JPH10202103A - Oxidation catalyst for diesel engine and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To retain high performance of oxidizing and decomposing HC at low temperature and suppress production of sulfates. SOLUTION: This catalyst is constituted of a porous carrier particle, a thermally treated carrier particle produced by heating a porous carrier at 600 deg.C or higher, and Pt deposited on the thermally treated carrier particle and the particle size of Pt is set to be within a range from 5 to 50nm. Since the particle size of Pt is large, SO2 oxidization activity is lowered and since a large quantity of the porous carrier whose property is not changed by the thermal treatment exists, the HC-adsorbing capacity is retained to be high.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxidation catalyst for a diesel engine and a method for producing the same, and more particularly, to harmful components such as carbon monoxide (CO) and hydrocarbons (HC) contained in exhaust gas from a diesel engine. ) And a soluble organic component (SOF), and an exhaust gas purification catalyst that reduces the emission of sulfate (sulfate).
Since SOF is a relatively high molecular weight hydrocarbon,
Hereinafter, low molecular weight HC and SOF are collectively referred to as HC.

[0002]

2. Description of the Related Art With respect to gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of coping with the regulations. However, for diesel engines, the harmful components are PM
(Particulate materials such as carbon-based particles, sulfur-based particles such as sulfate, and high-molecular-weight hydrocarbon particles)
Due to the peculiar circumstances of being discharged as gas, regulation and technological progress are behind the gasoline engine, and the development of an exhaust gas purifying device that can surely purify exhaust gas is desired.

[0003] As exhaust gas purifying devices for diesel engines that have been developed to date, trap type exhaust gas purifying devices using trap type exhaust gas purifying catalysts and open type exhaust gas purifying devices using open type exhaust gas purifying catalysts have been roughly classified. A purifying device is known. As a trap-type exhaust gas purifying catalyst, a ceramic plugged honeycomb body (diesel particulate filter (DP
F)) and the like are known. In an exhaust gas purifying apparatus using the exhaust gas purifying catalyst, the exhaust gas is filtered by a DPF or the like and the PM is filtered.
Is collected, and if the pressure loss increases, the PM accumulated by a burner or the like is burned to regenerate the DPF or the like. It also oxidizes CO and HC while collecting PM.
To decompose, a catalyst supporting layer is formed on a carrier base material such as DPF using alumina or the like, and platinum (Pt) is formed on the catalyst supporting layer.
Exhaust gas purifying catalysts carrying the same have also been studied.

On the other hand, as an open type exhaust gas purifying catalyst, a carrier substrate made of a ceramic straight flow type honeycomb body or the like, a coat layer formed of alumina or the like on the carrier substrate, There is known a gasoline engine comprising Pt carried similarly to a gasoline engine. According to this open type exhaust gas purification apparatus, Pt
CO and HC can be oxidized and decomposed by the catalytic action of.

However, in the trap-type or open-type exhaust gas purifying apparatus having Pt, the catalyst supporting layer adsorbs SO ₂ in the exhaust gas, and at a high temperature, SO ₂ is oxidized by the catalytic action of Pt to SO ₃ . In particular, in a diesel engine, sufficient oxygen is present in the exhaust gas, and this oxygen easily oxidizes SO ₂ . In addition, SO ₃ easily reacts with steam present in a large amount in the exhaust gas to form a sulfuric acid mist. For these reasons, these exhaust gas purifiers
There is a problem that the amount of PM increases due to the emission of sulfate at high temperatures.

[0006] Under such circumstances, Japanese Patent Application Laid-Open No. 62-56783.
In the publication, Pt is supported on a coat layer made of alumina and then heat-treated at 700 to 1000 ° C.
A method for producing a catalyst for removing particulates in diesel exhaust in which t is sintered to thereby reduce the oxidizing action of SO ₂ is disclosed. As a method for supporting a catalytic metal such as Pt on a porous carrier, an adsorption supporting method in which an aqueous solution such as a platinum complex is brought into contact with the porous carrier, or a predetermined amount of an aqueous solution such as a platinum complex is applied to the porous carrier. There is known a water absorption supporting method in which water is evaporated and dried to dryness and then supported.

[0007]

However, in the adsorption-supporting method or the water-absorbing supporting method, the average particle size of the supported Pt is extremely small, 1 nm or less, and the oxidizing activity of SO ₂ is increased. When the thus-supported material is subjected to heat treatment, the particle size of Pt increases and the oxidation activity of SO ₂ decreases, but the crystal structure, specific surface area, or acid sites of the coat layer changes, and the amount of Pt in the exhaust gas is reduced. There was also a problem that the amount of harmful components adsorbed and the cracking performance of HC during adsorption were reduced.

For this reason, the catalyst disclosed in Japanese Patent Application Laid-Open No. Sho 62-56783 has a great effect of suppressing the formation of sulfate. However, the catalyst has a low oxidizing activity of Pt and a low adsorption performance due to a change in the coating layer during heat treatment. There is a problem that the HC oxidation activity in a low temperature range is low. The present invention has been made in view of the above circumstances, and while maintaining high HC oxidation and decomposition performance in a low temperature range,
The purpose is to suppress the production of sulfate.

[0009]

The features of the diesel oxidation catalyst according to the first aspect of the present invention, which solves the above-mentioned problems, include porous carrier particles and heat-treated carrier particles obtained by heat-treating the porous carrier at 600 ° C. or higher. , Pt supported on heat-treated carrier particles
And the particle size of Pt is in the range of 5 to 50 nm.

The method for producing a diesel oxidation catalyst according to claim 2 for preparing the oxidation catalyst is characterized in that a supporting step of preparing Pt-supported carrier by supporting Pt on some porous carrier particles. A heat treatment step of heat-treating the Pt-supported carrier at 600 ° C. or more to prepare heat-treated carrier particles having a Pt particle size of 5 to 50 nm, and a mixing step of mixing the remaining porous carrier particles with the heat-treated carrier particles. To include.

The diesel oxidation catalyst according to claim 3,
The method for producing a diesel oxidation catalyst according to claim 4 is characterized in that the heat-treated carrier particles are at least 5% by weight to 40% by weight.
% By weight.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Comparing the oxidizability of SO ₂ and HC, SO ₂ has a characteristic that it is harder to oxidize than HC. That is, when Pt is brought into contact with exhaust gas in which SO ₂ and HC coexist, HC is preferentially oxidized. Therefore, when HC is not present, SO ₂
Is oxidized, and sulfate is easily generated. This phenomenon becomes remarkable especially when the exhaust gas is heated to a high temperature.

In Pt, the smaller the particle size, the more H
The oxidation activity of both C and SO ₂ increases. On the other hand, when the particle size increases, the oxidation activity of HC slightly decreases at low temperatures but hardly decreases at high temperatures. On the other hand, the oxidation activity of SO ₂ at high temperatures greatly decreases and the formation of sulfate occurs. Can be greatly suppressed. Therefore, in the oxidation catalyst according to the first aspect, the particle size of Pt is set in the range of 5 to 50 nm. As a result, while maintaining high oxidation activity of HC, SO
_Since the oxidizing activity of ₂ becomes low, HC can be oxidized and purified reliably while suppressing the production of sulfate.

If the particle size of the loaded Pt is smaller than 5 nm, the oxidizing activity is too high, and it is oxidized to SO ₂ , which is not preferable. If it exceeds 50 nm, the purification performance of HC at low temperatures is reduced. In addition, the oxidation catalyst according to the first aspect includes at least a portion of the porous carrier particles to which heat of 600 ° C. or higher does not act. Since the adsorption performance of the porous carrier particles does not decrease due to the heat treatment, the HC in the exhaust gas is adsorbed well, so that the HC is more reliably oxidized and purified.

The porous carrier can be used by selecting from refractory inorganic oxides such as alumina, silica, titania, zeolite, silica-alumina and titania-alumina. This porous carrier preferably has an average particle size of 20 μm or less and a specific surface area of 10 m ² / g or more. If the porous carrier has an average particle size of more than 20 μm and a specific surface area of less than 10 m ² / g, there is a possibility that sufficient HC purification performance may not be obtained. A honeycomb-shaped or pellet-shaped carrier substrate may be formed from these porous carriers, or a coat layer made of the porous carrier may be formed on a honeycomb-shaped substrate surface formed of cordierite or metal. It can be formed into a carrier substrate.

The amount of Pt supported is preferably 0.01 to 10.0 g / L per unit volume of the oxidation catalyst.
If the amount of supported Pt is less than 0.01 g / L, sufficient oxidation
There is a possibility that decomposition performance may not be obtained. Conversely, 10.0 g /
Even if Pt exceeds L, the oxidation / decomposition performance is slightly improved, and the oxidation catalyst becomes expensive. In particular, when the loading amount of Pt is 0.1 to 3.0 g / L, it is preferable in terms of both oxidation / decomposition performance and cost.

In the production method according to the second aspect of the present invention for preparing the oxidation catalyst according to the first aspect, first, Pt is supported on some of the porous carrier particles in the supporting step.
A carrier is prepared. The particle size of the supported Pt is extremely fine, generally 1 nm or less. At this time, the amount of the porous carrier on which Pt is supported is 5% by weight of the total amount of the carrier used.
It is preferable that the amount is not less than 40 wt%. As the amount increases, the amount of the porous carrier particles in the oxidation catalyst decreases, and the HC purification performance decreases.
It is desirable to be less than. When it is less than 5% by weight,
The amount of the porous carrier that does not carry Pt becomes too large, and the carrying density of Pt in the entire oxidation catalyst decreases. Therefore, the probability of contact between the exhaust gas and Pt decreases, and the purification performance of HC becomes insufficient.

The Pt-supported carrier prepared in the supporting step is heat-treated at a temperature of 600 ° C. or more in the next heat-treating step to obtain heat-treated carrier particles. Since Pt is supported on a small amount of the porous carrier at a high density, the particle size easily becomes 5 to 50 nm by sintering. This heat treatment temperature is 600 ° C
If it is less than sintering, sintering hardly occurs and it is difficult to make the particle size of Pt 5 to 50 nm. The heat treatment time until the particle size of Pt becomes 5 to 50 nm differs depending on the heat treatment temperature, and the higher the temperature, the shorter the heat treatment time.

In this heat treatment step, the porous carrier carrying Pt is also transformed by heat. For example, if the porous carrier is γ-alumina, it is transformed into α-alumina, and if it is rutile-type titania, it is transformed into anatase-type titania and the adsorption performance is reduced. Therefore, in the present invention, the remaining porous carrier particles are mixed with the heat-treated carrier particles in the next mixing step. Most of the oxidation catalyst thus produced can be made of porous carrier particles, so that the HC adsorption performance is improved.

The obtained oxidation catalyst can be put to practical use by coating a carrier substrate having a honeycomb shape or the like by using a paste method or the like.

[0021]

The present invention will be described more specifically with reference to examples and comparative examples. (Embodiment 1) FIG. 1 shows a schematic configuration diagram of an oxidation catalyst of this embodiment. This exhaust gas purifying catalyst is composed of a cordierite honeycomb carrier (1) and a coating layer (2) coated on the surface of the honeycomb carrier (1). The coating layer (2) is composed of α-alumina particles (20) and γ-alumina particles (21).
Carry Pt (22). The component ratio between the α-alumina particles (20) and the γ-alumina particles (21) is as follows:
Α-alumina: γ-alumina = 10: 90 by weight ratio.

The method for producing the oxidation catalyst will be described in detail below, and will replace the detailed description of the configuration of the oxidation catalyst of the present embodiment. <Supporting Step> 50 g of γ-alumina powder was immersed in 100 ml of an aqueous dinitrodiammineplatinum nitrate solution containing 7.5 g of Pt, and stirred for 1 hour. After stirring, the mixture was evaporated to dryness in a drying oven at 120 ° C., and further subjected to a heat treatment at 250 ° C. for 1 hour.
A t-supported carrier powder was prepared. The specific surface area of the Pt-supported carrier powder is 200 m ² / g, and the average particle size of the supported Pt is 0.8 nm.

<Heat Treatment Step> Next, a heat treatment was performed to heat the Pt-supported carrier powder at 1200 ° C. for 6 hours in the air. The specific surface area of the obtained heat-treated carrier powder is 12 m.
² / g and its crystal structure was α-alumina.
The average particle size of Pt was grown to 45 nm.

<Mixing step>
350 g of γ-alumina powder, 500 g of alumina sol (alumina concentration: 20% by weight), and 100 g of water were mixed in a ball mill for 6 hours to prepare a slurry. <Coating step> A 1.7 L cordierite-made straight flow type honeycomb carrier was prepared, immersed in the slurry and pulled up to blow off excess slurry.
After drying at 120 ° C for 6 hours, baking at 500 ° C for 1 hour
A coat layer (2) of 72.05 g ± 5 g was formed. Pt
Is 1.5 g per liter of the honeycomb carrier, and the heat-treated carrier powder is contained at 10% by weight in the coat layer.

(Performance test) The above oxidation catalyst was attached to the exhaust system of a 2.6 L diesel engine, and the rotation speed was set to 2000 rpm.
pm, the aging treatment was performed at an input gas temperature of 500 ° C. for 1 hour, then the input gas temperature was lowered by 50 ° C., and the HC purification rates at 400 ° C., 300 ° C. and 200 ° C. and S
The O ₂ reduction rate was measured. Table 1 shows the results. Note that SO ₂
The reduction rate is reduced on the catalyst relative to the gas entering the catalyst.
It is the ratio of O ₂ and corresponds to the amount of sulfate produced.

(Example 2) In the heat treatment step, 1000
A heat-treated carrier powder was prepared in the same manner as in Example 1 except that the mixture was heated at 6 ° C. for 6 hours. The specific surface area of the heat-treated carrier powder was 42 m ² / g, and the crystal structure was such that α-alumina was mostly contained and γ-alumina was mixed. The average particle size of Pt was grown to 31 nm.

Using this heat-treated carrier powder, a mixing step and a coating step were performed in the same manner as in Example 1, to obtain an oxidation catalyst of this example. Then, a performance test was performed in the same manner as in Example 1, and the results are shown in Table 1. (Example 3) A heat-treated carrier powder was prepared in the same manner as in Example 1, except that heating was performed at 800 ° C for 6 hours in the heat treatment step. The specific surface area of the heat-treated carrier powder is 102 m ^2.
/ G, and its crystal structure was γ-alumina. The average particle size of Pt was growing to 16 nm.

Using the heat-treated carrier powder, a mixing step and a coating step were performed in the same manner as in Example 1, to obtain an oxidation catalyst of this example. Then, a performance test was performed in the same manner as in Example 1, and the results are shown in Table 1. (Comparative Example 1) The total amount of the Pt-supported carrier powder prepared in the supporting step of Example 1, 350 g of γ-alumina powder, 500 g of alumina sol (alumina concentration 20% by weight), and water 10
And 0 g with a ball mill for 6 hours to prepare a slurry.

Using this slurry, a coating process was carried out in the same manner as in Example 1 to obtain an oxidation catalyst of this comparative example. Then, a performance test was performed in the same manner as in Example 1, and the results are shown in Table 1. The carrier composition in the oxidation catalyst was γ-alumina, and the average particle size of Pt was 0.8 nm. (Reference Example 1) A heat-treated carrier powder was prepared in the same manner as in Example 1, except that heating was performed at 600 ° C for 6 hours in the heat treatment step. The specific surface area of this heat-treated carrier powder is 180 m ²
/ G, and its crystal structure was γ-alumina. The average particle size of Pt was 4 nm.

Using this heat-treated carrier powder, a mixing step and a coating step were performed in the same manner as in Example 1, to obtain an oxidation catalyst of this reference example. Then, a performance test was performed in the same manner as in Example 1, and the results are shown in Table 1. Example 4 <Supporting Step> 100 g of γ-alumina powder was immersed in 200 ml of an aqueous dinitrodiammineplatinum nitrate solution containing 7.5 g of Pt, and stirred for 1 hour. After stirring, the mixture was evaporated to dryness in a drying oven at 120 ° C., and further heat-treated at 250 ° C. for 1 hour to prepare a Pt-supported carrier powder. The specific surface area of the Pt-supported carrier powder is 200 m ² / g, and the average particle size of the supported Pt is 0.7 nm.

<Heat Treatment Step> Next, a heat treatment was performed to heat the Pt-supported carrier powder at 1200 ° C. for 6 hours in the air. The specific surface area of the obtained heat-treated carrier powder is 12 m.
² / g and its crystal structure was α-alumina.
The average particle size of Pt had grown to 44 nm.

<Mixing step>
300 g of γ-alumina powder, 500 g of alumina sol (alumina concentration: 20% by weight), and 100 g of water were mixed in a ball mill for 6 hours to prepare a slurry. Using this slurry, a coating process was performed in the same manner as in Example 1 to obtain an oxidation catalyst of this example. The supported amount of Pt was 1.5 g per liter of the honeycomb carrier, and the heat-treated carrier powder was contained in the coat layer at 20% by weight. And Example 1
A performance test was performed in the same manner as described above, and the results are shown in Table 1.

Reference Example 2 <Supporting Step> 200 g of γ-alumina powder was immersed in 400 ml of an aqueous solution of dinitrodiammineplatinum nitrate containing 7.5 g of Pt, and stirred for 1 hour. After stirring, the mixture was evaporated to dryness in a drying oven at 120 ° C., and further heat-treated at 250 ° C. for 1 hour to prepare a Pt-supported carrier powder. The specific surface area of the Pt-supported carrier powder is 200 m ² / g, and the average particle size of the supported Pt is 0.6 nm.

<Heat Treatment Step> Next, a heat treatment was performed to heat the Pt-supported carrier powder at 1200 ° C. for 6 hours in the air. The specific surface area of the obtained heat-treated carrier powder is 12 m.
² / g and its crystal structure was α-alumina.
The average particle size of Pt was growing to 40 nm.

<Mixing step>
200 g of γ-alumina powder, 500 g of alumina sol (alumina concentration: 20% by weight), and 100 g of water were mixed in a ball mill for 6 hours to prepare a slurry. Using this slurry, a coating process was carried out in the same manner as in Example 1 to obtain an oxidation catalyst of this reference example. The supported amount of Pt was 1.5 g per liter of the honeycomb carrier, and the heat-treated carrier powder was contained in the coat layer at 40% by weight. And Example 1
A performance test was performed in the same manner as described above, and the results are shown in Table 1.

Reference Example 3 <Supporting Step> 20 g of γ-alumina powder was immersed in 30 ml of an aqueous solution of dinitrodiammineplatinum nitrate containing 7.5 g of Pt, and stirred for 1 hour. After stirring, the mixture was evaporated to dryness in a drying oven at 120 ° C., and further heat-treated at 250 ° C. for 1 hour to form Pt.
A supported carrier powder was prepared. The specific surface area of the Pt-supported carrier powder is 200 m ² / g, and the average particle size of the supported Pt is 2 nm.

<Heat Treatment Step> Next, a heat treatment was performed to heat the Pt-supported carrier powder at 1200 ° C. for 6 hours in the atmosphere. The specific surface area of the obtained heat-treated carrier powder is 12 m.
² / g and its crystal structure was α-alumina.
The average particle size of Pt had grown to 48 nm.

<Mixing step>
380 g of γ-alumina powder, 500 g of alumina sol (alumina concentration: 20% by weight), and 100 g of water were mixed in a ball mill for 6 hours to prepare a slurry. Using this slurry, a coating process was carried out in the same manner as in Example 1 to obtain an oxidation catalyst of this reference example. The supported amount of Pt was 1.5 g per liter of the honeycomb carrier, and the heat-treated carrier powder was contained in the coat layer at 4% by weight. Then, a performance test was performed in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 2 400 g of γ-alumina powder
And 500 g of alumina sol (alumina concentration 20% by weight)
And 100 g of water were mixed in a ball mill for 6 hours to prepare a slurry. Using this slurry, a coating process was performed in the same manner as in Example 1 to form a coating layer.

Next, the honeycomb carrier having the coated layer formed thereon was immersed in an aqueous solution of dinitrodiammineplatinum nitrate containing 2.55 g of Pt for 1 hour, evaporated to dryness at 120 ° C., and then heat-treated at 250 ° C. for 1 hour. And then 500 ° C
For 1 hour. The supported amount of Pt was 1.5 g per liter of the honeycomb carrier, and the average particle size of Pt was 0.6.
nm. Then, a performance test was performed in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 3 The oxidation catalyst of Comparative Example 2 was subjected to a heat treatment at 1200 ° C. for 6 hours in the air. The average particle size of Pt is 43 nm. Then, a performance test was performed in the same manner as in Example 1, and the results are shown in Table 1. (Evaluation)

[0042]

[Table 1]

In Comparative Examples 1 and 2, Pt has a fine particle size of 0.8 nm and 0.6 nm since it was not heat-treated. As a result, although the purification rate of HC is high, SO ₂
It is oxidized to a high degree and the SO ₂ reduction rate is high. In Comparative Example 3 in which the oxidation catalyst of Comparative Example 2 was heat-treated at 1200 ° C., Pt
Although the SO ₂ reduction rate is low due to grain growth, the adsorption performance is reduced due to the deterioration of the porous carrier, and the HC purification rate is also significantly reduced.

In Reference Example 1, the growth of the Pt particle size did not occur because the heat treatment time at 600 ° C. was insufficient.
Oxidation of O ₂ produces a large amount of sulfate. However, if the heat treatment time is further increased, the particle size of Pt becomes 5n.
m or more, and it has been confirmed that the same catalytic performance as in the other examples was obtained. Further, in Reference Example 2, since the amount of the Pt-supported carrier was as large as 40% by weight, the portion that had been altered by the heat treatment increased, and the HC purification rate decreased. In Reference Example 3, since the amount of the Pt-supported carrier was as small as 4% by weight, the Pt-supporting density of the entire oxidation catalyst was low, so that the probability of contact between the exhaust gas and Pt was reduced and the HC purification performance was insufficient. Becomes

On the other hand, in Examples 1 to 4, a high HC purification rate and a low SO ₂ reduction rate were secured, and the effect of the present invention was clearly shown.

[0046]

According to the diesel oxidation catalyst of the present invention, the activity of purifying HC in a low temperature range is excellent, and the oxidation of SO ₂ in a high temperature range is suppressed, so that the emission of sulfate can be suppressed. Therefore H
The emission amount of PM can be reduced over a wide temperature range while maintaining the purification activity of C at a high level.

According to the production method of the present invention, the diesel oxidation catalyst can be produced easily and reliably.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view illustrating the configuration of an exhaust gas purifying catalyst according to one embodiment of the present invention.

[Explanation of symbols]

1: Honeycomb carrier 2:
Coat layer 20: α-alumina particles (heat-treated carrier particles) 21: γ-alumina particles (porous carrier particles) 2
2: Pt

Claims (4)

[Claims] 1. The method according to claim 1, wherein the porous carrier particles and the porous carrier are 600
An oxidation catalyst for diesel, comprising heat-treated carrier particles heat-treated at a temperature of not less than C and platinum supported on the heat-treated carrier particles, wherein the particle diameter of the platinum is in the range of 5 to 50 nm. 2. A supporting step of preparing a Pt-supporting carrier by supporting platinum on some of the porous carrier particles, and heat-treating the Pt-supporting carrier at a temperature of 600 ° C. or higher to reduce the platinum particle diameter to 5 to 50 nm. A method for producing an oxidation catalyst for diesel, comprising: a heat treatment step of preparing heat-treated carrier particles obtained as described above; and a mixing step of mixing the remaining heat-treated carrier particles with the remaining porous carrier particles. 3. The diesel oxidation catalyst according to claim 1, wherein the heat-treated carrier particles are contained in a range of 5% by weight or more and less than 40% by weight of the whole. 4. The method for producing a diesel oxidation catalyst according to claim 2, wherein the heat-treated carrier particles are contained in a range of 5% by weight or more and less than 40% by weight of the whole.