Note: Descriptions are shown in the official language in which they were submitted.
1142S02
HOE 80/H 007
For various technical uses, it is desirable to have
relatively large-structured catalysts which are easy to
handle and make by mass-production. Another property de-
manded of catalysts is low heat capacity which is necessary
for them soon to reach the state of catalytic activity, under
the action of heat. Concerned of this are more particularly
those catalysts which are used in the decontamination of
automotive exhaust gases. To comply with these two require-
ments, it is possible to use monolithic, large-surfaced me-
tal structures formed of corrugated thin sheet metal orwire gauze. Depending on the wor~ing temperature the cata-
lyst is exposed to, it is necessary to select metal material
which has satisfatory stability under the operating condi-
tions. It is good practice, for example, to select for use
as a carrier of automotive exhaust gas catalysts, material
which has practically non-scaling properties at temperatu-
res of up to 1000C in the presence of up to 5 ~olume % oxy-
gen and up to 15 volume % hydrogen. It is indispensable for
the material to have this property in order not only not to
affect the mechanical strength of the structure but also
to prevent t~e catalytically active layer applied to the
surface of the metal structure from becoming peeled off,
under increasing scaling. Materials very suitable for this
purpose are nickel-free steel grades containing more than
10 weight % chromium and 3 weight % aluminum together with
further constituents, if desired, ALUCHROM W (this is a
registered Trade Mark; produc~ no. 1.4725, a ~roduct of
Vereinigte Deutsche Metallwerke AG) containing 13 to 15
.
11425(~2
weight % Cr; 3.5 to 5 weight % Al; ~ 1 weight ~ Mn; c 0.5
weight % Si; C 0.1 weight % C; ~ 0.045 weight ~ P; ' 0.03
weight % S; the balance being Fe, is a typical construction
material, for e~ample.
The catalytically active constituents, e.g. platinum,
palladium, rhodium,are required to be applied to carrier
material having an inner surface area as large as possible
so that it is naturally necessary for the metallic sub-
strate to have a tenaciously adhering layer of carrier
material first applied thereto. On subjecting a metal so
coated (catalyst carrier) to impregnation with noble metal
compounds, it is naturally not allowable for the latter to
react with the metal surface and, by cementation~ to cause
deposition of coarsely crystalline metal particles. In addi-
tion to this, it is not allowable for the active metal which
is to be applied as the catalytically active constituent,
to undergo subsequent recrystallization at higher tempera-
ture.
Typical of the process just described is the use of
cerium dioxide as a coating material. As to the catalyst,
it is possible for it to be made as follows, for example:
An ALUCH~OM-W sheet metal strip 76 mm wide and O.04
mm thick is treated on a roll mill provided with two toothed
roll~rs of which each has 24 teeth, the modulus being 1.
The rolled corrugated plate coming from the roll mill is
wound-up jointly with an unrolled flat sheet metal strip,
the resulting cylindrical coil presenting alternating
layers of corrugated and flat material. The coil is
brought to desirable thickness, the two sheet metal strips
are cut off, and the coil is forced into a cylindrical
114ZS~Z
sleeve. Next, the structure so made is annealed in contact
with air at 1000C for 20 hours, for example. In this way,
the material becomes superficially coated with a thin gray
oxide layer. The coil so modified is dipped into a cerium
~xide hydrate (cerium hydroxide) suspension, which may be
used while hot, in an aqueous ammonium nitrate solution.
After removal from the suspension which is allowed to drop
off, the metal structure is annealed once again at 800C
for 10 hours, for example. Depending on the thickness de-
sired for the coating, it is possible for the impregnationwith suspension, drying and annealing operations at 800C
to be repeated several $imes. The suspension needed for
impregnation can be made, for example, from an aqueous
solution of cerium-III-nitrate which has been admixed, with
mechanical agitation, l~ith ammonia until strongly alkaline
(pH = 9), ammonia in excess being caused to evaporate by
injection of air. Resulting ammonium nitrate is retained
in the solution. By impregnating the metal structures three
times, it is possible for them to be coated with cerium
dioxide layers with a weight exceeding 50 % of the metal
weight. Cerium dioxide permits the metal surface to be
covered with a tight coating relatively fast to wiping and
scraping.
Prior to impregnating the coated metal structure with a
noble metal compound solution, it is good practice to test
the coating for its volume of pores, To this end, the coa-
ted metal structure is dipped into water and the increase
in weight is determined, after superficial removal of the
water. Next, the coated metal structure is dried once again.
Needless to say, it is necessary for the quantity of noble
... . . . . . .
1~42502
metal solution which is used for applying a desirable quan-
tity of noble metal to the metal structure has to correspond
to the quantity of water absorbed in the above test. In
other words, the noble metal concentration in the impregnat-
ing solution should be selected so that the volume of liquidultimately absorbed by the coated metal structure actually
contains the quantity of noble metal for which is it desi-
rable to be applied to the coated structure.
In the manufacture in accordance with this invention
of the catalyst carrier or its use for the production of a
catalyst, it is naturally possible for the structure of
corrugated sheet metal to be replaced by a coil of metal
wire which may take an irregular form or consist of super-
posed layers of wire cloth or gauze, and is forced into and
tensionally held in position in a sheet metal container.
A technically very beneficial property of cerium di-
oxide which is used as a coating material resides in its
very good thermal stability; in other words, the inner sur-
face area of cerium dioxide is scarcely reduced upon expo-
sure of the catalyst to automotive exhaust gas at tempera-
tures of up to 1000C, for e~ample. Even in the event of the
exhaust gas containing lead, the inner surface area of the
present catalysts is not liable to be significantly reduced.
Even after haYing been contacted with exhaust gas, the cata-
lyst coating continues to combine in itself satisfactorymechanical strength with adhesiveness.
A still further technically beneficial effect which
accompanies the use of cerium dioxide as a coating material
resides in the fact that the compound itself has catalytic
activity for the reacti~n of carbon-monoxide with oxygen
(cf. Example 1 hereinafter). This is desirable especially
~14250Z
in those cases in which a platinum-containing catalyst
on a cerium dioxide coating has been poisoned in contact
with exhaust gas containing lead. In this event, the dimi-
nished platinum activity is partially compensated by the ca-
talytic activity of cerium dioxide.
The present invention relates more particularly to a
process for the production of a catalyst carrier, wherein
a chrome/aluminum-steel structure is annealed over a period
of 2 to 50 hours, at temperatures within the range 800 to
1100C with admission of air, and a cerium dioxide layer is
applied to the surface of the annealed structure, which
comprises: annealing a moulded structure made up of an
alloy containing 10 to 30 weight % Cr, 3 to 8 weight ~ Al;
up to 5 weight % Mn, Ti, Si, C, P and S, the balance being
Fe; wetting the annealed structure with a suspension of
cerium oxide hydrate in an aqueous ammonium nitrate solu-
tion; drying the structure and annealing it for 5 to 20
hours at temperatures within the range 600 to 1000C.
Preferred features of the present process provide:
a) for the cerium oxide hydrate suspension to be made by
mixing an aqueous cerium~ nitrate solution with an
aqueous ammonia solution until the resulting mixture
produces a strongly alkaline reaction, and evaporating
ammonia in excess by passing air through the mixture;
b) for the wetting, drying and annealing operations to
be repeated up to 5 times at temperatures within the
range 600 to 1000C;
c) for the wetted structure to be dried at temperatures
within the range 40 to 80C and preheated at tempera-
tures within the range 250 to 400C; and
^- ` ` ` 1142S0Z
d) for the structure to be coated with a 10 to 150 ~6,
preferably a 50 to 100 % proportion of cerium dioxide,
based on its weight.
The invention also relates to the use of the catalyst
carrier obtained by the present process in the production
of a catalyst for the decontamination of exhaust gases of
internal combustion engines and production facilities,
which comprises: coating the catalyst carrier with 0.05
to 0.5 weight % of at least one noble metal by impregnating
it with an aqueous solution of at least one noble metal
compound,drying it at 40 to 80C and heating it at 250 to
400C.
Preferred features provide:
e) for the catalyst carrier to be first tested for its
power of absorbing water and for it to be then impregna
ted with the solution of the at least one noble metal
compound which is so concentrated that the quantity of
solution just absorbable by the catalyst carrier has
the desirable proportion of noble metal therein; and
f) for the noble metal compound to be selected from ni-
trates or acetates of rhodium, palladium or platinum,
or from hexachloropalladium-IV-acid or hexachloro-
platinum-IV-acid. German Patent Specification "Auslege-
schrift'l 2,458,111 discloses a process for making a
catalyst carrier, wherein
a) a substrate of an aluminum-containing ferritic alloy
is contacted with a dispersion of a carrier for the
catalytic material in a liquid medium capable of at
least partially transforming the carrier into a gel;
.. . . .
1142S02
b) for a coating of carrier material separated from the
dispersion and consisting at least partially of gel
to be applied to the substrate; and
c) for the coating so applied to the substrate to be
baked with the resultant formation of a coherent
adhesive layer of carrier material on the substrate.
As results from column 4, line 40 of that Spec~fica-
tion, it is possible for cerium dioxide to be applied as
carrier material or surface layer to the alloy which pre-
ferably consists of Fe, Cr, Al and Y. The Specification is
silent however as to how apply the cerium dioxide. It has
merely been stated that the carrier should be used as a
dispersion in the form of a gel or sol. The final cata-
lyst is used for the decontamination of exhaust gases of
internal combustion engines. Following Example 1 of German
Patent Specification "Auslegeschrift" 2,458,111, it is made
as follows: platinum coming from a platinum source is ato-
mi~ed with the aid of an argon ion jet and in this way de-
posited on a sheet metal prov'ded with an Al20~-surface
coating (catalyst carrier material).
Details of the present invention are described in the
Examples hereinafter.
In contrast with comparative Example 5, it is shown
in Examples 1 to 4 that c~rium dioxide adheres very tena-
ciously to annealed aluminum-containing steel if use is
made of a cerium oxide hydrate (cerium hydroxide) suspen-
sion which has ammonium nitrate dissolved therein. As it
would appear, on evaporating the suspension on the metal
surface, the ammonium nitrate is transformed into a melt
~0 inducing the CeO2 to "grow" onto the metal surface.
114Z502
~YAMPLE 1:
Two cylindrical wound structures as referred to here-
in of ALUCHROM-W with the following dimensions were used:
Thickness o~ sheet metal = 0.04 mm; diameter = 24 mm;
width = 76 mm; weight = 19 g. They were annealed in an
annealing furnace over 20 hours at 1000C with admission
of air. Next, the superficially oxidized metal structures
were dipped into a cerium oxide hydrate (cerium hydroxide)
suspension at 90C, which had been prepared as follows:
1 mol (= 434 g) Ce(N03)3 . 6 H20 was dissolved in 2 l water
and the solution was admixed, with thorough agitation, with
a concentrated ammonia solution until the reaction was
strongly alkaline (pH 9). Next, air was passed through
the suspension which was concentrated to about 800 ml with
evaporation of the ammonia in excess. The preannealed wound
structures were dipped into this NH4N03-containing cerium
hydroxide suspension,then dried at 60C, preheated for 1
hour at 300C and annealed for 16 hours at 800C. After
having been dipped once into the suspension, 3 g of CeO2
in ~he form of a coating fast to wiping was found to have
been absorbed by each of the two metal structures. The
dipping operation was repeated altogether 5 times and 19 g
CeO2, corresponding to a 100 % increase in weight, was
found to have been absorbed. The wound metal structures
were tested for their catalytic activity in the oxidation
of CO. To this end, a synthetic gas mixture of 2 vol. ,~
CO, 3 vol. ~ 32' 2.5 vol. % H20, the balance being N, was
passed through two serially arranged metal structures. The
space/time load was 17 000 l gas per liter catalyst carrier
per hour at 0C under 1 bar. The temperature at which ~0
11~25(~2
and 90 %, respectively, of the C0 contained in the syn-
thetic gas were found to have underwent conversion, were
determined and the following results were obtained:
50 % C0 = 315C to 320C
CgO % C0 = 350C.
EXAMPLE 2:
Two preannealed wound metal structures were coated,
each structure with 11 g CeO2, as described in Example 1
by dipping them three times into a cerium hydroxide sus-
pension and immediately thereafter subjecting them to
thermal treatment. Each of the two metal structures so
coated with CeO2 was able to absorb 4.15 g H20. Next, 0,51 g
Pt (in the form of platinum nitrate) and 0.20 g Pd (in the
form Of pa ladium acetate) were dissolved in 50 ml water
and the solution was used for impregnation of the two metal
~tructures. Together with the 4.15 g H20, each of the
wol~nd structures absorbed 42.3 mg Pt and 16.6 mg Pd. They
were dried for 15 h at 60C and heated for 10 minutes to
300 C. Next, the catalyst was contacted under the conditions
described in Example 1 with test gas which additionally con-
tained 1000 ppm n-hexane. The fQllowing temperatures were
determined for 50 % and 90 % conversion, respectively, of
C0 and n-hexane:
C50 % ~0 = 155C
90 % C0 = 165C
C50 o~ n-hexane = 175C
C90 % n-hexane = 215C
, . . . . . . . .. . . . . .
EXAMPLE 3:
The procedure was as in Examples 1 and 2. 6.0 g CeOs
was applied to each of the metal structures by dipping
them twice into the NH4N03-containing cerium hydroxide
suspension. Each of them had a water absorption power of
3.1 g. Next they were impregnated with a solution of hexa-
chloroplatinum-IV-acid. Together with the 3.1 ml solution,
each metal structure absorbed ~0 mg Pt. The two structures
were dried and subjected to thermal treatment for 1 hour
at 300C and then contacted with test gas (2 vol. % CO,
3 vol. % 2' 2. 5 vol. % H20, 1000 ppm n-hexane, the balance
being N2) at a space velocity of 17000 h 1. The following
temper~tures were determined:
C50 % CO = 1 25C
90 % CO = 155C
C50 % n-hexane = 1 43C
CgO % n-hexane = 1 58C .
EXAMPLE 4:
The procedure was as in Examples 1 and 2 herein, unless
otherwise stated. Two wire gauzes tightly compressed to-
gether of ALUCHROM-O (No. 1. 4765, a product of Ver~inigte
Deutsche Metallwerke AG; 22 to 25 weight ,~ Cr; 4.5 to 6
weight % Al; < 1 weight % Ti; c 0.1 weight ,6 C; 0.3 to 1.0
weight % Si; < 0.~ w~ight % Mn; 0.045 weight % P; 0.03
weight % S, the balance being iron) with a wire thic~ness
of 0. 25 mm and the following dimensions were used: Diameter
= 24 mm; width = 76 mm; volume (including pores) = 34.4 ml;
weight = 17 g. T~ey were annealed for 16 hours at 800C
and coated with CeO2 by dipping them six times into the
cerium hydroxide suspension described in Example 1, drying
Z~2
and annealing them. 6.4 g CeO2 was found to have been
applied to each of the two wire gauzes, of which each
was able to absorb 1.7 g H~O. The impregnation was effected
with the use of a platinum nitrate solution. Together with
the 1.7 ml solution, each wire gauze absorbed 60 mg Pt.
After the customary thermal after-treatment, the follow-
ing temperatures were determined, under the conditions
already described, for 50 and 90 % conversion, respecti-
vely, of CO and n-hexane:
50 ~ CO = 115C
90 % CO = 145C
C50 % n-hexane = 130C
CgO % n-hexane = 145 C
EXAMPLE 5: (Comparative Example)
The NH4N03-containing cerium hydroxide suspension of
Example 1 was used. Cerium oxide hydrate (cerium hydroxide)
was suction-filtered therefrom, washed with considerable
water, the filter cake was made up to 800 ml with the use
~0 of water, thoroughly suspended and then used for impregna-
tion. Dipped into this suspension was an ALUCHROM sheet
metal 0.04 mm thick, preannealed at 1000C. It was succes-
sively dried, heated for 1 hour to 300C and ultimately
annealed for 16 hours at 800C. The CeO2 so applied to
the metal surface was found to have little adhesiveness
for metals from which it was easy to remove by wiping. In
clear contrast with this, a tenaciously adhering CeO2-
coating fast to wiping was obtained with the use of the
NH4N03-containing suspension described in Example 1.
