U.S. patent application number 12/588722 was filed with the patent office on 2010-04-22 for catalyst for decomposing nitrous oxide and method for performing processes comprising formation of nitrous oxide.
Invention is credited to Oystein Nirisen, Dag Ovrebo, Klaus Schoffel, David Waller.
Application Number | 20100098611 12/588722 |
Document ID | / |
Family ID | 19911354 |
Filed Date | 2010-04-22 |
United States Patent
Application |
20100098611 |
Kind Code |
A1 |
Nirisen; Oystein ; et
al. |
April 22, 2010 |
Catalyst for decomposing nitrous oxide and method for performing
processes comprising formation of nitrous oxide
Abstract
The present invention relates to a catalyst comprising 0.1-10
mol % CO.sub.3-xM.sub.xO.sub.4, where M is Fe or Al and x=0-2, on a
cerium oxide support for decomposition of N.sub.2O in gases
containing NO. The catalyst may also contain 0.01-2 weight %
ZrO.sub.2. The invention further comprises a method for performing
a process comprising formation of N.sub.2O. The N.sub.2O containing
gas is brought in contact with a catalyst comprising 0.1-10 mol %
CO.sub.3-xM.sub.xO.sub.4, where M is Fe or Al and x=0-2, on a
cerium oxide support, at 250-1000.degree. C. The method may
comprise that ammonia is oxidized in presence of an oxidation
catalyst and that the thereby formed gas mixture is brought in
contact with the catalyst comprising the cobalt component on cerium
oxide support at a temperature of 500-1000.degree. C.
Inventors: |
Nirisen; Oystein; (Brevik,
NO) ; Schoffel; Klaus; (Porsgrunn, NO) ;
Waller; David; (Porsgrunn, NO) ; Ovrebo; Dag;
(Porsgrunn, NO) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
1030 15th Street, N.W.,, Suite 400 East
Washington
DC
20005-1503
US
|
Family ID: |
19911354 |
Appl. No.: |
12/588722 |
Filed: |
October 26, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10312993 |
Mar 21, 2003 |
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PCT/NO2001/000283 |
Jul 4, 2001 |
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12588722 |
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Current U.S.
Class: |
423/237 ;
423/239.1; 502/304 |
Current CPC
Class: |
B01D 2255/20746
20130101; B01J 23/83 20130101; B01J 2523/00 20130101; B01J 2523/00
20130101; B01D 53/9427 20130101; B01J 23/75 20130101; B01D
2255/2065 20130101; Y10S 502/524 20130101; Y02C 20/10 20130101;
B01J 2523/31 20130101; B01J 2523/3712 20130101; B01J 2523/845
20130101; B01J 2523/845 20130101; B01J 2523/00 20130101; B01J
2523/31 20130101; B01J 23/002 20130101; B01D 53/8628 20130101 |
Class at
Publication: |
423/237 ;
502/304; 423/239.1 |
International
Class: |
B01D 53/56 20060101
B01D053/56; B01J 23/10 20060101 B01J023/10; B01D 53/58 20060101
B01D053/58; B01D 53/86 20060101 B01D053/86 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2000 |
NO |
20003467 |
Claims
1-12. (canceled)
13. A catalyst for the decomposition of N.sub.2O at temperatures of
250-1000.degree. C., wherein the catalyst comprises 0.1-10 mol %
CO.sub.3-xAl.sub.xO.sub.4, where 0.25.ltoreq.x<2, on a cerium
oxide support, wherein the catalyst decomposes N.sub.2O, and
wherein the catalyst has a high selectivity for decomposing
N.sub.2O without significant destruction of NO.
14. The catalyst according to claim 13, wherein the supported
catalyst contains 1-5 mol % of the cobalt component.
15. The catalyst according to claim 13, wherein the surface area of
the cerium oxide support used in the preparation of the catalyst is
larger than 10 m.sup.2/g at operating temperature.
16. The catalyst according to claim 13, wherein the catalyst
further comprises 0.01-2 weight % ZrO.sub.2.
17. A method for decomposing N.sub.2O, without significant
destruction of NO, comprising contacting an N.sub.2O containing gas
with a catalyst comprising 0.1-10 mol % CO.sub.3-xAl.sub.xO.sub.4,
where 0.25.ltoreq.x<2, on a cerium oxide support, at a
temperature in the range of 250-1000.degree. C., in an oxidizing
environment.
18. The method according to claim 17, wherein the catalyst further
comprises 0.01-2 weight % ZrO.sub.2.
19. The method according to claim 17, comprising oxidizing ammonia
in the presence of an oxidation catalyst and contacting the thereby
formed gas mixture with the catalyst at a temperature in the range
of 500-1000.degree. C.
20. The method according to claim 17, comprising contacting a tail
end gas from an absorption unit downstream an ammonia oxidation
unit with the catalyst at a temperature in the range of
250-500.degree. C.
21. The method according to claim 17, comprising contacting an
N.sub.2O containing gas mixture from an adipic acid process with
the catalyst at a temperature in the range of 500-800.degree.
C.
22. The catalyst according to claim 14, wherein the catalyst
further comprises 0.01-2 weight % ZrO.sub.2.
23. The catalyst according to claim 15, wherein the catalyst
further comprises 0.01-2 weight % ZrO.sub.2.
24. The method according to claim 18, comprising oxidizing ammonia
in the presence of an oxidation catalyst and contacting the thereby
formed gas mixture with the catalyst at a temperature in the range
of 500-1000.degree. C.
25. The method according to claim 18, comprising contacting a tail
end gas from an absorption unit downstream an ammonia oxidation
unit with the catalyst at a temperature in the range of
250-500.degree. C.
26. The method according to claim 18, comprising contacting an
N.sub.2O containing gas mixture from an adipic acid process with
the catalyst at a temperature in the range of 500-800.degree.
C.
27. The catalyst according to claim 15, wherein the surface area of
the cerium oxide support is larger than 50 m.sup.2/g.
Description
[0001] The present invention relates to a catalyst for decomposing
nitrous oxide (N.sub.2O) to nitrogen and oxygen at temperatures of
250-1000.degree. C. The invention also comprises a method for
performing processes comprising formation of nitrous oxide.
[0002] In recent years there has been increasing focus on how to
decompose N.sub.2O as it is an atmospheric ozone depletion gas
(greenhouse gas). N.sub.2O will be formed during the catalytic
oxidation of ammonia in connection with nitric acid production and
during oxidation of alcohols and ketones, for instance in
connection with the adipic acid production. Also in connection with
the use of N.sub.2O, for instance as an anaesthetic gas, the
effluent N.sub.2O should not be discharged to the atmosphere, but
decomposed.
[0003] Though N.sub.2O will decompose homogeneously to some extent
at high temperatures, most processes are comprised of the
application of various types of catalysts for its decomposition.
However, a catalyst which may function well in a certain
temperature range and/or gas mixture containing N.sub.2O, will not
necessarily function for other operating conditions. The
selectivity of the catalyst is also of great importance, especially
if the catalyst is applied in connection with ammonia oxidation, up
front of the absorption unit, in a nitric acid plant. In that case
the catalyst should not decompose the main product, i.e. the
nitrogen oxide (NO).
[0004] Numerous N.sub.2O decomposition catalysts are known and most
of these are based on various metal oxides such as cerium oxide,
cobalt oxide, cupric oxide, chromium oxide, manganese dioxide and
nickel oxide as the active component. Furthermore, there are known
catalysts based on metal oxides on zeolite carriers and transition
metal ion exchanged zeolites.
[0005] A catalyst for reducing nitrogen oxide compounds is known
from Japanese application JP 48089185. Though this application does
not mention nitrous oxide specifically, its definition also covers
this nitrogen oxide. The catalyst contain Co and Ce oxides as its
main components. In an example a mixture of 249 parts cobalt
acetate, and 315 parts cerium acetate was dissolved in water.
ZrO.sub.2 was soaked with this solution and pyrolyzed at
900.degree. C. for 5 hours to give a catalyst containing CeO.sub.2
and Co.sub.3O.sub.4 on the surface of the ZrO.sub.2 support.
[0006] From the application WO 93/15824 it is known to contact a
N.sub.2O containing gas with a catalyst containing nickel oxide
plus cobalt oxide on a zirconia substrate at 280.degree. C. The
ratio nickel oxide to cobalt oxide is 0.5-1:3-1. Pure and diluted
N.sub.2O containing gases can be treated according to this
application.
[0007] It is further known from EP 207857B1 that a catalyst
comprised of ceria and 1-20 weight % of at least Al, Si Zr, Th or
rare earth metals as oxides. This composition which essentially
contains ceria and preferably 1-5 weight % of said metal oxides can
be used for the synthesis of methanol over an extended period
without loss of surface area.
[0008] From U.S. Pat. No. 4,738,947 it is known that a p-type metal
oxide being dispersed on a refactory oxide such as ceria improves
the oxidation of hydrocarbons and carbon monooxide. It is further
claimed that the dispersion with addition of platinum on the
refactory oxide results in a catalyst suitable for a catalytic
reduction of nitrogen oxide with hydrocarbons and/or carbon
monoxide. No reference is made to a catalytic decomposition of
nitrogen oxide without reductants. No example refers to nitrous
oxide.
[0009] Application WO98/32524 describes an invention related to the
catalytic reduction of nitrogen oxide and the use of a catalyst for
the reduction of nitrogen oxide and the oxidation of carbon
monoxide and hydrocarbons. The essential ingredient is gold which
is complexed by a transition metal and anchored to an oxide
support. No reference is made to a catalytic decomposition of
nitrogen oxide without reductants. No example refers to nitrous
oxide.
[0010] Applied Catalysis B: Environmental 13 (1977) 69-79 R. S.
Drago et al. describes a catalyzed decomposition of N.sub.2O on
metal oxide supports. Decomposition of N.sub.2O using metal oxides
supported on silica, magnesium oxide, calcium oxide and
hydrotalcite-like supports were studied. CoO was found to be a most
active catalyst when supported on silica at temperatures of
500.degree. C. The silica supported catalysts were prepared by pore
filling the silica support with nitrates of the metals, drying at
180.degree. C. and decomposition of the nitrates to oxides at
500.degree. C.
[0011] When supporting CoO on MgO a much more active catalyst was
attained. However, the activity of the catalyst decreased by
calcination at 1000.degree. C. Catalysts calcined at 500.degree. C.
gave 99% conversion of N.sub.2O, while catalysts calcined at
1000.degree. C. gave 50% conversion of N.sub.2O. Preparation of
CO.sub.3Mg.sub.5Al.sub.2(OH).sub.2OCO.sub.3.y.H.sub.2O
"hydrotalcite-like" compound is also described. This precursor was
calcined at 500.degree. C. or 800.degree. C. BET analysis of
CO.sub.2O/2MgO catalysts calcined at 500.degree. C. and
1000.degree. C. showed a surface area of 118 m.sup.2/g and 4
m.sup.2/g, respectively.
[0012] When the catalyst for N.sub.2O decomposition comprises
cobalt oxide, as reported in Journal of Chem. Soc. Faraday Trans.
1, 74(7), 1595-603, which studied the structure and activity of
Co.sub.xMg.sub.1-xAl.sub.2O spinel solid solutions for use as
catalysts in decomposing N.sub.2O, the catalyst activity generally
increases when a greater amount of cobalt ions is incorporated into
octahedral sites in the structure.
[0013] The main object of the present invention was to arrive at a
versatile, active and thermally stable catalyst for decomposing
N.sub.2O at temperatures above 250.degree. C., especially at
temperatures of 800-1,000.degree. C.
[0014] Another object was that the catalyst should be stable and
retain its activity for at least a normal cycle, i.e. the length of
time between change of the ammonia oxidation catalyst.
[0015] A further objective was to produce a catalyst which could be
applied at high space velocities and having a high selectivity for
decomposing N.sub.2O without decomposing NO.
[0016] It was also an objective to arrive at a method for reducing
the amount of nitrous oxide from processes comprising formation of
nitrous oxide, such as nitric acid processes, adipic acid processes
and combustion of hydrocarbons in vehicle engines.
[0017] Another object was to remove nitrous oxide from the tail end
gas from nitric acid plants and other exhaust gases.
[0018] The various known N.sub.2O decomposition catalysts were
first evaluated with regard to activity and thermal stability.
Cobalt oxide was known from the literature to possess high
activity, at least initially for some gas compositions and at
relatively low temperatures. N.sub.2O decomposition in the tail end
gas from nitric acid plants was reported to perform at high
activity by using catalysts prepared from hydrotalcite precursors
containing cobalt and being lightly calcined, i.e. at
200-500.degree. C. The inventors therefore started by further
investigating this type of catalyst for preparation of a process
gas catalyst. An essential requirement for a new catalyst was that
it should be thermally stable at ammonia oxidation operating
conditions. This means that the catalyst must be active, selective
and stable at temperatures of 800-1000.degree. C. and for the gas
mixtures formed during the catalytic ammonia oxidation.
[0019] Several cobalt oxide containing precursors were then made
and calcined for at least 5 hours, at temperatures of about
900.degree. C. Such catalysts were compared with other known
catalysts in an initial test of N.sub.2O decomposition for a gas
containing 2932 ppm N.sub.2O and 2574 ppm NO and where the ratio
NO:N.sub.2O was 0.88 and hourly space velocity, GHSV, was 280,000
h.sup.-1. The rate constants for the decomposition were measured at
700.degree. C.
[0020] These tests confirmed that Co is the essential metal in the
Co--Mg--Al (ex-hydrotalcite). The oxides were tested under
different GHSV and for the most active oxides with a conversion too
high to give accurate rate constants. Tests of the thermal
stability of a catalyst based on calcined hydrotalcite containing
Co, Mg and Al were performed for 48 hours at about 900.degree. C.
The tests were performed at GHSV 108,440 h.sup.-1 and with 2932 ppm
N.sub.2O, 2575 ppm NO, the rest of the gas mixture being argon. For
these types of catalysts the N.sub.2O conversion and their rate
constants were reduced during the test period. The surface area of
the catalyst was reduced, from 9.3 down to 1.3 BET m.sup.2/g. These
results clearly indicate that the stability of such catalysts is
questionable.
[0021] The inventors then started to investigate catalysts based on
active components on a support, for instance metal oxides like
zirconia, alumina, ceria and mixtures thereof. One advantage of
these types of catalysts will be the material cost reduction if it
is possible to substantially reduce the amount of the active
component in the decomposition of nitrous oxide.
[0022] Firstly, the cobalt-aluminium system was systematically
evaluated in laboratory reactor tests. The composition of the
system Co.sub.3-xAl.sub.xO.sub.4 was varied from x=0 to x=2.
[0023] The results of laboratory activity data are shown in FIG. 1.
It was observed that there is an increase in the activity, measured
after approximately 90 hours operation, as aluminium is added to
the spinel structure. However, when the cobalt/aluminium ratio was
less than 1, a decrease in activity was observed. This was
surprising as there is a continuous increase in surface area with
increase in the aluminium content. Therefore, in terms of intrinsic
reaction rate, it seemed advantageous to work with a cobalt rich
spinel though these materials tend to have a low surface area.
[0024] Another oxide system which was also thoroughly examined was
the Co.sub.3-xFe.sub.xO.sub.4 system, x could vary from 0 to 2.
These materials were tested in a laboratory microreactor and the
results are shown in FIG. 2. CoFe.sub.2O.sub.4 showed the highest
activity. This particular composition may be described as "cobalt
stabilised magnetite".
[0025] Although these two types of spinels show high activities,
they were not considered practical for use as pure phases in a
plant for the following three reasons: Any catalyst containing a
high cobalt content will be prohibitively expensive, all the above
active phases, except for the aluminium rich spinel, have low
surface areas and they also deactivate, even at the relatively low
temperature 800.degree. C. Accordingly, these phases can only be
considered useful if they can be success-fully combined with an
appropriate support phase. However, many conventional catalyst
supports can not be used for this application. The following
properties are required: The support should be a refractory
material, preferably with a melting point above 1800.degree. C., so
that it resists sintering and maintains a high surface area under
the process conditions. Further, the support should not react
significantly with the active phase resulting in loss of activity
and/or selectivity. Finally, the support should be readily
available at a price substantially lower than that of the active
phase.
[0026] Selection of a suitable support proved to be more
complicated than expected and it was soon realised that possible
combinations of active phase and support material had to be
thoroughly evaluated. The first support material examined was
magnesium oxide. A pilot activity test on a cobalt aluminate spinel
phase, with a nominal composition of CO.sub.2AlO.sub.4 was then
performed. It was observed that the initial activity of this
catalyst was good. However, during further testing it was found
that there was a continuous reduction in performance with time.
Detailed analysis of the catalyst after the pilot test indicated
that there was transport of cobalt from the spinel active phase
into the magnesia support. Further investigations revealed that the
aluminium rich spinel and the cobalt-magnesia solid solution
exhibit a lower activity than the CO.sub.2AlO.sub.4 spinel and this
explained the deactivation. This process will continue until the
chemical activity of cobalt in the spinel and the magnesia support
are the same. Based on these observations and tests magnesia was
excluded as a support for the catalyst to be used in a process gas
environment.
[0027] Another commonly used support material is alumina. However,
as with a magnesia support, transport of transition metal from the
active phase to the alumina was found to occur, leading to the
formation of alumina rich spinels which exhibit a lower intrinsic
rate than the cobalt rich spinel or perovskite active phases.
Therefore, alumina had to be excluded as a realistic support
material. Similar arguments are made against the use of
alumino-silicates and alumino-magnesium silicate supports.
[0028] Zirconia, ceria and mixtures of these have also been used as
support material in some catalysts for oxidation of carbon monoxide
and hydrocarbons, (WO 96/14153), the active catalyst being a noble
metal and possibly also a transition metal. As referred above
cerium oxide is also used in a catalyst for methanol production.
This catalyst contains 1-20% of at least Al, Si Zr or Th. In view
of the physical properties of ceria it was decided to investigate
this support material further. The solubility of cobalt and iron in
ceria is low, and the rate of diffusion of these elements into
ceria is reported to be very slow, therefore, ceria remained an
interesting candidate. Ceria will in most cases be in the form of
CeO.sub.2, but can also be in the form of Ce.sub.2O.sub.3.
Laboratory tests with CO.sub.3O.sub.4/CeO.sub.2 were then performed
and the activity and stability of the catalyst were most promising.
Further laboratory tests and pilot plant tests were then performed
in order to establish the optimum composition of this type of
catalyst.
[0029] Pure cerium oxide samples, without an active phase
component, was also tested for activity towards N.sub.2O
decomposition in the laboratory microreactors under standard test
conditions. At a temperature of 890.degree. C., conversion of 70%
was achieved, compared with conversions of greater than 95% for the
best supported spinel catalysts. These results indicate an
additional advantage or synergy of using cerium oxide as a support
material. The whole area of the catalyst, both the active phase and
the support material will be contributing to the decomposition of
the nitrous oxide. Contrary to cerium oxide, other support
materials such as alumina and magnesia were found to be completely
inert towards nitrous oxide decomposition.
[0030] Ceria supported catalysts could be made in several ways
using conventional catalyst manufacturing methods. Cobalt salts,
cobalt-aluminium-salts and cobalt-iron-salts could be precipitated
on or impregnated into cerium oxide powder and the resulting slurry
could be dried and calcined. The catalyst particles could then be
formed into useful shape by tableting, compacting, extrusion etc. A
high surface area of the ceria will be advantageous and as it will
be reduced during calcination, a ceria with high initial surface
area should be used. At operating temperature the surface area of
the ceria should be larger than 10 m.sup.2/g, preferably larger
than 50 m.sup.2/g.
[0031] The invention is further explained and elucidated in the
following experiments and corresponding tables and figures.
[0032] FIG. 1 shows N.sub.2O conversion of the
Co.sub.3-xAl.sub.xO.sub.4 active phases at 890.degree. C. in a
laboratory microreactor.
[0033] FIG. 2 shows N.sub.2O conversion of the
Co.sub.3-xFe.sub.xO.sub.4 active phases 890.degree. C. in a
laboratory microreactor.
[0034] FIG. 3 shows N.sub.2O conversion of
Co.sub.3O.sub.4--CeO.sub.2 catalysts in a laboratory
micro-reactor.
[0035] FIG. 4 shows pilot plant activity data for N.sub.2O
conversion of for various catalysts at 5 bar, 900.degree. C. and
GHSV=66,000 h.sup.-1.
[0036] FIG. 5 shows data from a laboratory microreactor for
N.sub.2O conversion using various catalysts according to the
invention.
[0037] FIG. 6 shows effect of catalyst composition on loading on
N.sub.2O conversion in a laboratory microreactor.
[0038] FIG. 7 shows the effect on N.sub.2O conversion of the
addition of ZrO.sub.2 to the Co.sub.2ALO.sub.4/CEO.sub.2
catalyst.
[0039] The catalyst according to the invention consists essentially
of 0.1-10 mol % Co.sub.3-xM.sub.xO.sub.4, where M is Fe or Al and
x=0-2, on a cerium oxide support. A preferred catalyst also
contains 0.01-2 weight % ZrO.sub.2.
[0040] The supported catalyst contains preferably 1-5 mol % of the
cobalt component.
[0041] The cerium oxide support used in the preparation of the
catalyst should preferably have a surface area larger than 10
m.sup.2/g, preferably larger than 50 m.sup.2/g at operating
temperature.
[0042] Preferred cobalt components in the catalyst are,
CO.sub.3O.sub.4, Co.sub.3-xAl.sub.xO.sub.3 where x=0.25-2 or
Co.sub.3-xFe.sub.xO.sub.4 where x=0.25-2.
[0043] The main feature of the method according to the invention
for performing processes comprising formation of N.sub.2O is that
the N.sub.2O containing gas is brought in contact with a catalyst
comprising 0.1-10 mol % Co.sub.3-xM.sub.xO.sub.4, where M is Fe or
Al and x=0-2, on a cerium oxide support, at 250-1000.degree. C. It
is preferred to use a catalyst which also contains 0.01-2 weight %
ZrO.sub.2.
[0044] When the method according to the invention is applied in a
nitric acid plant, ammonia is oxidised in presence of an oxidation
catalyst and then the thereby formed gas mixture is brought in
contact with the catalyst comprising the cobalt component on cerium
oxide support at a temperature of 500-1000.degree. C.
[0045] The tail end gas from an absorption unit downstream an
ammonia oxidation unit, can be brought in contact with the N.sub.2O
decomposition catalyst comprising 0.1-10 mol %
CO.sub.3-xM.sub.xO.sub.4, where M is Fe or Al and x=0-2, on a
cerium oxide support at a temperature of 250-500.degree. C.
[0046] The N.sub.2O containing gas mixture from an adipic acid
process can also be treated according to the invention by bringing
the said gas in contact with the N.sub.2O decomposing catalyst
comprising 0.1-10 mol % Co.sub.3-xM.sub.xO.sub.4, where M is Fe or
Al and x=0-2, on a cerium oxide support at a temperature of
500-800.degree. C.
EXAMPLE 1
[0047] This example shows the results from tests performed on
laboratory scale using catalysts having the respective 1, 5 or 10
mol % concentration of Co.sub.3O.sub.4 on a ceria support. N.sub.2O
conversion % as function of time is shown in FIG. 3. The tests were
performed at a pressure of 3 bar, GHSV=560,000 h.sup.-1 and a gas
composition of:
N.sub.2O=1200 ppm
NO=10000 ppm
Oxygen=4%
H.sub.2O=1.7%
[0048] the balance being nitrogen.
[0049] The tests were performed at 800.degree. C. and 890.degree.
C. The results of the tests are shown in FIG. 3. These tests show
that the conversion of N.sub.2O was very high, about 98%. The
stability of the catalyst was also promising. Best results were
obtained when the catalyst contained 5 mol % of the cobalt
component.
EXAMPLE 2
[0050] The catalysts used in the tests of example 1 were then
tested in a pilot plant having ammonia oxidation conditions.
Further, the tests comprised investigations of unsupported
CO.sub.2AlO.sub.4 catalyst and a catalyst being CO.sub.2AlO.sub.4
on MgO support The N.sub.2O conversion catalyst was placed right
below the ammonia oxidation catalyst and platinum recovery gauze's.
The tests were performed at the following standard conditions:
Pressure 5 bar, temperature 900.degree. C., GHSV 55.000
h.sup.-1-110,000 h.sup.-1. The gas composition was:
N.sub.2O=1200-1400 ppm, NO=10%, Oxygen=4%, H.sub.2O=16% and the
balance being nitrogen (plus Ar, CO.sub.2, etc. from air).
[0051] The results of these tests are shown in FIG. 4 and show that
for the best catalyst the N.sub.2O conversion was about 95% after
100 days on stream. The NO decomposition was well below 0.5% which
was considered an acceptable level.
[0052] FIG. 4 further shows that the unsupported CO.sub.2AlO.sub.4
catalyst and the same active phase on MgO both lost most of their
activity after a few days of operation.
EXAMPLE 3
[0053] This example show the results from tests performed for 90
hours in a laboratory microreactor. The operating conditions were
as in example 1 and the tests were run at temperatures of
800.degree. C. and 890.degree. C. The results are shown in FIG. 5
which shows that the cobalt aluminate on ceria is a more stable
catalyst than cobalt oxide on ceria.
EXAMPLE 4
[0054] This example shows the effect on catalyst composition and
loading on N.sub.2O conversion in a laboratory reactor. The
operating conditions were as in Example 1. The tests were run at
890.degree. C. For all the catalysts according to the invention the
best results are obtained at a catalyst loading of about 2 mol %,
but for some high activity is achieved already at very low loading
as illustrated in FIG. 6. N.sub.2O conversion of more than 95% can
be obtained at very low catalyst loading and even at as much as 10
mol %, but nothing seems to be gained by increasing the catalyst
loading above 5 mol %. The catalyst loading will accordingly also
depend on practical and economical evaluations.
EXAMPLE 5
[0055] This example shows the effect of ZrO.sub.2 addition to the
performance of the CO.sub.2AlO.sub.4/CeO.sub.2 catalyst installed
in an amonia oxidation pilot plant. The catalyst was prepared by
mixing the ingredients as in previous examples plus adding 0.01
weight %-2 weight % fine ZrO.sub.2 powder (particle size 1-3
.mu.m). The tests were performed under the same standard conditions
as for Example 2. In FIG. 7 the results are presented for an
addition of 0.22, 0.28 and 0.90 weight % ZrO.sub.2 compared to
catalysts without ZrO.sub.2. The optimum concentration in this case
was 0.2 weight %. The effect of adding ZrO.sub.2 is reduced
degradation of catalyst activity over time.
[0056] The inventors have by the present invention succeeded in
arriving at a versatile, active and thermally stable N.sub.2O
decomposition catalyst. The catalysts according to the invention
can be applied at a wide temperature range and will also be stable
at varying gas composition. Presence of water, which often is a
problem, for instance in connection with catalyst for exhaust from
car engines, have been found to be no serious problem for these
catalysts. Accordingly, the new catalysts can be applied for
decomposing N.sub.2O from processes comprising formation of
N.sub.2O. The catalysts are especially useful in connection with
nitric acid production as the N.sub.2O can be decomposed in the
process gas formed after ammonia oxidation without significant
destruction of NO and also be used in the tail gas from the
subsequent absorber unit.
* * * * *