CA1142502A

CA1142502A - Production and use of a catalyst carrier

Info

Publication number: CA1142502A
Application number: CA000372117A
Authority: CA
Inventors: Wilhelm Vogt; Hermann Glaser; Helmut Dyrschka
Original assignee: Hoechst AG
Current assignee: Vinnolit Monomer GmbH and Co KG
Priority date: 1980-03-21
Filing date: 1981-03-02
Publication date: 1983-03-08
Also published as: EP0036938A3; EP0036938A2; EP0036938B1; ATE3953T1; DK125981A; DE3010950A1; DE3160506D1; JPS56147634A

Abstract

PRODUCTION AND USE OF A CATALYST CARRIER

ABSTRACT OF THE DISCLOSURE
The invention relates to the production of a cata-lyst carrier. To this end, a chrome/aluminum-steel struc-ture is annealed over a period of 2 to 50 hours, at tempe-ratures within the range 800 to 1100°C with admission of air, and a cerium dioxide layer is applied to the surface of the annealed structure. More specifically a 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; is annealed, the annealed structure is impregnated with a suspension of cerium oxide hydrate in an aqueous ammonium nitrate solution; dried and anne-aled for 5 to 20 hours at temperatures within the range 600 to 1000°C.
The invention also relates to the use of the cata-lyst carrier in the production of a catalyst for the de-contamination of exhaust gases of internal combustion engines or production facilities. To this end the cata-lyst carrier is coated 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 80°C and heating it at 250 to 400°C.

Description

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;

.. . . .

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.

~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.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the production of a catalyst carrier which has a cerium dioxide layer applied to its surface, 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;
over a period of 2 to 50 hours, at temperatures within the range 800 to 1100°C with admission of air; wetting the annealed structure with a suspen-sion of of cerium oxide hydrate in an aqueous ammanium nitrate solution; drying the structure and annealing it for 5 to 20 hours at temperatures within the range 600 to 1000°C.

2. A process as claimed in claim 1, wherein the cerium oxide hydrate suspension is made by mixing in aqueous cerium-III-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.

3. A process as claimed in claim 1, wherein the wetting, drying and annealing operations are repeated up to 5 times at temperatures within the range 600 to 1000°C.

4. A process as claimed in claim 1, wherein the wetted structure is dried at temperatures within the range 40 to 80°C and preheated at tempera-tures within the range 250 to 400°C.

5. A process as claimed in claim 1, wherein the structure is coated with a 10 to 150 % proportion of cerium dioxide, based on its weight.

6. A process for making a catalyst for use in the deconta-mination of exhaust gases of internal combustion engines and production facilities, which comprises: coating the catalyst carrier of claim 1 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 80°C and heating it at 250 to 400°C.

7. A process as claimed in claim 6, wherein the catalyst carrier is first tested for its power of absorbing water and then impregnated with the solution of the at least one noble metal compound which is so concentrated that the quantity of solution just absorbable by the cata-lyst carrier has the desirable proportion of noble metal contained therein.

8. A process as claimed in claim 6, wherein nitrates or ace-tates of rhodium, palladium or platinum, hexachloropal-ladium-IV-acid or hexachloroplatinum-IV-acid are used as the noble metal compound.