Method of purifying exhaust gases

The exhaust gases, such as of an internal combustion engine are contacted with a catalyst containing copper in the presence of hydrocarbons in an oxidizing atmosphere, whereby nitrogen oxides are reduced from the exhaust gases. Then, the exhaust gases are preferably contacted with an oxidizing catalyst. The former catalyst contains copper loaded on a porous support formed from alumina, silica, silica-alumina or zeolite or mixture thereof. The support is preferably a monolithic body having a first portion loaded with copper and a second portion loaded with a metal or metals defining the oxidizing catalyst.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of reducing nitrogen oxides 
efficiently from the exhaust gases of an internal combustion engine in an 
automobile, etc., a plant for producing nitric acid, or the like, and a 
catalyst used for carrying it out. 
2. Description of the Prior Art 
The exhaust gases of an internal combustion engine in an automobile, etc., 
a plant for producing nitric acid, or the like contains harmful components 
of nitrogen oxides (NO.sub.X) which give rise to environmental pollution. 
Attempts have, therefore, been made in various fields of industry to 
reduce nitrogen oxides from such exhaust gases. 
There is known a method which employs a catalyst for reducing nitrogen 
oxides form exhaust gases. According to this method, exhaust gases are 
brought into contact with a catalyst, whereby the nitrogen oxides 
contained therein are adsorbed on the surface of the catalyst and are 
decomposed to nitrogen and oxygen. The oxygen is then reacted with a 
reducing substance, such as carbon monoxide or hydrogen. 
The catalyst which has hitherto been employed for reducing nitrogen oxides 
comprises a metal, such as copper, palladium, platinum or rhodium, loaded 
on a support formed from a porous material, such as alumina, zirconia or 
zeolite, as disclosed in e.g. Japanese Laid-Open Patent Specifications 
Nos. 11063/1976, 23474/1976 and 86693/1978. 
The known method has, however, not been able to remove nitrogen oxides 
effectively in an oxidizing atmosphere which contains a greater amount of 
oxygen than is required for oxidizing a reducing substance (such as 
ammonia, carbon monoxide or hydrogen) completely to water (H.sub.2 O) or 
carbon dioxide (CO.sub.2). This is because the oxygen which the atmosphere 
contains tends to react with the reducing substance more quickly than the 
oxygen which has been separated from the nitrogen oxides and prevents the 
latter from reacting with the reducing substance effectively. Such an 
oxidizing atmosphere is, for example, produced by an automobile engine if 
it is supplied with a fuel-air mixture having a higher ratio of air to the 
fuel and containing a greater amount of oxygen than is required for the 
complete combustion of any unburned fuel. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the drawbacks of the 
prior art as hereinabove pointed out and provide a method which can purify 
exhaust gases by reducing nitrogen oxides therefrom effectively in an 
oxidizing atmosphere. 
It is another object of the present invention to provide a catalyst which 
can effectively reduce nitrogen oxides from exhaust gases in an oxidizing 
atmosphere. 
According to a first aspect of the present invention, there is provided a 
method of purifying exhaust gases containing nitrogen oxides which 
comprises contacting the exhaust gases with a catalyst containing copper 
in the presence of hydrocarbons in an oxidizing atmosphere to reduce the 
nitrogen oxides from the exhaust gas. 
According to a second aspect of the present invention, the exhaust gases 
which have been contacted with the catalyst containing copper is further 
contacted with an oxidizing catalyst. 
According to a third aspect of the present invention, there is provided a 
catalyst for purifying exhaust gases containing nitrogen oxides which 
comprises copper loaded on a support composed of a porous material, such 
as alumina, silica or zeolite. 
The catalyst containing copper promotes the reaction between the nitrogen 
oxides and hydrocarbons and thereby enables the efficient reduction of the 
nitrogen oxides from the exhaust gases. The oxidizing catalyst reduces any 
unreacted carbon monoxide and hydrocarbons from the exhaust gases by 
oxidation. 
The present invention is useful for purifying any exhaust gas containing 
nitrogen oxides, including the exhaust gases of an internal combustion 
engine in an automobile, various kinds of combustion facilities and a 
plant for producing nitric acid.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is based on the discovery of the fact that in an 
oxidizing atmosphere, a catalyst containing copper promotes selectively 
the reaction between nitrogen oxides and hydrocarbons in exhaust gases. 
The catalyst according to the present invention comprises copper loaded on 
a porous support. The porous support may be composed of one or more porous 
materials, such as alumina, silica, silica-alumina and zeolite. It may 
take various shapes, including granular and honey-combed. 
The catalyst preferably contains 0.1 to 50 g of copper per liter of the 
porous support. If it contains only less than 0.1 g of copper, it cannot 
be expected to fulfill the purpose for which it is employed. It cannot be 
expected to produce any better result even if it may contain over 50 g of 
copper. 
The porous support is preferably loaded with copper in a way which will now 
be described. A copper compound, such as copper nitrate or acetate, is 
dissolved in a solvent, such as water or alcohol. The porous support is 
immersed in its solution so that the solution may impregnate the support. 
Then, the support is dried and heated. 
The reduction of nitrogen oxides from exhaust gases is carried out by 
contacting it with the catalyst in the presence of hydrocarbons when an 
oxidizing atmosphere prevails. The oxidizing atmosphere means an 
atmosphere containing a greater amount of oxygen than is required for 
oxidizing the existing reducing substances, i.e. the carbon monoxide, 
hydrogen and hydrocarbons which exhaust gases contains and the 
hydrocarbons which are added for carrying out the method of the present 
invention, to H.sub.2 O and CO.sub.2 completely. If the exhaust gases are, 
for example, of an internal combustion engine for an automobile, an 
oxidizing atmosphere is produced by a lean fuel-air mixture. When any such 
oxidizing atmosphere prevails, the catalyst containing copper promotes the 
reaction of hydrocarbons (HC) and nitrogen oxides (NO.sub.X), which is 
shown by the following formula, rather than the reaction of hydrocarbons 
and oxygen, so that the nitrogen oxides may be effectively reduced: 
EQU .mu.HC+vNO.sub.X .fwdarw.wH.sub.2 O+yCO.sub.2 +zN.sub.2 
The hydrocarbons which the exhaust gases contain may be utilized as a 
reductant which is employed for the purpose of the present invention. The 
coercive addition of hydrocarbons should, however, be made if the exhaust 
gases do not contain any hydrocarbons, or if it contains only a smaller 
amount of hydrocarbons than is required for the reaction as shown by the 
formula above. The reaction is promoted still more effectively if some 
excess of hydrocarbons exist. Thus, the exhaust gases in which the 
reaction occurs preferably contains 100 to 5000 ppm of hydrocarbons in 
terms of CH.sub.4. 
The catalyst containing copper has a high activity for reducing nitrogen 
oxides, but a low activity for oxidizing the carbon monoxide, 
hydrocarbons, etc. which the exhaust gases contain. Therefore, the exhaust 
gases which have been contacted with the catalyst containing copper is 
preferably contacted with an oxidizing catalyst which can effectively 
reduce any such carbon monoxide, hydrocarbons, etc. The oxidizing catalyst 
may comprise one or more metals selected from among Pr, Pd, Rh, etc. The 
metal (or the metals) is preferably loaded on a porous support composed of 
alumina, silica, zirconia, etc. The catalyst preferably contains 0.1 to 10 
g of the metal per liter of the porous support. If it contains only less 
than 0.1 g of the metal, it cannot be expected to fulfill the purpose for 
which it is employed. It cannot be expected to produce any better result 
even if it may contain over 10 g of the metal. 
Referring more specifically to a method embodying the present invention, a 
catlalyst containing copper is placed in a reaction vessel and exhaust 
gases are introduced into the vessel so that they may be contacted with 
the catalyst. An oxidizing atmosphere is caused to prevail in the vessel, 
and an appropriate amount of hydrocarbons is supplied into the vessel if 
required. The purified gases are removed from the vessel. 
Another method embodying the present invention employs both a catalyst 
containing copper and an oxidizing catalyst. A reaction vessel holding the 
catalyst containing copper is positioned upstream of a reaction vessel 
holding the oxidizing catalyst. The two catalysts may be formed by using a 
monolithic support in which an upstream portion is loaded with copper and 
a downstream portion is loaded with the metal or metals defining the 
oxidizing catalyst. This construction has the advantage that only a single 
support is required for realizing both the catalyst for reducing nitrogen 
oxides from exhaust gases and the catalyst for reducing hydrocarbons, 
carbon monoxide, etc. 
The catalyst bed of the catalyst containing copper is preferably heated to 
a temperature of 300.degree. C. to 600.degree. C. to promote the desired 
reaction. The catalyst bed of the oxidizing catalyst is preferably heated 
to a temperature of 200.degree. C. to 800.degree. C. to promote the 
desired reaction. 
The exhaust gases to be purified are preferably introduced into the 
catalyst bed of the catalyst containing copper at a space velocity (SV) of 
10,000 to 100,000 hr.sup.-1, and into the layer of the oxidizing catalyst 
at a space velocity of 10,000 to 100,000 hr.sup.-1, too. 
If the exhaust gases to be purified are of an internal combustion engine 
for an automobile, the catalyst containing copper, or the catalyst 
containing copper and the oxidizing catalyst, are preferably positioned 
downstream of an exhaust manifold. The invention is effective with an air 
fuel ratio of at least 18:1. 
There is no particular limitation to the shape or structure of the catalyst 
containing copper or the oxidizing catalyst. They may be, for example, in 
form of granules, pellets, or a honeycombed body. 
The invention will now be described more specifically with reference to 
several examples thereof. It is to be understood that the following 
examples are not intended to limit the scope of the present invention. 
EXAMPLE 1 
50 cc of Y-type zeolite (product of Union Carbide) were dipped in 100 cc of 
an aqueous solution containing 0.5 mol of copper nitrate per liter and 
having a temperature of 80.degree. C. and after 24 hours, the zeolite was 
removed from the solution and washed by water. This procedure was repeated 
five times. Then, the zeolite was dried at 110.degree. C. for 12 hours and 
calcined at 600.degree. C. for three hours in the air to yield a catalyst 
containing copper and embodying the present invention (Sample No. 1). The 
catalyst contained 2% by weight of copper. 
For the sake of comparison, a catalyst containing palladium (Comparative 
Sample No. Cl) was prepared by repeating substantially the foregoing 
procedure, but employing an acidic aqueous solution of palladium nitrate 
instead of the copper nitrate solution. This catalyst contained 0.2% by 
weight of palladium. 
The two catalysts were each examined for selectivity between the reaction 
of HC and NO.sub.X and the reaction of HC and O.sub.2. A fixed catalyst 
bed was formed from 7 cc of each catalyst in a quartz reactor having an 
inside diameter of 18 mm. The catalyst bed was heated to 400.degree. C. 
and a gas containing 1000 ppm of NO and 2% of O.sub.2, the balance being 
N.sub.2, was introduced into the reactor at a space velocity (SV) of 
30,000 hr.sup.-1. At the same time, propylene was also introduced into the 
reactor in a quantity stepwisely increasing up to 1800 ppm in terms of 
CH.sub.4, or THC (total hydrocarbon). The resulting conversions of each of 
NO and O.sub.2 were measured. The results are shown in the drawing. The 
ordinate represents the conversion of NO and the abscissa represents the 
conversion of O.sub.2 and the increasing quantity of propylene. The 
diagonal line which appears in the drawing defines a 50% selectivity 
between the reaction of HC and NO.sub.X and the reaction of HC and 
O.sub.2. 
The curve obtained by plotting the results of the reaction which was 
promoted by the catalyst embodying the present invention (Sample No. 1) 
lies at a very high region above the diagonal line. It is, therefore, 
obvious that the catalyst of the present invention has a high selectivity 
for the reaction of HC and NO.sub.X. On the other hand, the curve obtained 
by plotting the results of the use of the comparative catalyst (sample No. 
C1) lies on the abscissa showing the conversion of O.sub.2. This means 
that the comparative catalyst hardly promoted the reaction of HC and 
NO.sub.X. 
EXAMPLE 2 
Four samples of catalyst embodying the present invention (Samples Nos. 2 to 
5) were prepared by impregnating four different types of porous supports, 
respectively, with an aqueous solution of copper nitrate, as shown in the 
table which will hereinafter appear. Four comparative samples of catalyst 
(Samples Nos. C2 to C5) containing four different metals, respectively, 
were also prepared. 
Each sample was tested for its effectiveness in the conversion of NO. The 
reaction was carried out by employing the same apparatus and conditions as 
those used in EXAMPLE 1 and introducing into the catalyst bed a gas 
containing 1000 ppm of NO, 0.3% of CO, 1300 ppm of propylene in terms of 
CH.sub.4, 2.1% of O.sub.2, 12% of CO.sub.2 and 3% of H.sub.2 O, the 
balance being N.sub.2. The results are shown in the table. As is obvious 
therefrom, all of the samples of the present invention showed a higher 
conversion of NO than any of the comparative samples. 
TABLE 
______________________________________ 
Sample 
Catalyst Amount of Conversion 
No. Metal Support metal (wt. %) 
of NO (%) 
______________________________________ 
2 Cu .gamma.-alumina 
5 15 
3 Cu Silica 5 20 Inven- 
4 Cu Silica-alumina 
5 10 tion 
5 Cu L-type zeolite 
5 25 
C2 Co .gamma.-alumina 
5 0 
C3 Mn " 5 0 Compar- 
C4 Pt " 5 0 ative 
C5 Rh " 5 0 
______________________________________ 
EXAMPLE 3 
Each of Samples Nos. 1 and C1, which had been prepared in EXAMPLE 1, was 
tested for its conversion of NO.sub.X in the exhasut gas of a practical 
engine. A catalytic converter having a volume of 1.9 liters was filled 
with each sample of catalyst (Sample No. 1). A 2000 cc engine was operated 
at a rotating speed of 1600 rpm and a manifold pressure of -400 mmHg, 
while it was supplied with a lean fuel-air mixture having an air to fuel 
ratio of 18:1. The exhaust gas of the engine entering the catalytic 
converter had a temperature of 400.degree. C. and contained 1500 ppm of 
NO.sub.X. Sample No. 1 showed a NO.sub.X conversion of 40%, while 
Comparative Sample No. C1 could not reduce NO.sub.X at all (i.e. it showed 
a conversion of 0%). 
These results confirm that the present invention can effectively remove 
NO.sub.X in an oxidizing atmosphere. 
EXAMPLE 4 
A combination of each of Samples Nos. 1 and C1 with an oxidizing catalyst 
was tested for its conversion of NO.sub.X, CO and THC in the exhaust gas 
of a practical engine. A catalytic converter having a volume of 1.9 liters 
was filled with each sample and positioned downstream of the exhaust 
manifold of the engine. A catalytic converter also having a volume of 1.9 
liters was filled with the oxidizing catalyst and positioned downstream of 
the converter holding Sample No. 1 or C1. The engine having a volume of 
2000 cc was driven at a rotating speed of 2000 rpm and a manifold pressure 
of -350 mmHg. It was supplied with a lean fuel-air mixture having an air 
to fuel ratio of 18:1. The exhaust gas entering the catalytic converter 
had a temperature of 500.degree. C., a NO.sub.X content of 2000 ppm, a CO 
content of 0.2% and a THC content of 1500 ppm. The combination of Sample 
No. 1 and the oxidizing catalyst showed a NO.sub.X conversion of 45%, a CO 
conversion of 99% and a HC conversion of 98%. The combination of 
Comparative Sample No. C1 and the oxidizing catalyst did, however, not 
reduce NO.sub.X at all, though it was as effective for the conversion of 
CO and HC as the combination including Sample No. 1. 
These results confirm that the present invention can effectively reduce 
NO.sub.X in an oxidizing atmosphere. 
EXAMPLE 5 
A catalyst was formed by loading copper and palladium (as an oxidizing 
catalyst) on upstream and downstream portions of a monolithic support, 
respectively and tested for its conversions of NO.sub.X, CO and HC. The 
monolithic support (400 mesh) was a product of Nippon Gaishi formed from 
cordierite (30 mm in diameter, 50 mm in length and about 15 g). It had an 
upperstream portion and a downstream portion each having a length of about 
25 mm. The upperstream portion was coated with about 2.5 g of slurry which 
had been prepared by mixing 80 parts of Y-type zeolite (SK-40 of Union 
Carbide) and 20 parts of an alumina sol (AS200 of Nissan Chemical), and 
was calcined at 500.degree. C. The downstream portion was coated with 
about 3 g of a slurry which had been prepared by mixing 80 parts of a 
powder of .gamma.-alumina (which had been obtained by crushing KHA-24 of 
Sumitomo Chemical) and 20 parts of the same alumina sol as hereinabove 
mentioned. 
The upperstream portion of the support was loaded with copper by employing 
the method which had been employed for the preparation of Sample No. 1 in 
EXAMPLE 1. The downstream portion was loaded with palladium by employing 
the method which had been employed for the preparation of Comparative 
Sample No. C1 in EXAMPLE 1. The resulting catalyst contained 0.24% by 
weight of copper and 0.03% by weight of palladium. 
The catalyst was heated to 500.degree. C. and a gas containing 1000 ppm of 
NO, 0.3% of CO, 1300 ppm of propylene in terms of CH.sub.4, 2.5% of 
O.sub.2, 10% of CO.sub.2 and 10% of H.sub.2 O, the balance being N.sub.2, 
was supplied to the catalyst at a flow rate of 50 liters per minute so 
that it might flow first through its upperstream portion and then through 
its downstream portion. The purified gas leaving the catalyst showed a NO 
conversion of 30%, a CO conversion of 98% and a propylene conversion of 
99%. 
For the sake of comparison, the downstream portion of the catalyst was cut 
away from its upperstream portion and was likewise tested. It could not 
reduce NO at all, though it achieved a CO conversion of 95% and a 
propylene conversion of 96%. 
These results confirm that the present invention can effectively remove NO 
in an oxidizing atmosphere.