JP2002336656A - Catalyst for cleaning exhaust gas from diesel oil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure the durability of NOx cleaning ability of a continuous regeneration type diesel particulate filter(DPF) by further depositing a NOx occluding material on the coating layer of the DPF while the pressure loss is restrained from being increased. SOLUTION: The coating layer of 100-200 g/L is formed on the DPF having >=60% porosity and >=22 μm average pore diameter and the NOx occluding material is deposited on the coating layer to obtain a filter body. The pressure loss can be restrained from being increased by using this filter body. Even when the coating layer of 100-200 g/L is formed. The durability of the NOx purifying ability of the filter can be secured since the thick coating layer is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diesel exhaust gas purifying catalyst for collecting particulates (particulate matter) contained in exhaust gas from a diesel engine and purifying harmful components in the exhaust gas.

[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, with regard to diesel engines, regulations and technological advances have been limited due to the unique situation that harmful components are emitted as particulates (particulate matter: carbon fine particles, sulfur-based fine particles such as sulfate, high-molecular-weight hydrocarbon fine particles, etc.). It is behind the gasoline engine.

As exhaust gas purifying devices for diesel engines which have been developed to date, trap type exhaust gas purifying devices (wall flow) and open type exhaust gas purifying devices (straight flow) are known. . Among them, as a trap type exhaust gas purifying device, a plugged honeycomb body made of ceramic (diesel particulate filter (hereinafter, referred to as DPF)) is known. This DPF is formed by alternately plugging both ends of an opening of a cell of a ceramic honeycomb structure in a checkered pattern, and filtering exhaust gas through pores of the cell partition to collect particulates in the cell partition. This will reduce emissions.

However, in the DPF, since the pressure loss increases due to the accumulation of the particulates, it is necessary to periodically remove and regenerate the particulates accumulated by some means. Therefore, conventionally, when the pressure loss increases, the DPF is regenerated by burning the accumulated particulates by a burner or an electric heater. However, in this case, as the amount of deposited particulates increases, the temperature during combustion rises, and due to the resulting thermal stress, the DP increases.
F may be damaged.

Therefore, in recent years, a continuous regeneration type DPF in which a coat layer is formed from alumina or the like on the cell partition walls of the DPF and the coat layer carries a catalytic metal such as platinum (Pt) has been developed. According to this continuous regeneration type DPF, the particulates collected by the catalytic reaction of the catalytic metal are oxidized and burned, and thus the DPF can be regenerated by burning simultaneously with or continuously with the collection. Further, since the catalytic reaction occurs at a relatively low temperature and can be burned while the trapping amount is small, there is an advantage that thermal stress acting on the DPF is small and breakage is prevented.

[0006] As such a continuous regeneration type DPF, for example, Japanese Patent Application Laid-Open No. 9-220423 discloses a cell partition having a porosity of 40 to 40.
65%, the average pore diameter is 5 to 35 μm, and the porous oxide constituting the coating layer has a particle diameter smaller than the average pore diameter of the cell partition walls and occupies 90 wt% or more. ing. By coating with such a porous oxide having a high specific surface area, a coat layer can be formed not only on the surface of the cell partition but also on the inner surface of the pores. Further, if the coating amount is constant, the coating thickness can be reduced, so that an increase in pressure loss can be suppressed.

[0007]

[SUMMARY OF THE INVENTION In However continuous regeneration type DPF described above, we are difficult to purify the NO _x in the exhaust gas. Therefore, by applying the NO _x storage reduction catalyst used in the exhaust system of a gasoline engine, a continuous regeneration type DPF
It is conceivable to coat layer further carrying _the NO _x storage material.

In the NO _x storage-reduction catalyst, NO is oxidized on the catalyst metal in an exhaust gas in an oxygen-rich lean atmosphere, and the generated NO ₃ or NO ₄ is stored in the NO _x storage material.
Then, by supplying intermittently a rich gas containing excess reducing components, NO _x is released from the NO _x storage material, and the NO _x reacts with the reducing components on the catalytic metal to be reduced to N ₂ . Thereby the NO _x in the exhaust gas from a lean burn engine to efficiently reduced and removed.

Accordingly the use of the continuous regeneration type DPF was further carrying _the NO _x storage material in the coating layer, it is believed to be able to purify NO _x in the exhaust gas of a diesel engine by the same operation as described above.

However, the DPF has a higher pressure loss due to the wall flow structure as compared with a straight flow structure such as an NO _x storage reduction catalyst. In the case of the continuous regeneration type DPF, there is a problem that the pressure loss is further increased because a coating layer is further formed on the DPF. Therefore, the thinner the coat layer, the more desirable it is about 40 to 80 g per liter of DPF volume.

[0011] According to the inventors' study, NO _x storage reduction catalyst is improved as the durability greater the amount of the coating layer,
It has been clarified that the durability decreases as the thickness of the coat layer decreases. And in coating amount of about volume per liter 40~80g of DPF, the durability of the NO _x purifying ability is revealed that impractical low.

Namely in the case of a continuous regeneration type DPF was further carrying _the NO _x storage material in the coating layer, the coating layer in order to lower the pressure loss thinner is preferable, in order to ensure the durability of the NO _x purification performance There was a contradiction that the thicker the coat layer, the better.

[0013] The present invention has been made in view of such circumstances, further NO _x coat layer of the continuous regeneration type DPF
An object of the present invention is to support an occlusion material and to secure the durability of the NO _x purification ability while suppressing an increase in pressure loss.

[0014]

The feature of the catalyst for purifying diesel exhaust gas of the present invention that solves the above-mentioned problems is that a filter body in which both ends of a cell opening of a ceramic honeycomb structure are alternately plugged in a checkered pattern, more it becomes a coating layer made of formed in the cell partition walls porous oxide, a catalytic metal and NO _x storage material that is supported on the coating layer, the filter body porosity of 60%, an average pore diameter be more than 22μm , The coating layer is 1 volume per liter of the ceramic honeycomb structure.
00 to 200 g.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The catalyst for purifying diesel exhaust gas of the present invention uses a filter body having a porosity of 60% or more and an average pore diameter of 22 μm or more. By using such a filter main body, an increase in pressure loss can be suppressed even when the coat layer is formed at 100 to 200 g / L. If the porosity is less than 60% or the average pore diameter is less than 22 μm, the coating amount capable of suppressing an increase in pressure loss is less than 100 g / L, and NO _x
A decrease in the durability of the storage capacity is inevitable.

If the coating amount is small, NO_x The durability of the storage capacity
The decrease is due to the high density of the catalyst metal supported,
It is considered that the main cause is that grains grow easily at times. to this
Therefore, the sulfur poisoned NO_x SO from occlusion materials_x Desorption
The reaction is inhibited, resulting in NO _x NO for occlusion material_x Low storage capacity
Down. Also, with the growth of catalytic metal grains, NO_x The occlusion material is also grain
Grow up, no_x It is thought that the decrease in storage sites is also a factor
It is.

Therefore, according to the catalyst of the present invention in which the coating amount is increased, the catalyst metal is supported with high dispersibility, so that the grain growth of the catalyst metal is suppressed and the above-mentioned problem does not occur.
The durability of the NO _x purification capacity is improved.

The porosity is preferably in the range of 60 to 75%,
The average pore diameter is desirably in the range of 22 to 35 μm. When at least one of the porosity and the average pore diameter exceeds this upper limit,
Problems such as difficulty in collecting particulates and reduction in strength are caused.

The filter main body is one in which both ends of the opening of the cell of the ceramic honeycomb structure are alternately plugged in a checkered pattern, and can be manufactured from a heat-resistant ceramic such as cordierite. For example, a clay-like slurry containing cordierite powder as a main component is prepared, molded by extrusion or the like, and fired to obtain a honeycomb structure.
Instead of the cordierite powder, each powder of alumina, magnesia and silica can be blended so as to have a cordierite composition. Thereafter, cell openings on one end face of the honeycomb structure are plugged in a checkered pattern with the same clay-like slurry, and cell openings not plugged on one end face are plugged in a checkered pattern on the other end face. Thereafter, the filter body can be manufactured by fixing the plugging material by firing or the like.

In order to form pores in the cell partition walls of the filter body, flammable material powders such as carbon powder, wood powder, starch, and polymer are mixed in the above slurry, and the flammable material powder is baked during firing. By disappearing, pores can be formed.

The porosity and the average pore diameter of the cell partition walls can be formed as specified in the present invention by adjusting the particle diameter and amount of the combustible powder. For example, in order to make the porosity 60%, the combustible material powder may be used in an amount of 65% by volume or more based on the ceramic powder in the slurry. Also, in order to make the average pore diameter 22 μm or more, the combustible powder having a particle size of 22 to 26 μm is contained in all the combustible powders.
It is advisable to mix 70 to 80% by volume.

In addition to the adjustment of the combustible powder, it is desirable to adjust the viscosity of the clay-like slurry, the particle size of the powder in the slurry, and the like.

A coating layer made of a porous oxide is formed on the cell partition walls of the filter body. This coat layer serves as a carrier for supporting the catalyst metal, and is composed of Al ₂ O ₃ , Zr
It can be formed from an oxide such as O ₂ , CeO ₂ , TiO ₂ , SiO ₂ or a composite oxide composed of a plurality of these.

This coat layer is desirably formed not only on the surface of the cell partition wall but also on the surface in the pores formed by the disappearance of the combustible powder. In order to form the coating layer in this way, as described in, for example, JP-A-9-220423, oxide powder having a particle size smaller than the average pore diameter of the cell partition accounts for 90% by weight or more, or It is preferable to use a composite oxide powder. If more than 10% by weight has a particle diameter larger than the average pore diameter, it becomes difficult to form a coat layer on the surface inside the pores, and it becomes difficult to oxidize and burn the particulates that have entered the pores. .

The coating layer formed on the filter body has a coating amount of 100 to 200 g / L. Coating amount can not be avoided deterioration of the durability of the NO _x storage capacity is less than 100 g / L, not practical and pressure loss becomes too high when it exceeds 200 g / L.

In order to form a coat layer, an oxide powder or a composite oxide powder is formed into a slurry together with a binder component such as alumina sol and water, and the slurry is applied to the cell partition walls and then fired. A general immersion method can be used to attach the slurry to the cell partition walls, but it is desirable to remove excess slurry from the pores by air blowing or suction.

As the catalyst metal supported on the coat layer, any metal that can reduce NO _x by a catalytic reaction and promote the oxidation of particulates can be used, but platinum such as Pt, Rh, and Pd can be used. It is particularly preferred to carry one or more selected from the group noble metals. The amount of catalyst metal supported is 2 per liter of filter body volume.
It is preferably in the range of ８8 g. If the amount is less than this, the activity is too low to be practical, and if the amount is more than this range, the activity is saturated and the cost is increased.

In order to carry the catalyst metal, a solution in which a nitrate of the catalyst metal or the like is dissolved may be carried on the coat layer by an adsorption carrying method, a water absorbing carrying method or the like. Alternatively, a catalyst layer may be previously supported on the oxide powder or the composite oxide powder, and the coat layer may be formed using the catalyst powder.

Examples of the NO _x occluding material supported on the coating layer include alkali metals such as K, Na, Cs, and Li, Ba, Ca, Mg, and the like.
It can be selected from alkaline earth metals such as Sr or rare earth elements such as Sc, Y, Pr, and Nd. Among them, it is desirable to use at least one of an alkali metal and an alkaline earth metal having excellent NO _x storage ability.

The amount of the NO _x storage material to be carried is preferably in the range of 0.25 to 0.45 mol per liter of the filter body. If the amount is less than this, the activity is too low to be practical, and if it is more than this range, the catalyst metal is covered and the activity is reduced.

To support the NO _x storage material, acetate,
A solution in which nitrate or the like is dissolved may be supported on the coat layer by a water absorption supporting method or the like. The advance carries advance _the NO _x storage material on the oxide powder or a composite oxide powder, it is possible to form the coating layer by using the powder. Note that
The NO _x occluding material is supported as a carbonate, oxide or nitrate.

The diesel exhaust gas purifying catalyst of the present invention comprises:
Although it can be used in general diesel exhaust gas in a lean atmosphere, it is preferable to use it in an exhaust gas which is always in a lean atmosphere and is controlled intermittently to a stoichiometric to rich atmosphere. In a lean atmosphere, collected particulates together are removed by oxidative combustion on the catalyst metal, HC and CO in the exhaust gas is purified, and NO _x is occluded in _the NO _x storage material. The intermittent stoichiometric to rich atmosphere makes it possible to recover the NO _x storage ability of the NO _x storage material, and to maintain a high NO _x purification ability for a long period of time.

In order to attain the stoichiometric to rich atmosphere intermittently, the air-fuel ratio may be controlled,
It is also preferable to introduce a reducing component such as fuel into the exhaust gas.

[0034]

Hereinafter, the present invention will be described in detail with reference to test examples.

As the filter body, a plurality of cordierite DPFs having different porosity and average pore diameter were prepared. The porosity is 9 levels between 50 and 70% and the average pore size is 9 levels between 10 and 40 μm.

Next, an Al ₂ O ₃ powder having an average particle size of 1 to ₃ μm was prepared.
100 parts by weight, 100 parts by weight of ZrO ₂ -TiO ₂ composite oxide powder having an average particle diameter of 1 to 3 μm, 20 parts by weight of CeO ₂ powder having an average particle diameter of 1 to 3 μm, and ZrO ₂ having an average particle diameter of 1 to 3 μm. Is prepared by preparing a slurry containing 50 parts by weight of Rh / ZrO ₂ powder carrying 1% by weight and 52 parts by weight of alumina nitrate, dipping the DPF, pulling it up, and vacuum-suctioning to remove excess slurry.
And baked at 450 ° C for 120 minutes
To form a coat layer. The coating amount was 8 levels between 75 and 270 g per liter of DPF volume.

The coating layer was impregnated with a predetermined amount of an aqueous solution of dinitrodiamine platinum nitrate having a predetermined concentration, dried and fired to carry 5 g of Pt per liter of DPF. Next, using an aqueous barium acetate solution, an aqueous potassium acetate solution and an aqueous lithium acetate solution, similarly, 0.1 mol of Ba, 0.1 mol of K and
0.2 mol of Li was supported on each.

(Test Example 1) Eight types of catalysts were placed in the engine evaluation device shown in FIG.
An endurance test was conducted to allow time distribution.

[0039] For each of the catalysts after the durability test, as shown in FIG. 2, the concentration of NO _x exhausted at 1600 rotation lean normal combustion was occluded NO _x to stabilize. Followed by reduction of the occluded NO _x is switched to control the air-fuel ratio 5 seconds rich atmosphere. After that, lean normal combustion again (catalyst gas temperature
300 ° C) and NO until the exhausted NO _x concentration stabilizes
_{The x} storage amount was measured as RSNO _x storage amount. FIG. 3 shows the measurement results of the amount of occluded RSNO _x .

FIG. 3 shows that the greater the coating amount, the greater the amount of RSNO _x occlusion after the durability test, and the better the durability. When the coating amount is 100 g / L or more, the amount of occluded RSNO _x per 1.3 liter of catalyst volume becomes 110 mg or more, which is a preferable range.

(Test Example 2) In order to investigate the cause of the decrease in durability as described above, the coating amount used in Test Example 1 was 75%.
A durability test was performed in the same manner as in Test Example 1 for two types of catalysts, g / L and 270 g / L. Next, the catalyst after the durability test was cut into a test piece having a diameter of 30 mm and a length of 50 mm. Then, the amount of SO _x released when heated from 200 ° C. to 700 ° C. at a heating rate of 20 ° C./min was continuously measured, and the results are shown in FIG.

On the other hand, the coating amount used in Test Example 1 was 75 g /
For two types of catalysts, L and 270 g / L, the particle size of the supported Pt was measured using the CO adsorption method, and the results are shown in FIG. The Pt particle size after the durability test was performed in the same manner as in Test Example 1, and the result is shown in FIG.

As shown in FIG. 4, the catalyst having a coating amount of 75 g / L is
Coating amount is released SO _x at a high temperature range as compared with the catalyst of 250 g / L, it can be seen that a lower desorption of SO _x. Also, from FIG. 5, the catalyst having a coating amount of 75 g / L has a coating amount of 250 g / L.
Pt particle size after endurance test is extremely large compared to
The endurance test shows that grains have grown.

From the above results, it is considered that in a state where the coating amount is small, Pt is carried at a high density, so that the grain grows easily during the durability test. Then, as a result, it is considered that the contact area with the NO _x storage material was reduced, and the amount of released SO _x was reduced.

[0045] Then when considered comprehensively the results of Test Example 1 and Test Example 2, the grain growth of Pt occurs during the durability test and less coating amount, NO _x storage material is eliminated in the sulfur poisoning difficult It is considered that the catalyst deteriorated and the amount of occluded RSNO _x decreased. If the coating amount is 100 g / L or more, the Pt grain growth is suppressed, so that such a problem does not occur, and a high RSNO _x occlusion amount can be secured even after the durability test.

(Test Example 3) The coating amount was 150 g / L, and the porosity and average pore diameter of DPF were various levels.
Each kind of catalyst was used and arranged in the exhaust system as shown in FIG. Then, when the engine was operated at 1600 rpm lean normal combustion, the exhaust gas pressure difference before and after the catalyst was measured by a differential pressure gauge. Then, the ratio of the actually measured pressure loss value to the target pressure loss value is calculated, and the results are shown in FIGS.

FIGS. 6 and 7 show that when the porosity is 60% or more and the average pore diameter is 22 μm or more, the measured pressure loss becomes less than the target value.

That is, if a DPF having a porosity of 60% or more and an average pore diameter of 22 μm or more is used, the coating layer can be
It is clear that the increase in pressure loss is suppressed even when 150 g / L is formed. And if the coat layer is formed 150g / L,
From the results of Test Example 1, it is clear that RSNOs _x storage amount after the durability test is kept high, it is clear that can ensure the durability of the NO _x purifying capability while suppressing an increase in pressure loss.

[0049]

According to i.e. the catalyst for purification of diesel exhaust gas of the present invention, it is possible to ensure the durability of the NO _x purifying capability while suppressing an increase in pressure loss.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an evaluation device used in a test example.

FIG. 2 is an explanatory diagram showing the definition of the amount of occluded RSNO _x used in a test example.

FIG. 3 is a graph showing a relationship between a coating amount and an RSNO _x occlusion amount.

4 is a graph showing the relationship between the temperature and the released SO _x.

FIG. 5 is a graph showing a relationship between a coating amount and a Pt particle size.

FIG. 6 is a graph showing the relationship between the porosity of DPF and pressure loss.

FIG. 7 is a graph showing the relationship between the average pore diameter of DPF and pressure loss.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B01J 35/04 301 F01N 3/02 301C 35/10 301 301E F01N 3/02 301 321A 3/08 A 321 3 / 24 E 3/08 3/28 301C 3/24 301P 3/28 301 B01D 53/36 104A B01J 23/56 301A F term (reference) 3G090 AA02 AA03 BA01 EA02 3G091 AA18 AB06 AB13 BA01 BA07 BA14 BA38 BA39 GA06 GA17 GA19 GB02W GB03W GB04W GB05W GB06W GB07W GB17Y HA14 4D019 AA01 BA02 BA06 BB07 BB10 BC05 BC07 CA01 CB04 4D048 AA06 AA14 AB02 BA14X BA15X BA19X BA30X BA33X BB02 CD05 EA04 4G069 AA03 BC04BC03BC03BC

Claims (2)

[Claims] 1. A filter body in which both ends of an opening of a cell of a ceramic honeycomb structure are alternately plugged in a checkered pattern,
A coating layer formed on the cell partition wall and made of a porous oxide,
More becomes supported catalyst metal and NO _x storage material in the coating layer, the filter body porosity of 60%, an average pore diameter of 22μ
m or more, and the coating layer is coated in an amount of 100 to 200 g per liter of the volume of the ceramic honeycomb structure. 2. The catalyst for purifying diesel exhaust gas according to claim 1, wherein the NO _x storage material is at least one selected from an alkali metal and an alkaline earth metal.