JP2005246216A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent a solid solution from being formed between a noble metal and an oxide carrier and thoroughly restrain different noble metals from being alloyed. <P>SOLUTION: This catalyst contains first catalyst powder obtained by depositing Pt on powder of a first porous oxide, second catalyst powder obtained by depositing Rh on powder of a second porous oxide and powder of a third porous oxide having the particle size smaller than those of the powder of the first porous oxide and the powder of the second porous oxide. A particle 3 of the third porous oxide is apt to interpose between a first catalyst particle 1 of the first catalyst powder and a second catalyst particle 2 of the second catalyst powder. Therefore, a possibility that the first catalyst particle 1 is in contact with the second catalyst particle 2 becomes small since the particle 3 of the third porous oxide becomes an obstacle. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to exhaust gas purification catalysts such as automobile three-way catalysts and NO _x storage reduction catalysts, and more particularly to exhaust gas purification catalysts carrying two or more kinds of noble metals.

For example, a three-way catalyst is known as an exhaust gas purification catalyst for automobiles. This three-way catalyst is a porous oxide such as alumina or zirconia that carries a noble metal such as Pt, Rh, or Pd. It can purify by oxidizing HC and CO in the exhaust gas in an atmosphere near the stoichiometric atmosphere. At the same time, NO _x can be reduced and purified. In order to mitigate atmospheric fluctuations, oxides having oxygen absorption / release capability such as ceria and ceria-zirconia solid solution are often mixed with alumina.

  This three-way catalyst is generally produced by forming a coating layer made of a porous oxide on a honeycomb structure substrate formed of cordierite or the like by a wash coating method and supporting a noble metal on the coating layer. There is also a method in which a catalyst powder in which a noble metal is supported in advance on a porous oxide powder is prepared and the catalyst powder is coated on a honeycomb substrate.

Of the noble metals, Pt and Pd mainly contribute to the oxidation and purification of CO and HC, and Rh mainly contributes to the reduction and purification of NO _x . Rh also has the effect of preventing sintering of Pt or Pd. That is, it has been found that the combined use of Pt or Pd and Rh suppresses the disadvantage that the activity is lowered due to the reduction of the active sites due to sintering, and improves the heat resistance. Therefore, it is known that it is desirable to use Pt or Pd together with Rh in the three-way catalyst.

  By the way, in order to respond to the recent tightening of exhaust gas regulations, many two-catalyst systems comprising a startup catalyst and an underfloor catalyst have been adopted. However, in this two-catalyst system, the startup catalyst is mounted directly under the engine, so the temperature of the catalyst during use rises considerably compared to the underfloor catalyst, and the effect of suppressing sintering of Pt and Pd by Rh is reduced. . In addition, when Pt and Rh are used in combination, Pt and Rh are alloyed at a high temperature, and it has been revealed that there is a problem that the activities of Pt and Rh are reduced.

  Further, there are unfavorable combinations between the noble metal species and the support species depending on the use conditions. For example, in a catalyst in which Rh is supported on alumina, there is a problem that Rh is dissolved in alumina in a high-temperature oxidizing atmosphere of 900 ° C. or more, and the performance is significantly reduced.

  Three-way catalysts are strongly required to have high temperature durability of 900 ° C or higher. For that purpose, it is an important subject to suppress the deterioration of the catalyst. Furthermore, Rh is extremely rare in terms of resources, and it is desired to use Rh efficiently and to suppress its deterioration and increase heat resistance.

  Therefore, for example, as described in JP-A 63-197546, a catalyst layer in which Rh is supported on zirconia is mixed with alumina to form a coat layer. By supporting Rh on zirconia in this manner, solid solution of Rh in alumina described above can be prevented, and deterioration of Rh can be suppressed. Further, if a coating layer is formed by mixing a catalyst powder having Rh supported on zirconia and a catalyst powder having Pt supported on alumina, alloying of Pt and Rh can be suppressed.

  Furthermore, an exhaust gas purifying catalyst has been proposed in which the coat layer has a two-layer structure and separates and supports plural kinds of noble metals. For example, Japanese Patent Laid-Open No. 05-293376 discloses an exhaust gas purifying catalyst in which Rh is supported on the outermost layer of a coat layer and Pt or Pd is supported on the inner layer. Japanese Laid-Open Patent Publication No. 06-063403 discloses a first coat layer containing Pt or Pd and a second coat layer provided on the first coat layer and containing Rh. In the second coat layer, cerium and An exhaust gas purifying catalyst containing a metal oxide powder mainly containing zirconium has been proposed.

  By thus supporting Pt and Rh separately, it is possible to suppress a decrease in the activity of Pt and Rh due to alloying. In addition, since a carrier having a good compatibility with the noble metal species can be selected, a decrease in purification ability due to the interaction with the carrier is suppressed.

However, in the catalyst in which Pt and Rh are separately supported on the coat layer, there is a problem that the effect of using both is reduced and Pt is easily sintered. Even when a coating layer is formed by mixing a catalyst powder in which Rh is supported on zirconia and a catalyst powder in which Pt is supported on alumina, the alloy is often alloyed because of the high probability that Pt and Rh are close to each other. . Accordingly, there is a demand for further suppressing alloying and further suppressing a decrease in activity.
JP 63-197546 JP 05-293376 A JP 06-063403

  The present invention has been made in view of the above circumstances, and it is an object to be solved to prevent solid solution of a noble metal in an oxide carrier and to well suppress alloying of different kinds of noble metals.

  The exhaust gas purifying catalyst of the present invention that solves the above problems is characterized in that a support substrate, a first catalyst powder in which a first noble metal is supported on a first porous oxide powder, and a second porous oxide powder. And a second catalyst powder having a second noble metal supported thereon, a first porous oxide powder, and a third porous oxide powder having a particle size smaller than that of the second porous oxide powder. And a catalyst layer formed.

  The weight of at least one of the first porous oxide powder and the second porous oxide powder in the catalyst layer is desirably four times or more the weight of the third porous oxide powder.

Further, the particle diameters of the first porous oxide powder and the second porous oxide powder in the catalyst layer are set to a ratio of 50% average particle diameter (D ₅₀ ) to the particle diameter of the third porous oxide powder. It is desirable that it is 4 times or more.

  According to the exhaust gas purifying catalyst of the present invention, alloying of the first noble metal and the second noble metal can be well suppressed, and a decrease in activity can be prevented. Moreover, since the oxide powder most suitable for each noble metal can be selected, the malfunction that a noble metal dissolves in an oxide powder can be prevented. Therefore, durability is improved and long-term use is possible.

  The catalyst layer is formed by a wash coat method in which a catalyst powder in which a noble metal is supported on a porous oxide powder is used as a slurry, and a support base material is dipped in the slurry, and then the excess slurry is removed by suction or the like, and dried and fired. It is common. Therefore, when powders having different particle sizes are present in the slurry, each powder is in a state close to fine packing, and the state is maintained even in the catalyst layer.

  Therefore, the exhaust gas purifying catalyst of the present invention includes the third porous oxide powder having a particle diameter smaller than that of the first porous oxide powder and the second porous oxide powder. The third porous oxide particles 3 are likely to be interposed between the first catalyst particles 1 and the second catalyst particles 2. Therefore, the third porous oxide particle 3 becomes a barrier, and the catalyst layer is formed in a state where the probability that the first catalyst particle 1 and the second catalyst particle 2 are in contact with each other is reduced. Therefore, the probability that the first noble metal and the second noble metal are close to each other is also reduced, and mutual alloying is well suppressed. When the particle diameter of the third porous oxide particle 3 is equal to or larger than the particle diameter of the first catalyst particle 1 or the second catalyst particle 2, the first catalyst particle 1 and the second catalyst particle 2 are shown in FIG. Is likely to come into contact, and alloying is likely to occur.

The weight of at least one of the first porous oxide powder and the second porous oxide powder in the catalyst layer is preferably at least four times the weight of the third porous oxide powder. The particle diameters of the porous oxide powder and the second porous oxide powder are desirably 4 times or more as the ratio of D ₅₀ to the particle diameter of the third porous oxide powder. The weight of both the first porous oxide powder and the second porous oxide powder is less than four times the weight of the third porous oxide powder, or the first porous oxide powder and the second porous oxide powder When the particle size of the product powder is less than 4 times as the ratio of D _{50 to} the particle size of the third porous oxide powder, the probability that the first catalyst powder and the second catalyst powder come into contact with each other is increased. Is likely to occur.

  As the carrier substrate, a straight flow structure having a foam shape or a honeycomb shape is generally used, but a wall flow structure having a honeycomb shape in which both ends are alternately packed can also be used. The carrier base material is made of a heat-resistant ceramic such as cordierite or a heat-resistant metal.

As the first porous oxide and the second porous oxide, oxidation of γ-Al ₂ O ₃ , θ-Al ₂ O ₃ , α-Al ₂ O ₃ , SiO ₂ , ZrO ₂ , TiO ₂ , MgO, etc. things, _{zeolite, CeO 2 -ZrO 2, CeO 2} -ZrO 2 -Y 2 O 3, Al 2 O 3 -CeO 2, Al 2 O 3 -ZrO 2, Al 2 O 3 -CeO 2 -ZrO 2, Al 2 _{O 3 -CeO 2 -ZrO 2 -MgO,} Al 2 O 3 -CeO 2 -ZrO 2 -Y 2 O 3, Al 2 O 3 -CeO 2 -ZrO 2 -La 2 O 3, Al 2 O 3 -ZrO 2 A composite oxide such as -TiO ₂ or a rare earth oxide such as MgAl ₂ O ₄ or Y ₂ O ₃ or La ₂ O ₃ can be used. The types of the first porous oxide and the second porous oxide are selected according to the types of the first noble metal and the second noble metal, and may be different or the same.

  The third porous oxide powder can also be selected from the various oxides exemplified above. The third porous oxide powder may be different from or the same as the first porous oxide powder or the second porous oxide powder.

The first noble metal and the second noble metal are selected from Pt, Rh, Pd, Ir, and the like. Pt or Pd is desirably supported on alumina or a composite oxide containing alumina having a particularly large specific surface area. Thereby, gas diffusibility improves and high activity is expressed. Rh is preferably supported on zirconia or a composite oxide containing zirconia. As a result, it is possible to avoid the problem that Rh is dissolved in the porous oxide. Further, since the catalyst in which Rh is supported on zirconia generates hydrogen from the exhaust gas by the steam reforming reaction, there is an advantage that the reduction purification activity of NO _x is improved by the generated H ₂ .

The particle diameters of the first porous oxide powder and the second porous oxide powder are preferably 2 to 30 μm at D ₅₀ . If the particle size is smaller than this range, the catalyst layer tends to fall off, and if the particle size is larger than this range, the slurry stability is remarkably deteriorated and coating may not be possible. Since the particle diameter of the first porous oxide powder and the second porous oxide powder is desirably 4 times or more as the ratio of D ₅₀ to the particle diameter of the third porous oxide powder, Therefore, the particle size of the third porous oxide powder is desirably 1 to 7.5 μm at D ₅₀ . The third porous oxide powder does not normally carry a catalyst metal, but is a catalyst metal that does not alloy with the first noble metal and the second noble metal, and does not dissolve in the third porous oxide. It can also be supported.

  The catalyst layer can be formed in an amount of 80 to 300 g per liter of the carrier base material. If the catalyst layer is too thick, the exhaust pressure loss increases, which is not preferable. If the catalyst layer is too thin, the noble metal is supported at a high concentration, so that it is easy to sinter at high temperatures and the activity tends to decrease.

  The supported amount of the first noble metal and the second noble metal is preferably 0.1 to 10 g in the case of Pt or Pd and 0.05 to 5 g in the case of Rh per 1 L of the carrier base material.

  In order to form a catalyst layer, first, a first catalyst powder in which a first noble metal is supported on a first porous oxide powder and a second catalyst powder in which a second noble metal is supported on a second porous oxide powder are prepared. . The supporting method is generally an evaporation to dryness method. A predetermined amount of the first catalyst powder, the second catalyst powder, and the third porous oxide powder are mixed with water and a binder, and milled as necessary to prepare a slurry. A catalyst layer can be formed by wash-coating this on a carrier substrate and baking.

  Hereinafter, the present invention will be specifically described with reference to test examples.

(Sample 1)
CeO ₂ —ZrO ₂ —Y ₂ O ₃ solid solution powder as the first porous oxide (molar ratio CeO ₂ : ZrO ₂ : Y ₂ O ₃ = 65: 30: 5, D ₅₀ = 30 μm) (hereinafter referred to as CZ1 ) Is impregnated with a predetermined amount of dinitrodiamineplatinum aqueous solution at a predetermined concentration, evaporated to dryness, and heat treated at 250 ° C. for 1 hour so that Pt as the first noble metal becomes 1.0 part by weight with respect to 150 parts by weight of CZ1. A Pt / CZ1 catalyst powder supported on a catalyst was prepared.

Further, CeO ₂ —ZrO ₂ —Y ₂ O ₃ solid solution powder as the second porous oxide (molar ratio CeO ₂ : ZrO ₂ : Y ₂ O ₃ = 30: 65: 5, D ₅₀ = 30 μm) (hereinafter, CZ2) is impregnated with a predetermined amount of rhodium chloride solution of a predetermined concentration, evaporated to dryness and heat-treated at 250 ° C for 1 hour, so that Rh as the second noble metal is 0.2 parts by weight with respect to 40 parts by weight of CZ2. An Rh / CZ2 catalyst powder supported as described above was prepared.

Then, 150 parts by weight of the Pt / CZ1 catalyst powder obtained above and 40 parts by weight of the Rh / CZ2 catalyst powder were mixed with pure water and milled to prepare a slurry A having a particle size of D ₅₀ = 6 μm.

On the other hand, 60 parts by weight of γ-Al ₂ O ₃ powder as the third porous oxide, 3 parts by weight of alumina hydrate, and 44 parts by weight of 40% aluminum nitrate aqueous solution are mixed with pure water and milled. A slurry B having a particle size of D ₅₀ = 1 μm was prepared.

  Next, a cordierite straight flow structure honeycomb substrate (cell density 600 cpsi, wall thickness 75 μm, 1.1 L) is prepared, dipped in a mixed slurry in which the total amount of slurry A and total amount of slurry B are well mixed, and then pulled up. Excess slurry was removed by suction, dried at 120 ° C., and calcined at 500 ° C. for 3 hours to form a catalyst layer. The catalyst layer is formed in an amount of 260 g per 1 L of the honeycomb base material, and the supported amount of the noble metal is 1.0 g of Pt and 0.2 g of Rh per 1 L of the honeycomb base material.

(Sample 2)
A catalyst layer was formed at 260 g / L in the same manner as in Sample 1 except that the particle size of slurry A was adjusted to D ₅₀ = 9 μm and the particle size of slurry B was adjusted to D ₅₀ = 4 μm.

(Sample 3)
A catalyst layer of 260 g / L was formed in the same manner as Sample 1 except that the particle size of the slurry A was adjusted to D ₅₀ = 3 μm and the particle size of the slurry B was adjusted to D ₅₀ = 8 μm.

(Sample 4)
A catalyst layer of 260 g / L was formed in the same manner as Sample 1 except that the particle size of slurry A was adjusted to D ₅₀ = 4 μm and the particle size of slurry B was adjusted to D ₅₀ = 11 μm.

(Sample 5)
260 g / L of a catalyst layer was formed in the same manner as Sample 1 except that the particle size of the slurry A was adjusted to D ₅₀ = 12.5 μm and the particle size of the slurry B was adjusted to D ₅₀ = 3 μm.

(Sample 6)
A catalyst layer of 260 g / L was formed in the same manner as Sample 1 except that the particle size of the slurry A was adjusted to D ₅₀ = 15 μm and the particle size of the slurry B was adjusted to D ₅₀ = 4.5 μm.

(Sample 7)
A catalyst layer of 260 g / L was formed in the same manner as Sample 1 except that the particle size of slurry A was adjusted to D ₅₀ = 30 μm and the particle size of slurry B was adjusted to D ₅₀ = 4 μm.

(Sample 8)
A catalyst layer of 190 g / L was formed in the same manner as in Sample 1 except that the amount of Pt / CZ1 catalyst powder in slurry A was 120 parts by weight and the amount of γ-Al ₂ O ₃ powder in slurry B was 30 parts by weight.

(Sample 9)
A catalyst layer of 210 g / L was formed in the same manner as Sample 1 except that the amount of Pt / CZ1 catalyst powder in slurry A was 120 parts by weight and the amount of γ-Al ₂ O ₃ powder in slurry B was 50 parts by weight.

(Sample 10)
A catalyst layer of 260 g / L was formed in the same manner as in Sample 1 except that the amount of Pt / CZ1 catalyst powder in slurry A was 120 parts by weight and the amount of γ-Al ₂ O ₃ powder in slurry B was 100 parts by weight.

(Sample 11)
A catalyst layer was formed at 230 g / L in the same manner as in Sample 1 except that the amount of γ-Al ₂ O ₃ powder in slurry B was 30 parts by weight.

(Sample 12)
270 g / L of a catalyst layer was formed in the same manner as Sample 1 except that the amount of γ-Al ₂ O ₃ powder in slurry B was 70 parts by weight.

(Sample 13)
A catalyst layer of 180 g / L was formed in the same manner as Sample 1 except that the amount of Pt / CZ1 catalyst powder in slurry A was 100 parts by weight and the amount of γ-Al ₂ O ₃ powder in slurry B was 40 parts by weight.

(Sample 14)
A catalyst layer of 240 g / L was formed in the same manner as in Sample 1 except that the amount of Pt / CZ1 catalyst powder in slurry A was 180 parts by weight and the amount of γ-Al ₂ O ₃ powder in slurry B was 20 parts by weight.

(Sample 15)
Sample 1 except that the amount of Pt / CZ1 catalyst powder of slurry A was 180 parts by weight, the amount of γ-Al ₂ O ₃ powder of slurry B was 20 parts by weight, and the particle size of slurry B was adjusted to D ₅₀ = 1 μm. In the same manner as above, a catalyst layer was formed at 240 g / L.

(Sample 16)
The amount of Pt / CZ1 catalyst powder of slurry A is 120 parts by weight, the particle size of slurry A is set to D ₅₀ = 6 μm, the amount of γ-Al ₂ O ₃ powder of slurry B is 30 parts by weight, and the particle size of slurry B A catalyst layer of 190 g / L was formed in the same manner as in Sample 1 except that D ₅₀ = 1.5 μm.

<Test and evaluation>
Each catalyst is mounted on the exhaust system of V-type 8-cylinder, 4.0L engine bench, and exhaust gas with A / F = 15.0 and A / F = 14.0 vibrated at 1Hz is distributed for 100 hours at a catalyst bed temperature of 1050 ° C. An endurance test was conducted.

  Next, each in-line 4-cylinder, 2.4-liter engine bench exhaust system is loaded with each of the catalysts after the endurance test, operated at a theoretical air-fuel ratio of A / F = 14.6, and the catalyst bed temperature is adjusted via a heat exchanger. While increasing the temperature from 200 ° C. to 450 ° C. at 10 ° C./min, the purification rate of the HC component was continuously measured. And the temperature which can purify 50% of HC is calculated, and it is set as HC ignition temperature, and each result is shown in Table 1, FIG.3, and FIG.4.

  Each catalyst after the durability test was subjected to X-ray diffraction analysis, and the degree of alloying of Pt and Rh was measured. Since the lattice constant of Pt is 3.923mm and the lattice constant of Rh is 3.821mm, when alloyed, the lattice constant will be between them. The closer the lattice constant of noble metals measured by X-ray diffraction is to 3.821mm, the more alloyed Means that is in progress. The results are shown in Table 1 and FIG.

In sample 3 and sample 4, since the particle size of the γ-Al ₂ O ₃ powder contained in the slurry B is larger than the particle size of the catalyst powder contained in the slurry A, the HC ignition temperature is higher than other samples. I understand. Therefore, the particle size of the γ-Al ₂ O ₃ powder contained in the slurry B needs to be smaller than the particle size of the catalyst powder contained in the slurry A.

FIG. 3 shows the results of samples 8 to 14, and FIG. 4 shows the results of samples 1 to 7. FIG. 3 shows that the weight of the catalyst powder is desirably four times or more the weight of the γ-Al ₂ O ₃ powder. FIG. 4 also shows that the particle size of the catalyst powder is desirably 4 times or more as the ratio of D ₅₀ to the particle size of the γ-Al ₂ O ₃ powder.

  Further, FIG. 5 shows that the alloying of Pt and Rh does not proceed as compared with the other samples in the above-described desirable range, and the volume and particle size of each powder are controlled as in the present invention. It is clear that the alloying of Pt and Rh can be suppressed by this, and as a result, the activity after the durability test is improved.

And each is the weight and D ₅₀ ratio from the results of the sample there is an optimum range, it is considered most preferable that a 4 about the weight and D ₅₀ ratio as in Sample 16.

The present invention can be used not only for a three-way catalyst but also for a catalyst supporting a plurality of noble metals, such as an oxidation catalyst, a NO _x storage reduction catalyst, and a filter catalyst.

It is explanatory drawing which shows the effect | action of the catalyst of this invention. It is explanatory drawing which shows the effect | action of the catalyst which has a composition contrary to this invention. It is a graph showing the relationship between the weight ratio and HC light-off temperature of CZ1 powder to the weight of γ-Al ₂ O ₃ powder. CZ1 for γ-Al ₂ O ₃ powder of D _50, is a graph showing the relationship between the ratio and the HC light-off temperature of CZ2 powder D50. γ-Al ₂ O ₃ powder CZ1 powder weight ratio of to the weight of, CZ1 for γ-Al ₂ O ₃ powder of D _50, CZ2 ratio of powder D _50, respectively X-axis the HC light-off temperature, Y-axis and Z It is a three-dimensional graph which shows the position of each sample when it is set as an axis.

Explanation of symbols

  1: First porous oxide particles 2: Second porous oxide particles 3: Third porous oxide particles

Claims (3)

A carrier substrate;
A first catalyst powder in which a first noble metal is supported on a first porous oxide powder; a second catalyst powder in which a second noble metal is supported on a second porous oxide powder; and the first porous oxidation And a catalyst layer coated on the carrier base material, and a catalyst layer for coating an exhaust gas, comprising a catalyst powder and a third porous oxide powder having a particle size smaller than that of the second porous oxide powder. .   2. The weight of at least one of the first porous oxide powder and the second porous oxide powder in the catalyst layer is four times or more the weight of the third porous oxide powder. The catalyst for exhaust gas purification as described in 1. The particle diameters of the first porous oxide powder and the second porous oxide powder in the catalyst layer are 50% average particle diameter (D ₅₀ ) with respect to the particle diameter of the third porous oxide powder. The exhaust gas purifying catalyst according to claim 1 or 2, wherein the ratio of the ratio is 4 times or more.

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