oxygen in an amount exceeding 
the amount of oxygen necessary for completely oxidizing carbon monoxide 
and hydrocarbons and hydrogen contained in the exhaust gas. 
2. Description of the Related Art 
Nitrogen oxides, carbon monoxide and hydrocarbons, which are toxic 
substances contained in an exhaust gas discharged from internal combustion 
engines, are removed, for example, through the use of a three-way catalyst 
comprising Pt, Rh, Pd, etc., supported on a carrier material. In the case 
of an exhaust gas discharged from diesel engines, however, no effective 
catalyst exists for removing nitrogen oxides because the exhaust gas 
contains a large amount of oxygen, and thus a purification of the exhaust 
gas by a catalyst has not been realized. 
In recent gasoline engines, a lean burn combustion is used for lowering the 
fuel consumption and reducing the amount of exhausted carbon dioxide gas, 
but an exhaust gas from this lean burn gasoline engine comprises an 
atmosphere containing an excessive amount of oxygen, and therefore, it is 
impossible to use the above-mentioned conventional three-way catalyst, and 
thus a method of removing toxic components from the exhaust gas has not 
been put to practical use. 
Examples of the method of removing particularly nitrogen oxides in an 
exhaust gas containing an excessive amount of oxygen include that wherein 
a reducing agent such as ammonia is added, and that wherein the nitrogen 
oxides are absorbed in an alkali to remove same. These methods are not 
effective for automobiles, which are a moving nitrogen oxides source, and 
thus the application thereof is limited. 
Recently it has been reported that a zeolite catalyst subjected to an ion 
exchange with a transition metal can remove nitrogen oxides in an exhaust 
gas containing an excessive amount of oxygen, without the addition of a 
special reducing agent such as ammonia. For example, Japanese Unexamined 
Patent Publication (Kokai) Nos. 63-283727 and 1-130735 propose a catalyst 
able to selectively reduce nitrogen oxides even in an exhaust gas 
containing an excessive amount of oxygen and minor amounts of reducing 
agents such as unburnt carbon monoxide and hydrocarbons. 
The activity of the above-mentioned catalysts proposed in the art, however, 
is remarkably deteriorated when used at a high temperature for a long 
time, and thus it is necessary to improve the durability and catalytic 
performance thereof. 
Accordingly, to solve the above-described problems, a catalyst for 
purifying an exhaust gas comprising a zeolite having an SiO.sub.2 
/Al.sub.2 O.sub.3 mole ratio of at least 15, and incorporated therein 
cobalt and an alkaline earth metal, has been proposed (see Japanese Patent 
Application No. 1-337249). 
Although the exhaust gas purification catalyst proposed in Japanese Patent 
Application No. 1-337249 has an improved durability, the temperature 
region in which the nitrogen oxides can be removed is relatively narrow. 
Therefore, a higher capability of removing nitrogen oxides in a broader 
temperature region, particularly at a low temperature, is required from a 
catalyst for purifying an exhaust gas discharged, in particular, from 
automobiles. 
SUMMARY OF THE INVENTION 
Accordingly, the objects of the present invention are to eliminate the 
above-mentioned disadvantages of the prior art and to provide a catalyst 
for purifying an exhaust gas capable of simultaneously removing nitrogen 
oxides, carbon monoxide and hydrocarbons from an exhaust gas discharged 
from, for example, internal combustion engines of automobiles, which 
catalyst is less susceptible to thermal deterioration and has a high 
catalytic activity. 
Other objects and advantages of the present invention will be apparent from 
the following description. 
The present inventors have found that the incorporation of any one of 
20 (1) silver, 
(2) nickel and/or zinc, 
(3) platinum and/or manganese, and 
(4) copper and/or rhodium, 
in the above-mentioned catalyst for purifying an exhaust gas comprising a 
zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of at least 15, 
and incorporated therein cobalt and an alkaline earth metal, improves the 
capability of the catalyst of removing nitrogen oxides, and thus completed 
the present invention. 
Accordingly, the present invention provides a catalyst for purifying an 
exhaust gas to remove nitrogen oxides, carbon monoxide and hydrocarbons 
from an oxygen-rich exhaust gas containing nitrogen oxides, carbon 
monoxide and hydrocarbons, comprising (i) a zeolite having an SiO.sub.2 
/Al.sub.2 O.sub.3 mole ratio of at least 15 and (ii), incorporated 
thereinto, (a) cobalt and (b) an alkaline earth metal and (c) any one of 
(1) silver, 
(2) nickel and/or zinc, 
(3) platinum and/or manganese, and 
(4) copper and/or rhodium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in more detail. 
The catalyst for purifying an exhaust gas according to the present 
invention comprises (i) a zeolite having an SiO.sub.2 /Al.sub.2 O.sub.3 
mole ratio of at least 15 and (ii) incorporated therein, (a) cobalt, (b) 
an alkaline earth metal (e.g., Ca, Mg, Sr, Ba) and (c) any one of 
(1) silver, 
(2) nickel and/or zinc 
(3) platinum and/or manganese, and 
(4) copper and/or rhodium. 
The above-mentioned zeolite generally has the following composition: 
EQU XM.sub.2/n O.Al.sub.2 O.sub.3.ySiO.sub.2.zH.sub.2 O 
wherein n is a valency of the cation, x is 0.8 to 1.2, y is at least 2, and 
z is at least 0 (zero). In the zeolite used in the present invention, the 
SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio is preferably at least 15. There is 
no particular limitation of the upper limit of the SiO.sub.2 /Al.sub.2 
O.sub.3 mole ratio, but when the SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio is 
less than 15, the heat resistance and durability of the zeolite per se are 
low, and thus the heat resistance and durability of the catalyst are 
unsatisfactory. The SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio is more 
preferably about 15 to 1000. 
The zeolite constituting the catalyst of the present invention may be a 
naturally occurring zeolite or a synthetic zeolite. There is no particular 
limitation of the method of producing the zeolite. Representative examples 
of the zeolite used in the present invention include ferrierite, Y, ZSM-5, 
ZSM-11, ZSM-12 and ZSM-20. These zeolites per se may be used as the 
catalyst of the present invention, or used after treatment with an 
ammonium salt, a mineral acid or the like for ion exchange to form an 
NH.sub.4 or H type zeolite. 
The zeolite used in the present invention contains (a) cobalt, (b) alkaline 
earth metal and (c) any one of 
(1) silver, 
(2) nickel and/or zinc, 
(3) platinum and/or manganese, and 
(4) copper and/or rhodium. 
There is no particular limitation of the method of incorporating the 
above-described metals in the zeolite, and in general, the above-mentioned 
metals can be incorporated by an ion exchange method, an impregnation 
method, and an evaporation-to-dryness method through the use of a water 
soluble salt. The above-mentioned metals may be incorporated at one time, 
or may be successively incorporated. 
When incorporating the above-mentioned metals in the zeolite, the 
concentration of individual metal ions in the aqueous solution can be 
properly selected depending upon the intended percentage ion exchange of 
the catalyst. Examples of the alkaline earth metal ions include Ca, Mg, Sr 
and Ba. The above-mentioned metal ions may be used in the form of a 
soluble salt, and suitable examples of the soluble salt include nitrate, 
acetate, oxalate and chloride. 
Regarding the contents of the above-mentioned metals in terms of mole ratio 
to the alumina in the zeolite, the contents of cobalt and an alkaline 
earth metal are preferably 0.1 to 1.5 times, more preferably 0.2 to 1.4 
times and 0.1 to 1 time, more preferably 0.2 to 1 time respectively. 
Further, 
1) preferably the content of silver is 0.05 to 2 times, more preferably 0.1 
to 1.8 times, and the total content of cobalt, alkaline earth metal and 
silver is 1.0 to 2.5 times, more preferably 1.0 to 2.3 times, 
2) preferably the content of nickel and/or zinc is 0.05 to 2 times, more 
preferably 0.1 to 1.8 times, and the total content of cobalt, alkaline 
earth metal and nickel and/or zinc is 1.0 to 2.5 times, more preferably 
1.0 to 2.3 times, 
3) preferably the content of platinum and/or manganese is 0.05 to 1.5 
times, more preferably 0.1 to 1.4 times, and the total content of cobalt, 
alkaline earth metal and platinum and/or manganese is 1.0 to 2.5 times, 
more preferably 1.0 to 2.3 times, and 
4) preferably the content of copper and/or rhodium is 0.05 to 1.5 times, 
more preferably 0.1 to 1.4 times, and the total content of cobalt, 
alkaline earth metal and copper and/or rhodium is 1.0 to 2.5 times, more 
preferably 1.0 to 2.3 times. 
The sample containing the above-mentioned metals is generally used after 
solid-liquid separation, washing and drying, and if necessary, can be used 
after calcination. 
The catalyst for purifying an exhaust gas according to the present 
invention may be used after mixing with a binder, such as a clay mineral, 
and then molding. Alternatively, the zeolite may be previously molded, and 
the above-mentioned metals may be incorporated into the molding. Examples 
of the binder used in molding of the zeolite include clay minerals such as 
kaolin, attapulgite, montmorillonite, bentonite, allophane and sepiolite, 
silica and alumina. Alternatively, the catalyst may be a binder-less 
zeolite molding directly synthesized without the use of a binder. Further, 
the zeolite may be wash-coated on a honeycomb-structured base material 
made of cordierite, a metal or the like. 
The nitrogen oxides, carbon monoxide and hydrocarbons contained in an 
oxygen-rich exhaust gas can be removed by bringing the exhaust gas into 
contact with the exhaust gas purification catalyst according to the 
present invention in any conventional manner. Specific examples of such an 
exhaust gas include exhaust gases discharged, for example, from the 
internal combustion engines of automobiles, particularly exhaust gases 
produced at a high air/fuel ratio (i.e., in the lean burn region). 
There is no particular limitation in the operating conditions of the 
catalyst according to the present invention, but the preferable 
temperature is 100.degree. C. to 900.degree. C., more preferably 
150.degree. C. to 800.degree. C. and the preferable space velocity is 
1,000 to 500,000 hr.sup.-1. The "space velocity" means a value of a gas 
flow rate (cc/hr) divided by a catalyst volume (cc). 
The above-mentioned catalyst for removing an exhaust gas exhibits no change 
in the performance even when applied to an exhaust gas containing carbon 
monoxide, hydrocarbons and hydrogen but not containing an excessive amount 
of oxygen. 
EXAMPLES 
The present invention will now be further illustrated by, but is by no 
means limited to, the following Examples. 
Comparative Example 1: Preparation of Comparative 
Catalyst 1 
A ZSM-5-like zeolite was synthesized according to the method described in 
Example 5 of Japanese Unexamined Patent Publication (Kokai) No. 59-54620. 
The zeolite had the following composition in terms of mole ratios of 
oxides on an anhydrous basis: 
EQU 1.1Na.sub.2 O.Al.sub.2 O.sub.3.40 SiO.sub.2 
The zeolite was ion-exchanged with an aqueous ammonium chloride solution, 
200 g of the resultant ammonium type ZSM-5 was put in 1800 ml of a 1.09 
mol/liter aqueous barium chloride solution, and the mixture was stirred at 
80.degree. C. for 16 hrs. The stirred mixture was subjected to 
solid-liquid separation, washed with water, subsequently put in 700 ml of 
a 0.23 mol/liter aqueous cobalt (II) acetate tetrahydrate solution, and 
the mixture was stirred at 80.degree. C. for 16 hrs. The slurry was 
subjected to solid-liquid separation, the resultant zeolite cake was put 
in a freshly prepared aqueous solution having the above-mentioned 
composition, and the above-mentioned procedure was repeated. The slurry 
was subjected to solid-liquid separation, washed with water, and dried at 
110.degree. C. for 10 hrs, to prepare a comparative catalyst 1. The 
catalyst was subjected to a chemical analysis to determine the barium and 
cobalt contents, and as a result, it was found that the barium and cobalt 
contents were respectively 0.58 time and 0.49 time as a divalent cobalt, 
based on the number of moles of Al.sub.2 O.sub.3 in the zeolite. 
Example 1: Preparation of Catalyst 1 
A 15 g amount of comparative catalyst 1 prepared in Comparative Example 1 
was put in 22 ml of a 0.025 mol/liter aqueous silver nitrate solution, 
dried under a reduced pressure while stirring, and further dried at 
110.degree. C. for 16 hrs, to thereby prepare a catalyst 1. The catalyst 
was subjected to a chemical analysis to determine the contents of barium, 
cobalt and silver, and as a result, it was found that barium, cobalt and 
silver were contained in respective amounts of 0.58 time, 0.49 time as 
divalent cobalt and 0.1 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 2: Preparation of Catalyst 2 
A 15 g amount of comparative catalyst 1 prepared in Comparative Example 1 
was put in 43 ml of a 0.05 mol/liter aqueous silver nitrate solution, 
dried under a reduced pressure while stirring, and further dried at 
110.degree. C. for 16 hrs, to thereby prepare a catalyst 2. The catalyst 
was subjected to a chemical analysis to determine the contents of barium, 
cobalt and silver, and as a result, it was found that barium, cobalt and 
silver were contained in respective amounts of 0.58 time, 0.49 time as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 3: Preparation of Catalyst 3 
A 15 g amount of comparative catalyst 1 prepared in Comparative Example 1 
was put in 43 ml of a 0.05 mol/liter aqueous silver nitrate solution, and 
the mixture was stirred at 80.degree. C. for 16 hrs. The stirred mixture 
was subjected to solid-liquid separation, washed with water, and dried at 
110.degree. C. for 16 hrs, to thereby prepare a catalyst 3. The catalyst 
was subjected to a chemical analysis to determine the contents of barium, 
cobalt and silver, and as a result, it was found that barium, cobalt and 
silver were contained in respective amounts of 0.57 time, 0.48 time as 
divalent cobalt and 0.13 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 4: Preparation of Catalyst 4 
A 20 g amount of ammonium type ZSM-5 prepared in Comparative Example 1 was 
put in 180 ml of a 1 09 mol/liter aqueous barium chloride solution, and 
the mixture was stirred at 80.degree. C. for 16 hrs. The stirred mixture 
was subjected to solid-liquid separation, thoroughly, washed with water, 
subsequently put in 180 ml of a 0.23 mol/liter aqueous cobalt (II) nitrate 
tetrahydrate solution, and the mixture was stirred at 80.degree. C. for 16 
hrs. The slurry was subjected to solid-liquid separation, the resultant 
zeolite cake was put in a freshly prepared aqueous solution having the 
above-mentioned composition, and the above-mentioned procedure was 
repeated. The slurry was subjected to solid-liquid separation, thoroughly 
washed with water, and dried at 110.degree. C. for 10 hrs, and the 
procedure of Example 2 was repeated to prepare a catalyst 4. The catalyst 
was subjected to a chemical analysis to determine the contents of barium, 
cobalt and silver, and as a result, it was found that barium, cobalt and 
silver were contained respectively in amounts of 0.52 time, 0.32 time as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 5: Preparation of Catalyst 5 
A 20 g amount of ammonium type ZSM-5 prepared in Comparative Example 1 was 
put in 180 ml of a 0.2 mol/liter aqueous silver nitrate solution, and the 
mixture was stirred at 80.degree. C. for 16 hrs. The stirred mixture was 
subjected to solid-liquid separation, the resultant zeolite cake was put 
in a freshly prepared aqueous solution having the above-mentioned 
composition, and the above-mentioned procedure was repeated. The slurry 
was subjected to solid-liquid separation, thoroughly washed with water, 
subsequently put in 180 ml of a 1.09 mol/liter aqueous barium chloride 
solution, and the mixture was stirred at 80.degree. C. for 16 hrs. The 
slurry was subjected to solid-liquid separation, thoroughly washed with 
water, put in 180 ml of a 0.23 mol/liter aqueous cobalt (II) acetate 
tetrahydrate solution, and the mixture was stirred at 80.degree. C. for 16 
hrs. The slurry was subjected to solid-liquid separation, thoroughly 
washed with water, and dried at 110.degree. C. for 20 hrs, to prepare a 
catalyst 5. The catalyst was subjected to a chemical analysis to determine 
the contents of barium, cobalt and silver, and as a result, it was found 
that barium, cobalt and silver were contained respectively in amounts of 
0.67 time, 0.58 time as divalent cobalt and 0 09 time, based on the number 
of moles of Al.sub.2 O.sub.3 in the zeolite. 
Example 6: Preparation of Catalyst 6 
A 20 g amount of ammonium type ZSM-5 prepared in Comparative Example 1 was 
put in 180 ml of a 1.09 mol/liter aqueous barium chloride solution, and 
the mixture was stirred at 80.degree. C. for 16 hrs. The slurry was 
subjected to solid-liquid separation, subsequently put in 180 ml of a 0.2 
mol/liter aqueous silver nitrate solution, and the mixture was stirred at 
80.degree. C. for 16 hrs. The slurry was subjected to solid-liquid 
separation, subsequently put in 180 ml of a 0.1 mol/liter aqueous cobalt 
(II) acetate tetrahydrate solution, and the mixture was stirred at 
80.degree. C. for 16 hrs. The slurry was subjected to solid-liquid 
separation, thoroughly washed with water, and dried at 110.degree. C. for 
20 hrs, to prepare a catalyst 6. The catalyst was subjected to a chemical 
analysis to determine the contents of barium, cobalt and silver, and as a 
result, it was found that barium, cobalt and silver were contained 
respectively in amounts of 0.56 time, 0.58 time as divalent cobalt and 
0.21 time, based on the number of moles of Al.sub.2 O.sub.3 in the 
zeolite. 
Comparative Example 2: Preparation of Comparative Catalyst 2 
A 200 g amount of ammonium type ZSM-5 prepared in Comparative Example 1 was 
put in 1800 ml of a 1.09 mol/liter aqueous strontium chloride solution, 
and the mixture was stirred at 80.degree. C for 16 hrs. The slurry was 
subjected to solid-liquid separation, subsequently put in 1800 ml of a 
0.23 mol/liter aqueous cobalt (II) acetate tetrahydrate solution, and the 
mixture was stirred at 80.degree. C. for 16 hrs. The slurry was subjected 
to solid-liquid separation, the resultant zeolite cake was put in a 
freshly prepared aqueous solution having the above-described composition, 
and the above-mentioned procedure was repeated. The slurry was subjected 
to solid-liquid separation, thoroughly washed with water, and dried at 
110.degree. C. for 10 hrs, to prepare a comparative catalyst 2. The 
catalyst was subjected to a chemical analysis to determine the contents of 
strontium and cobalt, and as a result, it was found that strontium and 
cobalt were contained respectively in amounts of 0.23 time and 1.12 times 
as divalent cobalt, based on the number of moles of Al.sub.2 O.sub.3 in 
the zeolite. 
Example 7: Preparation of Catalyst 7 
A 15 g amount of comparative catalyst 2 prepared in Comparative Example 2 
was put in 43 ml of a 0.05 mol/liter aqueous silver nitrate solution, 
dried under a reduced pressure while stirring, and further dried at 
110.degree. C. for 16 hrs, to thereby prepare a catalyst 7. The catalyst 
was subjected to a chemical analysis to determine the contents of 
strontium, cobalt and silver, and as a result, it was found that 
strontium, cobalt and silver were contained in respective amounts of 0.23 
time, 1.12 times as divalent cobalt and 0.4 time, based on the number of 
moles of Al.sub.2 O.sub.3 in the zeolite. 
Example 8: Evaluation of Activity of Catalysts 
Catalysts 1 to 7 and comparative catalyst 1 and 2 were each press-molded 
and then crushed to regulate the size of granules to 12 to 20 meshes, and 
an atmospheric fixed bed reaction tube was packed with 1 g of each of the 
granular catalysts. The temperature of the catalyst bed was raised to 
500.degree. C. while passing a gas having the following composition 
(hereinafter referred to as "reaction gas") through the reaction tube at a 
flow rate of 1000 ml/min, and the temperature was maintained at 
500.degree. C. for 0.5 hrs, to thereby conduct a pretreatment. Thereafter, 
the temperature of the catalyst bed was raised from 300.degree. C. to 
500.degree. C. In this case, the temperature was kept constant at each 
50.degree. C. increment to measure the catalytic activity at respective 
temperatures. The NO.sub.x conversions at respective temperatures after 
the state had become steady are shown in Table 1. The NO.sub.2 conversion 
can be determined by the following equation. 
##EQU1## 
wherein NO.sub.xin : NO.sub.x concentration at inlet of fixed bed reaction 
tube; and 
No.sub.xout : NO.sub.x concentration at outlet of fixed bed reaction tube. 
In all of the catalysts, little carbon monoxide was detected at 450.degree. 
C. or above, and few hydrocarbons were detected at 400.degree. C. or 
above. 
______________________________________ 
Composition of reaction gas: 
______________________________________ 
NO 700 ppm 
O.sub.2 
4% 
H.sub.2 
330 ppm 
CO 1000 ppm 
H.sub.2 O 
3% 
CO.sub.2 
10% 
N.sub.2 
balance 
______________________________________ 
TABLE 1 
______________________________________ 
Results of Evaluation of Activity 
NO.sub.x conversion (%) 
Composi- 300.degree. 
350.degree. 
400.degree. 
450.degree. 
500.degree. 
Catalyst 
tion C. C. C. C. C. 
______________________________________ 
Catalyst 1 
Ba Co Ag 11 35 63 50 39 
Catalyst 2 
Ba Co Ag 20 70 70 57 44 
Catalyst 3 
Ba Co Ag 14 48 65 52 38 
Catalyst 4 
Ba Co AgC 19 69 71 57 43 
Catalyst 5 
Ag Ba o 15 36 40 41 34 
Catalyst 6 
Ba Ag Co 23 52 51 41 29 
Catalyst 7 
Sr Co Ag 14 51 54 42 32 
Comp. Ba Co -- 7 34 52 42 30 
Catalyst 1 
Comp. Sr Co -- 8 33 51 43 31 
Catalyst 2 
______________________________________ 
Example 9: Evaluation of Durability of Catalysts 
Catalyst 1 and comparative catalyst 1 were subjected to an endurance 
treatment at 800.degree. C. for 5 hrs, while flowing the above-mentioned 
reaction gas, and then subjected to a measurement of the catalytic 
activity in the same manner as that of Example 8. The NO.sub.x conversions 
at respective temperatures after the state had become steady are shown in 
Table 2. 
TABLE 2 
______________________________________ 
Results of Evaluation of Activity 
NO.sub.x conversion (%) 
Composi- 300.degree. 
350.degree. 
400.degree. 
450.degree. 
500.degree. 
Catalyst 
tion C. C. C. C. C. 
______________________________________ 
Catalyst 
1Co Ba Ag 10 29 46 51 42 
Comp. 1Co Ba -- 8 27 45 42 33 
Catalyst 
______________________________________ 
Example 10: Preparation of Catalyst 8 
A 15 g amount of comparative catalyst 1 prepared in Comparative Example 1 
was put in 43 ml of a 0.05 mol/liter aqueous nickel nitrate solution, 
dried under a reduced pressure while stirring, and further dried at 
110.degree. C. for 16 hrs, to thereby prepare a catalyst 8. The catalyst 
was subjected to a chemical analysis to determine the contents of barium, 
cobalt and nickel, and as a result, it was found that barium, cobalt and 
nickel were contained in respective amounts of 0.58 time, 0.49 times as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 11: Preparation of Catalyst 9 
A 20 g amount of ammonium type ZSM-5 prepared in Comparative Example 1 was 
put in 180 ml of a 1.09 mol/liter aqueous barium chloride solution, and 
the mixture was stirred at 80.degree. C for 16 hrs. The stirred mixture 
was subjected to solid-liquid separation, thoroughly washed with water, 
subsequently put in 180 ml of a 0.23 mol/liter aqueous cobalt (II) nitrate 
tetrahydrate solution, and the mixture was stirred at 80.degree. C. for 16 
hrs. The slurry was subjected to solid-liquid separation, the resultant 
zeolite cake was put in a freshly prepared aqueous solution having the 
above-described composition, and the above-mentioned procedure was 
repeated. The slurry was subjected to solid-liquid separation, thoroughly 
washed with water, and dried at 110.degree. C. for 10 hrs, and the 
procedure of Example 10 was repeated to prepare a catalyst 9. The catalyst 
was subjected to a chemical analysis to determine the contents of barium, 
cobalt and nickel, and as a result, it was found that barium, cobalt and 
nickel were contained respectively in amounts of 0.52 time, 0.32 time as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 12: Preparation of Catalyst 10 
A catalyst 10 was prepared in the same manner as that of Example 11, except 
that nickel chloride was used instead of nickel nitrate. The catalyst was 
subjected to a chemical analysis to determine the contents of barium, 
cobalt and nickel, and as a result, it was found that barium, cobalt and 
nickel were contained respectively in amounts of 0.52 time, 0.32 time as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 13: Preparation of Catalyst 11 
A catalyst 11 was prepared in the same manner as that of Example 11, except 
that nickel acetate was used instead of nickel nitrate. The catalyst was 
subjected to a chemical analysis to determine the contents of barium, 
cobalt and nickel, and as a result, it was found that barium, cobalt and 
nickel were contained respectively in amounts of 0.52 time, 0.32 time as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 14: Preparation of Catalyst 12 
A 15 g amount of comparative catalyst 2 prepared in Comparative Example 2 
was put in 43 ml of a 0.05 mol/liter aqueous nickel nitrate solution, 
dried under a reduced pressure while stirring, and further dried at 
110.degree. C. for 16 hrs, to thereby prepare a catalyst 12. The catalyst 
was subjected to a chemical analysis to determine the contents of 
strontium, cobalt and nickel, and as a result, it was found that 
strontium, cobalt and nickel were contained in respective amounts of 0.23 
time, 1.12 times as divalent cobalt and 0.4 time, based on the number of 
moles of Al.sub.2 O.sub.3 in the zeolite. 
Comparative Example 3: Preparation of Comparative Catalyst 3 
A comparative catalyst 3 was prepared in the same manner as that of 
Comparative Example 2, except that magnesium was used as the alkaline 
earth metal. The comparative catalyst was subjected to a chemical analysis 
to determine the contents of magnesium and cobalt, and as a result, it was 
found that magnesium and cobalt were contained in respective amounts of 
0.18 time and 1.08 times as divalent cobalt, based on the number of moles 
of Al.sub.2 O.sub.3 in the zeolite. 
Example 15: Preparation of Catalyst 13 
A 15 g amount of comparative catalyst 3 prepared in Comparative Example 3 
was put in 43 ml of a 0.05 mol/liter aqueous nickel nitrate solution, 
dried under a reduced pressure while stirring, and further dried at 
110.degree. C. for 16 hrs, to thereby prepare a catalyst 13. The catalyst 
was subjected to a chemical analysis to determine the contents of 
magnesium, cobalt and nickel, and as a result, it was found that 
magnesium, cobalt and nickel were contained in respective amounts of 0.18 
time, 1.08 times as divalent cobalt and 0.4 time, based on the number of 
moles of Al.sub.2 O.sub.3 in the zeolite. 
Comparative Example 4: Preparation of Comparative Catalyst 4 
A comparative catalyst 4 was prepared in the same manner as that of 
Comparative Example 2, except that calcium was used as the alkaline earth 
metal. The comparative catalyst was subjected to a chemical analysis to 
determine the contents of calcium and cobalt, and as a result, it was 
found that calcium and cobalt were contained in respective amounts of 0.16 
time and 1.04 times as divalent cobalt, based on the number of moles of 
Al.sub.2 O.sub.3 in the zeolite. 
Example 16: Preparation of Catalyst 14 
A 15 g amount of comparative catalyst 4 prepared in Comparative Example 4 
was put in 43 ml of a 0.05 mol/liter aqueous nickel nitrate solution, 
dried under a reduced pressure while stirring and further dried at 
110.degree. C. for 16 hrs, to thereby prepare a catalyst 14. The catalyst 
was subjected to a chemical analysis to determine the contents of calcium, 
cobalt and nickel, and as a result, it was found that calcium, cobalt and 
nickel were contained in respective amounts of 0.16 time, 1.04 times as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Comparative Example 5: Preparation of Comparative Catalyst 5 
A 20 g amount of ammonium type ZSM-5 prepared in Comparative Example 1 was 
put in 180 ml of a 0.23 mol/litter aqueous nickel acetate tetrahydrate 
solution, and the mixture was stirred at 80.degree. C. for 16 hrs. The 
stirred mixture was subjected to solid-liquid separation, the resultant 
zeolite cake was put in a freshly prepared aqueous solution having the 
above-mentioned composition, and the above-mentioned procedure was 
repeated. The slurry was subjected to solid-liquid separation, washed with 
water, and dried at 110.degree. C. for 10 hrs, to prepare a comparative 
catalyst 5. The catalyst was subjected to a chemical analysis to determine 
the nickel content, and as a result, it was found that nickel was 
contained as divalent nickel in an amount of 1.40 times the number of 
moles of Al.sub.2 O.sub.3 in the zeolite. 
Example 17: Preparation of Catalyst 15 
A catalyst 15 was prepared in the same manner as that of Example 10, except 
that zinc nitrate was used instead of nickel nitrate. The catalyst was 
subjected to cobalt and zinc, and as a result, it was found that barium, 
cobalt and zinc were contained in respective amounts of 0.58 time, 0.49 
times as divalent cobalt and 0.4 time, based on the number of moles of 
Al.sub.2 O.sub.3 in the zeolite. 
Example 18: Preparation of Catalyst 16 
A catalyst 16 was prepared in the same manner as that of Example 11, except 
that zinc nitrate was used instead of nickel nitrate. The catalyst was 
subjected to a chemical analysis to determine the contents of barium, 
cobalt and zinc, and as a result, it was found that barium, cobalt and 
zinc were contained in respective amounts of 0.52 time, 0.32 times as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 19: Preparation of Catalyst 17 
A catalyst 17 was prepared in the same manner as that of Example 14, except 
that zinc nitrate was used instead of nickel nitrate. The catalyst was 
subjected to a chemical analysis to determine the contents of strontium, 
cobalt and zinc, and as a result, it was found that strontium, cobalt and 
zinc were contained in respective amounts of 0.23 time, 1.12 times as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 20: Evaluation of Activity of Catalysts 
Catalysts 8 to 17 and comparative catalysts 1 to 5 were subjected to a 
measurement of the catalytic activity in the same manner as that of 
Example 8. The NO.sub.x conversions at respective temperatures after the 
state had become steady are shown in Table 3. 
In all of the catalysts, little carbon monoxide was detected at 450.degree. 
C. or above, and few hydrocarbons were detected at 400.degree. C. or 
above. 
TABLE 3 
______________________________________ 
Results of Evaluation of Activity 
NO.sub.x conversion (%) 
Composi- 300.degree. 
350.degree. 
400.degree. 
450.degree. 
500.degree. 
Catalyst 
tion C. C. C. C. C. 
______________________________________ 
Catalyst 8 
Co Ba Ni 12 43 56 44 34 
Catalyst 9 
Co Ba Ni 16 54 54 43 32 
Catalyst 10 
Co Ba Ni 11 35 51 42 30 
Catalyst 11 
Co Ba Ni 14 45 53 44 32 
Catalyst 12 
Co Sr Ni 15 52 54 44 33 
Catalyst 13 
Co Mg Ni 13 50 53 43 33 
Catalyst 14 
Co Ca Ni 14 51 54 42 32 
Catalyst 15 
Co Ba Zn 9 35 62 52 43 
Catalyst 16 
Co Ba Zn 7 15 63 51 42 
Catalyst 17 
Co Sr Zn 8 25 58 50 40 
Comp. Co Ba -- 7 34 52 42 30 
Catalyst 1 
Comp. Co Sr -- 8 33 51 43 31 
Catalyst 2 
Comp. Co Mg -- 6 27 49 41 29 
Catalyst 3 
Comp. Co Ca -- 6 27 48 39 29 
Catalyst 4 
Comp. -- -- Ni 10 31 36 32 28 
Catalyst 5 
______________________________________ 
Example 21: Evaluation of Durability of Catalysts 
Catalysts 8 to 17 and Comparative catalysts 1 to 5 were subjected to an 
endurance treatment at 800.degree. C. for 5 hrs while flowing the 
above-mentioned reaction gas, and then subjected to a measurement of the 
catalytic activity in the same manner as that of Example 20. The NO.sub.x 
conversions at respective temperatures after the state had become steady 
are shown in Table 4. 
TABLE 4 
______________________________________ 
Results of Evaluation of Activity 
NO.sub.x conversion (%) 
Composi- 300.degree. 
350.degree. 
400.degree. 
450.degree. 
500.degree. 
Catalyst 
tion C. C. C. C. C. 
______________________________________ 
Catalyst 8 
Co Ba Ni 10 33 54 60 52 
Catalyst 9 
Co Ba Ni 8 25 53 59 50 
Catalyst 10 
Co Ba Ni 7 23 50 61 51 
Catalyst 11 
Co Ba Ni 7 21 49 51 44 
Catalyst 12 
Co Sr Ni 7 23 52 58 46 
Catalyst 13 
Co Mg Ni 6 22 48 53 44 
Catalyst 14 
Co Ca Ni 7 21 49 54 45 
Catalyst 15 
Co Ba Zn 8 26 47 45 40 
Catalyst 16 
Co Ba Zn 7 20 40 43 39 
Catalyst 17 
Co Sr Zn 7 20 41 44 38 
Comp. Co Ba -- 8 27 45 42 33 
Catalyst 1 
Comp. Co Sr -- 7 26 44 40 32 
Catalyst 2 
Comp. Co Mg -- 5 21 40 37 29 
Catalyst 3 
Comp. Co Ca -- 6 23 41 39 29 
Catalyst 4 
Comp. -- -- Ni 2 7 21 32 28 
Catalyst 5 
______________________________________ 
Example 22: Preparation of Catalyst 18 
A 15 g amount of comparative catalyst 1 prepared in Comparative Example 1 
was put in 43 ml of a 0.05 mol/liter aqueous tetraamminedichloroplatinum 
solution, dried under a reduced pressure while stirring, and further dried 
at 110.degree. C. for 16 hrs, to thereby prepare a catalyst 18. The 
catalyst was subjected to a chemical analysis to determine the contents of 
barium, cobalt and platinum, and as a result, it was found that barium, 
cobalt and platinum were contained in respective amounts of 0.58 time, 
0.49 times as divalent cobalt and 0.4 time, based on the number of moles 
of Al.sub.2 O.sub.3 in the zeolite. 
Example 23: Preparation of Catalyst 19 
A 15 g amount of comparative catalyst 1 prepared in Comparative Example 1 
was put in 22 ml of a 0.025 mol/liter aqueous tetraamminedichloroplatinum 
solution, dried under a reduced pressure while stirring, and further dried 
at 110.degree. C. for 16 hrs, to thereby prepare a catalyst 19. The 
catalyst was subjected to a chemical analysis to determine the contents of 
barium, cobalt and platinum, and as a result, it was found that barium, 
cobalt and platinum were contained in respective amounts of 0.58 time, 
0.49 times as divalent cobalt and 0.1 time, based on the number of moles 
of Al.sub.2 O.sub.3 in the zeolite. 
Example 24: Preparation of Catalyst 20 
A 200 g amount of ammonium type ZSM-5 prepared in Comparative Example 1 was 
put in 1800 ml of a 1.09 mol/liter aqueous barium chloride solution, and 
the mixture was stirred at 80.degree. C. for 16 hrs. The stirred mixture 
was subjected to solid-liquid separation, thoroughly washed with water, 
subsequently put in 1800 ml of a 0.23 mol/liter cobalt (II) nitrate 
tetrahydrate solution, and the mixture was stirred at 80.degree. C. for 16 
hrs. The slurry was subjected to solid-liquid separation, the resultant 
zeolite cake was put in a freshly prepared aqueous solution having the 
above-mentioned composition, and the above-mentioned procedure was 
repeated. The slurry was subjected to solid-liquid separation, thoroughly 
washed with water, and dried at 110.degree. C. for 20 hrs, to prepare 
ZSM-5 containing cobalt and barium. 15 g of the ZSM-5 containing cobalt 
and barium was put in 43 ml of a 0.05 mol/liter aqueous 
tetraamminedichloroplatinum solution, dried under a reduced pressure while 
stirring, and further dried at 110.degree. C. for 16 hrs, to thereby 
prepare a catalyst 20. The catalyst was subjected to a chemical analysis 
to determine the contents of barium, cobalt and platinum, and as a result, 
it was found that barium, cobalt and platinum were contained in respective 
amounts of 0.52 time, 0.32 time as divalent cobalt and 0.4 time, based on 
the number the moles of Al.sub.2 O.sub.3 in the zeolite. 
Example 25: Preparation of Catalyst 21 
A 15 g of the ZSM-5 containing cobalt and barium prepared in Example 24 was 
put in 43 ml of a 0.05 mol/liter aqueous manganese acetate solution, dried 
under a reduced pressure while stirring, and further dried at 110.degree. 
C. for 16 hrs, to thereby prepare a catalyst 21. The catalyst was 
subjected to a chemical analysis to determine the contents of barium, 
cobalt and manganese, and as a result, it was found that barium, cobalt 
and manganese were contained in respective amounts of 0.52 time, 0.32 time 
as divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 26: Preparation of Catalyst 22 
A catalyst 22 was prepared in the same manner as that of Example 25, except 
that manganese nitrate was used instead of manganese acetate. The catalyst 
was subjected to a chemical analysis to determine the contents of barium, 
cobalt and manganese, and as a result, it was found that barium, cobalt 
and manganese were contained in respective amounts of 0.52 time, 0.32 time 
as divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 27: Preparation of Catalyst 23 
A catalyst 23 was prepared in the same manner as that of Example 25, except 
that manganese chloride was used instead of manganese acetate. The 
catalyst was subjected to a chemical analysis to determine the contents of 
barium, cobalt and manganese, and as a result, it was found that barium, 
cobalt and manganese were contained in respective amounts of 0.52 time, 
0.32 time as divalent cobalt and 0.4 time, based on the number of moles of 
Al.sub.2 O.sub.3 in the zeolite. 
Example 28: Preparation of Catalyst 24 
A 15 g amount of comparative catalyst 2 prepared in Comparative Example 2 
was put in 43 ml of a 0 05 mol/liter aqueous tetraamminedichloroplatinum 
solution, dried under a reduced pressure while stirring, and further dried 
at 110.degree. C. for 16 hrs, to thereby prepare a catalyst 24. The 
catalyst was subjected to a chemical analysis to determine the contents of 
strontium, cobalt and platinum, and as a result, it was found that 
strontium, cobalt and platinum were contained in respective amounts of 
0.23 time, 1.12 times as divalent cobalt and 0.4 time, based on the number 
of moles of Al.sub.2 O.sub.3 in the zeolite. 
Example 29: Preparation of Catalyst 25 
A catalyst 25 was prepared in the same manner as that of Example 287, 
except that manganese nitrate was used instead of 
tetraamminedichloroplatinum. The catalyst was subjected to a chemical 
analysis to determine the contents of strontium, cobalt and manganeses, 
and as a result, it was found that strontium, cobalt and manganese were 
contained in respective amounts of 0.23 time, 1.12 times as divalent 
cobalt and 0.4 time, based on the number of moles of Al.sub.2 O.sub.3 in 
the zeolite. 
Example 30: Evaluation of Activity of Catalysts 
With respect to catalysts 18 to 25 and comparative catalyst 1 and 2, in the 
same manner as that of Example 8, the temperature of the catalyst bed was 
raised from 250.degree. C. to 450.degree. C. In this case, the temperature 
was kept constant at each 50.degree. C. increment to measure the catalytic 
activity at respective temperatures. The NO.sub.x conversions at 
respective temperature after the state had become steady are shown in 
Table 5. 
In the comparative catalysts, little carbon monoxide was detected at 
450.degree. C. or above, and few hydrocarbons were detected at 400.degree. 
C. or above. On the other hand, in the catalysts of examples of the 
present invention, little carbon monoxide was detected at 400.degree. C. 
or above, and few hydrocarbons were detected at 350.degree. C. or above. 
TABLE 5 
______________________________________ 
NO.sub.x conversion (%) 
Composi- 250.degree. 
300.degree. 
350.degree. 
400.degree. 
450.degree. 
Catalyst 
tion C. C. C. C. C. 
______________________________________ 
Catalyst 18 
Co Ba Pt 42 36 30 18 10 
Catalyst 19 
Co Ba Pt 40 25 20 15 9 
Catalyst 20 
Co Ba Pt 38 30 24 15 8 
Catalyst 21 
Co Ba Mn 20 41 52 45 36 
Catalyst 22 
Co Ba Mn 19 35 54 41 34 
Catalyst 23 
Co Ba Mn 16 23 47 45 36 
Catalyst 24 
Co Sr Pt 41 34 28 17 10 
Catalyst 25 
Co Sr Mn 18 34 53 43 35 
Comp. Co Ba -- 4 7 35 52 42 
Catalyst 1 
Comp. Co Sr -- 4 8 33 51 43 
Catalyst 2 
______________________________________ 
Example 31: Evaluation of Durability of Catalysts 
Individual catalysts were subjected to an endurance treatment at 
800.degree. C. for 5 hrs, while flowing the above-described reaction gas, 
and the subjected to a measurement of the catalytic activity in the same 
manner as that of Example 30. The NO.sub.x conversions at respective 
temperatures after the state had become steady are shown in Table 6. 
TABLE 6 
______________________________________ 
NO.sub.x conversion (%) 
Composi- 250.degree. 
300.degree. 
350.degree. 
400.degree. 
450.degree. 
Catalyst 
tion C. C. C. C. C. 
______________________________________ 
Catalyst 18 
Co Ba Pt 37 37 29 17 9 
Catalyst 19 
Co Ba Pt 23 30 22 14 9 
Catalyst 20 
Co Ba Pt 45 38 27 18 10 
Catalyst 21 
Co Ba Mn 10 12 35 40 38 
Catalyst 22 
Co Ba Mn 10 14 32 39 38 
Catalyst 23 
Co Ba Mn 10 16 35 40 39 
Catalyst 24 
Co Sr Pt 38 34 27 15 10 
Catalyst 25 
Co Sr Mn 10 14 33 39 38 
Comp. Co Ba -- 4 8 27 45 42 
Catalyst 1 
Comp. Co Sr -- 4 7 26 44 40 
Catalyst 2 
______________________________________ 
Example 32: Preparation of Catalyst 26 
A 200 g amount of ammonium type ZSM-5 prepared in Comparative Example 1 was 
put in 1800 ml of a 1.09 mol/liter aqueous barium chloride solution, and 
the mixture was stirred at 80.degree. C. for 16 hrs. The stirred mixture 
was subjected to solid-liquid separation, thoroughly washed with water, 
subsequently put in 1800 ml of a 0.23 mol/liter cobalt (II) nitrate 
tetrahydrate solution, and the mixture was stirred at 80.degree. C. for 16 
hrs. The slurry was subjected to solid-liquid separation, the resultant 
zeolite cake was put in a freshly prepared aqueous solution having the 
above-mentioned composition, and the above-mentioned procedure was 
repeated. The slurry was subjected to solid-liquid separation, thoroughly 
washed with water, and dried at 110.degree. C. for 20 hrs, to prepare 
ZSM-5 containing cobalt and barium. 15 g of the ZSM-5 containing cobalt 
and barium was put in 43 ml of a 0.05 mol/liter aqueous copper acetate 
solution, dried under a reduced pressure while stirring, and further dried 
at 110.degree. C. for 16 hrs, to thereby prepare a catalyst 26. The 
catalyst was subjected to a chemical analysis to determine the contents of 
barium, cobalt and analysis to determine the contents of barium, cobalt 
and copper, and as a result, it was found that barium, cobalt and copper 
were contained in respective amounts of 0.52 time, 0.32 time as divalent 
cobalt and 0.4 time, based on the number of moles of Al.sub.2 O.sub.3 in 
the zeolite. 
Example 33: Preparation of Catalyst 27 
A catalyst 27 was prepared in the same manner as that of Example 32, except 
that rhodium nitrate was used instead of copper acetate. The catalyst was 
subjected to a chemical analysis to determine the contents of barium, 
cobalt and rhodium, and as a result, it was found that barium, cobalt and 
rhodium were contained in respective amounts of 0.52 time, 0.32 time as 
divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 34: Preparation of Catalyst 28 
A 15 g amount of comparative catalyst 2 prepared in Comparative Example 2 
was put in 43 ml of a 0.05 mol/liter aqueous copper nitrate solution, 
dried under a reduced pressure while stirring, and further dried at 
110.degree. C. for 16 hrs, to thereby prepare a catalyst 28. The catalyst 
was subjected to a chemical analysis to determine the contents of 
strontium, cobalt and copper, and as a result, it was found that 
strontium, cobalt and copper were contained in respective amounts of 0.23 
time, 1.12 times as divalent cobalt and 0.4 time, based on the number of 
moles of Al.sub.2 O.sub.3 in the zeolite. 
Example 35: Preparation of Catalyst 29 
A catalyst 29 was prepared in the same manner as that of Example 34, except 
that rhodium nitrate was used instead of copper nitrate. The catalyst was 
subjected to a chemical analysis to determine the contents of strontium, 
cobalt and rhodium, and as a result, it was found that strontium, cobalt 
and rhodium were contained in respective amounts of 0.23 time, 1.12 time 
as divalent cobalt and 0.4 time, based on the number of moles of Al.sub.2 
O.sub.3 in the zeolite. 
Example 36: Evaluation of Activity of Catalysts 
Catalysts 26 to 29 comparative catalysts 1 and 2 were subjected to a 
measurement of the catalytic activity in the same manner as that of 
Example 8. The NO.sub.x conversions at respective temperatures after the 
state had become steady are shown in Table 7. 
In the comparative catalysts, little carbon monoxide was detected at 
450.degree. C. or above, and few hydrocarbons were detected at 400.degree. 
C. or above. On the other hand, in the catalysts of examples of the 
present invention, little carbon monoxide was detected at 400.degree. C. 
or above, and few hydrocarbons were detected at 350.degree. C. or above. 
TABLE 7 
______________________________________ 
NO.sub.x conversion (%) 
Composi- 250.degree. 
300.degree. 
350.degree. 
400.degree. 
450.degree. 
Catalyst 
tion C. C. C. C. C. 
______________________________________ 
Catalyst 26 
Co Ba Cu 27 36 49 37 30 
Catalyst 27 
Co Ba Rh 20 26 21 20 15 
Catalyst 28 
Co Sr Cu 26 37 47 38 31 
Catalyst 29 
Co Sr Rh 19 25 22 21 16 
Comp. Co Ba -- 4 7 35 52 42 
Catalyst 1 
Comp. Co Sr -- 4 8 33 51 43 
Catalyst 2 
______________________________________ 
As apparent from Tables 1 to 7, the catalysts of the present invention are 
superior to the comparative catalysts in the capability thereof of 
purifying an exhaust gas containing an excessive amount of oxygen, in 
particular, the capability of removing nitrogen oxides. 
Specifically, 
1) the addition of silver to cobalt and an alkaline earth metal contributes 
to an improvement in the capability of removing nitrogen oxides, 
2) the addition of nickel and/or zinc to cobalt and an alkaline earth metal 
contributes to an improvement in the capability of removing nitrogen 
oxides at a high temperature of 350.degree. C. or above, 
3) the addition of platinum and/or manganese to cobalt and an alkaline 
earth metal contributes to an improvement in the capability of removing 
nitrogen oxides at a low temperature of 350.degree. C. or below, and 
4) the addition of copper and/or rhodium to cobalt and an alkaline earth 
metal contributes to a slight improvement in the capability of removing 
nitrogen oxides at a low temperature. 
Therefore, nitrogen oxides, carbon monoxide and hydrocarbons can be removed 
with a high conversion by bringing the catalyst of the present invention 
into contact with an exhaust gas even when the exhaust gas contains an 
excessive amount of oxygen.