JP2004261681A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning exhaust gas inhibiting the catalyst from being poisoned by phosphor, particularly inhibiting alumina as a support material from being poisoned by phosphor utilizing Zr without impairing activity of the catalyst. <P>SOLUTION: Zirconia particles 6 having a particle diameter of 10 nm or smaller are dispersed and carried on an outer surface of γ-alumina particles 5 as the support material carried with a catalyst noble metal. A ration of a projection area of the zirconia particles occupied on the outer surface of this γ-alumina particles 5 is made to 80% or less. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purifying catalyst.
[0002]
[Prior art]
A three-way catalyst is one type of exhaust gas purifying catalyst for automobiles. This is generally because a noble metal such as Pt as a catalyst metal, γ-alumina as a support material for stabilizing the noble metal and increasing the contact area with exhaust gas to improve purification performance, and oxygen storage such as ceria In many cases, it is composed of a substance (OSC).
[0003]
By the way, when the engine oil contains phosphorus (for example, zinc dialkyldithiophosphate), the phosphorus forms a glassy compound, covers the catalyst surface, inhibits diffusion of gas into the catalyst, and purifies the catalyst. There is a problem that performance is reduced. In addition, this phosphorus has a problem that it reacts with alumina or ceria to form a compound and deteriorates the compound, and further, it is combined with an active component such as Pt to deteriorate the compound.
[0004]
With respect to such a problem of phosphorus poisoning, it has been proposed that the surface of the catalyst be coated with TiO ₂ to prevent phosphorus in the exhaust gas from forming a glassy compound on the catalyst surface. (See Patent Document 1).
[0005]
In addition, a proposal is known in which activated alumina as a support material carries a noble metal and at least one of Ba, La, Zr, and Nd to increase the heat resistance of a three-way catalyst (Patent Document 2). reference).
[0006]
[Patent Document 1]
JP-A-55-167046 [Patent Document 2]
JP-A-10-277390
[Problems to be solved by the invention]
An object of the present invention is to suppress phosphorus poisoning of a catalyst, particularly phosphorus of alumina as a support material, without impairing the activity of the catalyst by using Zr. That is, as described above, it has been conventionally known that a Zr compound is contained in a catalyst. However, this is intended to improve the heat resistance of the catalyst. It is not possible to effectively prevent the poisoning of alumina with phosphorus without impairing its activity. In addition, it is conceivable to increase the noble metal loading of the catalyst to delay performance degradation due to phosphorus poisoning and extend the useful life of the catalyst, but this would increase the cost.
[0008]
[Means for Solving the Problems]
In order to solve such a problem, the present invention has a structure in which fine zirconia particles are dispersed and supported on the surfaces of γ-alumina particles.
[0009]
That is, the present invention is an exhaust gas purifying catalyst in which a catalyst layer containing a noble metal and γ-alumina particles is formed on a cell surface of a honeycomb catalyst carrier, and purifies an exhaust gas containing phosphorus.
Zirconia particles having a particle size of 10 nm or less are dispersed and supported on the outer surface of the γ-alumina particles,
The ratio of the projected area of the zirconia particles to the outer surface of the γ-alumina particles is 80% or less.
[0010]
In the case of the present invention, zirconia is more likely to generate a phosphate compound than γ-alumina, so that phosphorus poisoning of γ-alumina particles, that is, aluminum phosphate is generated and pores of γ-alumina particles are crushed. Be suppressed. Further, since the zirconia particles are dispersed and supported on the outer surface of the γ-alumina particles, the zirconia particles contribute to the improvement of the activity by increasing the specific surface area of the catalyst, and also function to prevent the sintering of the γ-alumina. In addition, since the zirconia particles are stable compounds, even when the catalyst is exposed to a high temperature, the poisoning of γ-alumina by phosphorus is effectively suppressed.
[0011]
Then, since the zirconia particles are 10 nm or less and the ratio of the projected area of the zirconia particles to the outer surface of the γ-alumina particles is 80% or less, the exhaust gas diffuses into the pores of γ-alumina. There is little hindrance to movement, which is advantageous for ensuring the activity of the catalyst. The ratio of the projected area is preferably 5% or more in order to prevent the phosphorus poisoning and increase the specific surface area of the catalyst. The lower limit of the particle size of the zirconia particles is about 1 nm as a standard.
[0012]
The projected area is the area when the zirconia particles are projected in the normal direction to the outer surface of the γ-alumina particles. However, the area ratio is determined as a ratio of the total area of the individual zirconia particles that can be projected to the external surface of the γ-alumina particles with respect to the total external surface area of the γ-alumina particles contained in the catalyst. Things.
[0013]
The γ-alumina particles in which the zirconia particles having a particle size of 10 nm or less are dispersed and supported on the outer surface are obtained by heating and stirring a raw material solution obtained by mixing γ-alumina powder into a zirconium alkoxide solution, That is, it can be obtained by firing a powder in which a zirconia precursor is supported on individual γ-alumina particles. In this case, the amount of γ-alumina and the amount of zirconium alkoxide in the raw material solution are adjusted so that the ratio of the projected area of the zirconia particles to the outer surface of the γ-alumina particles is 80% or less.
[0014]
The γ-alumina particles carry at least one noble metal selected from Pt, Pd, and Rh, in addition to the zirconia particles, and the catalyst further includes a Ce—Zr composite oxide (Ce and (A mixed oxide containing Zr) is preferably contained. Thus, the catalyst can be used as a three-way catalyst that simultaneously purifies HC, CO, and NOx in the exhaust gas of an automobile. In this case, the Ce-Zr composite oxide functions as an oxygen storage material that enlarges the window of the three-way catalyst (the width of the air-fuel ratio centered on the stoichiometric air-fuel ratio at which the three-way catalyst works effectively).
[0015]
【The invention's effect】
As described above, according to the present invention, a catalyst layer containing a noble metal and γ-alumina particles is formed on the cell surface of the honeycomb catalyst carrier, and the outer surface of the γ-alumina particles has a particle size of 10 nm. Since the following zirconia particles are dispersed and supported, and the ratio of the projected area of the zirconia particles to the outer surface of the γ-alumina particles is 80% or less, phosphorus poisoning of γ-alumina can be performed without impairing the activity of the catalyst. In addition, it is not necessary to particularly increase the amount of noble metal in order to increase the service life of the catalyst, which is advantageous in reducing the cost.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0017]
The exhaust gas purifying catalyst 1 for an automobile shown in FIG. 1 is a three-way catalyst. In the catalyst 1, reference numeral 2 denotes a cordierite honeycomb catalyst carrier, and a first catalyst layer 3 is formed on a cell surface thereof. The second catalyst layer 4 is formed on the surface of the first catalyst layer 3. The first catalyst layer 3 contains a catalyst component obtained by supporting Pd as a noble metal on a support material. The second catalyst layer 4 supports a catalyst component in which Pt is supported as a noble metal on a support material, and Rh as a noble metal in a Ce-Zr composite oxide (complex oxide or mixed oxide) having Ce and Zr. And a catalyst component comprising:
[0018]
As shown in the schematic diagram of FIG. 2, the support material has a large number of zirconia particles 6 dispersedly supported on the outer surface of γ-alumina particles 5.
[0019]
<Support material manufacturing method>
The manufacturing method of the above support material is as follows.
[0020]
-Raw material preparation process-
A raw material solution is prepared by adding γ-alumina powder to a zirconium alkoxide solution.
[0021]
-Zirconia precursor generation step-
The raw material solution is stirred while being heated to generate a zirconia precursor on the outer surface of each γ-alumina particle. That is, the zirconium alkoxide is hydrolyzed on the outer surface of the γ-alumina particles to generate a zirconia precursor.
[0022]
-Separation process-
The precipitate (concentrated sol) obtained in the previous step is washed with a centrifuge until the dispersion medium is replaced with water.
[0023]
-Drying process-
The cake obtained in the previous step is dried.
[0024]
-Baking process-
The dried cake obtained in the previous step is fired to obtain the target support material. This calcined support material is pulverized and used as a catalyst.
[0025]
<Catalyst production method>
Basically, a catalyst noble metal is supported on the above-mentioned support material, mixed with an oxygen storage material, a binder, and water to prepare a slurry, and the slurry is coated on a catalyst carrier, followed by drying and calcining to prepare a catalyst. Can be obtained. Alternatively, a slurry is prepared by mixing the above-mentioned support material, oxygen storage material, binder and water, the slurry is coated on a catalyst carrier, dried and calcined, and then the coating layer is impregnated with a solution of a catalyst noble metal. , Drying and calcination to obtain a catalyst.
[0026]
The catalyst 1 shown in FIG. 1 can be manufactured by the following method.
[0027]
-Formation of first catalyst layer 3-
Pd is supported on the support material by an impregnation method, an evaporation to dryness method or the like, and is dried, calcined, and pulverized to form a catalyst powder. The catalyst powder is mixed with a binder and water to prepare a slurry, the honeycomb carrier 1 is immersed in the slurry, pulled up to remove excess slurry, and dried and fired to form the first catalyst layer 3.
[0028]
-Formation of second catalyst layer 4-
Pt is supported on the support material by an impregnation method, an evaporation to dryness method, or the like, and is dried, calcined, and pulverized to form a Pt catalyst powder. On the other hand, Rh is supported on the Ce-Zr composite oxide by an impregnation method, an evaporation to dryness method, or the like, and is dried, calcined, and pulverized to form an Rh catalyst powder. The Pt catalyst powder and the Rh catalyst powder are mixed, and a binder and water are mixed therein to prepare a slurry. The honeycomb carrier 1 is immersed in the slurry and pulled up to remove the excess slurry, followed by drying and firing. Thereby, the second catalyst layer 4 is formed on the surface of the first catalyst layer 3.
[0029]
<Example>
15 g of γ-alumina powder was heated to a temperature of 150 ° C. for 1 hour. On the other hand, 10 g of a 70% zirconium-n-propoxide solution in propanol was diluted with 100 mL of propanol. After stirring the diluted propanol solution at a temperature of 27 ° C. for 10 minutes, the raw material solution was prepared by mixing the γ-alumina powder therewith. By stirring this raw material solution at a temperature of 80 ° C. for 2 hours, a zirconia precursor was generated on the outer surfaces of the γ-alumina particles. The resulting mixture was centrifuged and washed. The obtained powder was dried by heating at a temperature of 90 ° C. for 1 hour in a drier, and further heated at a temperature of 500 ° C. for 2 hours in an electric furnace and fired. The obtained fired product was pulverized to obtain a support material (ZrO ₂ / Al ₂ O ₃ ) having a large number of zirconia particles dispersedly supported on the outer surface of the γ-alumina particles.
[0030]
FIG. 3 is a transmission electron micrograph of the obtained support material (ZrO ₂ / Al ₂ O ₃ ). The particle size of the zirconia particles is 10 nm or less. In this support material, the ratio of the projected area of the zirconia particles to the outer surface of the γ-alumina particles (hereinafter, simply referred to as area ratio) was 29%. This area ratio is determined based on the particle size of the γ-alumina particles, the total of the outer surface area of the γ-alumina particles, while the zirconia particles based on the particle size of the zirconia particles with respect to the outer surface of the γ-alumina particles Is calculated as a percentage of the total of the projected areas with respect to the total of the external surface areas. The average particle size of the γ-alumina particles is 100 to 1000 nm for primary particles and 1 to 10 μm for secondary particles.
[0031]
A Pd catalyst powder obtained by supporting Pd on the support material, a Pt catalyst powder obtained by supporting Pt on the support material, and an Rh catalyst powder obtained by supporting Rh on the Ce-Zr composite oxide were prepared. . Then, the Pd catalyst powder was mixed with a binder and water to prepare a slurry, which was wash-coated on the honeycomb carrier 2 to form the first catalyst layer 3. Next, the Pt catalyst powder and the Rh catalyst powder are mixed with a binder and water to prepare a slurry, which is wash-coated on the honeycomb carrier 2 on which the first catalyst layer 3 is formed to form a second catalyst layer 4. Formed.
[0032]
The total supported amount of the first catalyst layer 3 and the second catalyst layer 4 per 1 L of the honeycomb carrier was 140 g, and the proportion of each component in the total supported amount was 2.9% by mass of Pd, Pt Is 0.1% by mass, Rh is 0.2% by mass, the support material of the first catalyst layer 3 is 41.5% by mass, the support material of the second catalyst layer 4 is 17.3% by mass, and the Ce-Zr composite oxidation is used. Is 38.0% by mass.
[0033]
<Relationship between area ratio of zirconia particles and specific surface area of catalyst>
FIG. 4 shows the BET specific surface area of the catalyst (heat-treated at 900 ° C. for 8 hours in the air atmosphere) when the area ratio of the zirconia particles to the outer surface of the γ-alumina particles was changed. The impact is seen. According to the figure, when zirconia particles are dispersed and supported on γ-alumina particles, the specific surface area of the catalyst is increased. Usually, the specific surface area of γ-alumina for a catalyst is about 180 g / m ^{2 in a} state where heat treatment is not performed, and it is reduced to about 100 g / m ² by heat treatment. It can be said that the supported zirconia particles have the effect of suppressing a decrease in specific surface area and increasing the activity of the catalyst.
[0034]
As can be seen from FIG. 4, the area ratio is preferably 5% or more. Even when a large amount of zirconia particles having an area ratio exceeding 80% are supported on the γ-alumina particles, the activity of the catalyst is improved. Is not valid for
[0035]
In the area ratio of 200%, the total area of the zirconia particles that can be projected on the outer surface of the γ-alumina particles is twice as large as the total area of the outer surfaces of the γ-alumina particles. It means that the zirconia particles are excessively supported on the particles.
[0036]
<Evaluation of catalyst performance>
Based on the above results, the performance of the catalyst of the example was evaluated in comparison with Comparative Examples 1 and 2 described below.
[0037]
-Comparative Example 1-
The same as the example except that instead of the support material (ZrO ₂ / Al ₂ O ₃ ) of the example, γ-alumina not supporting zirconia (hereinafter simply referred to as Al ₂ O ₃ ) is used as the support material. The catalyst of Comparative Example 1 was prepared according to the conditions and the method.
[0038]
-Comparative Example 2-
Instead of using the support material (ZrO ₂ / Al ₂ O ₃ ) of the example, a mixture of γ-alumina powder and zirconia powder (hereinafter referred to as ZrO ₂ .Al ₂ O ₃ ) is used as the support material. Prepared a catalyst of Comparative Example 2 under the same conditions and method as in the example. The average particle diameter of the zirconia particles constituting the support material of Comparative Example 2 is about 50 nm. The ratio between the γ-alumina powder and the zirconia powder was adjusted so as to be equivalent to the support material (ZrO ₂ / Al ₂ O ₃ ) of the example.
[0039]
-Evaluation of catalyst-
Each of the catalysts of Examples and Comparative Examples 1 and 2 was subjected to a heat treatment of holding at a temperature of 900 ° C. for 8 hours in the air atmosphere, and a catalyst coated with phosphorus at a temperature of 900 ° C. for 5 hours in the air atmosphere. A heat treatment and a phosphorus poisoning treatment of performing a poisoning treatment and then maintaining the temperature at 900 ° C. for 3 hours in an air atmosphere were prepared. Hereinafter, a catalyst that has been subjected to only heat treatment is referred to as a catalyst before poisoning, and a catalyst that has been subjected to heat treatment and phosphorus poisoning is referred to as a catalyst after poisoning.
[0040]
The phosphorus poisoning treatment was performed by a phosphorus poisoning treatment device 12 passed through an electric furnace 11 as shown in FIG. That is, the placing test catalyst 13 inside the treatment of poisoning by phosphorus 12 which is adapted to circulate N ₂ gas, the vessel 14 containing the P ₂ O ₅ to the front is disposed, flowing N ₂ gas While heating the P ₂ O ₅ container 14, the P ₂ O ₅ is vaporized and supplied to the test catalyst 13. This phosphorus poisoning treatment was performed for 1 hour at a temperature of the electric furnace 11 of 500 ° C.
[0041]
For each catalyst before the poisoning and each catalyst after the poisoning, a light-off temperature T50 and a purification rate C400 relating to purification of HC, CO and NOx were measured using a model gas flow reactor and an exhaust gas analyzer. That is, each catalyst was attached to a model gas flow reactor, and a model gas rich in air-fuel ratio (temperature: 600 ° C.) was flowed for 10 minutes, and then T50 and C400 were measured by flowing the model gas.
[0042]
T50 is the gas temperature at the catalyst inlet when the purification rate reaches 50% by gradually increasing the temperature of the model gas flowing into the catalyst from room temperature. C400 is the purification rate when the catalyst inlet gas temperature is 400 ° C. The model gas was A / F = 14.7 ± 0.9. That is, the A / F is forcibly controlled at an amplitude of ± 0.9 by adding a predetermined amount of the variation gas in a pulsed manner at 1 Hz while constantly flowing the main stream gas having the A / F = 14.7. Vibrated. The space velocity SV is 120,000 h ^-1 and the heating rate is 30 ° C./min.
[0043]
The result of T50 is shown in FIG. 6, and the result of C400 is shown in FIG. 6 and 7, Al ₂ O ₃ is Comparative Example 1, ZrO ₂ .Al ₂ O ₃ is Comparative Example 2, and ZrO ₂ / Al ₂ O ₃ is Example. According to FIG. 6, the T50 after phosphorus poisoning is lowest in the embodiment (ZrO ₂ / Al ₂ O ₃ ) for all of HC, CO and NOx, and the temperature of T50 before and after phosphorus poisoning for HC and CO. The difference is small in the embodiment. According to FIG. 7, although there is no significant difference between the catalysts with respect to NOx, the embodiment (ZrO ₂ / Al ₂ O ₃ ) shows a high C400 with respect to HC and CO even after phosphorus poisoning, and Also, there is little reduction in C400 due to phosphorus poisoning.
[0044]
Comparative Example 2 (ZrO ₂ .Al ₂ O ₃ ) contains ZrO ₂ as in the example, but does not differ much from Comparative Example 1 (Al ₂ O ₃ ).
[0045]
From the above, it is said that dispersing and supporting fine zirconia particles on the outer surface of γ-alumina particles as in the example is effective in preventing phosphorus poisoning of the catalyst, and particularly effective in preventing phosphorus poisoning of γ-alumina. be able to.
[0046]
<Effect of area ratio of zirconia particles>
By the same manufacturing method as the support material of the above example, a support material having an area ratio of zirconia particles occupying 80% of the outer surface of the γ-alumina particles and a support material of 100% were prepared. To prepare a catalyst. For the two types of catalysts having different area ratios, T50 and C400 before and after phosphorus poisoning were measured by the method described in the above-mentioned "Evaluation of catalyst". The result of T50 is shown in FIG. 8, and the result of C400 is shown in FIG. 8 and 9, “80%” and “100%” represent the area ratio of the zirconia particles in the support material.
[0047]
According to FIG. 8 and FIG. 9, both T50 and C400 are worse than the case of 80% when the area ratio of the zirconia particles is 100%. This is not a problem of the phosphorus poisoning prevention effect. In the case of 100%, since the zirconia particles have covered the outer surface of the γ-alumina particles, the exhaust gas by the γ-alumina particles It is considered that the trapping ability was reduced. Further, comparing the ZrO ₂ / Al ₂ O ₃ (after poisoning) in which the area ratio of the zirconia particles in FIG. 7 is 29% and the same area ratio 100% (after poisoning) in FIG. The purification rate of 100% is slightly lower than that of 29%.
[0048]
<Others>
Although the above embodiment relates to a three-way catalyst, the present invention can be applied to other exhaust gas purifying catalysts such as an oxidation catalyst, and also applicable to exhaust gas purifying catalysts other than automobiles.
[Brief description of the drawings]
FIG. 1 is a sectional view showing an exhaust gas purifying catalyst according to an embodiment of the present invention.
FIG. 2 is a schematic view showing a support material of the exhaust gas purifying catalyst.
FIG. 3 is a transmission microscope photograph of the support material.
FIG. 4 is a graph showing the relationship between the area ratio of zirconia particles and the BET specific surface area of a catalyst.
FIG. 5 is an explanatory view showing a phosphorus poisoning treatment apparatus.
FIG. 6 is a graph showing T50 of an example of the present invention and a comparative example.
FIG. 7 is a graph showing C400 of an example of the present invention and a comparative example.
FIG. 8 is a graph showing T50 before and after phosphorus poisoning when the area ratio of zirconia particles is 80% and 100%.
FIG. 9 is a graph showing C400 before and after phosphorus poisoning when the area ratio of zirconia particles is 80% and 100%.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Exhaust gas purification catalyst 2 Support 3 First catalyst layer 4 Second catalyst layer 5 γ-alumina particles 6 Zirconia particles

Claims (3)

A catalyst layer containing a noble metal and γ-alumina particles is formed on the cell surface of the honeycomb catalyst carrier, and is an exhaust gas purification catalyst that purifies exhaust gas containing phosphorus.
Zirconia particles having a particle size of 10 nm or less are dispersed and supported on the outer surface of the γ-alumina particles,
An exhaust gas purifying catalyst, wherein a ratio of a projected area of the zirconia particles to an outer surface of the γ-alumina particles is 80% or less. In claim 1,
By heating and stirring a raw material solution obtained by mixing γ-alumina powder into a zirconium alkoxide solution, a powder in which a zirconia precursor is supported on individual γ-alumina particles is obtained, and the powder is calcined. Zirconia particles having a particle size of 10 nm or less are dispersed and supported on the outer surface of the γ-alumina particles,
An exhaust gas purifying catalyst, wherein the raw material solution is prepared such that the ratio of the projected area of the zirconia particles to the outer surface of the γ-alumina particles is 80% or less. In claim 1 or claim 2,
The γ-alumina particles carry at least one selected from Pt, Pd and Rh as the noble metal,
Furthermore, an exhaust gas purifying catalyst characterized by containing a Ce-Zr composite oxide.

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