Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~065~23
The present invention relates to the oxidation of carbon
monoxide and hydrocarbons, especially of those being contained
in the exhaust gas of internal combustion engines, with
oxygen-containing gases to carbon dioxide and water.
Carbon monoxide and hydrocarbons are oxidized in contact
with catalysts, wherein the active ingredients are either noble
metals belonging to group VIII of the Periodic System or are
mixed oxides, e.g. of copper, manganese and nickel, the mixed
oxides, which are less costly, being preferred so as to enable
wide use to be made of those catalysts, e.g. for the decontamination
of automobile exhaust gas. The catalysts used to this end are required
to be abrasionproof, to be thermally shockproof, to be sinter-
proof, to have a low starting temperature, if possible lower than
200 C, and to have a constant activity, even if heated for pro-
longed periods of time to temperatures of at least 800& .
United States Patent Specification 3 493 325 describes a
catalyst deposited, e.g. on gel-like or activated alumina, for the
catalytic oxidation of exhaust gas of internal combustion engines.
The active ingredients applied to the carrier are selected from
oxides of copper~ nickel, cobalt, iron, chromium, manganese or
mixtures thereof.
A further catalyst has been described in German Patent
Specification 1 272 896, which is deposited on a carrier contain-
ing at least 30 weight % of alumina together with alkali metal
and alkaline earth metal oxides, and heat-resistant filler
materials. The catalytically active ingredients inter alia
include oxides of copper, nickelJ cobalt, manganese and cerium.
- 2 -
.. . . ..
~065~23 ~ ~
These prior art catalysts are not fully satisfactory,
however, as their activity varies depending on whether they are
contacted with an oxidizing or reducing gas. More particularly,
they are highly active if contacted with a mixture consisting of `
the gas to undergo oxidation and a stoichiometric or slightly
understoichiometric proportion of oxygen, whilst they are less
active if contacted with a mixture containing an excess of oxygen.
In other words, it is necessary for those prior art
catalysts to be contacted with a gas mixture containing a stoi- `
chiometric proportion of oxygen, based on the carbon monoxide
hydrocarbons present in the gas mixture to undergo oxidation. This
is more especially necessary in an attempt (a) to maintain the cat-
alyst active and (b) to ensure complete combustion of the gas to
undergo oxidation. It should be borne in mind, however, that the
exhaust gases of internal combustion engines always contain variable `
proportions of carbon monoxide and hydrocarbons so that considerable
expenditure in respect of equipment is at least necessary to achieve
this, if at all.
It is accordingly an object of the present invention
to provide a process for the oxidation of carbon monoxide and hydro- ~ -
carbons with oxygen-containing gas to carbon dioxide and water in `
contact with a catalyst whose activity remains unaffected in contact -
with gas mixtures, irrespective of the oxygen concentration therein,
and which additionally has a low starting temperature.
To this end, the invention provides a process for the
oxidation of carbon monoxide and hydrocarbons contained in exhaust
gas of internal combustion engines with oxygen-containing gas to
r - 3 -
1065~23
carbon dioxide and water, which comprises carrying out the oxida~
tion at temperatures within the range 150 and 800C in contact with .
a carrier-supported catalyst, wherein alumina containing at most
0.2 weight % of alkali and converted to a mixture of ~-alumina and .:
a-alumina is the carrier and cerium in oxide form is the catalytically
active ingredient, the catalytically active ingredient constituting
0.2 to 10 weight % of the carrier being deposited thereon.
Further preferred features of the present process pro-
vide:
(a) for the oxidation to be carried out at temperatures
within the range 200 and 700C; :
~b) for the oxidation to be carried out at temperatures
within the range 300 and 600C;
(c) for the use of a catalyst wherein 0.5 to 8 weight %
of cerium in oxide form is deposited on the carrier;
~d) for the use of a catalyst produced by compressing
an alumina selected from the group consisting of ~-alumina and
boehmite into shapes; annealing the shapes for periods within the
range 10 and 20 hours at temperatures within the range 1000 and 1250C;
impregnating the annealed shapes with an aqueous solution of a cerium
salt of a readily decomposable acid; drying the cerium salt so ap-
plied to the shapes at temperatures within the range 130 and 150C;
decomposing the dry cerium salt by gradually heating the shapes in a
two-stage procedure to temperatures within the range 200 and 300C
and 450 and 550C, respectively; and annealing the resulting shapes
containing cerium oxide at temperatures within the range 700 and
900C;
(e~ for the use of a catalyst produced by compressing the
- 4 -
1065123
alumina carrier in admixture with graphite;
(f) for the use of a catalyst produced by compressing
the alumina carrier in admixture with between 3 and 10 weight %,
preferably 5 weight %, of graphite;
(g) for the use of a catalyst produced by burning off
the graphite at temperatures within the range 550 and 750C, pre- ~
ferably 650C prior to annealing the shapes; and -~- -
(h) for the use of a catalyst, wherein cerium nitrate
is the cerium salt of a readily decomposable acid.
The process of the present invention, which is par-
ticularly useful for the oxidation of carbon monoxide and hydro- ;
carbons with an excess of oxygen, ensures complete combustion of
these substances.
Vital to the catalyst used in the present process is
more particularly the alumina carrier, which is partially converted
to -A1203. Carrier-supported catalysts having the catalytically
active cerium oxide applied to incompletely dehydrated aluminum oxide
monohydrate or to highly crystalline a-A1203 are considerably less
active catalytically.
In addition to this, it is necessary for the alumina
carrier to contain little alkali as alkalies are known to increase
the starting temperature of carrier-supported catalysts.
The cerium salt used in aqueous solution for impregnat-
ing the carrier and applying the catalytically active ingredient
thereonto should have an anion which can be completely removed
later, during calcination. The preferred cerium salt is cerium
nitrate. Cerium sulfate produces catalysts of
5 -
10651Z3
delayed activity.
The carrier should preferably be impregnated with a quantity of
cerium salt solution which just corresponds to its volume of pores. In -
other words, it is the concentration of the cerium salt solution used for
impregnation and the volume of pores of the carrier which determine the
cerium content of the resulting carrier-supported catalyst.
With respect to catalysts having a low starting temperature, it is ~
necessary for them to be made with the use of very pure cerium salt. So- `
called cerium mixed metal salts should conveniently not be used. -
Qualitatively highly reliable carrier-supported catalysts for use ~.
in the present process are obtained in those cases in which the cerium salt
applied to the carrier is dried over a prolonged period of time within the `
range 16 and 64 hours, for example.
The carrier-supported catalysts used in the ollowing Examples were
cylindrical shapes 3 mm in diameter and 3 mm long. 17 normal liters (S.T.P.),
per cc of catalyst per hour, of a test gas mixture was oxidized in contact
therewith. The gas mixture contained 3 % by volume of 2~ 2 % by volume of
cn, looo ppm of n-hexane, 2.5 % by volume of steam, the balance being N2, and
was heated in each particular case to the testing temperature. The residual
concentrations of C0 and n-hexane were identified in the oxidized gas and the
conversion in % of these two substances was calculated therefrom.
The temperatures at which 50 and 90 %, respectively, of C0 and n-
hexane, respectively, were found to have been converted to C02 and H20
(U50 G0; U90 C0; U50 Hex.; U90 Hex) were used as the activity indexes of the
catalyst.
EXAMPLE 1: (Comparative Example~
Ce(N03)3 6 H20 was dried until the nitrate commenced decomposition
and compressed into shapes. The shapes were heated for 10 hours to temp-
eratures within the range 500 and 800C, with the supply of air. The
catalytic activity was determined and the following results were obtained:
U50 C0 - 430C U50 Hex ~ 560C
UgO C0 - 540C UgO Hex - 660C
,,
-- 6 --
106SlZ3
EXAMPLE 2: (Invention)
~-A1203 ~Aluminum oxide "C", a product o~ Degussa) ~as suspended in water and
compacted therein. The resulting highly viscous magma was dried, admixed with
3 weight ~ of graphite and ground. The ground material was compressed into
shapes which were first heated to 650C to burn off the graphite and then
annealed for 20 hours at 1100C. The carrier so made was impregnated with a
cerlum tIII) nitrate solution and dried for 64 hours at 140& . To decompose
the cerium nitrate, the carrier was first heated for 3 hours to 250C and
then for 10 hours to 500C. The resulting cerium oxide-containing shapes -
were finally annealed for 10 hours at 800C. Catalysts containing 2, 4 and
6 weight %, respectively, of cerium were made in this manner. Their activity `
was determined and the following results were obtained: -
Ce-content in weight %
2 4 6
~ . . . s~ .
U50 C0 ~C] 190 170 160 ~;
.... __
UgO C0 ~C] 270 240 _ 205 _
U50 Hex ~C] 565 540 505
EXAMPLE 3: (Invention)
~-A1203 ~Aluminum oxide "C", a product of Degussa) was made into shapes in
the manner described in Example 2. The graphite used as a compression aid
was burnt o~f and shape specimens were annealed in each case for 20 hours at
temperatures of 950C, 1000C, 1050C, lloo&, 1150C, 1200C, 1250C and
1300C, respectively. Following this, 6 weight % of cerium was applied to
the individual specimens, in the manner described in Example 2. The activity
of the resulting catalysts was determined and the following results were
obtained:
1065123
Carrier annealed [ C ]
at C
50 CO 90 CO 50 Hex 90 Hex
(gS0) 500 730 620 740
1000 450 580 620 740
1050 260 380 560 700
1100 160 205 505 640
1150 190 270 560 670
1200 190 305 565 680
1250 240 350 570 690
(1300) 270 390 600 730 : :
EXAMPLE 4: (Invention)
Boehmite (AlOOH) containing 0.03 weight % of sodium was compressed
into shapes in the manner described in Example 2. The graphite used as a
compression aid was burnt off and shape specimens were annealed in each
particular case for 20 hours at temperatures of 900C, 1000C, 1100CJ 1200C
and 1250C, respectively. Following this, 6 weight % of cerium was applied
to the individual specimens, in the manner described in Example 2. The
activity of the resulting catalysts was determined and the following results
were obtained: :`
[ C 1 Carrier annealed at &
~ 900 _1000 _~_ 1100 _ 1200_ 1250
U50 CO 460 420 400 360 280
._ . . __
U90 CO 610 530 510 450 450
. . . .__ ..
U50 Hex . 590 600 580 610 __ _600
. .
EXAMPLE 5: (Carrier containing alkali)
The carrier-supported catalyst prepared in the manner described in
Example 2 was impregnated with the quantity of potassium carbonate solution
which was necessary to incorporate 0.5 weight % of K2C03 therein, then dried
at 140C and annealed for 10 hours at 800C. The activity was determined
and the following results were obtained:
U50 CO= 335C
-- 8 --
-
~O~;SlZ3
UgO CO ~ 450 C
U50 Hex = 620 C
EXAMPLE 6: (Carrier containing alkali)
Bayerite ("Martifin" , a product of Martinswerk, Berheim) containing
about 0.25 weight % of sodium (this is sodium originating from the particular
process used for making the product) was compressed into shapes, in the
manner described in Example 2. The graphite used as a compression aid was
burnt off and shape specimens were annealed in each particular case for 20
hours at temperatures of 900C, 1000C, 1100C, 1200C and 1300C, respectively. ;-
Following this, 6 weight % of cerium was applied to the individual carrier
specimens, in the manner described in Example 2. The activity of the
resulting catalysts was determined and the following results were obtained:
[ C ] Carrier annealed at C -
.,,____ . .. :
900 1000 1100 1200 1300
. ' .
U50 C0 510 430 410 320 330
UgO C0 620 510 530 450 460
... _._ . ,._,.. , _ ,_ . .
U50 Hex 590 570 610 600 610
EXAMPLE 7: (Less readiiy decomposable cerium salt)
A catalyst containing 6 weight % of cerium was prepared in the
manner described in Example 2 save that cerium sulfate was substitued for
cerium nitrate. The activity of the catalyst so made was determined in a
series of tests. Between the individual tests, the catalyst below the test
gas mixture was annealed in each particular case for 10 minutes at 800 & .
Activity deter- U50 CO [ C ] U90 CO [ C ] U50 Hex [ C ]
mination
1 430 530 590
2 350 450 580 _
3 290 390 560
260 360 540
_ 240 340 530
_
6 220 330 530
* Trade mark
_ g _
1065~23
.:
EXAMPI.E 8: (Mixture of rare earths)
_
Catalysts containing 6 weight % of rare earths were prepared in
the manner described in Example 2 save t~at the cerium nitrate was replaced
by salt solutions by dissolving cerium mixed metals of the following
composition:
Alloy A: 56 % of Ce Alloy B: 49 % of Ce
28 % of La 23 % of La
10 % of Nd 15 % of Nd ~;
4 % of Pr 12 % of Pr ~ Y
2 % of further r.e. 1 % of
further r.e.
in nitric acid. The activity was determined and the following résults were
obtained:
Cerium mixed metal alloy:
A B
, . .
U50 C0 ~~C ~ 280 330
UgO C0 [~C ] 380 430
U50 Hex [ C ] 550 525
EXAMPLE 9: ~Annealing period and temperature)
The catalyst prepared in the manner described in Example 2 which
had the following activity:
U50 CO = 160C
UgO C0 = 205C
U50 Hex = 505 C
was annealed for 13 hours at 1100C and then had the following activity:
U50 C0 - 185 C
UgO C0 = 280C
U50 Hex = 530 C
A catalyst was prepared in the manner described in Example 2,
save that the annealing step at 700 - goo& was omitted and replaced by heat
treatment at 300 - 400C after decomposition of the cerium salt.
The catalyst so made had the following activity:
- 10 -
1065~23
U50 C0 ~ 185 C
UgO C0 - 355C
U50 Hex = 535 C
eXAMPL~ 10:
Basic aluminum chloride A12(0H)5CL . 3 H20 ("Locron" , a product
of Farbwerke Hoechst AG) was dissolved in water and the solution was admixed
with the quantity of cerium nitrate necessary to obtain a catalyst containing
6 % of cerium. The whole was admixed with ammonia to jointly precipitate the ~;
Al and Ce hydroxides. The precipitate was filtered off, scrubbed with water
and dried. After the addition of 5 weight % of graphite, the dry precipitate :
was compressed into shapes. The graphite was burnt off at 650C and the ~ -;
shapes were annealed for 20 hours at 800C. The activity was determined and
the following results were obtained:
U C0- 330C
Ugo CO ' 450C
U50 Hex = 540C
UgO Hex ~ 640C-.
* Trade mark
