JPH03202154A - Catalyst for purifying exhaust gas of engine - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a catalyst for purifying engine exhaust gas.

[Prior Art] A noble metal or other metal catalyst material (catalyst material) that promotes the oxidation of unburned components (hydrocarbons, etc.) in exhaust gas is coated on the surface of a honeycomb carrier made of a material such as cordierite.
For example, engine exhaust gas purifying catalysts have been known in which an alumina-based catalyst coat layer containing platinum, rhodium, etc. (hereinafter such substances are simply referred to as catalytic noble metals) is formed.

However, such an exhaust gas purifying catalyst cannot obtain sufficient catalytic activity unless its temperature rises above a certain level (for example, 150° C. or higher).

For this reason, when the exhaust gas temperature is low, such as when the engine is cold, the temperature of the exhaust gas purification catalyst also becomes low, resulting in a problem that the activity of the exhaust gas purification catalyst decreases and the emission performance of the engine decreases.

Therefore, in the exhaust passage upstream of the exhaust gas purification catalyst,
An exhaust gas purification device including an adsorbent brick that adsorbs unburned components (hydrocarbons) in exhaust gas has been proposed (see, for example, Japanese Utility Model Application Publication No. 109616/1983). Specifically, in such an exhaust gas purification device, as shown in FIG. 9, in the exhaust passage 101, from upstream to
a first container 102 with an adsorbent brick 103;
A second container 104 equipped with an exhaust gas purifying catalyst brick 105 (so-called two containers 2
Brick type), or as shown in FIG. 10, an adsorbent brick 109 and an exhaust gas purification catalyst brick 110 are provided in order from upstream in one container 108 in which an exhaust vent wj1071 is installed (so-called Container 2 brick type).

In such an exhaust gas purification device, adsorbent brick I 03. +09 adsorbs unburned components in the exhaust gas when the exhaust gas temperature is low, that is, when the adsorbent temperature is low, and on the other hand, when the exhaust gas temperature is high, the adsorbed unburned components are desorbed (from the catalyst). withdrawal)
let Therefore, when the exhaust gas temperature is low, although the activity of the exhaust gas purifying catalyst bricks 105 and 110 decreases, unburned components are absorbed into the adsorbent bricks 103 and 103.
9, unburned components are not emitted into the atmosphere. Thereafter, when the exhaust gas temperature becomes high, the unburned components adsorbed on the adsorbent bricks 103 and 109 are desorbed and removed from the exhaust gas purifying catalyst bricks 105 and 110.
However, at this point, the activity of the exhaust gas purifying catalyst bricks 105 and 110 has increased, so the unburned components are oxidized (purified) by the exhaust gas purifying catalyst bricks 105 and 110.

[Problem to be solved by the invention] However, as shown in FIG. 9 or 1O,
In conventional exhaust gas purification devices equipped with adsorbent bricks 103, 109, the adsorbent bricks 103, 109 significantly increase the flow resistance of the exhaust gas, which increases engine back pressure and reduces engine output. (For example, a 12% reduction in the output of an aNV type 6-cylinder engine). Furthermore, since the exhaust gas purification device becomes large in size, there is a problem that it becomes difficult to mount it on the engine.

The present invention has been made in view of the above-mentioned conventional problems, and the present invention has been made in view of the above-mentioned conventional problems. It is an object of the present invention to provide an engine exhaust gas purification device or an exhaust gas purification catalyst having a compact structure and capable of increasing the purification rate of components (hydrocarbons, etc.).

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a first coat layer made of an alumina layer containing a catalyst component on the surface of a catalyst carrier,
The present invention provides a catalyst for purifying engine exhaust gas, characterized in that a second coat layer containing cerium oxide and an adsorbent that adsorbs unburned engine gas components is provided on the first coat layer.

[Operations and Effects of the Invention] According to the present invention, when the exhaust gas is at a low temperature such as when the engine is cold, unburned components (hydrocarbons, etc.) in the exhaust gas are adsorbed by the adsorbent in the second coat layer, and the exhaust gas is When the temperature (exhaust gas purification catalyst temperature) rises, the adsorbed unburned components are desorbed from the adsorbent, but these desorbed unburned components are absorbed into the first coat layer adjacent to the second coat layer. It diffuses into the catalyst and comes into contact with the noble metal, thereby being oxidized (purified).

Here, the unburned components that are desorbed from the adsorbent and diffused into the first coat layer are not substantially mixed with the main stream of exhaust gas, so the concentration of unburned components on the surface of the exhaust gas purification catalyst is relatively high. Therefore, the oxidation reaction is promoted, the temperature of the exhaust gas purifying catalyst is rapidly increased, and the purification rate of unburned components is increased.

In addition, since the second coat layer containing an adsorbent is formed on the first coat layer of the exhaust gas purification catalyst, there is no need to separately provide an adsorbent brick in the exhaust passage, and therefore the exhaust gas flow resistance is reduced. is reduced. Therefore, an increase in engine back pressure is suppressed, and engine output is increased. Furthermore, the exhaust gas purification device is made more compact.

Therefore, when the exhaust gas is at a low temperature, it is possible to increase the engine output, make the exhaust gas purification device more compact, and increase the purification rate of unburned components.

[Example] Examples of the present invention will be specifically described below.

6. As shown in FIG. 1, an exhaust passage 2 is connected to the downstream end of an exhaust manifold 1 of an engine (not shown). Slightly downstream of the connection with the exhaust manifold 1, the exhaust passage 2 is provided with a substantially cylindrical converter 3 for pre-treating the exhaust gas. A deposit catalyst 4 is arranged.

In addition, in the exhaust passage 2 downstream of the converter 3,
In addition to converting hydrocarbons (HC), -carbon oxides (CO), etc. in exhaust gas into carbon dioxide and water, nitrogen oxides (N
A generally cylindrical main converter 5 that converts oxygen (Ox) into nitrogen is interposed, and inside this main converter 5, approximately cylindrical first converters having an oxidation/reduction promoting effect are arranged in order from the upstream. Second
Exhaust gas purifying catalysts 6 and 7 and a substantially cylindrical oxidation catalyst 8 having only an oxidation promoting effect are arranged.

As shown in FIG. 2, the first exhaust gas purifying catalyst 6 has a honeycomb carrier 11 on the surface of which a large number of pores 9 (not shown in detail) extending in the axial direction are formed. 1 inch (25.4 mm), as explained later.
Its axial length is 2 inches (50.8 mm). Further, the pores 9 of the honeycomb carrier 11 have a cross-sectional area of 1
400 pieces are formed per square inch.

(2) The honeycomb carrier 11 is taken out of the alumina slurry and is subjected to air blowing to remove excess alumina slurry.

(2) The air-blown honeycomb carrier 11 is dried at 250°C for 2 hours and then fired at 650°C for 2 hours to form a first coat layer I2 on the surface of the honeycomb carrier 11. Here, the first coat layer 12 is applied to the honeycomb carrier 11 at a rate of 1.
Adjust to 4wt%.

■ Prepare a platinum chloride solution with an appropriate concentration and a rhodium chloride solution with an appropriate concentration, and immerse the honeycomb carrier 11 on which the first coat layer 12 is formed in the platinum chloride solution and the rhodium chloride solution in order. One coat layer 12 supports platinum and rhodium. Here, the amount of platinum and rhodium supported in the first coat layer 12 is 1.6 g/(l), and the main components are platinum and rhodialumina, and the platinum-based component (
A first coat layer 12 whose catalyst components are a rhodium-based component (hereinafter simply referred to as platinum) and a rhodium-based component (hereinafter simply referred to as rhodium).
is formed, and on this first coat layer 12, a second layer consisting essentially of cerium oxide (Cen2) and an adsorbent is formed.
A coat layer 13 is formed. Note that the second exhaust gas purification catalyst 7 also has the same configuration as the first exhaust gas purification catalyst 6.

Hereinafter, a method for manufacturing the first exhaust gas purification catalyst 6 (second exhaust gas purification catalyst 7) will be described.

■After mixing γ-alumina (γ-AI20G) 54 a9, 60 g of boehmite, and 1 part water (!), add 10 g of nitric acid.
In addition to rice, an alumina slurry for the first coating layer 12 is prepared. Note that, of course, it is possible to make any amount of alumina slurry by increasing or decreasing the amount of each material used while maintaining the above ratio.

(2) Immerse the honeycomb carrier 11 in the alumina slurry. This honeycomb carrier 11 is made of cordierite and has a cylindrical shape, and its diameter is adjusted by adjusting the concentrations of both solutions and the immersion time so that the ratio of ram to ram is 5:1.

(2) The honeycomb carrier 11 carrying platinum and rhodium is dried and fired to fix platinum and palladium to the first coat layer I2.

■Cerium oxide (Cent), boehmite, and water lQ
A slurry for the second coat layer 13 is prepared by mixing nitric acid 10+y(2) and further adding an adsorbent (e.g., molecular sieve, silicalite).Here, cerium oxide, boehmite, and adsorbent The amount of addition is set so that, for example, the amount of boehmite added is 60g and the total amount of these three additions is 6009.By increasing or decreasing the amount of each material used while maintaining the above ratio, any amount can be obtained. Of course, it is possible to make a slurry of

■The honeycomb carrier 11 on which the first coat layer 12 is formed is dipped in the slurry, dried and fired, and then the second coat layer 13 is formed.
form. Here, the second coat layer 13 is adjusted to have a content of 28 wt% with respect to the honeycomb carrier If.

A temperature sensor 22 for detecting the temperature of the synthesis gas is provided inside the container 16 on the upstream side of the exhaust gas purification catalyst 15.

In the warm-up performance test, first, the synthesis gas is allowed to bypass the container +6 (exhaust gas purification catalyst 15) and flowed to the gas bypass passage I8 side, and at an appropriate point, the heater 21 is turned on and the synthesis gas supply path is turned on. is switched to the container 16 (exhaust gas purification catalyst 15) side, and from this point on, the hydrocarbons (HC) and -carbon oxide (
This was done by measuring the purification rate (conversion) of CO) and nitrogen oxides (NOx) at regular intervals.

The test results are shown in FIGS. 4 to 7. Here, Figure 4 shows the warm-up performance against hydrocarbons (HC), Figure 5 shows the warm-up performance against carbon monoxide (CO), and Figure 6 shows the warm-up performance against nitrogen oxides (NOx). , and FIG. 7 shows the temperature of the synthesis gas entering the exhaust gas purifying catalyst 15.

As is clear from FIG. 4, four types of exhaust gas purifying catalysts according to the present invention (Example 1 - 4), and as a comparative example, an exhaust gas purification catalyst containing no adsorbent was also prepared, and a warm-up performance (low-temperature activity) test was conducted on these.

FIG. 3 shows the test equipment used in the above test. In this test device, an exhaust gas purifying catalyst 15 (volume: 24 ffc) is placed in a container 16, and at the upper end of the container 16, a catalyst with a volume almost the same as that of exhaust gas at an air-fuel ratio (A/F) of 14.5 is placed. A gas supply passage 17 is connected to the exhaust gas purification catalyst 15 for supplying synthesis gas having a composition such that the gas flow rate is 60,000 t/H, while a gas discharge passage 19 is connected to the lower end. There is. Further, a gas bypass passage 18 is connected to the gas supply passage +7r, which bypasses the container 16 and discharges the synthesis gas. A heater 21 for heating the synthesis gas flowing in the gas supply passage 17 is installed on the upstream side of the branching part to the gas bypass passage 18 to warm up as the adsorbent content of the second coat layer 13 increases. Performance (purification rate) has improved.

This is because hydrocarbons have a small number of carbon atoms. The hydrocarbons are desorbed from the adsorbent and diffused into the first coat layer I2 adjacent to the second coat layer I3 to form catalyst noble metals (platinum, platinum,
rhodium), but since these hydrocarbons do not substantially mix with the main synthesis gas stream, the catalyst surface? This is probably because the concentration of hydrocarbons in the exhaust gas purification catalyst 15 increases, thereby promoting the oxidation reaction and rapidly raising the temperature of the exhaust gas purifying catalyst 15.

As is clear from FIGS. 5 and 6, the warm-up performance for carbon oxides and nitrogen oxides decreases as the adsorbent content of the second coat layer 3 increases. It is declining. Here, the reason why the purification rate of carbon oxide decreases is that cerium oxide generally has a water gas reaction (CO + H2O
-4C02+H9), but as the adsorbent content increases, the cerium oxide content decreases, making water gas reactions less likely to occur, and as a result - inhibiting the oxidation of carbon oxide. In addition, the purification rate of nitrogen oxides decreases because nitrogen oxides are reduced by H2 generated by the water gas reaction, and the reduction of nitrogen oxides is suppressed as the water gas reaction slows down. Probably.

From the above two results, it can be seen that the mixing ratio of cerium oxide and adsorbent in the second coat layer 13 is preferably set within the range of 50:50 to 40.60.

By the way, the inventors of the present invention have discovered that adding a palladium-based catalyst (hereinafter simply referred to as palladium) to the second coat layer further improves the warm-up performance against hydrocarbons. , shows the results of a test on warm-up performance against hydrocarbons for an exhaust gas purifying catalyst on which a second coat layer containing palladium was formed.

The method for producing this palladium-containing catalyst for exhaust gas purification is the same as the above-described method except for the method for forming the second coat layer, so that the second coat can be improved in the following.

[Brief explanation of drawings]

FIG. 1 is an explanatory longitudinal cross-sectional view of an engine exhaust gas purification device equipped with an exhaust gas purification catalyst according to the present invention. FIG. 2 is a cross-sectional explanatory diagram of the first (second) exhaust gas purifying catalyst. FIG. 3 is a schematic diagram of a test device used for a warm-up performance test. FIGS. 4 to 7 are diagrams showing the characteristics of the purification rates of hydrocarbons, carbon monoxide, and nitrogen oxides with respect to time, and the characteristics of gas temperature with respect to time, respectively, in the warm-up performance test. FIG. 8 is a diagram showing the characteristics of hydrocarbon purification rate and gas temperature versus time when a warm-up performance test was conducted using an exhaust gas purification catalyst in which palladium was added to the second coat layer. FIG. 9 and FIG. 10 are longitudinal cross-sectional explanatory views of a conventional exhaust gas purification device in which an adsorbent splitter is provided upstream of an exhaust gas purification catalyst. Only the method of forming the layer will be explained. (2) A palladium chloride aqueous solution is added to cerium oxide (CeO2) powder, stretched, dried and crushed to prepare cerium oxide (CeO7) powder on which palladium is supported (immobilized). ■Cerium oxide powder 270 supporting the above palladium
g, 60 g of boehmite, adsorbent (for example, molecular sieve, silicalite) 2709, and water IQ are mixed, and 10 if2 of nitric acid is added thereto to prepare a slurry. (2) The honeycomb carrier on which the first coat layer is formed is immersed in the slurry, and then dried and fired to form the second coat layer. Here, the second coat layer is applied to two layers of the honeycomb carrier.
The amount of palladium was adjusted to 8 wt%, and the amount of palladium was adjusted to 1.09A. As is clear from FIG. 8, the warm-up performance against hydrocarbons is improved. As described above, according to the present invention, when the exhaust gas is at a low temperature, the engine output is increased and the exhaust gas purification device is made more compact, while the purification rate of unburned components is l...exhaust manifold, 2...exhaust passage, 3... Pre-converter, 5.
... Main converter, 6, 7 first and second exhaust gas purification catalysts, 9 ... Pores, 11 ... Honeycomb carrier, 12
...First coat layer, 13...Second coat layer.

Claims (1)

[Claims] (1) A first coat layer made of an alumina layer containing a catalyst component is provided on the surface of the catalyst carrier, and a first coat layer containing cerium oxide and an adsorbent that adsorbs unburned gas components of the engine is placed on the first coat layer. A catalyst for purifying engine exhaust gas characterized by having two coat layers.