FI75744C - Process for producing a catalyst cell used for purification of exhaust gases. - Google Patents

Description

75744

A method of making a catalyst cell for exhaust gas purification

The invention relates to a process for preparing a metal-based exhaust gas catalyst, in which a thin strip of aluminum-containing stainless steel is heat-treated at about 800-1000 ° C for 2-24 hours to form a thin oxide layer on which the support is applied, after which the honeycomb is dried. and coated with a catalyst.

Ceramic honeycombs are usually used as the bodies of the exhaust gas catalysts, but such honeycombs are very brittle and thus break easily during handling and use. Instead of ceramic honeycombs, new metallic honeycombs according to the present invention can be used, which are more durable and easier to manufacture.

Platinum metals are most commonly used as catalysts. In order to make them adhere sufficiently, firmly and evenly to the surface of the metal catalyst, a support layer is used between the catalyst layer and the metal foil. The support layer also increases the active surface area. The support is an alumina-containing substance that is slurried in water and its pH is adjusted to the acidic side. The support is then ground, and the excipients are added as usual

Ce (N0), as a result of which the thermal stability is improved and the 3 3 oxygen reserve is better when the car is running at different speeds. After grinding, the support is applied to the surface of the metal foil and calcined.

The prior art discloses methods for coating a honeycomb made of metal foil first with a support and then with a catalyst layer. Prior to the application of the support, the metal foil is annealed at a high temperature, forming an oxide layer on its surface, which thickens as oxygen and metals diffuse through it. Oxidation continues on both sides of the layer at a constant rate, forming a protective oxide film on the metal surface. Chromium is particularly effective in protecting steel from oxidation, forming a film containing chromium oxide on the surface of the metal at high temperatures. In order to improve the fire resistance, a little aluminum is often added to the metals in addition to chromium, which enhances the protective effect of the oxide film, thus allowing less chromium alloying. The diffusion rate of aluminum is more than 100 times that of chromium, which is why the oxidizing oxide layer on the surface of the aluminum / chromium steels used in the invention according to the application is almost completely Al 0.

2 3

A technique such as the one described above is already known from several publications, such as textbooks and manuals (see, e.g., Miekk-Oja, H.M., Metallioppi, Helsinki 1960, pp. 618-622).

U.S. Patent 4,279,782 describes a method of coating a metal foil made of an aluminum-containing stainless steel alloy with gamma aluminum having a heat-treated Al 2 layer obtained on the surface of the foil. Method 3 A mass of alumina-faced metal foil is first wetted with a gel formed by mixing about 4-6% by weight of aluminum monohydrate with water and this mixture is acidified with nitric acid to a pH below 2.0. GB 2,094,656 discloses a similar process in which an oxidized metal foil is first coated with a gel consisting of aluminum monohydrate in water and acidified with nitric acid to a pH of 2.0. The foil is then coated with aluminum slurry.

DE-A-2 339 513 discloses a process in which a metal cell is coated with a slurry prepared by stirring before adding a catalyst layer.

Ce (NO) .6H 0 with water-activated Al O powder 3 3 2 2 3. This slurry was dried, ground and calcined, then mixed with water and nitric acid. In this mixture, the honeycomb was dipped before coating with catalyst.

DE-A-2 306 114 discloses a process in which a monolithic support is coated with a catalyst using Al 0 as support and Pt, Pd, Cu-2 3 oxide and / or Ni oxide as active ingredients. Solution containing Cu (NO).

3 2 3H Buy, Ni (NO) .9H Buy and water were mixed with 2 3 3 2 HNO and stabilized with gamma-Al O silicon. Monolith 3 2 3 was coated with this mixture, dried and reduced to CO 3 11a.

When using the old technique mentioned above, in which the pH of the support slurry is adjusted to about 2 with nitric acid, this causes corrosion at the hole points formed by heat treatment on the surface of the oxide layer due to metal inhomogeneity and mechanical treatment of the metal strip. Under the conditions of use of the catalyst, this corrosion begins to spread. It has now surprisingly been found that the onset of corrosion during the coating step can be prevented in the process according to the present invention by using acetic acid instead of nitric acid. According to the invention, aluminum hydrate, such as boehmite, is used as a support, which is slurried in dilute acetic acid, after which any excipients are added to the ground mixture. The concentration of acetic acid used to adjust the pH is about 1-4%, preferably about 1.5%. The pH of the slurry is then slightly acidic.

When acetic acid is used, the sludge thickens on standing. By controlling the viscosity, solids content and grinding time of the slurry, the particle size distribution of the product can be affected. By applying the above-mentioned properties, support layers of different thicknesses can be produced in one coating. For example, for grinding the support, a slurry containing less acetic acid can be made first to obtain the most suitable conditions for grinding, and then acetic acid can be added to the support layer to obtain the most suitable slurry. The support can either be applied as a paste or sprayed on the surface of the metal strip.

Catalysts prepared using acetic acid have the same efficiency as catalysts prepared by conventional methods. The invention will now be described by way of example.

Example 1 225 g of boehmite were mixed with 385 g of 1.5% acetic acid at room temperature. The pH of the slurry was 4-5. The boehmite mixture was ground in a ball mill to an average particle size of less than 5. The slurry was allowed to stand, thickening to an easy to apply paste. The resulting paste was applied to the surface of the heat-treated metal foil and dried at 0 ° C and calciner for 4 h at 550 ° C. The obtained support layer had a thickness of 20-25 μm and good mechanical strength. The support coated strip was immersed in Pt, Rh solution, dried, annealed and reduced with hydrogen.

A catalyst with 0.2% Pt and 0.02% Rh was prepared from the strip. Its efficiency was tested with a gas mixture of 1.7% CO, 10.0% CO, 0.03% CH, 0.07% NO, 1.0% O, 10.0% 2 3 8 o 2 H 2 O and the rest N ^. The gas temperature was 400 C and the turnover 37000 / h. The CO, hydrocarbon and NO conversions were 95, 94 and 99%, respectively. The catalyst was aged in a 20 m test engine for 300 h, after which it was tested as described above. The conversions were then the same as with the new catalyst.

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Claims (3)

5,75744 A process for producing a metal-backed exhaust gas catalyst, which comprises heat-treating a thin strip of aluminum-containing stainless steel at about 800-1000 ° C for 2-24 hours to form a thin oxide layer on the surface of which the support is applied, after which the honeycomb is dried, and coating with a catalyst, characterized in that aluminum hydrate, preferably boehmite, is used as a support, which is slurried in water and the pH of the slurry is adjusted to the acid side with acetic acid, after which any excipients are added to the ground mixture. Process according to Claim 1, characterized in that the concentration of acetic acid used to make the slurry is about 1 to 4%, preferably about 1.5%. Process according to one of the preceding claims, characterized in that Ce (NO) is used as auxiliary in the slurry. 3 3