JPS61291040A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To enhance the mechanical strength of a catalyst at low temp., by supporting a catalytic substance by a ceramic carrier obtained by applying heat treatment to a composition consisting of rehydratable alumina, alkali titanate and fused silica. CONSTITUTION:A composition comprising a mixture of rehydratable alumina, alkali titanate and fused silica is heat-treated at 1,100-1,300 deg.C to form a ceramics catalyst carrier. The aforementioned composition pref. consists of 5-20wt% of rehydratable alumina, 1-10wt% of alkali titanate and 70-94wt% of fused silica. As rehydratable alumina, for example, there are pi-Al2O3 and amorphous alumina, etc. As alkali titanate, potassium titanate is designated and pref. fibrous.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a decaramic molded body having a small coefficient of thermal expansion used as a catalyst for purifying combustion exhaust gas for automobiles, industrial use, and consumer use.

BACKGROUND OF THE INVENTION Most of the ceramics with a small coefficient of thermal expansion used in conventional catalyst carriers for exhaust gas purification are cordierite ceramics. In addition, alumina ceramics and mullite ceramics are also used. but,
These are alumina-based.

The coefficient of thermal expansion of mullite ceramics is 3 to 5 times larger than that of cordierite ceramics, and there are few practical examples of this. Cordierite is 2 MgO・2A1
203.5 is a ceramic represented by SiO□,
This is talc (M75(Si40, (,)(o'
K)2), kaolin (A12Si205(OH)4), and alumina (k1203) are mixed in the desired ratio of cordierite components, mixed, dehydrated, and molded.

31, - Manufactured by drying and sintering. By the way, the weight percent of each component in the theoretical composition of cordierite is Mg○13.8%.

A40334.9%, 5i0251.3 crab. The sintering temperature is usually about 1400°C for about IJ (d).The coefficient of thermal expansion of this material is 1.2 to 1.
4X 10-6/deg (room temperature to 100Q°C).

The sintering temperature for alumina-based and mullite-based ceramics is also high, exceeding 1300°C (Special Publication Publication No. 54-156).
No. 4, Japanese Patent Publication No. 51-20358).

In this way, cordierite type and alumina type.

Conventional catalyst carriers made of mullite-based ceramics were subjected to high-temperature sintering to produce a material with a small coefficient of thermal expansion and further improve mechanical strength. The carriers of these conventional methods are:

During sintering, it became extremely vitrified and its specific surface area was extremely small (the specific surface area of cordierite mink is less than 1 m/y). Therefore, it is difficult to directly support a catalyst substance on these conventional catalyst carriers, and even if the catalyst can be supported, it is insufficient as a catalyst for exhaust gas purification, and a catalyst sufficient for WtII cannot be obtained. It is thought that for this reason, the noble metals that are held together tend to move and diffuse on the carrier due to heat, causing rapid deterioration of the catalytic ability. Therefore, conventional catalyst carriers made of cordierite, alumina, and mullite are subjected to a pretreatment called wash coating (surface coating with powder such as γ-ke20s). The current situation is that a catalyst material is then supported. In addition, when sintering the catalyst carrier made of conventional cordierite ceramics, approximately 3
There is zero-temperature sintering shrinkage, making it difficult to maintain dimensional accuracy, making the manufacturing process complicated, reducing product yield, and increasing costs due to the high sintering temperature. One of the other drawbacks is that the coefficient of thermal expansion is 1.2 to 1.4 x 1.
o''''/deg (room temperature to 100°C), there were problems such as cracking or breaking during actual use depending on the application. Particularly in recent years, the thermal shock properties required for automotive catalysts have become more severe as the performance of engines has improved, and the reduction in the coefficient of thermal expansion has been reduced to the expected level of 5. The invention was made in view of these points, and provides a low-cost exhaust gas purification catalyst that has a smaller coefficient of thermal expansion than conventional catalysts, provides mechanical strength at a lower heat treatment temperature, and has a smaller specific surface area. It is.

Means for Solving the Problems In order to solve the above-mentioned problems, the exhaust gas purifying catalyst of the present invention is provided by heat-treating a composition comprising at least rewaterable alumina, an alkali titanate, and fused silica at a relatively low temperature. In this way, a ceramic having excellent physical properties as a carrier is obtained, and this is used as a catalyst carrier for exhaust gas purification, and a catalytic material is supported on the ceramic as a catalyst for exhaust gas purification. explain. Rehydratable alumina, which is an essential component in order to obtain the catalyst carrier of the present invention, is α-ke2, which is obtained by thermally decomposing alumina hydrate.
Transition alumina other than 03, e.g. ρ-ke20s
and amorphous alumina, etc. ○Industrially, for example, alumina hydrate such as alumina trihydrate obtained from the Bayer 1 process is used at a concentration of about 400 to 1200
' C hot gas is usually brought into contact with a fraction of a second to 10 seconds,
Or alumina hydrate under reduced pressure at about 250~9o○°
Examples include those having a loss on ignition of about 0.5 to 15% by weight, which can be obtained by heating and holding C for 1 minute to 4 hours.

Next, alkali titanate has the general formula M20・nTi02
(In the formula, M' [, lithium, strium, potassium.

Represents an alkali metal atom selected from rubidium, cesium, barium, strontium, and calcium, n
td is an integer greater than or equal to 1. ).

Next, another essential component, fused silica, will be described. Fused silica has a thermal expansion coefficient of 0.5X 10-6
/ deg (room temperature to 1000°C) and is generally known as a material that reduces thermal expansion. , alkaline components such as potassium and calcium, and alumina, tetanya, etc.)
When heat-treated at a temperature of 1,000°C or higher in a mixture with
From this point of view, a composition consisting of at least the above three components from Okajicho, which has not been used conventionally as a ceramic or Nox with excellent thermal shock resistance as in the present invention, was heated at room temperature to 1000 to 1000 liters. By heat-treating in the range of 1300''C, a catalyst carrier with excellent thermal shock resistance and a relatively large specific surface area was obtained, although the reason is not clear. In addition to alumina, alkali titanate, and fused silica, it is also possible to optionally add molding aids (for example, cMC, Mc), plasticizers (glycerin, vaseline), and the like.

Next, the physical properties of the catalyst carrier manufactured from five or more components are shown. The coefficient of thermal expansion is 1.2 x 10-6/deg (from room temperature to 10
00"C) or less. The thermal shock resistance temperature was also so
It had excellent thermal shock resistance, exceeding O'C. An example of specific surface is shown.

[Example 1] Wall thickness 0.3 words, − side 1. A honeycomb formed body consisting of EJM square cells was heated to 1200°C at a heating rate of 100'C/hour.
The temperature was raised at ``C'' and further heat treated at 1200°C for 1 hour.

A catalyst was prepared by using the thus obtained honeycomb-shaped ceramics as a catalyst carrier, and carrying platinum (Pt) and rhodium (Rh) in an amount of 1.0 g and 20oJ9 per catalyst carrier 1e. Note that platinum is chloroplatinic acid (H2PtCd6
) is rhodium nitrate (Rh(NOs) s
), a 5:1 ratio of PT:Rh was mixed, impregnated, dried, and activated (heat treated at 500'C in N2 atmosphere) to make an exhaust gas purification catalyst.For comparison, commercially available cordierite was used. A comparative example in which platinum and rhodium catalyst materials were supported in the same manner as described above using a catalyst carrier based on the above-described method. The above two exhaust gas purification catalysts were evaluated using the following two evaluation methods.

■ Using the above exhaust gas purification catalyst, the catalyst temperature is 200°.
Space velocity: 20,000 [”, Go
1. Initial purification performance and electric furnace under conditions of inlet concentration: 600 ppm (in air). ○OO'C Purification performance after heat treatment for 6o hours was measured and evaluated.

■ Each of the above exhaust gas purification catalysts was installed in the exhaust gas path of a car equipped with a 2800CC engine, and the air-fuel ratio was adjusted to 14. ○
~15.5 with a width of 0.1, Go, H
The purification rate of G, NoX gas was measured and evaluated at the initial stage and after 6 hours of bench durability.

Table 1 shows the results obtained using evaluation method (■). The results of evaluation method (2) are shown in Figures 1 (A) and (B). (A) is the result of the catalyst of the example of the present invention, and (B) is the result of the comparative example of the catalyst made from the cordierite carrier. Thermal deterioration was extremely poor compared to the example catalyst.

Similarly, in the bench durability test results shown in FIGS. 1(A) and (B), the comparative example catalyst had Go, He,
Deterioration was observed from the beginning for each component gas of NOx.

[Example 2] The proportions of rehydrating aluminum, potassium titanate, and fused silica were varied.As in Example 1, the molding aid, plasticizer,
A honeycomb-shaped ceramic was prepared in the same manner as in Example 1 by adding water, and the thermal shock resistance was evaluated. Platinum and rhodium catalyst materials were also supported in the same manner as in Example 1, and evaluated using the evaluation method (2).

As a result, the composition ranges with excellent thermal shock resistance are shown in the shaded area in Figure 2, where rehydratable alumina is 5-20% by weight, alkali titanate is 1-10% by weight, and fused silica is 70-9% by weight.
It was in the range of 4 weight class. The thermal expansion and contraction coefficients at temperatures outside this range were as shown in FIG. 3(a). The composition of the present invention was as shown in (b). From the curve in (a), it is easy to think that substances such as cristobalite and trichimanite were partially crystallized from fused silica, and the coefficient of thermal expansion was 0.1, which coincided with the poor thermal shock resistance.

Table 2 shows the catalyst performance results according to evaluation method (■).

As is clear from Table 2, the compositions outside the range of the present invention showed poor thermal deterioration. (However, thermal deterioration is small compared to catalysts made from conventional cordierite catalyst carriers.) The reason for this is not clear, but it is thought that some of the precious metals such as platinum and palladium diffuse into the carrier, resulting in a decrease in catalytic performance. [Example 3] Rehydratable alumina 10% by weight, alkali titanate 5% by weight (the alkali component is a salt selected from lithium, sodium, potassium, rubidium, cesium, barium, strontium, and calcium) and fused Silica 86% by weight0)) Methyl cellulose as a molding aid 4. ○ parts by weight, 2.0 parts by weight of glycerin as plasticizers, and 32 parts by weight of water were added to produce honeycomb-shaped ceramics in the same manner as in Example 1, and platinum and rhodium were supported in the same manner as in Example 1, and evaluated. Evaluation was made using method ■. The results are shown in Table 3.

(The following is a blank space) 13B-7 As is clear from Table 3, the compositions consisting of each alkali titanate salt and fused silica are
It was excellent with little deterioration due to heat treatment. Among all, those using potassium titanate showed extremely little thermal deterioration.

[Example 4] Table 4 shows the physical properties of each alkali titanate salt in Example 3 when it is fibrous. It was observed that the thermal deterioration rate of the catalyst tended to decrease as the temperature increased.

[Example 6] Purification ability after O heat treatment was evaluated by measuring the catalytic performance of the Nl 3 -Nicum molded product of Example 4 at a heat treatment temperature range of 1000 to 1400°C using the evaluation method (2) above. The results are shown in 015 in Figure 4. From Figure 4, thermal deterioration of the catalyst is significant when the heat treatment temperature is below 1100°C. Unfortunately, thermal deterioration was poor even at temperatures above 1300°C.The reason for this is not clear, but ceramic supports obtained by heat treatment at temperatures below 1100°C are unstable to heat, and the supported platinum and rhodium It seems to cause a reaction with precious metals and reduce the catalyst performance.On the contrary, at 1000°C
The ceramic carrier obtained by the above heat treatment temperature is very stable against heat, but the surface is vitrified and the specific surface area is 1. Onf/g or less, supported platinum.

It is thought that the rhodium catalyst material migrated and diffused on the surface of the carrier, causing aggregation and rapid deterioration.

In addition, scanning electron micrographs showing the surface conditions such as the particle state of the cross section of the honeycomb-shaped Ceraconox carrier obtained by heat-treating at 1000°C and the honeycomb-shaped Ceraconox support obtained by heat-treating at 1400°C are shown below. It is shown in Figure 6. Figure 6 (A
) is a 3000 times magnified scanning electron micrograph showing the surface state such as the particle state of a honeycomb-shaped Cera ε Nox cross section obtained by heat treatment at 10°C;
3. Cross-section of honeycomb-shaped ceramics obtained by heat treatment at °C
This is a scanning electron micrograph with a magnification of 000 times. The above two scanning electron micrographs also show that the physical properties of the ceramics obtained (particularly the properties as a catalyst carrier) differ greatly depending on the heat treatment temperature.

Effects of the Invention As described above, the present invention is extremely useful in that it is possible to obtain catalyst carrier ceramics with excellent catalytic performance, low coefficient of thermal expansion, and excellent thermal shock resistance at a relatively low temperature and at low cost. This is a great invention.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between the purification rate and the air-fuel ratio of an exhaust gas purifying catalyst according to an embodiment of the present invention, Fig. 2 is a state diagram showing the composition range of the carrier of the exhaust gas purifying catalyst, and Fig. 3 is a state diagram showing the composition range of the carrier of the exhaust gas purifying catalyst. Relationship diagram of expansion/contraction rate with respect to temperature of carrier of purification catalyst, 4th
The figure is a relationship diagram showing the heat treatment temperature for obtaining the carrier of the exhaust gas purifying catalyst and the catalyst performance, and FIG. 5 is a scanning electron micrograph showing the state of surface particles of the carrier of the exhaust gas purifying catalyst. Go to 17 / A: Rehydratable alumina, B: Alkaline titanate, C: Fused silica Name of agent: Patent attorney Toshio Nakao and 1 other person 1
Diagram (1,000 ri) (A) Sky? :Le Figure 1 (+su2) (B) Figure 2 A-11 丙7 に〉&Full ξβ---Satan changes °now 4 G-1~Rokurenri Sword No. Figure 3 fLLt' Figure 4 /ρθθ l/ρθ /2ρθ /3θl /4θρ
Heat treatment

Claims (5)

[Claims] (1) A catalyst for exhaust gas purification in which a catalyst carrier is a ceramic obtained by heat treating a composition consisting of at least rehydratable alumina, an alkali titanate, and fused silica. (2) The exhaust gas purifying catalyst according to claim 1, comprising 5 to 20% by weight of rehydratable alumina, 1 to 10% by weight of alkali titanate, and 70 to 94% by weight of fused silica. (3) The exhaust gas purifying catalyst according to claim 1 or 2, wherein the alkali titanate salt is potassium titanate. (4) The exhaust gas purifying catalyst according to claim 1, 2, or 3, wherein the alkali titanate salt is fibrous. (5) The exhaust gas purifying catalyst according to claim 1, 2, 3, or 4, wherein the heat treatment temperature is 1100 to 1300°C.