JP2021016854A - Catalyst for exhaust purification - Google Patents

Abstract

To provide a catalyst for exhaust purification that can purify carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) from the stage at which a catalyst-containing gas temperature is low-temperature, and is more inexpensive.SOLUTION: A catalyst for exhaust purification has a CO oxidation catalyst containing cobalt oxide particles, and a three-way catalyst.SELECTED DRAWING: None

Description

The present invention relates to an exhaust gas purification catalyst containing a three-way catalyst.

A catalyst made of metal oxide as an exhaust gas purification catalyst that can oxidize carbon monoxide (CO) and hydrocarbon (HC) in gas emitted from internal combustion engines such as automobile engines and at the same time reduce nitrogen oxides (NOx). A so-called three-way catalyst in which a noble metal is supported on a carrier is known. However, since this three-way catalyst has low exhaust gas purification activity at low temperatures, there is a problem that the exhaust gas is not sufficiently purified and is discharged when the temperature of the gas containing the catalyst is low, such as when starting an engine. It was.

Therefore, Japanese Patent Application Laid-Open No. 2009-7942 (Patent Document 1) describes an exhaust gas in which a CO oxidation catalyst is provided on the upstream side of the exhaust gas flow and an HC trap catalyst is provided on the downstream side of the three-way catalyst provided in the exhaust gas flow path. Purification catalyst devices have been proposed. In this exhaust gas purification catalyst device, a CO oxidation catalyst showing high catalytic activity even at a low temperature is provided on the upstream side of the exhaust gas flow from the three-way catalyst, so that CO can be purified from the stage where the temperature of the gas containing the catalyst is low. In addition, the reaction heat generated by the purification of CO raises the gas input temperature of the three-way catalyst and activates the three-way catalyst, so that HC and NOx are also sufficiently purified. However, in the exhaust gas purification catalyst device, since the CO oxidation catalyst and the three-way catalyst are arranged in series, the scale of the exhaust gas purification catalyst device is larger than that in the case where only the three-way catalyst is arranged. There was a problem.

Further, Japanese Patent Application Laid-Open No. 10-504370 (Patent Document 2) describes an exhaust device including a CO oxidation catalyst and a hydrocarbon oxidation catalyst, and more preferably, a three-way catalyst is provided. It is also described that the CO oxidation catalyst containing platinum and / or palladium is preferable. However, the exhaust gas purification catalyst containing a CO oxidation catalyst containing platinum and / or palladium has a problem of high cost.

Further, Japanese Patent Application Laid-Open No. 2006-289213 (Patent Document 3) describes an exhaust gas purification catalyst in which a plurality of noble metal particles are supported on a carrier and the average particle size and the average inter-particle distance of the noble metal particles are within a specific range. It is also described that this suppresses the aggregation of precious metal particles in a high temperature environment and obtains high catalytic activity. However, even if the average particle size of the noble metal particles and the distance between the average particles are adjusted within a specific range, it is difficult to obtain sufficient catalytic activity at the stage where the temperature of the gas containing the catalyst is low.

JP-A-2009-7942 Special Table No. 10-504370 Japanese Unexamined Patent Publication No. 2006-289213

The present invention has been made in view of the above-mentioned problems of the prior art, and purifies carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) from the stage where the catalyst-containing gas temperature is low. It is an object of the present invention to provide an exhaust gas purification catalyst which is possible and cheaper.

As a result of diligent research to achieve the above object, the present inventors have added a CO oxidation catalyst containing cobalt oxide particles to the three-way catalyst so that the temperature of the gas containing the catalyst is low. We have found that it is possible to purify carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx), and that a cheaper exhaust gas purification catalyst can be obtained, and have completed the present invention.

That is, the exhaust gas purification catalyst of the present invention is characterized by containing a CO oxidation catalyst containing cobalt oxide particles and a three-way catalyst.

In such an exhaust gas purification catalyst, the average value of the distance between the centers of gravity of the CO oxidation catalyst is preferably 400 μm or less. Further, the CO oxidation catalyst is at least one other selected from the group consisting of the cobalt oxide particles, zirconium oxide particles, cerium oxide particles, cerium-zirconium solid solution oxide particles, and aluminum oxide particles. It is preferable that it contains the metal oxide particles of the above, and it is more preferable that the cobalt oxide particles are supported by the other metal oxide particles.

Further, in the exhaust gas purification catalyst of the present invention, examples of the three-way catalyst include those containing a catalyst carrier made of a metal oxide and a noble metal supported on the catalyst carrier. Further, the mass ratio of the CO oxidation catalyst to the three-way catalyst (CO oxidation catalyst / three-way catalyst) is preferably 1/5 to 5/1.

In the present invention, the average value of the distance between the centers of gravity of the CO oxidation catalyst is obtained by the following method. That is, first, the exhaust gas purification catalyst of the present invention is subjected to SEM-EDS analysis using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDS), and a cobalt mapping image is acquired. Next, the obtained cobalt mapping image was binarized at 8 bits using image analysis processing software (for example, "imageJ" developed by the National Institute of Public Health (NIH)), and obtained. The binarized image is subjected to particle detection processing with a threshold value of 50 pix or more, and then mask processing is performed with the black region as a CO oxidation catalyst and the white region as a three-way catalyst. For images in which 15 or more CO oxidation catalysts (black regions) are dispersed, use the dispersion measurement function of image analysis software (for example, "A image-kun (R)" manufactured by Asahi Kasei Engineering Co., Ltd.) to use the CO oxidation catalyst (black region). Measure the distance between the centers of gravity of the area) and calculate the average value.

Further, the reason why carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) can be purified from the stage where the temperature of the gas containing the catalyst is low by using the exhaust gas purification catalyst of the present invention is Although it is not always clear, the present inventors infer as follows. That is, the exhaust gas purification catalyst of the present invention contains a CO oxidation catalyst containing cobalt oxide particles and a three-way catalyst. Since the CO oxidation catalyst containing the cobalt oxide particles exhibits high oxidation catalytic activity even at a low temperature, it is presumed that CO and HC can be purified by the CO oxidation catalyst from the stage where the temperature of the gas containing the catalyst is low. .. Further, CO and HC are purified by being oxidized. At this time, heat of reaction is generated, and this heat of reaction raises the temperature of the three-way catalyst. Therefore, from the stage where the temperature of the gas containing the catalyst is low, the above. It is presumed that the three-way catalyst is activated and NOx can be purified.

According to the present invention, it is possible to purify carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) from the stage where the temperature of the gas containing the catalyst is low, and the exhaust gas purification catalyst is cheaper. Can be obtained.

6 is a graph showing the relationship between the catalyst-containing gas temperature of the exhaust gas purification catalyst before the durability test and the CO purification rate obtained in Examples 1 to 4 and Comparative Examples 1 to 3. It is a graph which shows the relationship between the catalyst-containing gas temperature of the exhaust gas purification catalyst before the endurance test and the C ₃ H ₆ purification rate obtained in Examples 1 to 4 and Comparative Examples 1 to ₃ . 6 is a graph showing the relationship between the catalyst-containing gas temperature of the exhaust gas purification catalyst before the durability test and the NO purification rate obtained in Examples 1 to 4 and Comparative Examples 1 to 3. It is a graph which shows the CO50% purification temperature of the exhaust gas purification catalyst before the endurance test and after the endurance test obtained in Examples 1 to 4 and Comparative Examples 1 to 3. It is a graph showing the C _{3 H} 6 _50% purification temperature of the exhaust gas purifying catalyst after the durability test before and durability test obtained in Examples 1-4 and Comparative Examples 1-3. It is a graph which shows NO50% purification temperature of the exhaust gas purification catalyst before the endurance test and after the endurance test obtained in Examples 1 to 4 and Comparative Examples 1 to 3. It is a binarization image (CO oxidation catalyst dispersion image) after masking the electron micrograph of the exhaust gas purification catalyst obtained in Example 6. It is a binarization image (CO oxidation catalyst dispersion image) after masking the electron micrograph of the exhaust gas purification catalyst obtained in Example 8. It is a binarization image (CO oxidation catalyst dispersion image) after masking the electron micrograph of the exhaust gas purification catalyst obtained in Example 9. It is a graph which shows the CO50% purification temperature of the exhaust gas purification catalyst before the endurance test and after the endurance test obtained in Examples 5-9 and Comparative Examples 4-6. It is a graph showing the C _{3 H} 6 _50% purification temperature of the exhaust gas purifying catalysts of Examples 5-9 and before the durability test obtained in Comparative Examples 4-6 and after the endurance test. It is a graph which shows the NO50% purification temperature of the exhaust gas purification catalyst before the endurance test and after the endurance test obtained in Examples 5-9 and Comparative Examples 4-6.

Hereinafter, the present invention will be described in detail according to its preferred embodiment.

The exhaust gas purification catalyst of the present invention contains a CO oxidation catalyst containing cobalt oxide particles and a three-way catalyst. By blending a CO oxidation catalyst containing cobalt oxide particles with a three-way catalyst, CO, HC and NOx can be purified from the stage where the temperature of the gas containing the catalyst is low.

(CO oxidation catalyst)
The CO oxidation catalyst used in the present invention contains cobalt oxide particles. The cobalt oxide particles are excellent in CO-oxidizing activity at low temperatures, and can sufficiently purify CO and HC even at a stage where the temperature of the catalyst-containing gas is low, such as when starting an engine. In addition, the heat of reaction generated at this time can raise the temperature of the three-way catalyst, which will be described later, so that the activity of the three-way catalyst can be expressed at an early stage.

Examples of such cobalt oxide particles include tricobalt tetroxide (Co ₃ O ₄ ) particles, cobalt oxide (Co O) particles, and dicobalt tricobalt (Co ₂ O ₃ ) particles. These cobalt oxide particles may be used alone or in combination of two or more. Further, among these cobalt oxide particles, tricobalt tetraoxide (Co ₃ O ₄ ) particles and cobalt oxide (Co O) particles are preferable from the viewpoint that the oxidation state of cobalt is easily changed.

The average particle size of the cobalt oxide particles is preferably 1 to 500 nm, more preferably 5 to 100 nm. When the average particle size of the cobalt oxide particles is less than the lower limit, the grain growth of the cobalt oxide particles due to heat tends to be remarkable, while when the average particle size exceeds the upper limit, the number of active points in the CO oxidation catalyst is insufficient. Tend to do. The average particle size of the cobalt oxide particles can be obtained from the half width of the peak caused by the cobalt oxide particles in the X-ray diffraction pattern obtained by the XRD measurement by Scheller's formula.

As the specific surface area of the cobalt oxide particles is preferably ^{5~100m 2 / g, 5~75m 2 /} g is more preferable. When the specific surface area of the cobalt oxide particles is less than the lower limit, the activity of the CO oxidation catalyst tends to decrease, while when the specific surface area exceeds the upper limit, the specific surface area of the cobalt oxide particles is significantly reduced by heat. It tends to be. The specific surface area of the cobalt oxide particles can be determined by the BET method.

The CO oxidation catalyst, in addition to the cobalt oxide particles, zirconium oxide particles (ZrO ₂ particles), cerium oxide particles (CeO ₂ particles), cerium - zirconium solid solution oxide particles _(CeO 2 -ZrO ₂ particles) , And at least one other metal oxide particle selected from the group consisting of aluminum oxide particles (Al ₂ O ₃ particles) may be further contained. By using such other metal oxide particles in combination with the cobalt oxide particles, the cobalt oxide particles can be maintained in a highly dispersed state. Among these other metal oxide particles, zirconium oxide particles (ZrO ₂ particles), cerium oxide particles (CeO ₂ particles), and the like, from the viewpoint of not causing excessive interaction with the cobalt oxide particles. cerium - zirconium solid solution oxide particles _(CeO 2 -ZrO ₂ particles) are preferred, ZrO ₂ particles is more preferable.

Further, in the CO oxidation catalyst, the cobalt oxide particles may be simply physically mixed with the other metal oxide particles, but the cobalt oxide particles are maintained in a higher dispersed state. From the viewpoint, it is preferable that the particles are supported by the other metal oxide particles.

The average particle size of the other metal oxide particles is preferably 5 to 200 nm, more preferably 10 to 200 nm. When the average particle size of the other metal oxide particles is less than the lower limit, the other metal oxide particles themselves tend to grow due to heat, while when the upper limit is exceeded, the cobalt oxide particles are grown. It tends to be difficult to maintain a highly dispersed state. The average particle size of the other metal oxide particles can be obtained by Scheller's formula from the half-value width of the peak caused by the other metal oxide particles in the X-ray diffraction pattern obtained by the XRD measurement.

As the specific surface area of the other metal oxide particles, preferably ^{10~200m 2 / g, 20~150m 2 /} g is more preferable. When the specific surface area of the other metal oxide particles is less than the lower limit, the ability to hold the cobalt oxide particles in a highly dispersed state tends to decrease, while when the specific surface area exceeds the upper limit, the other metal oxide particles are heated. The specific surface area of the metal oxide particles themselves tends to decrease. The specific surface area of the other metal oxide particles can be determined by the BET method.

In the CO oxidation catalyst, the content of the cobalt oxide particles is preferably 5 to 95% by mass, more preferably 10 to 90% by mass, based on the entire CO oxidation catalyst. When the content of the cobalt oxide particles is less than the lower limit, the activity of the CO oxidation catalyst is lowered, and the purification of CO and HC tends not to proceed sufficiently at the stage where the temperature of the gas containing the catalyst is low. Further, since the reaction heat due to purification is not sufficiently generated, the temperature of the three-way catalyst does not rise, and the purification of NOx at the stage where the temperature of the gas containing the catalyst is low tends not to proceed sufficiently. As a result, the 50% purification temperature of CO, HC and NOx tends not to be sufficiently lowered. On the other hand, when the content of the cobalt oxide particles exceeds the upper limit, it tends to be difficult to keep the cobalt oxide particles in a highly dispersed state.

The method for preparing such a CO oxidation catalyst is not particularly limited, and a known impregnation method or coprecipitation method can be adopted. For example, the other metal oxide particles are immersed in an aqueous solution containing a cobalt salt (for example, cobalt carbonate, cobalt nitrate, cobalt chloride, cobalt sulfate) as needed, and the obtained aqueous solution is heated to provide a solvent. CO composed of the cobalt oxide particles (preferably the cobalt oxide particles supported on the other metal oxide particles) by firing the produced solid component in the air after removing (water). An oxidation catalyst can be obtained. It also contains a cobalt salt (eg, cobalt carbonate, cobalt nitrate, cobalt chloride, cobalt sulfate) and, if necessary, salts of the other metals (eg, carbonate, nitrate, chloride, sulfate). An acid (for example, carbon dioxide, nitric acid, hydrochloric acid, sulfuric acid) was added dropwise to the aqueous solution to co-precipitate the cobalt salt and the salt of the other metal, and then the solvent (water) was removed by heating to obtain the product. By firing the solid component in the air, a CO oxidation catalyst composed of the cobalt oxide particles (preferably a co-deposit of the cobalt oxide particles and the other metal oxide particles) can be obtained. ..

(Three-way catalyst)
The three-way catalyst used in the present invention is not particularly limited as long as it is used for purifying CO, HC and NOx. For example, a known three-way catalyst in which a noble metal is supported on a catalyst carrier made of a metal oxide. Can be mentioned.

The metal oxide is not particularly limited, and for example, aluminum oxide (Al ₂ O ₃ ), cerium oxide (CeO ₂ ), zirconium oxide (ZrO ₂ ), and cerium-zirconium solid solution oxide (CeO _2- ZrO). ₂ ) can be mentioned. These metal oxides may be used alone or in combination of two or more. Among these metal oxides, aluminum oxide (Al ₂ O ₃ ) and zirconium oxide (ZrO ₂ ) are preferable, and Al ₂ O ₃ is more preferable, from the viewpoint of heat resistance.

The average particle size of the catalyst carrier made of such a metal oxide is preferably 5 to 100 nm, more preferably 5 to 50 nm. When the average particle size of the catalyst carrier is less than the lower limit, the heat resistance tends to be insufficient, while when the average particle size exceeds the upper limit, the noble metal tends to be unable to be supported in a high dispersion. The average particle size of the catalyst carrier can be obtained by Scheller's formula from the half-value width of the peak caused by the catalyst carrier in the X-ray diffraction pattern obtained by the XRD measurement.

As the specific surface area of the catalyst support, preferably ^{5~200m 2 / g, 20~150m 2 /} g is more preferable. When the specific surface area of the catalyst carrier is less than the lower limit, the noble metal tends to be unable to be supported in a high dispersion, while when the specific surface area exceeds the upper limit, the metal oxide itself tends to grow grains by heat. The specific surface area of the catalyst carrier can be determined by the BET method.

Further, in the exhaust gas purification catalyst of the present invention, when the CO oxidation catalyst contains the other metal oxide particles, the metal oxide constituting the catalyst carrier is separate from the other metal oxide particles. It may be the above-mentioned other metal oxide particles. That is, in the former case, the exhaust gas purification catalyst of the present invention comprises the other metal oxide particles and the cobalt oxide particles (preferably the cobalt oxide particles supported on the other metal oxide particles). It contains a CO oxidation catalyst and a ternary catalyst in which the noble metal is supported on a catalyst carrier made of the metal oxide. On the other hand, in the latter case, the exhaust gas purification catalyst of the present invention comprises the other metal oxide particles and the cobalt oxide particles (preferably the cobalt oxide particles supported on the other metal oxide particles). It contains the noble metal and is further supported on the other metal oxide particles.

The noble metal is not particularly limited, and examples thereof include platinum (Pt), palladium (Pd), rhodium (Rh), and iridium (Ir). These precious metals may be used alone or in combination of two or more. Of these precious metals, platinum (Pt), palladium (Pd), and rhodium (Rh) are preferable from the viewpoint of efficiently purifying CO, HC, and NOx.

The average particle size of the noble metal is preferably 1 to 100 nm, more preferably 2 to 50 nm. When the average particle size of the noble metal is less than the lower limit, the noble metal itself tends to grow due to heat, while when it exceeds the upper limit, the number of active points in the three-way catalyst tends to be insufficient. The average particle size of the noble metal can be obtained by randomly extracting the noble metal particles in the TEM image, measuring the particle size, and averaging them.

In the three-way catalyst, the content (supported amount) of the noble metal is preferably 0.01 to 4 parts by mass, more preferably 0.1 to 1 part by mass with respect to 100 parts by mass of the catalyst carrier. When the content of the noble metal is less than the lower limit, the number of active points in the three-way catalyst is insufficient, and the purification of NOx tends not to proceed sufficiently. On the other hand, if the upper limit is exceeded, the purification performance tends to be saturated.

The method for preparing such a three-way catalyst is not particularly limited, and a known impregnation method or coprecipitation method can be adopted. For example, the metal oxide is immersed in an aqueous solution containing a salt of the noble metal (for example, carbonate, nitrate, chloride, sulfate), and the obtained aqueous solution is heated to remove the solvent (water). By calcining the produced solid component in the air, a ternary catalyst in which the noble metal is supported on a catalyst carrier made of the metal oxide can be obtained. Further, an acid (for example, carbonate) is added to an aqueous solution containing the salt of the noble metal (for example, carbonate, nitrate, chloride, sulfate) and the salt of the metal (for example, carbonate, nitrate, chloride, sulfate). (Carbonate, nitrate, hydrochloric acid, sulfate) is added dropwise to co-precipitate the noble metal salt and the metal salt, and then the solvent (water) is removed by heating, and the obtained solid component is fired in the air. This also makes it possible to obtain a ternary catalyst in which the noble metal is supported on a catalyst carrier made of the metal oxide.

[Exhaust gas purification catalyst]
The exhaust gas purification catalyst of the present invention contains the CO oxidation catalyst and the three-way catalyst. Such an exhaust gas purification catalyst of the present invention exhibits good CO, HC and NOx purification performance from the stage where the temperature of the gas containing the catalyst is low, at least before the durability test. The method for preparing such an exhaust gas purification catalyst is not particularly limited, and examples thereof include a method of stirring and physically mixing the CO oxidation catalyst and the three-way catalyst. Further, after the mixture of the CO oxidation catalyst and the three-way catalyst obtained in this manner is pressure-molded, the obtained molded product may be pulverized and sieved to form catalyst pellets.

In addition, as another method for preparing the exhaust gas purification catalyst of the present invention, for example, a cobalt salt (for example, cobalt carbonate, cobalt nitrate, cobalt chloride, cobalt sulfate) and a salt of the noble metal (for example, carbonate, nitrate, chloride) are used. The other metal oxide particles are immersed in an aqueous solution containing a substance (sulfate), the obtained aqueous solution is heated to remove the solvent (water), and then the produced solid component is fired in the atmosphere. Thereby, the other metal oxide particles and the cobalt oxide particles supported on the other metal oxide particles are contained, and further, the noble metal supported on the other metal oxide particles is contained. The exhaust gas purification catalyst of the present invention can be obtained. Further, a cobalt salt (eg, cobalt carbonate, cobalt nitrate, cobalt chloride, cobalt sulfate), a salt of the noble metal (eg, carbonate, nitrate, chloride, sulfate) and a salt of the other metal (eg, salt). , Carbonate, Nitrate, Chloride, Sulfate) by dropping an acid (eg, carbonic acid, nitric acid, hydrochloric acid, sulfate) into a cobalt salt, the noble metal salt, and the other metal salt. The solvent (water) is removed by heating after co-precipitation, and the obtained solid component is also supported on the other metal oxide particles and the other metal oxide particles by firing in the air. It is possible to obtain the exhaust gas purification catalyst of the present invention containing the above-mentioned cobalt oxide particles and further containing the above-mentioned noble metal supported on the above-mentioned other metal oxide particles.

In the exhaust gas purification catalyst of the present invention, the average value of the distance between the centers of gravity of the CO oxidation catalyst is preferably 400 μm or less, more preferably 100 μm or less, and further preferably 50 μm or less. The exhaust gas purification catalyst in which the average value of the distance between the centers of gravity of the CO oxidation catalyst is within the above range has good CO and HC from the stage where the temperature of the catalyst-containing gas is low not only before the durability test but also after the durability test. And NOx purification performance, whereas the exhaust gas purification catalyst in which the average value of the distance between the centers of gravity of the CO oxidation catalyst exceeds the upper limit is CO, at the stage where the catalyst-containing gas temperature is low after the durability test. The purification performance of HC and NOx tends to decrease. The reason for this is not always clear, but the present inventors speculate as follows. That is, in the exhaust gas purification catalyst containing the CO oxidation catalyst and the three-way catalyst, the reaction heat generated by the purification of CO and HC in the CO oxidation catalyst is transferred to the three-way catalyst. At this time, in the exhaust gas purification catalyst in which the average value of the distance between the centers of gravity of the CO oxidation catalyst is within the above range, most of the generated reaction heat is transferred to the three-way catalyst, so that the temperature of the three-way catalyst rises. Even if the three-way catalyst is thermally deteriorated (for example, grain growth of noble metal) by the durability test, the NOx purification performance at the stage where the temperature of the catalyst-containing gas is low is significantly due to the large temperature rise of the three-way catalyst. It is presumed that the decline will be suppressed. On the other hand, in the exhaust gas purification catalyst in which the average value of the distance between the centers of gravity of the CO oxidation catalyst exceeds the upper limit, the reaction heat transferred from the CO oxidation catalyst to the three-way catalyst is small, and the temperature rise of the three-way catalyst is high. It is presumed to be small. Before the durability test, since the three-way catalyst has not been thermally deteriorated, even if the temperature rise of the three-way catalyst is small, good NOx purification performance is exhibited from the stage where the temperature of the catalyst-containing gas is low. When the three-way catalyst is thermally deteriorated (for example, grain growth of noble metal) by the heat resistance test, a small temperature rise of the three-way catalyst causes a large NOx purification performance at a stage where the temperature of the catalyst-containing gas is low. It is presumed that the decline cannot be sufficiently suppressed.

The lower limit of the average value of the distance between the centers of gravity of the CO oxidation catalyst is not particularly limited, but the cobalt contained in the CO oxidation catalyst covers the noble metal or promotes particle coarsening of the noble metal. From the viewpoint of preventing the purification performance from being deteriorated, 0.1 μm or more is preferable, and 0.5 μm or more is more preferable.

In the exhaust gas purification catalyst of the present invention, the mass ratio of the CO oxidation catalyst to the three-way catalyst is preferably CO oxidation catalyst / three-way catalyst = 1/5 to 5/1, preferably 1/2 to 2/1. More preferred. When the mass ratio of the CO oxidation catalyst to the three-way catalyst is less than the lower limit, the amount of the catalyst of the CO oxidation catalyst is small, and the purification of CO and HC tends not to proceed sufficiently at the stage where the temperature of the gas containing the catalyst is low. Further, since the reaction heat due to purification is not sufficiently generated, the temperature of the three-way catalyst does not rise, and the purification of NOx at the stage where the temperature of the gas containing the catalyst is low tends not to proceed sufficiently. As a result, the 50% purification temperature of CO, HC and NOx tends not to be sufficiently lowered. On the other hand, when the upper limit is exceeded, the amount of the catalyst of the three-way catalyst is small and the purification of NOx does not proceed sufficiently, so that the 50% purification temperature of NOx tends not to be sufficiently lowered.

Hereinafter, the present invention will be described in more detail based on Examples and Comparative Examples, but the present invention is not limited to the following Examples.

(Example 1)
[Preparation of CO oxidation catalyst]
Cobalt cobalt nitrate hexahydrate (Co (NO ₃ )) so that the mass ratio of tricobalt tetraoxide (Co ₃ O ₄ ) to zirconium oxide (ZrO ₂ ) is Co ₃ O ₄ : ZrO ₂ = 5: 95. _{2 · 6H 2 O, 99.5%} , Fuji film manufactured by Wako pure Chemical Industries, Ltd.) 1.81 g was prepared by dissolving in ion-exchanged water 50ml Co _{(NO 3) 2} aqueous solution of zirconium oxide particles (ZrO _2, the "RC-100" manufactured by Ichiraku Element Chemical Industry Co., Ltd., specific surface area: 100 m ^{2 /} g) 9.5 g is immersed to impregnate ZrO ₂ particles with a Co (NO ₃ ) ₂ aqueous solution, and then set at 200 ° C. The solvent (water) was evaporated on the hot plate, and further fired in the air at 400 ° C. for 5 hours to obtain CO oxidation catalyst particles in which Co ₃ O ₄ particles were carried on ZrO ₂ particles.

[Preparation of three-way catalyst]
Palladium nitrate (Pd (NO ₃ ) ₂ ) solution (Tanaka Kikinzoku Kogyo) so that the mass ratio of palladium (Pd) to aluminum oxide (Al ₂ O ₃ ) is Pd: Al ₂ O ₃ = 0.5: 100. 10 g of aluminum oxide particles (Al ₂ O ₃ , "PURALOX TH100-L1" manufactured by Sasol, BET specific surface area: 100 m ² / g) is immersed in a solution obtained by diluting 0.61 g of (manufactured by Co., Ltd.) with 50 ml of ion-exchanged water. After impregnating Al ₂ O ₃ particles with a Pd (NO ₃ ) ₂ solution, the solvent is evaporated on a hot plate set at 200 ° C., and the mixture is further fired in the air at 400 ° C. for 5 hours to obtain Al ₂ O. Pd particles was obtained a three-way catalyst particles supported on ₃ particles.

[Preparation of exhaust gas purification catalyst]
0.75 g of the CO oxidation catalyst particles and 0.75 g of the three-way catalyst particles were stirred in a mortar for 10 minutes for physical mixing. The obtained mixture was pressure-molded at a pressure of 1000 kg / cm ² , and then the obtained molded product was pulverized and sieved to prepare an exhaust gas purification catalyst pellet having a particle size of 0.5 to 1.0 mm.

(Example 2)
Exhaust gas purification catalyst pellets were prepared in the same manner as in Example 1 except that the mass ratio of Co ₃ O ₄ and ZrO ₂ in the CO oxidation catalyst particles was changed to Co ₃ O ₄ : ZrO ₂ = 10: 90.

(Example 3)
Exhaust gas purification catalyst pellets were prepared in the same manner as in Example 1 except that the mass ratio of Co ₃ O ₄ and ZrO ₂ in the CO oxidation catalyst particles was changed to Co ₃ O ₄ : ZrO ₂ = 33: 67.

(Example 4)
Exhaust gas purification catalyst pellets were prepared in the same manner as in Example 1 except that the mass ratio of Co ₃ O ₄ and ZrO ₂ in the CO oxidation catalyst particles was changed to Co ₃ O ₄ : ZrO ₂ = 50: 50.

(Comparative Example 1)
Same as Example 1 except that 0.75 g of aluminum oxide particles (Al ₂ O ₃ , "PURALOX TH100-L1" manufactured by Sasol, BET specific surface area: 100 m ^{2 /} g) was used instead of the CO oxidation catalyst particles. To prepare exhaust gas purification catalyst pellets.

(Comparative Example 2)
Exhaust gas purification catalyst pellets were produced in the same manner as in Example 1 except that the CO oxidation catalyst particles were not used and the amount of the three-way catalyst particles was changed to 1.5 g.

(Comparative Example 3)
Same as Example 3 except that 0.75 g of aluminum oxide particles (Al ₂ O ₃ , "PURALOX TH100-L1" manufactured by Sasol, BET specific surface area: 100 m ^{2 /} g) was used instead of the three-way catalyst particles. To prepare exhaust gas purification catalyst pellets.

<Durability test>
1.5 g of the obtained exhaust gas purification catalyst pellet (particle size: 0.5 to 1.0 mm) was filled in a quartz glass tube, and hydrogen-containing gas (H ₂ (2%) + N ₂ (remaining)) and oxygen-containing gas. (O ₂ (1%) + N ₂ (remaining portion)) was circulated at a flow rate of 0.5 L / min while switching every 5 minutes, and a durability test was conducted at 800 ° C. for 5 hours.

<Performance evaluation test of exhaust gas purification catalyst>
A catalyst layer filled with 1.5 g of exhaust gas purification catalyst pellets (particle size: 0.5 to 1.0 mm) before or after the durability test is fixed to the bed distribution reactor (Best Instruments Co., Ltd. "CATA-5000-" 8 ”) was set. A flow rate of 7 L / min of stoichiometric gas (CO (1.00%) + C ₃ H ₆ (1200 ppm C) + NO (1200 ppm) + O ₂ (6200 ppm) + CO ₂ (10.0%) + N ₂ (remaining)) is applied to this catalyst layer. in supplying, while entering the catalyst gas temperature was increased at a heating rate of 25 ° C. / min from 100 ° C. to 400 ° C., measured CO _{concentration,} C 3 _{H 6} concentration and NO concentration of the catalyst exiting gas, the catalyst CO in incoming gas temperature _to calculate the purification ratio of C 3 _{H 6} and NO. Figure 1A~-1C, entering the catalyst gas temperature and CO in the exhaust gas purifying catalyst pellets before the durability test obtained in Examples 1-4 and Comparative Examples _1-3, the relationship between the purification rate of C 3 _{H 6} and NO It is a graph which shows. Further, based on the graph shown entering the catalyst gas temperature and CO in the exhaust gas purifying catalyst pellets before and after the durability test and durability test, the relationship between the C ₃ H ₆ and NO purification rate, CO, of the C ₃ H ₆ and NO The temperature at which the purification rate was 50% (50% purification temperature) was determined. The results are shown in Table 1 and FIGS. 2A to 2C. In the bar graphs in FIGS. 2A to 2C, the left side is the result before the durability test, and the right side is the result after the durability test.

As shown in Table 1 and FIGS. 2A to 2B, the exhaust gas purification catalyst (Examples 1 to 4 and Comparative Example 3) containing the CO oxidation catalyst according to the present invention before the durability test is the CO oxidation according to the present invention. It was found that the purification temperature of CO and C ₃ H ₆ was 50% lower than that of the exhaust gas purification catalyst (Comparative Examples 1 and 2) before the durability test containing no catalyst. It is considered that this is because the CO oxidation catalyst according to the present invention was able to purify CO and C ₃ H ₆ at a lower temperature than the three-way catalyst. Further, as is clear from the results of Examples 1 to 4, it was found that the 50% purification temperature of CO and C ₃ H ₆ tends to decrease as the amount of Co ₃ O ₄ supported increases.

Further, as shown in FIG. 1C, the exhaust gas purification catalysts (Examples 1 to 4 and Comparative Examples 1 and 2) containing the three-way catalyst before the durability test purify almost 100% of NO at a temperature of less than 400 ° C. However, it was difficult to purify NO in the exhaust gas purification catalyst (Comparative Example 3) before the durability test, which did not contain a three-way catalyst, at 400 ° C. or lower. This result indicates that the purification of NO is proceeding on the three-way catalyst.

Further, as shown in Table 1 and FIG. 2C, the exhaust gas purification catalyst (Examples 1 to 4) containing the CO oxidation catalyst and the three-way catalyst according to the present invention before the durability test contains the three-way catalyst. It was found that the purification temperature of NO was 50% lower than that of the exhaust gas purification catalyst (Comparative Examples 1 and 2) before the durability test, which did not contain the CO oxidation catalyst according to the present invention. This is because the purification of CO and C ₃ H ₆ on the CO oxidation catalyst according to the present invention was started from the stage where the temperature of the gas containing the catalyst was low, and the reaction heat generated by the purification increased the temperature of the three-way catalyst. As a result, it is probable that the purification of NO was started on the three-way catalyst whose temperature had risen from the stage where the temperature of the catalyst-containing gas was low. On the other hand, when the CO oxidation catalyst is not contained (Comparative Examples 1 and 2), the reaction heat due to purification is not generated and the temperature of the three-way catalyst does not rise. Therefore, when the temperature of the gas containing the catalyst is low, the three-way catalyst is used. It is probable that the purification of NO did not start above. It was also found that the 50% purification temperature of NO was significantly increased in the case of using only the CO oxidation catalyst (Comparative Example 3).

Further, as is clear from the results of Examples 1 to 4 shown in Table 1 and FIG. 2C, it was found that the 50% purification temperature of NO tends to decrease as the amount of Co ₃ O ₄ supported increases. It was. This was caused by the purification of CO and C ₃ H ₆ on the CO oxidation catalyst started from the stage where the temperature of the catalyst-containing gas was lower as the amount of supported Co ₃ O ₄ increased. It is probable that the heat of reaction raised the temperature of the three-way catalyst, and as a result, the purification of NO started on the three-way catalyst whose temperature had risen from the stage where the temperature of the gas containing the catalyst was lower.

Furthermore, as shown in Table 1 and FIGS. 2A to 2C, these tendencies were also observed in the exhaust gas purification catalyst after the durability test, and the effect of the CO oxidation catalyst, the effect of the three-way catalyst, and the CO oxidation catalyst. It was confirmed that the effect of mixing with the three-way catalyst was maintained even after the durability test.

(Example 5)
[Preparation of CO oxidation catalyst]
The mass ratio of _Co 3 _{O 4} and ZrO ₂ in the CO oxidation catalyst particles _{Co 3 O 4: ZrO 2 =} 10: was changed to 90 in the same manner as in Example 1 on the ZrO ₂ grains _Co 3 _{O 4} particles The CO oxidation catalyst particles carried by the above were prepared, and the CO oxidation catalyst particles were pulverized in a dairy pot so that the particle size was 75 μm or less, and then sized by sieving.

[Preparation of three-way catalyst]
In the same manner as in Example 1, three-way catalyst particles (Pd: Al ₂ O ₃ = 0.5: 100 (mass ratio)) in which Pd particles are supported on Al ₂ O ₃ particles are prepared, and these three-way catalyst particles are prepared. Was crushed in a dairy pot so that the particle size was 75 μm or less, and then sized by sieving.

[Preparation of exhaust gas purification catalyst]
The particle size is 0.5 to 0.5 as in Example 1 except that 0.75 g of the CO oxidation catalyst particles (particle size: 75 μm or less) and 0.75 g of the three-way catalyst particles (particle size: 75 μm or less) are used. A 1.0 mm exhaust gas purification catalyst pellet was prepared.

(Example 6)
[Preparation of CO oxidation catalyst]
The mass ratio of _Co 3 _{O 4} and ZrO ₂ in the CO oxidation catalyst particles _{Co 3 O 4: ZrO 2 =} 10: was changed to 90 in the same manner as in Example 1 on the ZrO ₂ grains _Co 3 _{O 4} particles The CO oxidation catalyst particles carried by the above were prepared, and the CO oxidation catalyst particles were pulverized in a dairy pot so that the particle size was 75 μm or less, and then sized by sieving.

[Preparation of three-way catalyst]
Palladium nitrate (Pd (NO ₃ ) ₂ ) solution (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) so that the mass ratio of palladium (Pd) and zirconium oxide (ZrO ₂ ) is Pd: ZrO ₂ = 0.5: 100. 10.0 g of zirconium oxide particles (ZrO ₂ , "RC-100" manufactured by Daiichi Rare Element Chemical Industry Co., Ltd., BET specific surface area: 100 m ^{2 /} g) was immersed in a solution obtained by diluting 0.61 g with 50 ml of ion-exchanged water. After impregnating the ZrO ₂ particles with the Pd (NO ₃ ) ₂ solution, the solvent is evaporated on a hot plate set at 200 ° C., and further, the ZrO ₂ particles are baked in the air at 400 ° C. for 5 hours to be placed on the ZrO ₂ particles. A ternary catalyst particle carrying Pd particles was prepared. The three-way catalyst particles were crushed in a mortar so that the particle size was 75 μm or less, and then sized by sieving.

[Preparation of exhaust gas purification catalyst]
The particle size is 0.5 to 0.5 as in Example 1 except that 1.0 g of the CO oxidation catalyst particles (particle size: 75 μm or less) and 1.0 g of the three-way catalyst particles (particle size: 75 μm or less) are used. A 1.0 mm exhaust gas purification catalyst pellet was prepared.

(Example 7)
Exhaust gas purification catalyst pellets having a particle size of 0.5 to 1.0 mm were prepared in the same manner as in Example 6 except that the CO oxidation catalyst particles and the three-way catalyst particles were pulverized and sized so as to have a particle size of 25 μm or less. ..

(Example 8)
[Preparation of exhaust gas purification catalyst]
Nitrate so that the mass ratio of palladium (Pd), tricobalt tetraoxide (Co ₃ O ₄ ) and zirconium oxide (ZrO ₂ ) is Pd: Co ₃ O ₄ : ZrO ₂ = 0.5: 10: 90. palladium (Pd _{(NO 3) 2)} solution (Tanaka Kikinzoku Co., Ltd.) 0.61 g and cobalt nitrate hexahydrate _{(Co (NO 3) 2 ·} 6H 2 O, 99.5%, Fuji film Wako pure Chemical Zirconium oxide was added to an aqueous solution containing Pd (NO ₃ ) ₂ and Co (NO ₃ ) ₂ prepared by dissolving 1.81 g of (manufactured by Co., Ltd.) in 50 ml of ion-exchanged water and further adding 50 ml of ion-exchanged water. Immerse 20.0 g of particles (ZrO ₂ , "RC-100" manufactured by Daiichi Rare Element Chemical Industry Co., Ltd., specific surface area: 100 m ^{2 /} g) in ZrO ₂ particles with Pd (NO ₃ ) ₂ and Co (NO _3). ) After impregnating with an aqueous solution containing ₂ and, the solvent is evaporated on a hot plate set at 200 ° C., and further fired in the air at 400 ° C. for 5 hours to obtain Pd particles and Co on ZrO ₂ particles. Exhaust gas purification catalyst particles carried by ₃ O ₄ particles were prepared. The exhaust gas purification catalyst particles were crushed in a mortar so that the particle size was 75 μm or less, and then sized by sieving.

The obtained exhaust gas purification catalyst particles (particle size: 75 μm or less) were pressure-molded at a pressure of 1000 kg / cm ² , and then the obtained molded body was pulverized and sieved to have a particle size of 0.5 to 1.0 mm. Exhaust gas purification catalyst pellets were prepared.

(Example 9)
CO having a particle size of 0.5 to 1.0 mm in the same manner as in Example 6 except that the CO oxidation catalyst particles and the three-way catalyst particles were pulverized and sized so as to have a particle size of 0.5 to 1.0 mm. Oxidation catalyst particles and three-way catalyst particles were prepared, respectively. 0.75 g of the CO oxidation catalyst particles and 0.75 g of the three-way catalyst particles were stirred in a dairy pot for 10 minutes and physically mixed to prepare exhaust gas purification catalyst pellets having a particle size of 0.5 to 1.0 mm.

(Comparative Example 4)
Instead of the CO oxidation catalyst particles, ceria-zirconia solid solution particles (CeO _2- ZrO ₂ (CZ)) crushed and sized so that the particle size is 75 μm or less, CeO _2- manufactured by Solvay Special Chem Japan Co., Ltd. Exhaust gas purification catalyst pellets having a particle size of 0.5 to 1.0 mm were prepared in the same manner as in Example 5 except that 1.0 g of ZrO ₂ composite oxide and BET specific surface area: 60 m ^{2 /} g) were used.

(Comparative Example 5)
Instead of the CO oxidation catalyst particles, zirconium oxide particles crushed and sized so that the particle size is 75 μm or less (ZrO ₂ , “RC-100” manufactured by Daiichi Rare Element Chemical Industry Co., Ltd., BET specific surface area: 100 m Exhaust gas purification catalyst pellets having a particle size of 0.5 to 1.0 mm were prepared in the same manner as in Example 6 except that ^{2 /} g) 1.0 g was used.

(Comparative Example 6)
Instead of the three-way catalyst particles, zirconium oxide particles crushed and sized so that the particle size is 75 μm or less (ZrO ₂ , “RC-100” manufactured by Daiichi Rare Element Chemical Industry Co., Ltd., BET specific surface area: 100 m Exhaust gas purification catalyst pellets having a particle size of 0.5 to 1.0 mm were prepared in the same manner as in Example 6 except that ^{2 /} g) 1.0 g was used.

<Durability test>
1.5 g of the obtained exhaust gas purification catalyst pellet (particle size: 0.5 to 1.0 mm) was filled in a quartz glass tube, and hydrogen-containing gas (H ₂ (2%) + N ₂ (remaining)) and oxygen-containing gas. (O ₂ (1%) + N ₂ (remaining portion)) was circulated at a flow rate of 0.5 L / min while switching every 5 minutes, and a durability test was conducted at 800 ° C. for 5 hours.

<Measurement of the distance between the centers of gravity of the CO oxidation catalyst>
A part of the exhaust gas purification catalyst pellet (particle size: 0.5 to 1.0 mm) after the durability test is sampled and embedded in a curable resin (epoxy resin), and the surface is polished to expose the cross section of the catalyst pellet. Further, the cross section of the exposed catalyst pellet was mirror-polished. The obtained polished cross section is subjected to an energy dispersive X-ray analyzer (EDS, "X-max ^N " manufactured by Horiba Seisakusho Co., Ltd., effective element area: 50 mm ² ) and a scanning electron microscope (SEM, manufactured by Hitachi High Technologies Co., Ltd.). Observation using "SU3500") (magnification: 400 times (40 times in Example 9)) was performed and SEM-EDS analysis (EDS sweep count (integration count): 20 times) was performed to obtain a cobalt mapping image. .. The obtained cobalt-mapped image was binarized in 8 bits using image analysis processing software (“imageJ” developed by the National Institutes of Health (NIH)), and the obtained binarized image was obtained. After performing the particle detection treatment with 50 pix or more as the threshold value, the black region was used as a CO oxidation catalyst and the white region was used as a three-way catalyst for mask treatment. 3A to 3C show binarized images (CO oxidation catalyst dispersion images) of the exhaust gas purification catalyst pellets obtained in Examples 6, 8 and 9 after mask treatment. Regarding this CO oxidation catalyst dispersion image, CO is used for 15 or more CO oxidation catalysts (black area) by using the dispersion degree measurement function of image analysis software ("A image-kun (R)" manufactured by Asahi Kasei Engineering Co., Ltd.). The distance between the centers of gravity of the oxidation catalyst (black region) was measured, and the average value was calculated. The results are shown in Table 2. The solid line in FIGS. 3A to 3C is a straight line connecting the centers of gravity of the CO oxidation catalyst (black region), and the length of this straight line was measured as the distance between the centers of gravity.

<Performance evaluation test of exhaust gas purification catalyst>
A catalyst layer filled with 1.5 g of exhaust gas purification catalyst pellets (particle size: 0.5 to 1.0 mm) before or after the durability test is fixed. Bed flow reactor (model gas generator: manufactured by Best Instruments Co., Ltd.) It was set in "CATA-5000-SP", gas analyzer: "BEX5900C-SP" manufactured by Best Instruments Co., Ltd.). A flow rate of stoichiometric gas (CO (1.00%) + C ₃ H ₆ (1200 ppm C) + NO (1200 ppm) + O ₂ (6200 ppm) + CO ₂ (10.0%) + N ₂ (remaining)) is applied to this catalyst layer at a flow rate of 10 L / min. in supplying, while entering the catalyst gas temperature was increased at a heating rate of 25 ° C. / min from 100 ° C. to 450 ° C., measured CO _{concentration,} C 3 _{H 6} concentration and NO concentration of the catalyst exiting gas, the catalyst CO in incoming gas temperature _to calculate the purification ratio of C 3 _{H 6} and NO. Entering the catalyst gas temperature and CO in the exhaust gas purifying catalyst pellets before and after the durability test and durability test, based on the graph showing the relationship between the purification rate of C ₃ H ₆ and NO, CO, purification rate of C ₃ H ₆ and NO The temperature at which is 50% (50% purification temperature) was determined. The results are shown in Table 2 and FIGS. 4A to 4C. In the bar graphs in FIGS. 4A to 4C, the left side is the result before the durability test, and the right side is the result after the durability test.

As shown in Table 2 and FIGS. 4A to 4B, the exhaust gas purification catalyst (Example 5) containing the CO oxidation catalyst according to the present invention before the durability test has an oxidation catalyst or oxygen absorption instead of the CO oxidation catalyst. It was found that the purification temperature of CO and C ₃ H ₆ was 50% lower than that of the exhaust gas purification catalyst (Comparative Example 4) before the durability test, which contained a solid solution of ceria-zirconia having high activity as a release material. It is considered that this is because the CO oxidation catalyst according to the present invention was able to purify CO and C ₃ H ₆ at a lower temperature than the ceria-zirconia solid solution. In addition, this tendency was also observed in the exhaust gas purification catalyst after the durability test, and it was confirmed that the effect of the CO oxidation catalyst was maintained even after the durability test.

Further, as shown in Table 2 and FIG. 4C, the exhaust gas purification catalyst (Example 5) containing the CO oxidation catalyst and the three-way catalyst according to the present invention before the durability test is ceria instead of the CO oxidation catalyst. -It was found that the purification temperature of NO was 50% lower than that of the exhaust gas purification catalyst (Comparative Example 4) containing the zirconia solid solution before the durability test. This is because, in the exhaust gas purification catalyst containing the CO oxidation catalyst (Example 5) according to the present invention, the purification of CO and C ₃ H ₆ on the CO oxidation catalyst was started from the stage where the temperature of the catalyst-containing gas was low. As a result, the reaction heat generated by the purification raises the temperature of the three-way catalyst, and as a result, NO purification starts on the three-way catalyst whose temperature has risen from the stage where the temperature of the gas containing the catalyst is low. In the exhaust gas purification catalyst (Comparative Example 4) containing a solid solution of ceria-zirconia instead of the CO oxidation catalyst, the reaction heat due to purification is not generated and the temperature of the three-way catalyst does not rise, so that the gas temperature containing the catalyst At the low stage, it is considered that NO purification did not start on the three-way catalyst. In addition, this tendency was also observed in the exhaust gas purification catalyst after the durability test, and it was confirmed that the effect of mixing the CO oxidation catalyst and the three-way catalyst was maintained even after the durability test.

Further, as shown in Table 2 and FIGS. 4A to 4C, even when ZrO ₂ is used as the carrier of the three-way catalyst, the CO oxidation catalyst according to the present invention is used as in the case where Al ₂ O ₃ is used. The exhaust gas purification catalyst before the durability test (Example 6) containing the above has CO, C ₃ H ₆ and CO, C ₃ H ₆ and as compared with the exhaust gas purification catalyst before the durability test (Comparative Example 5) containing the CO oxidation catalyst according to the present invention. It was found that the purification temperature of NO was lowered by 50%. Furthermore, when ZrO ₂ is used as the carrier of the three-way catalyst (Example 6), low-temperature purification of CO, C ₃ H ₆ and NO equal to or higher than that when Al ₂ O ₃ is used (Example 5). It turned out that performance can be obtained. In addition, these tendencies are also observed in the exhaust gas purification catalyst after the durability test, and the effect of the CO oxidation catalyst, the effect of the three-way catalyst, and the effect of mixing the CO oxidation catalyst and the three-way catalyst are the durability test. It was confirmed that it was maintained even afterwards.

Further, as shown in Table 2 and FIGS. 4A to 4C, the exhaust gas purification catalyst (Example 9) in which the average value of the distance between the centers of gravity of the CO oxidation catalyst exceeds a predetermined range is the distance between the centers of gravity of the CO oxidation catalyst. compared to an exhaust gas purifying catalyst having an average value is within a predetermined range (examples 6-8), before the durability test, _CO, but 50% purification temperature of _C 3 H ₆ and NO were comparable, in after the durability test, _CO, be 50% purification temperature of C 3 _{H 6} and NO increases were found. This is because the 50% purification temperature of NO of the exhaust gas purification catalyst (Example 9) after the durability test in which the average value of the distance between the centers of gravity of the CO oxidation catalyst exceeds a predetermined range does not include the CO oxidation catalyst according to the present invention. Since it is about the same as the 50% purification temperature of NO of the exhaust gas purification catalyst (Comparative Example 5) after the test, the exhaust gas purification catalyst after the durability test (the average value of the distance between the centers of gravity of the CO oxidation catalyst exceeds a predetermined range). In Example 9), it is considered that the effect of mixing the CO oxidation catalyst and the three-way catalyst is not sufficiently obtained. That is, in the exhaust gas purification catalyst (Examples 6 to 8) in which the average value of the distance between the centers of gravity of the CO oxidation catalyst is within a predetermined range, the reaction generated by the purification of CO and C ₃ H ₆ on the CO oxidation catalyst. Since the amount of heat transferred to the three-way catalyst is large, even if the three-way catalyst is thermally deteriorated by the durability test, the temperature of the three-way catalyst rises sufficiently due to the transferred reaction heat, and the activity of the three-way catalyst decreases. Is considered to be sufficiently suppressed. On the other hand, in the exhaust gas purification catalyst (Example 9) in which the average value of the distance between the centers of gravity of the CO oxidation catalyst exceeds a predetermined range, the distance between the CO oxidation catalyst and the three-way catalyst becomes long, so that the CO oxidation catalyst is used. It is considered that the amount of the generated reaction heat transferred to the three-way catalyst is reduced. As a result, before the durability test, the three-way catalyst is not thermally deteriorated, so that even if the amount of heat transferred from the CO oxidation catalyst to the three-way catalyst is small, the decrease in the activity of the three-way catalyst is suppressed. When the three-way catalyst was thermally deteriorated by the durability test, the amount of heat transferred from the CO oxidation catalyst to the three-way catalyst was small, so the temperature of the three-way catalyst did not rise and the activity of the three-way catalyst decreased. it is conceivable that.

As described above, according to the present invention, it is possible to purify carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) from the stage where the temperature of the gas containing the catalyst is low.

Therefore, the exhaust gas purification catalyst of the present invention is a catalyst for purifying exhaust gas containing carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx) emitted from an internal combustion engine such as an automobile engine. It is useful as such.

Claims (6)

An exhaust gas purification catalyst characterized by containing a CO oxidation catalyst containing cobalt oxide particles and a three-way catalyst. The exhaust gas purification catalyst according to claim 1, wherein the average value of the distance between the centers of gravity of the CO oxidation catalyst is 400 μm or less. The CO oxidation catalyst is at least one other metal selected from the group consisting of the cobalt oxide particles, zirconium oxide particles, cerium oxide particles, cerium-zirconium solid solution oxide particles, and aluminum oxide particles. The exhaust gas purification catalyst according to claim 1 or 2, wherein the catalyst contains oxide particles. The exhaust gas purification catalyst according to claim 3, wherein the cobalt oxide particles are supported on the other metal oxide particles. The exhaust gas according to any one of claims 1 to 4, wherein the three-way catalyst contains a catalyst carrier made of a metal oxide and a noble metal supported on the catalyst carrier. Purification catalyst. The item according to any one of claims 1 to 5, wherein the mass ratio of the CO oxidation catalyst to the three-way catalyst (CO oxidation catalyst / three-way catalyst) is 1/5 to 5/1. Exhaust gas purification catalyst described.