JP2007330864A - Catalyst for cleaning exhaust gas and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning exhaust gas, which is heated up and activated quickly so that the weight of the catalyst is hardly increased or the catalyst is hardly deteriorated with time when an internal-combustion engine is cold-started. <P>SOLUTION: The catalyst for cleaning exhaust gas is provided with: a noble metal particle 11 having catalytic activity; and an endoergic/exoergic material 13. The endoergic/exoergic material 13 is composed of: a first compound particle 14 for generating heat by a hydration reaction; and a second compound 15 which has pores, through each of which a molecule of water to be used in the hydration reaction of the first compound particle 14 can be permeated, and does not form a solid solution together with the first compound particle. The first compound particle 14 is covered with the second compound 15. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying catalyst and a method for producing the same.

In order to remove harmful substances such as hydrocarbon compounds (HC), carbon monoxide (CO), and nitrogen oxides (NO _x ) contained in the exhaust gas discharged from the internal combustion engine, alumina (Al ₂ O ₃₎ Exhaust gas purification catalysts in which noble metal particles such as platinum (Pt) are supported on a metal oxide carrier such as) are widely used.

  These exhaust gas purifying catalysts are formed on the surface of the inner wall of the honeycomb substrate disposed in the middle of the exhaust pipe of the internal combustion engine. During operation of the internal combustion engine, the exhaust gas purifying catalyst is heated by contact with the exhaust gas, and converts harmful substances in the exhaust gas into harmless gas within a predetermined activation temperature range. However, when the exhaust gas purifying catalyst does not reach the activation temperature, such as when starting the internal combustion engine (cold start), the exhaust gas purification may not be sufficient.

Therefore, various measures have been attempted to raise the temperature of the exhaust gas purifying catalyst to an active temperature as quickly as possible in order to sufficiently purify the exhaust gas even at a cold start. For example, heated by increasing the amount of noble metal in the exhaust gas purifying catalyst, HC contained in the exhaust gas, CO, is activated oxidation reaction at a low temperature of H _2, at an early stage a catalyst for purifying exhaust gas by the oxidation reaction heat There is an exhaust gas purifying catalyst. In addition, a first coat layer made of a dielectric material that can be heated by microwaves is provided on the inner wall surface of the honeycomb substrate, and the dielectric material of the first coat layer is heated by microwaves generated by a magnetron. There is an exhaust gas purifying catalyst for heating a catalyst component layer on one coat layer (Patent Document 1). Further, the atmosphere around the catalyst to which the rare earth element is added is made a reducing atmosphere when the internal combustion engine is stopped, and is switched to an oxidizing atmosphere immediately before the internal combustion engine is started, and the catalyst is generated by the oxidation reaction heat of the catalyst itself by the oxidizing atmosphere. There is an exhaust gas purification method that raises the temperature of the gas early (Patent Document 2).
Japanese Patent Laid-Open No. 4-131137 JP 2005-337133 A

  However, it is not preferable to use a large amount of precious metal in the exhaust gas purification catalyst for early heating of the catalyst from the viewpoint of preserving the resources of precious metals with low output, and also to increase the cost of the exhaust gas purification catalyst. Connected. In addition, induction heating of the first coat layer made of a dielectric by microwaves requires that a magnetron for generating microwaves be mounted on the vehicle, which increases the weight of the vehicle. Furthermore, the control for switching the atmosphere of the exhaust gas purifying catalyst before and after the start of the internal combustion engine needs to be injected into a reducing agent catalyst such as an organic substance such as gasoline, ammonia or hydrogen simultaneously with or immediately after the stop of the internal combustion engine. In addition, since a device for injection and its control is required, the weight of the vehicle is increased and the exhaust gas purification device is complicated.

  The exhaust gas-purifying catalyst of the present invention comprises noble metal particles having catalytic activity and a heat-absorbing / dissipating material. The heat-absorbing / dissipating material includes first compound particles that generate heat by a hydration reaction, and the first compound particles. The first compound particles have pores that are permeable to water molecules used in the hydration reaction and are composed of a second compound that is insoluble in the first compound particles. The gist is that it is covered with the second compound.

  In the method for producing an exhaust gas purifying catalyst of the present invention, after preparing a colloidal solution in which a precursor of first compound particles that generates heat due to a hydration reaction is dispersed, the precursor of the first compound in the colloidal solution is prepared. A step of forming a second compound around the body particles and then baking to produce a heat absorbing / dissipating material; an active point material containing a noble metal having catalytic activity; and the heat absorbing / dissipating material And a step of supporting with an oxide.

  According to the exhaust gas purifying catalyst of the present invention, since the heat absorbing / dissipating material is contained together with the noble metal particles, the heat absorbing / dissipating material generates heat when the internal combustion engine is started. It is possible to raise the temperature up to.

  Hereinafter, embodiments of the exhaust gas purifying catalyst of the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional view schematically showing the particle structure of an embodiment of an exhaust gas purifying catalyst according to the present invention. The exhaust gas purifying catalyst shown in FIG. 1 includes noble metal particles 11. The noble metal particles 11 are made of a noble metal having catalytic activity, and have an effect of detoxifying when in contact with exhaust gas containing harmful components. The noble metal particles 11 are supported on the surface of a carrier 12 made of a heat resistant oxide. As shown in the drawing, the structure in which the noble metal particles 11 are supported on the carrier 12 is a general exhaust gas purifying catalyst.

  A heat absorbing / dissipating material 13 is disposed in the vicinity of the noble metal particles 11 carried on the carrier 12. The heat absorbing / dissipating material 13 includes a first compound particle 14 and a second compound 15 that covers the first compound particle 14 by being formed around the first compound particle 14.

  The first compound particles 14 generate heat during the hydration reaction in which moisture is added to form a hydrate, and the original compound substance 14 is converted into the original substance by a dehydration reaction (endothermic reaction) by heating the hydrate. It changes reversibly. The material of the first compound particles 14 will be described in detail later, and for example, there is magnesia (MgO). Further, the second compound 15 has a plurality of pores 16 that lead from the surface of the second compound 15 to the first compound particles 14 inside. The pores 16 have a size capable of transmitting water molecules (water vapor molecules) used for the hydration reaction of the first compound particles. The material of the second compound 15 is a compound that is insoluble in the first compound particles 14 and will be described in detail later. Examples thereof include ceria (CeO2) and zirconia (ZrO2). The heat absorbing / dissipating material 13 shown in FIG. 1 is schematically shown in order to facilitate understanding of the heat absorbing / dissipating material according to the present invention. The thickness of the second compound 15 is not limited to that shown in FIG.

  The effect of the exhaust gas purifying catalyst according to the present embodiment will be described. This exhaust gas-purifying catalyst is formed in layers on the inner wall surface of the through hole of the honeycomb substrate. By providing this honeycomb substrate in the middle of the exhaust pipe of the internal combustion engine, the noble metal particles 11 contained in the exhaust gas purification catalyst remove harmful substances in the exhaust gas that are discharged from the internal combustion engine and pass through the through holes of the honeycomb substrate. , Converted into a harmless gas within a predetermined activation temperature range And by arranging the heat absorbing / dissipating material 13 in the vicinity of the noble metal particles 11, when the internal combustion engine is cold-started, the heat absorbing / dissipating material 13 generates an exothermic reaction due to water vapor contained in the exhaust gas, thereby generating a hydrate. In addition, the noble metal particles 11 near the heat absorbing / dissipating material 13 are heated. Therefore, in the exhaust gas purifying catalyst according to the present embodiment, the noble metal particles 11 are quickly heated and activated by the heat-absorbing and dissipating material 13, so that the exhaust gas can be exhausted quickly and sufficiently even at low temperatures such as during cold start. It becomes possible to purify.

Further, since the heat absorbing / dissipating material 13 is a material that generates a hydrate (for example, hydroxide) by a hydration reaction, the heat absorbing / dissipating material 13 reacts with water vapor in the exhaust gas at the cold start. As a result, the concentration of water vapor in the exhaust gas temporarily decreases. Water vapor (moisture) in the exhaust gas is not preferable for exhaust gas purification because it inhibits adsorption of harmful components (HC, CO, NO _x, etc.) in the exhaust gas to the catalyst active points (noble metal particles) of the exhaust gas purification catalyst. It is an ingredient. In the exhaust gas purifying catalyst according to the present embodiment, the concentration of water vapor in the exhaust gas temporarily decreases due to the reaction with the heat absorbing and radiating material 13 at the cold start, so that it is possible to suppress the adsorption inhibition of harmful components due to water vapor, Therefore, further early activation of the exhaust gas purifying catalyst can be achieved.

  During the steady operation after the internal combustion engine is started, the heat absorbing / dissipating material 13 in the exhaust gas purifying catalyst receives heat from the exhaust gas to cause a dehydration reaction, and reversibly changes from the hydrate to the original oxide or the like. Return. This state of oxide or the like is maintained even after the internal combustion engine is stopped, and when the internal combustion engine is cold-started, it reacts with the water vapor in the exhaust gas and generates heat as described above.

  When the internal combustion engine has been stopped for a long period of time, a hydration reaction may gradually occur due to moisture absorption / radiation material 13 in the exhaust gas purification catalyst absorbing moisture in the air. In order to prevent such a hydration reaction of the heat-absorbing / dissipating material 13 when the internal combustion engine is stopped, it is possible to take measures to prevent air containing water vapor from flowing into the vicinity of the exhaust gas purifying catalyst. For example, a detachable lid is provided on each of the gas inlet side portion and the outlet side portion of the honeycomb substrate on which the exhaust gas purification catalyst is applied and formed, and a driving device for attaching and removing the lid body to and from the honeycomb substrate; In addition, a control device for operating the driving device according to the operating condition can be provided, and the attachment / detachment of the lid body to the honeycomb substrate according to the operating condition of the internal combustion engine can be enabled. In addition, a valve capable of opening and closing the flow path can be provided before and after the honeycomb substrate of the exhaust pipe provided with the honeycomb substrate so that movement of air before and after the honeycomb substrate can be blocked.

  In the heat absorbing / dissipating material 13, the first compound particles 14 are covered with the second compound. The first compound particle is a material that reversibly changes to a hydrate by a hydration reaction and a dehydration reaction, for example, MgO. Even if only the first compound is used, the first compound particle functions as the heat absorbing / dissipating material 13. Can be achieved. However, the inventors' research has found that when the heat absorbing / dissipating material 13 is composed only of the first compound particles 14, the function as the heat absorbing / dissipating material may be deteriorated by long-term use. More specifically, when an ordinary heat absorbing / dissipating material such as MgO (magnesium oxide) generates heat due to its hydration reaction, the surface energy of each particle is reduced, that is, the surface area of the particle is reduced. Mass transfer occurs in the direction. Thereby, adjacent MgO particles are bonded to each other and the MgO particles themselves are aggregated. When the MgO particles themselves aggregate, the surface area of the MgO particles decreases, so the contact area that reacts with water molecules as the heat absorbing / dissipating material decreases. It has been found that due to the reduction of the contact area with water molecules due to the aggregation of the heat absorbing / dissipating material particles, the function as the heat absorbing / dissipating material may be deteriorated when only the first compound particles are used as the heat absorbing / dissipating material. This agglomeration of the heat absorbing / dissipating material particles cannot be solved even if the heat absorbing / dissipating material particles are simply of high purity or have a uniform particle diameter.

  Therefore, in the exhaust gas purification catalyst 13 of the present embodiment, the first compound particles 14 having the function of absorbing and releasing heat are covered with the second compound 15 in order to suppress aggregation of the first compound particles. It has been broken. As a result, the contact between the first compound particles 14 is suppressed by the second compound 15 and aggregation is suppressed. Further, since the second compound 15 has a large number of pores 16 that can penetrate water molecules, the water molecules can contact the first compound particles 14 through the pores 16. Even if the second compound 15 is formed, a decrease in the contact area between the water molecules and the first compound particles 14 is suppressed. From the above, the exhaust gas purifying catalyst of the present embodiment includes the first compound particles 14 of the heat absorbing / dissipating material 13 by inclusion and fixation with the second compound, thereby aggregating the first compound particles 14. Since it is possible to absorb and release heat with high energy density while suppressing deterioration and reducing deterioration over time, early activation of the exhaust gas purifying catalyst can be stably maintained for a long period of time.

  In the embodiment shown in FIG. 1, the heat absorbing / dissipating material 13 is not supported on the carrier 12 on which the noble metal particles 11 are supported, that is, the heat absorbing / dissipating material 13 is not in contact with the carrier 12. In the exhaust gas purifying catalyst of such an embodiment, for example, a slurry obtained by mixing the powder of the carrier 12 on which the noble metal particles 11 are supported and the powder of the heat absorbing / dissipating material 13 is applied and formed on the inner wall surface of the honeycomb substrate. Is obtained.

  The exhaust gas purifying catalyst according to the present invention is not limited to the embodiment shown in FIG. For example, an embodiment in which the heat absorbing / dissipating material 13 is disposed on the same carrier as the carrier 12 on which the noble metal particles 11 are carried can be employed. An example of such an embodiment is shown in FIG.

  In the exhaust gas purifying catalyst of the embodiment shown in FIG. 2, the heat absorbing / dissipating material 13 is supported on and in contact with the carrier 12 supporting the noble metal particles 11 as compared with the exhaust gas purifying catalyst shown in FIG. 1. About another structure, it is the same as that of embodiment shown in FIG. Therefore, the description which overlaps already described about each member shown in FIG. 2 is abbreviate | omitted.

  In the exhaust gas purifying catalyst of the embodiment shown in FIG. 2, the heat absorbing / dissipating material 13 is disposed on the same carrier as the carrier 12 carrying the noble metal particles 11. For this reason, since the heat absorbing / dissipating material 13 is reliably located in the vicinity of the noble metal particles 11, the heat generating action of the heat absorbing / dissipating material 13 can be further utilized.

  Another embodiment of the exhaust gas purifying catalyst according to the present invention is shown in a schematic sectional view in FIG. The exhaust gas-purifying catalyst shown in FIG. 3 includes noble metal particles 21, a porous heat-resistant inorganic oxide 22 formed around the noble metal particles 21, and noble metal particles 21 inside the heat-resistant inorganic oxide 22. And a compound particle 23 disposed in the vicinity of.

  The noble metal particles 21 and the compound particles 23 are supported by being encapsulated in a porous heat-resistant inorganic oxide 22, and can come into contact with exhaust gas through a large number of holes of the heat-resistant inorganic oxide 22.

  The compound particles 23 generate heat during the hydration reaction to form a hydrate by adding water, and reversibly return to the original substance by a dehydration reaction (endothermic reaction) by heating the hydrate. It is a first compound of the heat absorbing / dissipating material in the present invention.

  The heat-resistant inorganic oxide 22 is a non-solid oxide with the compound particles 23. This heat-resistant inorganic oxide 22 covering the compound particles 23 corresponds to the second compound of the heat absorbing / dissipating material in the present invention.

  In the exhaust gas purifying catalyst of the embodiment shown in FIG. 3, contact and aggregation of the noble metal particles 21 are suppressed by the inclusion of the noble metal particles 21 with the heat-resistant inorganic oxide 22. Therefore, it is possible to suppress a decrease in catalytic activity due to a decrease in the surface area of the noble metal particles 21.

  Further, since the compound particles 23 are provided close to the noble metal particles 21, the compound particles 23 undergo an exothermic reaction by water vapor contained in the exhaust gas when the internal combustion engine is cold-started, as in the embodiment described in FIG. This produces hydrate and generates heat, and heats the noble metal particles 21 in the vicinity of the compound particles 23. Therefore, in the exhaust gas purifying catalyst according to the present embodiment, since the noble metal particles 21 are quickly heated by the compound particles 23 and activated, the exhaust gas can be quickly and sufficiently even at low temperatures such as during cold start. It becomes possible to purify. At the cold start, the compound particles 23 react with the water vapor in the exhaust gas, so that the concentration of water vapor in the exhaust gas temporarily decreases. Therefore, the adsorption | suction inhibition of the harmful | toxic component by water vapor | steam can be suppressed, Therefore The further early activation of the exhaust gas purification catalyst can be aimed at.

  Furthermore, the inclusion and aggregation of the noble metal particles 21 are suppressed by the inclusion of the compound particles 23 by the heat-resistant inorganic oxide 22. Therefore, it is possible to absorb and release heat with high energy density while reducing deterioration with time of the compound particles 23, so that early activation of the exhaust gas purifying catalyst can be stably maintained for a long period of time. Moreover, since the compound particles 23 are surrounded by the heat-resistant inorganic oxide 22, the compound particles 23 are surely positioned in the vicinity of the noble metal particles 21, so that the heat generation action of the heat absorbing / dissipating material 13 is further utilized. Is possible.

  In order to obtain the exhaust gas purifying catalyst of the embodiment shown in FIG. 3, for example, after preparing the noble metal particles 21 and the compound particles 23, the noble metal particles 21 and the compound particles 23 are made of the heat-resistant inorganic oxide 22. After adding to the solution containing the raw material, the solid content in the solution may be fired.

  The exhaust gas purifying catalyst in which the noble metal particles 21 are included by the heat-resistant inorganic oxide 22 is not limited to the embodiment shown in FIG. FIG. 4 is a schematic cross-sectional view of an exhaust gas purifying catalyst according to another embodiment. In the embodiment shown in FIG. 4, another compound particle 24 is formed around the heat-resistant inorganic oxide 22 as compared with the embodiment shown in FIG. 3. Other configurations are the same as those of the embodiment shown in FIG. Therefore, the description which overlaps already described about each member shown in FIG. 4 is abbreviate | omitted.

  The other compound particles 24 are made of another material that is substantially non-solid solution with the heat-resistant inorganic oxide 22, and the other compound particles 24 are disposed around the heat-resistant inorganic oxide 22. The particles of the heat-resistant inorganic oxide 22 are prevented from contacting and aggregating. Therefore, the catalytic activity of the noble metal particles 21 and the exothermic action of the compound particles 23 can be further utilized.

  A more preferable aspect of the exhaust gas purifying catalyst of each embodiment described so far with reference to FIGS. 1 to 4 will be described below.

  The particle diameters of the first compound particles 14 in the embodiment shown in FIGS. 1 and 2 and the compound particles 23 in the embodiments shown in FIGS. 3 and 4 are preferably 10 nm or less. When the particle diameter of the first compound particles 14 and the compound particles 23 having the function of absorbing and releasing heat is 10 nm or less, the surface area is dramatically increased as compared with the case where the particle diameter exceeds 10 nm, and excellent heat absorbing and releasing characteristics are exhibited. By reducing the particle size of the first compound particles 14 and the compound particles 23, the reaction area with water molecules increases, and as a result, a remarkable temperature increase effect can be obtained during an exothermic reaction. Furthermore, by controlling the particle diameter to a predetermined particle diameter of 10 nm or less, it becomes possible to control the amount of heat generation (temperature increase) or the heat generation rate.

  The sizes of the pores 16 included in the second compound 15 in the embodiment shown in FIGS. 1 and 2 and the heat-resistant inorganic oxide 22 in the embodiment shown in FIGS. It is preferable that the particle diameter of the compound particle 14 and the compound particle 23 is smaller. Since the pores of the second compound 15 and the heat-resistant inorganic oxide 22 are smaller than those of the first compound particles 14 and the compound particles 23, the first compound particles 14 and the compound particles 23 have the second The second compound 15 and the heat-resistant inorganic oxidation are suppressed while moving through the pores 16 of the compound 15 and the heat-resistant inorganic oxide 22 and suppressing aggregation between the first compound particles 14 and the compound particles 23. It becomes possible to maintain the water molecule permeation function of the object 22.

  The lower limit of the size of the pores 16 of the second compound 15 and the heat-resistant inorganic oxide 22 is determined as the size through which water molecules can pass. Since the diameter of the water molecule is very small, less than 0.3 nm, the lower limit of the size of the pores 16 is less than 0.3 nm. However, in a normal material that can be used as the second compound 15 or the heat-resistant inorganic oxide, the size of the pores 16 of the second compound 15 or the heat-resistant inorganic oxide 22 is set to the first compound particle. 14 or the particle diameter of the compound particles 23, the size of the pores 16 does not hinder the water molecule permeation function.

The first compound particles 14 and the compound particles 23 are preferably a compound of at least one metal selected from alkali metals, alkaline earth metals, and rare earth metals. A material that generates heat from the hydration reaction and reversibly changes to the original compound by the endothermic reaction can be applied to the first compound particles 14 and the compound particles 23. These compounds can be arbitrarily selected depending on the operating temperature range of the exhaust gas purifying catalyst. The exhaust gas temperature of the internal combustion engine varies depending on the output level of the internal combustion engine and the distance from the exhaust valve. For example, in the endothermic process, the temperature range in which the first compound particles 14 as the heat absorbing and radiating material can be obtained is 150. CaO can be applied at 300 ° C., MgO at 300 ° C., Pr ₂ O ₃ at 650 ° C., Li ₂ O at 1000 ° C., and the like.

Among the various metal compounds listed above, MgO is excellent in the endothermic heat generation per unit weight. Furthermore, MgO also has the property that the Mg (OH) ₂ → MgO + H ₂ O reaction proceeds (can absorb heat) even in a temperature range of about 300 ° C., which is relatively difficult to use waste heat. Therefore, although depending on the operating temperature of the exhaust gas purifying catalyst, it is preferable to employ a magnesium compound, especially MgO, as the first compound particles 14 and the compound particles 23.

The second compound 15 and the heat-resistant inorganic oxide 22 are preferably at least one compound selected from CeO ₂ , ZrO ₂ , TiO ₂ and SiO ₂ . The material of the second compound 15 and the heat-resistant inorganic oxide 22 covers the first compound particles 14 and the compound particles 23 and suppresses aggregation of the first compound particles 14 and the compound particles 23. In order to exert the effect, a compound that does not cause a solid-phase reaction (so-called solid solution) with the first compound particle 14 or the compound particle 23 can be selected, and CeO ₂ , ZrO ₂ , TiO ₂ listed above can be selected. And SiO ₂ meet that requirement. Further, CeO ₂ , ZrO ₂ , TiO ₂ and SiO ₂ can all be formed around the first compound particles 14 as a collection of fine particles or fibers, and thus the predetermined pores described above are formed. It is a material that can be formed. As an example of a combination of the first compound particle 14 and the second compound 15 or an example of a combination of the compound particle 23 and the heat-resistant inorganic oxide 22, the first compound particle 14 and the compound particle 23 are MgO. If present, CeO ₂ is arranged around the MgO as the second compound 15 or the heat-resistant inorganic oxide 22 so as to suppress the coarsening of the MgO particles and keep them stable without reacting with the MgO particles. Is possible.

  As the noble metal particles 11 and 21, a conventionally known noble metal used for an exhaust gas purifying catalyst can be applied. In addition, a conventionally known carrier material used for an exhaust gas purification catalyst can also be applied to the carrier 12 of the noble metal particles 11.

  Next, an embodiment of the method for producing the exhaust gas purifying catalyst of the present invention will be described with reference to the explanatory diagram of the production method shown in FIG. In one embodiment of the method for producing an exhaust gas purifying catalyst according to the present invention, after preparing a colloidal solution in which a precursor of first compound particles that generates heat due to a hydration reaction is dispersed, the first colloidal solution in the colloidal solution is prepared. A step of forming a second compound around the precursor particles of the compound, and then baking to produce a heat-absorbing and heat-dissipating material; an active site material containing a noble metal having catalytic activity; With a heat-resistant inorganic oxide.

  In the production method including these steps, first, in order to produce a heat absorbing / dissipating material, first compound particles 31 of 10 nm or less or a precursor thereof are formed in a colloidal state finely dispersed in a liquid containing a colloid material. (FIG. 5 (a)). By dispersing the minute first compound particles 31 or the precursors thereof in the colloidal solution, the first compound particles 31 or the precursors thereof are prevented from contacting and aggregating with each other during the manufacturing process. The surface area of the first compound particles 31 can be remarkably increased in the heat absorbing / dissipating material after manufacture. In particular, when the particle diameter of the first compound particles 31 is 10 nm or less, the surface area of the first compound particles 31 is significantly larger than when the particle diameter exceeds 10 nm. As the first compound particles 31 or their precursors, conventionally known ones can be used, and for the colloidal material 32, a known material such as PVP (polyvinylpyrrolidone) can be used. In addition, the particle diameter of the first compound particles 31 can be adjusted by adjusting the quantitative ratio between the first compound particles 31 or a precursor thereof and the colloid material 32.

  Next, the second compound 32 is formed around the first compound particle 31 or its precursor in the colloidal solution. By mixing the colloidal solution in which the first compound particle 31 or its precursor is dispersed and the separately prepared second compound 32 or its precursor solution, the periphery of the first compound particle 31 or its precursor is mixed. To form the second compound 32 (FIG. 5B). In order to form the second compound, for example, a sol-gel method can be used. For this reason, it is preferable to mix a colloidal solution and a solution containing the metal alkoxide of the second compound 32. The pore size of the second compound 32 can be adjusted by adjusting the concentration of the solution of the second compound 32 or a precursor thereof (for example, a metal alkoxide solution). The adjustment of the pore size is not limited to the concentration adjustment of the second compound 32 or the precursor thereof in the solution of the second compound 32 or the precursor thereof. It can also be adjusted by adjusting.

  Thereafter, the colloidal solution is filtered, and the gel is dried and fired to obtain a heat-absorbing and heat-dissipating material 34 (FIG. 5C). The heat absorbing / dissipating material 34 may be pulverized to adjust the particle diameter as necessary.

  The obtained heat absorbing / dissipating material 34 is supported on a heat-resistant inorganic oxide together with an active point material containing a noble metal having catalytic activity. For example, a sol-gel method can be used as the supporting method, but the method is not limited to the sol-gel method. This process allows the heat-absorbing / dissipating material and the catalyst active point to coexist on the same carrier (heat-resistant inorganic oxide), resulting in less heat dissipation from the heat-absorbing / dissipating material, and heat storage energy from the heat-absorbing / dissipating material Can be used efficiently.

  The exhaust gas-purifying catalyst obtained through the above steps is applied on the inner wall surface of the honeycomb substrate 41 as shown in the perspective view of FIG. FIG. 6B is an enlarged view of a part of a cross section obtained by cutting the honeycomb substrate 41 along a plane perpendicular to the through holes. As shown in the figure, the exhaust gas purifying catalyst coat layer 42 is formed on the surface of the through hole having a rectangular cross section formed by the lattice-shaped inner wall of the honeycomb substrate 41.

[Example 1]
<Preparation of heat absorbing / dissipating material>
An aqueous solution of Mg acetate, polyvinylpyrrolidone (PVP), and ion-exchanged water are put into a container in an amount such that the PVP / Mg molar ratio is 8, stirred for 1 hour, and then heated to 100 ° C at a rate of 5 ° C / min. The temperature was raised to prepare a 5 wt% -PVP-Mg colloidal solution (average particle size 4 nm).

  On the other hand, cerium isopropoxide was added to another container in an amount adjusted to 10 wt% in the solvent (hexylene glycol), stirred at 100 ° C. for 1 hour, and then the PVP-Mg colloidal solution described above in this container. Was introduced.

Thereafter, the obtained gel was dried and fired to obtain a heat absorbing / dissipating material for Example 1. The composition of the obtained heat absorbing / dissipating material powder was MgO (65%) / CeO ₂ (residue). The particle diameter of MgO in this powder was 4 nm, and the pore diameter of CeO ₂ was measured to be 3 nm.

<Catalyst powder preparation>
Γ-alumina is added to a predetermined amount of dinitrodiamine Pt nitric acid aqueous solution while stirring, and after continuous stirring for 1 hour, it is dried in air at 120 ° C for 1 hour, and further calcined in air at 400 ° C for 1 hour. As a result, a Pt (0.3 wt%) / γ-alumina catalyst powder was obtained.
90.9 g of the heat absorbing / dissipating material of Example 1 above, 90.9 g of the above catalyst powder, 24.6 g of boehmite and 293.6 g of ion-exchanged water were taken, put into a magnetic alumina pot with 10 mmφ alumina balls, pulverized, A catalyst slurry was obtained. The obtained slurry was applied to a 0.119 L cordierite honeycomb carrier (400 cpsi / 6 mil), excess slurry was removed with an air stream, dried at 80 ° C, and then up to 400 ° C, 3-5 ° C / The temperature was increased at a temperature increase rate of min, and firing was performed in an air stream for 1 hour.

  The obtained catalyst honeycomb carrier of Example 1 had a Pt amount of 0.3 g / L and a heat absorbing / dissipating material of 100 g / L per liter of the carrier.

[Example 2]
<Catalyst metal containing-Preparation of heat absorbing / dissipating material>
An aqueous solution of Mg acetate, polyvinyl pyrrolidone (PVP)) and ion-exchanged water are charged into a container in an amount such that the PVP / Mg molar ratio is 8, stirred for 1 hour, and then heated to 100 ° C at a rate of 5 ° C / min. The temperature was raised at a temperature of 5 wt% -PVP-Mg colloidal solution (average particle size 4 nm).

  Further, dinitrodiamine Pt was introduced into a commercially available ceria sol having an average particle diameter of 5 nm to prepare Pt (1.5%) / ceria sol.

  On the other hand, cerium isopropoxide was added to another container in an amount adjusted to 10 wt% in the solvent (hexylene glycol), stirred at 100 ° C. for 1 hour, and then the PVP-Mg colloidal solution described above in this container. And a mixed solution of Pt / ceria sol.

Thereafter, the obtained gel was dried and calcined to obtain a catalyst metal-containing / heat-absorbing / dissipating material for Example 2. The composition of the obtained heat absorbing / dissipating material powder was Pt (0.3 wt%) / MgO (65%) / CeO ₂ (residue). The particle diameter of MgO in this powder was 4 nm, and the pore diameter of CeO ₂ was measured to be 3 nm.

<Honeycomb catalyst>
90.9 g of the heat absorbing / dissipating material for Example 2 above, 90.9 g of ceria powder, 24.6 g of boehmite and 293.6 g of ion-exchanged water were taken, put into a magnetic alumina pot together with 10 mmφ alumina balls, pulverized, A catalyst slurry was obtained. The obtained slurry was applied to a 0.119 L cordierite honeycomb carrier (400 cpsi / 6 mil), excess slurry was removed with an air stream, dried at 80 ° C, and then up to 400 ° C, 3-5 ° C / The temperature was increased at a temperature increase rate of min, and firing was performed in an air stream for 1 hour.

  The obtained catalyst honeycomb carrier of Example 2 had a Pt amount of 0.3 g / L and a heat absorbing / dissipating material of 100 g / L per liter of the carrier.

[Example 3]
A catalyst honeycomb carrier of Example 3 was obtained in the same manner as in Example 1 except that the PVP / Mg molar ratio was 1.2 and the solution concentration of cerium isopropoxide was 40 wt%. The composition of the obtained heat absorbing / dissipating material powder was MgO (65%) / CeO ₂ (residue). The particle diameter of MgO in this powder was 10 nm, and the pore diameter of CeO ₂ was measured to be 9 nm.

[Example 4]
An aqueous solution of Pr acetate, polyvinylpyrrolidone (PVP)) and ion-exchanged water are put into a container in an amount such that the molar ratio of PVP / Pr is 8, and after stirring for 1 hour, the rate of temperature increase to 100 ° C is 5 ° C / min. The temperature was raised to 5 wt% -PVP-Pr colloidal solution (average particle size 6 nm).

  On the other hand, zirconium isopropoxide was put into another container in an amount adjusted to 30 wt% in a solvent (hexylene glycol), stirred at 100 ° C. for 1 hour, and then the PVP-Pr colloidal solution described above in this container Was introduced.

Thereafter, the obtained gel was dried and fired, and the honeycomb carrier of Example 4 was obtained in the same manner as Example 1. The composition of the heat absorbing / dissipating material powder of the exhaust gas purifying catalyst of Example 4 was PrO ₂ (65%) / ZrO ₂ . The particle diameter of PrO ₂ in this powder was 6 nm, and the pore diameter of ZrO ₂ was measured to be 10 nm.

[Example 5]
An aqueous solution of La acetate, polyvinylpyrrolidone (PVP) and ion-exchanged water are put into a container in such an amount that the PVP / La molar ratio is 6, stirred for 1 hour, and then heated to 100 ° C at a rate of 5 ° C / min. The temperature was raised to prepare a 5 wt% -PVP-La colloidal solution (average particle size 4 nm).

  On the other hand, a predetermined amount of cerium isopropoxide was added to another container in an amount adjusted to 30 wt% in a solvent (hexylene glycol), stirred at 100 ° C. for 1 hour, and then the PVP- La colloid solution was added.

Thereafter, the obtained gel was dried and fired, and the honeycomb carrier of Example 5 was obtained in the same manner as Example 1. The composition of the heat absorbing / dissipating material powder of the exhaust gas purifying catalyst of Example 5 was La ₂ O ₃ (65%) / CeO ₂ (residue). The particle diameter of La ₂ O ₃ in this powder was 6 nm, and the pore diameter of CeO ₂ was measured to be 10 nm.

[Example 6]
An aqueous solution of Mg acetate, polyvinylpyrrolidone (PVP), and ion-exchanged water are put into a container in an amount such that the PVP / Mg molar ratio is 8, stirred for 1 hour, and then heated to 100 ° C at a rate of 5 ° C / min. The temperature was raised to prepare a 5 wt% -PVP-Mg colloidal solution (average particle size 4 nm).

  On the other hand, cerium isopropoxide was put into another container in an amount adjusted to 40 wt% in the solvent (hexylene glycol), stirred for 1 hour at 100 ° C., and then the PVP-Mg colloidal solution described above in this container Was introduced.

Thereafter, the obtained gel was dried and fired, and the honeycomb carrier of Example 6 was obtained in the same manner as Example 1. The composition of the heat absorbing / dissipating material powder of the exhaust gas purifying catalyst of Example 6 was MgO (65%) / CeO ₂ (residue). The particle diameter of MgO in this powder was 4 nm (when the pore diameter of this powder was measured, it was 9 nm.

[Comparative Example 1]
As compared with the exhaust gas purifying catalyst of Example 1, a catalyst honeycomb carrier of Comparative Example 1 was obtained in the same manner except that the step of including MgO with cerium was not included in the heat absorbing / dissipating material.

[Comparative Example 2]
Comparative Example 2 is an example of an exhaust gas purifying catalyst that does not include a heat absorbing / dissipating material.

<Catalyst powder preparation>
Γ-alumina is added to a predetermined amount of dinitrodiamine Pt nitric acid aqueous solution while stirring, and after continuous stirring for 1 hour, it is dried in air at 120 ° C for 1 hour, and further calcined in air at 400 ° C for 1 hour. Pt (0.3 wt%) / γ-alumina catalyst powder was obtained.

<Honeycomb catalyst>
90.9 g of the catalyst powder, 90.9 g of γ-alumina, 24.6 g of boehmite, and 293.6 g of ion-exchanged water were sampled and put into a magnetic alumina pot together with 10 mmφ alumina balls and pulverized to obtain a catalyst slurry. The obtained slurry was applied to a 0.119 L cordierite honeycomb carrier (400 cpsi / 6 mil), excess slurry was removed with an air stream, dried at 80 ° C, and then up to 400 ° C, 3-5 ° C / The temperature was increased at a temperature increase rate of min, and firing was performed in an air stream for 1 hour.

  The obtained catalyst honeycomb carrier of Comparative Example 3 had a Pt amount of 0.3 g / L per liter of the carrier.

The exhaust gas purifying catalysts 40 cc of each of the examples and comparative examples thus obtained were incorporated into a simulated exhaust gas circulation device, and the temperature was raised while circulating simulated exhaust gas having the composition shown in Table 1 below. A cold start mock test was conducted. The space velocity SV of the simulated exhaust gas in this simulation test was 90000 h ⁻¹ , and the temperature was increased from 150 to 450 ° C. at a temperature increase rate of 5 ° C./min. In addition, water addition to the reaction gas was started at the start of temperature increase. The HC purification rate (ηHC)% at 500 ° C. of each exhaust gas purification catalyst was calculated from the HC concentrations at the inlet side and outlet side at 500 ° C. in this simulation test.

  Further, the cycle endurance test shown in FIG. 7 was performed 100 times, and the post-cycle endurance state was regarded as the initial state and the post-cycle endurance state, and the HC purification rate was examined.

  In the cycle endurance test shown in FIG. 7, each sample powder was held for 30 minutes at 200 ° C. in an atmosphere of 10% water vapor / remainder He and a predetermined temperature (water of the first compound) in the He atmosphere. This is a test in which the holding time for 20 minutes at a temperature equal to or higher than the decomposition temperature of the oxide is set to one cycle with a heating time of 5 minutes and a cooling time of 2 minutes. Corresponding model test.

The evaluation results of each Example and Comparative Example are shown in Table 2 as the initial state and the results after the cycle durability test. In Table 2, YES / NO indicates whether or not the heat absorbing / dissipating material is supported on the same carrier powder as the noble metal particles.

  As is apparent from Table 2, Examples 1 to 6 according to the present invention have an excellent HC purification rate in the initial state as compared with Comparative Example 2 that does not include a heat absorbing / dissipating material, and the heat absorbing / dissipating material at the cold start. Therefore, it has excellent catalytic activity even at low temperatures. Moreover, these Examples 1-6 are also excellent in the catalyst activity after a cycle endurance test, and this point is clear by comparison with the comparative example 1. That is, although the comparative example 1 includes the heat absorbing / dissipating material, since the heat absorbing / dissipating material does not include the second compound, the heat absorbing / dissipating material aggregates to reduce the surface area, thereby causing a cycle durability test. Later, it is considered that the heat generation effect due to the inclusion of the heat absorbing / dissipating material was significantly reduced. On the other hand, Examples 1 to 6 show a superior performance over a long period of time with little decrease in the heat generation effect even after the cycle durability test.

  Among Examples 1 to 6, an example in which the particle diameter of the first compound particles of the heat absorbing / dissipating material is larger than the pore diameter of the second compound covering the first compound particles is exothermic after the cycle durability test. The degree of deterioration of the effect was smaller.

  Further, Example 2 in which the heat absorbing / dissipating material is supported on the same carrier powder as the noble metal particles has a low exhaust gas purification temperature in the initial state, and has particularly excellent catalytic activity from the low temperature.

It is typical sectional drawing of the exhaust gas purification catalyst of one Embodiment of this invention. It is typical sectional drawing of the catalyst for exhaust gas purification of other embodiment of this invention. It is typical sectional drawing of the catalyst for exhaust gas purification of other embodiment of this invention. It is typical sectional drawing of the catalyst for exhaust gas purification of other embodiment of this invention. It is explanatory drawing of an example of the manufacturing method of the catalyst for exhaust gas purification of this invention. It is explanatory drawing of the honeycomb base | substrate with which the exhaust gas purification catalyst was apply-formed. It is explanatory drawing of a cycle durability test.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Noble metal particle 12 Carrier 13 Heat absorbing / dissipating material 14 First compound particle 15 Second compound 21 Noble metal particle 22 Heat-resistant oxide 23 First compound particle

Claims (9)

Noble metal particles having catalytic activity;
With heat absorbing and dissipating materials,
The heat absorbing / dissipating material includes first compound particles that generate heat by a hydration reaction;
The first compound particle has a pore that can permeate water molecules used for the hydration reaction, and is composed of a second compound that is insoluble in the first compound particle. An exhaust gas purifying catalyst, wherein one compound particle is covered with the second compound.   The exhaust gas-purifying catalyst according to claim 1, wherein the heat-absorbing and heat-dissipating material is disposed on the same carrier as the carrier on which the noble metal particles are supported.   3. The exhaust gas purifying catalyst according to claim 1, wherein a particle diameter of the first compound particles of the heat absorbing / dissipating material is 10 nm or less. The exhaust gas purification apparatus according to any one of claims 1 to 3, wherein the pore size of the second compound of the heat absorbing / dissipating material is smaller than the particle diameter of the first compound particle. catalyst.   2. The exhaust gas purifying catalyst according to claim 1, wherein the first compound particles of the heat absorbing / dissipating material are a compound of at least one metal selected from an alkali metal, an alkaline earth metal, and a rare earth metal. The second compound of absorbing heat materials, CeO _2, ZrO _2, the exhaust gas purifying catalyst according to claim 1, characterized in that at least one compound selected from TiO ₂ and SiO _2.   6. The exhaust gas purifying catalyst according to claim 5, wherein the first compound particles of the heat absorbing / dissipating material are made of a magnesium compound. After preparing the colloidal solution in which the precursor of the first compound particle that generates heat due to the hydration reaction is dispersed, the second compound is formed around the particle of the precursor of the first compound in the colloidal solution. , And then baking to produce a heat-absorbing / dissipating material,
A method for producing an exhaust gas purifying catalyst, comprising: an active site material containing a noble metal having catalytic activity; and a step of supporting the heat absorbing / dissipating material with a heat-resistant inorganic oxide.   The method for producing an exhaust gas purifying catalyst according to claim 8, wherein the precursor of the first compound particles has a particle size of 10 nm or less.

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