JP2016198759A - Exhaust purification catalyst and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust purification catalyst for exhaust purification, particularly, a composite metal fine particle contained therein and a method of producing the same.SOLUTION: A exhaust purification catalyst has composite metal fine particles comprising Rh and Pd, and when fine particles in the exhaust purification catalyst are analyzed by STEM-EDX, an average ratio of Pd to the total of Rh and Pd is 1.7 atom% or more and 24.8 atom% or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an exhaust gas purification catalyst and a method for producing the same. More specifically, the present invention relates to an exhaust gas purification catalyst with improved exhaust gas purification performance and a method for producing the same.

  There are harmful components such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides in the exhaust gas discharged from internal combustion engines for automobiles, for example, gasoline engines or diesel engines. (NOx) and the like are included.

  For this reason, in general, an exhaust gas purification device for decomposing and removing these harmful components is provided in the internal combustion engine, and these harmful components are almost harmless by the exhaust gas purification catalyst mounted in the exhaust gas purification device. It has become. As such an exhaust gas purification catalyst, for example, a three-way catalyst and a NOx storage reduction catalyst are known.

  The three-way catalyst is a catalyst that simultaneously performs oxidation of CO and HC and reduction of NOx in a stoichiometric (theoretical air-fuel ratio) atmosphere.

The NOx occlusion reduction catalyst is a catalyst that oxidizes and stores NO in exhaust gas to NO ₂ in a lean atmosphere and reduces it to nitrogen (N ₂ ) in a stoichiometric atmosphere and a rich atmosphere. And the change of the exhaust gas component in the rich atmosphere is skillfully used.

  However, even when these catalysts are employed, purification of exhaust gas is still a problem, and various studies have been made.

  The composite metal colloid of Patent Document 1 includes a plurality of metal elements, the average particle diameter of the composite metal colloid is 2 to 12 nm, and the plurality of metal elements are substantially uniformly distributed in the particles of the composite metal. ing. Specifically, Patent Document 1 discloses a composite metal colloid dispersion in which a palladium chloride solution and a rhodium chloride solution are mixed at a molar ratio of 1: 1.

  The exhaust gas purification catalyst for CO or HC purification of Patent Document 2 has an alloy containing Pd and Ag, and the alloy is supported on a carrier.

JP 2002-102679 A JP 2011-78857 A

  An object of the present invention is to provide an exhaust gas purification catalyst for exhaust gas purification, in particular, composite metal fine particles contained therein and a method for producing the same.

  The present inventors have found that the above problem can be solved by the following means.

<1> An exhaust gas purification catalyst having composite metal fine particles containing Rh and Pd,
When the fine particles in the exhaust gas purification catalyst are analyzed by STEM-EDX, the average ratio of Pd to the total of Rh and Pd is 1.7 atomic percent or more and 24.8 atomic percent or less.
The average ratio is determined by randomly selecting 10 or more fine particles from the exhaust gas purification catalyst, measuring all the selected fine particles by STEM-EDX, and further selecting Rh and Pd from all the selected fine particles. All the composite metal fine particles contained are selected, the ratio of Pd to the total of Rh and Pd of each composite metal fine particle is totaled, and this sum is divided by the total number of all the selected composite metal fine particles. Is a calculated value,
Exhaust gas purification catalyst.
<2> The exhaust gas purification catalyst according to <1>, further comprising a powder carrier, wherein the composite metal fine particles are supported on the powder carrier.
<3> The powder carrier is a powder carrier selected from the group consisting of SiO ₂ , MgO, ZrO ₂ , CeO ₂ , Al ₂ O ₃ , TiO ₂ , solid solutions thereof, and combinations thereof, <2 Exhaust gas purification catalyst according to item>.
<4> The exhaust gas purification catalyst according to <2> or <3>, wherein the powder carrier contains CeO ₂ in an amount of more than 0% by mass and 40% by mass or less with respect to the mass of the powder carrier.
<5> In a stoichiometric atmosphere, an exhaust gas containing HC, CO, and NOx is brought into contact with the exhaust gas purification catalyst according to any one of <1> to <4>, whereby HC and CO are brought into contact with each other. An exhaust gas purification method that purifies by oxidizing and reducing NOx.
<6> In a lean atmosphere, the exhaust gas purifying catalyst according to any one of <1> to <4> is brought into contact with exhaust gas containing NOx, and the NOx is reduced and purified in a rich atmosphere. An exhaust gas purification method.
<7> A method for producing an exhaust gas purification catalyst, comprising producing composite metal fine particles by heating and refluxing a solution containing Rh ions, Pd ions, a reducing agent, and a protective agent,
The molar ratio of the Rh ion and the Pd ion is 70:30 to 99: 1.
A method for producing an exhaust gas purification catalyst.
<8> The method according to <7>, comprising supporting the composite metal fine particles on a powder carrier.
<9> The powder carrier is a powder carrier selected from the group consisting of SiO ₂ , MgO, ZrO ₂ , CeO ₂ , Al ₂ O ₃ , TiO ₂ , solid solutions thereof, and combinations thereof, <8 The method according to item>.
<10> The method according to <8> or <9>, wherein the powder carrier contains CeO ₂ in an amount of more than 0% by mass and 40% by mass or less based on the mass of the powder carrier.

  ADVANTAGE OF THE INVENTION According to this invention, the exhaust gas purification catalyst for exhaust gas purification, especially the composite metal fine particle contained in this, and its manufacturing method can be provided.

Fig.1 (a) is a STEM image of the exhaust gas purification catalyst of Example 1 analyzed by the scanning transmission electron microscope (STEM-EDX) with an energy dispersive X-ray analyzer, FIG.1 (b) is implementation. It is a figure which shows the ratio (atomic%) of Pd with respect to the sum total of Rh and Pd of each fine particle about ten fine particles extracted at random from the exhaust gas purification catalyst of Example 1. FIG. 2A is an STEM image of the exhaust gas purification catalyst of Example 2 analyzed by STEM-EDX, and FIG. 2B is a graph of 10 samples randomly extracted from the exhaust gas purification catalyst of Example 2. It is a figure which shows the ratio (atomic%) of Pd with respect to the sum total of Rh and Pd of each fine particle about fine particles. FIG. 3A is an STEM image of the exhaust gas purification catalyst of Example 3 analyzed by STEM-EDX. FIG. 3B is a graph of 10 samples randomly extracted from the exhaust gas purification catalyst of Example 3. It is a figure which shows the ratio (atomic%) of Pd with respect to the sum total of Rh and Pd of each fine particle about fine particles. 4A is an STEM image of the exhaust gas purification catalyst of Comparative Example 1 analyzed by STEM-EDX, and FIG. 4B is a graph of 10 samples randomly extracted from the exhaust gas purification catalyst of Comparative Example 1. It is a figure which shows the ratio (atomic%) of Pd with respect to the sum total of Rh and Pd of each fine particle about fine particles. FIG. 5A is an STEM image of the exhaust gas purification catalyst of Comparative Example 3 analyzed by STEM-EDX, and FIG. 5B is a graph of 10 samples randomly extracted from the exhaust gas purification catalyst of Comparative Example 3. It is a figure which shows the ratio (atomic%) of Pd with respect to the sum total of Rh and Pd of each fine particle about fine particles. FIG. 6 is a diagram showing the relationship between time (minutes) and temperature (° C.) regarding the evaluation conditions for the three-way catalyst. FIG. 7 is a graph showing the relationship between the temperature (° C.) and the NOx purification rate (%) when the activities of the exhaust gas purification catalysts of Examples 1 to 3 and Comparative Examples 1 to 3 are evaluated. FIG. 8 is a diagram showing the relationship between the ratio (atomic%) of Pd to the total of Rh and Pd of the exhaust gas purification catalysts of Examples 1 to 3 and Comparative Example 1 and the NOx 50% purification temperature (° C.). FIG. 9 is a diagram showing the relationship between the exhaust gas purification catalysts of Examples 1 to 3 and Comparative Examples 1 to 3 and the NOx 50% purification temperature (° C.). FIG. 10 shows Examples 1 ′ to 3 ′ and Comparative Examples in which the carriers of Examples 1 to 3 and Comparative Example 1 were replaced with Al ₂ O ₃ —ZrO ₂ —TiO ₂ and barium acetate was added thereto. It is a figure which shows the relationship between time (second) and NOx discharge | emission amount (ppm) when the sample of the exhaust gas purification catalyst of Example 1 'is exposed to lean atmosphere and rich atmosphere. FIG. 11 is an enlarged view of a part of FIG. FIG. 12 is a schematic diagram showing one cycle of a NOx storage reduction cycle composed of a lean atmosphere and a rich atmosphere in the performance evaluation B as a NOx storage reduction catalyst. FIG. 13 shows the content (mass%) of CeO _{2 in} the powder carrier and NOx emissions / NOx occlusion for the exhaust gas purification catalyst samples of Examples 4 to 11 (●) and Comparative Examples 4 to 11 (■). It is a figure which shows the relationship with quantity (au). FIG. 14 shows the content (mass%) of CeO _{2 in} the powder carrier and the NOx purification rate (%) for the exhaust gas purification catalyst samples of Examples 4 to 11 (●) and Comparative Examples 4 to 11 (■). It is a figure which shows the relationship. FIG. 15 is a diagram illustrating the relationship between time and temperature with respect to the evaluation conditions by the temperature-programmed reduction (TPR) method. FIG. 16 shows the temperature (° C.) and H ₂ consumption (TCD signal) when the temperature reduction method was applied to the exhaust gas purification catalyst samples of Example 6 ′ (solid line) and Comparative Example 6 ′ (dotted line). It is a figure which shows the relationship with (au).

  Hereinafter, embodiments of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments, and can be implemented with various modifications within the scope of the gist of the present invention.

<Exhaust gas purification catalyst>
The exhaust gas purification catalyst of the present invention has composite metal fine particles containing Rh and Pd, and when the fine particles in the exhaust gas purification catalyst are analyzed by STEM-EDX, the average ratio of Pd to the total of Rh and Pd Is 1.7 atomic percent or more and 24.8 atomic percent or less.

With respect to conventional exhaust gas purification catalysts, a platinum group element such as platinum (Pt), rhodium (Rh), and palladium (Pd) is supported on a porous oxide carrier such as alumina (Al ₂ O ₃ ). Catalysts are widely known.

  Among them, Rh has a high NOx reduction ability and is useful as a catalyst metal constituting an exhaust gas purification catalyst.

  However, Rh is easily oxidized, and this oxidation may reduce the NOx reduction ability. For this reason, an excess amount of Rh is contained in the exhaust gas purification catalyst in anticipation of oxidation of Rh. However, since Rh is a very expensive rare metal, its excessive use is undesirable from an economic and environmental point of view.

  Therefore, the present inventors have focused on Pd, which has a relatively weak affinity with oxygen, and as a result of diligent efforts, the present inventors contain Rh and Pd, and the average ratio of Pd to the total of Rh and Pd is 1.7 atomic% or more. We have developed composite metal microparticles of 24.8 atomic% or less.

  In this composite metal fine particle, since Pd suppresses the oxidation of Rh, the metal state of Rh can be maintained for a long time. Thus, conventionally, Rh's catalytic activity can be maintained or improved even under conditions in which Rh is easily oxidized, for example, conditions such as gas composition, pressure, and temperature, and Rh can be added in an appropriate amount. It became possible to use.

  Further, the inventors have high exhaust gas purifying ability, and as a result, the composite metal fine particles containing Rh and Pd exert a synergistic exhaust gas purifying effect by these two kinds of metal elements. I found out that I can do it.

  The exhaust gas purification catalyst of the present invention optionally further has a powder carrier, and the composite metal fine particles are supported on the powder carrier.

  When the composite metal fine particles are supported on this powder carrier, the contact surface between the exhaust gas and the composite metal fine particles can be increased because the specific surface area of the powder carrier is large. Thereby, the performance of the exhaust gas purification catalyst can be improved.

<Composite metal fine particles>
The composite metal fine particles contain Rh and Pd.

  When the particle diameter of the composite metal fine particles is sufficiently small, the specific surface area is increased, the number of Rh NOx active sites and the number of Pd active sites are increased, and the NOx reducing ability of the exhaust gas purification catalyst may be improved. is there.

  Moreover, when the particle diameter of the composite metal fine particles is moderately large, there is a possibility that the NOx purification ability of the exhaust gas purification catalyst can be sufficiently exhibited.

  Accordingly, the average particle size of the plurality of composite metal fine particles is not particularly limited, and examples include an average particle size of more than 0 nm, 1 nm or more, or 2 nm or more. Further, the average particle diameter of the plurality of composite metal fine particles is not particularly limited, and examples thereof include an average particle diameter of 100 nm or less, 70 nm or less, 40 nm or less, 10 nm or less, 7 nm or less, 5 nm, 4 nm, or 3 nm or less. .

  Specifically, the particle diameter of the composite metal fine particles is preferably in the range of 1 nm to 10 nm, more preferably in the range of 2 nm to 5 nm, and still more preferably in the range of 2 nm to 3 nm.

  By using composite metal fine particles having such a particle size as a catalyst component, it is possible to realize composite metal fine particles in which Rh and Pd coexist at the nano level, and thereby, the effect of suppressing oxidation of Rh by Pd. Can be demonstrated. Therefore, an exhaust gas purification catalyst with improved NOx purification performance can be obtained.

  In the present invention, the “average particle diameter” means a circle equivalent diameter (Heywood diameter) of 10 or more particles randomly selected using means such as a scanning transmission electron microscope (STEM) unless otherwise specified. ) Is the arithmetic average value of those measured values.

  When the average ratio of Pd to the total of Rh and Pd of the composite metal fine particles is sufficiently large, the effect of suppressing the oxidation of Rh by Pd is easily exhibited. Moreover, when this ratio is not too large, it is possible to secure a sufficient number of Rh NOx active points.

  In particular, the average ratio of Pd to the sum of Rh and Pd of the plurality of composite metal fine particles is 1.7 atomic% or more, 2 atomic% or more, 3 atomic% or more, 4 atomic% or more, and 5 atomic% or more, and And / or 24.8 atomic% or less, 20 atomic% or less, 15 atomic% or less, 13 atomic% or less, 10 atomic% or less, 8 atomic% or less, and 6 atomic% or less.

  Further, the fact that the ratio of Pd to the sum of Rh and Pd in a large number of composite metal fine particles is close to the average ratio of Pd to the sum of Rh and Pd in a plurality of composite metal fine particles, respectively, is close to the average ratio of Pd. This means that there are a large number of composite metal fine particles having the above ratio. For example, when the average ratio of Pd is such a ratio that the effect of the exhaust gas purifying catalyst of the present invention is remarkably exhibited, the composite metal fine particles having a Pd ratio close to the average ratio of Pd The presence of a large number means that the effects of the exhaust gas purifying catalyst of the present invention can be further improved.

  In this regard, the ratio of Pd to the total of Rh and Pd of the composite metal fine particles of 70%, 75%, 80%, 85%, 90%, or 95% or more on the basis of the number is not particularly limited. 20% or more, 30% or more, 40% or more, and 50% or more, and 190% or less, 180% or less, 170% or less, and 160% or less of the average ratio of Pd to the sum of Rh and Pd in the metal fine particles A percentage can be mentioned.

  As a result, the effect of suppressing the oxidation of Rh by Pd can be efficiently exhibited while maintaining the number of Rh active points sufficient for NOx purification. Therefore, it is possible to obtain an exhaust gas purification catalyst having a significantly improved NOx reduction ability.

  Note that the ratio of Pd to the total of Rh and Pd in the plurality of composite metal fine particles is intentionally separated from the average ratio of Pd to the total of Rh and Pd in the plurality of composite metal fine particles (for example, Rh in the plurality of composite metal fine particles Needless to say, it is typically easy that the average ratio of Pd to the total of Pd and Pd is 1% or less, or 400% or more.

  In the present invention, the “ratio of Pd to the sum of Rh and Pd” means the ratio of the number of Pd atoms to the total number of Rh atoms and Pd atoms contained in the composite metal fine particles. The “ratio of Pd to the total of Rh and Pd” in the present invention is a value calculated by analyzing composite metal fine particles using an optical method such as STEM-EDX, for example. Further, in the present invention, “the average ratio of Pd to the total of Rh and Pd” is, for example, randomly selected from 10 or more, 100 or more, or 1000 or more fine particles from the exhaust gas purification catalyst. Each of the composite metal fine particles is measured by STEM-EDX or the like, and all the composite metal fine particles containing Rh and Pd are further selected from all the selected fine particles, and Pd relative to the sum of Rh and Pd of each composite metal fine particle is selected. Is calculated by dividing the total by the total number of all selected composite metal fine particles.

  In the present invention, the “number basis” ratio means the ratio of the number of composite metal fine particles having a specific composition to the number of all composite metal fine particles of the exhaust gas purification catalyst, unless otherwise specified. The composite metal fine particles of the exhaust gas purification catalyst of the present invention have an excellent exhaust gas purification ability. Therefore, when at least 70% or more of the composite metal fine particles have a preferred composition on a number basis, the exhaust gas purification catalyst of the present invention is more converted in terms of its specific mass, specific volume, or specific surface area. It should be understood that a large amount of exhaust gas can be purified.

<Powder carrier>
The composite metal fine particles of the present invention can be supported on a powder carrier.

  The powder carrier on which the composite metal fine particles are supported is not particularly limited, but may be any metal oxide generally used as a powder carrier in the technical field of exhaust gas purification catalysts.

Examples of such powder carriers include silica (SiO ₂ ), magnesium oxide (MgO), zirconia (ZrO ₂ ), ceria (CeO ₂ ), alumina (Al ₂ O ₃ ), titania (TiO ₂ ), and the like. And a combination thereof.

An acidic carrier such as SiO ₂ has good compatibility with a catalyst metal that reduces NOx. A basic carrier such as MgO has good compatibility with K and Ba that occludes NOx. ZrO ₂ suppresses the sintering of other powder carriers at a high temperature at which other powder carriers cause sintering, and combines with Rh as a catalytic metal to cause a steam reforming reaction to generate Hr. ₂ can be produced and NOx can be reduced efficiently. CeO ₂ has an OSC (Oxygen Storage Capacity) characteristic of occluding oxygen in a lean atmosphere and releasing oxygen in a rich atmosphere. Therefore, it can be suitably used for a three-way catalyst or the like. Since an acid-base amphoteric carrier, for example, Al ₂ O ₃ has a high specific surface area, it can be used to efficiently store and reduce NOx. TiO ₂ can exhibit the effect of suppressing sulfur poisoning of the catalyst metal.

  According to the above characteristics of the powder carrier, the exhaust gas purification ability of the exhaust gas purification catalyst of the present invention, in particular, the NOx purification ability can be improved depending on the type, composition, combination and ratio and / or amount of the selected powder carrier. Please understand that there is sex.

<Relationship between Rh fine particles, CeO ₂ , and NOx storage reduction mechanism>
(NOx storage reduction mechanism)
As described above, the NOx occlusion / reduction mechanism is a mechanism that oxidizes and stores NO in exhaust gas into NO ₂ in a lean atmosphere and reduces the NO ₂ to nitrogen (N ₂ ) in a stoichiometric atmosphere and a rich atmosphere.

(Rh and NOx behavior when Rh fine particles are used in the NOx storage and reduction mechanism)
As described above, Rh is a metal that easily reduces NOx, while it is a metal that is relatively easily oxidized. The Rh fine particles are almost oxidized in a lean atmosphere, particularly in a lean atmosphere at a low temperature. It is considered that when the Rh oxide fine particles are exposed to a rich atmosphere, the Rh oxide fine particles are reduced to Rh metal fine particles, thereby expressing the NOx reducing ability of Rh. That is, in order for Rh to exhibit the NOx reduction ability, Rh needs to be in the state of metal fine particles, not its oxide fine particles.

(CeO ₂ and NOx behavior when CeO ₂ is employed in the NOx storage and reduction mechanism)
CeO ₂ has the characteristic of easily adsorbing NO ₂ in addition to the above OSC characteristics. Therefore, CeO ₂ performs oxygen storage and NO ₂ adsorption in a lean atmosphere, and releases oxygen and NO ₂ in a rich atmosphere. Incidentally, easily why CeO ₂ adsorbs NO ₂ is, CeO ₂ is considered because it is basic compared to the _Al 2 _{O 3.}

The inventors have found that CeO ₂ suppresses the reduction of Rh oxide in a rich atmosphere, resulting in insufficient NOx reduction, which makes certain combinations of Rh and CeO ₂ unsuitable for NOx storage and reduction mechanisms. I found it.

Therefore, for example, in an exhaust gas purification catalyst containing Rh fine particles and CeO ₂ , NOx occluded in a lean atmosphere cannot be sufficiently reduced in a rich atmosphere with respect to the NOx occlusion / reduction mechanism. Are exhausted, which can result in high NOx spikes being observed. The “NOx spike” means a phenomenon in which the NOx emission amount instantaneously increases when the lean atmosphere is switched to the rich atmosphere.

In this regard, the present inventors have intensively studied, and when an exhaust gas purifying catalyst containing composite metal fine particles containing Rh and Pd and CeO ₂ as a powder carrier is adopted, the occurrence of such a problem is caused. It was found that it can be suppressed.

  Although not limited by any principle, this is because, in the above-mentioned composite metal fine particles, Pd can suppress oxidation of Rh, so that Rh oxide can be easily converted into Rh metal in a rich atmosphere, particularly in a rich atmosphere at low temperature. It is thought that it is reduced.

Therefore, when the exhaust gas purifying catalyst of the present invention includes the composite metal fine particles and a powder carrier containing CeO ₂ , the NOx adsorption amount in a lean atmosphere is improved, In particular, the catalytic activity of Rh can be improved in a rich atmosphere at low temperatures. Therefore, when the exhaust gas purification catalyst of the present invention contains CeO ₂ as a powder carrier, the purification ability of exhaust gas, particularly exhaust gas containing NOx, can be improved.

In particular, in the exhaust gas purification catalyst of the present invention, the powder carrier preferably contains CeO ₂ in an amount of more than 0% by mass and 40% by mass or less based on the mass of the powder carrier.

When the powder carrier contains CeO ₂ in a relatively large proportion with respect to the mass of the powder carrier, for example, more than 0% by mass, the above NOx adsorption ability can be improved. Therefore, the amount of CeO ₂ contained in the powder carrier is more than 0% by mass, 5% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, or 24 with respect to the mass of the powder carrier. The mass% or more may be sufficient.

When the powder carrier contains CeO ₂ in a relatively small proportion with respect to the mass of the powder carrier, for example, 40% by mass or less, oxidation of Rh with CeO ₂ and / or CeO _{2 in a} rich atmosphere. Consumption of a reducing agent such as a hydrocarbon due to released oxygen can be sufficiently suppressed. Therefore, the amount of CeO ₂ contained in the powder carrier may be 40% by mass or less, 35% by mass or less, or 29% by mass or less based on the mass of the powder carrier.

  The amount of the composite metal fine particles supported by the powder carrier is not particularly limited, but for example, generally 0.01 parts by mass or more, 0.05 parts by mass or more, 0.1 parts by mass with respect to 100 parts by mass of the powder carrier. The supported amount may be not less than 0.5 parts by mass, not less than 0.5 parts by mass, or not less than 1 part by mass, and / or not more than 5 parts by mass, not more than 3 parts by mass, or not more than 1 part by mass.

<NOx storage material>
The exhaust gas purification catalyst of the present invention optionally contains a NOx storage material.

  The NOx storage material is not particularly limited, but may be a basic material. Examples of NOx storage materials include alkali metals and salts thereof, such as potassium (K) and potassium acetate; alkaline earth metals and salts thereof, such as barium (Ba) and barium acetate; and combinations thereof Can do.

<Exhaust gas purification method>
In the method of the present invention for purifying exhaust gas, the exhaust gas containing HC, CO, and NOx is brought into contact with the exhaust gas purification catalyst of the present invention in a stoichiometric atmosphere, thereby oxidizing HC and CO, and Reduce and purify.

The method of the present invention is preferably applied to an internal combustion engine operating in a stoichiometric atmosphere. In the stoichiometric atmosphere, HC and CO as reducing agents and NOx as oxidizing agents react at a theoretical equivalent ratio, and these can be converted into H ₂ O, CO ₂ , and N ₂ .

  The exhaust gas purification catalyst of the present invention may be contacted with exhaust gas in a stoichiometric atmosphere, or may be an optional method.

  Further, the method of the present invention for purifying exhaust gas brings NOx-containing exhaust gas into contact with the exhaust gas purification catalyst of the present invention in a lean atmosphere, and reduces and purifies the NOx in a rich atmosphere.

  The method of the present invention is preferably applied to an internal combustion engine operating in a lean atmosphere. This is because, in a lean atmosphere, HC and CO are easily oxidized and purified, while NOx is difficult to reduce and purify, so a large amount of NOx is generated.

  As a method of bringing the exhaust gas containing NOx into contact with the exhaust gas purification catalyst of the present invention in a lean atmosphere, an optional method can be adopted.

<Method for producing exhaust gas purification catalyst>
The method of the present invention for producing an exhaust gas purification catalyst includes a step of producing composite metal fine particles by heating and refluxing a solution containing Rh ions, Pd ions, a reducing agent, and a protective agent. The molar ratio of the Pd ions is 70:30 to 99: 1.

  In general, nano-sized metal fine particles have an electron energy structure different from that of the bulk due to the quantum size effect, and thus exhibit electrical and optical characteristics depending on the particle size. Furthermore, it is expected that nano-sized metal fine particles having a very large specific surface area will work as a highly active catalyst.

  With regard to such a method for producing nano-sized metal fine particles, a reducing agent such as alcohol is added to a mixed solution containing a salt of each metal element, and the mixed solution is heated in the mixed solution as necessary. A chemical reduction method is known in which ions of each metal element contained are simultaneously reduced.

  However, when different metal ions are chemically reduced, generally, ions of metal elements having a high redox potential are reduced first. For this reason, a metal element having a high redox potential is present in the center region of the fine particles, and a composite metal fine particle having a metal element having a low redox potential around it, that is, a composite metal fine particle having a so-called core-shell structure may be generated. is there.

  In the composite metal fine particles having the core-shell structure, since the elements are not uniformly distributed, the ability of the catalyst may not be sufficiently exhibited.

  Therefore, for example, in Patent Document 1, a composite metal colloid dispersion in which a palladium chloride solution and a rhodium chloride solution are mixed at a molar ratio of 1: 1 is irradiated with a laser beam to uniformly distribute the elements in the composite metal colloid fine particles. The method of making is disclosed.

  In contrast, the method of the present invention does not use a laser or the like. Therefore, composite metal fine particles containing Rh and Pd and having a uniform element distribution can be generated with a smaller number of steps. For this reason, the manufacturing method of an economical and environmentally friendly exhaust gas purification catalyst can be provided.

  Although not limited by any principle, the reason why composite metal fine particles containing Rh and Pd and having a uniform element distribution can be generated from the liquid phase without using a laser or the like is the oxidation of Rh This is probably because the reduction potential (0.758 V) and the redox potential (0.99 V) of Pd are close to each other, so that reduction of each metal ion occurs almost simultaneously.

  Although it does not specifically limit as time to heat-reflux the solution containing Rh ion, Pd ion, a reducing agent, and a protective agent, 0.5 hours or more, 1 hour or more, 1.5 hours or more, and 3 hours The above and / or 48 hours or less, 24 hours or less, 12 hours or less, and 6 hours or less can be mentioned.

  In addition, the method of the present invention may optionally further include a step of supporting the composite metal fine particles on the powder carrier during or after the step of generating the composite metal fine particles.

  The order and method of supporting the composite metal fine particles on the powder carrier may be any order and method. The order and method of supporting the composite metal fine particles on the powder carrier is, for example, heating and refluxing a solution containing Rh ions, Pd ions, a reducing agent, and a protective agent, and then adding the powder carrier to this solution and stirring. The order and method of supporting the composite metal fine particles on the powder carrier may be used. Thereby, the composite metal fine particles can be efficiently supported on the powder carrier.

<Rh ion and Pd ion>
Rh ions and Pd ions are contained in a solution containing a reducing agent and a protective agent.

  Although it does not specifically limit as a raw material of Rh ion, For example, the salt of Rh, Rh halide, etc., and these combinations can be mentioned. As raw materials for Rh ions, inorganic salts of Rh, such as nitrates, phosphates, and sulfates; organic salts of Rh, such as oxalates and acetates; halides of Rh, such as fluorides , Chlorides, bromides, and iodides; and combinations thereof.

  Regarding the Pd ion raw material, the above description of the Rh ion raw material can be referred to.

  The concentration of Rh ions and Pd ions is not particularly limited. The concentrations of Rh ions and Pd ions are preferably in the range of 0.01M to 0.20M.

  Although it does not specifically limit as molar ratio of Rh ion and Pd ion, It may correlate with the molar ratio of Rh and Pd in the target composite metal microparticles, for example, a molar ratio of 70:30 to 99: 1, 75 : The molar ratio of 25-95: 5 and the molar ratio of 80: 20-90: 10 can be mentioned.

  The molar ratio of Rh ions and Pd ions is not particularly limited as long as the composite metal fine particles of the above-described exhaust gas purification catalyst of the present invention can be produced. These molar ratios may be correlated with the average ratio of Pd to the total of Rh and Pd of the composite metal fine particles of the exhaust gas purification catalyst of the present invention. In this case, these molar ratios may be determined in consideration of a reduction scale of those ions, for example, a redox potential and easiness of solid solution of each element.

<Reducing agent>
The reducing agent is contained in a solution containing Rh ions, Pd ions, and a protective agent.

  A reducing agent can be used to reduce Rh ions and Pd ions to produce composite metal particulates. Further, the reducing agent may optionally have a function as a solvent.

  Although it does not specifically limit as a reducing agent, For example, the reducing agent which has a boiling point of 95 degreeC or more, 100 degreeC or more, 110 degreeC or more, or 120 degreeC or more in a standard state is preferable.

  When the reducing agent has the above boiling point, Rh ions and Pd ions can be efficiently reduced, and composite metal fine particles having a uniform elemental distribution of Rh and Pd can be generated. Therefore, the temperature at which the solution containing the Rh ion, Pd ion, reducing agent, and protective agent is heated to reflux is preferably a temperature equal to or higher than the boiling point of the reducing agent.

  Although it does not specifically limit as a reducing agent, Alcohol, glycol, an aldehyde, etc., and these combinations can be mentioned. Examples of the reducing agent include alcohols such as propanol, butanol, and pentanol; glycols such as ethylene glycol; aldehydes such as valeraldehyde; and combinations thereof.

  The amount of the reducing agent is not particularly limited, but the molar amount in the range of 1 to 100000 times, the molar amount in the range of 1 to 50000 times, and 1 to 10000 times the total molar amount of Rh and Pd. A mol amount in the double range can be mentioned.

<Protective agent>
The protective agent is contained in a solution containing Rh ions, Pd ions, and a reducing agent.

  The protective agent prevents excessive aggregation of the composite metal fine particles, and can appropriately disperse the composite metal fine particles in the solution. Therefore, the protective agent can appropriately disperse a plurality of substantially uniform nano-sized composite metal fine particles in the exhaust gas catalyst.

  The protective agent is not particularly limited, but polyvinyl pyrrolidone (PVP), polyvinyl pyrrolidone K25 (PVP-K25), polyethyleneimine, polyallylamine, poly (N-carboxymethyl) allylamine, poly (N, N-dicarboxymethyl) , Allylamine, and poly (N-carboxymethyl) ethyleneimine, and the like, and combinations thereof. Among these, PVP is preferable from the viewpoint of high solubility.

  The concentration of the protective agent is not particularly limited as long as aggregation of metal fine particles can be prevented. For example, the concentration is 1 to 1000 times the molar amount of Rh and Pd. Mention may be made of a mol amount in the range of ˜500 times and a mol amount in the range of 1 to 100 times. Here, when the protective agent is a polymer such as PVP, the molar amount of the protective agent means the molar amount of the monomer unit.

<solvent>
The solvent is optionally contained in a solution containing Rh ions, Pd ions, a reducing agent, and a protective agent.

  The solvent is not particularly limited. The boiling point of the solvent is preferably higher than the boiling point of the reducing agent.

<Others>
Regarding the components of the method of the present invention, reference can be made to the above description of the exhaust gas purification catalyst.

  The present invention will be described in more detail with reference to the following examples, but it goes without saying that the scope of the present invention is not limited by these examples.

<< Example 1 (liquid phase reduction method) >>
<Preparation of mixed solution containing catalytic metal>
0.078 mmol of Pd chloride (PdCl ₂ ) as Pd ions and 5 mL of distilled water were weighed and mixed in a 100 mL beaker. In addition, 1.477 mmol of Rh chloride (RhCl ₃ ) as Rh ions and 5 mL of distilled water were weighed and mixed in a 100 mL beaker. Solution A was prepared by mixing these Pd and Rh solutions.

  Solution B was prepared by measuring 15052 mL of PVP-K25 as a protective agent and 150 mL of 1-propanol as a reducing agent, stirring them in a 500 mL separable flask, and dissolving PVP-K25.

The solution A was washed into a 500 mL separable flask containing the solution B with 150 mL of 1-propanol, and the mixed solution was stirred. Next, a 500 mL separable flask containing the mixed solution was immersed in an oil bath at 102 ° C., and heated under reflux for 1.5 hours while performing N ₂ bubbling. After heating to reflux, the above mixed solution was cooled to room temperature.

<Supporting catalyst metal on powder carrier>
On the other hand, the beaker 500 mL, the _{Al 2 O} 3 -CeO 2 -ZrO 2 as powder carrier took 80g weighed. The above mixed solution was added to the 500 mL beaker and stirred, and the solvent was evaporated by hot water bath. Moreover, after drying this residue overnight in a drying furnace, this was crushed and fired at 500 ° C. for 2 hours in a baking furnace.

<Press of fired product>
The fired product was taken out from the firing furnace, placed in a bag for CIP (cold isostatic processing), and vacuum packed. This was pressed at 1 ton / cm ² , sieved and struck with a pestle to pelletize. This pellet was used as a sample of the exhaust gas purification catalyst. When the fired product is aggregated when the fired product is taken out from the firing furnace, the fired product may be crushed with a pestle.

<< Examples 2 and 3 and Comparative Examples 1 and 2 (Liquid Phase Reduction Method) >>
Exhaust gas purification catalyst samples of Examples 2 and 3 and Comparative Examples 1 and 2 were prepared in the same manner as in Example 1 except that the molar amounts of Rh and Pd were changed.

<< Comparative Example 3 (impregnation method) >>
<Preparation of mixed solution containing catalytic metal>
0.09 g of Pd nitrate (8.2% by mass) as Pd ions was weighed and 1.57 g of Rh nitrate (2.75% by mass) as Rh ions was weighed. These were stirred together with 50 mL of ion-exchanged water in a 100 mL beaker to prepare solution C.

<Supporting catalyst metal on powder carrier>
On the other hand, the _{Al 2 O} 3 -CeO 2 -ZrO 2 as powder carrier took 25g weighed. This was stirred with a 500 mL beaker together with 150 mL of ion exchange water to prepare a solution D.

  The solution C and the solution D were mixed while being washed with ion-exchanged water to prepare a solution E. This solution E was evaporated to dryness. The dried product was dried overnight in a drying furnace, and then baked at 500 ° C. for 2 hours in a baking furnace.

<Press of fired product>
The fired product was taken out from the firing furnace, placed in a bag for CIP (cold isostatic processing), and vacuum packed. This was pressed at 1 ton / cm ² , sieved and struck with a pestle to pelletize. This pellet was used as a sample of the exhaust gas purification catalyst. When the fired product is aggregated when the fired product is taken out from the firing furnace, the fired product may be crushed with a pestle.

  The molar amounts of Rh and Pd used in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1 below.

“85 ^: 15 ^* ” in Table 1 indicates that Rh fine particles and Pd fine particles are individually present in the sample of Comparative Example 3, and Rh and Pd are contained at a molar ratio of 85:15. Show.

<STEM-EDX analysis>
STEM-EDX is applied to the exhaust gas purification catalyst samples prepared in Examples 1 to 3 and Comparative Examples 1 to 3, thereby extracting a plurality of metal fine particles as measurement points from the STEM image. The composition and particle size of the metal fine particles were evaluated. The results of Examples 1 to 3 and Comparative Examples 1 and 3 are shown in FIGS. In addition, the average particle diameter of the sample of any example was about 3 nm.

  FIG. 1 (a) shows that fine particles are dispersed in the exhaust gas purification catalyst.

  FIG. 1B shows that the fine particles are composite metal fine particles containing Rh and Pd. Further, FIG. 1B shows that the ratio of Pd to the total of Rh and Pd in the composite metal fine particles is in the range of 2 atomic% to 13.5 atomic%. Further, the average ratio of Pd to the total of Rh and Pd in the plurality of composite metal fine particles is 7.5 atomic% by arithmetically averaging the ratio of Pd to the total of Rh and Pd in each composite metal fine particle. Is understood.

  Therefore, it is understood that the ratio of Pd to the sum of Rh and Pd in the composite metal fine particles is in the range of 27% to 180% of the average ratio of Pd to the sum of Rh and Pd in the plurality of composite metal fine particles. .

  FIG. 2A shows that fine particles are dispersed in the exhaust gas purification catalyst.

  FIG. 2B shows that the fine particles are composite metal fine particles containing Rh and Pd. Further, FIG. 2B shows that the ratio of Pd to the total of Rh and Pd in the composite metal fine particles is in the range of 12 atomic% to 31 atomic%. Further, the average ratio of Pd to the total of Rh and Pd in the plurality of composite metal fine particles is 16.5 atomic% by arithmetically averaging the ratio of Pd to the total of Rh and Pd in each composite metal fine particle. Is understood.

  Therefore, it is understood that the ratio of Pd to the sum of Rh and Pd in the composite metal fine particles is in the range of 72.7% to 188% of the average ratio of Pd to the sum of Rh and Pd in the plurality of composite metal fine particles. Is done.

  FIG. 3A shows that fine particles are dispersed in the exhaust gas purification catalyst.

  FIG. 3B shows that the fine particles are composite metal fine particles containing Rh and Pd. 3B shows that the ratio of Pd to the total of Rh and Pd in the composite metal fine particles is in the range of 5.5 atomic% to 46.4 atomic%. Further, the average ratio of Pd to the total of Rh and Pd in the plurality of composite metal fine particles is 24.8 atomic% by arithmetically averaging the ratio of Pd to the total of Rh and Pd in each composite metal fine particle. Is understood.

  Therefore, it is understood that the ratio of Pd to the total of Rh and Pd in the composite metal fine particles is in the range of 22% to 187% of the average ratio of Pd to the total of Rh and Pd in the plurality of composite metal fine particles. .

  As described above, the ratio of Pd to the total of Rh and Pd in the composite metal fine particles is in the range of about 20% to about 190% of the average ratio of Pd to the total of Rh and Pd in the plurality of composite metal fine particles. In other words, this means that there are a large number of composite metal fine particles having a Pd ratio close to the average ratio of Pd.

  FIG. 4A shows that fine particles are dispersed in the exhaust gas purification catalyst. FIG. 4B shows that the fine particles are metal fine particles containing only Rh.

  FIG. 5A shows that fine particles are dispersed in the exhaust gas purification catalyst. FIG. 5B shows that the fine particles are metal fine particles containing only Rh or only Pd.

《Catalyst evaluation A》
About the sample of the exhaust gas purification catalyst of Examples 1-3 and Comparative Examples 1-3, the performance as a three-way catalyst was evaluated, and about the sample of the exhaust gas purification catalyst of Examples 1-3 and Comparative Example 1, NOx occlusion reduction The performance as a catalyst was evaluated.

<Performance evaluation A as a three-way catalyst>
In performance evaluation A as a three-way catalyst, a gas flow type catalyst evaluation apparatus was used. Specifically, the composition of the test gas after contacting the sample was measured by circulating the test gas through the catalyst evaluation apparatus and using infrared spectroscopy (FT-IR).

Note that the mass of the above sample is 3 g, and the test gases in the evaluation of the three-way catalyst are NO: 0.15%, O ₂ : 0.70%, CO ₂ : 10.00%, CO: 0.00. 65%, C ₃ H ₆ : 0.10%, H ₂ O: 3.00%, N ₂ : Balance.

  Moreover, the flow rate of the test gas was set to 20 L / min, the temperature increase rate during evaluation was set to 20 ° C./min, and the temperature of the evaluation was in the range of 100 ° C. to 400 ° C. An outline of the measurement conditions is shown in FIG.

  Specifically, the evaluation of the three-way catalyst is performed by measuring the temperature (° C.) and the NOx purification rate (%); and the average ratio (atomic%) of Pd to the total of Rh and Pd and the NOx 50% purification temperature (° C.). Performed by measurement. The results are shown in FIGS. 7 and 8, respectively.

(Measurement of temperature (° C) and NOx purification rate (%))
FIG. 7 is a graph showing the relationship between the temperature (° C.) and the NOx purification rate (%) when the activities of the exhaust gas purification catalysts of Examples 1 to 3 and Comparative Examples 1 to 3 are evaluated. From FIG. 7, the following (i) to (iii) can be seen:

  (I) Compared to the temperature of the sample of Comparative Example 1 having only Rh fine particles in a range where the NOx purification rate is about 0% to about 60%, composite metal fine particles containing Rh and Pd The temperature of the samples of Examples 1 to 3 having a lower temperature is lower. In particular, in the range where the NOx purification rate (%) is about 0% to about 95%, the temperature of the samples of Examples 1 and 2 is lower than the temperature of the sample of Comparative Example 1;

  (Ii) Compared with the temperature of the sample of Comparative Example 2 having only Pd fine particles in the range where the NOx purification rate is about 5% to 95%, the composite metal fine particles containing Rh and Pd The temperature of the samples of Examples 1 to 3 is lower;

  (Iii) The NOx purification rate of the samples of Examples 1 to 3 containing the composite metal fine particles of Rh and Pd as compared with the NOx purification rate of the sample of Comparative Example 3 containing the Rh fine particles and the Pd fine particles Is higher in the range of at least about 260 ° C to 400 ° C;

  Therefore, from the above (i) to (iii), the NOx purification rate of the exhaust gas purification catalyst having composite metal fine particles containing Rh and Pd is the same as that of conventional Rh fine particles, Pd fine particles, or Rh fine particles and Pd fine particles. It is understood that the NOx purification rate of the exhaust gas purification catalyst containing NO is higher and the activity is higher.

  This is presumably because the NOx purification ability was maintained or improved by suppressing the oxidation of Rh and maintaining the Rh metal state for a long time.

(Measurement of the average ratio (atomic%) of Pd to the total of Rh and Pd and NOx 50% purification rate temperature (° C.))
FIG. 8 is a graph showing the relationship between the average ratio (atomic%) of Pd and the NOx 50% purification temperature (° C.) with respect to the sum of Rh and Pd in the exhaust gas purification catalysts of Examples 1 to 3 and Comparative Example 1. FIG. 8 shows Examples 1 to 3 having composite metal fine particles containing Rh and Pd as compared with the sample of Comparative Example 1 containing Rh fine particles (Pd: 0 atomic%). It can be seen that the NOx purification rate of 50% is achieved at a lower temperature in this sample.

  A curve showing the relationship between the average ratio (atomic%) of Pd to the total of Rh and Pd and the NOx 50% purification temperature (° C.) is a downwardly convex curve. From FIG. 8, it can be seen that there is a temperature range where the NOx 50% purification temperature is lower than the conventional temperature, that is, a temperature range where the exhaust gas purification catalyst is activated. Therefore, it is understood that the average ratio of Pd to the sum of Rh and Pd in the region is the average ratio of Pd to the sum of Rh and Pd suitable for purifying NOx.

  When comparing the NOx 50% purification temperature of the samples of Examples 1 to 3 and the sample of Comparative Example 1, the NOx 50% purification temperature (294.8 ° C.) of the sample of Example 3 (Pd: 24.8 atomic%) is compared. It is lower than the 50% NOx purification temperature (296.9 ° C.) of the sample of Example 1 (Pd: 0 atomic%). Further, from the curve of FIG. 8, when the average ratio of Pd to the total of Rh and Pd is more than 0 atomic% and less than about 27 atomic%, the NOx 50% purification temperature may be lower than 296.9 ° C. As can be seen, there is an average ratio of Pd to the sum of Rh and Pd optimal for purifying NOx in the region of the temperature of 294.8 ° C. or less in Example 3, which is about 1 from the curve of FIG. It is understood that it is from 7 atomic% to 24.8 atomic%.

  As described above, the ratio of Pd to the total of Rh and Pd in the composite metal fine particles is in the range of about 20% to about 190% of the average ratio of Pd to the total of Rh and Pd in the plurality of composite metal fine particles. Therefore, in the samples of Examples 1 to 3, the Pd ratio close to the average ratio (about 1.7 atomic% to 24.8 atomic%) of Pd effective in purifying NOx is obtained. It is understood that a large number of composite metal fine particles are present. Therefore, the ratio of Pd to the total of Rh and Pd in the plurality of composite metal fine particles is in the range of about 20% to about 190% of the average ratio of Pd to the total of Rh and Pd in the multiple composite metal fine particles. It is understood that the NOx purification ability of the samples of Examples 1 to 3 is further improved.

  The relationship between the samples of Examples 1 to 3 and Comparative Examples 1 to 3 and the NOx 50% purification temperature (° C.) is shown in Table 2 and FIG.

In Table 2, “85 ^: 15 ^* ” indicates that the sample of Comparative Example 3 contains Rh fine particles and Pd fine particles separately, and Rh and Pd are contained at a molar ratio of 85:15. Show.

  From Table 2 and FIG. 9, compared with the NOx 50% purification temperature of the samples of Comparative Examples 2 and 3, the NOx 50% purification temperature of the samples of Examples 1 to 3 is low. It can be seen that the NOx purification capacity of the sample is higher.

  In particular, the sample of Example 2 adopting the liquid phase reduction method and the sample of Comparative Example 3 adopting the impregnation method have substantially the same molar ratio of Rh and Pd in the sample. The NOx 50% purification temperature is 35 ° C. or more lower.

  This is considered to be because in the sample of Example 2, the fine particles of the catalytic metal contain Rh and Pd, and Rh and Pd coexist at the nano level. That is, since Pd suppresses oxidation of Rh and maintains the metal state of Rh, it is considered that Rh is activated and the NOx 50% purification temperature of the sample of Example 2 is lowered.

  Moreover, since Pd itself has a high exhaust gas purification ability, as a result, the composite metal fine particles containing Rh and Pd exhibit a synergistic exhaust gas purification effect by these two kinds of metal elements. Conceivable.

<Performance evaluation A as NOx storage reduction catalyst>
In the performance evaluation as a NOx occlusion reduction catalyst, 32 g of Al ₂ O ₃ —ZrO ₂ —TiO ₂ was used as a powder carrier, and barium acetate was used at 11% by mass as a NOx occlusion material based on the mass of the sample. Except this, the same samples as those of Examples 1 to 3 and Comparative Example 1 used in the performance evaluation of the three-way catalyst were employed. Hereinafter, the sample (3 g) for the performance evaluation is referred to as the samples of Examples 1 ′, 2 ′, and 3 ′, and Comparative Example 1 ′.

  In addition, in the performance evaluation as the NOx occlusion reduction catalyst, a gas flow type catalyst evaluation device was used. Specifically, the composition of the test gas after contacting the sample was measured by circulating the test gas through the catalyst evaluation apparatus and using infrared spectroscopy (FT-IR).

  The test gas was composed of a lean atmosphere and a rich atmosphere. In this evaluation, a lean atmosphere of 60 seconds and a rich atmosphere of 6 seconds were alternately repeated at a temperature of 450 ° C. The composition of the test gas in the lean atmosphere and the rich atmosphere is shown in Table 3 below, and the evaluation results are shown in FIGS.

In addition, the flow rate of the test gas was set to 20 L / min, and the space velocity (SV) was set to 200000 h- ¹ . The space velocity means a value obtained by dividing the flow rate (volume / h) of the test gas by the volume of the sample.

  Note that “λ”, which is an index of the strength of the lean atmosphere, is defined as “oxidant equivalent / reducing agent equivalent”. For example, rich, stoichiometric, and lean atmospheres can be represented by λ <1, λ = 1, and λ> 1, respectively.

  From FIG. 10, it can be seen that during the lean atmosphere for 60 seconds, the occlusion of NO into the sample is saturated and the NOx emission (ppm) tends to increase. Further, FIG. 10 shows that a so-called rich spike was performed during a rich atmosphere of 6 seconds (in this period, NO is not present in the test gas, so the NOx emission amount is rapidly decreased). It can be seen that during the next 60-second lean atmosphere, NO is occluded by the sample and the occlusion of NO into the sample is saturated.

  Note that the rich spike means rich combustion in a very short time, whereby NOx stored in the NOx storage material can be reduced.

  FIG. 11 is an enlarged view of a part of FIG. From FIG. 11, it can be seen that the NOx emissions of the samples of Examples 1 'to 3' are smaller than the NOx emissions of the sample of Comparative Example 1 '.

  This is because the fine particles of the catalytic metal contain Rh and Pd, and Rh and Pd coexist at the nano level, so that Pd suppresses the oxidation of Rh and maintains the metal state of Rh. Conceivable. Therefore, in the samples of Examples 1 'to 3', the NOx purification capacity is maintained or improved, and NOx is sufficiently purified, so it is considered that the NOx emission amount is reduced.

  In contrast, in the sample of Comparative Example 1 ', it is considered that the NOx purification ability of Rh deteriorates due to oxidation of Rh or the like, and NOx cannot be sufficiently purified under a rich atmosphere. Therefore, it was considered that the NOx emission amount of the sample of Comparative Example 1 ′ was larger than the NOx emission amount of the samples of Examples 1 ′ to 3 ′ because NOx that was not purified was discharged as it was. .

  From FIG. 11, it can be seen that the peak of the rich spike becomes smaller in the order of Examples 1 'to 3'. This is presumably because the higher the Pd content in the composite metal fine particles, the more the oxidation of Rh is suppressed.

<< Examples 4 to 11 >>
Of the catalytic metal Example 2 in the process of carrying the powder _carrier, in place of Al ₂ O ₃ -CeO 2 -ZrO _2, employs a powder carrier 16g which is composed of _Al 2 _{O 3} and CeO _2, and fired Exhaust gas purification catalyst samples of Examples 4 to 11 (pellet type, 1 mm to 1.7 mm diameter, 3 g, except that the product was immersed in an 11% by mass barium acetate solution and barium as a NOx storage material was supported on the catalyst. ) Was prepared in the same manner as the sample of the exhaust gas purification catalyst of Example 2. The mass ratios of Al ₂ O ₃ and CeO _{2 in} the powder carrier contained in the exhaust gas purification catalyst samples of Examples 4 to 11 are 100: 0, 90:10, 76:24, and 71:29, respectively. 60:40, 50:50, 40:60, and 20:80.

<< Comparative Examples 4-11 >>
In carrying the process into powder carrier of the catalyst metal in Comparative Example _1, in place of _{Al 2 O 3 -CeO 2 -ZrO 2} , employs a powder carrier 16g which is composed of _Al 2 _{O 3} and CeO _2, and Samples of exhaust gas purification catalysts of Comparative Examples 4 to 11 (pellet type, 1 mm to 1.7 mm diameter, except that the calcined product was immersed in an 11% by mass barium acetate solution and barium as a NOx storage material was supported on the catalyst. 3 g) was prepared in the same manner as the sample of the exhaust gas purification catalyst of Comparative Example 1. The mass ratios of Al ₂ O ₃ and CeO _{2 in} the powder carriers included in the exhaust gas purification catalyst samples of Comparative Examples 4 to 11 are 100: 0, 90:10, 76:24, and 71:29, respectively. 60:40, 50:50, 40:60, and 20:80.

《Catalyst evaluation B》
Regarding the exhaust gas purification catalyst samples of Examples 4 to 11 and Comparative Examples 4 to 11, the performance as a NOx storage reduction catalyst was evaluated, and temperature-programmed reduction (TPR) was performed on the exhaust gas purification catalyst samples. The reduction temperature was evaluated by applying the method.

<Performance evaluation B as NOx storage reduction catalyst>
In the performance evaluation B as the NOx occlusion reduction catalyst, a gas flow type catalyst evaluation apparatus was used. Specifically, the composition of the test gas after contacting the sample was measured by circulating the test gas through the catalyst evaluation apparatus and using infrared spectroscopy (FT-IR).

The test gas was composed of a lean atmosphere and a rich atmosphere, and in this evaluation, five cycles were repeated with a lean atmosphere of 60 seconds and a rich atmosphere of 6 seconds as one cycle at a temperature of 400 ° C. The flow rate of the test gas was set to 20 L / min, and the space velocity was set to 200000 h- ¹ . The composition of the test gas in the lean atmosphere and the rich atmosphere is shown in Table 4 below, and the evaluation results are shown in Table 5 below, FIG. 13 and FIG.

Table 5 shows the details of the powder carrier (the fine particle form, the ratio of Rh and Pd, the CeO ₂ in the powder carrier) included in the exhaust gas purification catalyst samples of Examples 4 to 11 and Comparative Examples 4 to 11. And the ratio of the NOx emission amount to the NOx occlusion amount (NOx emission amount / NOx occlusion amount). In Table 5, “NOx emission amount / NOx occlusion amount” is a value obtained by arithmetically averaging these NOx emission amount / NOx occlusion amount by extracting 2 to 4 cycles out of the above five cycles.

  With reference to FIG. 12, “NOx emission amount / NOx occlusion amount” in Table 5 will be described. FIG. 12 is a schematic diagram showing a NOx storage reduction cycle composed of a lean atmosphere 60 seconds and a rich atmosphere 6 seconds in the performance evaluation B as a NOx storage reduction catalyst. In FIG. 12, the dotted line means the NO content (constant) in the lean atmosphere before coming into contact with the exhaust gas purification catalyst sample; the region of “S1” is occluded by the exhaust gas purification catalyst sample in the lean atmosphere; And / or means the total amount of NOx adsorbed (NOx occlusion amount); the region of “S2” (also referred to as NOx spike) is exhausted without being reduced from the exhaust gas purification catalyst sample in a rich atmosphere. It means the total amount of NOx (NOx emission amount). That is, the above “NOx emission amount / NOx occlusion amount” corresponds to “S2 / S1” in FIG.

(Result of performance evaluation B as NOx storage reduction catalyst)
FIG. 13 shows the content (mass%) of CeO _{2 in} the powder carrier and NOx emissions / NOx occlusion for the exhaust gas purification catalyst samples of Examples 4 to 11 (●) and Comparative Examples 4 to 11 (■). It is a figure which shows the relationship with quantity (au). Incidentally, the content of CeO ₂ in the powder carrier and (mass%), with respect to the relationship between the NOx emissions / NOx absorption amount (a.u.), may refer to Table 5.

From FIG. 13, regarding the exhaust gas purification catalyst samples of Comparative Examples 4 to 11 (■), as the CeO ₂ content in the powder carrier increases, the value of NOx emission amount / NOx occlusion amount increases linearly. I understand that This is because as the CeO ₂ content in the powder carrier increases, the reduction of the Rh oxide is suppressed by CeO ₂ and a reducing agent, for example, a reducing agent such as a hydrocarbon, is consumed by such oxygen. Conceivable.

From FIG. 13, regarding the exhaust gas purification catalyst samples of Examples 4 to 7 (●), when the CeO ₂ content in the powder carrier is in the range of more than 0 mass% and 29% or less, NOx It can be seen that the value of the discharge amount / NOx occlusion amount is small and almost the same. This is because, in the composite metal fine particles containing Rh and Pd, Pd can suppress the oxidation of Rh. Therefore, the oxidation of Rh by CeO ₂ is suppressed in a rich atmosphere. This is probably because most of the product was purified.

From FIG. 13, regarding the exhaust gas purification catalyst samples of Examples 8 to 11 (●), as the CeO ₂ content in the powder carrier increases within the range of 40% by mass to 80% by mass, NOx It can be seen that the value of the discharge amount / NOx occlusion amount increases linearly. However, from FIG. 13, regarding the value of NOx emission / NOx occlusion, Example 8 and Comparative Example 8; Example 9 and Comparative Example 9; Example 10 and Comparative Example 10; and Example 11 and Comparative Example 11 When comparing each, it is understood that the value of the NOx emission amount / NOx occlusion amount in the example is smaller. A small value of NOx emission amount / NOx occlusion amount indicates that most of NOx occluded in the exhaust gas purification catalyst is purified.

FIG. 14 shows the content (mass%) of CeO _{2 in} the powder carrier and the NOx purification rate (%) for the exhaust gas purification catalyst samples of Examples 4 to 11 (●) and Comparative Examples 4 to 11 (■). It is a figure which shows the relationship.

From FIG. 14, when the exhaust gas purification catalysts of Examples and Comparative Examples having the same CeO ₂ content in these powder carriers were compared (for example, a combination of Example 7 and Comparative Example 7), It can be seen that the NOx purification rate of the exhaust gas purification catalyst of the example is higher than the NOx purification rate of the exhaust gas purification catalyst of the comparative example.

Moreover, from the curve of the Example of FIG. 14, regarding the content rate of CeO _{2 in} the powder carrier, the NOx purification rate is particularly high in the range of 0% by mass (Example 4) to 40% by mass (Example 8) or less. It can be seen that This is presumably because, in the exhaust gas purification catalyst in which composite metal fine particles and CeO ₂ are combined, the NOx adsorption amount is improved in a lean atmosphere, and the catalytic activity of Rh is improved in a rich atmosphere.

<Evaluation B of reduction temperature>
The reduction temperature was evaluated and the results are shown in FIG.

In addition, evaluation of reduction temperature was specifically performed by performing the procedure of following (1)-(6) in this order:
(1) A 50 mg sample of the exhaust gas purification catalyst is introduced into the sample tube.
(2) An evaluation apparatus is comprised in order of a gas generator, a sample tube, a desiccant, and a thermal conductivity detector (TCD: Thermal Conductivity Detector).
(3) The gas composed of 10 vol% O ₂ and 90 vol% helium is circulated from the gas generator to the sample tube at 30 mL / min, and the gas temperature is increased to 500 ° C. at a rate of temperature increase of 20 ° C./min. Raise the temperature.
(4) After the temperature of the gas reaches 500 ° C., maintain the temperature for 10 minutes, and then cool the temperature of the gas to 50 ° C.
(5) A gas composed of Ar 100 vol% is circulated from the gas generator to the sample tube.
(6) The gas composed of 1 vol% H ₂ and Ar 99 vol% is circulated from the gas generator to the sample tube at 30 mL / min, and the gas temperature is increased to 500 ° C. at a rate of 10 ° C./min. Let warm.

  The role of the desiccant (2) above is to trap water contained in the gas that has passed through the sample tube. The thermal conductivity detector (TCD) measures a change in the thermal conductivity of the gas accompanying a change in the gas component, and thereby calculates the gas concentration.

  In addition, the sample of the exhaust gas purifying catalyst (1) described above employs Example 6 'and Comparative Example 6', respectively. Example 6 'and Comparative Example 6' are samples produced in the same manner as Example 6 and Comparative Example 6, respectively, except that the operation of supporting barium as a NOx storage material on the catalyst was not performed.

  Note that a series of flows regarding the procedures (1) to (6) is shown in FIG.

(Result of reduction temperature evaluation B)
FIG. 16 shows the temperature (° C.) and H ₂ consumption (TCD) when the temperature-programmed reduction (TPR) method is applied to the exhaust gas purification catalyst samples of Example 6 ′ and Comparative Example 6 ′. signal) (au).

In FIG. 16, the solid line shows the sample of the exhaust gas purification catalyst of Example 6 ′, and the dotted line shows the sample of the exhaust gas purification catalyst of Comparative Example 6 ′. From Figure 16, it can be seen that Example 6 'peak of H ₂ consumption temperature is about 115 ° C., and Comparative Example 6' peak of H ₂ consumption temperature of about 170 ° C.. Therefore, it is understood that the composite metal fine particles of the exhaust gas purification catalyst of Example 6 ′, particularly Rh therein, are converted from the oxide to the metal at a lower temperature.

  This is presumably because Rh and Pd coexisted at the nano level in the composite metal fine particles, and the effect of inhibiting Rh oxidation by Pd was remarkably exhibited.

  Although the preferred embodiment of the present invention has been described in detail, it is possible to change the manufacturer, grade, quality, etc. of the apparatus, equipment, chemicals, etc. used in the present invention without departing from the scope of the claims. Those skilled in the art understand that.

Claims (10)

An exhaust gas purification catalyst having composite metal fine particles containing Rh and Pd,
When the fine particles in the exhaust gas purification catalyst are analyzed by STEM-EDX, the average ratio of Pd to the total of Rh and Pd is 1.7 atomic percent or more and 24.8 atomic percent or less.
The average ratio is determined by randomly selecting 10 or more fine particles from the exhaust gas purification catalyst, measuring all the selected fine particles by STEM-EDX, and further selecting Rh and Pd from all the selected fine particles. All the composite metal fine particles contained are selected, the ratio of Pd to the total of Rh and Pd of each composite metal fine particle is totaled, and this total is divided by the total number of all the selected composite metal fine particles. Is a calculated value,
Exhaust gas purification catalyst.   The exhaust gas purification catalyst according to claim 1, further comprising a powder carrier, wherein the composite metal fine particles are supported on the powder carrier. The powder carrier _is a _{SiO 2, MgO, ZrO 2,} CeO 2, Al 2 O 3, TiO 2, and their solid solutions, as well as powder carrier selected from the group consisting of a combination thereof, according to claim 2 Exhaust gas purification catalyst. The exhaust gas purification catalyst according to claim 2 or 3, wherein the powder carrier contains CeO ₂ in an amount of more than 0 mass% and not more than 40 mass% with respect to the mass of the powder carrier.   In a stoichiometric atmosphere, the exhaust gas purifying catalyst according to any one of claims 1 to 4 is contacted with exhaust gas containing HC, CO, and NOx, thereby oxidizing HC and CO, and An exhaust gas purification method that reduces and purifies.   An exhaust gas purification method in which an exhaust gas containing NOx is brought into contact with the exhaust gas purification catalyst according to any one of claims 1 to 4 in a lean atmosphere, and the NOx is reduced and purified in a rich atmosphere. A method for producing an exhaust gas purification catalyst, comprising producing composite metal fine particles by heating and refluxing a solution containing Rh ions, Pd ions, a reducing agent, and a protective agent,
The molar ratio of the Rh ion and the Pd ion is 70:30 to 99: 1.
A method for producing an exhaust gas purification catalyst.   The method according to claim 7, comprising supporting the composite metal fine particles on a powder carrier. The powder carrier _is a _{SiO 2, MgO, ZrO 2,} CeO 2, Al 2 O 3, TiO 2, and their solid solutions, as well as powder carrier selected from the group consisting of a combination thereof, according to claim 8 the method of. The method according to claim 8 or 9, wherein the powder carrier contains CeO ₂ in an amount of more than 0% by mass and 40% by mass or less based on the mass of the powder carrier.