JP2007000812A - Waste gas cleaning system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning system in which the noble metal particles of a nano size disposed in catalyst sections can hardly be sintered even in a high-temperature region of about 900°C in the temperature of the exhaust gas at the inlets of the catalyst sections arranged at the uppermost stream, and a satisfactory exhaust gas cleaning performance can be exhibited. <P>SOLUTION: The exhaust gas cleaning system includes an upstream catalyst section and a downstream catalyst section provided with the noble metal particles of the nano size carried on a porous inorganic oxide, and in which the grain size ratio of the noble metal particles between the upstream catalyst section and the downstream catalyst section is 2:1 to 10:1. The noble metal particles of 4 to 20 nm are carried on the porous inorganic oxide of the upstream catalyst section and those of 2 to 10 nm are carried on the porous inorganic oxide of the downstream catalyst section. The difference between the temperature T<SB>U</SB>°C of the exhaust gas flowing into the upstream catalyst section and the temperature T<SB>L</SB>°C of the exhaust gas flowing into the downstream catalyst section is 50 to 100°C. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification system, and more particularly to an exhaust gas purification system that efficiently purifies gas exhausted from an internal combustion engine.

Since the exhaust gas regulations for automobiles are expanding worldwide, noble metal fine particles such as platinum (Pt), palladium (Pd), and rhodium (Rh) are supported by a porous oxide such as alumina (Al ₂ O ₃ ). The three-way catalyst supported on the catalyst is used for the purpose of purifying hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in the exhaust gas.

However, the noble metal that is the active point of the catalyst is agglomerated at a high temperature of several hundred degrees C. Therefore, the surface area of the active point is reduced.
Therefore, in order to suppress aggregation, control of the distance between the noble metal fine particles and control of the particle diameter of the noble metal fine particles can be mentioned. Among these, with respect to the control of the distance between the noble metal particles, not only the outer surface of the carrier but also the inner surface of the carrier, that is, the surface of the pores, from the viewpoint that if the noble metal particles are supported on the outer surface of the carrier, the aggregation tends to proceed. However, it is important to support noble metal fine particles.

In addition, regarding the control of the particle diameter of the noble metal fine particles, it is known that a certain degree of particle diameter is required because the melting point of the fine particles is lowered if the initial noble metal fine particles are too small (for example, non-patent document). 1).
Ph. buffat et. al. Phys. Rev. A, Vol. 13, no. 6). (1976)

Furthermore, it is known that if the particle size of the supported metal fine particles is non-uniform, aggregation is more likely to proceed with the metal particles having a large particle size present in a part as a nucleus (for example, non-patent document). 2).
M.M. Che, J. et al. F. Dutel et. al. J. et al. Phys. Chem, 80, p2371. (1976)

  Therefore, in order to suppress aggregation of the noble metal fine particles, it is effective that the noble metal fine particles have a certain size and are uniformly dispersed and supported on the surface of the carrier.

Therefore, a catalyst has been proposed in which metal colloid is formed by a chelating agent and the metal is dispersed and supported on the surface of the carrier (see Patent Document 1).
A catalyst in which a quaternary ammonium salt is used as a protective colloid and a colloidal salt is impregnated and supported inside the pores of the carrier has been proposed (see Patent Document 2).
JP 2000-279824 A (2nd page) JP 2002-1119 A (page 2)

However, in the technique disclosed in the above-mentioned patent document, a polymer having a molecular size larger than the pore diameter of alumina as a carrier is used as a protective agent for the noble metal fine particles. I could not.
Further, when a quaternary ammonium salt is used as a protective colloid, there is a problem in the stability of the colloidal salt, such as a part of the colloid aggregates and precipitates due to long-term storage.
Furthermore, even if the noble metal colloidal particles are simply supported inside the carrier pores, the movement of the noble metal particles themselves could not be suppressed. That is, there has been a problem that aggregation degradation of noble metals occurs.

  The present invention has been made in view of the above-described problems of the prior art. The object of the present invention is to provide a catalyst even in a high temperature range where the exhaust gas temperature at the inlet of the catalyst section disposed in the uppermost stream is about 900 ° C. An object of the present invention is to provide an exhaust gas purification system in which nano-sized noble metal particles provided in the part are difficult to sinter and can exhibit good exhaust gas purification performance.

  As a result of intensive studies to solve the above problems, the present inventor has increased the precious metal particles in the upstream catalyst portion where the exhaust temperature is high and the particle aggregation is likely to proceed, and conversely the downstream catalyst portion where the exhaust temperature is low and the particle aggregation is difficult to proceed. The inventors have found that the above-mentioned problems can be solved by reducing the size of the noble metal particles, and have completed the present invention.

That is, the exhaust gas purification system of the present invention is an exhaust gas purification system in which an upstream catalyst portion and a downstream catalyst portion are arranged in this order from the upstream side of the exhaust passage of an internal combustion engine or a combustion device,
The upstream catalyst part and the downstream catalyst part comprise nano-sized noble metal particles supported on a porous inorganic oxide,
The particle size ratio of the noble metal particles in the upstream catalyst portion and the noble metal particles in the downstream catalyst portion is 2: 1 to 10: 1.

Further, the preferred form of the exhaust gas purification system of the present invention is to carry precious metal particles having a particle diameter of 4 to 20 nm on the porous inorganic oxide in the upstream catalyst part,
It is characterized in that noble metal particles having a particle diameter of 2 to 10 nm are supported on the porous inorganic oxide in the downstream catalyst part.

Furthermore, in another preferred embodiment of the exhaust gas purification system of the present invention, the difference between the exhaust gas temperature T _U ° C flowing into the upstream catalyst part and the exhaust gas temperature T _L ° C flowing into the downstream catalyst part is 50 to 100 ° C. It is characterized by being.

Furthermore, in another preferred embodiment of the exhaust gas purification system of the present invention, in the noble metal particles used in the upstream catalyst portion and the downstream catalyst portion, the following formula (1)
Exposure rate (%) = 0.895 × (A × B × C × D) / (E × F) × 100 (1)
(In the formula, A is the CO adsorption amount [cc / g], B is the supported noble metal cross-sectional area [nm ² ], C is the supported noble metal density [g / cc], D is the TEM particle radius [nm], and E is the stoichiometric amount. The theoretical ratio, F, indicates the loading concentration [% / g])
The exposure rate represented by the formula is 50 to 85%.

  Another preferred embodiment of the exhaust gas purification system of the present invention is that the precious metal particles are initially and after endurance when they are brought into contact with the exhaust gas at 900 ° C. for 3 hours in the upstream catalyst part and / or the downstream catalyst part. The diameter ratio is from 3:10 to 7:18.

  The inlet of the catalyst part is arranged at the most upstream by increasing the precious metal particles in the upstream catalyst part where the exhaust temperature is high and the particle aggregation is easy to proceed, and conversely by reducing the precious metal particles in the downstream catalyst part where the exhaust temperature is low and the particle aggregation is difficult to proceed. Even in the high temperature range where the exhaust gas temperature is about 900 ° C., the nano-sized noble metal particles provided in the catalyst part are difficult to sinter and can exhibit good exhaust gas purification performance.

  Hereinafter, the exhaust gas purification system of the present invention will be described in detail. In the present specification and claims, “%” indicates a mass percentage unless otherwise specified.

As described above, the exhaust gas purification system of the present invention includes the upstream catalyst portion and the downstream catalyst portion arranged in this order from the upstream side of the exhaust passage of the internal combustion engine or the combustion apparatus.
In addition, the upstream catalyst portion and the downstream catalyst portion include a porous inorganic oxide on which nano-sized noble metal particles are supported, and a part of the noble metal particles are buried in the porous inorganic oxide.
Furthermore, the particle size ratio of the noble metal particles in the upstream catalyst part and the noble metal particles in the downstream catalyst part is 2: 1 to 10: 1.
The nano-size indicates that the particle size is generally 1 to 100 nm, and 10 to 50 nm in the porous inorganic oxide used in the present application.

As described above, since the noble metal particles supported on the surface of the porous inorganic oxide have a size suitable for the exhaust gas temperature flowing through each catalyst part, the change rate of the particle size of the supported noble metal particles is low, and the catalyst The activity is maintained for a long time. In particular, the noble metal particles are prevented from agglomerating even at a high temperature of about 700 to 1000 ° C. in the upstream catalyst portion, and the catalyst activity per unit weight is increased in the range where the particle aggregation does not occur in the downstream catalyst portion.
Therefore, this system is excellent in heat resistance, and the exhaust gas purification efficiency is remarkably improved.

Here, the surface (supporting surface) of the porous inorganic oxide includes not only the outer surface but also the inner surface of pores such as a hollow shape and a slit shape possessed by the porous inorganic oxide.
And since the noble metal particles can be supported not only on the outer surface of the porous inorganic oxide but also on the inner surface, the surface area of the entire catalyst can be effectively utilized. That is, the catalytic activity per unit weight of the catalyst is improved, and the amount of exhaust gas adsorbed to be reacted is increased.
In general, the noble metal particles supported on the outer surface of the porous inorganic oxide have a short distance between the particles and tend to agglomerate. When supported, the distance between particles becomes long and aggregation is suppressed.

Further, the noble metal particles supported by the porous inorganic oxide in the upstream catalyst part preferably have a particle size of 4 to 20 nm, and the noble metal particles supported by the porous inorganic oxide in the downstream catalyst part have a particle size of Is preferably 2 to 10 nm.
At this time, the rate of change in the particle size of the noble metal particles tends to be lower, and the catalytic activity can be maintained for a long time.

Furthermore, the difference between the exhaust gas temperature T _U ° C flowing into the upstream catalyst part and the exhaust gas temperature T _L ° C flowing into the downstream catalyst part is preferably 50 to 100 ° C.
By disposing the upstream catalyst portion and the downstream catalyst portion so as to have such an exhaust gas temperature difference, purification treatment in a wide temperature range becomes possible.

  The inflow exhaust gas temperature can be adjusted by controlling the shape of the exhaust passage, the interval between the catalyst portions, the combustion operation using the temperature sensor, and the like.

Furthermore, in one or both of the upstream catalyst part and the downstream catalyst part, the noble metal particles supported by the porous inorganic oxide are represented by the following formula (1):
Exposure rate (%) = 0.895 × (A × B × C × D) / (E × F) × 100 (1)
(In the formula, A is the CO adsorption amount [cc / g], B is the supported noble metal cross-sectional area [nm ² ], C is the supported noble metal density [g / cc], D is the TEM particle radius [nm], and E is the stoichiometric amount. The theoretical ratio, F, indicates the loading concentration [% / g])
It is preferable that the exposure rate represented by is 50 to 85%.

  At this time, aggregation of the noble metal particles dispersed and supported in the porous inorganic oxide can be suppressed.

  Moreover, the anchor effect with respect to the noble metal particle of a porous inorganic oxide is notably exhibited, and the state at the time of catalyst manufacture can be maintained over a longer period even after heating.

Here, the anchor effect means that the porous inorganic oxide surrounding the noble metal particles acts as an anchor for immobilizing the noble metal particles. Specifically, as shown in FIG. 1B, the noble metal particles are supported on the surface of the porous inorganic oxide in a state in which the noble metal particles are partially embedded in the porous inorganic oxide.
Thereby, the distance between noble metal particles is maintained, and aggregation of the noble metal particles is effectively suppressed.

In general, when the noble metal particles are fine particles having a particle diameter of 10 nm or less, the noble metal particles are likely to aggregate, but since the noble metal particles are partially embedded in the surface of the porous inorganic oxide, the anchor effect is improved. It is exhibited remarkably, and the state at the time of catalyst production is maintained even after heating.
Therefore, since the rate of change of the particle size of the noble metal particles is low, the catalyst activity is maintained for a long time, and the exhaust gas purification system is excellent in heat resistance.

  From the viewpoint of exerting the anchor effect, the porous oxide preferably contains a transition metal, and more preferably ceria.

In addition, when the porous oxide is ceria, the particle size is preferably 10 to 50 nm, and more preferably 20 to 30 nm.
At this time, an appropriate amount of noble metal particles is easily buried in the ceria, and at the same time, the oxygen storage function of the ceria is sufficiently exhibited, so that a sufficient catalytic activity is obtained.

Furthermore, the particle diameter of the noble metal particles supported on ceria is desirably 3 to 10 nm.
At this time, an appropriate amount of noble metal particles is easily embedded in ceria, so that sufficient catalytic activity can be obtained.

In the exhaust gas purification system of the present invention, when the exposure rate of the noble metal particles is 50 to 85%, as described above, a part of the noble metal particles is supported in a state of being buried in the porous inorganic oxide. Will be.
Usually, atoms that effectively function as a catalyst are atoms present on the surface, and when the exposure rate is too low, the stability of the noble metal particles is high, but the noble metal particles may sufficiently contact the reactants. As a result, sufficient catalytic activity cannot be obtained.
For this reason, in order to maintain the performance as a catalyst, the exposure rate is preferably at least 50% or more.

On the other hand, when the exposure rate of the noble metal particles is too high, the initial activity of the catalyst is high, but the noble metal particles supported on the surface of the porous inorganic oxide may be aggregated by heating, resulting in poor durability.
For this reason, it is preferable that the exposure rate does not exceed 85%.

Further, in one or both of the upstream catalyst part and the downstream catalyst part, it is preferable that the amount of the noble metal particles supported is 0.72 g or less per 1 L of the catalyst part.
Conventionally, with this loading amount, noble metal particles agglomerate with use and sufficient catalytic activity has been obtained, but in this system, noble metal particles do not easily agglomerate at each catalyst part, so sufficient exhaustion is possible with the above loading amount. Gas purification ability can be demonstrated.
Furthermore, as described above, when the anchor effect is exerted, since a part of the nano-sized noble metal particles are buried in the porous inorganic oxide, the amount of noble metal used is reduced as compared with the conventional case. However, sufficient catalytic activity can be maintained.

Moreover, as said noble metal particle, it is suitable to use platinum (Pt), palladium (Pd), rhodium (Rh), and these in arbitrary combinations.
These noble metals have high catalytic activity and are effective for purification of exhaust gas.

Furthermore, in each catalyst part of the exhaust gas purification system, one or both of the porous inorganic oxide and the noble metal particles are manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper It is preferable to be in contact with (Cu) or zinc (Zn) and a transition metal arbitrarily combined with these.
In this case, since these transition metals assist the catalytic activity of the noble metal particles, the catalytic activity can be further improved.
In addition, when any one of a porous inorganic oxide and a noble metal particle is not contacting these transition metals, it is good to exist in the vicinity of this transition metal.

Furthermore, it is desirable that some or all of these transition metals form a composite compound with the carrier.
In this case, it is considered that the catalyst performance can be improved if the noble metal particles are in contact with the composite compound containing the transition metal. This is considered to be due to a phenomenon called spillover in which exhaust gas is adsorbed on the surface of the noble metal and then moves to the surface of the composite compound to purify the exhaust gas on the surface of the composite compound.

  That is, since the noble metal particles and the composite compound are in contact with each other, the noble metal not only serves as a catalyst but also serves as an adsorption site for adsorbing exhaust gas. May be activated to function as a catalyst site for performing a catalytic reaction.

As described above, in the present exhaust gas purification system, the catalytic activity can be improved since the composite compound containing the transition metal supplements the catalytic activity of the noble metal particles in each catalyst portion.
In addition, since part or all of the transition metal forms a composite compound with the carrier, the composite oxide can act as an anchor that suppresses the movement of the noble metal particles, so that aggregation of the noble metal is suppressed. Thus, deterioration of the catalyst active point can be suppressed.

In each catalyst part, the noble metal-supporting porous inorganic oxide is ceria (CeO ₂ ), alumina (Al ₂ O ₃ ), zirconia (ZrO ₂ ), silica (SiO ₂ ) or titania (TiO ₂ ), and these It can be used by supporting it on a carrier composed of any combination of oxides.
In this case, the adhesion with the porous inorganic oxide can be improved. Moreover, the effect which activates the noble metal particle and transition metal which are catalyst metals, and the heat resistance of a support | carrier can be exhibited.
These carrier constituent materials may be the same as or different from the porous inorganic oxide. Further, the noble metal-supported porous inorganic oxide powder itself can be formed into a honeycomb or the like.

Furthermore, the oxide as the carrier constituting material can include either or both of the porous inorganic oxide and the noble metal particles.
At this time, the movement and aggregation of the noble metal particles are further suppressed, and the dispersed state during the production of the catalyst can be maintained for a longer period.
For example, as shown in FIG. 2, fibrous oxides can be disposed around porous inorganic oxides, noble metal particles, and transition metals to be held.

In this way, the exhaust gas purification system increases the precious metal particles in the upstream catalyst portion where the exhaust temperature is high and the particle aggregation is likely to proceed, and conversely reduces the precious metal particles in the downstream catalyst portion where the exhaust temperature is low and the particle aggregation is difficult to proceed. Thus, even when the exhaust gas temperature at the inlet of the catalyst part disposed in the uppermost stream is high, the nano-sized noble metal particles provided in the catalyst part are difficult to sinter and can exhibit good exhaust gas purification performance.
Specifically, one or both of the upstream catalyst portion and the downstream catalyst portion is brought into contact with the exhaust gas at 900 ° C. for 3 hours, and then the initial and endurance particle size ratio of the noble metal particles is set to 3:10. It can be suppressed within the range of ˜7: 18.

  Next, the manufacturing method of the noble metal carrying | support porous inorganic oxide with which an upstream catalyst part and a downstream catalyst part are provided is demonstrated. There are roughly two types of methods for producing such noble metal-supported porous inorganic oxides: a colloid method and a reverse micelle method.

(Colloid method)
In this method, a noble metal colloid dispersed in a solvent is included in a ceria precursor, which is an example of a porous inorganic oxide, in a state where the periphery of the noble metal particles is protected by organic molecules, thereby forming a catalyst precursor. A precious metal-supporting porous inorganic oxide (precious metal-supporting ceria) is obtained by performing a step (inclusion step) and a step (firing step) of firing the catalyst precursor in an oxidizing atmosphere.

In this way, a colloid of noble metal particles is prepared, and after the formation of cerium hydroxide, which is a ceria precursor, around the noble metal colloid, the catalyst precursor is calcined in an oxidizing atmosphere, so It becomes possible to disperse and carry metal particles.
In addition, since the ceria precursor is formed around the noble metal colloid, a part of the noble metal particles are buried in the ceria.
As a result, the noble metal particles can be immobilized, and a catalyst having high heat resistance can be obtained.

  Here, as the organic molecule, for example, a highly stable compound such as polyvinyl pyrrolidone, polyvinyl alcohol, polyethylene imide, polyacrylic acid, oxalic acid, citric acid, maleic acid, or a mixture thereof can be used.

  Examples of the solvent include water, alcohols such as methanol and ethanol, esters such as acetic acid methyl ester and ethyl acetate, ethers such as diethyl ether, and mixtures thereof.

When the catalyst precursor is formed using these organic molecules and the dispersion solvent, the protective layer of the organic molecules is coordinated around the noble metal particles, so that the noble metal particles can be uniformly dispersed in the solvent. Further, the stability of the colloidal solution, that is, the sedimentation separation can be suppressed.
For this reason, the catalyst precursor is formed in a state where the noble metal particles are uniformly taken into the ceria precursor.

Furthermore, by firing this catalyst precursor in an oxidizing atmosphere, water evaporates from the ceria hydroxide, which is a ceria precursor, and many pores are formed after firing, so that ceria with a high specific surface area is formed. .
In this catalyst precursor, the noble metal particles are uniformly incorporated into the ceria precursor, so the catalyst produced by firing is in a state where the noble metal particles are uniformly supported on all ceria surfaces including the inner surface. It becomes.

Thus, even when the size of the noble metal colloid is larger than the pores of ceria, it becomes possible to impregnate and carry the noble metal particles inside the pores.
Further, by using such a loading method, a ceria barrier is formed around the noble metal particles, so that an anchor effect by the barrier is obtained and aggregation of the noble metal particles is suppressed.
Furthermore, a part of the noble metal particles is buried in ceria by creating a barrier.
Furthermore, when a part of this barrier is a composite compound of transition metal, the catalytic activity of the transition metal by the noble metal occurs, and as a result, the surface area of the catalytic activity point is further improved, and excellent catalytic performance after durability Will come to show.

  In order to obtain the above-mentioned effects, the noble metal colloid solution is composed of platinum (Pt), palladium (Pd) or rhodium (Rh), and a noble metal arbitrarily combined with manganese (Mn), iron (Fe), It is preferable that a composite metal compound with cobalt (Co), nickel (Ni), copper (Cu) or zinc (Zn), and a transition metal arbitrarily combined with these is included as colloidal particles.

  In this case, it is possible to prepare a catalyst material in which a composite compound containing a transition metal is selectively arranged around the noble metal by making the particles serving as the core of the noble metal colloid into a composite of the noble metal and the transition metal. .

The ceria precursor is in the form of a water-soluble inorganic salt such as nitrate or acetate, and is first introduced into an aqueous solution containing colloidal particles of a composite metal compound of a noble metal and a transition metal, and then ammonia water or tetramethylammonium (TMAH). ) A hydroxide may be formed by introducing a basic precipitating agent such as an aqueous solution.
Further, an alkoxide may be added to the aqueous colloidal solution to form a hydroxide around the colloidal particles of the composite metal compound.
Furthermore, if the polymer chain of the organic molecule constituting the noble metal colloid has, for example, an imino (NH) group and the colloid solution is basic, a precipitant and an aqueous solution containing a ceria precursor are added. By introducing a mixed aqueous solution of noble metal colloids, the noble metal colloids can be stably supported on ceria.

  Here, an example of the colloid method will be described with reference to a schematic process flow diagram shown in FIG. FIG. 4 shows a schematic diagram of the manufacturing process of the exhaust gas purification system.

As shown in FIG. 3, first, in STEP 1 (hereinafter referred to as “S1”), a noble metal salt and an organic molecule are put into a solvent and stirred to prepare a mixed solution.
Here, as the noble metal salt, inorganic salts such as noble metal complexes such as dinitrodiamine salt, triammine salt, tetraammine salt and hexaammine salt, nitrate, chloride and sulfate can be used. As the transition metal salt, acetate, nitrate, carbonate and the like can be used.
Moreover, as an organic molecule, polyvinyl pyrrolidone, polyvinyl alcohol, polyethylene imide, polyacrylic acid, oxalic acid, citric acid, maleic acid and the like can be used.
In addition, you may use these as a 2 or more types of mixed solution.
Here, as shown in FIG. 4A, noble metal ions 10 and organic molecules 11 exist in the solvent.

Next, in S2, a reducing agent is added to the mixed solution to reduce the noble metal ions 10, and as shown in FIG. 4B, a noble metal colloid in which organic molecules 11 are coordinated around the reduced noble metal particles 10a. 12 dispersion.
Here, as the reducing agent, hydrazine, sodium borohydride, hydrogen gas, or the like can be used.

In S3, as an inclusion process, ceria hydrate, which is a ceria precursor, is added to the dispersion, and after sufficient stirring, an aqueous precipitant solution is added dropwise to the solution until pH 7.0, The resulting solution is aged overnight.
As shown in FIG. 4C, the noble metal colloid 12 is taken into the hydroxide 13 which is a ceria precursor, and a precipitate of the catalyst precursor 14 is obtained. Further, a part 12 a of the noble metal colloid 12 is in a state in which a part is taken into the ceria hydroxide 13. The other noble metal colloid 12b is completely included in the ceria hydroxide 13.
Here, the ceria precursor may be a hydrate. As the precipitant aqueous solution, ammonia water, TMAH aqueous solution, or the like can be used.

Further, in S4, the obtained precipitate is filtered through a membrane filter, and then washed with a large amount of water.
In S5, the precipitate is dried at 120 ° C. for a whole day and night.
In S6, as a firing step, the dried precipitate is fired in an air stream at 400 ° C. for 1 hour to obtain a noble metal-supported ceria 15 as shown in FIG. 4 (d).

The obtained noble metal-supported ceria 15 is obtained by calcining the catalyst precursor 14 in which the noble metal colloid 12 is incorporated in the ceria hydroxide 13 which is a ceria precursor, so that water evaporates from the ceria hydroxide 13. Due to dehydration shrinkage.
At this time, many pores are formed in the ceria 17, and the noble metal colloid 12 taken into the ceria hydroxide 13 is buried in the outer surface of the ceria 17 and the inner surface inside the pores 16. It becomes.

  In this way, the noble metal particles 16 are uniformly dispersed and firmly fixed on the ceria 17, and the ceria 17 acts as an anchor for the noble metal particles 16, so that the aggregation of the noble metal particles 16 is suppressed and the catalyst performance is improved. A noble metal-supporting ceria 15 having a high specific surface area and a high catalytic activity can be obtained.

Further, in the inclusion step (S3 in FIG. 3, (c) in FIG. 4), a partial oxidizing agent may be added to the colloid solution in which the noble metal colloid is dispersed in the solvent.
The partial oxidizing agent refers to, for example, an aqueous solution containing an oxidizing substance such as hydrogen peroxide. This partial oxidizing agent partially oxidizes the surface of the noble metal particles once reduced to metal by the reducing agent or the composite compound of the noble metal and transition metal. By this oxidation, the noble metal particles or the composite compound have high affinity with the ceria hydrate which is a ceria precursor, and the immobilization of the noble metal particles or the composite compound supported on the ceria is promoted. And the exposure rate of a noble metal particle and a composite compound is controllable.

This effect is particularly effective in the case of a noble metal alone. Since noble metals are relatively stable as compared to transition metals, the affinity between ceria and sufficient noble metals cannot be controlled without partial oxidation. For this reason, it is difficult to control the exposure rate of the noble metal. However, when a partial oxidizing agent is added to the colloid solution and the surface of the noble metal is partially oxidized at a predetermined ratio, the affinity with the ceria precursor at the partially oxidized portion of the surface of the noble metal becomes higher. The exposure rate of the precious metal particles after firing is determined.
Thus, it becomes possible to control the exposure rate of the noble metal particles and the composite compound in the catalyst by the amount of the partial oxidant introduced into the colloidal solution.

  Further, another example of the colloid method will be described with reference to a schematic process flow diagram shown in FIG. FIG. 6 shows a schematic diagram of the manufacturing process of the noble metal-supporting ceria.

  In the process flow diagram shown in FIG. 5, the same operation as the process flow diagram of FIG. 3 is performed except that the partial oxidizing agent aqueous solution is added after S2.

First, in S1, a noble metal salt and an organic molecule are put into a solvent and stirred to prepare a mixed solution. Here, as the noble metal salt, a noble metal complex such as a dinitrodiamine salt, a triammine salt, a tetraammine salt, or a hexaammine salt is used. Inorganic salts such as nitrates, chlorides and sulfates can be used. As the transition metal salt, acetate, nitrate, carbonate and the like can be used.
Moreover, as an organic molecule, polyvinyl pyrrolidone, polyvinyl alcohol, polyethylene imide, polyacrylic acid, oxalic acid, citric acid, maleic acid and the like can be used.
In addition, you may use these as a 2 or more types of mixed solution. Further, here, metal ions and organic molecules exist in the dispersion medium.

Next, in S2, a reducing agent is added to the mixed solution to reduce the noble metal ions to obtain a dispersion of noble metal colloid in which organic molecules are coordinated around the reduced noble metal particles.
Here, as the reducing agent, hydrazine, sodium borohydride, hydrogen gas, or the like can be used.

In S3, a partial oxidizing agent aqueous solution is added to the dispersion to partially oxidize the noble metal particles. At this time, as the partial oxidant, any compound can be used as long as it is an oxidant having a property of giving oxygen atoms to metal particles such as hydrogen peroxide.
After adding ceria hydrate which is a ceria precursor and stirring sufficiently, an aqueous precipitant solution is added dropwise to this solution until the pH becomes 7.0, and the resulting solution is aged overnight.
Thereafter, a precipitate of the catalyst precursor in which the noble metal colloid is incorporated into the hydroxide which is the ceria precursor is obtained. At this time, a part of the noble metal colloid is in a state of being taken up by the ceria hydroxide, and the other is in a state of being completely enclosed by the hydroxide.
Here, the ceria precursor may be a hydrate. As the precipitant aqueous solution, ammonia water, TMAH aqueous solution, or the like can be used.

Further, in S4, the obtained precipitate is filtered through a membrane filter, and then washed with a large amount of water.
In S5, the precipitate is dried at 120 ° C. overnight.
In S6, the dried precipitate can be baked in an air stream at 400 ° C. for 1 hour to obtain noble metal-supported ceria.

The obtained noble metal-supported ceria is obtained by calcining a catalyst precursor in which a noble metal colloid is incorporated into ceria hydroxide as a ceria precursor, and thus dehydrates and shrinks as water evaporates from the ceria hydroxide.
At this time, many pores are formed in the ceria, and the noble metal colloid taken into the ceria hydroxide becomes noble metal particles buried in the outer surface of the ceria and the inner surface inside the pores.

  In this way, the noble metal particles are uniformly dispersed and firmly fixed on the ceria, and since the ceria acts as an anchor for the noble metal particles, aggregation of the noble metal particles is suppressed, a high specific surface area is obtained, and the catalytic activity is increased. Enhanced.

Further, as shown in FIG. 6, the exposure rate of the noble metal particles can be controlled by the partial oxidizing agent.
FIG. 6A is a partial cross-sectional view of the noble metal-supporting ceria 20 manufactured by reducing the amount of the partial oxidizing agent. In this noble metal-supporting ceria 20, a part of the noble metal particles 21 is supported on the surface of the ceria 22 in a state of being embedded in the ceria 22.
Here, since the amount of the partial oxidizing agent is small, the exposure rate of the noble metal particles 21 is high, and the amount of the ceria 22 surrounding the periphery is also small.

  FIG. 6B is a partial cross-sectional view of a noble metal-carrying ceria 30 manufactured by increasing the amount of the partial oxidizing agent compared to the noble metal-carrying ceria 20. In the noble metal-supporting ceria 30, the noble metal particles 31 are supported on the surface of the ceria 32 in a state of being embedded in the ceria 32 by about 50%.

FIG. 6C is a partial cross-sectional view of the noble metal-supported ceria 40 manufactured by further increasing the amount of the partial oxidizer than the noble metal-supported ceria 30. In this noble metal-supporting ceria 40, most of the noble metal particles 41 are supported on the surface of the ceria 42 while being embedded in the ceria 42.
Here, since the amount of the partial oxidizing agent is large, the exposure rate of the noble metal particles 41 is low, and the amount of ceria 42 surrounding the metal particles 73 is large.

  As shown in FIGS. 6A to 6C, since the affinity of the noble metal particles with ceria can be controlled by the amount of the partial oxidizing agent, the exposure rate of the noble metal particles supported on the ceria can be controlled.

(Reverse micelle method)
In this method, after depositing noble metal particles or composite metal particles of noble metal particles and a transition metal in reverse micelles having a droplet diameter of 20 nm or less, a step of adding a partial oxidizing agent in the reverse micelles And in a reverse micelle, the step of enclosing the noble metal particles or the composite metal particles of the noble metal particles and the transition metal with the precursor of ceria, which is an example of the porous inorganic oxide, Product (noble metal-supported ceria) is obtained.

When this method is adopted, the noble metal particles or the composite metal particles of the noble metal particles and the transition metal are partially oxidized in the reverse micelle, so that the composite metal particles of the noble metal particles or the noble metal particles and the transition metal and the ceria precursor. It becomes possible to increase the affinity.
Similarly to the colloid method, by using a partial oxidizing agent, it becomes possible to control the exposure rate of the noble metal particles or the composite metal particles of the noble metal particles and the transition metal.

  Here, an example of the reverse micelle method will be described with reference to a schematic process flow diagram shown in FIG. FIG. 8 shows a schematic diagram of the manufacturing process of the noble metal-supporting ceria.

As shown in FIG. 7, first, in STEP 1 (hereinafter referred to as “S1”), a mixed solution in which a surfactant is dissolved in an organic solvent is prepared.
Here, cyclohexane, cycloheptane, octanol, isooctane, n-hexane, n-decane, benzene, toluene, xylene, or the like can be used as the organic solvent.
As the surfactant, polyethylene glycol (5) mono-4-nonylphenyl ether, pentaethylene glycol dodecyl ether, or the like can be used.
In addition, you may use these in mixture of 2 or more types.

Next, in S2, a mixed aqueous solution of a noble metal salt and a transition metal salt is added to this mixed solution and stirred for 2 hours.
At this time, as shown in FIG. 8A, fine reverse micelles 50 are formed. In the reverse micelle 50, the surfactant 51 is arranged around a spherical droplet having a diameter of about several tens of nanometers so that the hydrophobic group is directed to the inside of the hydrophilic group and the hydrophobic phase is directed to the outside of the hydrophilic group. An aqueous solution 52 containing a transition metal salt is contained.
Here, nitrates, acetates, chlorides, amine compounds, carbonyl compounds, metal alkoxides, and the like can be used as the noble metal salts and transition metal salts.
In addition, you may use these as a 2 or more types of mixed solution.

In S3, a reducing agent of a noble metal and a transition metal is added to a mixed solution of an organic solvent containing the reverse micelle 50 and stirred for 2 hours.
At this time, as shown in FIG. 8B, the noble metal salt and the transition metal salt are simultaneously reduced and metallized in the reverse micelle 50.
Here, as the reducing agent, for example, ammonia, tetramethylammonium, alkali metal hydrate (such as sodium hydroxide), hydrazine, sodium borohydride and the like can be used.

Thereafter, in S4, a partial oxidizing agent aqueous solution is put into a mixed solution of an organic solvent containing reverse micelles, and a part of the composite metal grains of noble metal and transition metal is oxidized in the reverse micelles (FIG. 8C).
Here, as the partial oxidizer aqueous solution, any compound can be used as long as it is an oxidizer having a property of giving oxygen atoms to metal particles such as hydrogen peroxide as described above.

In S5, a cerium aqueous solution is mixed and stirred for 2 hours, and the cerium salt and water are contained in the reverse micelle 50 containing the composite metal grains of the partially oxidized noble metal 53 and the transition metal 54.
Here, nitrates, chlorides, acetates, amine compounds, and the like can be used as the cerium salts.

Furthermore, in S6, the precipitant aqueous solution is mixed in the mixed solution of the organic solvent containing the reverse micelle to precipitate the cerium salt inside the reverse micelle 50.
At this time, as shown in FIG. 8 (d), the cerium salt inside the reverse micelle 50 is metalized and precipitated as cerium 55, and the composite metal grains of the noble metal 53 and the transition metal 54 are clathrated with the cerium 55.
Here, hydrazine, sodium borohydride, ammonia and the like can be used as the precipitant.
In addition, you may use these as a 2 or more types of mixed solution.

In S7, alcohol is added to the mixed solution of the organic solvent containing the reverse micelles, and the mixture is stirred for 2 hours to collapse the reverse micelles.
At this time, as shown in FIG. 8 (e), a precipitate in which the composite metal particles of the noble metal 53 and the transition metal 54 are clathrated with the cerium 55 is obtained by the collapse of the reverse micelle.
Here, as alcohol, methanol, ethanol, etc. can be used, for example.

Next, in S8, the obtained precipitate is filtered with a membrane filter, and then washed with alcohol and water to remove impurities (for example, surfactant) contained in the precipitate.
In S9, the precipitate is dried at 120 ° C. for a whole day and night.
In S10, after drying, the precipitate is fired in an air stream at 400 ° C. for 1 hour to obtain a noble metal-supporting ceria 56 as shown in FIG. 8 (f).
In this noble metal-supporting ceria 56, a part of the noble metal and transition metal (transition metal composite particles) 58 is supported in a state of being buried in the surface of the ceria 57.

In the reverse micelle method described above, it is preferable to use platinum (Pt), palladium (Pd), rhodium (Rh), and any combination thereof as the noble metal.
As the transition metal, manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu) or zinc (Zn), and any combination thereof may be used. Is preferred.
Further, the ceria precursor is preferably a compound having aluminum (Al), titanium (Ti), zirconium (Zr) or lanthanum (La), and any combination thereof.

Thus, also in the manufacturing method of the noble metal carrying | support ceria using the reverse micelle method, since a ceria precursor is formed around the noble metal particle, a part of the noble metal particle is buried in the ceria.
For this reason, it becomes possible to fix the noble metal particles, and a catalyst part having high heat resistance can be obtained.

  In the above-described method for producing a noble metal-supported porous inorganic oxide, the catalytic activity of the porous inorganic oxide obtained varies depending on the type of raw material and the use conditions, etc. The desired catalyst part and exhaust gas purification system can be obtained by appropriately changing the type, reaction temperature, reaction time, stirring intensity and stirring method.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in full detail, this invention is not limited to these Examples.

Example 1
(1) Preparation of upstream catalyst part Pt colloid aqueous solution (Pt particle size 6 nm, Pt concentration 4%) was mixed at a 2-methyl-2,4-pentanediol / isopropoxide Al = 5/1 weight ratio and dried. Firing was performed to obtain Pt / alumina powder.

  This powder was impregnated with an aqueous solution of Ce nitrate and dried and fired to obtain a Pt / Ce / alumina powder.

This powder, water, and nitric acid were put into a magnetic ball mill, mixed and ground to obtain a slurry.
This slurry was applied to a 20 cc ceramic honeycomb catalyst to obtain an upstream catalyst portion.
In this catalyst part, the amount of Pt was 0.007 g, the amount of CeO ₂ was 0.2 g, and the amount of alumina was 1.8 g.

(2) Production of downstream catalyst part A dinitrodiammine Pt aqueous solution (Pt concentration 4%) was mixed with alumina powder, dried and fired to obtain Pt / alumina powder.

  This powder was impregnated with an aqueous solution of Ce nitrate and dried and fired to obtain a Pt / Ce / alumina powder.

This powder, water, and nitric acid were put into a magnetic ball mill, mixed and ground to obtain a slurry.
This slurry was applied to a 20 cc ceramic honeycomb catalyst to obtain a downstream catalyst portion.
In this catalyst part, the amount of Pt was 0.007 g, the amount of CeO ₂ was 0.2 g, and the amount of alumina was 1.8 g.

(Example 2)
(1) Production of upstream catalyst part A catalyst part similar to Example 1 was produced.

(2) Production of downstream catalyst part The downstream catalyst part was produced by repeating the same operation as in Example 1 except that the Pt colloid particle size was 8 nm.

(Comparative Example 1)
As the upstream catalyst portion, the same as the downstream catalyst portion obtained in Example 1 was used.
Further, as the downstream catalyst part, the same one as the upstream catalyst part obtained in Example 1 was used.

<Evaluation measurement>
(1) Durability test The upstream catalyst part and the downstream catalyst part were disposed on the flow path, and the gas was circulated so that the upstream catalyst part inlet was 900 ° C. and the downstream catalyst part inlet was 700 ° C., and was endured for 30 hours.
Table 1 shows the initial and post-end Pt particle sizes.

(2) Purification reaction test Under the conditions shown below, a simulated exhaust gas was passed through the catalyst to determine the conversion rate.
CO: 0.6 vol%
H _2: 0.2vol%
O ₂ : 0.63 vol%
NO: 1000 volppm
C ₃ H ₆ : 1665 volppm C
CO ₂ : 14 vol%
H ₂ O: 10vol%
The rest: N ₂
SV: 60000 ^-1
Reaction temperature: 550 ° C

(3) it was observed particle size of the Pt particles and CeO ₂ at Pt particles and CeO ₂ particle size measured TEM (transmission electron microscope).

  Table 1 shows the results of the above evaluation measurements (1) to (3).

As shown in Table 1, the catalyst powders obtained in Examples 1 and 2 are a preferred embodiment of the present invention, and the Pt particles were kept small even after the durability test. When Examples 1 and 2 are compared, it can be seen that there is a suitable range for the particle size ratio of the Pt particles.
Moreover, it was confirmed that exhaust gas can be purified over a long period of time by making the particle size of the Pt particles used for the upstream catalyst portion larger than the particle size of the Pt particles used for the downstream catalyst portion. As in Comparative Example 1, the purification rate was remarkably reduced when the Pt particles used in the initial upstream catalyst portion and the downstream catalyst portion had the same particle size.

  Further, as Reference Examples 1 to 5, the effects of the noble metal particle size and the durability temperature on the noble metal particle size were measured using various Pt / alumina powders. The results are shown in Table 2.

(Reference Example 1)
A dinitrodiammine Pt aqueous solution (Pt particle size 1 nm) was mixed with alumina powder, dried and fired to obtain Pt / alumina powder.

(Reference Example 2)
A Pt colloid aqueous solution (Pt particle size 3 nm) was mixed at a 2-methyl-2,4-pentanediol / isopropoxide Al = 5/1 weight ratio, dried and fired to obtain a Pt / alumina powder.

(Reference Examples 3-5)
Pt / alumina powder was obtained by repeating the same operation as in Reference Example 2, except that colloids having different Pt particle sizes (Pt particle sizes 6 nm, 8 nm, and 24 nm) were used.

<Evaluation measurement>
The powder obtained in each reference example was calcined in air at 700 ° C., 3 hours, 900 ° C., 3 hours.
Table 2 shows the initial and endurance Pt particle sizes.

  As shown in Table 2, it can be seen that there is a suitable range for the initial Pt particle size when the endurance temperature is different.

As mentioned above, although this invention was demonstrated in detail by the preferred Example, this invention is not limited to these Examples, A various deformation | transformation is possible within the range of the summary of this invention.
For example, the exhaust gas purification catalyst system of the present invention has a configuration in which the upstream catalyst portion and the downstream catalyst portion are arranged in tandem, and these upstream, downstream, and both catalyst portions are arranged in parallel (V-type). Etc.), another catalyst part capable of purifying the exhaust gas component is provided at any time, so that the arrangement configuration of three or more stages can be obtained.
Further, these catalyst parts can be supported on a honeycomb-shaped monolith carrier, a metal carrier or the like made of a heat-resistant material. A checkered honeycomb carrier can also be used.

It is the schematic which shows the support state of the conventional (a) noble metal particle, (b) the support state based on this invention. It is the schematic which shows the state in which the oxide has included the noble metal particle. It is a process flow figure showing an example of a colloid method. It is the schematic which shows (a) mixed solution, (b) noble metal colloid, (c) catalyst precursor, (d) catalyst. It is a process flow figure showing other examples of a colloid method. It is a schematic sectional drawing which shows a carrying state when the addition amount of a partial oxidizing agent is made to increase in order of (a)-(c). It is a process flow figure showing an example of a reverse micelle method. (A) reverse micelle, (b) state in which noble metal and transition metal are deposited in reverse micelle, (c) state in which salt and water of carrier precursor are contained in reverse micelle, (d) carrier in reverse micelle It is the schematic which shows the state which the precursor precipitated, (e) the precipitate obtained by collapsing a reverse micelle, and (f) a catalyst.

Explanation of symbols

1 PM particles (about 100nm)
2 noble metal 3 ceria 4 transition metal 5 oxide 10 noble metal ion 11 organic molecule 12 noble metal colloid 13 ceria hydroxide 14 catalyst precursor 15 noble metal supported ceria 16 noble metal particle 17 ceria 20, 30, 40 noble metal supported ceria 21, 31, 41 Noble metal particles 22, 32, 42 Ceria 50 Reverse micelle 51 Surfactant 52 Aqueous solution containing noble metal salt and transition metal salt 53 Noble metal 54 Transition metal 55 Cerium 56 Noble metal-supported ceria 57 Ceria 58 Transition metal composite particle

Claims (9)

An exhaust gas purification system comprising an upstream catalyst portion and a downstream catalyst portion arranged in this order from the upstream side of an exhaust passage of an internal combustion engine or a combustion device,
The upstream catalyst part and the downstream catalyst part comprise nano-sized noble metal particles supported on a porous inorganic oxide,
An exhaust gas purification system, wherein the particle size ratio of the noble metal particles in the upstream catalyst part and the noble metal particles in the downstream catalyst part is 2: 1 to 10: 1. The precious metal particles having a particle size of 4 to 20 nm are supported on the porous inorganic oxide in the upstream catalyst part,
2. The exhaust gas purification system according to claim 1, wherein noble metal particles having a particle size of 2 to 10 nm are supported on the porous inorganic oxide in the downstream catalyst portion. The difference between the exhaust gas temperature T _U ° C flowing into the upstream catalyst part and the exhaust gas temperature T _L ° C flowing into the downstream catalyst part is 50 to 100 ° C. An exhaust gas purification system according to one of the items. In the noble metal particles used in the upstream catalyst part and the downstream catalyst part, the following formula (1)
Exposure rate (%) = 0.895 × (A × B × C × D) / (E × F) × 100 (1)
(In the formula, A is the CO adsorption amount [cc / g], B is the supported noble metal cross-sectional area [nm ² ], C is the supported noble metal density [g / cc], D is the TEM particle radius [nm], and E is the stoichiometric amount. The theoretical ratio, F, indicates the loading concentration [% / g])
The exhaust gas purification system according to any one of claims 1 to 8, wherein an exposure rate represented by: is 50 to 85%.   The exhaust gas purification according to any one of claims 1 to 9, wherein the upstream catalyst part and / or the downstream catalyst part has a supported amount of noble metal particles of 0.72 g or less per liter of the catalyst part. system.   11. The upstream catalyst part and / or the downstream catalyst part, wherein the noble metal particles are at least one selected from the group consisting of platinum, palladium and rhodium. The exhaust gas purification system described in 1.   In the upstream catalyst part and / or the downstream catalyst part, the porous inorganic oxide and / or the noble metal particles are included in at least one oxide selected from the group consisting of ceria, alumina, zirconia, silica and titania. The exhaust gas purification system according to any one of claims 1 to 11, wherein   In the upstream catalyst part and / or the downstream catalyst part, the porous inorganic oxide and / or the noble metal particles are in contact with at least one transition metal selected from the group consisting of manganese, iron, cobalt, nickel, copper and zinc. The exhaust gas purification system according to any one of claims 1 to 12, wherein   In the upstream catalyst part and / or the downstream catalyst part, the particle size ratio of the precious metal particles after initial and durability when contacting with the exhaust gas at 900 ° C. for 3 hours is 3:10 to 7:18 The exhaust gas purification system according to any one of claims 1 to 13.

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