JP2010149112A - Catalyst for cleaning exhaust gas and exhaust gas cleaning method using this - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning an exhaust gas suppressing sintering by covering a noble metal with a covering substance and also having excellent catalytic performance, and to provide a method for efficiently cleaning an exhaust gas of an internal combustion engine using the catalyst for cleaning the exhaust gas. <P>SOLUTION: In the catalyst for cleaning the exhaust gas, a composite body comprising the noble metal 2 and cerium oxide 3 is covered by lanthanum-containing alumina 1. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a catalyst for purifying exhaust gas discharged from an internal combustion engine and an exhaust gas purification method using the same, and more particularly, efficiently purifies exhaust gas from an internal combustion engine even after high temperature durability treatment. The present invention relates to an exhaust gas purifying catalyst that can be used and an exhaust gas purifying method using the same.

A support using a composite oxide of Al ₂ O ₃ derived from aluminum alkoxide and CeO ₂ and La ₂ O ₃ has high surface area stability, and a catalyst supporting a noble metal on the support is used for purification of automobile simulated exhaust gas. High performance is disclosed (Patent Document 1).

  In addition, a catalyst in which a metal is supported on an oxide composite in which a carrier composed of a porous oxide derived from aluminum alkoxide and an oxygen storage / release material is combined at an atomic level allows the growth of metal and oxygen storage / release materials to grow. It is disclosed that it is suppressed (Patent Document 2).

On the other hand, it is disclosed that Pt / SiO ₂ catalyst prepared by the sol-gel method has Pt particles covered with SiO ₂ and can suppress sintering of Pt particles (Non-patent Document 1).

JP-A-1-230425 (published September 13, 1989) Japanese Patent Laid-Open No. 11-347410 (published on December 21, 1999)

M. Azomoza, T. Lopez, R. Gomez, RD Gonzalez, “Synthesis of high surface area supported Pt / SiO2catalysts from H2PtCl6 ・ 6H2O by the sol-gel method”, Catalysis Today, 21 October 1992, Volume 15, Issues 3- 4, p.547-554

  However, in any of the catalysts described in Patent Documents 1 and 2, since most of the noble metal particles are present on the surface of the supporting substrate, there are few physical barriers to the sintering of the noble metal, and when used at high temperatures or for a long time. During the high temperature durability treatment, there is a problem that the sintering of the noble metal particles is likely to occur and the performance is deteriorated.

On the other hand, by using the method described in Non-Patent Document 1, it is possible to suppress sintering of noble metal particles during high temperature durability treatment by coating the noble metal with a coating material such as SiO _2. Is not enough.

  The present invention has been made in view of the above problems, and an object of the present invention is not only to suppress sintering, but also to an exhaust gas purifying catalyst having excellent catalytic performance, and the exhaust gas purifying catalyst. An object of the present invention is to provide a method for efficiently purifying exhaust gas of an internal combustion engine using

  As a result of intensive studies in view of the above problems, the present inventors have found that sintering of noble metal particles during high temperature durability treatment can be suppressed by coating the noble metal with a coating substance. However, in such a coating state, it was considered that the fluctuation of the air-fuel ratio around the noble metal that is a catalytic active component may not be sufficiently relaxed. And, when a complex composed of noble metal and cerium oxide (hereinafter also referred to as “noble metal-cerium oxide complex”) is coated with lanthanum-containing alumina, not only sintering is suppressed but also excellent catalytic performance is achieved. The present inventors have found that an exhaust gas purifying catalyst can be obtained, and have completed the present invention.

  That is, the exhaust gas cleaning catalyst according to the present invention is characterized in that a composite composed of a noble metal and cerium oxide is coated with lanthanum-containing alumina.

  According to the above configuration, since the composite of noble metal and cerium oxide is coated using lanthanum-containing alumina, not only can the heat resistance of the catalyst be improved and sintering can be suppressed, but also oxygen absorption. The rate can be expressed high, and the catalyst performance can be kept high. Therefore, the exhaust gas from the internal combustion engine can be efficiently purified even after the high temperature durability treatment. In other words, it is possible to achieve both the heat resistance improvement by coating the noble metal with alumina and the suppression of the decrease in the ability to mitigate fluctuations in the air-fuel ratio, and the exhaust gas purification having a high oxygen storage / release capability even when the noble metal is coated. It is possible to provide a method for efficiently purifying exhaust gas from an internal combustion engine using the catalyst for exhaust gas and the exhaust gas purifying catalyst.

  In the exhaust gas cleaning catalyst according to the present invention, in the above composite, a compound composed of noble metal and cerium is preferably formed on the contact surface between the noble metal and cerium oxide. The above compound formed on the contact surface between the noble metal and cerium oxide serves as an indicator that the noble metal and cerium oxide are in contact with each other to form a complex. Therefore, it can be said that the catalyst in which the above compound is formed on the contact surface between the noble metal and cerium oxide is a catalyst having a high oxygen absorption rate and a high catalytic performance.

  In the exhaust gas cleaning catalyst according to the present invention, the content of the cerium oxide in the composite is preferably larger than the content of the compound. As described above, the compound formed on the contact surface between the noble metal and cerium oxide serves as an index indicating that the noble metal and cerium oxide are in contact with each other to form a complex. However, the formed compound no longer has a function as an oxygen storage / release material. Therefore, when the content of the cerium oxide is larger than the content of the compound in the composite, the oxygen absorption rate can be made high and the catalyst performance can be kept high.

  In the exhaust gas cleaning catalyst according to the present invention, the noble metal is preferably rhodium. Since rhodium has a high three-way catalyst performance, exhaust gas can be purified efficiently by selecting rhodium as the noble metal.

  In the exhaust gas cleaning catalyst according to the present invention, the compound is preferably CeRh. CeRh is an index indicating that rhodium and cerium oxide are in contact with each other to form a complex. Therefore, it can be said that a catalyst in which CeRh is formed on the contact surface between rhodium and cerium oxide is a catalyst having a high oxygen absorption rate and a high catalytic performance.

  In the exhaust gas cleaning catalyst according to the present invention, the content of the cerium oxide is preferably 1% by weight to 30% by weight with respect to the total amount of the noble metal, cerium oxide, lanthanum, and alumina.

  According to the above configuration, the air-fuel ratio fluctuation is preferably alleviated around the noble metal that is the catalytically active component.

  Even after the exhaust gas cleaning catalyst according to the present invention is exposed to the exhaust gas of an internal combustion engine having a temperature of 950 ° C. to 1000 ° C. (hereinafter, referred to as “after high temperature endurance treatment” in this specification), It preferably contains a composite coated with lanthanum-containing alumina.

  According to the said structure, the exhaust gas purification catalyst contains the said composite_body | complex coat | covered with the lanthanum containing alumina even after a high temperature durability process. Therefore, even after high-temperature durability treatment, oxygen storage / release can be performed in the coating layer. Therefore, the exhaust gas can be purified stably even after the high temperature durability treatment.

  In the exhaust gas cleaning catalyst according to the present invention, the exposed noble metal surface area after being exposed to the exhaust gas of an internal combustion engine having a temperature of 950 ° C. to 1000 ° C. is 0% with respect to the exposed noble metal surface area before being exposed to the exhaust gas. It is preferably reduced by ~ 87%.

  If the reduction rate of the exposed noble metal surface area is in the above range, it can be said that the thermal shrinkage of alumina and sintering of the noble metal are highly suppressed even after the high temperature durability treatment. Therefore, the exhaust gas from the internal combustion engine can be more efficiently purified even after the high temperature durability treatment.

  The exhaust gas cleaning catalyst according to the present invention preferably has an oxygen absorption rate of 30 to 100%. If the oxygen absorption rate is in the above range, it can be said that the catalyst has a high oxygen storage / release capability. Therefore, the exhaust gas from the internal combustion engine can be more efficiently purified even after the high temperature durability treatment.

  The exhaust gas cleaning catalyst according to the present invention is preferably supported on a three-dimensional structure. By using a three-dimensional structure having a large surface area, the exhaust gas-purifying catalyst can be efficiently supported. Therefore, the exhaust gas purification efficiency of the exhaust gas purification catalyst can be improved.

  The exhaust gas cleaning catalyst according to the present invention preferably further comprises platinum and / or palladium which is not coated with lanthanum-containing alumina. Since these noble metals have catalytic activity, when used in combination with rhodium, the exhaust gas purifying catalyst according to the present invention can be provided with higher exhaust gas purifying ability.

  The exhaust gas cleaning catalyst according to the present invention preferably further comprises a refractory inorganic oxide. Since the refractory inorganic oxide can disperse the exhaust gas purifying catalyst according to the present invention, the exhaust gas can be efficiently brought into contact with the exhaust gas purifying catalyst component. Therefore, exhaust gas purification at a high temperature can be performed more efficiently.

  The exhaust gas cleaning catalyst according to the invention preferably further comprises cerium oxide and / or ceria-zirconia composite oxide not covered with lanthanum-containing alumina.

  The cerium oxide and / or ceria-zirconia composite oxide functions as an oxygen storage / release material and can act as a co-catalyst to improve the heat resistance of the exhaust gas purifying catalyst or to improve the activity of the exhaust gas purifying catalyst. It has the effect of promoting oxidation / reduction reactions by seeds. Therefore, exhaust gas purification at a high temperature can be performed more efficiently.

  The exhaust gas purification method according to the present invention includes a step of exposing the exhaust gas purification catalyst according to the present invention to exhaust gas of an internal combustion engine.

  As described above, the exhaust gas purifying catalyst according to the present invention has both heat resistance and oxygen storage / release capability. Therefore, according to the said structure, the exhaust gas process which fully utilized the catalytic activity of a noble metal can be performed, and the exhaust gas purification at high temperature can be performed very efficiently.

  In the exhaust gas purification method according to the present invention, the temperature of the exhaust gas of the internal combustion engine is 0 ° C. to 750 ° C., and the temperature of the exhaust gas purification catalyst is 950 ° C. to 1000 ° C. before being exposed to the exhaust gas of the internal combustion engine. You may be exposed to the exhaust gas whose air-fuel ratio which is (C) is 14.1-15.1.

  According to the above configuration, the exhaust gas generated from the internal combustion engine during normal operation of an automobile or the like equipped with the internal combustion engine can be purified, as well as during the operation of the automobile or the durability test of the exhaust gas purification catalyst. Even if high-temperature exhaust gas of 950 ° C. to 1000 ° C. flows into the purification catalyst, the sintering of the noble metal is suppressed, and the oxygen storage / release capability can be exhibited around the noble metal. Relaxation occurs. The air-fuel ratio is a value close to the stoichiometric air-fuel ratio of the gasoline engine. Therefore, according to the above configuration, it is possible to efficiently purify the exhaust gas discharged from the gasoline engine.

  As described above, the exhaust gas purifying catalyst according to the present invention has a configuration in which the composite composed of the noble metal and cerium oxide is coated with the lanthanum-containing alumina. In addition, it is possible to suppress sintering of the noble metal and to have a high oxygen storage / release capability, and it is possible to efficiently purify the exhaust gas from the internal combustion engine even after the high temperature durability treatment.

  Further, as described above, the exhaust gas cleaning method according to the present invention has a configuration including the step of exposing the exhaust gas cleaning catalyst according to the present invention to the exhaust gas of the internal combustion engine, so that the catalytic activity of the noble metal is fully utilized. As a result, the exhaust gas treatment can be performed, and the exhaust gas purification at a high temperature can be performed very efficiently.

(A) is a schematic diagram which shows the state of the exhaust gas purifying catalyst concerning this invention before high temperature resistance processing, (b) is a schematic diagram which shows the state of the contact surface of a noble metal and cerium oxide. It is a schematic diagram showing a state in which a noble metal is coated with lanthanum-containing alumina and cerium oxide is supported without being in close proximity to the noble metal. It is a schematic diagram which shows the carrying | support state of the noble metal and an alumina in the conventional catalyst for exhaust gas purification produced using the impregnation method. It is a graph which shows 50% washing | cleaning temperature (T50) in temperature rising evaluation of the catalyst for exhaust gas purification manufactured in the Example of this invention. It is a graph which shows the oxygen absorption rate of the catalyst for exhaust gas purification manufactured in the Example of this invention.

  An embodiment of the present invention will be described in detail as follows, but the present invention is not limited to this. In addition, all the nonpatent literatures and patent literatures described in this specification are used as reference in this specification. In addition, “A to B” in the present specification means “A or more and B or less”. For example, if “1 to 30% by weight” is described in the specification, “1 to 30% by weight” "% By weight or less". In the present specification, “and / or” means either one or both.

[1. (Exhaust gas purification catalyst)
In the exhaust gas purifying catalyst according to the present invention, it is only necessary that the composite of noble metal and cerium oxide is coated with lanthanum-containing alumina. That is, the exhaust gas purifying catalyst according to the present invention only needs to contain a noble metal-cerium oxide composite coated with lanthanum-containing alumina.

  The state where “the noble metal particles and the cerium oxide particles form a composite and are covered with lanthanum-containing alumina” will be described with reference to FIG. Fig.1 (a) is a schematic diagram which shows the state of the exhaust gas purifying catalyst according to the present invention before the high temperature resistance treatment.

As shown in FIG. 1A, the lanthanum-containing alumina 1 is supported in the vicinity of the composite of the noble metal 2 and the cerium oxide 3. The above “supported in close proximity” refers to a state in which the noble metal 2 or cerium oxide 3 and the lanthanum-containing alumina 1 are partially in contact. In the “closely supported state” described above, molecules (for example, oxygen (O ₂ ) or This refers to the state where carbon monoxide (CO) cannot be adsorbed.

  1 and 2, the lanthanum-containing alumina 1 is preferably lanthanum-containing alumina particles, the noble metal 2 is preferably noble metal particles, and the cerium oxide 3 is preferably cerium oxide particles. Further, in FIG. 1A and FIG. 2, reference numeral 1 indicates only one lanthanum-containing alumina, but all substances expressed in white amorphous form are lanthanum-containing alumina. Moreover, although the code | symbol 3 points out only one cerium oxide, what is gray is all cerium oxide. As described above, the lanthanum-containing alumina, noble metal and cerium oxide are preferably particles. In such a case, the lanthanum-containing alumina and cerium oxide may be primary particles or secondary particles, but are more preferably secondary particles in which the primary particles are aggregated. The average particle size of the lanthanum-containing alumina and cerium oxide particles is 0.5 to 150 μm, preferably 1 to 50 μm.

  Here, the above-mentioned “complex comprising noble metal and cerium oxide” will be described with reference to FIGS. As shown in FIG. 1 (a), the “composite composed of noble metal and cerium oxide” is a complex in which at least a part of each atom of noble metal 2 and cerium oxide 3 is in contact with each other. And a mixture of noble metal and cerium oxide, a mixture of noble metal and cerium, a compound of noble metal and cerium, or a combination of two or more thereof. If noble metal particles and cerium oxide particles form a complex, cerium oxide 3 is present in the vicinity of noble metal 2, and therefore, when oxygen-excess gas flows in, cerium oxide 3 occludes oxygen. The gas atmosphere around the noble metal 2 can be relaxed from excess oxygen. On the other hand, in a reducing atmosphere in which oxygen is insufficient, cerium oxide 3 can effectively utilize oxygen by the noble metal 2 because the oxygen stored in the oxygen-excessive atmosphere exists around the noble metal 2.

  FIG. 1B is a schematic diagram showing the state of the contact surface between the noble metal and cerium oxide. As shown in FIG. 1B, in the composite composed of the noble metal and cerium oxide, a compound 4 composed of the noble metal and cerium oxide is formed on the contact surface between the noble metal and cerium oxide. The formation of such a compound can be confirmed by analyzing the “noble metal-cerium oxide composite coated with lanthanum-containing alumina” by X-ray diffraction. As will be described later, the presence of such a compound can be confirmed before and after the high temperature durability treatment.

  FIG. 2 is a schematic view showing a state in which the noble metal 2 is coated with the lanthanum-containing alumina 1 but the cerium oxide 3 is supported without being in close proximity to the noble metal 2. Compared with the exhaust gas purifying catalyst according to the present invention, in such a configuration, the cerium oxide 3 is not close to the noble metal 2, and therefore, when analyzed by X-ray diffraction before and after the high temperature endurance treatment, from the noble metal and cerium oxide. The presence of the compound cannot be confirmed. In this supported state, the cerium oxide 3 is not supported in the vicinity of the noble metal 2, so the cerium oxide 3 occludes oxygen when an excess gas of oxygen flows in, but an excess of the cerium oxide 3 around the noble metal 2. It is thought that oxygen will remain. On the other hand, under a reducing atmosphere in which oxygen is insufficient, cerium oxide 3 releases the stored oxygen in an oxygen-excess atmosphere, but oxygen is not released around the noble metal 2, so oxygen is not effectively utilized by the noble metal 2. it is conceivable that.

  FIG. 3 is a schematic diagram showing a supported state of the noble metal 2 and the alumina 1 in a conventional exhaust gas purifying catalyst produced using an impregnation method. In such a supported state, as shown in FIG. 3, most of the noble metal particles are supported on the surface of the carrier. In the supported state as shown in FIG. 3, the peak attributed to cerium oxide 3 is more clearly detected even if analysis is performed by X-ray diffraction before and after the high temperature durability treatment, and thus consists of noble metal and cerium oxide. Compound peaks are difficult to detect. In addition, since cerium oxide 3 is supported relatively close to noble metal 2, cerium oxide 3 absorbs oxygen around noble metal 2 in an oxygen-excess atmosphere, and releases from cerium oxide 3 in a reducing atmosphere. It is possible for the noble metal 2 to use the generated oxygen. However, since the noble metal 2 is not coated as shown in FIGS. 1A and 2, sintering is likely to occur.

The “lanthanum-containing alumina” is not particularly limited as long as lanthanum and alumina are mixed. Examples of the lanthanum-containing alumina include lanthanum oxide (La ₂ O ₃ ) and / or a mixture of lanthanum-alumina composite oxide (such as LaAlO ₃ ) and alumina. Lanthanum is preferably contained in the form of lanthanum oxide and / or lanthanum-alumina composite oxide (such as LaAlO ₃ ), and more preferably in the form of lanthanum oxide (La ₂ O ₃ ). The “lanthanum-containing alumina” more preferably includes both lanthanum oxide and a lanthanum-alumina composite oxide.

The content of lanthanum (in terms of La ₂ O ₃ ) in the “lanthanum-containing alumina” is preferably 0.5 to 30% by weight with respect to the total amount of lanthanum and alumina (in terms of Al ₂ O ₃ ). 2 to 20% by weight is more preferable. When the content of lanthanum is 0.5% by weight or more, it is preferable because the noble metal hardly dissolves in the alumina after the high temperature durability treatment. In addition, since the content of lanthanum is 30% by weight or less, the proportion of lanthanum having a small surface area as compared with alumina does not become too large, so it is difficult to reduce the dispersibility of other catalysts and / or promoter components. Therefore, it is preferable.

  The noble metal used in the exhaust gas purifying catalyst according to the present invention is not particularly limited. For example, gold, silver, platinum, ruthenium, rhodium, palladium, osmium, iridium, or a combination of two or more of these. Etc. Especially, since the three-way catalyst performance is high, the precious metal is more preferably rhodium, platinum, or palladium, and rhodium is more preferable because the purification rate of nitrogen oxides is high.

The content of cerium oxide (CeO ₂ equivalent) in the exhaust gas purifying catalyst according to the present invention is based on the total amount of noble metal (metal equivalent), cerium oxide, lanthanum (lanthanum equivalent), and alumina (Al ₂ O ₃ equivalent). It is preferably 1 to 30% by weight, and more preferably 5 to 20% by weight. It is preferable that the content of cerium oxide is 1% by weight or more because air-fuel ratio fluctuations are preferably mitigated around the noble metal that is a catalytically active component. Moreover, since it is easy to contact exhaust gas with rhodium because it is 30 weight% or less, it is preferable.

In the exhaust gas cleaning catalyst according to the present invention, the content of the cerium oxide is preferably larger than the content of the compound composed of the noble metal and cerium oxide in the composite composed of the noble metal and cerium oxide. . Here, the magnitude of the “content” can be determined by comparing peak intensities by X-ray diffraction analysis. Therefore, in the exhaust gas cleaning catalyst according to the present invention, at the diffraction angles indicating the maximum intensities of CeO ₂ and CeRh (CeO ₂ is 28.5547 ° ± 0.017 °, CeRh is 32.765 ° ± 0.017 °). The intensity of the peak is determined, and the intensity of the peak at 28.5547 ° ± 0.017 ° attributed to CeO ₂ is preferably stronger than the intensity of the peak at 32.7651 ° ± 0.017 ° attributed to CeRh. Further, when containing a substance having a diffraction angle at a position close to the diffraction angle belonging to the CeO ₂ and CeRh are the peak intensities after separation of the peak attributable to the peak and CeO ₂ and CeRh attributable to the substance We will ask for it.

  The ratio of the content of cerium oxide to the content of the compound composed of noble metal and cerium oxide varies depending on the content of cerium oxide in the exhaust gas purification catalyst, etc., but the content of cerium oxide is 1 to 30 as described above. In the range of% by weight, the cerium oxide content is larger than the content of the compound in the composite, so that the function of cerium oxide as an oxygen storage / release material is exhibited, and the air-fuel ratio fluctuation around the noble metal Is preferable because the relaxation of this occurs suitably.

In one embodiment of the invention, when the noble metal is rhodium, the compound is CeRh. In the X-ray diffraction analysis, the intensity value of the peak attributed to cerium oxide relative to the intensity value of the peak attributed to CeRh (hereinafter referred to as “CeO ₂ / CeRh”) is greater than 1.0 and less than 3.3. More preferably, it is greater than 1.0 and less than 2.8. If “CeO ₂ / CeRh” is within such a range, cerium oxide having an oxygen storage / release capability exists, and CeRh also exists, so that cerium oxide is supported in the vicinity of rhodium, and oxygen The occlusion / release ability can be exhibited effectively.

  The exhaust gas purifying catalyst according to the present invention preferably contains a composite coated with lanthanum-containing alumina even after being exposed to the exhaust gas of an internal combustion engine having a temperature of 950 ° C to 1000 ° C.

  In the example of the present invention, 950 ° C. to 1000 ° C. was set as the threshold value for the high temperature durability treatment. The threshold value of 1000 ° C. is the maximum temperature that the catalyst bed temperature reaches at about 1000 ° C. during high-speed operation or durability test. The catalyst bed temperature reaches above this temperature due to an engine control error or the like. This is because there is no need to assume during normal operation, such as when excessively supplied, or when soot or fuel adhering to the catalyst burns abnormally.

  In addition, in high temperature durability treatment at a temperature lower than 950 ° C., noble metal sintering is unlikely to occur, so in the present invention it is not used as a high temperature durability treatment temperature for confirming sintering of noble metal.

  Although it does not specifically limit about high temperature durability processing time, It is preferable that it is 5 to 100 hours, and it is more preferable that it is 10 to 50 hours. Less than 5 hours is not preferable because the high-temperature durability treatment is not sufficient and the sintering resistance of the noble metal may not be confirmed. Further, even if the high temperature durability treatment is performed for more than 100 hours, the cost for the high temperature durability treatment increases despite the small change in the state, which is not preferable.

  In one embodiment, the exhaust gas-purifying catalyst according to the present invention contains a composite coated with lanthanum-containing alumina even after being exposed to an oxygen-excess gas having a temperature of 950 ° C to 1000 ° C. That is, even if the catalyst bed temperature is exposed to the highest temperature that can be reached, sintering of the noble metal is suppressed, and oxygen storage and release by cerium oxide is performed in the vicinity of the noble metal particles, so that fluctuation of the air-fuel ratio is also reduced around the noble metal. Happens. Therefore, even after the high temperature durability treatment, it is possible to efficiently function the noble metal as an exhaust gas purifying catalyst.

  The method for exposing the exhaust gas purifying catalyst according to the present invention to the gas is not particularly limited. For example, the exhaust gas purification catalyst can be exposed to the gas by installing the exhaust gas purification catalyst in the middle of an exhaust pipe of an internal combustion engine.

The gas is not particularly limited, but is preferably exhaust gas from an internal combustion engine. Examples of exhaust gas components of internal combustion engines include nitrogen oxides (eg, NO, NO ₂ , N ₂ O), carbon monoxide, carbon dioxide, oxygen, hydrogen, ammonia, water, sulfur dioxide, and hydrocarbons in general. be able to.

  The internal combustion engine is not particularly limited. For example, a gasoline engine, a hybrid engine, an engine using natural gas, ethanol, dimethyl ether or the like as a fuel can be used. Of these, a gasoline engine is preferred.

  Here, the term “exposure to gas” in this specification means that the exhaust gas-purifying catalyst comes into contact with the gas, and not only when the entire surface of the catalyst is in contact with the gas, but also with a portion of the catalyst surface as the gas. It also includes the case of contact.

  The “sintering” means that particles are bonded and coarsened by being exposed to a high temperature. When the noble metal, which is a catalyst component, causes sintering, the surface area of the noble metal particles is reduced, so that the catalytic activity is lowered, which is not preferable.

  From the viewpoint of suppressing sintering of noble metal particles, the exposed noble metal surface area after being exposed to exhaust gas from a gasoline engine vehicle having a temperature of 950 ° C. to 1000 ° C. is the exposed noble metal surface area before being exposed to the exhaust gas. On the other hand, it is preferably 0% to 87%, more preferably 0% to 40%.

The “exposed noble metal surface area” is a value represented by the number of CO molecules adsorbed on 1 g of the exhaust gas purifying catalyst × (the lattice constant of the noble metal) ² , and the “number of CO molecules” is the CO pulse adsorption method ( Catalyst, 1986, vol. 28, No. 1). For example, when the noble metal is rhodium, the lattice constant is 3.8030.

  The “exposed noble metal surface area” can be measured according to the CO pulse adsorption method (Catalyst, 1986, vol. 28, No. 1) proposed by the Catalytic Society Reference Catalyst Committee. In general, since noble metal supported on a carrier by high temperature durability treatment undergoes sintering, it is considered that the amount of CO adsorption after the durability treatment is smaller than that before the durability treatment in the CO pulse adsorption method. The reduction rate of the exposed noble metal surface area can be defined by the following formula (1).

  If the reduction rate of the surface area of the exposed noble metal is smaller than 0%, it means that the exposed noble metal surface area is increased after the high temperature durability treatment. At this time, formation of a solid solution noble metal or alumina coated with the noble metal may not hold the coating structure due to heat shrinkage due to a high temperature durability treatment, which is not preferable. On the other hand, if the reduction ratio of the exposed noble metal surface area is larger than 87%, it is not preferable because the noble metal particles are significantly sintered by the high temperature durability treatment and the number of noble metal atoms contributing to the catalytic reaction is reduced. .

  In addition, the exhaust gas purifying catalyst according to the present invention may contain only a complex of noble metal and cerium oxide coated with lanthanum-containing alumina, or may contain other components. For example, a refractory inorganic oxide, a ceria-zirconia composite oxide, or the like may be included as appropriate. These components may or may not be coated with lanthanum-containing alumina. Further, it may contain cerium oxide not covered with lanthanum-containing alumina; precious metals such as platinum, palladium, and rhodium not covered with lanthanum-containing alumina. Furthermore, the exhaust gas purifying catalyst according to the present invention may or may not be supported on the three-dimensional structure.

  In the exhaust gas purifying catalyst according to the present invention, the oxygen absorption rate is preferably 30 to 100%, more preferably 40 to 100%, further preferably 50 to 100%, and 80 to 100%. Most preferably. Here, the “oxygen absorption rate” refers to a value determined by the method described in Examples described later, and is closely related to the purification performance of the exhaust gas purification catalyst. In general, the higher the oxygen absorption rate, the higher the so-called oxygen storage / release capability, so the higher the exhaust gas purification performance, while the lower the oxygen absorption rate, the lower the oxygen storage / release capability, the exhaust gas purification performance. It is considered low.

  If the oxygen absorption rate is 100%, it means that all the oxygen introduced into the exhaust gas purification catalyst has been used, and the exhaust gas purification performance of the exhaust gas purification catalyst is very high. On the other hand, when the oxygen absorption rate falls below 30%, oxygen that could not be absorbed is present around the noble metal. Therefore, in particular, when the concentration of oxygen present around the noble metal is equal to or higher than the stoichiometric air-fuel ratio, the exhaust gas catalytic reaction on rhodium may be difficult to proceed.

[2. (Manufacture of exhaust gas purification catalyst)
The exhaust gas purifying catalyst according to the present invention is not particularly limited as a method for producing an exhaust gas purifying catalyst, as long as the composite of noble metal and cerium oxide is covered with lanthanum-containing alumina. A well-known manufacturing method can be used. For example, a sol-gel method, an alkoxide method, a reverse micelle method, a hydrothermal synthesis method, or the like can be used.

  As the noble metal, for example, gold, silver, platinum, ruthenium, rhodium, palladium, osmium, iridium, a combination of two or more of these can be used, and the noble metal is particularly limited as long as it is a noble metal having catalytic activity. It is not a thing. Among these, it is preferable to use rhodium as a noble metal.

  Nitrate, chloride, acetate, organic salt and the like can be used as the noble metal raw material, and are not particularly limited.

  For example, rhodium nitrate, rhodium chloride, rhodium acetate, hexaammine rhodium or the like can be used as the raw material of the rhodium, and is not particularly limited.

  As a raw material of the cerium oxide, in addition to cerium oxide, cerium oxide may be obtained by drying and baking. Examples thereof include cerium nitrate (III) hexahydrate, cerium acetate (III) monohydrate, ceria sol and the like, and are not particularly limited.

  As the lanthanum raw material, lanthanum acetate (III) n hydrate, lanthanum nitrate hexahydrate, lanthanum chloride heptahydrate, lanthanum sulfate (III) n hydrate, lanthanum oxide, etc. can be used. It is not particularly limited.

  As the raw material of the alumina, aluminum isopropoxide, aluminum ethoxide, aluminum n-butoxide, aluminum s-butoxide, aluminum nitrate, basic aluminum nitrate, aluminum hydroxide, alumina sol and the like can be used, and are particularly limited. It is not a thing.

  In the present invention, the method of coating the composite of noble metal and cerium oxide with lanthanum-containing alumina is not particularly limited, and a coating method suitable for each alumina raw material is preferably used. For example, it is preferably manufactured by a method in which an aqueous solution containing a noble metal and an aqueous solution of cerium oxide is added to an alcohol solution of an aluminum alkoxide, and then water containing lanthanum is added, dried, and fired as necessary. can do.

  An example of the case where rhodium and cerium oxide are coated with aluminum isopropoxide containing lanthanum is described below.

  First, aluminum isopropoxide is added to an isopropanol solution having a mass similar to that of aluminum isopropoxide and stirred for about 10 minutes. Next, rhodium nitrate and cerium acetate are weighed so as to have a predetermined loading amount to prepare a rhodium nitrate-cerium acetate mixed aqueous solution, and the above predetermined mixed aluminum isopropoxide-isopropanol mixed solution is stirred for 10 minutes. A mixed aqueous solution of rhodium nitrate and cerium acetate is added so as to obtain a supported amount. Aluminum isopropoxide is weakly acidic to acidic and undergoes a hydrolysis reaction to become aluminum hydroxide. In this case, it is preferable that the hydrolysis reaction proceeds with water contained in the added rhodium nitrate-cerium acetate mixed aqueous solution. Since the hydrolysis reaction proceeds with the water contained in the aqueous rhodium nitrate solution, the composite composed of rhodium and cerium oxide is supported in a state of being covered with alumina.

  It is preferable to add lanthanum acetate after adding a rhodium nitrate-cerium acetate mixed aqueous solution and then causing a portion of aluminum isopropoxide to undergo a hydrolysis reaction. At this time, water necessary for hydrolyzing all of the aluminum isopropoxide is added at the same time, and stirring is continued until hydrolysis of the aluminum isopropoxide is completed. Since the hydrolysis reaction is an exothermic reaction, the time when the exotherm disappears is set as the time when the hydrolysis reaction is completed. When the hydrolysis reaction is completed, the sample is in the form of a gel, and the gel is preferably dried at 50 ° C to 200 ° C, more preferably 70 ° C to 150 ° C, and then preferably 300 ° C to 950 ° C, More preferably, firing is performed at 400 ° C. to 600 ° C. in an oxygen-excess atmosphere. Through these steps, an exhaust gas purifying catalyst containing a noble metal-cerium oxide composite coated with lanthanum-containing alumina can be obtained.

  Here, the content of noble metal (in noble metal conversion) in the exhaust gas cleaning catalyst is preferably 0.2 to 20% by weight, and 0.5 to 5% by weight, per lanthanum-containing alumina powder. Is more preferable. Because the precious metal content is 0.2% by weight or more, the coverage of the precious metal with the lanthanum-containing alumina does not become too high, and as a result, the precious metal particles exposed to the gas phase do not become too small. It is possible to prevent a decrease in the catalyst performance of the catalyst. On the other hand, the content of 20% by weight or less is preferable because the ratio of the noble metal particles that could not be covered with the lanthanum-containing alumina does not become too large.

  About content of the cerium oxide in the said exhaust gas purification catalyst, it is said [1. Exhaust gas purification catalyst].

  The exhaust gas purifying catalyst according to the present invention is not particularly limited, but is preferably supported on a three-dimensional structure. The three-dimensional structure is not particularly limited. For example, a heat-resistant carrier such as a honeycomb carrier can be used. Further, as the three-dimensional structure, an integrally molded type (integrated structure) is preferable. For example, a monolith carrier, a metal honeycomb carrier, a plugged honeycomb carrier such as a diesel particulate filter, a punching metal, or the like is preferably used. Note that the three-dimensional integrated structure is not necessarily required, and for example, a pellet carrier can be used.

  As the monolithic carrier, what is usually referred to as a ceramic honeycomb carrier may be used, and cordierite, mullite, α-alumina, silicon carbide, silicon nitride and the like are particularly preferable. Those are particularly preferred. In addition, an integrated structure using an oxidation-resistant heat-resistant metal including stainless steel, Fe—Cr—Al alloy, or the like is used.

  These monolithic carriers are manufactured by an extrusion molding method or a method of winding and solidifying a sheet-like element. The shape of the gas vent (cell shape) may be any of a hexagon, a square, a triangle, and a corrugation. A cell density (number of cells / unit cross-sectional area) of 100 to 1200 cells / in 2 is sufficient, preferably 200 to 900 cells / in 2.

  When the exhaust gas purifying catalyst according to the present invention is supported on a three-dimensional structure, the amount of the noble metal is preferably 0.01 to 10 g per liter of the three-dimensional structure, and 0.01 to 3 g. Is more preferable. The method for supporting the three-dimensional structure is not particularly limited. For example, a method such as a wash coat can be used.

  When the amount of the noble metal is 0.01 g or more per liter of the three-dimensional structure, it is preferable because the catalyst performance is excellent, and when it is 3 g or less, the contribution to the catalyst performance with respect to the amount used is large and the cost performance is good. .

  Further, the powder obtained by coating a composite of noble metal and cerium oxide with lanthanum-containing alumina is preferably 30 to 300 g, more preferably 70 to 150 g, per liter of the three-dimensional structure.

  The exhaust gas purifying catalyst according to the present invention may further contain platinum and / or palladium which is not coated with lanthanum-containing alumina. As a raw material of platinum, for example, platinum nitrate can be used. The amount of platinum is preferably 0.1 to 5 g per liter of the three-dimensional structure, and more preferably 0.5 to 1 g. When the amount of platinum is 0.1 g or more per liter of the three-dimensional structure, it is preferable because the catalyst performance is excellent, and when it is 5 g or less, the contribution to the catalyst performance with respect to the amount used is large, and the cost performance is good. Therefore, it is preferable.

  As a raw material of palladium, palladium nitrate, palladium chloride, palladium acetate, tetraammine palladium and the like can be used. The amount of palladium is preferably 0.5 to 10 g and more preferably 1 to 8 g per liter of the three-dimensional structure. When the amount of palladium is 0.5 g or more per liter of the three-dimensional structure, it is preferable because the catalyst performance is excellent, and when it is 10 g or less, the contribution to the catalyst performance with respect to the amount used is large, and the cost performance is good. Therefore, it is preferable.

The exhaust gas-purifying catalyst according to the present invention preferably further comprises a refractory inorganic oxide. The refractory inorganic oxide is not particularly limited as long as it is usually used as a catalyst carrier for exhaust gas. For example, γ-alumina (Al ₂ O ₃ ), silica (SiO ₂ ), silica-alumina (SiO ₂ -Al ₂ O ₃ ), titania (TiO ₂ ), magnesia (MgO), zeolite, and the like can be used. To the refractory inorganic oxide, a transition metal such as iron, nickel, cobalt, or manganese, an oxide of a rare earth element such as an alkali metal, an alkaline earth metal, or lanthanum can also be added. The amount of the refractory inorganic oxide used is preferably 30 to 300 g, more preferably 70 to 150 g, per liter of the three-dimensional structure.

  The exhaust gas purifying catalyst according to the present invention preferably further comprises cerium oxide and / or ceria-zirconia composite oxide not covered with lanthanum-containing alumina. Here, the ceria-zirconia composite oxide may or may not be coated with lanthanum-containing alumina, but is more preferably not coated with lanthanum-containing alumina. Thereby, exhaust gas can be purified more efficiently. The amount of cerium oxide and / or ceria-zirconia composite oxide not coated with lanthanum-containing alumina is preferably 5 to 100 g, more preferably 20 to 80 g, per liter of the three-dimensional structure. Note that “not covered with lanthanum-containing alumina” means that the cerium oxide and / or ceria-zirconia composite oxide does not exist between the rhodium particles and the alumina particles at the contact portion between the rhodium particles and the alumina particles. State.

  The “ceria-zirconia composite oxide” is a composite oxide in which zirconia is solid-dissolved in ceria, and can contain lanthanum, yttrium, praseodymium and the like at the same time. The ratio of ceria to zirconia is preferably 90:10 to 10:90 by weight, and the content of other components is preferably 20% by weight or less per ceria-zirconia composite oxide. The ceria-zirconia composite oxide can be prepared, for example, by a coprecipitation method (encyclopedia of catalysts, page 194, Asakura Shoten).

  Hereinafter, a method for preparing an exhaust gas purification catalyst will be described. The method for preparing the exhaust gas purification catalyst is not particularly limited. However, when the catalyst composition itself is used as the exhaust gas purification catalyst, the catalyst composition is sufficiently mixed and then formed into a cylinder, a spherical shape, or the like. Thus, a method for producing an exhaust gas purifying catalyst can be used. The catalyst composition is an aggregate of components that can constitute an exhaust gas purification catalyst, as well as the above-mentioned refractory inorganic oxide, ceria-zirconia composite oxide, cerium oxide not covered with lanthanum-containing alumina, lanthanum-containing alumina. It also refers to those containing precious metals that are not covered by.

  When using the three-dimensional structure, the catalyst composition is charged into a ball mill or the like in a lump, wet pulverized into an aqueous slurry, the three-dimensional structure is immersed in the aqueous slurry, dried, fired, etc. Can be mentioned.

[3. (Exhaust gas purification method)
The exhaust gas purification method according to the present invention includes a step of exposing the exhaust gas purification catalyst according to the present invention to exhaust gas of an internal combustion engine.

  The method for exposing the exhaust gas-purifying catalyst to the exhaust gas is not particularly limited. For example, the exhaust gas purifying catalyst can be installed in the exhaust passage of the exhaust port of the internal combustion engine, and the exhaust gas can be caused to flow into the exhaust passage. The time for which the exhaust gas purifying catalyst is exposed to the exhaust gas is not particularly limited as long as at least a part of the exhaust gas purifying catalyst can be in contact with the exhaust gas. The temperature of the exhaust gas is not particularly limited, but is preferably 0 ° C. to 750 ° C., that is, the temperature of the exhaust gas during normal operation.

  Moreover, in one embodiment, the exhaust gas purification method according to the present invention has a temperature before the exhaust gas purification catalyst according to the present invention is exposed to exhaust gas of an internal combustion engine (temperature of exhaust gas; 0 ° C. to 750 ° C.). An air fuel ratio of 950 ° C. to 1000 ° C. may be preferably exposed to the exhaust gas of 14.1 to 15.1.

  In this case, it is sufficient that at least a part of the exhaust gas purifying catalyst can be in contact with the exhaust gas having a temperature of 0 ° C. to 750 ° C. Therefore, the exhaust gas purifying catalyst has a temperature of 0 ° C. to 750 ° C. The time for exposure to the exhaust gas at 0 ° C. is not particularly limited. Also, the time for which the exhaust gas purifying catalyst is exposed to an oxygen-excess exhaust gas having a temperature of 950 ° C. to 1000 ° C. is not particularly limited. It may not be exposed at all, for example, it may be exposed for 5 to 100 hours which is a preferable time for the high temperature durability treatment time.

  As described above, the exhaust gas purifying catalyst according to the present invention has both heat resistance and oxygen storage / release capability. Therefore, even during the purification of exhaust gas during normal operation, even when exhaust gas having a temperature significantly higher than that generated during normal operation flows into the exhaust passage, such as exhaust gas having a temperature of 950 ° C. to 1000 ° C., Purification of exhaust gas during normal operation can be continued.

  The “0 ° C. to 750 ° C.” is preferably the exhaust gas temperature at the catalyst inlet, and “950 ° C. to 1000 ° C.” is preferably the exhaust gas temperature at the catalyst bed. The “catalyst inlet portion” refers to a central portion with respect to the exhaust pipe vertical direction from the catalyst end surface on the exhaust gas inflow side where the exhaust gas purification catalyst is installed to the 20 cm internal combustion engine side. The “catalyst bed” is the center of the distance from the exhaust gas inflow side end surface to the exhaust gas outflow side end surface and the center of the exhaust pipe in the vertical direction (if the exhaust pipe is not circular, the exhaust pipe vertical The center of gravity of the cross section in the direction).

  The above-mentioned “during normal operation” refers to a state excluding a state in which an automobile, a two-wheeled vehicle or the like equipped with an internal combustion engine is driven at a remarkably high speed and low speed. For example, this corresponds to the operation in the LA-4 mode.

  The air-fuel ratio of the exhaust gas (exhaust gas temperature: 0 ° C. to 750 ° C.) of the internal combustion engine is preferably 14.1 to 15.1.

  It should be noted that the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims, and obtained by appropriately combining technical means disclosed in different embodiments. Embodiments are also included in the technical scope of the present invention.

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

  The performance of the exhaust gas cleaning catalyst according to the present invention was evaluated by measuring the temperature when the NOx purification rate reached 50% (hereinafter also referred to as T50) and the oxygen absorption rate. Below, the measuring method of T50 and oxygen absorption rate is demonstrated.

<Measurement method of T50>
T50 refers to the temperature at which the NOx purification rate reaches 50%. As a measuring method of T50, an exhaust gas purifying catalyst is installed 30 cm downstream from the exhaust hole of a 3.0 L, 6 cylinder MPI engine, and the MPI engine has an A / F of 14 at a heating rate of 50 ° C./min. The temperature at the catalyst inlet was raised from 200 ° C. to 500 ° C. while varying the frequency from 0.1 to 15.1 at a frequency of 1.0 Hz. The CO, THC (total hydrocarbon, methane conversion), and NOx purification rates at this time were determined, and the temperature at which the NOx purification rate reached 50% was defined as T50. The “catalyst inlet portion” refers to a central portion from the catalyst end surface on the exhaust gas inflow side where the exhaust gas purifying catalyst is installed to the 20 cm internal combustion engine side and with respect to the exhaust pipe vertical direction. The “temperature at the catalyst inlet” refers to the temperature at which the position of the catalyst inlet is measured using a thermocouple. It can be said that the lower the value of T50 is, the higher the catalyst performance is.

<Measurement method of oxygen absorption rate>
The oxygen absorption rate is a value obtained by subtracting the ratio of the total length of the oxygen curve at the inlet portion of the exhaust gas purifying catalyst from the total length of the oxygen curve at the outlet portion of the exhaust gas purifying catalyst from 1, and multiplying by 100. 2).

  The method for obtaining the “oxygen curve at the outlet of the exhaust gas purifying catalyst” and the “oxygen curve at the inlet of the exhaust gas purifying catalyst” will be described in detail below. After the above T50 measurement, the temperature at the catalyst inlet is set to 400 ° C., and the oxygen at the catalyst inlet when the MPI engine is varied from 14.1 to 15.1 at a frequency of 0.5 Hz. The concentration and the oxygen concentration at the catalyst outlet were measured every 0.1 seconds. Subsequently, the temperature at the catalyst inlet is set to 400 ° C., and the oxygen concentration at the catalyst inlet and the catalyst outlet when the MPI engine is varied from 13.6 to 15.6 at a frequency of 0.5 Hz. The oxygen concentration of was measured every 0.1 seconds. Similarly, the temperature of the catalyst inlet is set to 500 ° C., and the catalyst inlet when the MPI engine is varied from 14.1 to 15.1 or 13.6 to 15.6 at a frequency of 0.5 Hz. The oxygen concentration at the catalyst and the oxygen concentration at the catalyst outlet were measured every 0.1 second. The oxygen concentration at the catalyst inlet under each of the conditions obtained above was curved by the least square method. Similarly, the oxygen concentration at the catalyst outlet was also curved by the least square method. The obtained curve is hereinafter referred to as an oxygen curve. For each oxygen curve, the length from 20 seconds to 180 seconds after the start of measurement was determined.

Specifically, assuming that the oxygen concentration at the catalyst inlet 20 seconds after the start of measurement is O ₂₀ and the oxygen concentration at the catalyst inlet 20 seconds after the start of measurement is O _20.1 , the measurement starts 20 seconds after the start of measurement. the length L ₂₀ of the oxygen curve between until after the measurement start 20.1 seconds, the Pythagorean theorem can be expressed by equation (3). The length of the oxygen curve L ₂₀ is a positive value.

Similarly, it is possible to determine the total length of oxygen curve from L ₂₀ obtains the length of the oxygen curve per 0.1 second by summing the L _179.9, from up to 180 seconds after starting measurement after 20 seconds . That is, the total length of the oxygen curve from 20 seconds after the start of measurement at the catalyst inlet portion to 180 seconds later can be expressed by equation (4).

  Using the same method, the total length of the oxygen curve from 20 seconds to 180 seconds after the start of measurement at the catalyst outlet is obtained, and the oxygen absorption rate can be obtained using the above equation (2).

  180 seconds after the start of the measurement of the oxygen curve representing the change in the oxygen concentration at the outlet of the catalyst with respect to the total length between 20 seconds and 180 seconds after the start of the measurement of the oxygen curve representing the change in the oxygen concentration at the catalyst inlet. The total length of the exhaust gas becomes shorter as the catalyst for exhaust gas purification having a higher oxygen absorption rate. As a result, the oxygen absorption rate value obtained is high.

[1. (Manufacture of exhaust gas purification catalyst)
[Example 1]
Aluminum isopropoxide was used as the alumina raw material, rhodium nitrate aqueous solution was used as the rhodium raw material, lanthanum acetate was used as the lanthanum raw material, and cerium (III) acetate was used as the cerium oxide raw material.

  Each raw material was weighed so that the weight ratio of alumina: rhodium: lanthanum oxide: cerium oxide was 82.5: 4.5: 3: 10. The weighed aluminum isopropoxide and an equal amount of ethanol were stirred for 10 minutes, and then a mixed solution of cerium acetate and an aqueous rhodium nitrate solution was added to the aluminum isopropoxide / ethanol solution.

  Next, after lanthanum acetate is dispersed in an amount of water required for the hydrolysis reaction of aluminum isopropoxide, the dispersion is added to an aluminum isopropoxide / ethanol / rhodium nitrate / cerium acetate solution and stirred for 2 hours. did. The obtained gel was dried at 120 ° C. for 8 hours, and then the obtained dried product was calcined at 500 ° C. for 1 hour in an air atmosphere, so that the composite composed of cerium oxide and rhodium was converted to lanthanum oxide-containing alumina. A coated powder A was obtained.

[Example 2]
From the cerium oxide and rhodium in the same manner as in Example 1 except that each raw material was weighed so that the weight ratio of alumina: rhodium: lanthanum oxide: cerium oxide was 87.5: 4.5: 3: 5. A powder B in which the composite was coated with lanthanum oxide-containing alumina was obtained.

[Comparative Example 1]
Aluminum isopropoxide was used as the alumina raw material, rhodium nitrate aqueous solution was used as the rhodium raw material, and lanthanum acetate was used as the lanthanum raw material.

  Each raw material was weighed so that the weight ratio of alumina: rhodium: lanthanum oxide was 92.5: 4.5: 3. The weighed aluminum isopropoxide and an equal amount of ethanol were stirred for 10 minutes, and then an aqueous rhodium nitrate solution was added to the aluminum isopropoxide / ethanol solution.

  Next, lanthanum acetate was dispersed in an amount of water required for the hydrolysis reaction of aluminum isopropoxide, and then added to the aluminum isopropoxide / ethanol / rhodium nitrate solution and stirred for 2 hours. After the obtained gel was dried at 120 ° C. for 8 hours, the obtained dried product was calcined at 500 ° C. for 1 hour in an air atmosphere, whereby rhodium was coated with alumina containing lanthanum oxide. Got.

[Comparative Example 2]
Lanthanum oxide-containing alumina was used as the alumina and lanthanum raw material, and rhodium nitrate was used as the rhodium raw material.

  Each raw material was weighed so that the weight ratio of alumina: rhodium: lanthanum oxide was 92.5: 4.5: 3. Subsequently, rhodium nitrate was supported on the above-mentioned weighed lanthanum oxide-containing alumina by a pore filling method (see Catalyst Encyclopedia, page 174, Asakura Shoten). After drying at 120 ° C. for 8 hours, firing was performed at 500 ° C. for 1 hour in an air atmosphere to obtain a powder D in which lanthanum oxide-containing alumina was impregnated and supported.

[Comparative Example 3]
Aluminum isopropoxide was used as the alumina raw material, rhodium nitrate aqueous solution was used as the rhodium raw material, lanthanum acetate was used as the lanthanum raw material, and cerium oxide was used as the cerium oxide raw material.

  Each raw material was weighed so that the weight ratio of alumina: rhodium: lanthanum oxide: cerium oxide was 82.5: 4.5: 3: 10. The weighed aluminum isopropoxide and an equal amount of ethanol were stirred for 10 minutes, and then an aqueous rhodium nitrate solution was added to the aluminum isopropoxide / ethanol solution.

  Next, after lanthanum acetate was dispersed in an amount of water required for the hydrolysis reaction of aluminum isopropoxide, the dispersion was added to an aluminum isopropoxide / ethanol / rhodium nitrate solution and stirred for 2 hours. After the reaction product was dried at 120 ° C. for 8 hours, the obtained dried product was calcined at 500 ° C. for 1 hour in an air atmosphere, and then weighed cerium oxide was mixed and pulverized in a mortar. Powder E containing cerium oxide and rhodium coated with lanthanum oxide-containing alumina was obtained. The powder E contains cerium oxide, but the cerium oxide is not covered with lanthanum oxide-containing alumina.

[Comparative Example 4]
Except that each raw material was weighed so that the weight ratio of alumina: rhodium: lanthanum oxide: cerium oxide was 87.5: 4.5: 3: 5, cerium oxide was contained by the same method as in Comparative Example 3. Powder F in which rhodium was coated with alumina containing lanthanum oxide was obtained. The powder F contains cerium oxide as in the powder of Comparative Example 3, but the cerium oxide is not covered with lanthanum oxide-containing alumina.

[2. (Confirmation of powder component)
<X-ray diffraction analysis>
The components of the powders obtained in the examples and comparative examples were analyzed by X-ray diffraction to confirm the presence of CeO2 and CeRh. As the X-ray diffractometer, X'Pert PRO manufactured by Spectris Co., Ltd. was used. Using CuKα as the X-ray source, the X-ray output was measured at 40 mA, 45 kV, step angle, 0.017 °.

In the obtained X-ray diffraction pattern, CeO ₂ and CeRh have peaks at angles shown in JCPDS files 00-034-0394 and 00-019-0296, respectively. If CeO ₂ and CeRh are included in the measurement sample, a peak is detected at the angle shown in the JCPDS file. In the present invention, in addition to CeO ₂ and Rh, Al ₂ O ₃ and La ₂ O ₃ are detected. Therefore, the peaks may not be detected as clear peaks at diffraction angles with a low intensity ratio. Therefore, in the examples of the present specification, the peak at the diffraction angle indicating the maximum intensity of CeO ₂ and CeRh (CeO ₂ is 28.5547 ° ± 0.017 °, CeRh is 32.752 ° ± 0.017 °). The strength was determined.

  As shown in the schematic diagram of FIG. 1 (a), powders A and B in which the composite of cerium oxide and rhodium prepared in Example 1 or Example 2 was coated with lanthanum-containing alumina were combined with noble metal 2 and Cerium oxide 3 is supported in close proximity to form a composite. As a result of qualitative analysis by X-ray diffraction analysis after high-temperature durability treatment of these powders, a peak attributed to CeRh was detected. Since the CeRh compound was formed on the contact surface between the cerium oxide particles and the rhodium particles, it was confirmed that the composite composed of cerium oxide and rhodium was coated with lanthanum-containing alumina.

On the other hand, the powder E or F prepared in Comparative Example 3 or Comparative Example 4 and coated with rhodium with lanthanum-containing alumina and then physically mixed with cerium oxide is cerium oxide as shown in the schematic diagram of FIG. 3 is not carried close to the noble metal 2. As a result of qualitative analysis by X-ray diffraction analysis after high-temperature durability treatment of these powders, no peak attributed to CeRh was detected, and the CeO ₂ peak intensity was detected relatively strong. This was thought to be because rhodium and cerium oxide were not carried in close proximity, and therefore most of them existed as CeO ₂ , not as CeRh compounds.

  In addition, as shown in the schematic diagram of FIG. 3, most of the noble metals are prepared as shown in the schematic diagram of FIG. The particles are supported on the carrier surface. As a result of qualitative analysis by X-ray diffraction analysis after high-temperature endurance treatment of these powders, it was difficult to detect the peak attributed to CeRh because the peak attributed to cerium oxide was more clearly detected.

Further, the ratio of the CeO ₂ peak intensity to the CeRh peak intensity was determined. The results are shown in Table 1.

As shown in Table 1, powders A and B in which a composite composed of cerium oxide and rhodium was coated with lanthanum-containing alumina were compared with powders E and F in which cerium oxide was not coated with lanthanum-containing alumina. Thus, it can be seen that the ratio of CeRh to CeO ₂ is large.

[3. (Measurement of exposed noble metal surface area)
<Measurement method of exposed noble metal surface area>
The exposed noble metal surface area was basically measured according to the CO pulse method (Catalyst, 1986, vol. 28, No. 1) proposed by the Catalytic Society Reference Catalyst Committee. However, in this example, since the noble metal was coated with alumina, the reduction treatment temperature was set to 500 ° C. in order to sufficiently advance the pretreatment for the noble metal reduction treatment.

<Measurement of exposed noble metal surface area>
The results of measuring the exposed noble metal surface areas of the powders A, B, C, and D before and after the high temperature endurance treatment, and the reduction rate of the exposed noble metal surface area before and after the high temperature endurance treatment were measured. Best Mode for Carrying Out] of [1. Table 2 shows the results obtained in accordance with the above-described equation (1) in Exhaust gas purification catalyst].

  As shown in Table 2, powders A and B in which a composite composed of cerium oxide and rhodium was coated with lanthanum oxide-containing alumina and powder C in which rhodium was coated with lanthanum oxide-containing alumina were subjected to pore filling method. It was found that the exposed rhodium surface area was smaller than that of the powder D impregnated with and supported by rhodium.

  Here, the powders A, B and C have the same amount of rhodium supported as the powder D, and the powders impregnated and supported with rhodium in a state where the durability at a temperature at which the rhodium particles are coarsened by sintering is not performed. Since the exposed rhodium surface area is significantly smaller than that of the body D, it can be seen that in the powders A, B and C, the rhodium particles are coated with alumina.

  On the other hand, after the powders A, B, C, and D were subjected to a high temperature durability treatment at 950 ° C. for 50 hours in an air atmosphere, as shown in Table 2, the composite composed of cerium oxide and rhodium was lanthanum oxide-containing alumina. In comparison with the powders A and B coated with lanthanum oxide containing alumina and the powder C coated only with rhodium containing lanthanum oxide, the powder D impregnated and supported on the lanthanum oxide containing alumina by the pore filling method, The exposed rhodium surface area was small.

  Powders A and B in which a composite composed of cerium oxide and rhodium was coated with lanthanum oxide-containing alumina and powder C in which rhodium was coated with lanthanum oxide-containing alumina had a reduced surface area of exposed noble metal after high-temperature endurance treatment. However, the reduction rate was smaller than that of powder D in which rhodium was impregnated and supported on alumina containing lanthanum by the pore filling method, and it was found that sintering of rhodium particles was suppressed and the exposed noble metal surface area was maintained.

  On the other hand, in the powder D in which rhodium was impregnated and supported on lanthanum-containing alumina, the reduction rate of the exposed noble metal surface area before and after the high temperature durability treatment was larger than 87%, and it was considered that the rhodium particles were significantly sintered.

[4. (Manufacture of exhaust gas purification catalyst)
Exhaust gas purification catalysts containing further components were produced using the exhaust gas purification catalysts (powder A to F) produced in Examples 1-2 and Comparative Examples 1-4.

Example 3
Pt: Pd: La ₂ O using a dinitrodiammine platinum aqueous solution as a platinum raw material, palladium nitrate as a palladium raw material, alumina containing 3 wt% of lanthanum as an alumina raw material, CeO ₂ —ZrO ₂ composite oxide, and the above powder A Each raw material was adjusted so that the weight ratio of _3- Al ₂ O ₃ : CeO ₂ —ZrO ₂ : La ₂ O ₃ : powder A was 0.06: 0.2: 31.2: 30: 5.04: 4. Weighed. Next, after mixing the respective raw materials, the mixture was stirred for 2 hours, and then wet pulverized to obtain slurry A. The obtained slurry A was wash-coated on 0.92 L of cordierite so as to be 70.5 g per liter of cordierite after firing at 500 ° C. for 1 hour. Next, after drying at 150 ° C. for 15 minutes, air calcination is performed at 500 ° C. for 1 hour, and the exhaust gas-purifying catalyst A containing 0.06: 0.2: 0.24 (g) of Pt: Pd: Rh per liter is obtained. Obtained.

Example 4
Exhaust gas purifying catalyst B was produced by using powder B instead of powder A by the same raw material ratio and method as in Example 3.

[Comparative Example 5]
An exhaust gas-purifying catalyst C was produced using the powder C instead of the powder A by the same raw material ratio and method as in Example 3.

[Comparative Example 6]
An exhaust gas purifying catalyst D was produced by using the powder D instead of the powder A by the same raw material ratio and method as in Example 3.

[5. Performance evaluation of exhaust gas purification catalyst)
The performance of the exhaust gas purification catalyst was evaluated by measuring the temperature at which the purification rates of CO, THC and NO _x reach 50% (hereinafter also referred to as T50) and the oxygen absorption rate. Below, the measuring method of T50 and oxygen absorption rate is demonstrated.

<Durability treatment>
Each catalyst produced in Example 3 and Example 4 and Comparative Example 5 and Comparative Example 6 was installed 40 cm downstream from the exhaust port of an in-line 6-cylinder, 2.4 L engine, and the temperature of the catalyst bed was 950 ° C. did. Durability treatment was performed by operating for 50 hours at an A / F at the catalyst inlet of ± 4.0 with respect to 14.6.

<Measurement of 50% purification temperature (T50)>
The results of measuring the 50% purification temperature (T50) in the temperature rise evaluation are shown in FIG. As shown in FIG. 4, the exhaust gas-purifying catalyst containing powder A or B in which a composite composed of cerium oxide and rhodium is coated with lanthanum-containing alumina is powder C in which only rhodium is coated with lanthanum-containing alumina. It can also be said that the exhaust gas purification catalyst has a 50% lower purification temperature and higher catalyst performance than the exhaust gas purification catalyst containing rhodium-impregnated alumina powder D in which rhodium nitrate is supported on lanthanum oxide-containing alumina.

<Measurement of oxygen absorption rate>
The result of measuring the oxygen absorption rate of the exhaust gas purifying catalyst is shown in FIG. In FIG. 5, the horizontal axis represents the temperature at the catalyst inlet (400 ° C., 500 ° C.) and the air-fuel ratio fluctuation range (± 0.5: A / F = 14.1 to 15.1, ± 1.0: A / F). = 13.6 to 15.6), and the vertical axis represents the oxygen absorption rate (%).

  As shown in FIG. 5, the exhaust gas-purifying catalyst containing powder A or powder B in which a composite of cerium oxide and rhodium is coated with lanthanum-containing alumina is a powder in which only rhodium is coated with lanthanum-containing alumina. It can be seen that the oxygen absorption rate (%) is higher than that of the exhaust gas-purifying catalyst containing the body C. On the other hand, the exhaust gas-purifying catalyst containing rhodium-impregnated alumina powder D in which rhodium nitrate is supported on lanthanum oxide-containing alumina, even though it does not contain a complex composed of cerium oxide and rhodium, as in powder C. It can be said that it is a catalyst for exhaust gas purification that exhibits a high oxygen absorption rate (%) and a high oxygen absorption rate.

  Since the exhaust gas purifying catalyst according to the present invention can include at least a composite of noble metal and cerium oxide coated with lanthanum-containing alumina, the rhodium coating state is maintained even when exposed to high temperatures, and oxygen It is possible to efficiently express the occlusion / release ability, and the exhaust gas from the internal combustion engine can be efficiently purified even at high temperatures. Therefore, the present invention can be widely applied to all industries using an internal combustion engine, such as automobiles, railways, ships, aviation, and various industrial machines.

1 Lanthanum-containing alumina 2 Precious metal 3 Cerium oxide 4 Compound

Claims (15)

  An exhaust gas purifying catalyst characterized in that a composite of noble metal and cerium oxide is coated with lanthanum-containing alumina.   2. The exhaust gas purifying catalyst according to claim 1, wherein a compound composed of noble metal and cerium is formed on a contact surface between the noble metal and cerium oxide in the composite.   The exhaust gas-purifying catalyst according to claim 2, wherein a content of the cerium oxide in the composite is larger than a content of the compound.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the noble metal is rhodium.   The exhaust gas purifying catalyst according to any one of claims 1 to 4, wherein the compound is CeRh.   The content of the cerium oxide is 1% by weight to 30% by weight with respect to the total amount of noble metal, cerium oxide, lanthanum, and alumina, according to any one of claims 1 to 5. Exhaust gas purification catalyst.   7. The composite according to claim 1, wherein the composite coated with lanthanum-containing alumina is contained even after being exposed to an exhaust gas of an internal combustion engine having a temperature of 950 ° C. to 1000 ° C. 7. The catalyst for exhaust gas purification as described in 1.   The exposed noble metal surface area after being exposed to exhaust gas of an internal combustion engine having a temperature of 950 ° C. to 1000 ° C. is reduced by 0% to 87% with respect to the exposed noble metal surface area before being exposed to the exhaust gas. The exhaust gas-purifying catalyst according to claim 7.   The exhaust gas-purifying catalyst according to claim 8, wherein the oxygen absorption rate is 30 to 100%.   The exhaust gas-purifying catalyst according to any one of claims 1 to 9, wherein the exhaust gas-purifying catalyst is supported on a three-dimensional structure.   The exhaust gas-purifying catalyst according to any one of claims 1 to 10, further comprising platinum and / or palladium not covered with lanthanum-containing alumina.   The exhaust gas-purifying catalyst according to any one of claims 1 to 11, further comprising a refractory inorganic oxide.   The catalyst for exhaust gas purification according to any one of claims 1 to 12, further comprising cerium oxide and / or ceria-zirconia composite oxide not covered with lanthanum-containing alumina.   An exhaust gas purification method comprising a step of exposing the exhaust gas purification catalyst according to any one of claims 1 to 13 to exhaust gas of an internal combustion engine. The temperature of the exhaust gas of the internal combustion engine is 0 ° C to 750 ° C,
The exhaust gas-purifying catalyst is exposed to an exhaust gas having an air-fuel ratio of 14.1 to 15.1 having a temperature of 950 ° C. to 1000 ° C. before being exposed to the exhaust gas of the internal combustion engine. The exhaust gas purification method according to claim 14.

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