JP2003117399A - Exhaust gas cleaning catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the migration of an alkali metal and to enhance the permanence of NOx removing power. SOLUTION: An NOx occluding material has a core-shell structure comprising a core body comprising an alkali metal and a shell layer coating the periphery of the core body, wherein the shell layer contains an element selected from the alkaline earth metals, the rare earth elements, Mn, Fe, Co and Ni. Since the shell layer is in the solid state at high temperature, the migration of the alkali metal is regulated by the shell layer.

Description

DETAILED DESCRIPTION OF THE INVENTION [0001] The present invention relates to a lean oxygen-excessive system.
NO in atmosphere_x In a rich atmosphere with excess reducing components
NO absorbed_x NO that can be released and reduced_x Occlusion reduction type
The present invention relates to an exhaust gas purifying catalyst. [0002] 2. Description of the Related Art In recent years, global warming caused by carbon dioxide
Has become a problem, and reducing carbon emissions has become an imperative.
It has become a problem. Carbon dioxide in exhaust gas also in automobiles
The challenge is to reduce the amount of element, and the fuel is diluted in an oxygen-rich atmosphere.
Lean-burn engines that burn are being developed. This
Lean burn engine reduces fuel consumption
Reduce carbon dioxide emissions
You. [0003] In this lean-burn engine,
Is burned under an oxygen-rich fuel lean condition,
Exhaust gas is reduced in a reducing atmosphere by stoichiometric to rich conditions.
NO as atmosphere_x Has been developed to reduce and purify
Has been used. And the best catalyst for this system
NO in a lean atmosphere_x Occluded and occluded NO_x The
NO released in toy or rich atmosphere_x Using occlusion material
NO_x A storage reduction type exhaust gas purifying catalyst has been developed. For example, Japanese Patent Application Laid-Open No. 5-317652 discloses Ba and the like.
Alkaline earth metal and Pt to γ-Al_TwoO_ThreeSuch as porous oxide
An exhaust gas purifying catalyst supported on a carrier has been proposed. Ma
Japanese Unexamined Patent Publication No. 6-31139 discloses that an alkali metal such as K is used.
Pt to γ-Al_TwoO_ThreeExhaust gas carried on a porous oxide carrier such as
Purification catalysts have been proposed. Furthermore, JP-A-5-18860
In the gazette, rare earth elements such as La and Pt are γ-Al_TwoO_ThreeMany
An exhaust gas purification catalyst supported on a porous oxide carrier was proposed.
ing. Alkali metal, alkaline earth metal and rare earth element
The element is NO_x NO has the property of absorbing and releasing NO._x Sucking
It is called a storage material. [0005] This NO_x If an occlusion reduction catalyst is used,
Change the ratio from lean to pulsed stoichiometric or rich.
Control to ensure that the exhaust gas is also lean.
A stoichiometric or rich atmosphere is created in a pulsed manner. But
That's NO on the lean side_x Is NO _x Occluded by the occlusion material, it
Is released on the stoichiometric or rich side to reduce HC and CO
Lean burn engine
NO even if exhaust gas from_x Purifying water efficiently
Can be. In addition, HC and CO in exhaust gas are
And NO_x HC and HC
And CO are also efficiently purified. [0006] NO_x With storage reduction catalysts, oxygen excess
NO in the exhaust gas in the atmosphere
NO_Two Which is converted to nitrite ion by steam.
NO or nitrate ion_x Reacts with occlusion material and absorbs
Is stored. Therefore noble metals and NO_x Close to the storage material
It is desirable to be supported in contact with
_x High NO with maximum storage capacity_x Purification rate is expressed
You. On the other hand, NO_x Alkali metals as occlusion materials
Is NO in high temperature range_x In recent years because of its excellent storage capacity
Used in high temperature exhaust gas_x For storage reduction catalysts
It is an essential ingredient. Therefore NO_x Storage reduction catalyst
Precious metals and alkali metals are essential
It is desirable that they are carried in close proximity. However, alkali metals generally have a low melting point.
At high temperatures, it moves on the carrier and covers the surface of the noble metal,
Precious metal activity is reduced by_x Decreased storage capacity
There was a problem. For example, potassium is K_TwoO state
Has a melting point of 402 ° C and NO_xK (NO_Three) Condition
The melting point is 337 ° C. Therefore, exhaust gas of about 400 ° C
It liquefies at temperature and flows over the carrier, covering the precious metal
U. Therefore, the noble metal and the alkali metal are separated from each other.
Separated on the carrier particles or separated on the carrier layer
Is carried out, but the
All NO_x NO storage capacity_x The question that the purification rate also decreases
There is a title. In addition, alkali metals that scatter at high temperatures,
Solidify on a substrate such as a porous oxide carrier or cordierite
Some alkali metals dissolve, and if this happens NO_x Occlusion
Performance is reduced, and NO_x Purification rate
Will drop. [0010] Therefore, it is most desirable to operate at high temperatures.
To prevent the movement of alkali metals
Effective measures have been found due to the characteristics of potassium metal itself.
It is not done at present. [0011] SUMMARY OF THE INVENTION
It was made in consideration of the situation, and the movement of the alkali metal
Suppress, NO_x Pursuing a catalyst with enhanced purification performance durability
Aim. [0012] [MEANS FOR SOLVING THE PROBLEMS]
The characteristics of Akira's exhaust gas purification catalyst are at least
NO containing alkali metals_x Supporting occlusion material and precious metal
Become NO_x An occlusion reduction type exhaust gas purifying catalyst, comprising NO_x
The storage material covers the core of alkali metal and the surroundings of the core.
It has a core-shell structure consisting of a shell layer
The layers are alkaline earth metals, rare earth elements, Mn, Fe, Co, and
It is to contain an element selected from Ni. [0013] BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention,
Alkali metal core and shell layer surrounding core
NO with core-shell structure consisting of_x Carrying occlusion material
I have. Since the melting point of the shell layer is higher than that of the alkali metal,
Solid even when the core of alkali metal becomes liquid at warm
The flow is regulated by the shell layer in the state and covers the precious metal.
Troubles such as solid solution or carrier or substrate
Be suppressed. Therefore high NO even after high temperature endurance_x With occlusion ability
High NO_x The purification rate is expressed and the durability is excellent. Alkali gold which is a core of a core-shell structure
The genus can be any of K, Na, Cs, Li, Rb, and Fr,
K, Na, Cs, Li are preferred, and K is particularly preferred. The shell layer of the core-shell structure is made of aluminum.
Selected from potassium earth metals, rare earth elements, Mn, Fe, Co, and Ni
Containing these elements, oxides of these elements,
Carbonate, etc., solid state even at high temperatures of 400 ° C or higher
Is used. Among them, Al such as Ba, Ca, Mg
It is particularly desirable to include potassium earth elements. by this
NO at low temperature_x Improved storage capacity, NO_x Purification activity is one
Improve layers. NO of core-shell structure_x The particle size of the occlusion material is
 0.1 μm or less, preferably 0.05 μm or less
It is more desirable that If the particle size is larger than this
NO due to surface area decrease_x NO per supported amount of storage material_x Sucking
Storage capacity decreases. Therefore, a large amount must be carried,
A swell may affect the amount of the noble metal carried. Also
The ratio between the core and the shell layer is not particularly limited, but the ratio by weight is
The nucleus / shell layer is preferably in the range of 1/4 to 1/2. Such a core-shell structure NO_x Occlusion material
Various methods are conceivable for manufacturing
There is a method using cells. This method starts with benzene
Any non-aqueous solvent contains alkaline earth metal etc. in the molecule
Add surfactant to form reverse micelles. Al there
When an aqueous solution of potassium metal salt is added, the center parent of the reverse micelle is added.
Drops of aqueous solution of alkali metal salt are held in the aqueous part
Solubilize. If this is fired, the alkali metal will become the core
Core shell with a shell layer containing alkaline earth metal, etc.
NO in well structure_x Occlusion material particles can be produced. In the above manufacturing method, the center of the reverse micelle is
In the part, the alkali metal salt is precipitated as water-insoluble and
Firing is preferred. With this, from alkali metal
Cores can be made even finer, core-shell
Structure NO_x As the occlusion material can be miniaturized, the surface area can be increased.
More no_x Storage capacity and NO_x Purification ability is further improved. this
To precipitate alkali metal salts, for example, tartaric acid
Mix the aqueous solution and add the alkali metal liquor in the center of the reverse micelle.
There is a method of forming hydrogen oxalate. Acid used in exhaust gas purifying catalyst of the present invention
Compound carrier, Al_TwoO_Three, TiO_Two, ZrO_Two, CeO_Two, SiO_Two,
One or more such as MgO, or selected from these
It is possible to use complex oxides composed of multiple types
You. Then, the oxide carrier powder is formed into a pellet,
No core / shell structure_x The storage material and precious metal
It may be used as a let catalyst, cordierite or gold.
Oxide carrier powder on honeycomb substrate formed from metal foil
Of a core-shell structure
NO_x Supporting the occlusion material and precious metal as a honeycomb catalyst
You can also. The oxide carrier has a core-shell structure of NO._x Occlusion
To support the material, oxide carrier powder and core-shell structure
NO_x Mix the powder consisting of the occluding material particles to form a coat layer
This can be done by performing Note that the core
NO in well structure_x The occlusion material is screened by carbonation, etc.
It is desirable to make the well layer water-insoluble. This
Core-shell structure in aqueous slurry during coating layer formation
NO_x Elution of the occluding material can be prevented. In addition, NO having a core-shell structure_x Loading of occlusion material
With the conventional NO_x Alkaline like storage reduction catalyst
The alkali metal is absorbed by absorbing the aqueous solution of the metal salt into the coating layer.
It is also preferable to carry it. This allows fine alkali gold
Genus can be carried, so the initial NO_x Improve purification activity
be able to. However, the alkali metal moves at high temperatures
To reduce the possibility of covering precious metals with
The loading of the lukali metal should be very small. NO of core-shell structure_x Amount of occlusion material carried
Is 0.01 to 2 per liter of catalyst as an alkali metal.
It is preferred that the molar ratio be used. NO_x Occupation material
NO if holdings are less than this_x Storage capacity and NO_x Purification rate is real
It is not practical, and if it is more than this,
Activity is reduced due to the decrease in activity. Also, the noble metal referred to in the exhaust gas purifying catalyst of the present invention.
A genus selected from Pt, Rh, Pd, Ir, Ru, etc.
Alternatively, a plurality of types can be used. Carry amount of this noble metal
Is preferably 0.5 to 20% by weight per liter of catalyst.
New If it is less than this, practical activity will not be exhibited.
Even if it carries more, the activity will be saturated and it will be expensive.
You. The method of supporting the noble metal can be performed in the same manner as before,
If supported on the coat layer by the adsorption support method or the impregnation support method
Good. [0024] The present invention will be described below in more detail with reference to Examples and Comparative Examples.
Will be explained. (Embodiment 1) FIG. 1 shows an embodiment of the present invention.
The expansion explanatory drawing of a medium is shown. This catalyst, Al_TwoO_Three, TiO_Twoas well as
ZrO_TwoAnd a core shell carried on the carrier 1.
NO of structure_x Occlusion material 2, Pt3 supported on carrier 1, etc.
It is composed of NO of core-shell structure _x Occlusion material 2
Is the K (K_TwoNucleus 20 composed of O) and formed on the surface of nucleus 20
Ca (Ca (CO_Three)_Two) And a shell layer 21 composed of
ing. Hereinafter, a method for producing this catalyst will be described with reference to FIG.
And a detailed description of the configuration will be used. 1,2-bis (2-ethylhexene) is added to 341 g of benzene.
Syl) calcium sulfosuccinate ([(C_TwoOH₃₇O_Four) SO_Four ^-]_Two
・ Ca²⁺) Is added and stirred to completely dissolve and
A solution A was prepared. This allows 1,2 bis (2-ethyl
Xyl) calcium sulfosuccinate with hydrophilic group inside
The hydrophobic groups associate outward to form reverse micelles. On the other hand, potassium carbonate is added to pure water and stirred.
Completely dissolved, 45-52% by weight aqueous potassium carbonate solution
B was prepared. And the weight ratio of benzene and water is benzene
Stir the whole amount of benzene solution A so that /water=3.5
While adding potassium carbonate aqueous solution B, prepare solution C
Was. Since the center of the reverse micelle is strongly hydrophilic, potassium carbonate
In the state where the droplet of the aqueous solution B exists in the center of the reverse micelle,
Solubilized. On the other hand, 1,2 bis (2
Lehexyl) 0.95 mol of calcium sulfosuccinate
Then, the mixture was stirred and completely dissolved to prepare a benzene solution D.
Thereby, reverse micelles are formed. On the other hand, tartaric acid in pure water
And dissolve completely by stirring, and tartar with a concentration of 50-58% by weight.
An aqueous acid solution E was prepared. And the weight ratio of benzene and water is
Total amount of benzene solution D so that benzene / water = 3.5
Aqueous solution E of tartaric acid is added while stirring to prepare solution F.
Was. The central part of the reverse micelle is strongly hydrophilic, so tartaric acid aqueous solution
Solubilization with liquid E droplets in the center of reverse micelles
Is done. Next, while stirring the solution C, the whole amount of the solution F was added.
In addition, stirring was continued for 1 hour. This allows the inside of the reverse micelle
Aqueous potassium carbonate solution B and tartaric acid solution E are mixed
Tartaric acid solution with low solubility in water
Potassium boron precipitates. Thereafter, the solvent is removed using a rotary evaporator.
And remove O_TwoContaining 1% N_Two2 at 550 ° C under gas atmosphere
After firing for a time, it is an aggregate of particles with a core-shell structure
A powder was prepared. The particles of this core-shell structure are K_TwoO
And a shell layer composed of CaO
ing. The obtained powder is treated with ammonium bicarbonate having a predetermined concentration.
The shell layer is treated by immersion in an aqueous
Salt (Ca (CO_Three)_TwoNO) with core-shell structure_x Occlusion material powder
Powder was prepared. Al_TwoO_Three100 parts by weight of powder and TiO_Two100 powder
Parts and ZrO_Two50 parts by weight of powder and the above NO_x Storage material powder 66 weight
Parts, mix an appropriate amount of alumina sol and pure water
Was prepared. And a cordierite honeycomb base
Material (diameter 100mm, length 163mm, cell density 400 / in^Two )
Prepare and wash coat this slurry, 250
Dry at 60 ° C for 60 minutes and bake at 450 ° C for 2 hours to form a coat layer
Done. The coating layer is 2 per liter of honeycomb substrate.
50g formed, K per liter of honeycomb substrate
0.1 mol and 0.5 mol of Ca are supported. The honeycomb substrate on which the above-mentioned coat layer has been formed
Impregnated with predetermined amount of dinitrodiamine platinum aqueous solution of predetermined concentration
After evaporating to dryness, baking at 450 ° C for 2 hours to support Pt
Was. The loading amount of Pt is 2 g per liter of honeycomb substrate
It is. Example 2 1,2 bis (2-ethylhexyl)
L) Calcium sulfosuccinate instead of 1,2 bis (2
Tylhexyl) magnesium sulfosuccinate ([(C_TwoOH
₃₇O_Four) SO_Four ^-]_Two・ Mg²⁺Example 1 except that the same amount was used.
NO in core-shell structure_x Preparation of occlusion material powder
In the same manner, a coat layer was formed and Pt was
Medium. Example 3 341 g of benzene was mixed with 1,2 bis
One model of calcium (2-ethylhexyl) sulfosuccinate
Benzene solution A
did. This allows 1,2 bis (2-ethylhexyl) sulfo
Calcium succinate has a hydrophilic group inside and a hydrophobic group outside
And form reverse micelles. On the other hand, sodium carbonate decahydrate is added to pure water.
Dissolve completely by stirring, and add 70-80% by weight of sodium carbonate.
Lithium aqueous solution G was prepared. And the weight of benzene and water
Benzene solution A so that the ratio becomes benzene / water = 3.5
While stirring the entire amount of
Solution H was prepared. The central part of the reverse micelle was strongly hydrophilic
The droplet of the aqueous solution of sodium carbonate G is located at the center of the reverse micelle.
Is solubilized in the state where it exists. Next, CO was added to the solution H._TwoWhile bubbling gas
After stirring for 1 hour, the reverse micelles were dissolved in carbonate (Ca (C
O_Three)_Two). After that, use a rotary evaporator to
Remove the medium and remove O_TwoContaining 1% N_TwoAt 550 ° C under gas atmosphere
After firing for 2 hours, it is an aggregate of core-shell structured particles.
NO_x An occluding material powder was prepared. This NO_x The storage material powder is
Na_TwoO core and Ca (CO_Three)_TwoShell layer consisting of
It is configured. This NO_x Except for using the occlusion material powder,
A coat layer was formed in the same manner as in Example 1, and Pt was
The catalyst of Example 3 was used. Example 4 1,2 bis (2-ethylhexyl)
L) Calcium sulfosuccinate instead of 1,2 bis (2
Use the same amount of magnesium sulfosuccinate (tylhexyl)
Except that the core-shell structure was the same as in Example 3.
NO_x Prepare an occlusion material powder, form a coat layer in the same way
The catalyst was carried to obtain a catalyst of Example 4. Comparative Example 1 Al_TwoO_Three100 parts by weight of powder and TiO
_Two100 parts by weight of powder and ZrO_TwoMix 50 parts by weight of powder
Was added to obtain a slurry. So
Then, the same honeycomb substrate as in Example 1 was prepared.
After washcoating, dry at 250 ° C for 60 minutes.
 It was baked at 450 ° C. for 2 hours to form a coat layer. Coat layer
Was formed in an amount of 250 g per liter of the honeycomb substrate. Next, the honeycomb base on which the above-mentioned coating layer is formed
Add a specified amount of dinitrodiamine platinum aqueous solution of specified concentration to the material.
Impregnated, evaporated to dryness, and calcined at 450 ° C for 2 hours to carry Pt
did. The loading amount of Pt is 2 per liter of honeycomb substrate.
g. A honeycomb having a coat layer carrying Pt
Predetermined amounts of potassium nitrate and calcium nitrate
Impregnate a predetermined amount of the mixed aqueous solution and evaporate to dryness.
Calcination was carried out at 50 ° C. for 2 hours to carry K and Ca. Loaded amount of K
Is 0.1 mol per liter of honeycomb substrate, and Ca
Is 0.5 mol per liter of honeycomb substrate.
is there. (Comparative Example 2) Formed similarly to Comparative Example 1.
Using a honeycomb substrate with a coat layer carrying Pt,
Magnesium nitrate solution instead of calcium acid solution
K and Mg were carried in the same amount as in Comparative Example 1 except that they were used.
I carried it. (Comparative Example 3) Formed in the same manner as in Comparative Example 1.
Using a honeycomb substrate with a coat layer carrying Pt,
Use an aqueous solution of sodium nitrate instead of an aqueous solution of potassium acid
Except that the same amount of Na and Ca was carried as in Comparative Example 1 except that
Was. (Comparative Example 3) Formed in the same manner as in Comparative Example 1.
Using a honeycomb substrate with a coat layer carrying Pt,
Use an aqueous solution of sodium nitrate instead of an aqueous solution of potassium acid
That magnesium nitrate instead of calcium nitrate aqueous solution
Except that an aqueous solution was used.
The same amount of Mg was supported. <Test / Evaluation> Touch of Examples and Comparative Examples
Table 1 shows the structure of the medium. [0046] [Table 1] The catalyst of each of the Examples and Comparative Examples was used with a displacement of 1800.
Installed in the exhaust system of the cc lean burn engine,
An endurance test was conducted in 50 hours of operation in 9 Lap mode. Endurance test
The maximum value of the catalyst bed temperature during the test was 800 ° C. After the endurance test, A / F = 22
Gas for 1 minute and rich gas equivalent to A / F = 12 for 1 second
NO in a return atmosphere_x Purification rate at various catalyst bed temperatures
Each was measured and the results are shown in FIG. FIG. 3 shows that the catalysts of the examples correspond to the respective comparative examples.
Higher at each catalyst bed temperature of 250-480 ° C than the example catalyst
NO_x It turns out that it shows the purification rate. This is the core
NO in well structure_x It is clear that the effect is due to the support of the occlusion material.
Pt is covered mainly by the movement of K during the durability test
It is considered that the effect is suppressed. [0050] According to the exhaust gas purifying catalyst of the present invention,
The transfer of lukari metal is suppressed, and NO_x Purification ability is more durable
Up.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing a configuration of a catalyst according to one embodiment of the present invention. FIG. 2 shows a core-shell structure according to an embodiment of the present invention.
It is an explanatory view showing a manufacturing method of the NO _x storage material. FIG. 3 is a graph showing the relationship between the catalyst bed temperature and the NO _x purification rate of the catalysts of Examples and Comparative Examples. [Description of Signs] 1: Carrier 2: NO _x storage material 3: Pt 20: Core 21: Shell layer

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Claims (1)

Patent Claims: 1. A NO _x storage-and-reduction type exhaust gas purifying catalyst obtained by carrying the _the NO _x storage material and the noble metal comprising at least alkali metal oxide support, said _the NO _x storage The material has a core-shell structure consisting of a core made of an alkali metal and a shell layer surrounding the core, and the shell layer is made of an alkaline earth metal, a rare earth element, Mn,
An exhaust gas purifying catalyst comprising an element selected from the group consisting of Fe, Co, and Ni.