JPH0582257B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention deals with carbon monoxide (hereinafter referred to as CO), hydrocarbons (hereinafter referred to as HC), and nitrogen oxides (hereinafter referred to as NO) in exhaust gas emitted from internal combustion engines, etc., especially automobiles. The present invention relates to a method for manufacturing an engine exhaust gas purifying catalyst used to reduce (referred to as _x ). (Conventional technology) Conventionally, CO, HC,
As catalysts for purifying NO _x , noble metals such as platinum (Pt), rhodium (Rh), and palladium (Pd) are supported on alumina (Al ₂ O ₃ ). In addition, in order to improve the catalytic performance of these precious metals, we have developed an oxygen storage effect (the effect of capturing oxygen from exhaust gas and contributing to the purification of the catalyst).
Catalysts have been manufactured in which cerium oxide (CeO ₂ ), which is a certain substance, is contained in an alumina coating layer along with precious metals to improve the purification rate of exhaust gas. However, catalyst components such as noble metals and base metals and oxygen storage capacity imparting agents (hereinafter referred to as OSC) such as cerium oxide
The method of coexisting and supporting the alumina coating layer has the following problems. (a) Since the OSC agent is uniformly supported on the alumina coat layer, it is not necessarily supported in areas near the pores that are likely to come into contact with exhaust gas. In other words, most of the OSC agents are carried in parts of the exhaust gas that are difficult to diffuse, and do not participate in the purification reaction, making it impossible to obtain a high purification rate. (b) The thermal instability of the OSC agent adversely affects the catalyst components and alumina due to the increased direct contact between the agent and the catalyst components. (c) Since the OSC agent and the catalyst component are supported together, the two form a compound, reducing the dispersibility of the catalyst component and reducing exhaust gas purification performance. In addition, a catalyst layer containing Pt, Pd, etc. is provided on the catalyst carrier, and in order to protect the catalyst layer, an oxide consisting of alumina or a mixture of alumina and cerium oxide (0.1 to 0.5% by weight relative to alumina) is used. A method in which a coating layer is provided has been proposed (see Japanese Patent Application Laid-Open No. 1983-5230). In the case of this known technique, the content of CeO ₂ is too low, so no oxygen storage effect can be expected. Therefore, a method can be considered to increase the CeO ₂ content in the coating layer, but in that case, the catalyst surface is exposed to exhaust gas and becomes high temperature, so the crystal growth of CeO ₂ contained in the coating layer is inhibited. As a result, sintering occurs and the particle size of CeO ₂ increases, which may lead to a problem of significantly reducing the catalytic activity. (Object of the Invention) The present invention has been made in view of the above-mentioned problems, and includes a method for forming a coating layer containing CeO ₂ on the upper surface of a catalyst layer containing a catalyst component made of a noble metal or a base metal. The purpose of this method is to heat-treat CeO ₂ at a high temperature to prevent a decrease in catalytic activity due to sintering of CeO ₂ at high temperatures. (Means for achieving the object) In the present invention, as a means for achieving the above object, a catalyst layer containing at least one catalyst component selected from the group consisting of platinum, palladium, and rhodium is provided on a catalyst carrier. A coating layer is formed on the catalyst layer by heating CeO ₂ at 800 to 1000° C. for 1 to 5 hours and then mixing and coating with alumina. Here, as the catalyst component, known components such as Pt, Rh, and Pd, and mixtures of two or more thereof are used. The high temperature heat treatment is performed at a temperature range of 800 to 1000°C for 1 to 5 hours. In addition, below 800℃, 1
If the heat treatment is performed for less than one hour, the effect of the heat treatment will be weak and the thermal stability cannot be sufficiently improved.
In addition, heat treatment at 1000℃ or more for 5 hours or more,
Crystallization of CeO ₂ is promoted and there is a risk of deterioration of catalyst performance due to thermal deterioration. By the way,
When the change in HC purification rate with respect to the heat treatment temperature of CeO ₂ was investigated, the results shown in Figure 2 were obtained. The catalyst used in this test was Pt1.0g/1,
After heat treatment on a catalyst carrier supporting Rh0.2g/1
It was coated with a mixture of CeO ₂ and alumina, and the heat treatment time was 3 hours at each temperature. Further, the durability test was conducted at 900° C. for 50 hours, and the activity test was conducted at an air-fuel ratio A/F=14.5 and a space velocity SV=60000/Hr. The results of the above tests indicate that the heat treatment temperature is preferably in the range of 800 to 1000°C. The composition of the coating layer is preferably 50 _{to 95% by weight of CeO 2 and} the balance activated alumina. conduct. An example of an exhaust gas purifying catalyst manufactured by the above manufacturing method is shown in an enlarged scale in FIG. Here, reference numeral 1 indicates a catalyst carrier, 2 indicates a catalyst layer, and 3 indicates a coating layer. As the catalyst carrier 1, a honeycomb structure made of ceramics such as cordierite or various carriers made of heat-resistant metals and heat-resistant inorganic fibers are used. (Effect) As mentioned above, CeO ₂ is heated in advance at 800 to 1000℃ for 1~
The heat treatment for 5 hours increases the thermal stability of CeO ₂ , suppresses the crystal growth of CeO ₂ at high temperatures, and maintains excellent performance against thermal deterioration. Hereinafter, preferred embodiments of the present invention will be described. Example γ-alumina 160g, boehmite 160g, water
500 cc of concentrated nitric acid and 4 cc of concentrated nitric acid were mixed and stirred for 10 hours using a homomixer to obtain a slurry for alumina wash coating. A honeycomb catalyst carrier (made of cordierite) was immersed in this slurry and pulled up, the excess slurry was removed by high-pressure air blowing, dried at 130°C for 1 hour, and then calcined at 550°C for 1 hour. This alumina-coated catalyst carrier was converted into platinum and rhodium, respectively, at 1.0 g/l and
It was immersed in a mixed aqueous solution of chloroplatinic acid and rhodium chloride with a concentration of 0.2 g/l, then pulled up, dried at 200°C for 1 hour, and then calcined at 600°C for 2 hours. The noble metal content after firing was 1.0 g/of platinum (Pt) and 0.2 g/of rhodium (Rh). Note that this catalyst layer is
The amount was 14% by weight based on the weight of the catalyst carrier. CeO ₂ powder was then heat-treated at 900°C for 3 hours.
800 g of hydrated alumina and 200 g of hydrated alumina were mixed using a ball mill, and 1100 cc of water was added to this mixed powder and kneaded to obtain a slurry liquid for coating the coating layer. The previously prepared catalyst carrier was immersed in the slurry while stirring, then pulled out, excess slurry was removed by high-pressure air blowing, and then dried at 150°C for 3 hours and calcined at 700°C for 3 hours. As a result, 80% by weight of CeO ₂ and 20% by weight of γ are added to the upper surface of the catalyst layer 2 (containing Pt and Rh) formed on the catalyst carrier 1.
- A catalyst for exhaust gas purification was obtained in which a coating layer 3 was formed of about 25% by weight of the catalyst carrier (see Fig. 1). In addition, an evaluation test was conducted to compare the HC purification performance of the exhaust gas purification catalyst obtained in the above example after the heat resistance test with that of a catalyst (comparative example) that was not subjected to CeO ₂ heating pretreatment. The results shown in FIGS. 3 and 4 were obtained. In addition, the activity measurement conditions are air-fuel ratio A/F = 14.7±
An evaluation test was conducted with the space velocity SV = 60000/Hr. According to the results of the above evaluation test, the performance difference between the exhaust gas purification catalyst manufactured according to the example of the present invention and the comparative example is not clear when the catalyst temperature is relatively low, but when the catalyst temperature is The higher the temperature, the more noticeable it becomes. This is nothing but a result of the sintering of CeO ₂ being suppressed by the heating pretreatment. In this example, the content of cerium oxide in the coating layer is 80% by weight, but CeO ₂
The content can range from 50 to 95% by weight. Generally, as the CeO ₂ content in the coating layer decreases, the catalyst performance gradually decreases, and below 50%, the catalyst performance decreases rapidly. The reason is that when the concentration of CeO ₂ decreases, it is no longer able to interact with the active ingredient. By the way, CeO ₂ in the coating layer
Figure 5 shows the results of measuring changes in the CO purification rate (%) at 400°C with respect to the content (% by weight) of CO. These results are based on the activation measurement conditions: air-fuel ratio A/F = 14.7±0.9, space velocity SV = 60000/Hr, and an exhaust gas temperature of 850°C in a bench engine.
This is an evaluation using a catalyst after a durability test that was operated for 300 hours. According to this, when CeO ₂ falls below 50%, the CO purification rate drops significantly. On the other hand, when the content of CeO ₂ increases, the catalytic activity improves, but because the binding force of CeO ₂ itself is weak,
Physical strength (peel resistance) decreases and durability decreases. Incidentally, when a peel test was conducted by varying the content (wt%) of CeO ₂ in the coating layer, the results shown in Table 1 were obtained. here,
Peeling amount = (Coat adhesion amount before test - Coat adhesion amount after test) / (Coat adhesion amount before test),
In addition, the test method involved heating a cylindrical test piece with a diameter of 1 inch and a height of 1 inch at 600℃ for 30 minutes, then cooling it in water at 25℃ three times, and then drying it thoroughly. A method was adopted to measure the

[Table] From the results of the above peel test, CeO ₂ in the coating layer
It can be seen that the content is preferably 95% or less. If the adhesion ratio of the coating layer to the catalyst carrier is less than 5% by weight, the surface of the catalyst layer cannot be effectively coated, resulting in a rapid decline in catalyst performance, and if it exceeds 40% by weight, active components and exhaust gas Since contact with gas is inhibited, catalyst performance rapidly decreases. Taking this into consideration, it is desirable that the adhesion ratio of the coating layer be 5 to 40% by weight. Note that the thickness of the coating layer is preferably 20 to 40 μm. (Effects of the Invention) As described above, according to the method of the present invention, a catalyst can be obtained in which a catalyst layer containing active catalyst components (Pt, Rh, etc.) and a coating layer containing CeO ₂ are separated. At the same time, the high-temperature heat treatment of CeO ₂ increases the thermal stability of CeO ₂ in the coating layer, which not only prevents the active catalyst component and CeO ₂ from forming a compound. This has the excellent effect of being able to prevent performance deterioration due to CeO ₂ sintering under high-temperature conditions, and significantly improving exhaust gas purification performance. Furthermore, since the upper surface of the catalyst layer is coated with a coating layer containing CeO ₂ , the catalyst layer is easily placed in a reducing atmosphere, which has the advantage of further improving the NO _x purification performance in the exhaust gas.

[Brief explanation of the drawing]

FIG. 1 is an enlarged view showing an example of a catalyst manufactured by the method for manufacturing an exhaust gas purifying catalyst according to the present invention, and FIG.
Figure 3 is a characteristic diagram showing changes in HC purification rate with respect to CeO ₂ heating pretreatment temperature, and shows that the HC purification rate with respect to catalyst heating temperature is 50% for the catalyst obtained by the example of the present invention and the comparative example. Figure 4 is a characteristic diagram showing changes in catalyst inlet temperature (after heat resistance test), and Figure 4 is a characteristic diagram showing changes in HC purification rate at catalyst inlet temperature of 400°C with respect to catalyst heating temperature in the examples of the present invention and comparative examples. , FIG. 5 is a characteristic diagram showing the change in the CO purification rate (%) in the exhaust gas with respect to the CeO ₂ content (% by weight) in the coating layer of the exhaust gas purification catalyst. 1...Catalyst carrier, 2...Catalyst layer, 3...Coating layer.

Claims (1)

[Claims] 1 A catalyst layer containing at least one catalyst component selected from the group consisting of platinum, palladium, and rhodium is supported on a catalyst carrier, and CeO ₂ is applied on the catalyst layer at 800 to 1000°C for 1 to 5 hours. A method for producing a catalyst for purifying engine exhaust gas, which comprises heating the catalyst and then mixing it with alumina to form a coated coating layer.