JP6851219B2 - Exhaust gas purification catalyst and its manufacturing method - Google Patents

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 in which deterioration of catalyst performance due to continued use is suppressed, and an effective and simple method for producing the catalyst.

As an exhaust gas purification catalyst for treating automobile exhaust gas, a three-way catalyst in which a noble metal is supported on an inorganic oxide is known. The three-way catalyst is widely used as a catalyst capable of efficiently removing hydrocarbons (HC), nitrogen oxides (NO _x ), and carbon monoxide (CO) at the same time.

However, it is known that the activity of an exhaust gas purification catalyst typified by a three-way catalyst decreases over time when it is used continuously under automobile driving conditions. It is considered that this decrease in activity is due to a decrease in the catalytic active site due to the growth of the particle size of the noble metal under high temperature conditions.

Therefore, in the prior art, various studies have been conducted on means for suppressing the grain growth of precious metals. For example, there are disclosed techniques for improving the heat resistance of a catalyst carrier or enhancing the interaction between a noble metal and a carrier to reduce the surface energy of the noble metal particles (Non-Patent Documents 1 and 2).

According to the techniques of these non-patent documents, the surface energy of the noble metal particles does not decrease significantly under the conditions of high temperature and fluctuation of atmosphere. Therefore, the effect of suppressing grain growth under such conditions is insufficient.

On the other hand, as an attempt to directly reduce the surface energy of the noble metal particles, a technique for controlling the particle size of the noble metal particles and / or the distance between the noble metal particles is disclosed (Patent Documents 1 to 3).

By the way, conventionally, the catalyst layer in the exhaust gas purification catalyst as described above has been formed on a base material which does not have an exhaust gas purification ability by itself, for example, a honeycomb base material made of Cordellite. However, in recent years, an exhaust gas purification catalyst in which a noble metal is supported on a base material composed of inorganic oxide particles has been proposed (Patent Document 4).

Japanese Unexamined Patent Publication No. 2006-289213 Japanese Unexamined Patent Publication No. 09-262467 Japanese Unexamined Patent Publication No. 10-118455 Japanese Unexamined Patent Publication No. 2015-85241

J. Catal. , 242,103 (2006) Catalyst, 52,465 (2010)

Even when the techniques of Patent Documents 1 to 3 are used, the particle growth of the noble metal catalyst is effective under high temperature of 1,000 ° C. or higher and under harsh conditions such as fuel cut and rich spike where the atmosphere fluctuates. Suppression is difficult. Further, in the case of a three-way catalyst in which a plurality of types of platinum group elements coexist, alloying also proceeds in the process of particle growth, which leads to a further decrease in catalytic activity.

The present invention has been made in an attempt to improve the current state of the prior art. The purpose is to grow catalyst metal particles and alloys associated therewith, even under conditions where the atmosphere fluctuates significantly at a high temperature of 1,000 ° C. or higher while containing a plurality of platinum group elements as catalytically active species. It is an object of the present invention to provide an exhaust gas purification catalyst which is suppressed from forming and can maintain high activity for a long period of time, and an effective and simple method for producing the catalyst.

The present inventors have grown the noble metal particles contained in the exhaust gas purification catalyst to a specific particle size in advance, so that even if the use is continued under harsh conditions, further grain growth and accompanying grain growth will occur. We have found that alloying is highly suppressed, and arrived at the present invention based on this finding. The present invention provides the following catalysts and methods in order to achieve the above object.

[1] An exhaust gas purification catalyst having a catalyst layer containing palladium particles, rhodium particles, and carrier particles.
The catalyst, wherein the palladium particles have a particle size of 10 nm or more, and the alloying ratio of the palladium measured by X-ray diffraction analysis is 45% or less.

[2] The catalyst according to [1], wherein the palladium particles and the rhodium particles are supported on separate carrier particles.

[3] The palladium particles are supported on carrier particles made of alumina.
The catalyst according to [2], wherein the rhodium particles are supported on carrier particles made of a ceria-zirconia composite oxide.

[4] The catalyst according to any one of [1] to [3], wherein the rhodium particles have a particle size of 0.7 to 10 nm.

[5] The catalyst according to any one of [1] to [4], wherein the catalyst layer is present on the substrate.

[6] The catalyst according to any one of [1] to [4], wherein the catalyst layer constitutes a part of a substrate.

[7] The first material is produced by contacting the first carrier particles with the palladium precursor and then heating in a reducing atmosphere to grow the palladium particles, and the first material is placed on the substrate. It is characterized by including a step of forming a catalyst layer containing the material and a second material formed by supporting rhodium particles on a second carrier particle, and a step of firing a substrate having the catalyst layer. A method for producing an exhaust gas purification catalyst.

[8] A step of producing a first material through a step of bringing the palladium precursor into contact with the first carrier particles and then heating in a reducing atmosphere to grow the palladium particles, and the first material and the above-mentioned first material. A method for producing an exhaust gas purification catalyst, which comprises a step of calcining a raw material mixture containing a second material formed by supporting rhodium particles on a second carrier particle.

[9] The second material is produced through a step of bringing the rhodium precursor into contact with the second carrier particles and then heating the rhodium precursor in an oxidizing atmosphere to grow the rhodium particles. [7] Or the method according to [8].

According to the present invention, while containing a plurality of types of platinum group elements as catalytically active species, grain growth of catalytic metal particles and accompanying grain growth even under high temperature of 1,000 ° C. or higher and conditions in which the atmosphere fluctuates significantly. Provided are an exhaust gas purification catalyst in which alloying is suppressed and high activity can be maintained for a long period of time, and an effective and simple method for producing the catalyst. The exhaust gas purification catalyst of the present invention is particularly excellent in the purification ability of HC (hydrocarbon).

FIG. 1 is a graph showing the relationship between the coarsening rate of Pd and the initial particle size of Pd. FIG. 2 is a graph showing the relationship between the alloying ratio of Pd and the initial particle size of Pd. FIG. 3 is a graph showing the relationship between the catalytic performance after durability and the initial particle size of Pd. FIG. 4 is a graph showing the relationship between the catalytic performance after durability and the initial particle size of Rh. FIG. 5 is a graph showing the relationship between the catalyst performance after durability and the Pd concentration. FIG. 6 is a graph showing the relationship between the catalytic performance after durability and the Rh concentration.

The exhaust gas purification catalyst of the present invention has a catalyst layer containing palladium particles, rhodium particles, and carrier particles.

The exhaust gas purification catalyst of the present invention may have a base material and may have a catalyst layer on the base material, or the catalyst layer may form a part of the base material.

As the base material in the exhaust gas purification catalyst of the present invention, those generally used as the base material of the exhaust gas purification catalyst can be used. For example, a monolith honeycomb base material composed of cordierite, SiC, stainless steel, metal oxide particles and the like can be mentioned. The capacity of the base material can be, for example, about 1 L. When the exhaust gas purification catalyst of the present invention has a catalyst layer on a substrate, the catalyst is preferably a monolith honeycomb catalyst having a catalyst layer on a monolith honeycomb substrate.

When the catalyst layer constitutes a part of the base material, for example, the base material contains palladium particles, rhodium particles, and carrier particles, and the particle size of the palladium particles is 10 nm or more, and X-ray diffraction analysis is performed. The alloying ratio of palladium measured according to the above may be 45% or less.

The catalyst layer in the exhaust gas purification catalyst of the present invention contains palladium (Pd) particles, rhodium (Rh) particles, and carrier particles. The Pd particles and Rh particles in the catalyst layer are supported on carrier particles, respectively.

The particle size of the Pd particles is 10 nm or more. This value may be 20 nm or more, 30 nm or more, or 40 nm or more, and may be 90 nm or less, 80 nm or less, 70 nm or less, 60 nm or less, or 50 nm or less. By adjusting the particle size of the Pd particles to this range, even if the use is continued under harsh conditions, the Pd particles do not become coarse, the degree of forming an alloy with other noble metals is small, and the catalyst. It is possible to obtain a catalyst with less deterioration in performance.

The particle size of the Pd particles is a value determined by X-ray diffraction analysis (XRD). Specifically, it is a value calculated from the half width of the Pd peak in the XRD chart.

The alloying ratio of Pd particles in the catalyst of the present invention is 45% or less. This value is preferably 25% or less, more preferably 20% or less, still more preferably 15% or less. When the alloying ratio of the Pd particles is in this range, good catalytic performance can be provided.

The alloying ratio of the above Pd particles is a value estimated by XRD analysis. That is, in the XRD chart, the alloying ratio of the Pd particles is estimated from the relative positions of the Pd (111) plane peaks with respect to the Pd reference and the Rh reference. When the Pd particles are not alloyed at all and Pd is 100%, the position of the Pd (111) plane peak coincides with the position of the Pd reference peak. The position of the Pd (111) plane peak when the alloying ratio of the Pd particles is 50% is intermediate between the peak position with reference to Pd and the peak position with reference to Rh. As described above, the Pd alloying ratio in the present specification is estimated by proportional calculation by investigating the position of the Pd (111) plane peak relative to the peak positions of the Pd reference and the Rh reference. It is a value that has been calculated.

Here, it is appropriate to adopt the values described in the ICDD database as the peak positions of the Pd reference and the Rh reference. That is, 2θ = 40.12 ° for Pd reference and 2θ = 41.17 ° for Rh reference.

As described above, the Pd particles in the catalyst of the present invention are extremely suppressed in the degree of increasing the particle size and the degree of alloying even if the use is continued under severe conditions. Therefore, the above-mentioned preferable range of particle size and alloying ratio for Pd particles is related to the initial (before use of the catalyst) Pd particles, and the catalyst is continuously used after the start of use of the catalyst and under severe conditions. It is a numerical range that is also valid for the later Pd particles.

Initially, the Pd particles in the conventionally known exhaust gas purification catalyst have a small particle size outside the range of the present invention, and the alloying ratio is also small. However, if the use is continued, the particle size will increase and alloying will proceed. Therefore, the known catalyst in use may satisfy the requirements of the catalyst of the present invention when looking only at the Pd particle size. However, a known catalyst having a large Pd particle size is also being alloyed with the Pd particles, and is distinguished from the catalyst of the present invention in this respect.

An exhaust gas purification catalyst as in the present invention, in which the degree of alloying of Pd particles is extremely small even if the use is continued under harsh conditions, has not been known so far.

The particle size of the Rh particles contained in the catalyst layer of the exhaust gas purification catalyst of the present invention is preferably 0.6 nm or more. The range of the particle size of the Rh particles may be 0.7 nm or more, 1 nm or more, 2 nm or more, or 3 nm or more. This value is preferably 50 nm or less, and may be 30 nm or less, 20 nm or less, 15 nm or less, 10 nm or less, or 8 nm or less. By adjusting the particle size of the Rh particles to this range, deterioration of the catalytic performance can be effectively suppressed even when the use is continued under harsh conditions, which is preferable.

The particle size of the Rh particles is a value estimated by the CO pulse method. That is, it is a method of repeatedly introducing a constant amount of CO gas into a catalyst sample in a pulsed manner and estimating the particle size of Rh particles from a change in the amount of exhaust gas. In the initial pulse, CO is chemically adsorbed on the Rh particles, so the amount of gas emitted is smaller than the amount of the introduced pulse. When the pulses are repeated and the CO adsorption equilibrates, most of the introduced pulses will be discharged. Here, the total difference between the introduced amount and the discharged amount is obtained as the adsorption amount, and the particle size of the Rh particles can be estimated from this value and the metal species (Rh) and the concentration (supported amount) of the metal.

As a specific method for calculating the Rh particle size, it is preferable to calculate by subtracting the CO adsorption contribution by the Pd particles according to the method described in Examples described later.

The Pd particles and Rh particles in the exhaust gas purification catalyst of the present invention are supported on carrier particles and contained in the catalyst layer, respectively.

Examples of the carrier particles include composite oxides containing alumina (single alumina), silica alumina, zeolite, titanium oxide, silica, ceria, zirconia, and rare earth elements. The preferred particle size and BET specific surface area of these carriers are as follows, respectively.
Particle size: The median diameter measured by the laser diffraction / scattering method is preferably 1 to 10 μm, more preferably 4 to 7 μm.
BET specific surface area: As _{a measured value using N 2} as the adsorption medium, it is preferably 10 to 300 m ² / g, more preferably 30 to 150 m ² / g.

As the carrier particles, one or more selected from alumina and CZ are preferably used.

In a more preferred embodiment, the Pd particles and the Rh particles are supported on separate carrier particles. More preferably, the Pd particles and the Rh particles are supported on different types of carrier particles. Particularly preferably, the Pd particles are supported on the carrier particles made of alumina, and the Rh particles are supported on the carrier particles made of CZ.

The concentration (supported amount) of the Pd particles is preferably 0.1% by weight or more when the weight of the carrier particles on which Pd is supported is 100% by weight. This value may be 0.15% by weight or more or 0.2% by weight or more. The concentration of the Pd particles is preferably 10% by weight or less when the weight of the carrier particles carrying Pd is 100% by weight. This value may be 8% by weight or less, 6% by weight or less, or 5% by weight or less. By setting the Pd particle concentration in this range, the degree of deterioration of the catalyst performance can be suppressed to the maximum even when the use is continued under severe conditions.

On the other hand, the concentration (supported amount) of the Rh particles is 0.05% by weight or more when the weight of the carrier particles carrying Rh is 100% by weight, so that a high degree of HC purification ability is exhibited. Preferred from the point of view. This value may be 0.10% by weight or more or 0.20% by weight or more. The higher the concentration of Rh particles, the better the catalytic ability. However, even if the Rh concentration is unnecessarily increased, the catalytic ability does not improve linearly, and the catalyst cost becomes excessive. Therefore, the Rh concentration is preferably 1.5% by weight or less, and more preferably 1.0% by weight or less when the weight of the carrier particles is 100% by weight.

The ratio of Pd particles to Rh particles used in the catalyst of the present invention is preferably in the range of 0.3 or more as the ratio of Pd particle concentration to Rh particle concentration (Pd / Rh, weight ratio). This value may be 0.4 or more or 0.5 or more. The weight ratio of Pd / Rh is preferably 8 or less. This ratio may be 7 or less, 6 or less, or 5 or less. By setting the ratio of both to be within this range, even if the use is continued under harsh conditions, the degree of increase in the particle size of the Pd particles is small, the progress of alloying is suppressed, and therefore the catalytic ability is reduced. The degree of deterioration over time is small.

The number of Pd particles and Rh particles is calculated by the following formula:

Can be calculated by Unit in the above formula is a unit representing the range in which the catalyst layer to be calculated for the number of particles is formed, and when the exhaust gas purification catalyst has a base material and a catalyst layer on the base material, for example, a group. It may be a unit such as the area of the coated surface of the material (m ² ) and the capacity (L) of the base material.

The number of Pd particles per unit volume in the monolith honeycomb catalyst is preferably ^{1.5 × 10 11 to} 1.0 × 10 ¹³ / L from the viewpoint of further improving the effect of suppressing noble metal alloying during durability. 1.9 × 10 ^{11 to} 5.0 × 10 ¹² pieces / L is more preferable. The number of Rh particles per unit volume in the monolith honeycomb catalyst is preferably ^{1.0 × 10 15 to} 7.0 × 10 ¹⁶ / L from the viewpoint of further improving the effect of suppressing noble metal alloying during durability. 1.4 × 10 ^{15 to} 3.0 × 10 ¹⁶ pieces / L is more preferable.

The ratio of the number of Pd particles to the number of Rh particles (number of Pd particles / number of Rh particles) is preferably 0.2 or less, preferably 0.05 or less, from the viewpoint of suppressing deterioration of catalytic ability due to continuous use. More preferably, it is more preferably 0.01 or less. The lower limit of the number of Pd particles / the number of Rh particles should be appropriately set according to the desired catalytic performance. The number of Pd particles / Rh particles can be, for example, 1 × 10 ^-6 or more, and may be 5 × 10 ^-6 or more or 1 × 10 ^-5 or more.

The catalyst layer in the exhaust gas purification catalyst of the present invention contains the above-mentioned Pd particles, Rh particles, and carrier particles in the above-mentioned proportions. The catalyst layer may optionally contain components other than these. Examples of such an optional component include a binder, a transition metal, an alkali metal compound, an alkaline earth metal compound, a rare earth compound, and the like.

The binder has a function of adhering between the carriers and between the carrier and the surface of the base material to impart mechanical strength to the catalyst layer in the catalyst of the present invention. Examples of such a binder include alumina sol, zirconia sol, silica sol, titania sol and the like.

The proportion of the binder used in the catalyst layer in the catalyst of the present invention is preferably 20% by weight or less, preferably 0.5 to 10% by weight, when the total weight of the catalyst layer is 100% by weight. More preferred.

Examples of the transition metal include nickel, copper, manganese, iron, cobalt zinc and the like. Of these, when nickel is used in combination, the effect of suppressing the production of hydrogen sulfide can be obtained. Examples of the alkali metal compound include potassium compounds and lithium compounds; examples of the alkaline earth metal compounds include calcium compounds, barium compounds and strontium compounds; examples of rare earth compounds include lanthanum oxide. , Oxidized placeodium, Oxidized neodymium and the like. These have the effect of improving the heat resistance of the obtained catalyst.

When the catalyst of the present invention has a base material and a catalyst layer on the base material, the thickness of the catalyst layer in the catalyst is, for example, 5 μm to 500 μm in order to secure the flow of exhaust gas and exhibit suitable purification performance. It is preferably 10 μm to 300 μm.

When the catalyst of the present invention has a base material and a catalyst layer on the base material, the amount of the catalyst layer (coating amount or coating texture) in the catalyst is per 1 L of the base material from the viewpoint of higher catalyst performance. The weight of the catalyst layer is preferably 20 to 400 g / L, more preferably 50 to 300 g / L.

The catalysts of the present invention as described above exhibit excellent catalytic performance. For example, even when it is continuously used under harsh conditions, the degree of deterioration of the purification performance of HC is extremely small. Quantitatively, for example, when an exhaust gas containing 1,000 volume ppm of HC is circulated at a flow rate of 23 L / min to a catalyst molded into a size of 30 mm in diameter and 50 mm in length, the purification rate of the HC The temperature (T50-HC (° C.)) at which is 50% can be "365 ° C. or lower." This value is preferably 360 ° C. or lower, more preferably 355 ° C. or lower. One of the advantages of the catalyst of the present invention is that this numerical range is valid not only for the initial performance of the catalyst but also after continuous use.

The catalyst of the present invention as described above may be produced by any method as long as it satisfies the above requirements. However, for example, the following two manufacturing methods can be exemplified.

(First manufacturing method)
A step of producing the first material through a step of bringing the palladium precursor into contact with the first carrier particles and then heating in a reducing atmosphere to grow the palladium particles, and a step of producing the first material on the substrate, A method comprising a step of forming a catalyst layer containing a second material obtained by supporting rhodium particles on a second carrier particle, and a step of firing a substrate having the catalyst layer.

(Second manufacturing method)
A step of producing the first material through a step of bringing the palladium particles into contact with the first carrier particles and then heating in a reducing atmosphere to grow the palladium particles, and the first material and the second A method comprising a step of calcining a raw material mixture containing a second material formed by supporting rhodium particles on carrier particles.

Hereinafter, the first manufacturing method and the second manufacturing method will be described in order.

(First manufacturing method)
The first carrier particles carrying Pd particles and the second carrier particles carrying Rh particles used in the first production method can be selected and used from those described above, respectively.

The first material can be produced by immersing the first carrier particles in a solution containing a Pd precursor and then heating the particles to grow the Pd particles.

As the Pd precursor, for example, a water-soluble Pd salt can be preferably used. Specific examples thereof include palladium nitrate, palladium chloride, palladium sulfate and the like, but palladium nitrate is preferable because it has high dispersibility in the carrier. Water is preferable as the solvent for the Pd precursor solution. The Pd concentration in the Pd precursor solution should be appropriately selected according to the desired amount of Pd supported in the obtained catalyst _{, and the concentration in terms of PdO 2} should be, for example, 0.1 to 20 g / L. It is possible, preferably 0.15 to 15 g / L.

The liquid temperature when the first carrier particles are immersed in the Pd precursor solution can be, for example, 5 to 90 ° C, preferably 25 to 60 ° C. The immersion time can be, for example, 1 minute to 6 hours, preferably 10 minutes to 1 hour.

Next, the immersed carrier particles are heated in a reducing atmosphere. By this heating step, the Pd precursor is converted into a Pd metal, and the Pd particles are grown to the predetermined particle size of the present invention.

The reducing atmosphere at this time can be formed by a reducing gas or a mixed gas of the reducing gas and the inert gas. Examples of the reducing gas include carbon monoxide (CO), hydrogen, hydrogen sulfide and the like; and examples of the inert gas include nitrogen, helium, argon and the like.

The heating temperature and heating time in the heating step (Pd particle growth step) should be appropriately selected according to the desired Pd particle size. The temperature and time are conditions determined by the balance between the two. When the temperature is high, it is preferable to heat it for a relatively short time, and when a relatively low temperature is adopted, it is preferable to heat it for a long time.

The heating temperature in the Pd particle growth step is preferably 800 to 1,200 ° C, more preferably 850 to 1,100 ° C, and even more preferably 900 to 1,050 ° C. The heating time is preferably 0.5 to 48 hours, more preferably 0.75 to 36 hours, and particularly preferably 1 to 24 hours.

Here, when the heating temperature exceeds approximately 1,000 ° C., it is preferable to adopt a heating time of less than 10 hours from the viewpoint of obtaining good catalytic ability. When the heating temperature exceeds 1,000 ° C., the heating time is more preferably 7.5 hours or less, and further preferably 5 hours or less. It is presumed that this is because Pd particles grow excessively and the number of catalytic active sites decreases when heated at a high temperature exceeding 1,000 ° C. for a long time.

On the other hand, when the heating temperature is 1,000 ° C. or lower, the Pd particles do not grow excessively even when heated for a long time, so that there is no time limit from the viewpoint of catalytic ability as described above. However, since the catalytic ability and its durability are not improved by long-term heating, the heating time in this case is preferably 24 hours or less, more preferably 20 hours or less, from the viewpoint of catalyst production cost. Yes, more preferably 15 hours or less.

The second material can be produced by immersing the second carrier particles in a solution containing a Rh precursor. It is preferable that the particles after immersion are heated to grow Rh particles.

As the Rh precursor, for example, a water-soluble Rh salt can be preferably used. For example, rhodium chloride, sodium rhodium chloride, rhodium pentaamine chloride, carbonylacetyl rhodium and the like can be mentioned, but rhodium chloride is preferable because it has high dispersibility in the carrier. Water is preferable as the solvent for the Rh precursor solution. The Rh concentration in the Rh precursor solution _{can be, for example, 0.01 to 2} g / L, preferably 0.025 to 1.5 g / L, as a concentration in terms of Rh 2 O _3.

The liquid temperature when the second carrier particles are immersed in the Rh precursor solution can be, for example, 5 to 90 ° C, preferably 25 to 60 ° C. The immersion time can be, for example, 1 minute to 6 hours, preferably 10 minutes to 1 hour.

Next, it is preferable to heat the carrier particles after immersion in an oxidizing atmosphere to grow the Rh particles to the predetermined particle size of the present invention.

The oxidizing atmosphere at this time can be formed by an oxidizing gas or a mixed gas of the oxidizing gas and the inert gas. Examples of the oxidizing gas include oxygen (O ₂ ), ozone, air, nitrogen dioxide and the like; and examples of the inert gas include nitrogen, helium, argon and the like.

The heating temperature and heating time in the heating step (Rh particle growth step) should be appropriately selected according to the desired Rh particle size. The heating temperature can be, for example, 800 to 1,200 ° C, preferably 850 to 1,100 ° C. The heating time is, for example, 1 to 48 hours, preferably 5 to 24 hours.

Next, a coating liquid (slurry) containing the first material and the second material produced as described above is prepared. In addition to containing the first material and the second material in a predetermined ratio, the coating liquid may contain any component of the catalyst layer described above. The coating liquid in the present invention preferably contains a predetermined amount of a binder as an optional component. Water is preferable as the solvent of the coating liquid.

The solid content concentration of the coating liquid can be, for example, 5 to 60% by weight, preferably 10 to 40% by weight.

The coating liquid can be prepared by suspending each of the above components in a solvent by an appropriate method. For this suspension step, for example, a known mixing device such as a stirring blade type or blade type mixing device or a mixer can be used.

The catalyst of the present invention can be obtained by applying the coating liquid prepared above on the surface of the base material, removing the solvent if necessary, and then firing to form a catalyst layer.

As the coating method, for example, a known method such as a dip method or a pouring method can be adopted without limitation. The coating amount is appropriately selected according to the desired catalyst layer amount.

The solvent can be removed after coating by a method of heating at a temperature of, for example, 60 to 200 ° C., preferably 120 to 150 ° C. for, for example, 5 to 120 minutes, preferably 10 to 60 minutes. The atmosphere during this heating is sufficient in the air.

The heating temperature in the subsequent firing step can be, for example, 300 to 1,000 ° C, preferably 400 to 1,000 ° C. The heating time can be, for example, 0.5 to 10 hours, preferably 1 to 5 hours. This firing step is preferably carried out in an oxidizing atmosphere, but it is sufficient to carry out this firing step in air.

(Second manufacturing method)
The step of manufacturing the first material in the second manufacturing method can be performed in the same manner as in the case of the first manufacturing method. The second material may be manufactured in the same manner as in the case of the first manufacturing method.

The second production method uses the first material and the second material, and can be carried out according to, for example, the method described in Patent Document 4. Specifically, for example, it can be obtained by firing a raw material mixture containing the desired first and second materials and, if necessary, further containing a binder. The raw material mixture may be extruded into a predetermined shape after kneading, and may be dried prior to firing.

Of the first and second manufacturing methods, the first manufacturing method is preferable in that the process is simple and easy, and the manufacturing cost is low.

As described above, the catalyst of the present invention can be easily and inexpensively produced. The catalyst of the present invention exhibits high exhaust gas purification ability, and is particularly excellent in HC purification ability. Even when the catalyst of the present invention is used for a long time under harsh conditions where the atmosphere is significantly changed at high temperature, coarsening and alloying of noble metal (particularly Pd) particles serving as catalyst active sites are suppressed. The degree of deterioration of the exhaust gas purification ability is small.

Hereinafter, the excellent performance of the catalyst of the present invention will be specifically verified in the form of an example.

In the following examples, the material on which the metal is supported on the carrier is, for example, "Pd (0.5) / Al ₂ O ₃ ", "Rh (0.1) / CZ", etc. in the case of alumina-supported palladium. It is expressed by the notation of. The element symbol at the beginning represents the catalyst metal, and the number in parentheses is a numerical value representing the concentration (supporting ratio) of the catalyst metal as a percentage (% by weight) with respect to the carrier weight. The carrier name is indicated by a chemical symbol or abbreviation following the slash "/".

The characteristics of the carriers used in the following examples and comparative examples are as follows, respectively.
Alumina: BET specific surface area: 100 m ² / g, average particle size: 30 μm
CZ: Ceria-zirconia composite oxide, BET specific surface area: 50 m ² / g, average particle size: 5 μm

<< Examples 1 to 12 and Comparative Example 1 >>
In Examples 1 to 12 and Comparative Example 1 below, the initial particle diameters of Pd and Rh are set to the coarsening rate of Pd particles and the alloy after durability, while maintaining the concentrations (supported amount) of Pd and Rh constant. The effect on the conversion rate and catalytic performance was investigated.

<Example 1>
_{Pd (0.5) / Al 2} O ₃ is prepared by immersing alumina in an aqueous solution of palladium nitrate and heated at 900 ° C. for 20 hours in a 5% by volume CO (N _{2 balance) atmosphere to grow Pd particles.} It was. Similarly, Rh (0.1) / CZ is prepared by immersing CZ in an aqueous solution of rhodium hydrochloride and heated at 900 ° C. for 1 hour _{in a 10% by volume oxygen (N 2 balance) atmosphere to grow Rh particles.} It was.

While stirring the distilled water, here, _{50 parts by weight of the durable Pd (0.5) / Al 2} O ₃ obtained above, 50 parts by weight of the durable Rh (0.1) / CZ: 50 parts by weight, and alumina sol. 1 part by weight was added and suspended to obtain a catalyst preparation slurry.

The catalyst preparation slurry is poured into a monolith honeycomb base material (capacity 1 L), water is blown off at 120 ° C. for 30 minutes in a dryer, and then calcined at 500 ° C. for 2 hours in an electric furnace on the base material. _{, Pd 1} g / L, Rh 0.2 g / L, Al ₂ O 3 100 g / L, and CZ 100 g / L catalyst layers were prepared.

<Examples 2 to 13 and Comparative Example 1>
Table 1 shows the heating conditions for growing Pd particles and Rh particles for _{Pd (0.5) / Al 2} O ₃ and Rh (0.1) / CZ prepared in the same manner as in Example 1 above. A catalyst having a catalyst layer on the substrate was prepared in the same manner as in Example 1 except as shown in 1. "-" In the durability condition in Table 1 indicates that the durability corresponding to the relevant column was not performed.

For the catalysts prepared in each of the above Examples and Comparative Examples, the particle sizes of Pd and Rh, the alloying ratio of Pd after the durability test under the following conditions, and the catalyst performance (T50-HC (° C. after durability)). )) Was evaluated. The evaluation results are shown in Table 1.

<Particle diameter of _Pd d Pd (nm)>
A small amount of the catalyst layer of the catalyst prepared in each Example and Comparative Example was scraped off, and it was calculated from the half width of the Pd peak in the chart obtained by using an X-ray diffraction measuring device manufactured by Rigaku.

<Rh particle size>
A small amount of the catalyst layer of the catalyst prepared in each Example and Comparative Example was scraped off, and at 400 ° C., _{oxidation treatment under an O 2} (20% by volume) -He (balanced) environment, and CO (20% by volume) -He ( The reduction treatment under the balanced environment was carried out alternately in this order for 2 cycles of 15 minutes each. Then, after purging with He 100% gas and lowering the temperature to 50 ° C., CO (10% by volume) -He (balance) gas was introduced in a pulsed manner to perform CO adsorption. From the CO adsorption amount V at this time, subtract the CO adsorption amount _{V Pd} by Pd calculated from the above obtained palladium particle size _d Pd (nm) by the following equation (1), the CO adsorption amount _{V Rh} by Rh Obtained (formula (2)).

Then, the particle size of Rh was calculated using the following mathematical formulas (3) and (4) from the CO adsorption amount _{VRh by Rh and the content of Rh in the catalyst.}

<Roughness rate>
The particle size of Pd and the particle size of Rh were measured before and after the durability test under the following conditions, respectively, and the coarsening rate (unit = times) before and after the durability test was calculated by the following mathematical formula (5). This value is shown in Table 2.
Coagulation rate = {(particle size after endurance test)-(particle size before endurance test)} / (particle size before endurance test) (5)

<Palladium alloying rate after durability test>
(1) Durability test The catalysts prepared in each Example and Comparative Example were hollowed out to a size of 30 mm in diameter and 50 mm in length, and the introduction gas temperature was set to 1,000 ° C., O ₂ (10% by volume), N ₂ (90 volumes). %) And a flow rate of 1 L / min oxidation treatment and CO (5% by volume), N ₂ (95% by volume), and a flow rate of 1 L / min reduction treatment are alternately repeated in this order for 5 minutes each. A 5-hour endurance test was conducted.

(2) Alloying rate A small amount of the catalyst layer of the catalyst after the durability test was scraped off, and the position of the Pd peak was examined using an X-ray diffraction measuring device manufactured by Rigaku. What is the position of the peak position of the Pd (111) plane near 2θ = 40 ° relative to the Pd reference (2θ = 40.12 °) and Rh reference (2θ = 41.07 °)? The alloying ratio of Pd was estimated. Here, when the peak position of the Pd (111) plane matches the Pd reference, the alloying rate is 0%, and when it matches the Rh reference, the alloying rate is 100%, which is in between. In this case, the alloying ratio was calculated in proportion to the shift amount at the peak position. For example, when the peak position was between the Pd reference and the Rh reference, the alloying ratio was set to 50%.

<Catalyst performance>
The catalysts prepared in each of the above Examples and Comparative Examples were incorporated into a model exhaust gas flow test apparatus, and an accelerated test assuming actual use was performed.

Each catalyst was hollowed out to a size of 30 mm in diameter and 50 mm in length, and incorporated into a simulated exhaust gas distribution device. This catalyst was subjected to a durability test under the same conditions as the above <Palladium alloying ratio after durability test> "(1) Durability test". The simulated exhaust gas having the composition shown in Table 2 below was circulated at a flow rate of 23 L / min while raising the temperature of the introduced gas from 550 ° C to 35 ° C / min with respect to the catalyst after the durability test. The temperature (T50-HC (° C.)) at which the purification rate of HC was 50% was examined. The content of each gas in Table 3 is a value based on the volume.

The graph showing the relationship between the coarsening rate of Pd and the initial particle size of Pd obtained above is shown in FIG. 1, and the graph showing the relationship between the Pd alloying rate and the initial particle size of Pd is shown in FIG. FIG. 3 shows a graph showing the relationship between the initial particle size of Pd and the catalytic performance when the initial particle size of Pd is adjusted to 2 nm together with the data of Comparative Example 1 (Rh particle size = 0.7 nm). A graph showing the relationship between the initial particle size of Rh and the catalytic performance when the diameters are adjusted to 25 nm is shown in FIG. 4, respectively.

With reference to FIG. 1, it can be seen that when the initial particle size of Pd is set to a value larger than that of the prior art, preferably 25 nm or more, the coarsening rate of Pd particles after durability can be suppressed extremely effectively. Further, referring to FIG. 2, it is understood that when the initial particle size of Pd is set to a value larger than that of the prior art, preferably 25 nm or more, alloying of Pd is suppressed. It is presumed that these are due to the fact that the movement of Pd particles is suppressed by setting the initial particle size of Pd to be large.

In order to suppress deterioration of HC removal performance due to continuous use of the catalyst, it is preferable to set the initial Pd particle size to 25 to 80 nm, and from FIG. 3, the initial Rh particle size is set to 0.7 to 10 nm. It can be read from FIG. 4 that it is preferable to do so.

<< Examples 14 to 23 >>
In the following Examples 14 to 23, the influence of each catalyst metal concentration on various evaluation results is influenced by keeping the initial particle sizes of Pd and Rh constant and using the Pd concentration (supporting rate) and Rh concentration (supporting rate) as variables. Examined.

<Examples 14 to 23>
The catalyst on the substrate was the same as in Example 1, except that the immersion conditions and particle growth conditions for preparing Pd / Al ₂ O _{3 and Rh / CZ were changed as shown in Table 3, respectively.} A catalyst with layers was prepared.

The catalysts prepared in each of the above Examples were measured and evaluated in the same manner as in Example 1, and various evaluation results are shown in Table 3 together with the results of Example 7.

FIG. 5 shows the relationship between the Pd concentration and the catalytic performance when the initial particle diameters of Pd and Rh were constant in the range of Rh concentration = 0.02% by weight to 1% by weight. In addition, a graph showing the relationship between the Rh concentration and the catalyst performance when the initial particle diameters of Pd and Rh are constant is shown in FIG. 6, respectively.

From FIGS. 5 and 6, the Pd-supported amount and the Rh-supported amount for maintaining high catalytic performance after durability shall be set to 0.1 to 5% by weight and 0.05 to 1.0% by weight, respectively. It can be read that is preferable.

<< Examples 24 and 25 >>
In Examples 24 and 25 below, the effect of carrier type on catalytic activity was investigated.

<Example 24>
Pd (0.5) / CZ was prepared by immersing CZ in an aqueous palladium nitrate solution and heated at 1,000 ° C. for 10 hours in a CO atmosphere to grow Pd particles. Similarly, Rh (0.1) / CZ is prepared by immersing alumina in an aqueous solution of rhodium hydrochloride, and Ph particles are grown by heating at 900 ° C. for 5 hours _{in a 10% by volume oxygen (N 2 balance) atmosphere.} It was. A catalyst of Pd 0.5 g / L, Rh 0.1 g / L, CZ 100 g / L _{, and Al 2} O ₃ 100 g / L was used on the substrate by the same method as in Example 1 except that the carrier after particle growth was used. A catalyst with a layer was obtained.

<Example 25>
_{Pd (0.5) / (Al 2} O ₃ + CZ) was prepared by mixing alumina and CZ with a weight ratio of 1: 1 and immersing the obtained mixture in an aqueous palladium nitrate solution. Pd particles were grown by heating at 000 ° C. for 10 hours. _{Rh (0.1) / (Al 2} O ₃ + CZ) is prepared by mixing alumina and CZ with a weight ratio of 1: 1 and immersing the obtained mixture in an aqueous solution of rhodium hydrochloride, and 10% by volume oxygen (N). Rh particles were grown by heating at 900 ° C. for 5 hours in a ( _{2 balance) atmosphere.}

On the substrate by the same method as in Example 1 except that Pd (0.5) / (Al ₂ O ₃ + CZ) and Rh (0.1) / (Al ₂ O _{3 + CZ) after the particle growth were used.} A catalyst having a catalyst layer of Pd 0.5 g / L, Rh 0.1 g / L, CZ _{100 g / L, and Al 2} O _{3 100 g / L was obtained.}

The catalysts obtained in Examples 24 and 25, respectively, were measured and evaluated in the same manner as in Example 1, and various evaluation results are shown in Table 4 together with the results of Examples 7 and 10 and Comparative Example 1.

From Table 4, it is possible to maintain high catalytic performance after durability regardless of whether palladium and rhodium are supported on either alumina or CZ, respectively, but both palladium and rhodium are supported on the same type of carrier. If so, it was found that it is not a preferable embodiment from the viewpoint of catalytic performance after durability. However, even in this case, the alloying ratio after durability was low.

Claims (9)

An exhaust gas purification catalyst having a catalyst layer containing palladium particles, rhodium particles, and carrier particles.
The initial particle size of the palladium particles is 10 nm or more and 90 nm or less.
The initial particle size of the rhodium particles is 2 nm or more and 15 nm or less.
The palladium particles and the rhodium particles are supported on separate and different types of carrier particles, and
_{Oxidation treatment of O 2} (10% by volume), N ₂ (90% by volume), and flow rate of 1 L / min, and CO (5% by volume), N ₂ (95% by volume) with the introduced gas temperature set to 1,000 ° C. And the reduction treatment at a flow rate of 1 L / min were repeated alternately in this order for 5 minutes each, and after performing a durability test for 5 hours, the alloying rate of the palladium measured by X-ray diffraction analysis was 45%. The catalyst, which is characterized by the following. The catalyst according to claim 1, wherein the ratio of the palladium particles to the rhodium particles used is 0.3 or more and 7 or less as the ratio (Pd / Rh, weight ratio) of the palladium particle concentration to the rhodium particle concentration. .. The palladium particles are supported on carrier particles made of alumina, and the palladium particles are supported on the carrier particles made of alumina.
The catalyst according to claim 1 or 2, wherein the rhodium particles are supported on carrier particles made of a ceria-zirconia composite oxide. The catalyst layer further comprises a component selected from binders, transition metals, alkali metal compounds, alkaline earth metal compounds, and rare earth compounds.
However, the transition metal is selected from nickel, copper, manganese, iron, cobalt, and zinc, and the rare earth compound is selected from lanthanum oxide, placeodymium oxide, and neodymium oxide.
The catalyst according to any one of claims 1 to 3. The catalyst according to any one of claims 1 to 4, wherein the catalyst layer is present on the substrate. The catalyst according to any one of claims 1 to 4, wherein the catalyst layer constitutes a part of a substrate. A method for producing the catalyst according to any one of claims 1 to 5.
A step of producing the first material through a step of contacting the first carrier particles with the palladium precursor and then heating in a reducing atmosphere to grow the palladium particles, and a step of producing the first material, and the first material and the above-mentioned first material on the substrate. forming a catalyst layer comprising a second material comprising carrying the rhodium particles on the second carrier particles, and viewed including the step of firing the substrate having the catalyst layer,
The first with carrier particles and the second carrier particles, characterized in that it is a different kind of carrier particles, said method. The method for producing the catalyst according to any one of claims 1 to 4 and 6.
A step of producing a first material through a step of bringing the palladium precursor into contact with the first carrier particles and then heating in a reducing atmosphere to grow the palladium particles, and the first material and the second a step of firing the raw material mixture containing a second material comprising carrying rhodium particles on the carrier particles seen including,
The first with carrier particles and the second carrier particles, characterized in that it is a different kind of carrier particles, said method. According to claim 7 or 8, the second material is produced through a step of bringing the rhodium precursor into contact with the second carrier particles and then heating the rhodium precursor in an oxidizing atmosphere to grow the rhodium particles. The method described.

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