CATALYST CARRIERS AND A PROCESS FOR PRODUCING THE SAME

Abstract: A catalyst carrier is disclosed, which comprises a substrate made of a stainless steel at least containing no aluminum, and an alumina thin layer formed on surfaces of the substrate through an aluminum-iron compound layer. The catalyst carrier is shaped as a porous body having gas permeability in at least one direction. A process for producing such a catalyst carrier is also disclosed, which comprises the steps of: (i) preparing a metal strip from a stainless steel at least containing no aluminum; (ii) plating surfaces of the metal strip with aluminum; (iii) rolling the aluminum-plated metal strip; (iv) converting the rolled metal strip to a corrugated porous body having gas permeability in at least one direction; and (v) forming an alumina thin film by oxidizing the plated aluminum layer through heat treating the porous body at 600 to 1,000°C for not less than 10 minutes.

Description: The present invention relates to a method of producing catalyst carriers using a metallic material (hereinafter referred to as "metallic catalyst carriers") for purification catalysts to purify combustion waste gases exhausted from automobiles, factories, etc. The invention also relates to such catalyst carriers produced according to this method. Catalyst carriers may be prepared by various methods. For example, EP-A-243702 discloses a process for producing a catalyst carrier, which comprises producing a honeycomb structure of sheets of stainless steel (preferably aluminium containing) and an aluminium/aluminium alloy in an outer structure of resistant steel. This structure is firstly heat treated in a reducing atmosphere to avoid oxide film formation. Following this, an alumina film may be subsequently applied to the treated structure. GB-A-2005149 describes catalyst carriers comprising iron or steel, coated with an aluminium layer of about 40 micrometers, and having an aluminium/iron diffusion layer formed by tempering. An alumina layer may be subsequently applied by wash coating if desired. Japanese Patent Application Laid-open No. 56-96,726 discloses a technique using, as a metallic catalyst carrier, a foil made of an aluminium-containing ferrite type stainless steel in which alumina whiskers are formed on surfaces of the foil through heat treatment. According to this technique, stainless steel which can withstand high temperatures is used, and the alumina whiskers are present on its surface. Therefore, when a catalyst such as γ-Aℓ₂O₃ is carried on such a catalyst carrier as a wash coat, the alumina whiskers function as wedges. Thus, the wash coat made of a catalyst such as γ-Aℓ₂O₃ can firmly be bonded to the metallic catalyst carrier. However, in the catalyst carrier disclosed in Japanese Patent Application Laid-open No. 56-96, 726, since the ferrite type stainless steel as a substrate contains aluminium, its plastic deformability is much lower than would otherwise be the case. Thus, it takes many steps to obtain a foil of the stainless steel of this kind, which makes mass production difficult. It is an object of the present invention to alleviate the above-mentioned problems, and to provide a method of producing catalyst carriers in which an alumina thin layer can firmly be bonded to a substrate and which is free from reduction in plastic deformability. According to the present invention, there is provided a method of producing a catlayst carrier as set out in Claim 1. With such method, no Aℓ is contained in the substrate to produce alumina whiskers. Instead, the surfaces of the substrate are plated with aluminum, and the alumina thin layer which has substantially the same performance as those of the alumina whiskers is formed by heat treating the plated aluminum layer under the specific conditions. Therefore, mass productivity can be improved without damaging the plastic deformability. In addition, in the case where plating is effected with aluminum as in the present invention, a pretreatment such as coarsening of the surfaces to be plated is unnecessary, which is not the case when the alumina is directly coated on the substrate. Accordingly, the production steps can be simplified. Although an aluminum-iron compound is formed between the plated aluminum layer and the substrate made of the stainless steel since the aluminum diffuses into the substrate, such a compound does not influence bondability between the plated aluminum layer and the substrate when its thickness is not more than 40 µm. If the heating temperature is less than 600°C, it takes a long time to form the alloy and a sufficient alumina thin layer is not formed. Thus, a temperature of less than 600°C is unpractical. On the other hand, if the heating temperature is more than 1,000°C, the alloy layer is peeled due to rapid alloying. If the heating time is less than 10 minutes, the formation of the alumina layer or the alloying are not sufficiently effected. Therefore, the heating conditions are limited to the heating temperature: 600 to 1,000°C and the heating time: not less than 10 minutes. These and other optional features, and advantages of the invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled in the art to which the invention pertains. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a flow chart explaining an example of the process for producing the catalyst carriers according to the present invention; and Figs. 2a and 2b are a perspective view and an enlarged view of a part of an embodiment of the catalyst carrier produced according to the present invention. The present invention will be explained in more detail below. As will be mentioned later in connection with specific examples, if the thickness of the plated aluminum layer exceeds 10 µm, the aluminum-iron compound layer is likely to peel off due to influences in the difference of thermal expansion. On the other hand, it is very difficult to control the thickness of plated aluminum layer when thickness is less than 1 µm. Therefore, the thickness of the plated aluminum layer is preferably in a range from 1 to 10 µm. Next, preferred embodiments of the respective constituent features of the present invention will be explained in more detail. (1) Substrate: a) As the shape of the substrate, those given in Table 1 are preferably used. Shape Use state of substrate Strip Thickness : 0.03 to 0.50 mm Surface conditions : flat, embossed, finely corrugated, perforated, or the like b) Composition range of the stainless steel (Units are all "% by weight"): It is preferred that the stainless steel is composed of the ingredients and additive elements given in the following Table 2. Among the stainless steels, the following ones in Table 3 are recited as preferred. SUS304 type Cr : 18∼20 Si : not more than 1.00 Mn : not more than 2.00 Ni : 9.0 ∼ 13.0 Fe : remainder SUS316 type Cr : 16 ∼ 18 Si : not more than 1.00 Mn : not more than 2.00 Ni : 12.0 ∼ 15.0 Mo : 2.0 ∼ 3.0 Fe : remainder SUS321 type Cr : 17 ∼ 19 Si : not more than 1.00 Mn : not more than 2.00 Ni : 9.00 ∼ 13.0 Fe : remainder SUS410 type Cr : 11.0 ∼ 13.5 Si : not more than 1.00 Mn : not more than 1.00 Fe : remainder Other type Cr : 13 ∼ 18 Fe : remainder (2) Shape of the porous body: As given in the following Table 4, preferred shapes of the substrates are present. Substrate Shape of porous body Strip A porous body is shaped by corrugating a thin strip of 0.03 to 0.50 mm in thickness to form numerous through holes in an axial direction. A porous body is shaped by preliminarily embossing, perforating, or finely corrugating a thin strip of 0.03 to 0.50 mm in thickness, further corrugating and shaping it to form numerous through holes in an axial direction. (3) Plated aluminum layer: The plated aluminum layer is composed of not less than 80% by weight of aluminum as a main ingredient, and as is clear from the examples mentioned later, its thickness is preferably in a range from 1 to 10 µm. The plated aluminum layer may be formed by a known process such as a hot dipping in a molten aluminum bath. Next, the process for producing the catalyst carriers according to the present invention will be roughly explained with reference to a flow chart of Fig. 1 by way of example. In this flow chart of Fig. 1, a corrugated honeycomb catalyst carrier is produced from a metal strip. First, the metal strip having a given composition as recited above is produced from a stainless steel containing no aluminum. A conventionally known process such as rolling may be used for producing the strip. Next, the surfaces of the strip are plated with aluminum in a given thickness. Then, the plated strip is rolled to obtain a metal foil. The metal foil is corrugated. The thus corrugated metal foils and the non-corrugated metal foils are alternatively piled one upon another, which is coiled to obtain a honeycomb substrate. The thus obtained honeycomb substrate is heat treated under given conditions, so that the plated aluminum layer is oxidized to give an alumina thin film and that an aluminum-iron compound layer is formed at the same time. Thereby, a catalyst carrier having gas permeability in at least one direction is obtained. Figs. 2a and 2b are a perspective view and an enlarged view of a part of the catalyst carrier according to the present invention in an uncoiled state. This catalyst carrier is obtained by placing a corrugated foil A' on a non-corrugated flat metal foil A, and coiling them together. In Fig. 2b, reference numerals 1, 2 and 3 are a stainless steel, an aluminum-iron compound layer, and an alumina thin film, respectively. In the following, actual examples of the catalyst carriers will be explained. In order to produce a metal strip as a substrate, a stainless steel having a specific composition was melt in a vacuum melting furnace to obtain an ingot. The obtained ingot was repeatedly subjected to cold rolling and annealing after hot rolling, thereby obtaining a metal plate having a thickness of about 1 mm. The metal plate was cut to 100x500 mm as a substrate to be plated by hot dipping of molten aluminum. The substrate was coated with alumina in the following procedure. First, in order to conduct hot dipping of molten aluminum bath containing aluminum alloy of 8% by weight of Si was prepared. Next, the metal substrate was treated with a mixed acid solution containing 10 wt% of nitric acid and 2 wt% of hydrofluoric acid for about 10 minutes as pretreatment. After the thus treated metal substrate was immersed into the molten aluminum bath prepared above, an amount of aluminum attached was adjusted by gas wiping. The thickness of the plated aluminum was further adjusted by rolling the hot-dipped substrate, and the substrate was cut into a test substrate of 100 mm in square. The thickness of the plated aluminum was adjusted to 5 µm in each example. Thereafter, the metal substrate having thus plated aluminum layer was heat treated. The heat treatment was carried out by heating the metal substrates at a temperature range from 500 to 1,100°C for various time periods in air as shown in Table 5. In order to evaluate characteristics, a section of each of test samples thus obtained was observed with a microscope. Results are shown in Table 5. It is seen that the heat treating conditions shall need to be 600 to 1,000°C for not less than 10 minutes. With respect to the stainless steel substrate meeting the composition range according to the present invention among the above-mentioned examples, test pieces were prepared by heat treatment at 900°C for 10 minutes in air, while the thickness of the plated aluminum layer being varied. In order to evaluate their characteristics, the thus obtained test pieces were subjected to thermal shock test through heating-cooling cycles by using a device for automatically placing them into an electric furnace at 900°C in air and extracting them therefrom. The test was repeated to 500 cycles between 900°C for 15 minutes and room temperature for 45 minutes. The surface state of each of the test pieces was observed with naked eyes. Results are shown in Table 6. From Table 6, it is clear that the thickness of the plated aluminum layer is preferably not more than 10 µm. As is evident from the foregoing explanation, according to the catalyst carriers and the producing process therefor in the present invention, no aluminum is added into the substrate to produce alumina whiskers. Instead, the surfaces of the substrate are plated with aluminum, and the plated aluminum layer is oxidized by heat treatment to form the alumina thin film. Thus, the catalyst carriers which have gas permeability in at least given one direction and are useful for the purification of various kinds of waste gases can be obtained by simpler producion steps without damaging plastic deformability.

Claims: A method of producing a catalyst carrier comprising the steps of: i) plating a metal strip made of a stainless steel containing no aluminum, with aluminum; ii) rolling the aluminum-plated metal strip; iii) corrugating the rolled metal strip to form a porous body having gas permeability in at least one direction; and iv) forming an aluminum-iron layer by reacting the plated aluminum with the stainless steel, and an alumina film on the rolled metal strip by oxidizing the plated aluminum, characterized in that in step (i) said metal strip is plated with aluminum to a thickness in the range 1 to 10 µm and in that in step (iv) said aluminum-iron layer and said alumina film are formed simultaneously by heat treating the porous body at 600-1,000°C in air for not less than 10 minutes. A method according to Claim 1, wherein the porous body is formed by corrugating the rolled metal strip in at least one direction and then coiling the corrugated strip. A method according to Claim 1, wherein the porous body is formed by corrugating the rolled metal strip, laminating the corrugated metal strip onto a non-corrugated rolled strip and coiling the laminate. A method according to any one of Claims 1 to 3, wherein the thickness of the aluminum-iron alloy layer is not more than 40 µm.

Inventors: SHIBATA, MOTONOBU
Applicants: SHIBATA, MOTONOBU
Application year: 1988
Main classification: B01J37
IPC codes: B01J35, B01D53, B01J37, F01N3