JP3965672B2 - Exhaust gas purification catalyst - Google Patents

Description

【００４７】
各実施例及び各比較例の触媒を排気量1800ccのリーンバーンエンジンの排気系にそれぞれ搭載し、9Lapモードで50時間運転する耐久試験を行った。耐久試験の際の触媒床温度の最大値は 800℃である。
【００４８】
そして耐久試験後に、 A/F＝22相当のリーンガス１分間と A/F＝12相当のリッチガス１秒間の繰り返し雰囲気下におけるNO_x 浄化率を種々の触媒床温度でそれぞれ測定し、結果を図３に示す。
【００４９】
図３より各実施例の触媒は対応する各比較例の触媒に比べて、 250〜 480℃の各触媒床温度で高いNO_x 浄化率を示していることがわかる。これはコア・シェル構造のNO_x 吸蔵材を担持した効果であることが明らかであり、主に耐久試験時にＫの移動によってPtが覆われるのが抑制された効果であると考えられる。
【００５０】
【発明の効果】
本発明の排ガス浄化用触媒によれば、アルカリ金属の移動が抑制され、NO_x 浄化能の耐久性が向上する。
【図面の簡単な説明】
【図１】本発明の一実施例の触媒の構成を示す説明図である。
【図２】本発明の一実施例におけるコア・シェル構造のNO_x 吸蔵材の製造方法を示す説明図である。
【図３】実施例及び比較例の触媒の触媒床温とNO_x 浄化率の関係を示すグラフである。
【符号の説明】
１：担体 ２：NO_x 吸蔵材 ３：Pt
20：核体 21：シェル層[0001]
BACKGROUND OF THE INVENTION
The present invention, oxygen excess occluding NO _x in lean atmosphere, to a NO _x storage-and-reduction type catalyst for purifying an exhaust gas which can be reduced by releasing NO _x occluded in the rich atmosphere of over-reduction component.
[0002]
[Prior art]
In recent years, the global warming phenomenon caused by carbon dioxide has become a problem, and it has been a challenge to reduce the amount of carbon dioxide emissions. Reducing the amount of carbon dioxide in exhaust gas is also an issue in automobiles, and lean burn engines that develop lean burn of fuel in an oxygen-rich atmosphere have been developed. According to this lean burn engine, since the amount of fuel used is reduced, the amount of carbon dioxide emissions can be suppressed.
[0003]
In this lean burn engine, a system for reducing and purifying NO _x using exhaust gas as a reducing atmosphere has been developed and put into practical use by constantly burning in an oxygen-excess fuel lean condition and intermittently changing to a fuel stoichiometric to rich condition. Yes. And as the best catalysts for this system, occludes NO _x in a lean atmosphere, is occluded NO _x stoichiometric or NO _x storage-and-reduction type exhaust gas purifying catalyst using _the NO _x storage material that releases a rich atmosphere Development Has been.
[0004]
For example, Japanese Patent Laid-Open No. 5-317652 proposes an exhaust gas purifying catalyst in which an alkaline earth metal such as Ba and Pt are supported on a porous oxide carrier such as γ-Al ₂ O ₃ . JP-A-6-31139 proposes an exhaust gas purifying catalyst in which an alkali metal such as K and Pt are supported on a porous oxide carrier such as γ-Al ₂ O ₃ . Further, JP-A-5-18860 proposes an exhaust gas purification catalyst in which a rare earth element such as La and Pt are supported on a porous oxide carrier such as γ-Al ₂ O ₃ . Since alkali metals, alkaline earth metals and rare earth elements has a characteristic of absorbing and desorbing NO _x, are referred to as _the NO _x storage material.
[0005]
If this NO _x storage reduction type catalyst is used, the exhaust gas is changed from a lean atmosphere to a stoichiometric or rich atmosphere from a lean atmosphere by controlling the air-fuel ratio to become a stoichiometric or rich side from the lean side. Therefore, on the lean side, NO _x is occluded in the NO _x occlusion material, and it is released on the stoichiometric or rich side and reacts with reducing components such as HC and CO to be purified, so the exhaust gas from the lean burn engine Even if it exists, NO _x can be purified efficiently. Further, HC and CO in the exhaust gas are oxidized by the noble metal and consumed for the reduction of NO _x , so that HC and CO are also efficiently purified.
[0006]
The NO _x storage-and-reduction type catalyst, oxygen excess NO in the exhaust gas in a lean atmosphere is oxidized by the catalytic action of the noble metal becomes NO _2, that it _the NO _x storage material becomes nitrite ion or nitrate ion by steam reaction And then occluded. Therefore, it is desirable that the noble metal and the NO _x storage material are supported in close proximity to each other, whereby the NO _x storage capacity is maximized and a high NO _x purification rate is exhibited.
[0007]
On the other hand, an alkali metal as a NO _x storage material has an excellent NO _x storage capability in a high temperature range, and is therefore an essential component for NO _x storage reduction catalysts used in recent high temperature exhaust gas. Therefore, the NO _x storage-reduction catalyst requires a noble metal and an alkali metal, and it is desirable that they are supported close to each other.
[0008]
However the alkali metal is generally low melting point, to cover the noble metal surface is moved on the carrier at a high temperature, thereby to decrease the activity of noble metal _the NO _x storage capacity there is a problem of a decrease. For example, potassium has a melting point of 402 ° C. in the state of K ₂ O and has a melting point of 337 ° C. in the state of K (NO ₃ ) that occludes NO _x . For this reason, it liquefies at an exhaust gas temperature of about 400 ° C. and flows on the carrier to cover the noble metal.
[0009]
Therefore or supported noble metal and the alkali metal is separated into separate carrier particles, have been carried out or to separate with carrier layers, NO _x storage ability is decreased as compared with the case of loading in proximity There is a problem that the NO _x purification rate also decreases. Further, an alkali metal or scattered at a high temperature, the alkali metal which forms a solid solution to a substrate such as a porous oxide carrier or cordierite also has, this occurs when _the NO _x storage ability is decreased, similarly to the reduction activity of the noble metal In addition, the NO _x purification rate decreases.
[0010]
Therefore, it is most desirable to prevent the migration of alkali metals at high temperatures, but no effective countermeasures have been found under the circumstances of the characteristics of the alkali metals themselves.
[0011]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a catalyst that suppresses the migration of alkali metal and has improved durability of NO _x purification ability.
[0012]
[Means for Solving the Problems]
Features of the exhaust gas purifying catalyst of the present invention for solving the aforementioned problems is 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 The NO _x storage material has a core-shell structure consisting of a core made of alkali metal and a shell layer covering the periphery of the core, and the shell layer is made of alkaline earth metal, rare earth element, Mn, Fe, Co, and Including elements selected from Ni.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
The exhaust gas purifying catalyst of the present invention, and a shell layer covering the periphery of the core body and karyoplast consisting of alkali metal, carrying the _the NO _x storage material of the core-shell structure. Since the melting point of the shell layer is higher than that of the alkali metal, even if the core made of the alkali metal becomes liquid at high temperatures, the flow is restricted by the solid shell layer, and the noble metal is covered or dissolved in the carrier or base material. Such problems are suppressed. Therefore, a high NO _x storage capacity and a high NO _x purification rate are exhibited even after high temperature durability, and the durability is excellent.
[0014]
Any of K, Na, Cs, Li, Rb and Fr can be used as the alkali metal as the core of the core-shell structure, but K, Na, Cs and Li are preferable, and K is particularly preferable.
[0015]
The shell layer of the core-shell structure contains an element selected from alkaline earth metals, rare earth elements, Mn, Fe, Co, and Ni, and oxides or carbonates of these elements, Those that are in a solid state even at a high temperature of 400 ° C. or higher are used. Among these, it is particularly desirable to contain alkaline earth elements such as Ba, Ca, and Mg. As a result, the NO _x storage capacity in the low temperature region is improved, and the NO _x purification activity is further improved.
[0016]
The particle size of the NO _x storage material having a core / shell structure is preferably 0.1 μm or less, and more preferably 0.05 μm or less. When the particle size is larger than this, the NO _x storage capacity per supported amount of the NO _x storage material decreases due to the decrease in the surface area. Therefore, it is necessary to carry a large amount, which may affect the amount of noble metal carried. The ratio between the nucleus and the shell layer is not particularly limited, but the nucleus / shell layer is preferably in the range of 1/4 to 1/2 by weight ratio.
[0017]
Various methods are conceivable for producing such a core / shell structure NO _x storage material. For example, there is a method using reverse micelles. In this method, first, a reverse micelle is formed by adding a surfactant containing an alkaline earth metal or the like in a molecule to a non-aqueous solvent such as benzene. When an alkali metal salt aqueous solution is added thereto, droplets of the alkali metal salt aqueous solution are held in the hydrophilic portion at the center of the reverse micelle and solubilized. If firing the alkali metal can be produced _the NO _x storage material particles of the core-shell structure having a shell layer including nuclei and alkali earth metals.
[0018]
In the said manufacturing method, it is preferable to deposit an alkali metal salt as water-insoluble in the center part of a reverse micelle, and to bake it. As a result, the core made of alkali metal can be further refined, and the NO _x storage material having a core / shell structure can be made finer. Therefore, the NO _x storage capacity and the NO _x purification capacity are further improved by increasing the surface area. In order to precipitate the alkali metal salt in this way, for example, there is a method in which an aqueous tartaric acid solution is mixed to form an alkali metal hydrogen tartrate at the center of the reverse micelle.
[0019]
As the oxide carrier used in the exhaust gas purifying catalyst of the present invention, Al ₂ O ₃ , TiO ₂ , ZrO ₂ , CeO ₂ , SiO ₂ , MgO and the like, or a plurality selected from them are used. A composite oxide or the like can be used. The oxide support powder was formed into pellets, it to the _the NO _x storage material and the noble metal of the core-shell structure may be carried to a pellet catalyst, oxidizing the honeycomb substrate formed of cordierite or a metal foil object support powder coat layer was formed consisting of, it may be carrying the _the NO _x storage material and the noble metal of the core-shell structure is a honeycomb catalyst.
[0020]
In order to carry the core / shell structure NO _x storage material on the oxide support, the oxide support powder and the powder comprising the core / shell structure NO _x storage material particles are mixed to form a coating layer. Can do. Incidentally, NO _x storage material of the core-shell structure, it is desirable that a non-water soluble shell layer such as by carbonation process. Thereby _the NO _x storage material of the core-shell structure in aqueous slurry at coating layer formed is prevented from eluting.
[0021]
In addition to supporting the core / shell structure NO _x storage material, it is also preferable to support the alkali metal by causing the aqueous coating solution of the alkali metal salt to absorb water in the coating layer as in the case of a conventional NO _x storage reduction catalyst. As a result, a fine alkali metal can be supported, so that the initial NO _x purification activity can be improved. However, in order to reduce the movement of the alkali metal at a high temperature to cover the noble metal, the amount of fine alkali metal supported should be very small.
[0022]
Loading amount of _the NO _x storage material of the core-shell structure is preferably set to be 0.01 to 2 mol per 1 liter catalyst as an alkali metal. If the loading amount of the NO _x storage material is less than this, the NO _x storage capacity and the NO _x purification rate are not practical, and if it exceeds this amount, the loading amount of the noble metal is relatively reduced, and the activity is lowered.
[0023]
In addition, as the noble metal referred to in the exhaust gas purifying catalyst of the present invention, one or more kinds selected from Pt, Rh, Pd, Ir, Ru and the like can be used. The amount of the precious metal supported is preferably 0.5 to 20% by weight per liter of the catalyst. If less than this, practical activity is not expressed, and even if it is supported more than this, the activity is saturated and expensive. The precious metal can be supported in the same manner as in the prior art, and it may be supported on the coat layer by an adsorption support method or an impregnation support method.
[0024]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
[0025]
Example 1
FIG. 1 is an enlarged explanatory view of a catalyst according to an embodiment of the present invention. This catalyst is composed of a support 1 made of Al ₂ O ₃ , TiO _2, and ZrO ₂ , a core / shell structure NO _x storage material 2 supported on the support 1, and Pt 3 supported on the support 1. Yes. The core-shell structure NO _x storage material 2 is composed of a core 20 made of K (K ₂ O) and a shell layer 21 formed on the surface of the core 20 and made of Ca (Ca (CO ₃ ) ₂ ). Has been. Hereinafter, this catalyst production method will be described with reference to FIG.
[0026]
Add 1 mol of calcium 1,2bis (2-ethylhexyl) sulfosuccinate ([(C ₂ OH ₃₇ O ₄ ) SO ₄ ⁻ ] ₂ · Ca ²⁺ ) to 341 g of benzene, stir and dissolve completely to dissolve benzene solution A Was prepared. As a result, calcium 1,2bis (2-ethylhexyl) sulfosuccinate associates with the hydrophilic group on the inside and the hydrophobic group on the outside to form reverse micelles.
[0027]
On the other hand, potassium carbonate was added to pure water and stirred to be completely dissolved to prepare a potassium carbonate aqueous solution B having a concentration of 45 to 52% by weight. Then, a solution C was prepared by adding potassium carbonate aqueous solution B while stirring the total amount of benzene solution A so that the weight ratio of benzene and water was benzene / water = 3.5. Since the central part of the reverse micelle is highly hydrophilic, the droplets of the potassium carbonate aqueous solution B are solubilized in a state where they exist in the central part of the reverse micelle.
[0028]
On the other hand, 0.95 mol of calcium 1,2 bis (2-ethylhexyl) sulfosuccinate was added to 325 g of benzene, and the mixture was stirred and completely dissolved to prepare benzene solution D. Thereby, reverse micelles are formed. On the other hand, tartaric acid was added to pure water and stirred to dissolve completely to prepare an aqueous tartaric acid solution E having a concentration of 50 to 58% by weight. And the tartaric acid aqueous solution E was added, stirring the benzene solution D whole quantity so that the weight ratio of benzene and water might become benzene / water = 3.5, and the solution F was prepared. Since the central part of the reverse micelle is highly hydrophilic, the droplet of the tartaric acid aqueous solution E is solubilized in the state where it exists in the central part of the reverse micelle.
[0029]
Next, the whole amount of the solution F was added while stirring the solution C, and stirring was continued for 1 hour. Thereby, the potassium carbonate aqueous solution B and the tartaric acid aqueous solution E are mixed at the center of the reverse micelle, and potassium hydrogen tartrate having a low solubility in water is deposited at the center of the reverse micelle.
[0030]
Thereafter, the solvent was removed using a rotary evaporator, followed by firing at 550 ° C. for 2 hours in an N ₂ gas atmosphere containing 1% of O ₂ to prepare a powder as an aggregate of particles having a core / shell structure. The core-shell structure particles are composed of a nucleus composed of K ₂ O and a shell layer composed of CaO. The resulting powder was treated by immersion in ammonium bicarbonate aqueous solution having a predetermined concentration, and the shell layer of _the NO _x storage material powder of core-shell structure was prepared as carbonate which is insoluble in water (Ca (CO _{3) 2)} .
[0031]
Mix 100 parts by weight of Al ₂ O ₃ powder, 100 parts by weight of TiO ₂ powder, 50 parts by weight of ZrO ₂ powder, and 66 parts by weight of the NO _x storage material powder, add an appropriate amount of alumina sol and pure water, and add slurry. Prepared. Prepare a cordierite honeycomb substrate (diameter 100mm, length 163mm, cell density 400 / in ² ), wash coat this slurry, dry at 250 ° C for 60 minutes, and fire at 450 ° C for 2 hours. A coat layer was formed. The coating layer is formed in an amount of 250 g per liter of the honeycomb substrate, and 0.1 mol of K and 0.5 mol of Ca are carried per liter of the honeycomb substrate.
[0032]
The honeycomb substrate on which the coating layer was formed was impregnated with a predetermined amount of a dinitrodiamine platinum aqueous solution having a predetermined concentration, evaporated and dried, and then fired at 450 ° C. for 2 hours to carry Pt. The amount of Pt supported is 2 g per liter of honeycomb substrate.
[0033]
(Example 2)
Instead of calcium 1,2bis (2-ethylhexyl) sulfosuccinate, the same amount of magnesium 1,2bis (2-ethylhexyl) sulfosuccinate ([(C ₂ OH ₃₇ O ₄ ) SO ₄ ⁻ ] ₂ · Mg ²⁺ ) was used. in the same manner as in example 1 to prepare _the NO _x storage material powder of core-shell structure except that, as a catalyst of example 2 was carried in the same manner to form a coating layer Pt.
[0034]
(Example 3)
One mole of calcium 1,2bis (2-ethylhexyl) sulfosuccinate was added to 341 g of benzene, and the mixture was stirred and completely dissolved to prepare benzene solution A. As a result, calcium 1,2bis (2-ethylhexyl) sulfosuccinate associates with the hydrophilic group on the inside and the hydrophobic group on the outside to form reverse micelles.
[0035]
On the other hand, sodium carbonate decahydrate was added to pure water and stirred to dissolve completely, thereby preparing a sodium carbonate aqueous solution G having a concentration of 70 to 80% by weight. And the sodium carbonate aqueous solution G was added, stirring the whole quantity of the benzene solution A so that the weight ratio of benzene and water might become benzene / water = 3.5, and the solution H was prepared. Since the central part of the reverse micelle is highly hydrophilic, the droplet of the sodium carbonate aqueous solution G is solubilized in the state where it exists in the central part of the reverse micelle.
[0036]
Next, the solution H was stirred for 1 hour while bubbling CO ₂ gas, and the reverse micelle was converted to carbonate (Ca (CO ₃ ) ₂ ) which is hardly soluble in water. Thereafter, the solvent is removed using a rotary evaporator, and the NO _x storage material powder, which is an aggregate of core-shell structure particles, is fired at 550 ° C. for 2 hours in an N ₂ gas atmosphere containing 1% O _2. Prepared. This NO _x storage material powder is composed of a nucleus composed of Na ₂ O and a shell layer composed of Ca (CO ₃ ) ₂ .
[0037]
A coating layer was formed in the same manner as in Example 1 except that this NO _x storage material powder was used, and Pt was supported to obtain a catalyst of Example 3.
[0038]
Example 4
NO _x occlusion material powder having a core / shell structure in the same manner as in Example 3 except that the same amount of magnesium 1,2bis (2-ethylhexyl) sulfosuccinate was used instead of calcium 1,2-bis (2-ethylhexyl) sulfosuccinate In the same manner, a coating layer was formed and Pt was supported to obtain a catalyst of Example 4.
[0039]
(Comparative Example 1)
100 parts by weight of Al ₂ O ₃ powder, 100 parts by weight of TiO ₂ powder, and 50 parts by weight of ZrO ₂ powder were mixed, and an appropriate amount of alumina sol and pure water were added to prepare a slurry. A honeycomb substrate similar to that in Example 1 was prepared, and this slurry was wash coated, dried at 250 ° C. for 60 minutes, and fired at 450 ° C. for 2 hours to form a coat layer. The coating layer was formed in an amount of 250 g per liter of honeycomb substrate.
[0040]
Next, the honeycomb substrate on which the coating layer was formed was impregnated with a predetermined amount of a dinitrodiamine platinum aqueous solution having a predetermined concentration, evaporated and dried, and then fired at 450 ° C. for 2 hours to carry Pt. The amount of Pt supported is 2 g per liter of honeycomb substrate.
[0041]
A honeycomb substrate having a coating layer carrying Pt is impregnated with a predetermined amount of a mixed aqueous solution in which a predetermined amount of potassium nitrate and calcium nitrate is dissolved, evaporated to dryness, and fired at 450 ° C. for 2 hours to be K and Ca. Was supported. The supported amount of K is 0.1 mol per liter of honeycomb substrate, and the supported amount of Ca is 0.5 mol per liter of honeycomb substrate.
[0042]
(Comparative Example 2)
In the same manner as in Comparative Example 1, except that a honeycomb base material having a coating layer on which Pt was supported formed in the same manner as in Comparative Example 1 was used and a magnesium nitrate aqueous solution was used instead of the calcium nitrate aqueous solution, K and Mg were changed. The same amount was supported.
[0043]
(Comparative Example 3)
The same Na and Ca were used as in Comparative Example 1 except that a honeycomb substrate having a coating layer carrying Pt formed as in Comparative Example 1 was used and a sodium nitrate aqueous solution was used instead of the potassium nitrate aqueous solution. The amount was supported.
[0044]
(Comparative Example 3)
Using a honeycomb substrate having a coating layer carrying Pt formed in the same manner as in Comparative Example 1, using a sodium nitrate aqueous solution instead of a potassium nitrate aqueous solution, and using a magnesium nitrate aqueous solution instead of a calcium nitrate aqueous solution Except for the above, the same amount of Na and Mg was supported in the same manner as in Comparative Example 1.
[0045]
<Test and evaluation>
Table 1 shows the configuration of the catalyst of each Example and each Comparative Example.
[0046]
[Table 1]

[0047]
Endurance tests were carried out in which the catalysts of the examples and comparative examples were respectively mounted on the exhaust system of a lean burn engine with a displacement of 1800 cc and operated in 9 Lap mode for 50 hours. The maximum value of the catalyst bed temperature during the durability test is 800 ° C.
[0048]
After the endurance test, the NO _x purification rate was measured at various catalyst bed temperatures in a repetitive atmosphere of lean gas equivalent to A / F = 22 for 1 minute and rich gas equivalent to A / F = 12 for 1 second. Shown in
[0049]
It can be seen from FIG. 3 that the catalyst of each example shows a higher NO _x purification rate at each catalyst bed temperature of 250 to 480 ° C. than the catalyst of each corresponding comparative example. This is found to be effective carrying _the NO _x storage material of the core-shell structure is mainly considered to be effective, which is inhibited from Pt is covered by the movement of K during the durability test.
[0050]
【The invention's effect】
According to the exhaust gas purifying catalyst of the present invention, the movement of the alkali metal is suppressed, and the durability of the NO _x purification ability is improved.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing a configuration of a catalyst according to an embodiment of the present invention.
FIG. 2 is an explanatory view showing a method of manufacturing a NO _x storage material having a core / shell structure in one embodiment of the present invention.
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.
[Explanation of symbols]
1: Carrier 2: NO _x storage material 3: Pt
20: Nucleus 21: Shell layer

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,
The NO _x storage material has a core-shell structure comprising a core made of alkali metal and a shell layer covering the periphery of the core, and the shell layer is made of alkaline earth metal, rare earth element, Mn, Fe, Co And an exhaust gas purifying catalyst comprising an element selected from Ni.

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