JP2007111640A - Exhaust gas purification catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purification catalyst capable of preventing a decrease of catalytic activity after long time use. <P>SOLUTION: The exhaust gas purification catalyst consists of noble metal particles 2, a carrier 3 coating the surface of the noble metal particles 2 and made of a porous oxide, and an agglomeration suppressing material 4 coating the surface of the carrier 3 and made of alumina in mesh-like state. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst that purifies exhaust gas containing hydrocarbons (HC), carbon monoxide (CO), nitrided oxides (NOx), and the like discharged from an internal combustion engine represented by an automobile.

  The exhaust gas emitted from internal combustion engines such as automobiles contains harmful gases such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Purified with a three-way catalyst.

The three-way catalyst is made of a slurry containing a catalyst powder in which noble metal particles (for example, Pt, Pd, Rh, etc.) are supported on the surface of a support (for example, porous alumina (γ-Al ₂ O ₃ )). (For example, cordierite honeycomb).

  The three-way catalyst is a catalytic reaction in which the reaction proceeds on the surface of the noble metal particles, and the activity of the catalyst is almost proportional to the surface area of the noble metal particles. For this reason, noble metal particles having a small particle diameter are uniformly dispersed in the support, and the surface area of the noble metal particles is increased to increase the activity of the catalyst.

  On the other hand, when the particle size of the noble metal particles is reduced, the surface energy of the noble metal particles is increased, and aggregation is likely to occur as the noble metal particles move. There are two factors that cause the aggregation of the noble metal particles, that is, the noble metal particles themselves move and aggregate, and that the noble metal particles supported on the surface of the carrier aggregate due to the contraction of the carrier. In order to suppress aggregation of the noble metal particles, it is effective to control the interparticle distance of the noble metal particles, the average particle diameter of the noble metal particles, the physical obstacle that inhibits the movement of the noble metal particles, or the chemical reaction constraint.

  However, the three-way catalyst is installed, for example, directly under the engine of an automobile, but is exposed to a high-temperature environment directly below the engine of 800 ° C. to 900 ° C., and in some cases exceeding 1000 ° C. For this reason, the movement of the noble metal particles themselves and the contraction of the carrier are likely to occur, and accordingly, the aggregation of the noble metal particles occurs, making it difficult to maintain the activity of the catalyst at the time of catalyst preparation.

Therefore, an exhaust gas purification catalyst having a core-shell structure in which noble metal ultrafine particles are used as a core substance and the periphery thereof is surrounded by a silica (SiO ₂ ) layer is disclosed (see Patent Document 1). According to this exhaust gas purifying catalyst, the precious metal ultrafine particles are coated with a silica layer, so that it has high thermal stability, hardly aggregates even when heated to a high temperature of about 800 ° C., and heat resistance is improved. .
JP-A-11-246227

  However, even the above-described exhaust gas purification catalyst has a risk that the activity of the catalyst is lowered after the endurance.

This is because the amorphous phase of SiO ₂ forming the silica layer forms a crystal phase due to the water generated during the purification reaction of the exhaust gas and the high-temperature atmosphere, and the core-shell structure may collapse.

  Further, even when the core-shell structure is not completely collapsed and remains partly, the silica layer is severely deteriorated and the pores may be clogged. For this reason, aggregation of the noble metal particles is likely to occur, and furthermore, the ratio of contact between the noble metal particles and the exhaust gas is reduced, which may reduce the activity of the catalyst.

  The present invention has been made to solve the above-described problems. That is, the exhaust gas purifying catalyst of the present invention is formed of a noble metal particle and a support formed by coating the surface of the noble metal particle and a porous oxide. And an aggregation-suppressing material formed of alumina, which is coated on the surface of the carrier in a mesh shape.

  According to the exhaust gas purification catalyst of the present invention, it is possible to prevent a decrease in the activity of the catalyst after durability.

  Hereinafter, an exhaust gas purification catalyst according to an embodiment of the present invention will be described.

  FIG. 1 shows a cross-sectional view of an exhaust gas purification catalyst according to an embodiment of the present invention. The exhaust gas purification catalyst 1 has a two-layer structure in which a support 3 formed of a porous oxide is coated around a noble metal particle 2, that is, a core-shell structure. The periphery of the carrier 3 is further coated with an agglomeration suppressing material 4 formed of alumina in a mesh shape.

In this way, the carrier 3 is coated around the noble metal particles 2 to form a core-shell structure, whereby the movement of the noble metal particles 2 is physically hindered, and the surface area of the noble metal particles 2 is reduced due to the aggregation of the noble metal particles 2. Can be suppressed. Further, when the aggregation suppressing material 4 formed of alumina (Al ₂ O ₃ ) is coated around the support 3, the shrinkage of the porous oxide which is the support 3 generated by the water generated during the purification reaction of the exhaust gas and the high temperature atmosphere. Is suppressed, and aggregation of the noble metal particles 2 accompanying the contraction of the carrier 3 can be prevented. As a result, even when the exhaust gas purification catalyst 1 is exposed to a high temperature environment of 800 ° C. or higher, the noble metal particles 2 are held by the carrier 3 and the aggregation suppressing material 4, and as a result, are surrounded by the carrier 3. A catalyst in which the noble metal particles 2 are uniformly dispersed can be prepared.

  The noble metal particles 2 are preferably at least one element selected from Pt, Pd and Rh, or two or more elements.

  The average particle diameter X of the noble metal particles 2 shown in FIG. 2 is preferably 1 nm to 20 nm. This is because if the average particle diameter X of the noble metal particles 2 exceeds 20 nm, the surface area of the noble metal particles 2 becomes small, and the activity of the catalyst decreases. The average particle diameter X of the noble metal particles 2 can be controlled by changing the chemicals used in the preparation of the reverse micelle solution in the production method described later, the order of use, and the like.

The support 3 is formed of an oxide different from alumina which is the aggregation suppressing material 4, and specifically, at least one oxide selected from ceria, silica, zircon and titania, or a composite of two or more. It is preferable to use an oxide. By making the support | carrier 3 and the aggregation suppression material 4 into a different oxide, integration with the support | carrier 3 and the aggregation suppression material 4 is prevented, and it prevents that the function as the aggregation suppression material 4 is impaired. The interface between the carrier 3 and the aggregation suppressing material 4 may be formed by partially forming a compound with a porous oxide (for example, SiO ₂ ) and alumina (Al ₂ O ₃ ).

  In addition, as shown in FIG. 2, the carrier 3 can obtain an effect of suppressing aggregation of the noble metal particles 2 as the average particle size Y becomes larger, but the average particle size Y of the carrier 3 becomes too large. As a result, the contact rate between the exhaust gas to be purified and the noble metal particles 2 decreases. Considering these, in order to maintain both the effect of suppressing aggregation of the noble metal particles 2 and the effect of contact between the noble metal particles 2 and the exhaust gas, the average particle diameter Y of the carrier 3 is set to 30 nm or less. It is preferable.

  In the exhaust gas purification catalyst 1, the average pore diameter of the carrier 3 is preferably 0.1 nm to 10 nm. When the average pore diameter of the support 3 is less than 0.1 nm, the ratio of contact between the noble metal particles 2 and the exhaust gas decreases. Conversely, when the average pore diameter of the support 3 exceeds 10 nm, the noble metal particles 2 move and aggregate. Because it will do.

  Furthermore, in the exhaust gas purification catalyst 1, the exposure rate of the noble metal particles 2 is preferably 10% to 80%. That is, the noble metal particles 2 are preferably exposed from the support 3 within a range of 10% to 80% of the surface area. Hereinafter, a method for measuring the exposure rate will be described.

  The exposure rate means the ratio of the precious metal particles 2 exposed on the surface of the support 3 to the precious metal particles 2 present in the exhaust gas purification catalyst 1, and specifically, the precious metal outer surface area (PMSA) calculated by CO adsorption described later. And the ratio of the particle surface area (TSA) theoretically calculated from the average particle diameter X of the noble metal particles 2 obtained from the TEM observation results. Note that the noble metal particles 2 observed by TEM can also observe the noble metal particles 2 not exposed on the surface of the carrier 3. For this reason, if all the noble metal particles 2 are exposed on the surface of the carrier 3, a gas adsorption amount stoichiometrically adsorbed to TSA is obtained, and TSA and PMSA have the same value. However, when the noble metal particles 2 are supported in a state of being covered with the carrier 4, a gas adsorption amount that is stoichiometrically adsorbed with respect to the noble metal surface area determined from the average particle diameter of the noble metal particles 2 cannot be obtained. . Therefore, the ratio of the surface area of the noble metal exposed from the carrier 3 is calculated from the average particle diameter of the noble metal particles 2 observed by TEM and the amount of gas adsorbed on the sample, and the exposure rate is obtained.

PMSA is calculated by the following formula.

TSA is calculated by the following method. The average particle diameter of the noble metal particles 2 by TEM observation is defined as [D]. [D] When the number of atoms of the noble metal constituting one is [A], the number (n) of [D] contained in the catalyst can be calculated from the number of noble metal atoms [N] charged at the time of preparation.

  The exposure rate is calculated from the ratio of PMSA and TSA obtained.

Exposure rate (%) = (PMSA / TSA) x 100
If this is simplified, the following equation is obtained, and the exposure rate can be obtained from this equation. The values used in the formula are shown in Table 1.

The unit CO adsorption amount is measured in order to obtain the above exposure rate. The unit CO adsorption amount is measured using a metal dispersion measuring device (BEL-METAL-3 manufactured by Nippon Bell Co., Ltd.) as follows. Measured according to the procedure. The sample was heated to 400 ° C. at 10 ° C./min in a He 100% gas stream, and then oxidized at 400 ° C. for 15 minutes in an O ₂ 100% gas stream. Thereafter, purging was performed with He100% gas for 5 minutes, and reduction treatment was performed at 400 ° C. for 15 minutes in a H ₂ 40% / He balance gas stream. Next, after the temperature was lowered to 50 ° C. in a He100% gas stream, CO10% / He balance gas was introduced in a pulsed manner to determine the unit CO adsorption amount.

  In addition, the exhaust gas purification catalyst according to the embodiment of the present invention is, for example, an exhaust gas purification catalyst having a catalyst layer by coating the catalyst slurry on a refractory inorganic base material after making the catalyst slurry containing the catalyst described above. It is preferably used in practice.

  The exhaust gas purification catalyst is manufactured using the reverse micelle method shown below.

  First, a surfactant and a noble metal salt aqueous solution are added to an organic solvent and stirred, and then an emulsion solution containing many reverse micelles (reverse micelle solution) is obtained. The reverse micelle is formed into a spherical shape with a surfactant, an oil phase is formed outside the reverse micelle, and an aqueous phase is formed inside, and the particle size of the reverse micelle is about several tens of nm.

  Thereafter, a fine particle precipitant is introduced into the emulsion solution (reverse micelle solution) to precipitate noble metal particles.

  Next, an organic solvent in which a carrier precursor is dispersed is introduced into the reverse micelle solution, and the periphery of the noble metal particles is covered with the carrier precursor.

Furthermore, the obtained solution is put into an organic solvent in which a precursor of an aggregation inhibitor (aluminum isopropoxide) is dispersed. As a result, a solution containing a catalyst precursor in which the support precursor is coated around the noble metal particles and the Al ₂ O ₃ precursor is coated around the support precursor is obtained inside the reverse micelle.

  The solution containing the obtained catalyst precursor is filtered and dried, and then fired in an air atmosphere at a temperature of 350 ° C. to 650 ° C. As a result, a catalyst powder can be obtained in which the support is coated around the noble metal particles and the aggregation inhibitor is coated around the support.

  Hereinafter, materials that can be used in the above-described method for producing an exhaust gas purification catalyst will be described, but the present invention is not limited to these materials.

  Precious metal salts include dinitrodiammine Pt (II) nitric acid aqueous solution, hexachloroPt (IV) acid solution, hexaammine Pt (IV) tetrachloride solution, Pd chloride aqueous solution, palladium nitrate aqueous solution, dinitrodiammine Pd dichloride solution, rhodium chloride solution And a rhodium nitrate solution, a ruthenium chloride solution, a ruthenium nitrate solution, and an aqueous hexachloroiridate solution.

  Examples of the organic solvent include cyclohexane, methylcyclohexane, cycloheptane, heptanol, octanol, dodecyl alcohol, cetyl alcohol, isooctane, n-heptane, n-hexane, n-decane, benzene, toluene, xylene, and the like. You may use the organic solvent of 1 type selected from these, or the organic solvent which mixed 2 or more types.

  Surfactants include polyoxyethylene nonyl phenyl ether, polyoxyethylene cetyl ether, magnesium laurate, zinc caprate, zinc myristate, sodium phenyl stearate, aluminum dicaprylate, tetraisoamyl ammonium thiocyanate, n-octadecyl tri n-butylammonium formate, n-amyltri-n-butylammonium iodide, sodium bis (2-ethylhexyl) succinate, sodium dinonylnaphthalene sulfonate, calcium cetyl sulfate, dodecylamine oleate, dodecylamine propionate, Cetyltrimethylammonium bromide, stearyltrimethylammonium bromide, cetyltrimethylammonium chloride, stearylto Methylammonium chloride, dodecyltrimethylammonium bromide, octadecyltrimethylammonium bromide, dodecyltrimethylammonium chloride, octadecyltrimethylammonium chloride, didodecyldimethylammonium bromide, ditetradecyldimethylammonium bromide, didodecyldimethylammonium chloride, ditetradecyldimethylammonium chloride, And (2-octyloxy-1-octyloxymethyl) polyoxyethylene ethyl ether. You may use the 1 type selected from these, or 2 or more types of solutions.

Reducing agents include hydrazine (N ₂ H ₄ ), sodium borohydride (NaBH ₄ ), sodium thiosulfate, citric acid, sodium citrate, L-ascorbic acid, formic acid, formaldehyde, methanol, ethanol, ethylene, vitamin B One type selected from these, or a mixed solution of two or more types may be used.

  As the precipitating agent, those capable of obtaining a noble metal and metal oxide such as aqueous ammonia and tetramethylammonium hydroxide can be used.

Examples of the carrier precursor include tetraethoxysilane which is a precursor of SiO ₂ , zirconium isopropoxide which is a precursor of ZrO ₂ , and titanium tetraisopropoxide which is a precursor of TiO ₂ .

  An example of the aggregation inhibitor is aluminum isopropoxide.

  As described above, the precursor of the carrier and the raw material of the aggregation suppressing material are not particularly limited, but can be stably prepared by using an alkoxide.

  Hereinafter, further specific description will be made using examples. In addition, of course, the exhaust gas purification catalyst which concerns on embodiment of this invention is not limited to an Example.

Example 1
Polyoxyethylene cetyl ether is added to 500 ml of cyclohexane to a concentration of 0.05 mol / L, and 0.001 mol of diluted platinum chloride solution is mixed in ion-exchanged water. After stirring for 2 hours, Pt ions are present inside. A solution containing reverse micelles (reverse micelle solution) was prepared.

80.4 cc of NH ₃ water was added to the obtained reverse micelle solution to precipitate Pt ions as Pt particles.

Thereafter, 80 ml of cyclohexane in which 2 g of tetraethoxysilane as a precursor of SiO ₂ was dispersed was dropped into the reverse micelle solution, and the periphery of the Pt particles was covered with the precursor of SiO ₂ .

The obtained solution was dropped into cyclohexane in which 250 g of aluminum isopropoxide was dispersed, and the catalyst precursor in which the precursor of SiO _{2 and} the precursor of Al ₂ O ₃ were coated around the Pt particles was reverse micelle. The solution contained inside.

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder carrying 0.3 wt% Pt.

Example 2
Add polyoxyethylene cetyl ether to 500 ml of cyclohexane to 0.05 mol / L, mix 0.001 mol of palladium chloride solution diluted in ion-exchanged water, stir for 2 hours, and reverse the presence of Pd ions inside. A solution containing micelles (reverse micelle solution) was prepared.

To the obtained reverse micelle solution, 80.4 cc of NH ₃ water was added to precipitate Pd ions as Pd particles.

Thereafter, 80 ml of cyclohexane in which 2 g of tetraethoxysilane as a precursor of SiO ₂ was dispersed was dropped into the reverse micelle solution, and the periphery of the Pd particles was covered with the precursor of SiO ₂ .

The obtained solution was dropped into cyclohexane in which 250 g of aluminum isopropoxide was dispersed, and the catalyst precursor in which the precursor of SiO _{2 and} the precursor of Al ₂ O ₃ were coated around the Pd particles was reverse micelle. The solution contained inside.

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder carrying 0.3 wt% Pd.

Example 3
Polyoxyethylene cetyl ether is added to 500 ml of cyclohexane to a concentration of 0.05 mol / L, and 0.001 mol of diluted rhodium chloride solution is mixed in ion-exchanged water. After stirring for 2 hours, Rh ions are present inside. A solution containing reverse micelles (reverse micelle solution) was prepared.

To the obtained reverse micelle solution, 80.4 cc of NH ₃ water was added to precipitate Rh ions as Rh particles.

Thereafter, 80 ml of cyclohexane in which 2 g of tetraethoxysilane as a precursor of SiO ₂ was dispersed was dropped into the reverse micelle solution, and the periphery of the Rh particles was covered with the precursor of SiO ₂ .

The obtained solution was dropped into cyclohexane in which 250 g of aluminum isopropoxide was dispersed, and the catalyst precursor in which the SiO ₂ precursor and the Al ₂ O ₃ precursor were coated around the Rh particles was placed inside the reverse micelle. The solution contained in

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder carrying 0.3 wt% Rh.

Example 4
Polyoxyethylene cetyl ether is added to 500 ml of cyclohexane to a concentration of 0.05 mol / L, and 0.001 mol of diluted platinum chloride solution is mixed in ion-exchanged water. After stirring for 2 hours, Pt ions are present inside. A solution containing reverse micelles (reverse micelle solution) was prepared.

80.4 cc of NH ₃ water was added to the obtained reverse micelle solution to precipitate Pt ions as Pt particles.

Thereafter, 80 ml of cyclohexane in which 6.7 g of tetraethoxysilane as a precursor of SiO ₂ was dispersed was dropped into the reverse micelle solution, and the periphery of the Pt particles was covered with the precursor of SiO ₂ .

The obtained solution was dropped into cyclohexane in which 250 g of aluminum isopropoxide was dispersed, and the catalyst precursor in which the precursor of SiO _{2 and} the precursor of Al ₂ O ₃ were coated around the Pt particles was reverse micelle. The solution contained inside.

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder carrying 0.3 wt% Pt.

Example 5
Polyoxyethylene cetyl ether is added to 500 ml of cyclohexane to a concentration of 0.05 mol / L, and 0.001 mol of diluted platinum chloride solution is mixed in ion-exchanged water. After stirring for 2 hours, Pt ions are present inside. A solution containing reverse micelles (reverse micelle solution) was prepared.

80.4 cc of NH ₃ water was added to the obtained reverse micelle solution to precipitate Pt ions as Pt particles.

Thereafter, 80 ml of cyclohexane in which 20 g of tetraethoxysilane as a precursor of SiO ₂ was dispersed was dropped into the reverse micelle solution, and the periphery of the Pt particles was covered with the precursor of SiO ₂ .

The obtained solution was dropped into cyclohexane in which 250 g of aluminum isopropoxide was dispersed, and the catalyst precursor in which the precursor of SiO _{2 and} the precursor of Al ₂ O ₃ were coated around the Pt particles was reverse micelle. The solution contained inside.

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder carrying 0.3 wt% Pt.

Example 6
Add polyoxyethylene cetyl ether to 500 ml of cyclohexane to 0.05 mol / L, mix 0.001 mol of diluted platinum chloride solution in ion-exchanged water, stir for 2 hours, and then add Pt ions inside. A solution containing micelles (reverse micelle solution) was prepared.

80.4 cc of NH ₃ water was added to the obtained reverse micelle solution to precipitate Pt ions as Pt particles.

Thereafter, 80 ml of cyclohexane in which 1.8 g of zirconium isopropoxide as a precursor of ZrO ₂ was dispersed was dropped into the reverse micelle solution, and the periphery of the Pt particles was covered with the precursor of ZrO ₂ .

The obtained solution was dropped into cyclohexane in which 250 g of aluminum isopropoxide was dispersed, and a catalyst precursor in which a ZrO ₂ precursor and an Al ₂ O ₃ precursor were coated around Pt particles was used as a reverse micelle. The solution contained inside.

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder carrying 0.3 wt% Pt.

Example 7
Add polyoxyethylene cetyl ether to 500 ml of cyclohexane to a concentration of 0.05 mol / L, mix 0.001 mol of diluted platinum chloride solution in ion-exchanged water, stir for 2 hours, and reverse the presence of Pt ions inside. A solution containing micelles (reverse micelle solution) was prepared.

80.4 cc of NH ₃ water was added to the obtained reverse micelle solution to precipitate Pt ions as Pt particles.

Thereafter, 80 ml of cyclohexane in which 2.1 g of titanium tetraisopropoxide as a precursor of TiO ₂ was dispersed was dropped into the reverse micelle solution, and the periphery of the Pt particles was covered with the precursor of TiO ₂ .

The obtained solution was dropped into cyclohexane in which 250 g of aluminum isopropoxide was dispersed, and a catalyst precursor in which a TiO ₂ precursor and an Al ₂ O ₃ precursor were coated around Pt particles was used as a reverse micelle. The solution contained inside.

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder carrying 0.3 wt% Pt.

Comparative Example 1
Add polyoxyethylene cetyl ether to 500 ml of cyclohexane to 0.5 mol / L, mix 001 mmol of platinum chloride solution diluted in ion-exchanged water, stir for 2 hours, and then reverse micelles containing Pt ions inside A solution (reverse micelle solution) containing was prepared.

80.4 cc of NH ₃ water was added to the obtained reverse micelle solution to precipitate Pt ions as Pt particles.

Thereafter, the reverse micelle solution was dropped into cyclohexane in which 2 g of tetraethoxysilane was dispersed, and the periphery of the Pt particles was covered with a precursor of SiO ₂ .

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder supporting 25 wt% Pt.

Further, a commercially available SiO ₂ powder was mixed with the obtained catalyst powder so that 0.3 wt% of Pt was supported, and a catalyst powder with 0.3 wt% of Pt supported was obtained.

Comparative Example 2
Add polyoxyethylene cetyl ether to 500 ml of cyclohexane to 0.5 mol / L, mix 001 mmol of palladium chloride solution diluted in ion-exchanged water, stir for 2 hours, and then reverse micelles containing Pd ions inside A solution (reverse micelle solution) containing was prepared.

To the obtained reverse micelle solution, 80.4 cc of NH ₃ water was added to precipitate Pd ions as Pd particles.

Thereafter, the reverse micelle solution was dropped into cyclohexane in which 2 g of tetraethoxysilane was dispersed, and the periphery of the Pd particles was covered with a precursor of SiO ₂ .

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder supporting 25 wt% Pd.

Further, commercially available SiO ₂ powder was mixed with the obtained catalyst powder so that Pd was 0.3 wt% to obtain catalyst powder carrying 0.3 wt% Pd.

Comparative Example 3
Add polyoxyethylene cetyl ether to 500 ml of cyclohexane to 0.5 mol / L, mix 001 mmol of diluted rhodium chloride solution in ion-exchanged water, stir for 2 hours, then reverse micelles with Pt ions inside A solution (reverse micelle solution) containing was prepared.

80.4 cc of NH ₃ water was added to the obtained reverse micelle solution to precipitate Pt ions as Pt particles.

Thereafter, the reverse micelle solution was dropped into cyclohexane in which 2 g of tetraethoxysilane was dispersed, and the periphery of the Rh particles was covered with a SiO ₂ precursor.

  The obtained solution was filtered and dried, and then calcined at 500 ° C. in an air atmosphere to obtain a catalyst powder supporting 25 wt% Rh.

Commercially available SiO ₂ powder was mixed with the obtained catalyst powder so that Rh was 0.3 wt% to obtain catalyst powder carrying 0.3 wt% Rh.

  50 g of the catalyst powder obtained from each of the above Examples and Comparative Examples, 10 g of boehmite, and 157 g of an aqueous solution containing 10% nitric acid were put into an alumina magnetic pot, and immersed and ground together with alumina balls to obtain a catalyst slurry. Thereafter, the obtained catalyst slurry was put into a honeycomb carrier (manufactured by cordierite, 900 cells / 2.5 mil, capacity 0.06 L), excess slurry was removed with an air stream, and then dried at 120 ° C. in an air stream. The catalyst was calcined at 400 ° C.

  Performance was evaluated about each obtained catalyst.

  First, the obtained catalyst was connected to the exhaust line of a V6 type 3.5L engine (manufactured by Nissan Motor Co., Ltd.) and operated at a durable temperature (catalyst inlet) of 700 ° C for 30 hours to conduct a simulated durability deterioration test. The catalyst was made durable. The fuel used at this time was unleaded gasoline.

After enduring the catalyst, a part of the catalyst was cut out, and each catalyst having a catalyst capacity of 40 cc was incorporated into a simulated exhaust gas distribution device. First, as a pretreatment, a simulated gas having the composition shown in Table 2 was circulated at 500 ° C. for 10 minutes and once replaced with N ₂ gas. Thereafter, a simulated gas having the composition shown in Table 2 was again introduced, and the simulated gas having the composition shown in Table 2 was again introduced when the catalyst outlet gas concentration was stabilized at 350 ° C. Then, when the catalyst outlet gas concentration was stabilized, the ηNOx conversion rate (%) at 350 ° C. was determined based on Equation 6.

  Further, the catalyst layer of the catalyst after endurance was scratched, and TEM-EDX measurement was performed.

  First, a small sample was pulverized on an agate mortar, mixed with a polymer dispersion medium, and kneaded well to obtain a paste. Thereafter, a small amount of paste was taken and applied to the microgrid, and then the microgrid from which the dispersion medium was removed with an organic solvent was observed.

  In the obtained image, the oxide portion forming a core-shell structure was identified by focusing on the oxide portion, and the particle size of the support was measured. After measuring the particle size of the carrier at several places, the average value was obtained and used as the average particle size Y of the carrier. At this time, HF-2000 (manufactured by Hitachi, Ltd.) was used as the transmission electron microscope, and the acceleration voltage was 200 kV and the cutting conditions were room temperature.

Table 3 shows the measurement results of the average particle diameter Y of the carrier before durability, the ηNOx conversion rate (%) at 350 ° C., and the average particle diameter X of the noble metal particles 2 after durability.

As shown in Table 3, Comparative Examples 1 to 3 in which the carrier is coated around the noble metal particles and the aggregation inhibitor is not coated around the carrier, the average particle diameter of the noble metal particles after durability is 10 nm. Above this, the ηNOx conversion at 350 ° C. was reduced. On the other hand, in each example in which the support and the aggregation inhibitor are coated around the noble metal particles, the average particle diameter of the noble metal particles is in the range of 1 nm to 10 nm, and the ηNOx conversion rate of the example compared to the comparative example Showed a high value. In particular, the ηNOx conversion rates of Examples 4 and 5 in which the average particle size of the carrier before durability exceeded 30 nm were lower than those in Example 1 in which the average particle size of the carrier was 30 nm or less. There was found. Further, it has been found that Examples 6 and 7 using ZrO ₂ and TiO ₂ as the support show particularly high values of NOx conversion. From these results, it was found that by reducing the activity of the catalyst after endurance, it was possible to make the exhaust gas purifying catalyst by coating the support around the noble metal particles and further covering the support with the aggregation inhibitor.

1 is an enlarged cross-sectional view of an exhaust gas purification catalyst according to an embodiment of the present invention. In the enlarged sectional view of the exhaust gas purification catalyst shown in FIG. 1, it is a diagram for explaining the average particle diameter X of noble metal particles and the average particle diameter Y of a carrier.

Explanation of symbols

1 ... Exhaust gas purification catalyst,
2 ... Precious metal particles,
3 ... carrier,
4 ... Aggregation inhibitor,

Claims (8)

Precious metal particles,
A support formed on a surface of the noble metal particles and formed of a porous oxide;
An agglomeration inhibitor formed of alumina coated on the surface of the carrier in a network;
An exhaust gas purifying catalyst characterized by comprising:   The exhaust gas purification catalyst according to claim 1, wherein the carrier is at least one oxide selected from ceria, silica, zircon, and titania, or two or more complex oxides.   The exhaust gas purification catalyst according to claim 1 or 2, wherein an average particle diameter of the noble metal particles is 1 nm to 20 nm.   The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein the noble metal particles are one kind or two or more kinds of elements selected from Pt, Pd, and Rh.   The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein the carrier has an average particle size of 30 nm or less.   The exhaust gas purification catalyst according to any one of claims 1 to 5, wherein an average pore diameter of the carrier is 0.1 nm to 10 nm.   The exhaust gas purification catalyst according to any one of claims 1 to 6, wherein an exposure rate of the noble metal particles is 10% to 80%.   It has a catalyst layer containing the catalyst of any one of Claim 1 thru | or 7, The exhaust gas purification catalyst characterized by the above-mentioned.

Images