JPH08332350A - Catalyst for exhaust gas purification - Google Patents

Abstract

PURPOSE: To improve the activity at a low temperature and the durability of an exhaust gas purifying catalyst at a high temperature by depositing palladium at a specified high density in an upper stream side of a carrier. CONSTITUTION: A catalytic converter 1 is composed of a monolithic catalyst 2, an outer cylinder 10 to house the monolithic catalyst 2, and a pair of cones 11, 12 extended from the outer cylinder 10 in a conical shape. Platinum and rhodium are deposited in the whole length of the monolithic catalyst 2 and palladium is further deposited in the upper stream part 20 arranged in the exhaust gas flowing-in side of the catalytic converter 1. The deposited density of the palladium in the upper stream part 20 is set to be 7-20g per 1 liter volume of the upper stream part 20 of the carrier. Consequently, unburned HC can be lessened by the catalyst for exhaust gas purification from immediately after starting of an engine to the time when the engine is warmed up and at the same time, the emission of platinum and CO, HC, NOx carried in the whole body of the catalyst can be lowered and thus the catalytic converter has high three-dimentional purification performance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for efficiently purifying hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO _x ) in exhaust gas discharged from an internal combustion engine. More specifically, it relates to an exhaust gas purifying catalyst having a high purifying activity at low temperatures.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by oxidizing CO and HC and reducing NO _x has been used as a catalyst for purifying exhaust gas of automobiles. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat resistant base material made of cordierite, and platinum (Pt) and rhodium (R) are formed on the porous carrier layer.
Those in which a catalytic noble metal such as h) and palladium (Pd) are supported are widely known.

By the way, there is a positive correlation between the catalytic activity of the catalytic noble metal and the reaction temperature, and the higher the temperature, the higher the catalytic activity. Therefore, the higher the temperature of the exhaust gas flowing into the exhaust gas purification catalyst, the better the purification performance.However, when the exhaust gas temperature is low, such as during winter startup, purification by the exhaust gas purification catalyst becomes insufficient and the components to be purified are high. Exhaust gas contained in the concentration was sometimes released.

As an exhaust gas purifying catalyst for solving such a problem, for example, Japanese Patent Laid-Open No. 63-84635 discloses Pd on the exhaust gas inflow side of 1/10 to 2/5 of the entire length of the carrier.
A catalyst for purifying exhaust gas in which Pt is carried on the exhaust gas outflow side of 3/5 to 9/10 of the entire length of the carrier and Rh is further carried over the entire length is disclosed. Since Pd has excellent HC and CO oxidizing activity even at low temperatures, the exhaust gas-purifying catalyst disclosed in JP-A-63-84635 has excellent purification performance at low temperatures. That is, when low-temperature exhaust gas flows in, the exhaust gas first comes into contact with Pd and Rh carried on the upstream side, and HC and CO are oxidatively purified by the catalytic reaction of Pd. Since the reaction heat at this time is transferred to the exhaust gas, the exhaust gas becomes hot and flows to the downstream side, and is further purified by the contact reaction with Pt and Rh carried on the outflow side. As a result, the exhaust gas can be purified with high efficiency.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the exhaust gas purifying catalyst disclosed in Japanese Patent No.
Temperature that can purify 50% of HC (HC50% purification temperature)
Was as high as about 400 ° C., which revealed that the low temperature activity was still insufficient. It was also clarified that the durability performance at high temperature was not sufficient and the degree of reduction in purification performance after the durability test was large.

The present invention has been made in view of such circumstances, and an object thereof is to provide an exhaust gas-purifying catalyst that has high activity at low temperatures and excellent durability performance at high temperatures.

[0007]

The exhaust gas-purifying catalyst of the present invention which solves the above-mentioned problems is provided with Pd and R on the upstream side of the exhaust gas flow.
An exhaust gas purifying catalyst that carries h and carries Pt and Rh on the downstream side, and has a carrying density of Pd on the upstream side of 7 to 20 per 1 liter volume of the upstream part of the carrier carrying Pd.
It is characterized in that it is g.

[0008]

[Action]

(Action of carrier upstream side portion) In the exhaust gas purifying catalyst of the present invention, 7 to 20 per volume of 1 liter is provided upstream of the carrier.
g of Pd is carried. Since Pd is loaded at a high density in this way, the HC50% purification temperature is 35%.
Since it can be lowered to around 0 ° C. and the low temperature activity is improved, it exhibits a high oxidation purification ability even when the exhaust gas temperature is low such as at the time of starting. Further, the presence of Rh and reducing components of HC and CO in the exhaust gas reduces and purifies NO _{x in} the exhaust gas.

At high temperature, since Pd is loaded at a high density, even if sintering (grain growth) of Pd occurs, many active points are present, so that durability is excellent. Also, the degree of deterioration of the oxidation catalyst performance is small. Therefore, even when it is arranged in the vicinity of the engine body, it shows good catalyst performance for a long period of time. If the loading density of Pd supported on the upstream side portion is less than 7 g per 1 liter volume of the carrier on the upstream side portion, the oxidation purification performance at low temperature is deteriorated and the durability is lowered. Since most of HC and CO are oxidized on the side, the NO _x reduction and purification performance deteriorates. A particularly desirable range is 7 to 10 g. (Operation of the downstream part of the carrier) The heat generated by the oxidation reaction on the upstream side is transmitted to the exhaust gas, and the sufficiently heated exhaust gas flows into the downstream side. And unpurified HC and CO on the upstream side
Is oxidized and purified by the catalytic reaction with Pt and Rh. Further, due to the presence of Pt and Rh and the reducing components of HC and CO in the exhaust gas, the remaining NO _x in the exhaust gas is further reduced and purified.

[0010]

【Example】

[Specific Example of the Invention] As the carrier, a conventionally used inorganic porous material can be used, and it can be selected from alumina, silica, titania, zirconia, silica-alumina, zeolite and the like. Above all, it is particularly preferable to use alumina which is excellent in heat resistance and noble metal dispersibility. This carrier can be used in the same manner as in the past by forming it on a monolith carrier substrate formed of cordierite or metal, or by forming it in the form of pellets.

The upstream side portion of the carrier means the side into which exhaust gas flows
For example, in the case of a monolith carrier, exhaust gas flows in
It is a range of 1/10 to 1/4 of the entire length from the end face.
If the length of the upstream part is shorter than this, the activity at low temperature decreases.
However, if it is longer than this, NO _xThe purification rate of is reduced. R
The loading density of h can be uniform over the entire carrier,
It can be 0.1 to 0.5 g per liter of body volume.
it can. NO if the Rh carrying density is less than this_xPurification of
The conversion rate decreases, and the effect is saturated even if more than this is loaded.
Not only does this lead to an increase in cost. 0.2-0.4 g is especially
preferable.

The Pt carrying density on the downstream side is preferably 0.5 to 2.0 g, and particularly preferably 1.0 to 1.5 g per liter volume of the downstream portion of the carrier. Even if the Pt carrying density is further increased, the activity is not improved and the Pt cannot be effectively used. Further, if the Pt carrying density is lower than this, sufficient activity for practical use cannot be obtained. It is preferable that Pt is also loaded on the upstream side portion. If you do this, P
The catalytic action of Pt and Rh on the upstream side is rapidly developed by the heat generated by the oxidation of HC by d, and the three-way purification performance is further improved. In this case, the loading density of Pt loaded on the upstream side is preferably 0.5 to 2.0 g, and particularly preferably 1.0 to 1.5 g per 1 liter of the carrier volume of the upstream side, as described above.

In order to support these catalytic noble metals on the carrier, the chloride, nitrate or the like thereof is used to impregnate, spray, or
It can be supported in the same manner as in the past by using a slurry mixing method or the like. [Examples] Hereinafter, specific examples will be described. (Embodiment 1) FIG. 1 shows a catalytic converter in which an exhaust gas purifying catalyst according to an embodiment of the present invention is arranged. The catalytic converter 1 includes a monolith catalyst 2, an outer cylinder 10 that houses the monolith catalyst 2, and a pair of cones 11 and 12 that extend in a truncated cone shape from both ends of the outer cylinder 10. The monolith catalyst 2 carries Pt and Rh over the entire length, and the upstream side portion 20 arranged on the exhaust gas inflow side of the catalytic converter.
Is further loaded with Pd. This upstream part 20
Is 1/5 of the total length of the monolith catalyst 2.

Next, a method for producing the monolith catalyst 2 will be described, and will be replaced with a detailed description of the structure of the monolith catalyst 2.
100 parts by weight of activated alumina powder, 65 parts by weight of 40% by weight aluminum nitrate aqueous solution and 80 parts by weight of water are mixed,
An alumina slurry for coating was prepared. And a metal carrier (volume 0.7 liter, 4 liters) produced by stacking and winding a ferritic stainless steel foil flat plate and a corrugated plate.
The above alumina slurry was applied to 100 cells (total length: 120 mm), dried at 250 ° C. for 1 hour, and then baked at 600 ° C. for 1 hour to form an alumina coat layer.

The monolithic carrier thus obtained was dipped in a mixed aqueous solution of dinitrodiammine platinum and rhodium chloride having a predetermined concentration for 1 hour, pulled up to blow off excess water droplets, and then dried at 250 ° C. for 1 hour. Then, a portion of 25 mm from the end face on the upstream side of the monolith carrier was immersed in an aqueous palladium nitrate solution having a predetermined concentration for 1 hour, pulled up to blow off excess water droplets, and then dried at 250 ° C. for 1 hour.
Monolith Catalyst 2 of was prepared.

The loading amount of each noble metal catalyst is 1.0 g of Pd and 1.0 of Pt per one monolith catalyst 2.
g and Rh are 0.21 g. Converted to the loading density,
Pd is supported in an amount of 7 g per 1 liter volume of the upstream portion 20 of the monolith catalyst 2, and Pt and Rh are the monolith catalyst 2
The total volume is 1.5 g per liter and 0.
It carries 3 g.

The catalytic converter 1 of the first embodiment described above is
For example, as shown in FIG. 3, the exhaust gas is mounted immediately below the exhaust manifold 50 of the engine body 5, and when the engine body 5 is started, the exhaust gas passes through the exhaust manifold 50, passes through the catalytic converter 1, and then further passes through the catalytic converter 6. HC, CO and NO _x in the exhaust gas are purified. At this time, the catalytic converter 1 is quickly warmed by the heat of the exhaust gas, and further, since the upstream side portion 20 carries Pd at a high density, the activation time is shortened, and immediately after the engine is started until it is warmed up. It is possible to reduce the emissions of HC, CO and NO _x . (Example 2) Except that the loading amount of Pd in the upstream portion 20 was 1.45 g per one monolith catalyst 2, that is, the loading density was 10 g per 1 liter volume of the upstream portion 20 of the monolith catalyst 2. The configuration is the same as in Example 1. (Example 3) The loading amount of Pd in the upstream portion 20 was 2.9 g per one monolith catalyst 2, that is, the loading density was 2 per volume of 1 liter of the upstream portion 20 of the monolith catalyst 2.
The configuration is the same as that of the example 1 except that it is set to 0 g. (Embodiment 4) FIG. 2 shows a catalytic converter in which the exhaust gas purifying catalyst of Embodiment 4 is arranged. In this catalytic converter, the first monolith catalyst 3 having a volume of 0.15 liters, 400 cells, and a total length of 25 mm is arranged on the upstream side of the exhaust gas flow path, and 10
Volume 0.55 liters on the downstream side with a space of 4 mm, 4
Example 2 is the same as Example 1 except that the second monolith catalyst 4 having 00 cells and a total length of 95 mm is arranged.

The first monolith catalyst 3 and the second monolith catalyst 4 were respectively manufactured in the same manner as in Example 1. Pd, Pt and Rh were carried on the first monolith catalyst 3 and Pt was carried on the second monolith catalyst 4. And Rh are carried. Pd is supported only on the first monolith catalyst 3, and the supported amount is 1.4.
At 5 g, the supported density is 10 g per 1 liter volume of the first monolith catalyst 3. Pt and Rh were loaded on the first monolith catalyst 3 and the second monolith catalyst 4 respectively at the same density, and the loading amounts of Pt were 0.23 g (first monolith catalyst) and 0.83 g (second monolith catalyst), respectively. , R
The amount of h carried is 0.05 g each (first monolith catalyst)
And 0.17 g (second monolith catalyst). (Comparative Example 1) Except that the loading amount of Pd in the upstream portion 20 was 0.73 g per one monolith catalyst 2, that is, the loading density was 5 g per 1 liter volume of the upstream portion 20 of the monolith catalyst 2. The configuration is the same as in Example 1. (Comparative Example 2) In the same manner as in Example 1, the upstream side portion 20
Example 1 is the same as Example 1 except that Pt and Rh are supported on the downstream side portion 95 mm except for and Pd and Rh are supported on the upstream side portion 20. 1.45 g of Pd is carried in the upstream portion 20, and 0.83 g and 0.17 g of Pt and Rh are carried in the downstream portion, respectively. Converted to the loading density, P
10 g is carried per 1 liter volume of the upstream side portion 20, and Pt and Rh are carried with 1.5 g and 0.3 g per 1 liter volume of the downstream side portion of the monolith catalyst 2, respectively. (Evaluation of purification performance) Durability of attaching each of the above catalytic converters to the exhaust system of an engine with a displacement of 2 liters and flowing exhaust gas for 100 hours under the conditions of exhaust gas temperature of 900 ° C and A / F = 14.6 (stoichiometric) The test was conducted. The HC50% purification temperature after durability test (T50), HC after the durability test, by measuring the purification efficiency of CO and NO _x, Table 1 Results
Shown in A graph showing the relationship between the HC50% purification temperature and the amount of Pd carried is shown in FIG. The HC50% purification temperature is a measured value when the exhaust gas temperature is between 250 and 450 ° C, and the purification rate is a measured value when the exhaust gas temperature is 450 ° C.

[0019]

[Table 1] From Table 1 and FIG. 4, it can be seen that the HC50% purification temperature decreases and the low temperature activity remarkably improves as the Pd carrying density in the upstream portion increases. It is also understood that when the Pd is less than 7 g / L, the HC50% purification temperature is around 400 ° C, which is not preferable.

Further, when the purification rates after the durability test were compared, it was found that in Comparative Example 1, the degree of decrease in the NO _x purification rate after the durability was higher than that in the Examples, which is due to the small loading amount of Pd. It is clear that Further, as is clear from the comparison between Example 2 and Comparative Example 2, by carrying Pt and Rh in addition to Pd on the upstream side, the purification rate of CO and NO _x is particularly improved. Example 4 also shows better purification performance than Example 2, which is the same as Example 4.
It is assumed that the effect is that the exhaust gas flow is disturbed by providing the space between the upstream side portion and the downstream side portion, and the contact efficiency between the exhaust gas flowing into the downstream side portion and the catalyst is improved. Therefore, it is preferable to divide the upstream side portion and the downstream side portion.

[0021]

Therefore, according to the exhaust gas purifying catalyst of the present invention, it is possible to reduce the emission of unburned substances (particularly HC) from immediately after the engine is started to when the engine is warmed up. CO, HC, and NO _x emissions can be reduced, and high three-way purification performance is achieved.

[Brief description of drawings]

FIG. 1 is a sectional view showing a state in which an exhaust gas purifying catalyst according to an embodiment of the present invention is housed in a catalytic converter.

FIG. 2 is a cross-sectional view showing a state in which an exhaust gas purifying catalyst according to a fourth embodiment of the present invention is housed in a catalytic converter.

FIG. 3 is an explanatory view showing a state in which the exhaust gas purifying catalyst of the present invention is attached to an exhaust system of an automobile engine.

FIG. 4 is a graph showing the relationship between the HC50% purification temperature and the Pd carrying density of the exhaust gas purifying catalyst of the example.

[Explanation of symbols]

1: Catalytic converter 2: Monolith catalyst (exhaust gas purification catalyst) 20: Upstream side part

Claims (1)

[Claims] 1. A catalyst for exhaust gas purification in which palladium and rhodium are supported on the upstream side of the exhaust gas flow and platinum and rhodium are supported on the downstream side, and the density of palladium on the upstream side is upstream of the carrier on which palladium is supported. 7-20g per liter volume of the side part
The exhaust gas purifying catalyst is characterized in that