JP2007253110A - Catalyst for exhaust gas purification, integral construction type catalyst for exhaust gas purification, and method for exhaust gas purification - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an oxidation catalyst for exhaust gas purification that can efficiently remove hydrocarbons (HC) and carbon monoxide (CO) in exhaust gases and has resistance against sulfur poisoning, a catalyst structure for exhaust gas purification containing the above catalyst, and a method for exhaust gas purification using the above catalyst structure. <P>SOLUTION: The catalyst for exhaust gas purification is a catalyst for exhaust gas purification containing a fibrous inorganic basic material/metal catalyst composite where a metal catalyst component (A) is supported by a whisker-like inorganic basic material (B). The metal catalyst component (A) contains at least a noble metal element and the whisker-like inorganic basic material (B) contains Al<SB>2</SB>O<SB>3</SB>as the main component and a zirconium component in the crystal and/or on the surface of the crystal and has a diameter of substantially 50 nm or less and an aspect ratio of 2 or higher. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst, a manufacturing method thereof, and an exhaust gas purification monolithic catalyst, and more particularly to HC, CO, SOF, NOx, etc. contained in exhaust gas of an internal combustion engine, a boiler, etc. The present invention relates to an exhaust gas purification catalyst that has an excellent purification ability and that can exhibit a stable purification ability even in a low temperature environment, a manufacturing method thereof, and an integral structure type catalyst for exhaust gas purification. is there.

  Internal combustion engines and boilers emit various harmful substances derived from fuels, lubricants, etc., depending on their structure and type. In addition to hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), these harmful substances include particulate matter typified by soot discharged from diesel engines and the like: Hereinafter, the HC may be a soluble organic component (hereinafter referred to as SOF), which is a component soluble in an organic solvent such as benzene, toluene, or a hydrocarbon having a long carbon chain length. Is included).

In recent years, environmental pollution due to exhaust gas from various internal combustion engines has become a social problem, and various methods have been proposed for purification. As one method for purifying harmful components in these exhaust gases, a contact treatment method for purifying exhaust gases by contacting them with a catalyst has been put into practical use.
Among these, exhaust gas purification catalysts for gasoline engines include oxidation catalysts that oxidize and purify harmful components such as HC, SOF, and CO, heat after temporarily storing harmful components such as NOx, and fuel A reduction catalyst that reduces and removes by supplying a reducing agent such as unburned hydrocarbon, urea, ammonia, etc., and a three-way catalyst that simultaneously purifies by oxidizing and reducing HC, CO, and NOx (for example, patents) are known. Reference 1).

In addition, there is a method of filtering soot discharged from a diesel engine from exhaust gas, supplying unburned HC or NO ₂ such as fuel when the soot accumulates, heat-treating, and oxidizing (combusting) the soot. A filter (Diesel Particulate Filter, hereinafter referred to as DPF) for filtering out particulate matter in exhaust gas, and a catalyzed filter (catalyzed soot filter, hereinafter referred to as CSF) catalyzed by DPF. Is known).

  Known active metals that contribute to exhaust gas purification include noble metals such as platinum, palladium and rhodium, transition metals such as nickel and iron, rare earth metals such as cerium and lanthanum, and oxides thereof. These active metals are supported on a support such as alumina such as β-alumina and γ-alumina and zeolite in order to increase the surface area and increase the strength. There are various types of zeolite such as MFI type and β type, and the internal cation may be exchanged with metal ions such as Ce and Fe. The catalyst composition is selected according to the required function, and the type and amount of the active metal and the carrier are selected. At the time of producing the catalyst, an alkali metal, alkaline earth metal, oxygen storage / release material represented by ceria, etc. May be added.

  As such an exhaust gas purifying catalyst carrier, a honeycomb flow-through carrier formed of metal corrugated plates or ceramics, or one end of a honeycomb often used for DPF or CSF was closed to form a checkered pattern. An integral structure type carrier of a wall flow type carrier is known. In addition, a felt-like noncombustible structure made of a thick fibrous material, a spherical carrier having a diameter of several millimeters to several centimeters, a molded carrier made of a columnar material, etc. are known (in the present invention, these are generically called This is called a structural carrier, and is distinguished from a monolithic carrier such as a flow-through carrier and a wall flow carrier).

These exhaust gas purifying catalysts can suppress pressure loss when the exhaust gas flows out, and can cover the carrier surface with a catalyst composition so that a large contact efficiency can be secured. It is installed in the gas flow path.
When an exhaust gas purifying catalyst is installed in an automobile, it is said that it is installed under the floor of the vehicle body where the temperature is relatively low, or a direct type installed immediately after it is exhausted from the engine, where hot exhaust gas passes. There is something to be said. Among them, in the direct type, the temperature of the exhaust gas is relatively high, and depending on conditions, the temperature may exceed 800 ° C. Therefore, in such a high temperature state, the activity of the catalyst is high, and the purification of the exhaust gas High efficiency.

However, even in the direct type, it is not rare that the exhaust gas temperature is low immediately after starting the engine or at a low speed, and it is less than 300 ° C. The catalyst does not work effectively in such a low temperature state, and the exhaust gas It may be difficult to purify harmful components. On the other hand, the catalyst placed under the floor is more serious because the temperature under the floor is originally low.
An exhaust gas purification catalyst mounted on an automobile is required to be active even when the exhaust gas temperature is low. As a situation where such low-temperature catalyst activity, that is, low-temperature activity is required, for example, a case where a vehicle that has been allowed to stand for half a day or more and has been cooled to the outside temperature is started. In this case, the catalyst and the engine combustion chamber are cooled to about the outside air temperature. When the engine is started in this state, immediately after that, the engine combustion chamber is not sufficiently warmed up, so a part of the combustion heat of the fuel is taken away by the engine warming up, and it is difficult to completely burn the fuel. Many unburned components and incompletely burned components are discharged.

Purifying these exhaust gas harmful substances is a low-temperature activity required for the catalyst. Here, in the case of a catalyst having a low low temperature activity, the catalyst is gradually warmed by the heat of the exhaust gas, and the harmful components in the exhaust gas cannot be purified until the temperature reaches a sufficiently functioning temperature.
In this way, it is important for exhaust gas purification catalysts mounted on automobiles to have sufficient low-temperature activity, which is also an important issue for satisfying increasingly severe regulations on exhaust gas in recent years. It was.
JP-A-5-340236 ([0002])

  In view of the above-described conventional problems, the object of the present invention is to have an excellent purifying ability for HC, CO, SOF, NOx, etc. contained in exhaust gas of an internal combustion engine, a boiler, etc., and the exhaust gas has a low temperature. An object of the present invention is to provide an exhaust gas purification catalyst capable of exhibiting a stable purification ability even in an environment, a method for producing the same, and an integral structure type catalyst for exhaust gas purification.

In order to solve the above-mentioned problems, the present inventors have used a fibrous inorganic base material-metal catalyst composite in which a metal catalyst component is supported on a whisker-like inorganic base material as an exhaust gas purification catalyst. As a result of intensive research, by using a catalyst having a specific diameter and aspect ratio, containing a zirconium component in a whisker-like inorganic base material whose main structural unit is Al ₂ O ₃ , even at low temperatures It has been found that HC, CO and NOx in the exhaust gas can be efficiently purified, and the present invention has been completed.

That is, according to the first aspect of the present invention, the exhaust gas purifying apparatus contains the fibrous inorganic base material-metal catalyst composite in which the metal catalyst component (A) is supported on the whisker-like inorganic base material (B). The catalyst, wherein the metal catalyst component (A) contains at least a noble metal element, while the whisker-like inorganic base material (B) has Al ₂ O ₃ as a main constituent unit, and in the crystal and / or Provided is an exhaust gas purifying catalyst comprising a zirconium component on the surface, having a diameter of substantially 50 nm or less and an aspect ratio of 2 or more.

  According to the second aspect of the present invention, there is provided an exhaust gas purifying catalyst characterized in that, in the first aspect, the noble metal element in the metal catalyst component (A) contains rhodium.

  According to the third aspect of the present invention, there is provided an exhaust gas purification catalyst according to the first or second aspect, wherein the whisker-like inorganic base material (B) is fibrous boehmite. .

  According to a fourth aspect of the present invention, in the first to third aspects, the exhaust gas purifying catalyst is characterized in that the whisker-like inorganic base material (B) has a diameter of substantially 30 nm or less. Is provided.

  Furthermore, according to the fifth invention of the present invention, in any one of the first to fourth inventions, the content of the zirconium component is 0.1 in terms of Zr oxide with respect to the whisker-like inorganic base material (B). An exhaust gas purifying catalyst characterized by being -30% by weight is provided.

  On the other hand, according to the sixth invention of the present invention, the exhaust gas purification integrated structure type catalyst obtained by coating the exhaust gas purification catalyst according to any one of the first to fifth inventions on the surface of the integral structure type carrier. Is provided.

  According to the seventh aspect of the present invention, in the sixth aspect, the amount of the metal catalyst component (A) is 0.01 to 10 g / L with respect to the monolithic structure type carrier. An integral structure type catalyst for exhaust gas purification is provided.

  Furthermore, according to the eighth invention of the present invention, in the sixth or seventh invention, the amount of the inorganic base material-metal catalyst composite is 10 to 400 g / L with respect to the monolithic structure type carrier. An integral structure type catalyst for purifying exhaust gas is provided.

  On the other hand, according to a ninth invention of the present invention, according to any one of the sixth to eighth inventions, the exhaust gas containing nitrogen oxides, hydrocarbons, and carbon monoxide is converted into the integrated structure for exhaust gas purification. There is provided an exhaust gas purification method characterized in that it is distributed and brought into contact with a type catalyst.

The exhaust gas purifying catalyst of the present invention is a fibrous inorganic base material-metal catalyst in which the metal catalyst component (A) is supported on a specific whisker-like inorganic base material (B) whose main constituent unit is Al ₂ O ₃ Because it is a catalyst for exhaust gas purification containing a composite, it exhibits excellent purification ability even at low temperatures for HC, NOx, CO, etc. in the exhaust gas, and simultaneously removes these harmful components it can. Therefore, it is particularly suitable as a three-way catalyst for purifying exhaust gas discharged from an automobile engine.

  Hereinafter, the exhaust gas purification catalyst, the exhaust gas purification integrated structure catalyst of the present invention, and the exhaust gas purification method using these catalysts, with a focus on a three-way catalyst for purifying exhaust gas from an automobile, are shown in the drawings. Will be described in detail.

1. Exhaust gas purification catalyst The exhaust gas purification catalyst of the present invention comprises an exhaust gas containing a fibrous inorganic matrix-metal catalyst composite in which a metal catalyst component (A) is supported on a whisker-like inorganic matrix (B). The catalyst for purification, wherein the metal catalyst component (A) contains at least a noble metal element, while the whisker-like inorganic base material (B) contains Al ₂ O ₃ as a main constituent unit, and has a crystal structure and / or Alternatively, a zirconium component is contained on the surface, the diameter is substantially 50 nm or less, and the aspect ratio is 2 or more.

  In the present invention, “substantially” used as an expression representing a size refers to an average value of a diameter and a length, or a state in which many components capable of taking the size can maintain a portion having the size. It means that.

  The “whisker shape” of the whisker-like inorganic base material refers to a shape that can be defined by an aspect ratio in a broad sense such as a columnar shape, a needle shape, or a fiber shape in addition to a so-called whisker shape (whisker shape).

(Metal catalyst component)
In the present invention, the metal catalyst component must contain a noble metal element, but may also contain a transition metal, a rare earth metal, or the like as long as it has activity for purifying exhaust gas. Examples of the noble metal element include rhodium, gold, silver, platinum, and palladium, and one or more of them can be used. Preferred metal catalyst components are rhodium, platinum and palladium, with rhodium being most preferred.
Specific examples of the transition metal include iron, nickel, cobalt, zirconium, and copper. As the rare earth metal, one or more of cerium, lanthanum, praseodymium, neodymium, and the like can be appropriately selected.

  The amount of the metal catalyst component varies depending on the kind of the noble metal element, the kind of the inorganic base material and the support, etc., but if it is a noble metal, 0.01 to 25% by weight with respect to the whole catalyst including the inorganic base material, preferably 0.1 to 10% by weight. When the amount of the metal catalyst component exceeds 25% by weight, the production cost of the catalyst increases, and when it is less than 0.01% by weight, the exhaust gas purification performance decreases. In the case of a monolithic catalyst described later, it is preferably 0.01 to 10 g / L, particularly 0.1 to 10 g / L per unit volume of the monolithic catalyst.

(Inorganic base material)
In the present invention, the whisker-like inorganic base material used as a raw material is an inorganic substance consisting of at least Al ₂ O ₃ and a thermally stable structural unit, or a precursor that can become whisker-like after firing. . In particular, it is an inorganic substance containing Al ₂ O ₃ as a structural unit and containing a zirconium component. Examples of such whisker-like inorganic substances include fibrous boehmite (including a precursor).

Here, the whisker-like inorganic base material desirably has a diameter of substantially 50 nm or less, more preferably 20 nm or less, and most preferably 10 nm or less. The lower limit of the diameter of the whisker-like inorganic base material is not particularly limited, but the effect of the present invention has been confirmed when an inorganic base material of 2 nm or more is used. In the case of a precursor, since it is usually an assembly of chain polymer compounds, the diameter cannot be defined.
The length of the inorganic base material is not particularly limited, but is preferably longer than 100 nm, for example, in the range of 100 to 10,000 nm. Moreover, the desirable aspect ratio of the whisker-like inorganic base material in the present invention is 2 or more, and more desirably 10 or more. Hereinafter, in the present invention, these shapes are collectively referred to as a whisker shape.

In the present invention, the whisker-like inorganic base material contains a zirconium component, but the zirconium component may be a zirconium metal, or a compound such as zirconia aluminate or zirconium oxide. Zirconium itself may be contained as a simple substance in the inorganic substance, but it is desirable that it is incorporated into the Al ₂ O ₃ crystal, which is the main structural unit of the fibrous boehmite.

  Here, the presence state of the zirconium component relative to the fibrous boehmite is a state included in the boehmite crystal, a state in which a complex oxide with aluminum oxide is formed, a state in which the zirconium component is present in the boehmite in an island shape, and a whisker shape There is a state in which the surface of the inorganic base material is coated (there is a state in which the surface is entirely coated and a state in which the surface is coated in spots), and a state in which some of these states are combined can also be taken.

  In addition, the addition method of the zirconium component to fibrous boehmite changes with what kind of state a zirconium component exists with respect to fibrous boehmite. If the zirconium component is present in the state of being included in the boehmite crystal, for example, a solution of zirconium salt such as zirconium nitrate is mixed and stirred in a slurry containing fibrous boehmite, and neutralized with an alkali agent such as ammonia. It can be obtained by drying, if necessary, and then firing. Here, drying is preferably performed at 50 to 200 ° C. for 5 to 20 hours, and baking conditions are preferably performed at 400 to 700 ° C. for 1 to 6 hours. Moreover, among the neutralization conditions with an alkaline agent, the amount of the alkaline agent used may be 0.1 to 6 times the amount of the oxide, and neutralization may be performed at a neutralization rate charged in 10 minutes to 360 minutes. The alkali agent is preferably added in 30 to 90 minutes. When the charging speed is less than 10 minutes, the zirconium component is agglomerated and the dispersibility of the fibrous boehmite and zirconium may be impaired. On the other hand, if it exceeds 360 minutes, productivity does not increase, which is not preferable.

  If a zirconium component is present in a state where a composite oxide with aluminum oxide is formed, the zirconium component is mixed as zirconia in aluminum oxide and uniformly dispersed, followed by firing at a high temperature.

The amount of the zirconium component added to the fibrous boehmite is 0.1 to 30% by weight in terms of zirconium oxide with respect to the inorganic base material, more preferably 0.5 to 20% by weight, and 1 to 10% by weight. Most preferred. If the zirconium component is less than 0.1% by weight, sufficient ternary activity cannot be obtained. If the zirconium component exceeds 30% by weight, a change that inhibits the interaction between the fibrous boehmite and zirconium occurs, and a crystallographic change occurs. As a result, there is a risk of changing the characteristics.
In the present invention, the whisker-like inorganic base material must contain a zirconium component, and catalytic activity can be obtained with an alkaline earth metal such as barium or a rare earth metal such as lanthanum instead of the zirconium component. Can not.

In the present invention, the preferred fibrous boehmite as the inorganic base material has a molecular formula mainly represented by Al ₂ O ₃ .1.05-3.0H ₂ O. Boehmite with a crystal water of less than 1.05 in alumina may have a plate shape, and boehmite with a crystal water exceeding 3.0 is not preferable because it is an aggregate of very small fibers. The preferred fibrous boehmite as the inorganic base material is one whose molecular formula is mainly represented by Al ₂ O ₃ · 1.3 to 3.0H ₂ O.

  The fibrous boehmite preferably has a diameter of substantially 50 nm or less, particularly 3 to 50 nm. Although the length of a fiber is not specifically limited, A thing longer than 100 nm is preferable, For example, the thing in the range of 100-10000 nm is preferable. The diameter and length of the fibrous boehmite are closely related to the pore structure of the obtained fired product. That is, the pore volume of the fired product obtained from boehmite of short fibers is small, and the pore volume is large in the case of long fibers. Moreover, the specific surface area of the baked product obtained from boehmite having fine fibers is large, and the specific surface area is small in the case of thick fibers. The desirable aspect ratio of the fibrous boehmite in the present invention is 2 or more, more desirably 10 or more.

Alumina hydrate composed of such fibrous boehmite can be prepared, for example, by a method described in International Patents (WO97 / 32817, WO01 / 056951). Specifically, after adding an acid to an aqueous suspension of an alumina raw material, hydrothermal treatment is performed at a temperature of 90 to 150 ° C. to prepare an alumina sol in which fibrous boehmite is dispersed.
As the alumina raw material, alumina having a ρ and / or χ crystal structure having at least a partial rehydration property is used. Above all, it is obtained by dehydrating alumina trihydrate such as gibbsite and bayerite by rapid high-temperature heating, has a specific surface area in the range of 50 to 500 m ² / g, and has partial rehydration properties. Alumina is preferred.

  The photograph in FIG. 1 is a photograph (magnification: × 100 k) of the whisker-like inorganic base material according to the present invention, taken with a scanning transmission electron microscope. When FIG. 1 is seen, although the fibrous boehmite containing the zirconium component used as a raw material is dispersed as a whisker-like inorganic base material, it can be seen that a plurality of pieces are combined and thickened. . When the shape and size of the whisker-like inorganic base material were visually measured under an electron microscope, the average value was 5 nm in diameter and 130 nm in length.

  Further, the direction of the fibers of the whisker-like inorganic base material may be one direction or random direction, but is usually obtained in a state of being aligned in one direction by firing. The length of the fiber and the width of the thick part are not particularly limited, but the longer the fiber and the wider the width, the more preferable the metal catalyst can be carried effectively.

  The fibrous boehmite obtained as described above is usually in the form of a sol dispersed in water. When the catalyst metal component is mixed in the sol, and the slurry is dried and fired, the catalyst is formed on the surface of the fibrous boehmite. A metal component is supported to form a stable whisker-like structure. The existence state of the catalytic metal component on the surface of the fibrous boehmite cannot be generally stated, but if it is supported in a needle shape, extremely good activity can be exhibited.

The reason why the present invention exhibits excellent exhaust gas purification performance even at low temperatures is not clear, but if the noble metal in the catalyst is, for example, rhodium, it is assumed that the reason is as follows.
One is that rhodium is effectively supported due to the whisker shape of the inorganic base material described above, and the other is that the whisker-like inorganic base material of the present invention prevents the decrease in the activity of rhodium in the catalyst itself. Is Rukoto. The reason why the decrease in the activity of rhodium itself is considered to be prevented is as follows.

  That is, it is known that the surface of zirconium oxide is in a negatively charged state, and in the present invention, rhodium particles supported on the whisker-like inorganic base material are mixed with the whisker-like inorganic base material. Electron donation is performed from zirconium oxide, the rhodium particles are in a negatively charged state, the affinity between the oxygen anions in the exhaust gas, which is also negatively charged, and the rhodium particles are reduced, and the oxidation of the rhodium particles is suppressed. It is considered that a highly active metal state can be maintained.

  In addition to the metal catalyst component and whisker-like inorganic base material that are the raw materials, alkali metal, alkaline earth metal, and the like can be appropriately added to the exhaust gas purification catalyst of the present invention in order to improve the catalyst performance. . Moreover, adsorbents such as zeolite, oxygen storage / release materials such as ceria and ceria-zirconia composite oxides, noble metal catalysts such as platinum and palladium, and other α-type, γ-type alumina, etc. An exhaust gas purifying catalyst having a higher level of function can be obtained.

  When an oxygen storage / release material is used, the amount thereof varies depending on the type, type of carrier, etc., but in the case of a ceria-zirconia composite oxide, 0.01 to 10 g / L, especially 0, per volume of the carrier. It is preferable that it is 1-10 g / L. As described above, when the present invention is used as an oxidation catalyst, the combined use with the oxygen storage / release material promotes the oxidizing action of HC, SOF, etc. by the oxygen released from the oxygen storage / release material. The steam reforming reaction is promoted by the combined use with a noble metal catalyst such as NOx, and the NOx purification performance is promoted.

  When γ-alumina is used, the amount thereof is preferably 5 to 150 g / L, particularly 10 to 80 g / L per volume of the carrier. By using γ-alumina, dispersibility with the fibrous inorganic base material-metal catalyst composite is improved, and adhesion to the carrier is also improved.

  The exhaust gas purifying catalyst of the present invention is desirably used as a structural catalyst in which the above composite is coated on the surface of the carrier. Here, the shape of the carrier is not particularly limited, and can be selected from a columnar shape, a cylindrical shape, a spherical shape, a honeycomb shape, a sheet shape, and the like. The size of the structure-type carrier is not particularly limited, and a structural carrier having a diameter of, for example, several millimeters to several centimeters can be used as long as it is in a columnar shape, a cylindrical shape, or a spherical shape.

2. Manufacturing method of exhaust gas purifying catalyst The exhaust gas purifying catalyst of the present invention is manufactured by mixing the metal catalyst component and the inorganic base material in an aqueous medium to prepare a slurry, and then firing the slurry. The The raw material for the metal catalyst component is usually used in the form of nitrate, sulfate, carbonate, acetate or the like.

  Here, the slurry is prepared by mixing an inorganic base material, a metal catalyst component, and an aqueous medium in a predetermined ratio. In the present invention, the slurry contains a noble metal such as rhodium with respect to 100 parts by weight of the inorganic base material. It is preferable to mix 0.01 to 25 parts by weight of the metal catalyst component. What is necessary is just to use the quantity which can disperse | distribute an inorganic base material and a metal catalyst component uniformly in a slurry for an aqueous medium.

At this time, an acid or an alkali for adjusting the pH, a surfactant for improving the viscosity or improving the slurry dispersibility, a dispersing resin, or the like can be blended as necessary. Further, as a mixing method of the slurry, pulverization and mixing by a ball mill or the like can be applied, but other pulverization or mixing methods may be applied.
In addition, although baking conditions are not specifically limited, 300-1200 degreeC is preferable and 400-800 degreeC is more preferable. About a heating means, it can carry out by well-known heating means, such as an electric furnace and a gas furnace.

  The structural catalyst of the present invention can be obtained by applying the slurry to the structural carrier and heating as described above, but after obtaining the calcined catalyst composition by calcining the slurry in advance, it is separately pulverized. Then, the catalyst can be obtained by supporting it on a structural support.

3. Exhaust gas purifying monolithic structure type catalyst The exhaust gas purifying monolithic structure type catalyst of the present invention is a catalyst in which the above-described catalyst composition is coated on the surface of an integral structure type carrier through which exhaust gas can flow.

Here, the monolithic structure type carrier is not particularly limited, and can be selected from known monolithic structure type carriers. As such an integral structure type carrier, there are a flow-through type carrier and a wall flow type carrier used for a DPF, and materials of these carriers include metals and ceramics. In addition, a sheet-like structure knitted from a fine fibrous material, or a felt-like non-combustible structure made of a relatively thick fibrous material can be used. Since these monolithic structural carriers have a large amount of supported metal catalyst components and a large contact area with the exhaust gas, they have a higher processing capacity than other structural carriers. When the exhaust gas purification catalyst of the present invention is used as a three-way catalyst for automobile exhaust gas purification, it is desirable to use a flow-through type carrier.
The overall shape of the monolithic structure type carrier is arbitrary, and can be appropriately selected according to the structure of the exhaust system to be applied, such as a columnar, quadrangular, or hexagonal column. Further, regarding the number of holes in the opening, an appropriate number of holes can be determined in consideration of the type of exhaust gas to be processed, gas flow rate, pressure loss or removal efficiency, etc. About 10 to 1500 per square inch.

In such a honeycomb-shaped carrier such as a flow-through type carrier and a wall-flow type carrier, the structural feature is expressed by the cell density. In the present invention, the cell density is 10 to 1500 cel / inch ² , more preferably 350. A carrier of ˜900 cel / inch ² can be used. When the cell density is 10 cel / inch ² or more, the pressure loss of the exhaust gas of the internal combustion engine does not occur and the performance of the internal combustion engine is not impaired. Further, when the cell density is 1500 cel / inch ² or less, the contact area between the exhaust gas and the catalyst can be secured, and a sufficient exhaust gas purification function can be obtained.
In the present invention, a catalyst in which a catalyst composition is coated on an integral structure type carrier such as a flow-through type carrier or a wall flow type carrier is hereinafter sometimes referred to as an integral structure type catalyst.

Further, in the monolithic structure type catalyst for exhaust gas purification of the present invention, when the monolithic structure type carrier has a cell density of 10 to 1500 cel / inch ² , the coating amount of the inorganic base material-metal catalyst composite is 10 to 400 g / L, In particular, it is preferably 30 to 300 g / L. If the coating amount exceeds 400 g / L, the pressure loss of the exhaust gas occurs when it is disposed in the exhaust gas flow path of the automobile, and if it is less than 10 g / L, the exhaust gas purification performance becomes insufficient.
When the metal catalyst component contains rhodium as a noble metal, the supported amount of the metal catalyst component is preferably 0.01 to 8 g / L, particularly preferably 0.1 to 5 g / L with respect to the monolithic structure type carrier. . When the amount of the catalyst supported exceeds 8 g / L, the production cost increases, and when it is less than 0.01 g / L, the exhaust gas purification performance becomes insufficient.

4). Manufacture of monolithic catalyst for exhaust gas purification The monolithic catalyst for exhaust gas purification of the present invention is a metal catalyst component or a precursor thereof, the inorganic matrix or a precursor thereof, and other An inorganic substance is used, and these are mixed with an aqueous medium to form a slurry mixture, and then the slurry mixture is applied to a monolithic structure type carrier, dried and fired.

That is, first, a metal catalyst component and an aqueous medium are mixed at a predetermined ratio to obtain a slurry mixture. In the present invention, it is preferable to mix 0.01 to 25 parts by weight of the metal catalyst component with respect to 100 parts by weight of the inorganic base material. As the inorganic substance, 1 to 50 parts by weight of γ-alumina or the like may be blended with respect to 100 parts by weight of the inorganic base material. What is necessary is just to use the quantity which can disperse | distribute an inorganic base material and a metal catalyst component uniformly in a slurry for an aqueous medium.
At this time, an acid or alkali for adjusting the pH, a surfactant for adjusting the viscosity or improving the slurry dispersibility, a dispersing resin or the like can be blended as necessary. As a mixing method of the slurry, pulverization and mixing by a ball mill or the like can be applied, but other pulverization or mixing methods may be applied.

  Next, the slurry-like mixture is applied to the monolithic structure type carrier. A coating method is not particularly limited, but a wash coat method is preferable. After coating, drying and firing are performed to obtain a monolithic structure type catalyst carrying the catalyst composition. In addition, 100-300 degreeC is preferable and, as for drying temperature, 100-200 degreeC is more preferable. Moreover, 300-1200 degreeC is preferable and baking temperature is 400-800 degreeC, Especially 400-600 degreeC is preferable. About a heating means, it can carry out by well-known heating means, such as an electric furnace and a gas furnace.

In addition to the above production steps, an undercoat layer may be provided on the monolithic structure type carrier for the purpose of improving the fixing property before coating with the catalyst composition of the present invention. Such an undercoat layer is not particularly limited in terms of its composition and material, and includes various types of alumina such as γ-type, and the coating amount is not particularly limited, but is generally about 1 to 100 g / L. Covered.
In addition, it cannot be overemphasized that the above-mentioned various catalyst materials may be suitably mix | blended with the said undercoat layer.

In the present invention, the catalyst composition containing the inorganic base material-metal catalyst composite obtained by the above method can be used by being laminated with a coating layer containing another catalyst component or another inorganic material. In that case, the layer of the catalyst composition of the present invention may be a surface layer that is in direct contact with the exhaust gas stream, and a coating containing another catalyst component or other inorganic material on the catalyst composition layer of the present invention. A layer may be formed.
In the coating layer containing such other catalyst components or other inorganic materials, as described above, catalyst metals such as platinum, palladium and rhodium, adsorbents such as zeolite, ceria and ceria-zirconia composite oxide, etc. An oxygen storage / release material, an alkali metal, an alkaline earth metal, or the like may be blended.

  Examples of the present invention and comparative examples are shown below, but the present invention is not construed as being limited to these examples.

Example 1
<Manufacture of monolithic catalyst>
First, 158.3 g of a slurry containing fibrous boehmite having a diameter of 5 nm, a length of 130 nm, and an aspect ratio of 26 (containing 95 g as Al ₂ O ₃ ), and 33.3 g of a zirconium nitrate solution that is 15% by weight in terms of ZrO ₂ (5 g contained as ZrO ₂ ) was added to water to prepare a 5 wt% slurry as a Zr / Al ₂ O ₃ composite oxide.
The slurry was neutralized by adding 300 g of 25 wt% aqueous ammonia in 60 minutes.
Next, the obtained slurry was filtered, sufficiently washed with water, calcined at 500 ° C. for 5 hours, and then pulverized. As a result, a powder having an average particle size (D50) of 55 μm or more of secondary particles or more formed by aggregation of whisker-like inorganic base materials was obtained.
In this way, a whisker-like inorganic base material in which 5% by weight of a zirconium component in terms of oxide was contained in the boehmite crystal was obtained. The photograph is shown in FIG.
Next, using this whisker-like inorganic base material, an aqueous medium was added to a rhodium nitrate aqueous solution and γ-alumina (specific surface area: 150 m ² / g) as a metal catalyst component, and then mixed using a ball mill to obtain a slurry. In addition, the aqueous medium was made into the quantity which can disperse | distribute an inorganic base material and a metal catalyst component uniformly in a slurry.
This slurry was coated on the following monolithic structure type carrier by a wash coat method, dried and then calcined at 500 ° C. for 1 hour to produce an monolithic structure type catalyst. The amount of each component “g / L” in a unit volume of this monolithic catalyst is shown in Table 1 below.
[Integrated structure type carrier]
・ Type of monolithic carrier: Flow-through carrier ・ Material of monolithic carrier: cordierite ・ Cell density of monolithic carrier: 900 cell / inch ²

(Comparative Examples 1 and 2)
<Manufacture of monolithic catalyst>
A monolithic catalyst of Comparative Example 1 was obtained in the same manner as in Example 1 except that the Zr component added to the whisker-like inorganic base material described in Example 1 was replaced with the Ba component. Moreover, the monolithic structure type catalyst of the comparative example 2 was obtained like Example 1 except having changed the Zr component added to the whisker-like inorganic base material of Example 1 into the La component. As in Example 1, a solution containing barium nitrate or lanthanum nitrate and a slurry containing fibrous boehmite were mixed, and Ba or La was added to the whisker-like inorganic base material.
A photograph of the whisker-like inorganic base material of Comparative Example 1 is shown in FIG. 2, and a photograph of the whisker-like inorganic base material of Comparative Example 2 is shown in FIG. In addition, the size and shape of the whisker-like inorganic base material used in Comparative Example 1 and Comparative Example 2 were almost the same as those in the examples.
The amount of each component “g / L” in a unit volume of this monolithic catalyst is shown in Table 1 below.

<Catalyst performance evaluation>
For each of the monolithic catalysts obtained in Example 1, Comparative Example 1, and Comparative Example 2, model gas imitating exhaust gas from a gasoline engine was circulated under the following conditions, and HC, CO, NOx in the exhaust gas was passed. The purification rate was measured. The catalyst performance at 200 to 300 ° C. is shown in FIGS.
・ Measurement mode: Model gas ・ Model gas composition:
CO: 1% by volume
C ₃ H ₆ : 0.1% by volume
C ₃ H _8: 0.025 vol%
NO: 0.05% by volume
H ₂ : 0.3% by volume
CO ₂ : 10% by volume
H ₂ O: 10% by volume
N ₂ : Measuring instrument for residual / exhaust gas: MEXA-7100 manufactured by Horiba, Ltd.
・ Space velocity: 60,000 / h
FIG. 4 shows the purification rate of HC in the exhaust gas under the above conditions, FIG. 5 shows the purification rate of CO, and FIG. 6 shows the purification rate of NOx.
From the graph, it can be seen that the catalyst of Example 1 is superior to the catalysts of Comparative Example 1 and Comparative Example 2 in purification capacity from low temperatures. In addition, the purification rate of HC is similarly bad for the Ba-added product and the La-added product, and the graphs of the purification rate are almost overlapped.

In this invention, it is a photograph which shows the external appearance of the inorganic base material containing Zr used as the raw material of a catalyst. It is a photograph which shows the external appearance of the inorganic base material containing Ba used as a raw material in the catalyst for comparison. It is a photograph which shows the external appearance of the inorganic base material containing La used as a raw material in the catalyst for comparison. It is a graph showing the conversion rate of HC when exhaust gas is processed with the catalyst of an Example and a comparative example. It is a graph showing the conversion rate of CO when exhaust gas is processed with the catalyst of an Example and a comparative example. It is a graph showing the conversion rate of NOx when exhaust gas is processed with the catalyst of an Example and a comparative example.

Claims (9)

An exhaust gas purifying catalyst containing a fibrous inorganic base material-metal catalyst composite in which a metal catalyst component (A) is supported on a whisker-like inorganic base material (B),
The metal catalyst component (A) contains at least a noble metal element, while the whisker-like inorganic base material (B) contains Al ₂ O ₃ as a main constituent unit and a zirconium component in the crystal and / or on the surface thereof. And an exhaust gas purifying catalyst characterized by having a diameter of substantially 50 nm or less and an aspect ratio of 2 or more.   The exhaust gas purifying catalyst according to claim 1, wherein the noble metal element in the metal catalyst component (A) contains rhodium.   The exhaust gas purifying catalyst according to claim 1 or 2, wherein the whisker-like inorganic base material (B) is fibrous boehmite.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the diameter of the whisker-like inorganic base material (B) is substantially 30 nm or less.   The exhaust gas according to any one of claims 1 to 4, wherein the content of the zirconium component is 0.1 to 30 wt% in terms of Zr oxide with respect to the whisker-like inorganic base material (B). Purification catalyst.   An integral structure type catalyst for exhaust gas purification, which is obtained by coating the surface of an integral structure type carrier with the exhaust gas purification catalyst according to any one of claims 1 to 5.   The monolithic catalyst for exhaust gas purification according to claim 6, wherein the amount of the metal catalyst component (A) is 0.01 to 10 g / L with respect to the monolithic support.   The monolithic structure type catalyst for exhaust gas purification according to claim 6 or 7, wherein the amount of the inorganic base material-metal catalyst composite is 10 to 400 g / L with respect to the monolithic structure type carrier.   Exhaust gas containing nitrogen oxides, hydrocarbons, and carbon monoxide is circulated and brought into contact with the exhaust gas purifying monolithic catalyst according to any one of claims 6 to 8. Purification method.

Images