JPH0113901B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an improvement in a metal carrier for a monolithic catalyst, which supports a catalyst component consisting of activated alumina and a noble metal by coating it on its surface. As catalysts for purifying exhaust gas from automobile engines, etc., so-called monolith catalysts, in which catalyst components consisting of activated alumina and precious metals are coated on the surface of a carrier, are widely used. Conventional carriers for monolithic catalysts are commonly extruded ceramic products called cordierite, which have excellent support for catalyst components (peel resistance); however, this type of ceramic body lacks its own strength. It had the disadvantage of being insufficient and having poor durability. As an alternative carrier, a metallic carrier has been proposed in which an alumina layer is formed on the surface of a metallic substrate and a catalyst component is coated on this alumina layer (for example, see Japanese Patent Publication No. 59-32183). However, in such metallic carriers, the alumina layer itself tends to peel off from the metallic substrate, and therefore,
Although there was no problem in terms of strength, the ability to support catalyst components was still insufficient, making it difficult to put it to practical use. The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a metal carrier for a monolith catalyst that improves the bonding strength between the metal substrate and the alumina layer and exhibits higher support capacity. It is said that The metal carrier according to the present invention contains 10 to 25% Cr, Al2
~5%, C0.04% or less, N0.005~0.05%, Ti0.1~
0.6%, Zr0.01~0.5% balance Fe, and (Ti+
An oxide layer containing Ti and Zr in solid solution and/or oxide form and mainly composed of α-Al ₂ O ₃ on the surface of a metal substrate having a composition with a Zr)/N value of 10 to 150. This oxide layer is wedged into the metal substrate and is strongly and densely bonded to the metal substrate, supporting the catalyst component to be coated without peeling. can do. The metal carrier of the monolithic catalyst according to the present invention is
A metal substrate having the above composition is heated preferably at 1000 to 1400℃.
It can be obtained by heating in an oxidizing atmosphere at about 1300° C. for an appropriate time, for example, about 2 hours or more in the air, and after the prescribed heating, cooling in a furnace or cooling in the air. If the heating temperature is not above 1000℃, Fe−Cr−
Due to the excellent oxidation resistance of Al-based alloys, it is not possible to obtain a desired oxide layer, and temperatures above 1400°C are undesirable because the metal substrate itself deteriorates. In addition,
The heating time may be appropriately determined depending on the oxygen potential in the atmosphere and the required layer thickness. The reason for limiting the composition of the metal substrate is as follows. Cr is an essential component for imparting oxidation resistance. If it is less than 10%, oxidation resistance will be insufficient, and if it exceeds 25%, workability will deteriorate, so 10 to 25% is appropriate, and about 20%. is most preferred. Al is also essential for imparting high-temperature oxidation resistance, and if it is less than ₂ %, the oxidation resistance will of course be _insufficient . ) In order to suppress the production of oxides with poor oxidation and corrosion resistance such _{as 2O3} , a minimum content of 2% is required. 5%
A value of around 3% is optimal. Note that Cr and Al are used to increase the creep resistance and oxidation resistance of the metal base itself against repeated high-temperature heating, rather than the oxide film, as in conventional Fe-Cr-Al electrothermal alloys, and the amounts added are also different from those of conventional products. They are the same and the same. C forms carbides that do not dissolve in the matrix with Ti, Zr, Cr, etc., and the generation of a large amount of these carbides not only causes grain boundary fracture, but also increases the amount of Ti and Zr added to strengthen the oxide film. Since it reduces the effect, it is desirable to keep the amount as small as possible, and the allowable range is 0.04% or less. N suppresses the diffusion of C and prevents the coarsening of carbide particles, but it has a stronger affinity with Ti and Zr than C, forming nitrides TiN and ZrN, and it has a stronger affinity with Ti and Zr than with C.
It is desirable to keep the amount as small as possible, as it reduces the effect of , and the upper limit is 0.05%. Further, as will be described later, since it is thought that N influences various properties of the oxide film using the value of (Ti+Zr)/N as a guideline, the lower limit was set at 0.005% from this viewpoint. As already mentioned, Ti and Zr are added to form a dense and strong peel-resistant coating on the surface of the metal substrate by their diffusion behavior when heated to high temperatures. Ti dissolves in α-Al ₂ O ₃ and makes the oxide film itself dense, and if it is less than 0.1%, it has no significant effect, while if it is more than 0.6%, it becomes difficult to work at room temperature.
0.1-0.6% is appropriate. Zr fixes N, improves the effectiveness of Ti, and improves the peeling resistance of the oxide film. Furthermore, like Ti, Zr dissolves in α-Al ₂ O ₃ and contributes to the densification of the oxide. From the standpoint of fixing N, at least 0.01% is necessary, but if it exceeds 0.5%, the oxidation resistance will decrease significantly, so 0.01%~
0.5% is appropriate. The value of (Ti+Zr)/N (weight % ratio) is an important parameter for the formation of an oxide layer during heat treatment and peeling resistance, and if it is less than 10 or more than 150, the necessary peeling resistance cannot be obtained. As schematically shown in FIGS. 1A to 1E, when the above value (Ti+Zr)/N is varied, the bonding state between the oxide layer produced by the heat treatment and the metal substrate changes in various ways. In type A shown in Fig. 1A, the metal substrate and the α-Al ₂ O ₃ layer in which ZrO ₂ is diffused are bonded by a relatively smooth uneven surface, and the α-Al ₂ O ₃ layer is ,
A thin film is produced in which Al ₂ O ₃ and TiO ₂ are compatible. Type B shown in Figure 1B has a metal base and α-Al ₂ O ₃ in which ZrO ₂ has been diffused, bonded in a complicated manner, and Type C shown in Figure 1C is different from Type B. A (Fe, Cr) ₂ O ₃ thin film is formed on a similar α-Al ₂ O ₃ layer, and the α-Al ₂ O ₃ layer is slightly smaller in the D type shown in Figure 1 D than in the C type. It becomes thinner, the boundary between the α-Al ₂ O ₃ layer and the (Fe, Cr) ₂ O ₃ layer becomes gently uneven, and ZrO ₂ becomes diffused in the (Fe, Cr) ₂ O ₃ layer. . Furthermore, in the E type shown in Figure 1E, the metal substrate and the α-Al ₂ O ₃ layer are bonded with a gently uneven surface, similar to the A type, and the α-Al ₂ O ₃ layer becomes thinner. ,
Conversely, the thickness of the (Fe, Cr) ₂ O ₃ layer in which ZrO ₂ is diffused increases. Among these bonding states, type A and type E are
While the metal substrate and the oxide layer mainly composed of α-Al ₂ O ₃ are bonded by a gently uneven surface,
Types B, C, and D have a metal base and oxide layer bonded in a complicated manner, and have much better bonding and adhesion than types A or E. Conceivable. As shown in the table below, the measurement results of the adhesion tests conducted on these types A to E indicate that in fact, B
Types C, C, and D exhibit superior peeling resistance compared to A and E types. For adhesion test, test sample 30mm x
After heating 10mm x 2mm at 1300℃ for 20 hours, sandblasting was performed for 5 minutes.
The oxidation weight gain and peeling amount in that case were measured.

[Table] As is clear from this table, type B, type C, and type D were created for reference as well as type A, E.
It exhibits much superior peeling resistance compared to the Fe-Cr-Al-Y alloy sample. Further, FIG. 2 shows the formation range of A to E type oxide layers when the Zr amount and the value of (Ti+Zr)/N were varied. As is clear from Figure 2, type A is formed in a region where (Ti+Zr)/N is relatively small compared to other types, while type E is formed in a region where (Ti+Zr)/N is relatively small compared to other types.
Type B, type C, and type D are formed in a region intermediate between type A and type E. The formation mechanism of types A to E as described above is not necessarily clear, but it depends on the diffusion rate of the alloying element in the metal substrate, the affinity with oxygen, and the value of (Ti+Zr)/N mentioned above. This is thought to be the result of a complex interrelationship. The metal substrate having the composition as described above is molded in advance into an optimal shape as a monolith catalyst and then heat-treated to form a B-type, C-type as described above.
A type or D-type oxide layer may be formed, and the metal carrier thus obtained may be coated with a catalyst component consisting of γ-Al ₂ O ₃ and a noble metal by dipping or the like. The monolithic catalyst obtained in this way has an oxide layer, which is a support for catalyst components, firmly and densely bonded to a metal substrate, so it is exposed to high-temperature exhaust gas and subjected to various vibrations. can also support catalyst components without causing peeling,
The supporting capacity can be significantly improved, and it can greatly contribute to the practical application of metal carriers that are advantageous in terms of strength.

[Brief explanation of drawings]

Figures 1A, B, C, D, and E are enlarged cross-sectional explanatory diagrams schematically showing the structure of the metal substrate and the oxide layer formed thereon, and Figure 2 shows the amount of Zr on the horizontal axis.
2 is a graph showing the formation region of the oxide layer shown in FIGS. 1A to 1E with (Ti+Zr)/N as the vertical axis.

Claims (1)

[Claims] 1 Cr10~25%, Al2~5%, C0.04% or less,
N0.005~0.05%, Ti0.1~0.6%, Zr0.01~0.5%,
The remainder is Fe, and the value of (Ti+Zr)/N is 10 to 150
An oxide layer of α-Al ₂ O ₃ containing Ti and Zr in solid solution and/or oxide form is formed on the surface of a metal substrate having a composition of A monolithic catalyst metal carrier coated with a catalyst component consisting of precious metals and precious metals.