JP3269535B2 - Exhaust gas purification catalyst for diesel engines - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diesel engine (hereinafter referred to as "DE") containing H
The present invention relates to a catalyst capable of burning and purifying C, CO and SOF (Soluble Organic Fraction), reducing the emission of diesel particulates and reducing the emission of sulfate.

[0002]

2. Description of the Related Art Regarding gasoline engines, harmful substances in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of coping with it. But DE
With regard to, the regulation and development of technology are behind in comparison with gasoline engines because of the peculiar circumstances that harmful components are mainly emitted as particulates, and there is a need for the development of an exhaust gas purification device that can reliably purify.

[0003] As a DE exhaust gas purifying apparatus which has been developed to date, a method using a trap (without a catalyst and with a catalyst) and an open type SOF decomposition catalyst are known. Of these, the method using a trap is
It traps diesel particulates and regulates emissions, and is particularly effective for exhaust gas with a high dry suit ratio. However, a regeneration treatment device is required, and many problems remain in practical use, such as cracking of the catalyst carrier during regeneration, blockage by ash, a complicated system, and a large pressure loss.

On the other hand, an open type SOF decomposition catalyst is, for example, a catalyst in which an oxidation catalyst such as a platinum group metal is supported on a catalyst support layer of activated alumina or the like as in a gasoline engine, as shown in JP-A-3-38255. Used, C
SO in diesel particulates with O and HC
F is oxidatively decomposed and purified. This open-type SOF cracking catalyst has a drawback in that the removal rate of the dry suit is low, but the amount of the dry suit can be reduced by improving the DE and the fuel itself, and a reprocessing device is not required.
Because there is a great advantage that pressure loss is small,
Further improvements in technology are expected in the future.

[0005]

By the way, it is known that the exhaust gas discharged from the DE contains a large amount of SOF in a low temperature range, but contains very little SOF in a high temperature range. Therefore, in the open type SOF decomposition catalyst, the effect of SOF decomposition can hardly be expected in a high temperature range.

The active alumina layer formed on the open type SOF decomposition catalyst has a property of adsorbing SO ₂ . Therefore, SO ₂ contained in exhaust gas of DE
Is adsorbed on the activated alumina layer, but in a high temperature region, the adsorbed SO ₂ is oxidized by the catalytic action of the catalytic metal to form SO ₃ , which is discharged as sulfate and particulates.

Therefore, the open type SOF decomposition catalyst is
There is a problem that a particulate reduction effect is hardly obtained in a high temperature range. The present invention has been made in view of such circumstances, and it is an object of the present invention to maintain a small pressure loss and reduce the emission of particulates in a whole range from a low temperature range to a high temperature range.

[0008]

An exhaust gas purifying catalyst for a diesel engine according to the present invention, which solves the above-mentioned problems, has a structure in which a catalytic metal is supported on a honeycomb carrier substrate having a large number of honeycomb passages arranged in parallel in an axial direction. An exhaust gas purifying catalyst for an engine, wherein a honeycomb core substrate has a crossflow portion in which a honeycomb passage base member has crossflow portions in which inlet and outlet openings of a honeycomb passage are alternately closed in a checkered pattern. The outer periphery of the portion has a straight flow portion where the inlet side opening and the outlet side opening of the honeycomb passage are respectively opened, the cross flow portion carries a palladium catalyst, and the straight flow portion carries a platinum catalyst. Features.

It is desirable that the cross-flow portion is provided in a range of 10 to 40% of the cross-sectional area of the honeycomb carrier substrate. If the number of cross-flow portions is smaller than this, the effect of reducing particulates in a high temperature range becomes difficult to obtain. Further, when the cross flow portion is provided in excess of 40%, the pressure loss increases. By setting the pressure in this range, the pressure loss is small, and the effect of reducing particulates can be obtained in all regions from a low temperature region to a high temperature region.

[0010]

The exhaust gas discharged during the low-speed operation of the DE is in a low temperature range and contains a large amount of SOF. Further, the pressure of the exhaust gas in this low temperature range is low. Therefore, the exhaust gas in the low-temperature region hardly flows into the cross flow portion and mainly flows through the straight flow portion. In this straight flow section, S
Since the platinum catalyst excellent in the low-temperature purification performance of the OF is supported, the SOF is easily decomposed and removed, and is prevented from being discharged as particulates.

On the other hand, during high-speed operation, the exhaust gas also has a high temperature range, and the contained SOF becomes small. Since the flow rate of the exhaust gas in the high temperature region is higher at the center and higher in the pressure, the exhaust gas flow also flows into the cross flow portion as in the straight flow portion. In other words, the inlet side end face of the cross flow portion flows into the unblocked honeycomb passage, passes through the pores of the wall portion, and is discharged from the adjacent outlet side passage face not closed. Therefore, particulates such as carbon particles are trapped in the cross flow portion, and are prevented from being discharged.

A palladium catalyst having a small oxidizing effect on SO ₂ even at a high temperature is supported on the cross flow section. Therefore, the production of sulfate is suppressed as compared with the conventional open-type SOF decomposition catalyst supporting only a platinum catalyst, and as a result, the emission of particulates in a high temperature range is reduced. Furthermore, since carbon particles and the like deposited in the honeycomb passage in the cross flow portion are burned by a palladium catalyst having an excellent oxidizing action during high-speed running, the catalyst can be easily regenerated.

[0013]

The present invention will be specifically described below with reference to examples. (Embodiment 1) FIG. 1 is a perspective view of an exhaust gas purifying catalyst according to an embodiment of the present invention, FIG. 2 is a schematic sectional view thereof, and FIG. 3 is an enlarged view of a main part of FIG. This catalyst is supported on a honeycomb carrier substrate 1 made of cordierite, an alumina coat layer 2 covering the honeycomb passage wall surface of the honeycomb carrier substrate 1, and an alumina coat layer 2 on a predetermined portion of the honeycomb carrier substrate 1. And a Pt catalyst 3 and a palladium catalyst 4.

The honeycomb carrier substrate 1 has a cylindrical shape with a diameter of 90 mm and a length of 130 mm, and the number of honeycomb passages is 20.
0 cells / in ² , the thickness of the cell wall is 0.3 mm, the average pore diameter of the cell wall is 30 μm, and the pore volume is 0.3 cm ³ / g. This honeycomb carrier substrate 1 has a diameter of 45 m from the central axis.
A cross-flow portion 10 in which inlet-side openings and outlet-side openings are alternately closed in a checkered pattern is formed in a columnar portion of m (25% in terms of cross-sectional area). A straight flow portion 11 is formed in the outer peripheral portion excluding the cross flow portion 10 so that both the inlet side opening and the outlet side opening are not closed.

Hereinafter, the structure of the exhaust gas purifying catalyst will be described in more detail while describing a method of manufacturing the exhaust gas purifying catalyst.
A commercially available alumina powder having an average particle size of 10 μm, alumina sol, distilled water and a surfactant were mixed to prepare an alumina slurry, and the above-mentioned honeycomb carrier substrate 1 was immersed. Then, after lifting and blowing off the excess slurry, 120
C. for 2 hours, and then calcined at 700.degree. C. for 2 hours to form an alumina coat layer 2. This alumina coat layer 2
Is calculated as 12 per liter of the honeycomb carrier substrate.
0 g is formed.

Next, the inlet-side opening of a portion having a diameter of 45 mm from the axis was alternately closed with a filler ("Aron Ceramic" manufactured by Toa Gosei Chemical Co., Ltd.) in a checkered pattern. The outlet opening of the honeycomb passage whose inlet opening was not closed at a portion having a diameter of 45 mm from the axis was similarly closed in a checkered pattern. As a result, a honeycomb structure was obtained in which 25% of the total cross-sectional area was a cross flow portion 10 having a cross flow type gas flow, and 75% was a straight flow portion 11 having a straight flow type flow.

Next, both end faces of the cross flow section 10 are masked, immersed in an aqueous solution of platinum dinitrodiammine, lifted up, and then sprayed with excess water droplets, dried and fired by a conventional method, and the volume 1 1.5 g of Pt was loaded per liter. Next, the cross flow section 10
Masking and straight flow section 1
Both ends were masked, and 1.8 g of Pd per liter of catalyst volume was supported on the cross flow section 10 in the same manner using an aqueous palladium nitrate solution. (Example 2) Using an aqueous solution of platinum dinitrodiammine and an aqueous solution of ammonium metavanadate, 1.5 g of Pt and 0.1 g of Pt per liter of catalyst volume were placed in the straight flow section 11.
The configuration is the same as that of Example 1 except that 3 mol of V is carried. (Comparative Example 1) The example was performed except that 1.5 g of Pt was supported per liter of catalyst volume in the cross flow section 10 and 1.8 g of Pd was supported in 1 liter of catalyst volume in the straight flow section 11. This is the same configuration as in FIG. (Comparative Example 2) The configuration is the same as that of Example 1 except that all are constituted only by the straight flow portion 11, and that 1.5 g of Pt is supported per liter of catalyst volume as a whole. (Comparative Example 3) Cross flow portion 1 carrying Pd, provided with 75% of straight flow portion 11 carrying Pt on the axis side
The configuration is the same as that of the first embodiment except that 25% is provided on the outer peripheral side. (Reference Examples 1 to 5) The configuration is the same as that of Example 1 except that the composition ratio of the cross flow portion 10 and the straight flow portion 11 is changed and no catalyst metal is supported.

Table 1 shows the configurations of the above Examples, Comparative Examples and Reference Examples.

[0019]

[Table 1] (Test Examples and Evaluations) First, the honeycomb structures of Reference Examples 1 to 5 were attached to an exhaust system of a vortex chamber type DE with a displacement of 2.4 liters, respectively, and operated under conditions of an inlet gas temperature of 200 to 500 ° C. The particulate reduction rate at that time was measured. The results are shown in FIG. The particulate reduction rate was determined by a gravimetric method by collecting particulates upstream and downstream of the catalyst by a direct sampling method.

In the honeycomb structures of Reference Examples 1 to 5, since the catalyst metal is not supported and the SOF is not oxidized, the reduction of the particulates mainly due to the adsorption is observed. FIG. 3 shows that Reference Example 1 does not have the cross-flow unit 10.
In Table 1, the rate of reduction of particulates decreased as the exhaust gas temperature rose, and became a negative rate above 400 ° C. This is because the ratio of SOF decreases as the exhaust gas temperature increases, and almost no adsorption of carbon particles or the like occurs due to the increase in the flow velocity. Although the reason why the adsorption rate becomes negative is unknown, it is assumed that the adsorption substance is desorbed at a high temperature.

On the other hand, in Reference Examples 2 to 5, an increase in the reduction rate due to the provision of the crossflow portion 10 is observed, and it is understood that the reduction ratio increases as the number of the crossflow portions 10 increases.
In the low temperature range, the difference in the reduction rate depending on the ratio of the crossflow portion 10 is small, but in the high temperature range, the difference in the reduction rate increases. This indicates that the exhaust gas flows through the straight flow section 11 having a small pressure loss in the low temperature range, and also flows through the cross flow section 10 in the high temperature range to perform filtration and collection.

FIG. 5 shows the pressure loss when the exhaust gas temperature is 500 ° C. for each reference example. From FIG. 5, it can be seen that when the ratio of the cross flow portion 10 increases, the pressure loss sharply increases. 4 and 5
From the results, it is understood that those having an effect of reducing particulates and having a small pressure loss are preferably in the range of Reference Examples 2 to 4, that is, in the range of the cross-flow portion 10 having a ratio of 10 to 40%.

FIG. 6 shows the particulate reduction rates of the exhaust gas purifying catalysts of the embodiment and the comparative example. In the catalyst of Comparative Example 2 in which Pt was supported entirely without the cross flow portion 10, the reduction rate was significantly negative, and it was found that the outgassing gas had a greater increase in particulates than the incoming gas. .
This is because the SO ₂ oxidation reaction occurred at an exhaust gas temperature of 400 ° C. or higher, and the generated SO ₃ was counted as particulates.

Further, Pt is carried on the cross flow section 10,
The catalyst of Comparative Example 1 in which Pd is supported on the straight flow portion 11 is improved as compared with Comparative Example 2, but is inferior to Examples 1 and 2. Furthermore, P
In the catalyst of Comparative Example 3 having the straight flow portion 11 carrying t and the cross flow portion 10 carrying Pd on the outer periphery, the particulate matter reduction rate in the low temperature range is excellent, but the reduction rate in the high temperature range is excellent. Is lower than that of the embodiment. This is because the high-pressure, high-temperature exhaust gas flowed through the shaft core, and the efficiency of capturing particulates deteriorated. In this case, the cross flow unit 10
It is conceivable to increase the ratio, but it is not preferable because the pressure loss particularly in a low temperature range increases.

On the other hand, the catalysts of Examples 1 and 2 are excellent in the particulate reduction rate at all gas temperatures, and the reduction in the reduction rate in the high temperature range is slight. This is because, in the low temperature region, P which is excellent in the low temperature purification capability of SOF mainly in the straight flow section 11 is used.
In the high temperature region, the carbon particles are captured by the cross flow section 10 and the Pd is loaded to reduce the SO.
_{This is because} the oxidation reaction of ₂ was suppressed.

[0026]

According to the exhaust gas purifying catalyst of the present invention, the pressure loss can be kept small, and the emission of particulates can be reduced in all regions from a low temperature region to a high temperature region. In addition, since carbon particles and the like deposited in the cross flow portion are oxidized and burned at a high temperature due to the presence of the Pd catalyst, the exhaust gas purifying catalyst is naturally regenerated and does not require a regenerator.

[Brief description of the drawings]

FIG. 1 is a perspective view of an exhaust gas purifying catalyst according to one embodiment of the present invention.

FIG. 2 is a cross-sectional view of an exhaust gas purifying catalyst according to one embodiment of the present invention.

FIG. 3 is an enlarged sectional view of a main part of FIG. 2;

FIG. 4 is a graph showing a relationship between an incoming gas temperature and a particulate reduction rate of a honeycomb structure of a reference example.

FIG. 5 is a graph showing a relationship between a closing rate and a pressure loss of a honeycomb structure of a reference example.

FIG. 6 is a graph showing the relationship between the gas inlet temperature and the particulate reduction rate of the honeycomb structures of Examples and Comparative Examples.

[Explanation of symbols]

1: Honeycomb carrier substrate 2: Alumina coat layer
3: Pt catalyst 4: Pd catalyst 10: Cross flow section 11:
Straight flow section

──────────────────────────────────────────────────続 き Continuation of front page (51) Int.Cl. ⁷ identification code FI F01N 3/02 ZAB B01D 53/36 104Z (58) Field surveyed (Int.Cl. ⁷ , DB name) B01J 21/00-38 / 74 B01D 53/94 F01N 3/02

Claims (1)

(57) [Claims] An exhaust gas purifying catalyst for a diesel engine comprising a honeycomb carrier substrate having a plurality of honeycomb passages arranged in parallel in an axial direction and carrying a catalytic metal on the honeycomb carrier substrate. The honeycomb passage has an inlet opening and an outlet opening each having a cross-flow portion which is alternately closed in a checkered pattern, and the outer periphery of the cross-flow portion has an inlet opening and an outlet opening of the honeycomb passage, respectively. An exhaust gas purifying catalyst for a diesel engine, comprising a straight flow portion that opens, a palladium catalyst supported on the cross flow portion, and a platinum catalyst supported on the straight flow portion.