Note: Descriptions are shown in the official language in which they were submitted.
~0;~67~4
Nitrogen oxides dis arged from various burners,
chemical plants, and cars represent a serious source of air
pollutants. There is, therefore a great need for an effective
method of removing these oxides from exhaust gas streams.
`Basically, the methods for removal of NOx may be divided into
two methods, one for suppressing the production of NOx and the
other for making NOx harmless after it is produced~ As for the
former method, two-step combustion processes, lo~-oxygen
combustion processes and exhaust gas circulation processes are
~0 being studied. The latter method includes a process for conver-
sion into nitrogen through a catalytic reaction and a second
process for absorption removal by using an absorptive liquid.
These processes, however, have their own merits and demerits.
No process has yet been established that is industrially satis-
factory. Seven different forms of nitrogen oxides are known,
but the principal air pollutants are NO and N02. These two
forms are collectively termed NOx. It is said that the NOx in
an ordinary exhaust gas contains 90 - 95% or more NO, the balance
being N02. Therefore, a method for removing NOx must be
primarily capable of removing at least NO.
One method of removal of NOx, as is known from U.S.
Patent No. 3,454,355 and other literature (e.g. Bartok et al,
"System Study of Nitrogen Oxide Control Methods for Stationary
Sources" Final Report-Vol. II, Esso Research and Engineering
Company Government Research Laboratory, November 20, 1969),
comprises using as a catalyst a metal, e.g. platinum, supported
on an alumina carrier and allowing NOx to react with carbon
monoxide, hydrogen or methane to reduce the NOx into nitrogen
thereby making it harmless. Such methods produce no harmful
by-products and present few problems in putting them into~
practical use.
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However, we have found that when tested with actual exhaust
gaseq these catalysts were not effective in removing NOx.
It appears that the catalyst becomes poisoned by the oxygen,
moisture and sulfur oxides in the exhaùst gases. This
is illuQtrated in the following comparative Example A.
In the attached drawingsj Figures 1 - 5 represent a
series of graphs showing amounts of N0 removed with different
catalysts and under differing conditions.
Experiment Example A
In this experiment, a typical catalyst found in
conventional literature, that is, a material comprising
copper carried on y-alumina was used. Such catalyst is pre-
pared by immersing commercially available y-alumina sieved
to 8 - 14 mesh in an aqueous solution of copper nitrate for
a fixed period of time, separating the aqueous solution,
drying at 100-120 C and baking at 540 C. The amount of Cu carried
in this catalyst was 5 wt%.
The reaction was conducted in a quartz pipe with an
inner d$ameter of 30 mm which was ins~alled within an annular
furnace, it being so arranged that the reaction temperature could
be preset at a predetermined level. The reductlon of NOx (in
the experiment, N0 being used) by C0 and H2 was carried out
by filling the reaction pipe with said catalyst, followed by
treatment with hydrogen gas at 540 for 1 hour. The reaction
gas was prepared by mixing bombed mixed gases C0 + N2, H2 + N2'
2 + N2- N0 ~ N2 and S02 + N2 of predetermined controlled
concentrations in accordance with the experimental conditions
and was then admitted into the reaction pipe.
In addition, moisture was added by detonating the
2 + N2 mixed gas in water in a bubbler installed in a
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thermostatic water tank controlled to a predetermined temperature
before the flows of said various mixed gases meet each other.
The amount of N0 removed was determined by gas
analyzers at the inlet and outlet ports of the reaction pipe,
an N02 meter produced by Mast Company being used for the analysis
of N0. The N0 in the sample gas was oxidized into N02 by an
oxidizin~ device and then admitted into the analyzer. S02, C0
and 2 were continuously nnalyzed using an infrared type
analyzed (produced by Horiba Saisaku-Sho) an infrared
type analyzer (produced by ~ugi Denkl Seizo) and a magnetic type
analyzer (produced by Slmazu Seisaku-Sho), respectively.
Further, H2 was analyzed by a gas chromatograph (produced
by Simazu Seisaku-Sho). In addition, the moisture concentration
was determlned by calculation.
The condltions and results of the experiment are
shown ln Tables 1 and la.
Table 1
Reactlon gas composition Reaction Space Rates of reaction
(the balance being N2) temperature velociOy of N0 and S02 (%)
(Dry gas basis) (at 20 C
N0 2 2 C0 - ( C) (hr~1) N0 ¦ S0
ppm % ppm % % 4
300 0 2500 0.75 0 540 10 98 100
300 1.0 1500 2.75 10 540 104
280 0 0 2.57 0 540 104100 __
_
280 1.1 0 2.57 l10 540 104 - -~ ~~
NOTE: H 0 concentration is indicated on a wet gas basis.
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Table la
~eaction gas composition Reaction ¦Space velocity j Rates of
(the balance being N2) temperature (converted for reaction of
(Dry gas basis) 20C) NO
¦ NO I H2 I 2 I H2 ¦ ( C) ¦ (hr ) ¦ (X)
ppm¦ % ~ % ¦ %
500 1.0 O I O 450 10 100
I . I I .
0.4 l 10 L 450 104 O
NOTE: H20 concentration is indicated on a wet gas basis.
10 ~ Table 1 corresponds to the case wherè CO~ is used
as a reducing agent, while Table la indicates the results
when H2 is used as a reducing agent. In each case, when there
is neither 2 nor H20 in the reaction gas, substantially
complete removal of NO and S02 is achieved. However, with 2
and H20 admitted into the reaction gas, this catalyst loses its
activity. This may also be said of the case of the single
reduction of NO.
Apart from S02, actual exhaust gases from boilers,
chemical plants and cars contain 2 and H20 without exception.
Further, it is almost impossible to remove these gases,
particularly H20. Therefore, it may be concluded that this
catalyst cannot be used in actual exhaust gases. In addition,
further tests were also carried out using this catalyst with
the reducing agent replaced by a hydrocarbon such as CH4, but
similarly the catalyst lost its activity under the influence of
2 and H20-
In our co-pending application Serial No. 181,840,
filed September 25, 1973 we have described a catalyst comprising
an alumina carrier, on which is carried copper together with
at least one further member selected from alkaline earth metals,
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alkali metals and transition elements. That catalyst was
e~fective in the presence of oxygen and moisture as illustrated
by the following Experiment Example B.
Experiment Example B
The equipment used in Experiment Example A was used
in this experiment, and the experimental method was also
approximately the same~ In this experiment, however, C02 was
also added to a reaction gas in order to make the latter resemble
the actual exhaust gas more closely. In addition, a gas chrom-
atograph was used for the analysis of C02 and CH4. In this case,an example is shown in which C0, H2 and CH4 are used~ as reducing
agents, but there was observed some difference in the
reaction temperature at which the effective reduction of N0
was achieved. That is, in the case of C0 and H2, temperatures
of about 300 C or above were required, whereas temperatures of
590 C or above in the case of CH4. Thus, the reduction experiments
were conducted at 450C for C0 and H2 and a~t 600C for CH4.
Further, the prepared catalyst was used in its state established
when it had been baked at 540C. In the previous experiment,
H2 reduction treatment was carried out, but there was no
substantial difference in the result, except that an activation
period of about 1 hour at maximum was taken before a fixed high
activity was obtained. The experimental conditions are as follows:
(1) C0 reduction experiment conditions
Reaction gas composition;
500ppmN0 + 4%2 + 1%C0 + 13%C02 + 13%H20 + N2
Reaction temperature; 450C
Space velocity (converted for 20C); 104 hr 1
(2) H2 reduction experiment conditions
Reaction gas composition;
500ppmN0 + 4%2 + 1.1%H2 + 15%C02 + 10%H20 + N2
Reaction temperature; 450 C
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~a3679~
Space velocity (converted for 20 C); 104 hr 1
(3) CH4 reduction experiment conditions
Reaction gas composition;
500 ppmNO + 4%2 + 2.3%CH4 + 13%C02 + 10%H20 + N2
Reaction temperature; 600C
Space velocity (converted for 20C); 104hr 1
The experimental results sre shown in Table 2. In addition,
the percen~age value preceeding each metal element indicates
the analy~ed value.
Table 2
Rate of reduction-wise
Catalyst removal of NO (%)
~ ~ CH4
5.3%Cu-4.4%Mg-A1203100L 100 ~:
5.0%Cu-5.0%Ba-A1203100 100 100
5.0%Cu-0.2%K-A1203 89 80
5.1XCu-5.4XCr-A1203100 100 85
8.lXCu-5.1%Mn-A1203100 100 90
5.8%Cu-6.8%Fe-A1203 100
5.6XCu-6.8%MO-A120389 85 85
5.1%Cu-0.2%Pr-A120340 40 10
As shown in Table 2, these are decidely superior
catalysts. However, when 500-lOOOppm S02 was added to said
reaction gas, all of these catalysts were gradually poisoned
by S02, so ehat the NO reduction activity was decreased.
When the concentration of the individual reducing agent was
extremely increased, however, the rate of reduction of NO was
achieved to some extent t30-80%), but S02 was reduced sub-
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stantially completely into H2S and COS. Therefore, when it is
desired to use these catalysts, except for the speclal case of
the exhaust gas not containing S02, it is necessary to remove
S2 in advance in order to prevent the production of H2S
and COS.
~ n ideal NOx reduction catalyse would have to be
such that it is immune from the influence of S02 contained
in reaction gases and that it has a suitable activity not
causing the conversion of S02 itself into H2S or COS-
The de8ree to which the various catalysts described
above are influenced by S02 differs wieh the kind of additive~
metals, that is, metals other than Cu, and also with the
concentration. Of these catalysts, those which are relatively
immune from the influence of S02 were Cu-Mg-A1203, Cu-Mn-A1203,
etc. Further, in the case of the Cu-Mg-A1203 catalyst, relatively
good results were obtained when the Mg concentration was about
3%. Thus, the results of a chec~ on the S02-poisoned conditions
of the 5%Cu-3%Mg-A120 catalyst are shown in Experiment Example
C.
ExPeriment Example C
The experimental equipment and method were the same
as those described in Experiment Example B. In order to observe
the influence of S02, pure S02 collected in advance was
intermittently in~ected at a fixed rate by a syringe into the
reaction gas immediately before the reaction pipe. In addltion,
the rate of in~ection was ad~usted so that the S02 concentra~ion
in the reaction gas was about lOOOppm. The experimental
conditions were:
Catalyst; 5Z Cu - 3% Hg - A1203
Reaction gas composition: 500ppm N0 + 0-4% 2 + 1.1~ H2
+ 15% C02 + 13% H20 + N2
Reaction temperature; 450 C
-- 7 --
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Space velocity; 104hr 1 (converted for 20 C)
The results of the experiment are shown in Fig. 1, in which
the number of in~ection of S02 is plotted on the horl~ontal
axis and the reduction of N0 on the vertical axis. While Fig. 1
shows the results when H2 is used as a reducing agent, there
was observed almost no difference in the degree of poisoning
by S02 when C0 or CH4 was used. That is; it has been
ascertained that the S02 poisoning has relation to S02 and
catalysts but has nothing to do with the kind of reducing gases.
On the other hand, as is generally known a catalyst
carrying thereon a precious metal such as platinum, ruthenium,
rhodium and palladium is effective for use as a reduction
catalyst and is utilized in various contact reactions. Thus,
we have made a check on the N0 reducing characteristics of such
catalysts. The results are shown in Experiment Example D.
Experiment Example D
The experimental equipment and method were the same
as those previously described, but in this expèriment, 2
was not contained in the reaction gas. The reaction gas
20 composition is as follows and the experimental results are shown
in Table 3.
Reaction gas composition;
300ppmNO + 1700ppm S02 + 0.6%C0 + 10%H20 + N2
Reaction temperature; 540 C
Space velocity (converted for 20C); 5000 hr 1
Table 3
Catalyst IRate of r~ action (%)
I N0 S02 ,
10.5%Ru-Al2o3 j 40 88 ¦
¦0.5%Rh-Al2o3 17 83
.5%Pd-A1203 10 84
.5%Pt-A1203 16 73
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The rate of removal of N0, though low, is retained.
Further, the rate of reaction of S02 is about 70 - 90% but the
amount of H2S detected on the outlet side was 1410ppm for
Ru-A1203, 1420ppm for Rh-A1203, 1400ppm for Pd-A1203, and
1200ppm for Pt-A1203~ Further, slight amounts of COS were detec-
ted. Similarly, when H2 was used as a reducing agent, S02 was
converted into H2S. NOx reduction experiments were conducted
under a condition in which S02 has been removed from the reaction
gas. However, in the case of C0 reduction and H2 reduction,
NH3 was formed in the reaction gas and the making harmless
of NOx could not be achieved. In C0 reduction, it is believed
that the reason for the production of NH3 is that the reaction
between C0 and H20 produces H2, which reacts with N0 to produce
NH3. As described above, if a catalyst prepared by carrying
a precious metal alone on a y-alumina carrier is used,
conversion of S02 or conversion of N0 into NH3 takes place
and under coe~istence with S02 it has not sufficient NO-reduction
activity. Thus, it may be concluded that it is not an
industrially satisfactory catalyst.
Summary of the Invention
According to one aspect of the present invention
there is provided a method of removing nitrogen oxides from
nitrogen oxide-containing exhaust gases, wherein the nitrogen
oxides in said exhaust gases are reacted with a reducing gas
in the presence of a reducing catalyst comprising an alumina
carrier on which is carried copper in an amount of less than
lOX by weight together with at least one member in an amount
of less than 10% by weight selected from alkaline earth metals,
alkali metals and transition elements, characterized in that
the catalyst also contains about 0.005 to 0.06% by weight of a
precious metal selected from rhodium and ruthenium.
According to another aspect of the invention there
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provided a process for treating an internal combustion engine
exhaust gas to decrease its content of nitrogen oxides which
comprises passing the exhaust gas under reducing conditions over
a catalyst comprising an alumina support on which is carried a
metal from the group iron, cobalt and nickel in an amount of less
than 10% by weight together with copper in an amount of less
than 10% by weight and a platinum group metal selected from
rhodium and ruthenlum in an amout~t of 0~005 to 0.06~ by weight.
Certain preferred embodlments of the inventive catalyst
are illustrated by the following examples:
Example 1
In this ex~ample, 5%Cu - 3%Mg - A1203 was used as the
catalyst base. Thus, catalysts prepared by immersing 5%Cu--
3%Mg - A1203 (~ - 14 mesh, crushed product) in aqueous solutions
of salts of Ru, Rh, Pd and Pt, respectively, were used. The
Ru, Rh, Pd and Pt contents were as follows.
0.051%Ru - Cu - Mg - A1203
0.051%Rh - Cu - Mg - A1203
0.053%Pd - Cu - Mg - A1203
0.052%Pt - Cu - Mg - A1203
The experimental equipment and method were substantially
the same as in the preceding experiment examples, and the
influence of S0 was determined by intermittently injecting
the same. C0 was used as a reducing agent. The conditions are
shown below and the experimental results are shown in Fig. 2.
The results in Figure 2 were obtained with the following con-
ditions:
Reaction gas composition: 660ppm N0 + 4%2 + 1%C0 +
13%C02 + 10%H20 + N2
Reaction temperature: 450 C
Space velocity: 10 hr (converted for 20C)
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_ _ . ,
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As seen in Fig. 2, even if S02 was injected, Rh - Cu - Mg - A1203
and Ru - Cu - Mg - A1203 did not exhibit any change in their
NO-removing capacities, and NO was completely removed. In
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contrast, Pt - Cu - Mg - A1203 and Pd - Cu - Mg - A1203 were
strongly influenced by S0. Indeed, they were even more adversely
influenced than was the Cu - M8 - A1203 not carrying such precious
metal. The fact that very good results are obtained when
Rh and Ru are added and that the results are not good in the
case of Pt and Pd, is contrary to our expectations and
shows that mere addition of the precious metals in slight
amounts is not effective to avoid the poisoning by S02.
Next, an example in which the rate of reduction-removal
of N0 at different temperatures while a reaction gas is contin-
uously flowing, by using H2 and CH4 as reducing agents, isshown in Example 2.
Example 2
In this example, the case of the 0.051% Rh - Cu - Mg -
A1203 is shown, but the conditions for reaction are as follows:
(i) H2 reduction experiment
Reaction gas composition; .-
SSOppmN0 + 600ppmS02 + 1.1%H2 + 4%2 + 15%C02+ 10%Y20 + the balance H2
Space velocity (converted for 20C); 10 hr 1
(ii) CH4 reduction experiment
Reaction gas composition;
SOOppmN0 + 600ppmS02 ~ 2.5%CH4 + 4%2 + 13%C02
+ 10%H20 +the balance N2
Space velocity; 104hr 1
Graphs corresponding to the above conditions are shown
in Figs. 3 and 4. As seen in these Figures, even under the
presence of S02, this catalyst was not influenced by S02 at all,
and extremely effective removal of NOx was possible provided
that the temperature was above 300 C for reduction with H2
and above 600C for reduction with CH4. These reaction
temperatures are as superior as ~ e results obtalned wlth the
Cu - Mg - A1203 catalyst, using a gas not containing S02.
These experiments were continuously conducted, each
expending about 50 hours, during which the catalysts themselves
displayed no sign of deterioration. Further,` S02 on the outlet
side was analyzed, and there was observed a decrease due to
adsorption by the catalyst itself in the initial stage of the
experiment, but about 2 hours after the start of passage of the
gas, ie coinclded with the inlet concentration. That is, as
previously described and as seen in the precious metal-A1203
~atalyst, there was observed almost no conversion into H2S.
Further, there was observed no NH3 caused by excessive reduction
of N0. These properties fully satisfy the requirements for an
industrial catalyst previously described.
Example 3
In this example, the amount of a precious metal to
be added is excessively decreased. The catalyst used was
0.005%Rh-Cu-Mg-A1203, and the reducing agent used was H2.
~he conditions for experiment are as follow~, and thc
result~ are sho~n in ~ig.5.
Reaction gas compo~itions;
700ppmN0 ~ 700ppmS02 ~ 1.2~H2 ~ -4~2 + 11%C~
+ 1O~6H2 0 ~ N2
Rcaction tempcraturc; 450C
Space velocity (converted for 20 C); 104hr~1
As seen in Fig. 5, the amount of N0 removed becomes
constant 40 hours after the start. There is observed some `
difference as compared with the fact that in a similar
experiment using a 0.051XRh - Cu - Mg - A1203 catalyst the removal
of N0 was maintained at a value of 100%. During that time,
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o~ course, ncithcr H2S nor ~I3 was dctected in the gas
on the reaction pipe outlet side.
Detail~d results have been described with reference
to Experiment Examples and examples in which sli~ht
amounts o~ Rh and Ru are carried on Cu-M~-Al203typc
catalysts ha~e been described. ~s for othor catalysts
in which a slight amount o~ Rh or ~u is carried on other
catalyst type, that is, the ca~alyst type which carries
Cu a~d the alkali metals, nlkaline earth me~als or
t~ansition metals, such as Cu-K-Al2 03~ Cu-Ba-A1203~
Cu-Cr-Al203, Cu-Mn-Al20~ and C~l-Mo-Al203, there were
obtained substantially the same xesults as in the case
of the Cu-Mg-A1203 type, though there was some difference
in the removal of NO~ Also the catalyst obtained by a precious
metal while using as the base the catalyst having no precious
metal carried thereon, that is, Cu-Me-A1203, which is less liable
to be influenced by S02, tended to a higher rate of removal
of NOx. . .
As for the method of producing catalysts, the catalysts
used in the experiment examples described herein were
prepared by the so-called immersion process in which ~ -
~lumina i~ immersed in an aqueous solution of the salt
of e motal to cerry it thereon. however~ the method of
!
jf
. .
^13-
~ 036794
producin~ cntaly5ts i~ not limitcd to t~o immcr~ion
proces~. In~lecd, ~ood rcsults were obtained by usin~ a
c~talyst havinG Cu and Rh or ~u imprc~nation-carricd on
Me-~ O~ (Mc: one or more mctals selected from the group
COnQisting of the ~lkali metals, alkaline earth met~ls
and transition met~ls) produced by the so-called
co-preci~itation process or kncading process. Further,
a cat~lyst prepared by impre~n~tion carrying Rh or Ru on
Cu-Me-Al~O~,produced by the kneading pxocess or co-pr~cipi-
ta~ion process, was also ef~ective. There~ore, thc term
'`carry`' aæ used in claim i9 not intendcd to ~imit the
p~ocess to the i~mersion process.
As ~or the reducing gas, examples have been described
in which H2, CH4 and Co were used, and only in the case
of CH~ it was necessary to raise the reaction temperature,
but there was no substantial difference in the required
temperature between CO and H2.
In the experiments, CH4 was selected as a typical
example of hydrocarbon, but it is known to use hydro-
carbons as reducing agents. ~or example, according to
a report by J.W. Ault, R.~. Ayen, et al. (A.I. Ch. E.
Journal Vol.17,No.2, page 26~- 271, 1971), CH4 belongs to
a group having the lo~est reducing power in the hydro- !
carbons. ~here~ore, whcn a catalyst according to t~e
present invcntion is used, it is readily seen that it is
possible to replace CH4 by other hydrocarbon~ while
achieving sufficient removal of N0x. Further, the use
of said gaseS, that is, H2, hydrocaxbons C0 and other
single gases, of course, makes possible the
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. . ~
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removal or NOx in an industrially satisractory m~nner.
llowever, it cnn be easily surmiscd that industrial gases
havin~ these gases as the principal in~redients, for
example, city gas may be used.
~ h~ ~eatures of the catalys~s of the present invention,
that is, Rh-Cu-Me-Al2 ~ and ~u-Cu-~le~ 03 catalysts (Me;
one or more metals selected ~rom the ~roup consisting of
the al~aline earth me~als, alXali metals and tran~ition
m~als) may be sun~ariz~d as follows.
~a) ~hey are not poisoned by 2 H20 and S02 contained
in actual ~ases, providing a stabili~ed NOx
removal rate. ~he activity is very high.
(b) S02 passes through the present catalysts as it isj
and S~2 is never converted into other substances~
; ~ (c) Reduction of ~Ox stops upon formation of N2 -
and there is no NH3 by-produced.
, ; ~d) As reducing agents, hydrocarbons such as H? and
, C~4 and, C0 gas may be used and industrial gases
` i having these 6ases as the principal in~redients may
also be used.
(e) ~he production of the catàlysts is easy, and in
use there is no need for preparatory treatment such
as H2 reduction treatment.
; As described above, the catalysts invented herein
have many features, and it is believed that they are also
industrially superior as they can be used for reducing or
making harmless the nitrogen oxides contained in exhaust
' gases discllurged ~rom steam-power plants, ~arious furnaces,
¦ chemical plants, etc.
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