JP2008221217A - Catalyst for cleaning exhaust gas and method of manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning exhaust gas capable of suppressing sintering of noble metal particles and maintaining high catalytic activity even in high temperature and oxidative environments, and a method of manufacturing the same. <P>SOLUTION: The catalyst comprises: a base material 2 formed from a compound containing alumina and ceria; noble metal particles 3 supported on the base material 2 and formed from at least a noble metal element selected from the group consisting of platinum, palladium, and rhodium; and a metal oxide layer 4 covering the surface of the noble metal particles 3 or a part thereof and formed from at least an oxide of the metal selected from the group consisting of Co, Ni, Fe, Cu, Sn, Mn, Ce, and Zr, wherein the amount of the metal oxide is such that the atomic ratio of the metal to the noble metal particles 3 is 0.005-6. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying catalyst that is mounted on an automobile and purifies exhaust gas, and a method for manufacturing the same.

In recent years, exhaust gas regulations for automobiles have been strengthened, and harmful components (for example, unburned hydrocarbons (HC) and carbon monoxide (CO)) contained in exhaust gases are purified and harmless components (for example, water ( Development of exhaust gas purification catalysts that convert to H ₂ O) and gas) is underway.

The exhaust gas purifying catalyst is configured by supporting noble metal particles (for example, platinum (Pt), palladium (Pd), rhodium (Rh)) on a base material (for example, alumina (Al ₂ O ₃ )). As a measure for improving the purification performance of the exhaust gas purification catalyst, there is a method of expanding the surface area of the noble metal particles, and the noble metal particles are atomized in order to increase the surface area of the noble metal particles. As a technique for atomizing noble metal particles, there is a reverse micelle method.

In the reverse micelle method, a surfactant and an aqueous solution containing a noble metal element are mixed in an organic solvent, and a large number of reverse micelles formed by assembling the surfactant in the organic solvent are dispersed, so that the inside of the reverse micelle In this method, an emulsion containing an aqueous solution containing a noble metal element is used, the noble metal particles are precipitated, reduced, or insolubilized, and the noble metal particles are precipitated inside the reverse micelles, whereby the noble metal is atomized.
JP 2000-42411 A

  According to the reverse micelle method described above, fine noble metal particles can be produced, but noble metal particles cannot be mass-produced.

  In addition, the initial size of the precious metal particles in the exhaust gas purification catalyst is in the form of ultrafine particles of several nanometers or less, but the exhaust gas purification catalyst has been in a high temperature oxidizing atmosphere environment of about 500 ° C to 600 ° C for a long time. When exposed, the surface of the noble metal particles is not only oxidized, but also the noble metal particles are sintered (aggregated), and there is a risk that the noble metal particles in the vicinity are united. When the noble metal particles are combined, the particle diameter of the noble metal particles is increased to about several tens of nanometers, and the purification performance of the exhaust gas purifying catalyst may be significantly lowered as the specific surface area of the noble metal particles is reduced.

  The present invention has been made to solve the above problems, that is, the exhaust gas-purifying catalyst of the present invention is supported on a substrate formed of a compound containing alumina and ceria, and on the surface of the substrate. Metal oxide formed from a metal oxide coated with a noble metal particle formed from at least one kind of noble metal element selected from platinum, palladium and rhodium, and a surface of the noble metal particle or a part of the surface of the noble metal particle. And the metal oxide layer is formed of an oxide of at least one metal selected from Co, Ni, Fe, Cu, Sn, Mn, Ce, and Zr. The amount of is summarized as the atomic ratio of the metal to the noble metal particles is 0.005 to 6.

  A method for producing an exhaust gas purifying catalyst includes a method of supporting noble metal particles formed of at least one element selected from platinum, palladium, and rhodium on a base powder of alumina powder and ceria powder. A method for producing an exhaust gas purifying catalyst as described above, wherein a metal oxide layer is coated on a surface or a part of a surface of a noble metal particle, the surface of the noble metal particle or a part of the surface of the noble metal particle, Using a selective precipitation method, at least one metal selected from Co, Ni, Fe, Cu, Sn, Mn, Ce and Zr is deposited, and then fired to oxidize the deposited metal to oxidize the noble metal. The gist is to form a metal oxide layer having a metal atomic ratio of 0.005 to 6 with respect to the particles.

  According to the exhaust gas purifying catalyst of the present invention, since the surface of the noble metal particles or a part of the surface of the noble metal particles is coated with the metal oxide layer, sintering of the noble metal particles is suppressed, and high catalytic activity even in a high-temperature oxidation environment. Can be maintained.

  According to the method for producing an exhaust gas purifying catalyst of the present invention, it is possible to obtain an exhaust gas purifying catalyst having high catalytic activity even in a high temperature oxidizing environment.

  Hereinafter, an exhaust gas purifying catalyst and a manufacturing method thereof according to an embodiment of the present invention will be described with reference to FIGS.

  1 (a) and 1 (b) are cross-sectional views showing the configuration of an exhaust gas purifying catalyst according to an embodiment of the present invention. As shown in FIG. 1 (a), in the exhaust gas purifying catalyst 1, noble metal particles 3 are supported on a base material 2, and a part of the surface of the noble metal particles 3 is covered with a metal oxide layer 4. Further, as shown in FIG. 1 (b), the exhaust gas purifying catalyst 5 is covered with a metal oxide layer 4 over the entire surface of the noble metal particles 3 supported on the substrate 2. Each exhaust gas-purifying catalyst is formed of, for example, the following materials.

  The substrate is formed from a compound containing alumina and ceria. The noble metal particles are formed of at least one or more noble metal elements selected from platinum, palladium, and rhodium. The metal oxide layer is formed of a metal oxide containing at least one selected from Co, Ni, Fe, Cu, Sn, Mn, Ce, and Zr.

  Although the surface of the noble metal particle 3 or a part of the surface of the noble metal particle 3 is covered with the metal oxide layer 4, the atomic ratio of the amount of metal in the metal oxide layer 4 to the noble metal particle 3 is in the range of 0.005 to 6. It is preferable that The atomic ratio of the amount of metal in the metal oxide layer 4 is defined within this range because when the atomic ratio is less than 0.005, aggregation of the noble metal particles 3 cannot be suppressed, and conversely, when the atomic ratio exceeds 6. This is because the catalytic activity of the noble metal particles 3 decreases. The atomic ratio of the amount of metal in the metal oxide layer 4 to the noble metal particles 3 is more preferably in the range of 0.1 to 3.

  Thus, the noble metal particles 3 can be prevented from agglomerating by covering the surfaces of the noble metal particles 3 with the metal oxide layer 4 and fixing the noble metal particles 3. In addition, when a part of the surface of the noble metal particle 3 is covered with the metal oxide layer 4, the metal oxide layer 4 becomes a physical barrier and the movement of the noble metal particle 3 is suppressed, and aggregation of the noble metal particle 3 can be prevented. . As a result, even when the exhaust gas purifying catalyst is used for a long time in a high-temperature oxidizing atmosphere, the reduction of the surface area accompanying the aggregation of the noble metal particles 3 is suppressed, and the activity of the catalyst can be prevented from being lowered.

  Next, a method for producing an exhaust gas purifying catalyst according to an embodiment of the present invention will be described.

  First, noble metal particles (for example, Pt particles) are supported on a substrate using an impregnation method, and then dried and fired. Next, a metal is deposited on the surface of the noble metal particle or a part of the surface of the noble metal particle by using a selective deposition method (for example, electroless plating method). Thereafter, the fired and precipitated metal is oxidized to form a metal oxide layer on the surface of the noble metal particle or a part of the surface of the noble metal particle.

  Specifically, a Pt-supported powder having Pt particles supported on a base material is prepared. After the Pt-supported powder prepared in the Co-dissolved solution is added and stirred, sodium borohydride is added and reduced to precipitate Co to support the Co on the substrate.

  Examples of the metal used in the electroless plating method include Co, Ni, Fe, Cu, Sn, Mn, Ce, and Zr. Among these, in particular, Co, Ni, Fe, Cu, and Sn are used. It is preferable.

  Hereinafter, although an Example demonstrates concretely, the exhaust gas purification catalyst which concerns on this invention is not limited to the illustrated Example.

Example 1
Alumina base material (composite of γ-alumina with 9% cerium oxide, 6% zirconium oxide, 6% lanthanum oxide) impregnated with dinitrodiamine platinum aqueous solution, dried and fired at 400 ° C, 0.44% Pt An alumina substrate containing was obtained. 150 g of the obtained Pt 0.44% alumina base material was added to a cobalt solution containing 0.002 g of cobalt nitrate dissolved in 200 g of water and stirred at room temperature for 1 hour. Thereafter, 0.0001 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., and the atomic ratio of Co to Pt was set to 0.01. % Alumina substrate was obtained.

  On the other hand, a ceria substrate was impregnated with a dinitrodiamine platinum aqueous solution, then dried and fired at 400 ° C. to obtain a ceria substrate containing 0.375% Pt. 100 g of the obtained Pt 0.375% ceria base material was dissolved in 200 g of water and cobalt nitrate hexahydrate was dissolved in a cobalt solution containing 0.0011 g of cobalt, and the mixture was stirred at room temperature for 1 hour. Thereafter, 0.000058 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., and the atomic ratio of Co to Pt was set to 0.01. Pt 0.375% A ceria substrate was obtained.

  After adding 124.8 g of Pt 0.44% alumina substrate and 48.6 g of Pt 0.375% ceria substrate and 1.6 g of boehmite alumina with an atomic ratio of Co to Pt of 0.01, 307.5 g of water and 10% Nitric acid aqueous solution (17.5 g) was added and the powder was pulverized to obtain a slurry having an average particle diameter of 3 μm. This is designated as slurry A.

  Next, rhodium nitrate was impregnated with a γ-alumina / zirconium oxide composite compound containing 3% of zirconium to obtain a 0.6% rhodium-supported powder. Further, a zirconia base material in which cerium oxide was compounded with zirconium oxide by 24% was obtained.

  116.55 g of the obtained rhodium 0.6% supported powder, 44.45 g of zirconia base material, 11 g of alumina base material, and 3 g of boehmite alumina were added to a ball mill, and further, 307.5 g of water and 17.5 g of 10% nitric acid aqueous solution were added. By grinding, a slurry having an average particle size of 3 μm was obtained. This is designated as slurry R.

  Example 1 A slurry carrier A was coated with 141 g / L of a honeycomb carrier (capacity 0.04 L) having a diameter of 36φ, 400 cells and 6 mil, dried, further coated with a slurry R of 59 g / L, dried and fired at 400 ° C. Samples were obtained. The obtained sample of Example 1 is a catalyst which carries Pt 0.587 g / L and Rh 0.236 g / L and has an atomic ratio of Co to Pt of 0.01.

Example 2
150 g of the Pt 0.44% alumina base material prepared in Example 1 was added to a cobalt solution containing 0.01 g of cobalt nitrate dissolved in 200 g of water and stirred at room temperature for 1 hour. Thereafter, 0.0005 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., and the atomic ratio of Co to Pt was 0.05. A 44% alumina substrate was obtained.

  On the other hand, 100 g of Pt 0.375% ceria substrate prepared in Example 1 was added to a cobalt solution containing 0.0057 g of cobalt nitrate dissolved in 200 g of water and stirred at room temperature for 1 hour. Thereafter, 0.00029 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., and Pt 0.375 with an atomic ratio of Co to Pt of 0.05. % Ceria substrate was obtained.

  124.8 g of Pt 0.44% alumina base material and 48.6 g of Pt 0.375% ceria base material with an atomic ratio of Co to Pt of 0.05 and 1.6 g of boehmite alumina were added to a ball mill, and further 307.5 g of water, 10% nitric acid 17.5 g of an aqueous solution was added and the powder was pulverized to obtain a slurry having an average particle size of 3 μm. This is designated as slurry B.

  A honeycomb carrier (capacity 0.04L) with a diameter of 36φ, 400 cells and 6 mils was coated with 141g / L of slurry B, dried, then coated with 59g / L of slurry R of Example 1, dried, and then fired at 400 ° C Thus, a sample of Example 2 was obtained. The obtained sample of Example 2 is a catalyst carrying 0.587 g / L of Pt and 0.236 g / L of Rh and having an atomic ratio of Co to Pt of 0.05.

Example 3
150 g of the Pt 0.44% alumina base material prepared in Example 1 was added to a cobalt solution containing 0.02 g of cobalt nitrate dissolved in 200 g of water and stirred at room temperature for 1 hour. Thereafter, 0.001 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, and calcined at 400 ° C., so that the atomic ratio of Co to Pt was 0.1. A .44% alumina substrate was obtained.

  On the other hand, 100 g of Pt 0.375% ceria base material prepared in Example 1 was added to a cobalt solution containing 0.0113 g of cobalt nitrate dissolved in 200 g of water and stirred at room temperature for 1 hour. Thereafter, 0.00058 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, and calcined at 400 ° C., so that the atomic ratio of Co to Pt was 0.1. A 0.375% ceria substrate was obtained.

  124.8 g of Pt 0.44% alumina base material with 0.1 atomic ratio of Co to Pt and 48.6 g of Pt 0.375% ceria base material and 1.6 g of boehmite alumina were added to a ball mill, and further 307.5 g of water, 10% nitric acid 17.5 g of an aqueous solution was added and the powder was pulverized to obtain a slurry having an average particle size of 3 μm. This is designated as slurry C.

  A honeycomb carrier (capacity 0.04L) having a diameter of 36φ, 400 cells and 6 mils was coated with 141 g / L of slurry C, dried, and further coated with 59 g / L of slurry R of Example 1 and dried at 400 ° C. The sample of Example 3 was obtained by firing. The obtained sample of Example 3 is a catalyst that supports Pt 0.587 g / L and Rh 0.236 g / L, and has an atomic ratio of Co to Pt of 0.1.

Example 4
149.72 g of Pt 0.441% alumina base material prepared using the same method as in Example 1 was added to a cobalt solution containing 0.20 g of cobalt nitrate dissolved in 200 g of water, and 1 at room temperature. Stir for hours. Thereafter, 0.0102 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., and Pt 0.44 with an atomic ratio of Co to Pt of 1 % Alumina substrate was obtained.

  On the other hand, 99.84 g of a Pt 0.3756% ceria base material prepared using the same method as in Example 1 was added to a cobalt solution containing 0.1132 g of cobalt dissolved in 200 g of water and containing 0.1132 g of cobalt. For 1 hour. Thereafter, 0.0058 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., and the atomic ratio of Co to Pt was set to 1. % Ceria substrate was obtained.

  124.8 g of Pt 0.44% alumina base material and 48.6 g of Pt 0.375% ceria base material with an atomic ratio of Co to Pt of 1 and 1.6 g of boehmite alumina were added to a ball mill, and further 307.5 g of water, 10% nitric acid 17.5 g of an aqueous solution was added and the powder was pulverized to obtain a slurry having an average particle size of 3 μm. This is designated as slurry D.

  A honeycomb carrier (capacity 0.04L) having a diameter of 36φ, 400 cells and 6 mils was coated with 141 g / L of slurry D and dried, and then 59 g / L of slurry R of Example 1 was further coated and dried at 400 ° C. The sample of Example 4 was obtained by firing. The obtained sample of Example 4 is a catalyst that carries Pt 0.587 g / L and Rh 0.236 g / L and that has an atomic ratio of Co to Pt of 1.

Example 5
149.44 g of Pt 0.442% alumina base material prepared using the same method as in Example 1 is added to a cobalt solution containing 0.3987 g of cobalt nitrate dissolved in 200 g of water and at room temperature. Stir for 1 hour. Thereafter, 0.0203 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., and Pt 0.44 with an atomic ratio of Co to Pt of 2 % Alumina substrate was obtained.

  99.68 g of a Pt 0.3762% ceria base material prepared using the same method as in Example 1 was added to a cobalt solution containing 0.2265 g of cobalt nitrate dissolved in 200 g of water, and 1 at room temperature. Stir for hours. Thereafter, 0.012 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., and Pt 0.375 with an atomic ratio of Co to Pt of 2. % Ceria substrate was obtained.

  124.8 g of Pt 0.44% alumina base material and 48.6 g of Pt 0.375% ceria base material with an atomic ratio of Co to Pt of 1 and 1.6 g of boehmite alumina were added to a ball mill, and further 307.5 g of water, 10% nitric acid 17.5 g of an aqueous solution was added and the powder was pulverized to obtain a slurry having an average particle size of 3 μm. This is designated as slurry E.

  A honeycomb carrier (capacity 0.04L) having a diameter of 36φ and 400 cells of 6 mils was coated with 141 g / L of slurry E and dried, and then 59 g / L of slurry R prepared in Example 1 was further coated and dried. The sample of Example 5 was obtained by baking at ° C. The obtained sample of Example 5 is a catalyst carrying 0.587 g / L of Pt and 0.236 g / L of Rh and having an atomic ratio of Co to Pt of 2.

Example 6
149.16 g of Pt 0.4425% alumina base material prepared using the same method as in Example 1 was added to a cobalt solution containing 0.5981 g of cobalt nitrate dissolved in 200 g of water, and at room temperature. Stir for 1 hour. Thereafter, 0.0304 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., and Pt 0.44 with an atomic ratio of Co to Pt of 3 % Alumina substrate was obtained.

  On the other hand, 99.52 g of Pt 0.3768% ceria base material prepared using the same method as in Example 1 was added to a cobalt solution in which cobalt nitrate hexahydrate was dissolved in 200 g of water and 0.3396 g of cobalt was contained. Stir at room temperature for 1 hour. Thereafter, 0.0173 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, fired at 400 ° C., and Pt 0.375 with an atomic ratio of Co to Pt of 3 % Ceria substrate was obtained.

  Add 124.8g of Pt 0.44% alumina base material and 48.6g of Pt 0.375% ceria base material with an atomic ratio of Co to Pt of 3 and 1.6g of boehmite alumina to the ball mill, and further add 307.5g of water, 17.5% nitric acid aqueous solution 17.5g g was added and the powder was pulverized to obtain a slurry having an average particle size of 3 μm. This is designated slurry F.

  The honeycomb carrier (capacity 0.04L) having a diameter of 36φ, 400 cells and 6 mils was coated with 141 g / L of slurry F and dried, and then the slurry R prepared in Example 1 was further coated with 59 g / L and dried. The sample of Example 6 was obtained by baking at ° C. The obtained sample of Example 6 is a catalyst that carries Pt 0.587 g / L and Rh 0.236 g / L, and has an atomic ratio of Co to Pt of 3.

Example 7
148.88 g of Pt 0.443% alumina base material prepared using the same method as in Example 1 was added to a cobalt solution containing 0.7974 g of cobalt nitrate dissolved in 200 g of water at room temperature. Stir for 1 hour. Thereafter, 0.041 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, and calcined at 400 ° C., so that the atomic ratio of Co to Pt was set to 4. A 0.44% alumina substrate was obtained.

  99.36 g of a Pt 0.3774% ceria base material prepared using the same method as in Example 1 was added to a cobalt solution containing 0.4528 g of cobalt dissolved in 200 g of water and containing 0.4528 g of cobalt at room temperature. Stir for 1 hour. Thereafter, 0.0231 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, and calcined at 400 ° C., so that the atomic ratio of Co to Pt was set to 4. A 0.375% ceria substrate was obtained.

  Add 124.8g of Pt 0.44% alumina substrate and 48.6g of Pt 0.375% ceria substrate and 1.6g of boehmite alumina to a ball mill with an atomic ratio of Co to Pt of 4, and then add 307.5g of water and 10% nitric acid aqueous solution. 17.5 g was added and the powder was pulverized to obtain a slurry having an average particle size of 3 μm. This is designated as slurry G.

  A honeycomb carrier (capacity 0.04 L) having a diameter of 36φ, 400 cells and 6 mils was coated with 141 g / L of slurry G and dried, and then the slurry R prepared in Example 1 was further coated with 59 g / L and dried. The sample of Example 7 was obtained by baking at 0 ° C. The obtained sample of Example 7 is a catalyst that carries Pt 0.587 g / L and Rh 0.236 g / L, and has an atomic ratio of Co to Pt of 4.

Example 8
148.60 g of Pt 0.444% alumina base material prepared using the same method as in Example 1 was added to a cobalt solution containing 0.997 g of cobalt nitrate dissolved in 200 g of water at room temperature. Stir for 1 hour. Thereafter, 0.051 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, fired at 400 ° C., and Pt 0.44 with an atomic ratio of Co to Pt of 5 % Alumina substrate was obtained.

  99.20 g of Pt 0.378% ceria base material prepared using the same method as in Example 1 was added to a cobalt solution containing 0.566 g of cobalt nitrate dissolved in 200 g of water at room temperature. Stir for 1 hour. Thereafter, 0.029 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, calcined at 400 ° C., Pt 0.375 with an atomic ratio of Co to Pt of 5 % Ceria substrate was obtained.

  124.8 g of Pt 0.44% alumina base material and 48.6 g of Pt 0.375% ceria base material with an atomic ratio of Co to Pt of 5 and 1.6 g of boehmite alumina were added to a ball mill, and further 307.5 g of water, 10% nitric acid 17.5 g of an aqueous solution was added and the powder was pulverized to obtain a slurry having an average particle size of 3 μm. This is designated as slurry H.

  A honeycomb carrier (capacity 0.04L) having a diameter of 36φ, 400 cells and 6 mils was coated with 141 g / L of slurry H and dried, and then 59 g / L of slurry R prepared in Example 1 was further coated and dried. The sample of Example 8 was obtained by firing at 0 ° C. The obtained sample of Example 8 is a catalyst supporting 0.587 g / L of Pt and 0.236 g / L of Rh and having an atomic ratio of Co to Pt of 5.

Example 9
150 g of the Pt 0.44% alumina base material prepared in Example 1 was added to a cobalt solution containing 0.001 g of cobalt nitrate dissolved in 200 g of water and stirred at room temperature for 1 hour. Thereafter, 0.000051 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, and calcined at 400 ° C., so that the atomic ratio of Co to Pt was 0.005. A .44% alumina substrate was obtained.

  100 g of the Pt 0.375% ceria substrate prepared in Example 1 was added to a cobalt solution containing 0.0006 g of cobalt nitrate dissolved in 200 g of water and stirred at room temperature for 1 hour. Thereafter, 0.00003 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, and calcined at 400 ° C., so that the atomic ratio of Co to Pt was 0.005. A 0.375% ceria substrate was obtained.

  124.8 g of Pt 0.44% alumina base material and 48.6 g of Pt 0.375% ceria base material with 1.6 atomic percent of Co to Pt and 1.6 g boehmite alumina were added to a ball mill, and further 307.5 g water, 10% nitric acid. 17.5 g of an aqueous solution was added and the powder was pulverized to obtain a slurry having an average particle size of 3 μm. This is designated as Slurry I.

  A honeycomb carrier (capacity 0.04L) having a diameter of 36φ, 400 cells and 6 mils was coated with 141 g / L of slurry I and dried, and then the slurry R prepared in Example 1 was further coated with 59 g / L and dried. The sample of Comparative Example 2 was obtained by firing at ° C. The obtained sample of Comparative Example 2 is a catalyst that carries Pt 0.587 g / L and Rh 0.236 g / L, and has an atomic ratio of Co to Pt of 0.005.

Example 10
148.32 g of Pt 0.445% alumina base material prepared using the same method as in Example 1 was added to a cobalt solution containing 1.196 g of cobalt nitrate dissolved in 200 g of water at room temperature. Stir for 1 hour. Thereafter, 0.061 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, and calcined at 400 ° C., so that the atomic ratio of Co to Pt was set to 6. A 0.44% alumina substrate was obtained.

  99.04 g of a Pt 0.3786% ceria substrate prepared using the same method as in Example 1 was added to a cobalt solution containing 0.679 g of cobalt dissolved in 200 g of water, and at room temperature. Stir for 1 hour. Thereafter, 0.035 mol of sodium borohydride was added to the cobalt solution and stirred for 1 hour, and then the cobalt solution was filtered, washed, dried, fired at 400 ° C., and the atomic ratio of Co to Pt was set to 6. A 0.375% ceria substrate was obtained.

  Add 124.8g of Pt 0.44% alumina substrate and 48.6g of Pt 0.375% ceria substrate and 1.6g of boehmite alumina with Co atomic ratio of P to Co to 6, and further add 307.5g of water and 10% nitric acid aqueous solution to the ball mill. 17.5 g was added and the powder was pulverized to obtain a slurry with an average particle size of 3 μm. This is designated as slurry J.

  A honeycomb carrier (capacity 004L) having a diameter of 36φ, 400 cells and 6 mils was coated with 141 g / L of slurry J, dried, and further coated with 59 g / L of slurry R prepared in Example 1, and then dried at 400 ° C. And a sample of Comparative Example 3 was obtained. The obtained sample of Comparative Example 3 is a catalyst carrying Pt 0.587 g / L and Rh 0.236 g / L and having an atomic ratio of Co to Pt of 6.

Comparative Example 1
124.8 g of Pt 0.44% alumina base material prepared according to Example 1, 48.6 g of Pt 0.375% ceria base material prepared according to Example 1, and 1.6 g of boehmite alumina were added to a ball mill, and further 307.5 g of water. Then, 17.5 g of a 10% nitric acid aqueous solution was added and the powder was pulverized to obtain a slurry having an average particle diameter of 3 μm. This is designated as slurry X.

  A honeycomb carrier (capacity 0.04L) having a diameter of 36φ, 400 cells and 6 mils was coated with 141 g / L of slurry X and dried, and then the slurry R prepared in Example 1 was further coated with 59 g / L and dried. The sample of Comparative Example 1 was obtained by baking at ° C. The obtained sample of Comparative Example 1 is a catalyst carrying Pt 0.587 g / L and Rh 0.236 g / L, and is an ordinary catalyst in which a support is impregnated with a noble metal.

  Using each catalyst prepared in Examples 1 to 10 and Comparative Example 1, after performing an endurance test, the purification performance of each catalyst was examined.

(An endurance test)
Using each of the catalysts of Examples 1 to 10 and Comparative Example 1, 5 catalysts per bank were installed in the exhaust system of a V-type engine with a displacement of 3500 cc. Using domestic regular gasoline, the catalyst inlet temperature was set at 650 ° C for 30 hours, and a durability test was conducted based on thermal history.

(Evaluation of purification performance)
Each catalyst after the durability test was incorporated into a simulated exhaust gas distribution device. After that, simulated exhaust gas (reactive gas) having the composition shown in Table 1 is circulated through the simulated exhaust gas circulation device, and the catalyst temperature is increased at a rate of 30 ° C./minute while NOx, CO, HC (C ₃ H ₆ ). The temperature T (° C.) at which each purification rate was 50% was examined.

The investigation results for Examples 1 to 10 and Comparative Example 1 are shown in Table 2, and the investigation results of the purification rate of HC (C ₃ H ₆ ) are shown in FIG.

  As shown in Table 2, the catalyst in which the Co oxide layer was not coated on the surface of the Pt particles with the atomic ratio of Co to Pt being 0 showed a high purification temperature. On the other hand, each catalyst of Examples 1 to 10 forms a Co oxide layer on the surface of Pt particles or a part of the surface of Pt particles, and Co atoms relative to Pt are in the range of 0.005 to 6 Therefore, the purification temperature was lower than that of the comparative example. In particular, the catalysts of Examples 3 to 6 in which the atomic ratio of Co to Pt was in the range of 0.1 to 3 had a lower purification temperature and better purification performance than the other examples. The reason is considered that the Co oxide layer coated on the surface of the substrate exists on the Pt particle side, and the Co oxide layer suppresses the movement of the Pt particles and prevents sintering. In contrast, in the catalyst of Example 9, the amount of Co with respect to Pt is too small and the surface of the Pt particles is not coated with the Co compound layer, resulting in a decrease in purification performance. In Example 10, the amount of Co with respect to Pt is reversed. This is thought to be due to the fact that the Pt particles are coated with a Co compound layer on the surface of the Pt particles and become a barrier, thereby inhibiting the diffusion of the reaction gas.

(Amount of reducing agent)
Next, an experiment was conducted to determine an appropriate amount of the reducing agent (sodium borohydride).

  In carrying out the experiment, the Pt 0.4425% alumina base material used in Example 6 was used, the atomic ratio of Co to Pt was set to 3, the amount of sodium borohydride was varied, and the catalyst was prepared. It was investigated whether or not cobalt was supported on a specified amount of alumina substrate. More specifically, the amount of sodium borohydride with respect to the number of moles of cobalt is gradually increased to 0.5 times, 1 time, 2 times, 3 times, 4 times, 5 times, The amount of Co supported on the alumina substrate was quantified by ICP (plasma emission spectroscopy). When the amount of reducing agent was 3 times mol or more of Co, the Co loading rate reached 100% (the specified amount), but 0.5 times the Co loading rate was 23%, and 1 times the Co loading rate was around 65%. The amount of the reducing agent was insufficient, and when doubled, the Co loading was 97%, which was slightly less than the specified amount. When sodium borohydride is used as a reducing agent, the number of moles of sodium borohydride may be 4 or 5 times the number of moles of Co in order to support Co. However, if the amount of sodium borohydride is too large, there is a possibility that boron or sodium, which is a decomposition product of sodium borohydride, remains during cleaning. For this reason, it is preferable that the amount of sodium borohydride is 3 times the mol of cobalt. In each of the above-described examples, 3 times mole of sodium borohydride is used with respect to the number of moles of cobalt.

  The sample of Comparative Example 1 is a catalyst prepared by impregnation with a Pt solution, which is usually performed, but the catalyst of each Example carries Co after carrying Pt. Co loading is carried out by adding Pt-supported powder to a solution in which Co is dissolved and stirring, and then adding sodium borohydride to reduce Co to precipitate it. When a sample carrying Co was examined by a transmission electron microscope (TEM), it was confirmed that Co was present at the place where Pt was carried.

  FIG. 3A is a diagram obtained by TEM observation of a Pt 0.44% alumina base material in which the atomic ratio of Co to Pt, which is the sample of Example 4, is 1. Moreover, the image which analyzed the sample of Example 4 with the energy dispersive X-ray fluorescence analyzer (EDX) is shown in FIG.3 (b) and FIG.3 (c). As shown in FIG. 3 (b), it was confirmed that Co particles were present as shown in FIG. 3 (c) at white positions where a large number of Pt particles 6 were present. The reason why Co exists at the position where Pt exists is considered to be that Pt particles supported on the base material become nuclei during the reduction of Co, and Co selectively precipitates on the surface of Pt particles. Further, since it is fired after Co deposition, it is considered that it exists as a Co oxide in the alumina base material.

(A) And (b) is sectional drawing which shows the structure of the exhaust gas purification catalyst which concerns on embodiment of this invention. For Examples 1 to 10 and Comparative Example 1, the purification rate of HC (C ₃ H ₆₎ is a diagram showing a temperature T (° C.) which is 50%. (a) is a figure which shows the TEM observation result of the alumina base material in the sample of Example 4, (b) and (c) are the energy dispersive X-ray fluorescence spectrometers (EDX) of the sample of Example 4. It is a figure which shows the image analyzed by this.

Explanation of symbols

1 ... Exhaust gas purification catalyst,
2 ... base material,
3 ... Precious metal particles,
4 ... Metal oxide layer,
5 ... Exhaust gas purification catalyst,

Claims (3)

A substrate formed from a compound comprising alumina and ceria;
Noble metal particles supported on the surface of the base material and formed from at least one kind of noble metal element selected from platinum, palladium and rhodium;
A surface of the noble metal particle or a part of the surface of the noble metal particle, and a metal oxide layer formed from a metal oxide,
The metal oxide layer is formed of an oxide of at least one metal selected from Co, Ni, Fe, Cu, Sn, Mn, Ce and Zr,
The exhaust gas purifying catalyst, wherein the amount of the metal oxide is such that an atomic ratio of metal to the noble metal particles is 0.005 to 6. After supporting noble metal particles formed of at least one element selected from platinum, palladium and rhodium on the base powder of alumina powder and ceria powder, the surface of the noble metal particles, or the surface of the noble metal particles A method for producing an exhaust gas purifying catalyst according to claim 1, wherein a part of the metal oxide layer is coated.
At least one or more metals selected from Co, Ni, Fe, Cu, Sn, Mn, Ce and Zr using a selective precipitation method on the surface of the noble metal particle or a part of the surface of the noble metal particle And then oxidizing the deposited metal to form a metal oxide layer having a metal atomic ratio of 0.005 to 6 with respect to the noble metal particles. Method.   The method for producing an exhaust gas purifying catalyst according to claim 2, wherein a powder containing alumina and ceria is used as the substrate powder.