JP4736162B2 - HC trap catalyst - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gas component reducing material and a hydrocarbon adsorbing material.
[0002]
[Prior art]
In order to purify exhaust gases from engines such as automobiles and various combustors, it has been studied to employ a zeolitic purification material in which various metals are supported on zeolite (crystalline aluminosilicate). However, for example, the exhaust gas temperature of an automobile engine may be 800 ° C. or higher, and further 900 ° C. or higher. On the other hand, zeolite has a low heat resistance, and deteriorates when exposed to high-temperature gas.
[0003]
In order to solve this problem, Japanese Patent Application Laid-Open No. 11-077686 discloses that the catalytic stability of the catalyst is enhanced by covering the zeolite catalyst particles carrying the active metal component with silica fine particles made from layered silicate. It is described. Further, as such zeolite catalyst particles, there are described particles in which Ag is supported on proton type β-type zeolite having a caivan ratio (a molar ratio of SiO ₂ to Al ₂ O ₃ ) of 40.
[0004]
Japanese Patent Laid-Open No. 6-287015 describes a method for producing a β-type zeolite having a Keiban ratio of 10 to 200.
[0005]
[Problems to be solved by the invention]
As described above, β-type zeolite can be used as an adsorbent or a catalyst. If various transition metals are supported on the zeolite, it is expected that the adsorption performance or the catalyst performance is improved. However, there is a problem in that it cannot be used effectively when it is exposed to a high-temperature gas due to the drawback of low thermal stability. That is, when the β-type zeolite carrying a metal is exposed to a high-temperature gas, the supported metal moves and causes sintering, and the β-type zeolite itself collapses due to a large distortion of its crystal lattice. .
[0006]
Accordingly, an object of the present invention is to improve the heat resistance of an HC trap catalyst using a hydrocarbon adsorbent obtained by loading a transition metal on a β-type zeolite.
[0007]
[Means for Solving the Problems]
The present inventor has found that this problem can be solved by increasing the caiban ratio of β-type zeolite, and has completed the present invention.
[0008]
The present invention is an HC trap catalyst provided in an exhaust passage of an engine,
An HC trap layer is formed on the carrier, and a three-way catalyst layer is formed on the HC trap layer.
The HC trap layer has a hydrocarbon adsorbent that makes the exhaust gas of the engine contact and adsorbs hydrocarbons in the exhaust gas,
The hydrocarbon adsorbent comprises β-type zeolite and Ag supported on the β-type zeolite,
The β-type zeolite has a cayban ratio of 120 or more,
The amount of Ag supported per liter of the carrier is 5 to 30 g .
[0009]
That is, Al constituting the β-type zeolite has one valence less than that of Si. Therefore, when the β-type zeolite is exposed to high-temperature exhaust gas, it becomes a factor that increases the distortion of the crystal lattice. It is considered that when the distortion of the crystal lattice of β-type zeolite increases, the transition metal supported on the β-type zeolite is likely to move and sinter.
[0010]
On the other hand, in the case of the present invention, the β-type zeolite having a high caiban ratio (120 or more) means that there is little Al that distorts the crystal lattice. Accordingly, even when exposed to high-temperature exhaust gas, movement and sintering of Ag is reduced, also become less likely to collapse β-type zeolite itself, Ru reduction in the function of the hydrocarbon adsorbent is suppressed.
[0011]
The β-type zeolite has a property of adsorbing hydrocarbons in engine exhaust gas well, but conventionally, when the exhaust gas temperature reaches about 150 ° C., the amount of adsorbed hydrocarbons increases. The hydrocarbon Where it is necessary to separately oxidized by the three-way catalyst, generally not in the state the three-way catalyst exhibits activity at about 0.99 ° C.. Therefore, a state occurs in which hydrocarbons are released into the atmosphere without being purified. On the other hand, when Ag is supported on β-type zeolite, the hydrocarbon release temperature shifts to the high temperature side, which is advantageous for the purification of hydrocarbons using a three-way catalyst. Since oxygen (oxidant) for the oxidation may be insufficient when the hydrocarbon is released, it is preferable to lean the air-fuel ratio of the engine or supply secondary air to the upstream side of the three-way catalyst.
[0012]
Further, if the amount of Ag supported is too small, the effect of increasing the hydrocarbon release temperature due to Ag cannot be obtained sufficiently. If the amount of Ag supported is too large, not only the cost is increased but also the movement of Ag Prone to sintering problems. The supported amount of Ag per liter of the carrier is preferably 5 to 30 g, or 7 to 25 g. According to the present invention, since there is little movement / sintering of Ag, it is possible to maintain a state in which the hydrocarbon release temperature of the β-type zeolite is increased without carrying a large amount of Ag.
[0013]
Thus, the exhaust gas of the engine may be 800 ° C. or higher, or 900 ° C. or higher, for example, when the engine is accelerated. Therefore, it is desirable that the hydrocarbon adsorbent does not cause Ag movement / sintering and β-type zeolite collapse, and suppresses a decrease in the amount of adsorbed hydrocarbons and a decrease in the release temperature of the adsorbed hydrocarbons. On the other hand, when the cayban ratio of β-type zeolite is set to 150 or more, and further to 200 or more, movement and sintering of Ag when the exhaust gas becomes high temperature is prevented. Collapse is also prevented. The Keiban ratio is more preferably 300 or more.
[0014]
In addition, as described above, β-type zeolite has a high silica ratio, and there is little movement of Ag when the exhaust gas becomes high temperature. Therefore, Ag binds to precious metals such as Pd and Pt constituting the three-way catalyst. There are few concerns about reducing the activity, for example, there is a concern that the purification start temperature of the three-way catalyst becomes high . The β- type zeolite can be increased to about 1000, which further increases the heat resistance of the hydrocarbon adsorbent.
[0015]
【The invention's effect】
As described above, in the HC trap catalyst of the present invention , the three-way catalyst layer is formed on the HC trap layer, and the HC trap layer contacts the exhaust gas of the engine to remove hydrocarbons in the exhaust gas. A hydrocarbon adsorbent to be adsorbed, the hydrocarbon adsorbent comprising β-type zeolite and Ag supported on the β-type zeolite, wherein the β-type zeolite has a caivan ratio of 120 or more; Since the loading amount per liter of the carrier is 5 to 30 g, the hydrocarbon release temperature of the β-type zeolite is shifted to the high temperature side, which is advantageous for hydrocarbon purification by a three-way catalyst, and the exhaust gas temperature is 800 ° C. even if the above more or 900 ° C., movement and sintering of Ag, avoid the collapse of the β-type zeolite, it becomes possible to reduce the Ag supported amount, Ag is sintered to the catalyst metal forming the three-way catalyst To concerns that reduces its activity, for example, concern the purification starting temperature of the three-way catalyst is high is small, maintaining a state of high hydrocarbon release temperature of β-type zeolite for a long period of time, a hydrocarbon by the three-way catalyst It becomes advantageous in purifying .
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0017]
This embodiment is an example in which the present invention is applied to purification of exhaust gas of a fuel injection gasoline engine of an automobile. In FIG. 1, 1 is the engine body, 2 is an intake passage, and 3 is an exhaust passage. The intake passage 2 has a surge tank 4, a throttle valve 5 is provided on the upstream side of the surge tank 4, and a fuel injection valve 6 is provided toward the intake port. A spark plug 9 is provided in the combustion chamber 7. An air-fuel ratio sensor 11, a three-way catalyst 12, and an HC (hydrocarbon) trap catalyst 13 are provided in the exhaust passage 3 in order from the upstream side.
[0018]
The three-way catalyst 12 is activated at a low temperature, and a mixed layer of alumina, ceria and praseodymium composite oxide and a binder (hydrated alumina) is formed on a cordierite honeycomb carrier, and Pt, Pd and Rh is impregnated and supported in a solution. The combined amount of these three metals (the amount supported per liter of support, hereinafter the same) is 4.0 g / L, and the mass ratio of the three metals is Pt: Pd: Rh = 1: 17: 3. The honeycomb carrier has 600 cells per cross-sectional area of about 6.45 cm ² (1 square inch), a wall thickness separating adjacent cells of about 0.15 mm (6 milliinches), and a capacity of 0.7 L. The upstream side catalyst 12 is configured by arranging two ones having a carrier capacity of 0.7 L in series.
[0019]
The HC trap catalyst 13 is a combination of a hydrocarbon adsorbent and a three-way catalyst material. As shown in FIG. 2, an HC trap layer 15 is formed on the surface of the carrier 14, and a three- way catalyst layer is formed thereon. 16 is formed. Carrier 14, the cross-sectional area of about 6.45 cm ² (1 square inch) the number of cells 400 per wall thickness of about 0.15 mm (6 mils) separating the Aitonaru cell cordierite honeycomb volume 1.0L It is a carrier.
[0020]
The HC trap layer 15 is formed by impregnating and supporting Ag in a mixed layer of β-type zeolite and a binder (hydrated alumina). The amount of β-type zeolite supported was 160 g / L, and the amount of binder supported was 20% of the amount of β-type zeolite supported. β-type zeolite has a caivan ratio of 120 or more and an Ag loading of 5 to 30 g / L.
[0021]
The three-way catalyst layer 16 is formed by forming a mixed layer of alumina, ceria-praseodymium composite oxide, and a binder, and impregnating and supporting Pd as a solution in the mixed layer. The supported amount of Pd is 5.7 g / L. The mass ratio of alumina to ceria / praseodymium composite oxide is 3: 1, and the binder loading is 10% of the total loading of alumina and ceria / praseodymium composite oxide.
[0022]
The manufacturing method of the HC trap catalyst 13 is as follows.
[0023]
-Formation of HC trap layer 15-
β-type zeolite and a binder are mixed, water and nitric acid are added thereto, and the mixture is stirred with a disperser to obtain a slurry. Nitric acid was 30 g per kg of β-type zeolite.
[0024]
A predetermined amount of slurry is coated on the carrier by immersing the honeycomb carrier in the slurry and lifting up and removing excess slurry by air blow. Then, after drying at 200 ° C., baking is performed at 500 ° C.
[0025]
Silver nitrate is dissolved in ion exchange water, and this is impregnated into the coating layer of the honeycomb carrier. Thereafter, the temperature is raised to a temperature of 200 ° C. over 1 hour, held at that temperature for 2 hours, then raised to a temperature of 500 ° C. over 5 hours, and held at that temperature for 2 hours, whereby the nitrate radical is obtained. Remove. When the Ag loading is 7.5 g / L, 12 g of silver nitrate is dissolved in 200 mL of ion exchange water.
[0026]
-Formation of the three-way catalyst layer 16-
The formation method of the three-way catalyst layer 16 is basically the same as the formation of the HC trap layer 15. That is, alumina, a ceria / praseodymium composite oxide and a binder are mixed, water and nitric acid are added thereto, and the mixture is stirred with a disperser to obtain a slurry. This slurry is coated on the HC trap layer 15 of the honeycomb carrier in the same manner as described above, and the same drying and firing treatment is performed. Thereafter, the coating layer is impregnated with an ion exchange aqueous solution of palladium nitrate, and dried and fired in the same procedure as described above.
[0027]
<Effects of β-zeolite Keiban ratio and Ag loading>
Various types of HC trap catalysts prepared with β-type zeolites having a cayban ratio of 25, 125, 150, and 300, and appropriately changing the amount of Ag supported between 0 and 30 g / L. Prepared. These HC trap catalysts were prepared by performing two types of aging treatment (heat treatment). One is held at 800 ° C. for 24 hours, and the other is held at 900 ° C. for 24 hours. Note that the β-type zeolite is a product of ZEOLYST, and the product names of β-type zeolites having a Keiban ratio of 25, 150 and 300 are as follows.
[0028]
Β-type zeolite with a Keiban ratio of 25; CP814E-22
Β-type zeolite with a Keiban ratio of 150; CP811E-150
Β-type zeolite with a Keiban ratio of 300; CP811C-300
[0029]
Each HC trap catalyst after the aging treatment was attached to a fixed bed flow reaction evaluation apparatus, and a pretreatment for the purpose of removing impurities or the like was performed. Thereafter, the simulated exhaust gas having the following composition is first allowed to flow at a temperature of 80 ° C. for 10 minutes, and then the gas temperature is increased to 400 ° C. at a rate of 30 ° C. per minute while continuing the flow of the gas. The amount of HC adsorbed was measured by measuring the HC (hydrocarbon) concentration of the gas that passed through the HC trap catalyst at this time, that is, the gas downstream of the catalyst.
[0030]
(Simulated exhaust gas composition)
Benzene; 82 ppm, NO; 1000 ppm, O ₂ ; 0.6%,
CO; 0.6%, H ₂ ; 0.2%, CO ₂ ; 13.9%, remaining N ₂
[0031]
The HC concentration of the downstream gas that has passed through the HC trap catalyst is about 25 ppm at the beginning of circulation of the simulated exhaust gas, but gradually increases as HC adsorption proceeds. After that, the HC concentration of the downstream gas exceeds the original HC concentration of 82 ppm of the simulated exhaust gas (apparent start of HC release), but the three-way catalyst layer of the HC trap catalyst starts to show activity, and HC oxidation begins. By starting, the HC concentration of the downstream gas rapidly decreases and finally becomes substantially zero (the HC purification rate becomes close to 100%).
[0032]
In this case, the HC concentration of the downstream side gas is oxidized and purified in the three-way catalyst layer as the timing of exceeding the original HC concentration of the simulated exhaust gas is delayed, that is, the gas temperature when exceeding the original HC concentration is higher. Since the amount to be produced increases, it is advantageous for the purification of HC. The amount of HC adsorption here is the total amount of adsorption from the start of circulation of the simulated exhaust gas until the HC concentration of the downstream gas exceeds the original HC concentration. This large amount of HC adsorption means that the degree of thermal degradation of the HC trap catalyst is low despite the above-described aging treatment, and it adsorbs HC until the HC trap layer reaches a relatively high temperature. Communicate with what you are doing.
[0033]
The measurement result of the HC adsorption amount is shown in FIG. First, the relationship between the Keiban ratio and the amount of HC adsorbed after the aging treatment is examined.
[0034]
Looking at the case where the HC trap catalyst was held at 800 ° C. for 24 hours (800 ° C. × 24 h aging), the β-type zeolite had a silica band ratio of 25 (hereinafter referred to as β25) and a silica band ratio of 300 (hereinafter referred to as β). , Β300)), in the latter case, the amount of HC adsorbed increases when the amount of Ag supported is 20 g / L or less, but there is no significant difference. The HC adsorption amount of the one having a Keiban ratio of 150 (hereinafter referred to as β150) is considerably high when the Ag loading is 20 g / L, but the reason is not clear.
[0035]
In the case of holding at 900 ° C. for 24 hours (aging at 900 ° C. × 24 h), the HC adsorption amount is larger than β25 at the Keiban ratio of 125 (referred to as β125) and β150, and when it becomes β300, It has increased significantly. Therefore, paying attention to the degree of decrease in the amount of HC adsorbed when aging is 800 ° C. × 24 h to 900 ° C. × 24 h, when β300, the degree of decrease is not so large, but when β150 and β25, The degree of decline is quite large.
[0036]
That is, although there is some variation in the vicinity of the Keiban ratio of 125 and 150, the HC adsorption amount after the aging treatment is larger than in the case of the Keiban ratio of 25, and when the Kaban ratio is 300, the HC adsorption amount becomes much larger. Yes. From this, it can be seen that setting the caiban ratio to 120 or more, 125 or more, 150 or 200 or more, and further 300 or more is advantageous for improving the heat resistance of the HC trap catalyst.
[0037]
Next, looking at the relationship between the Ag loading amount and the HC adsorption amount after the aging treatment, the Ag loading amount at which the HC adsorption amount becomes high at each Keiban ratio at the time of aging at 800 ° C. × 24 h is as follows.
β25 ... Ag loading 7.5g / L
β150 ... Ag loading 20.0 g / L
β300 ... Ag carrying amount 7.5-10.0 g / L
[0038]
At the time of aging at 900 ° C. × 24 h, it is as follows.
β25 ... Ag loading 7.5g / L
β125 ... Ag loading amount 7.5g / L
β150 ... Ag loading 7.5g / L
β300 ... Ag loading 20.0 g / L
[0039]
The measured value of the HC adsorption amount at aging at 800 ° C. × 24 h in the case of β150 and the Ag loading amount of 20.0 g / L is a little specific, but in other cases, the Ag loading amount is generally 7.5 to 20 g / L. The HC adsorption amount is high when the amount is in the range, and the HC adsorption amount is high when the loading amount is low in either range. The reason why the HC adsorption amount decreases when the Ag loading amount is lower than the above range is considered to be because the Ag amount is not sufficient for obtaining the effect of shifting the HC release temperature of the β-type zeolite possessed by Ag to the high temperature side.
[0040]
Moreover, when the amount of Ag carried increases to 20 g / L and 30 g / L, the amount of HC adsorbed tends to decrease because Ag becomes easier to sinter due to aging. This is thought to be because the catalyst moves from the trap layer to the three-way catalyst layer and combines with Pd to reduce its activity, that is, the HC purification start temperature of the three-way catalyst layer increases. However, when the Keiban ratio is increased to 150, 300, the HC adsorption amount is high even when the Ag loading is about 20 g / L (β 150 is 800 ° C. × 24 h aging, and β 300 is 900 ° C. × 24 h aging, the Ag loading is 20 g / L. A peak of HC adsorption appears.) This is considered to be because the stability of the crystal structure of β-type zeolite is increased and the movement or sintering of Ag is prevented by increasing the caivan ratio.
[0041]
In addition, as described above, when the Keiban ratio is high, the β-zeolite has higher thermal stability and higher HC adsorption capacity, which means that the amount of Ag supported for obtaining the desired adsorption capacity can be reduced than before. This is not only advantageous for cost reduction, but also reduces the amount of Ag transferred from the HC trap layer to the three-way catalyst layer, which is advantageous for preventing deterioration of the three-way catalyst layer. Means.
[0042]
When the HC trap catalyst according to the present invention is used to purify the exhaust gas of an automobile engine, when HC is released from the HC trap layer, HC is also discharged from the engine body. The amount of HC temporarily increases. Therefore, when the air-fuel ratio of the engine is not lean when HC is released from the HC trap layer, the air-fuel ratio of the engine is controlled to the lean side in order to secure an oxidant necessary for purification of the HC. Alternatively, it is preferable to separately provide a secondary air supply means and operate the means to supply secondary air to the HC trap catalyst in the exhaust passage.
[0043]
Such leaning of the air-fuel ratio or supply of secondary air may be performed for a predetermined time (for example, about 1 to 7 minutes) after detecting an increase in the amount of HC released from the HC trap catalyst. For example, a sensor for detecting the temperature of exhaust gas is provided in the exhaust passage, and a method for determining that the amount of released HC has increased when the exhaust gas temperature rises to 120 ° C. or 150 ° C. or higher, or an engine instead of a temperature sensor It is possible to employ a method of detecting an increase in the amount of HC released from the history of the operating state of the vehicle.
[0044]
In addition, when the position of the HC trap catalyst is disposed at the position of the three-way catalyst immediately downstream of the exhaust manifold 12a in FIG. 1 or the position of the HC trap catalyst immediately downstream of the three-way catalyst, the HC trap catalyst has a high temperature of 900 ° C. or higher. Prone. Therefore, the present invention is more suitable for such a layout.
[Brief description of the drawings]
FIG. 1 is a front view showing an intake / exhaust system configuration of an automobile engine according to the present invention.
FIG. 2 is a cross-sectional view showing a layer structure of an HC trap catalyst according to the present invention.
FIG. 3 is a graph showing the relationship between the amount of HC adsorbed after aging treatment and the amount of Ag supported when β-type zeolite having various Keiban ratios is used for HC adsorption.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Engine body 2 Intake passage 3 Exhaust passage 4 Surge tank 5 Throttle valve 6 Fuel injection valve 7 Combustion chamber 9 Spark plug 11 Air-fuel ratio sensor 12 Three-way catalyst 13 Trap catalyst 14 Carrier 15 HC trap layer 16 Three-way catalyst layer

Claims (6)

An HC trap catalyst provided in an exhaust passage of an engine,
An HC trap layer is formed on the carrier, and a three-way catalyst layer is formed on the HC trap layer.
The HC trap layer has a hydrocarbon adsorbent that makes the exhaust gas of the engine contact and adsorbs hydrocarbons in the exhaust gas ,
The hydrocarbon adsorbent comprises β-type zeolite and Ag supported on the β-type zeolite,
The β-type zeolite has a cayban ratio of 120 or more,
An HC trap catalyst , wherein the supported amount of Ag per 1 L of the carrier is 5 to 30 g. In the HC trap catalyst according to claim 1 ,
An HC trap catalyst characterized in that the engine has a maximum exhaust gas temperature of 900 ° C or higher. In the HC trap catalyst according to claim 1,
An HC trap catalyst characterized in that the engine has a maximum exhaust gas temperature of 800 ° C or higher . In the HC trap catalyst according to any one of claims 1 to 3,
  An HC trap catalyst, wherein the β-zeolite has a silica gel ratio of 150 or more. In the HC trap catalyst according to any one of claims 1 to 3,
  An HC trap catalyst, wherein the β-type zeolite has a quartz-ban ratio of 200 or more. In the HC trap catalyst according to any one of claims 1 to 5,
  The HC trap catalyst, wherein the three-way catalyst layer includes alumina, a ceria / praseodymium composite oxide, and Pd.

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