JP5606191B2 - Exhaust gas purification catalyst, method for producing the same, and exhaust gas purification method - Google Patents

Description

  The present invention relates to a catalyst for purifying exhaust gas discharged from an internal combustion engine, a method for producing the same, and a method for purifying exhaust gas using the same. Specifically, the present invention relates to a gas discharged from an internal combustion engine under a fuel excess condition with a low theoretical air-fuel consumption (A / F) even after high-temperature endurance treatment, particularly nitrogen oxide (NOx) in the gas. The present invention relates to an exhaust gas purification catalyst that can efficiently purify gas, a method for producing the same, and an exhaust gas purification method.

  As a catalyst for purifying exhaust gas from an internal combustion engine, a catalyst in which a noble metal component such as platinum (Pt), palladium (Pd) and rhodium (Rh) is supported on a support such as activated alumina is generally used. Among these catalysts, a catalyst using palladium (Pd catalyst) is a condition in which the theoretical air fuel consumption (A / F) is excessive as compared with a catalyst using platinum or rhodium (Pt / Rh catalyst). Then, it is reported that the purification rate of nitrogen oxide (NOx) and carbon monoxide (CO) is low (for example, refer nonpatent literature 1).

Also, an adsorbent comprising a low-temperature ignitable catalyst composition containing a substance having an electron-donating and / or nitrogen dioxide absorption or emission action such as magnesium, strontium, or barium and a noble metal such as platinum, palladium, or rhodium, and zeolite. Is disclosed in an exhaust pipe of an internal combustion engine, and since electrons are donated from an electron donating substance to a noble metal, hydrocarbon (HC) and / or carbon monoxide (CO) It has been shown that the adsorptive power to noble metals is weakened and the reaction between HC and / or CO and O ₂ is promoted (see, for example, Patent Document 1).

  Further, it is disclosed that, in an exhaust gas purification catalyst containing palladium and magnesium, adsorption of HC to palladium is suppressed when magnesium captures and adsorbs HC in the exhaust gas (for example, Patent Document 2). reference).

  Furthermore, a catalyst prepared using an aqueous solution in which palladium and at least one of calcium hydroxide and magnesium hydroxide are mixed is disclosed. Since calcium and magnesium react with a sulfur component to form a sulfide, the sulfur coverage of palladium is disclosed. It is disclosed that poison is suppressed (for example, refer patent document 3).

Japanese Patent Laid-Open No. 9-872 JP-A-8-229395 JP-A-7-136512

J. et al. C. Summers, J. et al. J. et al. White, W.W. B. Williamson, SAE PAPER, 890794 (1989)

  However, the catalyst described in Non-Patent Document 1 has a low NOx purification rate under conditions where the fuel is excessive and the A / F value is low, and at present, the exhaust gas emission regulation from the internal combustion engine is strengthened, the NOx purification rate Is not enough. Moreover, although rhodium has a higher NOx purification rate under conditions where the A / F value is lower than that of palladium, it is more expensive than palladium, so that reduction thereof is desired from an economical viewpoint.

  In addition, the catalysts described in Patent Documents 1 and 2 have most of the noble metal particles on the surface of the support (supporting base material), so that there are few physical barriers to sintering of the noble metal, and use at high temperatures and long-lasting time When exposed to high temperatures, there is a problem that sintering of noble metal particles is likely to occur, leading to performance degradation. In addition, all of the catalysts described in Patent Documents 1 to 3 are prepared using alumina sol, but the process of drying and firing the sol or gel is required at least twice, which may increase catalyst preparation costs. There is a problem that the catalyst preparation cycle time becomes long. Further, the catalysts described in Patent Documents 2 and 3 each have a required pore size of 5 to 20 nm and 20 to 60 nm, respectively. These catalysts are used as a support for a noble metal or the like which is an active component. Therefore, the region is different from the pore diameter required for the catalyst coated with the noble metal.

  The present invention has been made in view of the above problems, and its object is that the A / F value (air-fuel ratio) is less than the theoretical air-fuel ratio of 14.7, that is, it contains a large amount of reducing hydrocarbons. Exhaust gas purifying catalyst capable of efficiently purifying nitrogen oxide (NOx) and carbon monoxide (CO) from an internal combustion engine even when only palladium is used as a noble metal in a rich atmosphere with excessive fuel, Another object of the present invention is to provide a method for efficiently purifying exhaust gas from an internal combustion engine using the exhaust gas purifying catalyst.

  Another object of the present invention is to provide an exhaust gas purifying catalyst excellent in high temperature durability having excellent catalytic performance (thermal stability) even after being exposed to a high temperature, its inexpensive production method, and the exhaust gas. An object of the present invention is to provide a method for efficiently purifying exhaust gas from an internal combustion engine using a purifying catalyst.

  As a result of intensive studies in view of the above problems, the catalyst obtained by coating at least part of a composite of palladium or palladium oxide and magnesium or magnesium oxide with lanthanum-containing alumina is exposed to high temperatures. The present inventors have found that the catalyst has excellent catalytic performance (thermal stability) and high temperature durability, and has completed the present invention.

  The above problem is solved by an exhaust gas purifying catalyst comprising a composite containing palladium or palladium oxide and magnesium or magnesium oxide, and lanthanum-containing alumina, wherein at least a part of the composite is coated with the lanthanum-containing alumina. Is done.

  The exhaust gas purifying catalyst of the present invention has excellent catalytic performance (thermal stability) and high-temperature durability even after being exposed to high temperatures, and the A / F value (air-fuel ratio) is the stoichiometric air-fuel ratio. Efficiently purifies exhaust gas from internal combustion engines, especially nitrogen oxides (NOx) and carbon monoxide (CO) in exhaust gas, even in rich atmospheres with excess of fuel containing a large amount of reducing hydrocarbons, less than 7. it can.

It is a top view which shows typically the support state of the exhaust gas purification catalyst of this invention. FIG. 4 is a plan view schematically showing a state of a composite in which Pd / Pd oxide is coated with lanthanum-containing alumina, but Mg / Mg oxide is supported without being in close proximity to the Pd / Pd oxide. It is sectional drawing which shows typically the support state of the Pd / Pd oxide and alumina in the conventional exhaust gas purification catalyst produced using the impregnation method. It is a figure which shows the TEM-EDS analysis result of the catalyst A of the Example of this invention. It is a figure which shows the TEM-EDS analysis result of the catalyst B of the Example of this invention. It is a figure which shows the TEM-EDS analysis result of the catalyst D of a comparative example.

  The exhaust gas purifying catalyst according to the present invention includes a composite containing palladium or palladium oxide and magnesium or magnesium oxide, and lanthanum-containing alumina, and at least a part of the composite is coated with the lanthanum-containing alumina. It is a feature. The exhaust gas purifying catalyst of the present invention having such a structure has excellent catalytic performance (thermal stability) and high-temperature durability even after being exposed to high temperatures, and has an A / F value (air-fuel ratio) theoretically. Even when the air-fuel ratio is less than 14.7, that is, in a rich atmosphere containing a large amount of reducing hydrocarbons and excessive fuel, exhaust gas from the internal combustion engine, particularly nitrogen oxide (NOx) and carbon monoxide ( CO) can be efficiently purified. As described above, the reason why the exhaust gas purifying catalyst of the present invention has the above-mentioned effects is unknown, but it is inferred as follows. The present invention is not limited by the following inference.

That is, Mg / Mg oxide is present in the catalyst, but this Mg / Mg oxide, when selectively storing hydrogen (H ₂ ) in the exhaust gas, locally improves the output and purifies the exhaust gas. It is thought that the atmosphere is relaxed rather than a suitable stoichiometric state. In addition, when local atmospheric fluctuation occurs, it is possible to keep the Pd / Pd oxide in an oxidized state even in an atmosphere having a low A / F value (rich), using oxygen on palladium oxide, The reaction of NO + O → NO ₂ , HC + NO ₂ → N ₂ + H ₂ O + CO ₂ is considered to proceed. When the composite is coated with lanthanum-containing alumina, the Pd / Pd oxide and the Mg / Mg oxide are supported in close proximity even after being exposed to a high temperature. Conceivable. For this reason, it is considered that the reaction according to the present invention easily proceeds in an atmosphere having a low A / F value even after being exposed to a high temperature.

  In the present specification, “the exhaust gas purification catalyst of the present invention” is also referred to as “the catalyst of the present invention”; “palladium or palladium oxide” is also referred to as “Pd / Pd oxide”; and “magnesium or magnesium oxide” Are also referred to as “Mg / Mg oxides”, respectively.

  Embodiments of the present invention will be described below. However, the present invention is not limited to the following. In the present specification, “A to B” means “A or more and B or less”, and for example, “1% by mass to 30% by mass” or “1 to 30% by mass” is described in the specification. "1 mass% or more and 30 mass% or less". In addition, “C and / or D” in the present specification means to include one or both of C and D. Further, various physical properties mentioned in the present specification mean values measured by the methods described in Examples described later unless otherwise specified.

(I) Exhaust gas purification catalyst The catalyst of the present invention includes a composite in which a composite containing palladium or palladium oxide and magnesium or magnesium oxide is at least partially coated with lanthanum-containing alumina. Here, “a composite composed of palladium or palladium oxide and magnesium or magnesium oxide, at least partially coated with lanthanum-containing alumina” will be described with reference to FIG. In addition, since FIGS. 1-3 is the figure which showed the positional relationship of each component typically, it does not represent an actual distance, a shape, and a particle diameter.

  FIG. 1 is a plan view and a part schematically showing a state of a composite composed of palladium or palladium oxide and magnesium or magnesium oxide, at least partly covered with lanthanum-containing alumina contained in the catalyst of the present invention. It is an enlarged view. For example, in FIG. 1, reference numeral 1 indicates only one lanthanum-containing alumina, but all the substances represented by white amorphous shapes are lanthanum-containing alumina, and the same is true for the other reference numerals. The same applies to FIGS.

As shown in FIG. 1, the lanthanum-containing alumina 1 is supported in the vicinity of the periphery of a composite 2 of Pd / Pd oxide and Mg / Mg oxide. Here, “supported in close proximity” refers to a state in which the Pd / Pd oxide or Mg / Mg oxide and the lanthanum-containing alumina 1 are partially in contact. Further, in the “closely supported state”, the Pd / Pd oxide or Mg / Mg oxide can be adsorbed at the contact portion between the Pd / Pd oxide or Mg / Mg oxide and the lanthanum-containing alumina 1. Molecules (for example, oxygen (O ₂ ) and carbon monoxide (CO)) cannot be adsorbed.

  As shown in FIG. 1, the “composite composed of Pd / Pd oxide and Mg / Mg oxide” has at least a partial contact of at least one atom of each of Pd / Pd oxide and Mg / Mg oxide. And a solid solution of Pd / Pd oxide and Mg / Mg oxide, a mixture of Pd / Pd oxide and Mg / Mg oxide, or a combination of two or more thereof.

  If the Pd / Pd oxide and the Mg / Mg oxide form the composite 2, the Mg / Mg oxide is present in the vicinity of the Pd / Pd oxide. / F), nitrogen oxide (NOx) and carbon monoxide (CO) can be efficiently purified even under low conditions. In addition, since at least a part of the composite composed of Pd / Pd oxide and Mg / Mg oxide is coated with lanthanum-containing alumina, the lanthanum-containing alumina becomes a physical barrier against palladium sintering. Even if exposed to Pt, sintering of Pd particles can be suppressed / prevented and Pd / Pd oxide and Mg / Mg oxide can be supported in close proximity. Therefore, the catalyst of the present invention can exhibit excellent catalytic performance (thermal stability) and high temperature durability even after being exposed to high temperatures. The present invention is not limited to the above reasoning.

  In contrast, the Pd / Pd oxide and the Mg / Mg oxide do not form a composite. For example, as shown in FIG. 2, the Pd / Pd oxide 3 is covered with the lanthanum-containing alumina 1. However, when the Mg / Mg oxide 4 is supported in a state where it is not close to the Pd / Pd oxide 3 and is not covered with the lanthanum-containing alumina 1, many Pd / Pd oxides and Mg / Mg oxide is supported in close proximity, or Mg / Mg oxide is supported independently of Pd / Pd oxide. For this reason, under the condition of low theoretical air fuel consumption (A / F) with excessive fuel, only Pd's original catalytic performance is exhibited, and nitrogen oxide (NOx) and carbon monoxide (CO) cannot be efficiently purified.

  In addition, as shown in FIG. 3, when the Pd / Pd oxide 3 is supported on the surface of the lanthanum-containing alumina 1 as a support, the Mg / Mg oxide 4 is compared with the Pd / Pd oxide 3. Although Pd / Pd oxide 3 is not coated as shown in FIG. 1 and FIG. 2, sintering is likely to occur, and Mg / Mg oxide also has no physical barrier of a coating layer. Easy to move to. Therefore, even if the Pd / Pd oxide and the Mg / Mg oxide are supported in a close state at the time of catalyst preparation, it is difficult to maintain the close state when exposed to a high temperature. FIG. 3 is a plan view schematically showing a supported state of the Pd / Pd oxide 3 and the lanthanum-containing alumina 1 in a conventional exhaust gas purifying catalyst produced using an impregnation method.

  The “sintering” means that particles are bonded and coarsened by exposure to a high temperature. When Pd / Pd oxide, which is a catalyst component, causes sintering, the surface area of the Pd / Pd oxide particles decreases, and the catalytic activity decreases, which is not preferable.

  Hereafter, each component which comprises the said composite_body | complex which concerns on this Embodiment is demonstrated.

(I) Catalyst active component The catalyst of the present invention contains a composite of Pd / Pd oxide and Mg / Mg oxide as a catalyst active component. Here, the Pd / Pd oxide, which is one of the catalytically active components, may be present in the form of palladium or palladium oxide, but is preferably present in the form of palladium oxide. With such a configuration, carbon monoxide in the exhaust gas even in a fuel-rich atmosphere where the A / F value (air-fuel ratio) during acceleration or the like is less than 14.7, which is the stoichiometric air-fuel ratio, even when Rh is not used. (CO) and hydrocarbons (HC) can be purified. The Pd / Pd oxide is at least partially coated with lanthanum-containing alumina. However, the catalyst of the present invention may use the Pd / Pd oxide in the above-mentioned coated form. It is preferable to have a structure in which / Pd oxide is further supported on a three-dimensional structure. By using the three-dimensional structure having a large surface area in this way, the exhaust gas purification catalyst can be efficiently supported, and therefore the exhaust gas purification efficiency of the exhaust gas purification catalyst can be improved. Here, the amount of Pd / Pd oxide supported is not particularly limited. For example, when a catalytically active component is supported on a three-dimensional structure, one liter of the three-dimensional structure (hereinafter referred to as “L”). (In some cases, in terms of palladium), preferably in terms of palladium (in terms of Pd), preferably 0.05 to 20 g, more preferably 0.1 to 10 g, even more preferably 1 to 7 g / L, and particularly preferably 2 to 4 g / L. L. If the amount of Pd / Pd oxide supported is in the above range, NOx and CO can be efficiently purified, and combustion of hydrocarbons can proceed sufficiently to achieve HC combustibility commensurate with the input amount. The catalyst performance can be demonstrated. Moreover, since a sufficient effect can be obtained with a small amount, it is preferable in terms of cost performance. The amount of Pd / Pd oxide supported means the total amount of Pd / Pd oxide used in the catalyst of the present invention. For example, as described in detail below, Pd / Pd oxide not covered with lanthanum-containing alumina. When Pd oxide is present, it means the total loading of coated and uncoated Pd / Pd oxide.

  Further, the degree of coating of the Pd / Pd oxide coated with the lanthanum-containing alumina is not particularly limited, but the amount of the coated Pd / Pd oxide coated with the lanthanum-containing alumina is converted to palladium (Pd conversion). The mass ratio of Pd: lanthanum-containing alumina is preferably 4: 1 to 1: 100, more preferably 2: 1 to 1:50, particularly preferably 1: 2 to 1:12. . Here, when the mass ratio of Pd: lanthanum-containing alumina is 4: 1 or less, the Pd / Pd oxide can be coated with the lanthanum-containing alumina, which becomes a physical barrier against the movement of the Pd / Pd oxide. be able to. On the other hand, when the mass ratio of Pd: lanthanum-containing alumina is 1: 100 or more, the exhaust gas can be purified efficiently even with the coated Pd / Pd oxide.

  The catalyst of the present invention may comprise a Pd / Pd oxide not coated with lanthanum-containing alumina. When the catalytically active component according to the present embodiment is supported on a three-dimensional structure, the amount of Pd / Pd oxide not coated with lanthanum-containing alumina is converted to palladium (Pd) and the three-dimensional structure. It is preferable that it is 0.5-15g per liter, and it is more preferable that it is 1-10g. The amount of Pd / Pd oxide not coated with lanthanum-containing alumina when not supported on the three-dimensional structure is 0 to 90% by mass of the total palladium mass in terms of palladium (Pd). Is preferable, and it is more preferable that it is 0-85 mass%.

  The shape of the Pd / Pd oxide is not particularly limited, and the Pd / Pd oxide may be any of granular, fine particle, powder, cylindrical, conical, prismatic, cubic, pyramidal, indefinite, etc. The shape can be taken. In such a case, the Pd / Pd oxide particles may be primary particles or secondary particles, but are preferably secondary particles in which the primary particles are aggregated.

  The other catalytically active component, Mg / Mg oxide, may be present in the form of either magnesium or magnesium oxide, but is preferably present in the form of magnesium oxide. By supporting Pd / Pd oxide and Mg / Mg oxide close to each other in such a form, the A / F value (air-fuel ratio) at the time of acceleration or the like is less than 14.7 which is the theoretical air-fuel ratio. Even in a fuel-rich atmosphere, carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas can be purified only with Pd as the noble metal. The Mg / Mg oxide is at least partially coated with lanthanum-containing alumina. However, the catalyst of the present invention may use Mg / Mg oxide in the above-mentioned coated form. It is preferable to have a structure in which / Mg oxide is further supported on a three-dimensional structure. By using the three-dimensional structure having a large surface area in this way, the exhaust gas purification catalyst can be efficiently supported, and therefore the exhaust gas purification efficiency of the exhaust gas purification catalyst can be improved. Here, the amount of magnesium supported is not particularly limited, but the mass ratio of Pd (palladium equivalent (Pd equivalent)): Mg (magnesium oxide equivalent (MgO equivalent)) is preferably 10: 1 to 1: 5, more preferably. Is 5: 1 to 1: 2, particularly preferably 3: 1 to 2: 3. If the amount of Mg / Mg oxide is in the above range, the effect of Pd / Pd oxide can be sufficiently exhibited without excessively covering the surface of Pd / Pd oxide with Mg / Mg oxide. The amount of Mg / Mg oxide supported means the total amount of Mg / Mg oxide used in the catalyst of the present invention. For example, as detailed below, Mg / Mg oxide not covered with lanthanum-containing alumina When Mg oxide is present, it means the total loading of coated and uncoated Mg / Mg oxide.

  When the catalytically active component according to the present embodiment is supported on a three-dimensional structure, the amount of Mg / Mg oxide not coated with lanthanum-containing alumina is Pd (palladium equivalent (Pd equivalent)): Mg ( The mass ratio in terms of magnesium oxide (MgO) is preferably 10: 1 to 1: 5, more preferably 5: 1 to 1: 2, particularly preferably 3: 1 to 2: 3. .

  The Mg / Mg oxide can take any shape such as granular, fine particle, powder, cylindrical, conical, prismatic, cubic, pyramid, and indeterminate. Preferably, the Mg / Mg oxide is Mg / Mg oxide particles. In such a case, the Mg / Mg oxide particles may be primary particles or secondary particles, but more preferably secondary particles in which the primary particles are aggregated. The size of the Mg / Mg oxide particles is not particularly limited, but the average particle size of the Mg / Mg oxide particles before forming the slurry is preferably 0.5 to 150 μm, more preferably 1 to 50 μm. .

  Here, as described above, the Pd / Pd oxide and the Mg / Mg oxide are supported at positions where the Pd / Pd oxide and the Mg / Mg oxide are close to each other. In this state, the oxide is at least partially in contact. Here, “the Pd / Pd oxide and the Mg / Mg oxide are in a state of at least partially contacting” means that the Pd / Pd oxide and the Mg / Mg oxide are partially in contact with each other. In addition to the state, it includes a state in which the Pd / Pd oxide is coated with the Mg / Mg oxide, or a state in which the Mg / Mg oxide is coated with the Pd / Pd oxide. And Mg / Mg oxide partially contact each other, or Pd / Pd oxide covers Mg / Mg oxide. Furthermore, even if the Pd / Pd oxide and the Mg / Mg oxide are not partly in contact, they may be present in the vicinity of several nm or less. By making such a supported state, it becomes possible to support the Pd / Pd oxide in a state where the oxidation state is dominant. Further, in order to support the Pd / Pd oxide and the Mg / Mg oxide in a partially contacted state, the Pd / Pd oxide may be directly supported on the oxidized Mg / Mg oxide, or Pd / Pd An oxidized Mg / Mg oxide may be supported directly on the oxide. The state of being supported in close proximity can be confirmed by, for example, observation with an electron microscope equipped with an energy dispersive X-ray detector such as TEM-EDX, TEM-EDS, or X-ray diffraction analysis.

  In TEM-EDS, as shown in FIGS. 4 and 5, the close state can be confirmed by detecting the spectrum attributed to Mg on or near the particle where the spectrum attributed to Pd is detected. Further, as shown in FIGS. 4 and 5, Pd / Pd oxide and Mg / Mg oxide are visually present on the same particle by performing mapping measurement in the vicinity of the particle, and at least partially contact with each other. This can be confirmed.

  Furthermore, as shown in FIGS. 4 and 5, the lanthanum-containing alumina is supported in the vicinity of the Pd / Pd oxide and Mg / Mg oxide composite. This can be confirmed by the detection of Al and La.

On the other hand, in the X-ray diffraction analysis, when the Pd / Pd oxide and the Mg / Mg oxide are in a close state, a part of Pd is in a close state by confirming the formation of a compound with a part of the Mg. It can be confirmed that it is supported. That is, in the composite according to the present invention including Pd / Pd oxide and Mg / Mg oxide, it is preferable that a compound composed of palladium and magnesium (compound of Mg and Pd) is formed. Here, the compound of Mg and Pd is MgPd ₃ , MgPd ₂ , MgPd, Mg ₂ Pd, Mg ₃ Pd, Mg ₄ Pd, Mg ₅ Pd, Mg ₆ Pd, etc., preferably MgPd ₃ .

(Ii) Lanthanum-containing alumina The catalyst of the present invention contains lanthanum-containing alumina in addition to the Pd / Pd oxide and Mg / Mg oxide composite. Here, the lanthanum-containing alumina is not particularly limited as long as lanthanum and alumina are mixed. Examples of the lanthanum-containing alumina include a mixture of lanthanum oxide (La ₂ O ₃ ) and alumina (Al ₂ O ₃ ), lanthanum-alumina composite oxide (LaAlO ₃ etc.), lanthanum-alumina composite oxide (LaAlO ₃ etc.). ) And lanthanum oxide (La ₂ O ₃ ) and lanthanum-alumina composite oxides (LaAlO ₃ etc.) and alumina (Al ₂ O ₃ ). Among these, lanthanum is preferably contained in the form of lanthanum oxide and / or lanthanum-alumina composite oxide (LaAlO ₃ or the like), and more preferably contained in the form of lanthanum oxide (La ₂ O ₃ ). . The “lanthanum-containing alumina” more preferably contains both lanthanum oxide and a lanthanum-allemina composite oxide. The lanthanum-containing alumina may be used alone or in the form of a mixture of two or more.

The content of lanthanum in the lanthanum-containing alumina, lanthanum oxide in terms _(La 2 _{O 3} basis), based on the total amount of lanthanum _(La 2 _{O 3} equivalent) and alumina _(Al 2 _{O 3} conversion), 0. It is preferable that it is 2-30 mass%, and it is more preferable that it is 0.5-15 mass%. Since the content of lanthanum is 0.2% by mass or more, the Pd / Pd oxide and the Mg / Mg oxide can be supported close to each other even after high-temperature durability. The content of 15% by mass or less is preferable because the proportion of lanthanum having a small surface area compared to alumina does not become too large, and it is difficult to reduce the dispersibility of other catalysts and / or promoter components.

(Iii) Refractory Inorganic Oxide In addition to the Pd / Pd oxide and Mg / Mg oxide composite and lanthanum-containing alumina, the catalyst of the present invention comprises a refractory inorganic oxide, more preferably a refractory inorganic oxide. It is preferable that oxide powder is included. Thereby, since the said composite body is disperse | distributed by a refractory inorganic oxide, exhaust gas can be made to contact the exhaust gas purification catalyst component more efficiently. Therefore, exhaust gas purification at a high temperature can be performed more efficiently.

The refractory inorganic oxide is not particularly limited, and a known refractory inorganic oxide can be used, but an inorganic oxide having a high melting point, more preferably an inorganic oxide having a melting point of 1000 ° C. or more is preferably used. More preferably, an inorganic oxide having a melting point of 1000 to 3000 ° C., particularly preferably 1500 to 3000 ° C. is used. The refractory inorganic oxide is not particularly limited as long as it is usually used as a catalyst carrier for exhaust gas. For example, α-alumina (Al ₂ O ₃ ) or activated alumina such as γ, δ, η, θ, silica (SiO ₂ ), zeolite, titania (TiO ₂ ), magnesia (MgO), zirconia, titania, oxidation Silicon or a composite oxide thereof such as silica-alumina (SiO ₂ -A 1 ₂ O ₃ ) can be used, and preferably alumina, zirconia, and magnesia, more preferably alumina, especially alumina powder. It is. Here, the refractory inorganic oxide may be blended alone or in the form of a mixture of two or more.

  The refractory inorganic oxide may contain a rare earth metal. The contained form may be contained alone or in the form of a mixture of two or more. In the latter case, the mixed state may be a simple physical mixed state or a complex oxide of two or more rare earth metals. Here, as the rare earth metal, there are cerium (Ce), lanthanum (La), neodymium (Nd), yttrium (Y), scandium (Sc), praseodymium (Pr), etc., preferably cerium, lanthanum, neodymium, Yttrium, praseodymium, etc. The rare earth metal may be in the form of the metal as it is, in the form of an oxide, or in the form of a complex oxide with a refractory inorganic oxide. The amount of the rare earth metal used is not particularly limited, but is 5 to 150 g, preferably 5 to 80 g per liter of the three-dimensional structure in terms of oxide.

  Alternatively, the refractory inorganic oxide may be added with at least one selected from the group consisting of transition metals such as iron, nickel, cobalt, and manganese, alkali metals, alkaline earth metals, and oxides thereof. it can.

  Moreover, the said refractory inorganic oxide may be coat | covered with the lanthanum containing alumina, and does not need to be coat | covered. Note that “not covered with lanthanum-containing alumina” means that the substance does not exist between the Pd / Pd oxide and the lanthanum-containing alumina at the contact portion between the Pd / Pd oxide and the lanthanum-containing alumina. Say. That is, for example, the state “the refractory inorganic oxide is not covered with lanthanum-containing alumina” means the state “the refractory inorganic oxide is not present between the Pd / Pd oxide and the lanthanum-containing alumina”. means.

  The refractory inorganic oxide may be supported as it is or on lanthanum-containing alumina, but it is preferable that the refractory inorganic oxide is further supported on the three-dimensional structure in the above-described form. However, it is not excluded that a composite of Pd / Pd oxide and Mg / Mg oxide, which are catalytically active components, is partially supported on a refractory inorganic oxide. When the catalyst of the present invention further contains a refractory inorganic oxide, the amount of the refractory inorganic oxide supported is not particularly limited. For example, the refractory inorganic oxide is supported on a three-dimensional structure. Is preferably 10 to 300 g, more preferably 30 to 200 g, and particularly preferably 70 to 150 g per liter of the three-dimensional structure. Alternatively, the amount of the refractory inorganic oxide used when not supported on the three-dimensional structure is preferably 5 to 80% by mass, and more preferably 10 to 60% by mass based on the total catalyst mass. If the amount of the refractory inorganic oxide used is in the above range, the composite of Pd / Pd oxide and Mg / Mg oxide can be sufficiently dispersed, and sufficient durability can be secured. The contact state between the Mg / Mg oxide composite and the hydrocarbon introduced for temperature rise can be improved, and the temperature rise can be promptly promoted.

  The refractory inorganic oxide used in the present invention can take any shape such as granular, fine particle, powder, cylindrical, conical, prismatic, cubic, pyramid, and indeterminate. . Preferably, it is a refractory inorganic oxide, granular, fine particle, or powder, more preferably a powder. The average particle diameter of the refractory inorganic oxide when the refractory inorganic oxide is granular, particulate, or powder is not particularly limited, but is, for example, in the range of 0.5 to 150 μm, more preferably 1 to 100 μm. Preferably there is. Within such a range, the three-dimensional structure can be satisfactorily coated (coated). The “average particle diameter” of the refractory inorganic oxide in the present invention can be measured by the average value of the particle diameter of the refractory inorganic oxide measured by a known method such as classification.

The BET specific surface area of the refractory inorganic oxide is not particularly limited, but is preferably 50 to 750 m ² / g, more preferably 150 to 750 m ² / g.

(Iv) Cerium oxide, ceria-zirconia composite oxide The catalyst of the present invention preferably further contains cerium oxide and / or ceria-zirconia composite oxide. The cerium oxide and / or ceria-zirconia composite oxide functions as an oxygen storage / release material and can act as a co-catalyst, improve the heat resistance of the exhaust gas purification catalyst, Since it has an action of promoting the oxidation-reduction reaction by the active species of the catalyst, the exhaust gas purification at a high temperature can be performed more efficiently.

  Here, the cerium oxide and the ceria-zirconia composite oxide may be blended alone or in the form of a mixture of two or more. The cerium oxide and ceria-zirconia may be used as they are, but are preferably supported on a three-dimensional structure. In this case, the amount of cerium oxide and / or ceria-zirconia composite oxide supported is not particularly limited. For example, when a refractory inorganic oxide is supported on a three-dimensional structure, the three-dimensional structure is supported. It is preferably 250 g or less, more preferably 5 to 150 g, and particularly preferably 10 to 80 g per liter of body. That is, if the supported amount is within the above range, the exhaust gas purification or purification assisting effect by the cerium oxide and / or ceria-zirconia composite oxide can be sufficiently expressed, and the three-dimensional structure such as cordierite can be applied. Supportability or coatability can also be sufficiently secured. In addition, the exhaust gas reduction effect and cost performance can be sufficiently satisfied.

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

  The cerium oxide and / or ceria-zirconia composite oxide may contain a rare earth metal excluding cerium. The inclusion form may be a simple physical mixed state or a complex oxide with cerium. Here, as rare earth metals (excluding cerium), there are lanthanum (La), neodymium (Nd), yttrium (Y), scandium (Sc), praseodymium (Pr), etc., preferably lanthanum, neodymium, yttrium, Such as praseodymium. The rare earth metal may be a metal as it is or an oxide. The amount of the rare earth metal (excluding cerium) is not particularly limited, but is 5 to 80 g, preferably 10 to 50 g per liter of the three-dimensional structure in terms of oxide.

  The cerium oxide and / or ceria-zirconia composite oxide may be coated with lanthanum-containing alumina or may not be coated. Here, the ceria-zirconia composite oxide is more preferably not covered with lanthanum-containing alumina. As a result, the exhaust gas can be purified more efficiently. Note that “not covered with lanthanum-containing alumina” means that the substance does not exist between the Pd / Pd oxide and the lanthanum-containing alumina at the contact portion between the Pd / Pd oxide and the lanthanum-containing alumina. Say. That is, for example, the state “the ceria-zirconia composite oxide is not coated with lanthanum-containing alumina” means “the ceria-zirconia composite oxide does not exist between the Pd / Pd oxide and the lanthanum-containing alumina”. Means state.

(V) Other components The catalyst of the present invention comprises a composite of Pd / Pd oxide and Mg / Mg oxide, lanthanum-containing alumina and, if necessary, refractory inorganic oxide, cerium oxide and / or ceria. In addition to the zirconia composite oxide, an alkali metal and / or an alkaline earth metal may be contained. Here, examples of the alkali metal include lithium, sodium, potassium, rubidium, and cesium, and potassium is preferable. Examples of the alkaline earth metal include calcium, strontium, and barium, and barium is preferable. Here, the alkali metal and / or alkaline earth metal may be blended alone or in the form of a mixture of two or more. The alkali metal and / or alkaline earth metal may be used as it is, but is preferably supported on a three-dimensional structure. The amount of alkali metal and / or alkaline earth metal used when the alkali metal and / or alkaline earth metal is supported on the three-dimensional structure is not particularly limited. It is preferably 0.5 to 40 g per liter.

  The catalyst of the present invention can efficiently purify nitrogen oxides (NOx) and carbon monoxide (CO) from an internal combustion engine even when only palladium is used as a noble metal. For this reason, the catalyst of the present invention can achieve sufficient performance as described above if only palladium is included as a noble metal. However, the catalyst of the present invention may contain other noble metals in addition to the palladium. Here, the noble metal that can be used is not particularly limited, and examples thereof include gold, silver, platinum, ruthenium, rhodium, osmium, and iridium. The noble metal may be used alone or in the form of a mixture of two or more. Of the noble metals, rhodium and platinum are preferably used in view of the high performance of the three-way catalyst. In view of the high purification rate of nitrogen oxides and hydrocarbons, palladium is more preferable. The content of the noble metal when the catalyst of the present invention contains a noble metal is not particularly limited. For example, when a catalytically active component is supported on a three-dimensional structure, the metal per liter of the three-dimensional structure In terms of conversion, it is preferably 0.01 to 5 g, more preferably 0.05 to 3 g, and particularly preferably 0.1 to 2 g.

(Vi) Three-dimensional structure The catalyst of the present invention is preferably supported on a three-dimensional structure. That is, in the exhaust gas purifying catalyst according to the present embodiment, it is preferable that the above-described components are supported on the three-dimensional structure.

  The three-dimensional structure is not particularly limited, and 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 weir body) is preferable, and 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 in particular, cordierite, mullite, α-alumina, silicon carbide, silicon nitride, and the like are preferable. Particularly preferred are cordierite carriers. 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 quadrangle, 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.

  The method for supporting the catalyst of the present invention on the three-dimensional structure is not particularly limited. For example, a method such as a wash coat can be used.

(Vii) Characteristics of Exhaust Gas Purification Catalyst The various characteristics of the exhaust gas purification catalyst of the present invention are not particularly limited, but preferably have the following characteristics.

(Catalyst pore structure)
The pore volume of the pore of the exhaust gas purifying catalyst of the present invention is not particularly limited, but the pore volume of the pore having a diameter in the range of 160 nm or more and less than 1000 nm is 5% or more of the total pore volume 20 % Or less is preferable. More preferably, the pore volume of a pore having a diameter in the range of 160 nm or more and less than 800 nm is 5% or more and less than 18% of the total pore volume, more preferably in the range of 160 nm or more and less than 600 nm. The pore volume of the pores is 5% or more and less than 16% of the total pore volume. The pore volume of pores having a diameter of less than 160 nm is preferably 70% or more and 90% or less of the total pore volume. More preferably, they are 72% or more and 90% or less, More preferably, they are 77% or more and 89% or less. If the catalyst has such a pore distribution, the lanthanum-containing alumina can be supported close to the periphery of the composite of Mg / Mg oxide and Pd / Pd oxide, so that the exhaust gas is Pd / Pd oxide. In addition, it is easily adsorbed by Mg / Mg oxide, and sintering can be suppressed even after the endurance treatment, and an exhaust gas purification reaction is likely to occur. Therefore, according to the said structure, although Pd / Pd oxide and Mg / Mg oxide are coat | covered with the lanthanum containing alumina, exhaust gas can be purified efficiently. Further, according to the above embodiment, since Mg / Mg oxide exists in the vicinity of the Pd / Pd oxide, when oxygen-excess gas flows in, oxygen is occluded and the gas atmosphere around the Pd / Pd oxide is changed to oxygen. It is thought to ease from the excessive state. Similarly, in a reducing atmosphere in which oxygen is insufficient, oxygen stored in the oxygen-excessing atmosphere is released around the Pd / Pd oxide, so that oxygen is considered to be used effectively.

  In the present specification, exposing the catalyst to a temperature of 800 ° C. to 1000 ° C. is referred to as “endurance treatment”.

(Catalyst usage conditions)
The exhaust gas purifying catalyst of 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 800 ° C to 1000 ° C. That is, in this case, even if the catalyst bed temperature is exposed to the highest temperature that can be reached, the Pd / Pd oxide and the Mg / Mg oxide are supported in close proximity, so that the A / F during acceleration or the like is supported. Even in a fuel-rich atmosphere having a value (air-fuel ratio) of less than 14.7, which is the stoichiometric air-fuel ratio, carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas can be purified even with Pd alone as the noble metal. Therefore, even after being subjected to the high-temperature durability treatment, the exhaust gas can be efficiently purified by the Pd / Pd oxide.

(II) Method for Producing Exhaust Gas Purifying Catalyst The catalyst of the present invention is produced by coating at least part of Pd / Pd oxide and Mg / Mg oxide with lanthanum-containing alumina.

(I) Preparation of composite In the present invention, the method for producing a catalyst of the present invention, in which a composite composed of Pd / Pd oxide and Mg / Mg oxide is at least partially coated with lanthanum-containing alumina, Although not limited, for example, a sol-gel method, an alkoxide method, a reverse micelle method, a hydrothermal synthesis method, or the like can be used.

  In the above method, the palladium (Pd) source as a starting material for the Pd / Pd oxide is not particularly limited, and a material used in the field of exhaust gas purification can be used. Specifically, palladium; halides such as palladium chloride; inorganic salts such as nitrates, sulfates, ammonium salts, amine salts, and tetraammine salts of palladium; carboxylates such as acetates; and hydroxides, alkoxides And oxides. Nitrate, acetate, ammonium salt, amine salt, tetraammine salt, chloride and carbonate are preferable. Of these, nitrates (palladium nitrate), chlorides (palladium chloride), acetates (palladium acetate), and tetraammine salts (tetraammine palladium) are preferable, and palladium nitrate is more preferable. In the present invention, the palladium source may be a single source or a mixture of two or more types. Here, the addition amount of the palladium source is an amount carried on the three-dimensional structure in the amount as described above.

  Further, the magnesium raw material (Mg source) as a starting material for Mg / Mg oxide is not particularly limited, and a raw material used in the field of exhaust gas purification can be used. Specifically, magnesium; oxides such as magnesium oxide, magnesium peroxide, magnesium titanate, and magnesium chromate; halides such as magnesium chloride; magnesium sulfate, magnesium hydroxide, magnesium carbonate, magnesium acetate, magnesium nitrate, etc. A magnesium salt etc. are mentioned. Of these, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, and magnesium sulfate are preferably used. In the present invention, the magnesium source may be a single source or a mixture of two or more types. Here, the addition amount of the magnesium source is an amount carried on the three-dimensional structure in the amount as described above.

  In addition, when the catalyst of the present invention contains a noble metal other than palladium as a noble metal, the noble metal raw material as a starting material is not particularly limited, and a raw material used in the field of exhaust gas purification can be used. . Specifically, noble metals alone; halides; nitrates, dinitrodiammine salts, tetraammine salts, sulfates, ammonium salts, amine salts, carbonates, bicarbonates, nitrites, and other inorganic salts; formates and oxalates Carboxylates such as: organic salts such as bisethanolamine salts and bisacetylacetonate salts; and hydroxides, alkoxides, oxides, and the like. In the present invention, the noble metal source may be a single source or a mixture of two or more types. Here, the addition amount of the noble metal source is an amount carried on the three-dimensional structure in the amount as described above.

  The lanthanum source as a starting material in the composite is not particularly limited, and a raw material used in the field of exhaust gas purification can be used. Specifically, lanthanum acetate (III) n hydrate, lanthanum nitrate hexahydrate, lanthanum chloride heptahydrate, lanthanum sulfate (III) n hydrate, lanthanum oxide and the like can be used. Of these, lanthanum acetate (III) n hydrate and lanthanum nitrate hexahydrate are preferred, and lanthanum acetate (III) n hydrate is more preferred. The lanthanum source may be a single source or a mixture of two or more types. Here, the addition amount of the lanthanum source is an amount carried on the three-dimensional structure in the amount as described above. In the “n hydrate”, n means an integer of 1 or more.

  The aluminum source as a starting material in the composite is not particularly limited, and a material used in the field of exhaust gas purification can be used. Specifically, alumina sol, boehmite, aluminum isopropoxide, aluminum ethoxide, aluminum n-butoxide, aluminum sec-butoxide, aluminum nitrate, basic aluminum nitrate, aluminum hydroxide, and the like can be used. Of these, alumina sol, boehmite, and aluminum isopropoxide are preferable, and alumina sol is more preferable. The alumina source may be a single source or a mixture of two or more types. Here, the addition amount of the alumina source is an amount carried on the three-dimensional structure in the amount as described above.

  In the present embodiment, the method for coating the composite of Pd / Pd oxide and Mg / Mg oxide with lanthanum-containing alumina is not particularly limited, and depends on the type of aluminum source (alumina raw material). It is preferable to use an appropriate coating method. For example, after adding a mixture of a palladium source (including a solution, preferably in the form of an aqueous solution) and a magnesium source (including a solution, preferably in the form of an aqueous solution) to an alcohol solution of aluminum alkoxide, a lanthanum source (solution, A method of adding preferably an aqueous solution form, drying, and optionally calcining; an alumina sol, a palladium source (including a solution, preferably an aqueous solution form) and a magnesium source (solution, preferably an aqueous solution form). The mixture is added, the resulting mixture is added to gel the alumina sol, the palladium source and the magnesium source are gelled with the alumina gel, and then the lanthanum source (including solution, preferably in the form of an aqueous solution) is added. , Drying, and firing as necessary.

  In addition, what is necessary is just to use each component raw material which comprises the said exhaust gas purification catalyst so that it may become in the range of content of each component mentioned above.

(Ii) Method for Producing Exhaust Gas Purifying Catalyst The method for preparing the exhaust gas purifying catalyst according to the present embodiment is not particularly limited, but a catalyst composition containing the above-described composite is sufficiently used. Examples of the method include a method of forming a cylinder, a sphere, or the like after mixing and making an exhaust gas purifying catalyst.

  The catalyst composition includes refractory inorganic oxides such as alumina and ceria-zirconia composite oxide, Mg / Mg oxide not covered with lanthanum-containing alumina, Pd / Pd oxide not covered with lanthanum-containing alumina, etc. Components other than the complex described above can be included.

  Also, when using the above three-dimensional structure, the catalyst composition is collectively put into a ball mill or the like, wet-pulverized to form an aqueous slurry, the three-dimensional structure is immersed in the aqueous slurry, dried and fired. And the like.

<Production method in the case of using alumina sol as a raw material for the composite>
First, the sol is used in almost the same meaning as a colloidal solution and is in a state where it is dispersed in a liquid and exhibits fluidity. The gel means a state in which the colloidal particles have lost their independent motility (losing fluidity) to form a three-dimensional network structure.

  Specifically, when the fluidity is lost, the gel has a viscosity of 5,000 mPa · s (cP) to 500,000 mPa · s (cP), preferably 10,000 mPa · s (cP) to 100. 2,000 mPa · s (cP) or less, more preferably 12,000 mPa · s (cP) or more and 50,000 mPa · s (cP) or less. If it is 5,000 mPa · s (cP) or more, gelation is sufficiently advanced, and Pd / Pd oxide and Mg / Mg oxide can be well taken into the three-dimensional network structure. In this case, Pd / Pd oxide and Mg / Mg oxide are preferable because they are sufficiently covered with lanthanum-containing alumina. Moreover, if it is 500,000 mPa * s (cP) or less, the slurry viscosity in a slurrying process will not become high too much, and the washcoat to a three-dimensional structure can be performed favorably. The measurement of the viscosity of the gel is performed by a known method, and specifically, it is performed using a rotational viscometer such as a Brookfield viscometer.

  In order to change the sol into a gel, it is necessary to set the pH so that the sol cannot exist stably. For example, in order to gel a sol that is stable at pH 3 to 5, it is preferable that the pH is less than 3 or higher than 5.

When alumina sol is used as a raw material for the composite, the gel prepared by coating a composite made of Pd / Pd oxide and Mg / Mg oxide with a lanthanum-containing alumina gel is dried and sintered. And a method of mixing with another catalyst composition to form an aqueous slurry, immersing the three-dimensional structure in the aqueous slurry, drying, and firing. That is, in this case, the method for producing the exhaust gas purifying catalyst preferably includes a gel preparation step, and further includes a slurrying step, a wash coat step, and a dry sintering step. Therefore, the method for producing the catalyst of the present invention comprises:
A gel containing a composite in which at least a part of a composite containing palladium or palladium oxide and magnesium or magnesium oxide is coated with an alumina sol is obtained, the gel is further made into a slurry, and the slurry is wash-coated on a three-dimensional structure. It is preferable to further include drying and firing the three-dimensional structure obtained by wash-coating the slurry.

  In this method, the number of times of firing can be reduced by producing a catalyst supported on a three-dimensional structure in a state where a complex of a palladium source and a magnesium source is coated with lanthanum-containing alumina. Cost and manufacturing time can be reduced. For this reason, the exhaust gas purifying catalyst can be manufactured at a lower cost and with higher production efficiency.

(Gel preparation process)
The said gel preparation process is a process of producing the gel containing the composite_body | complex which the composite_body | complex consisting of a palladium source and a magnesium source coat | covered with the lanthanum containing alumina gel. Specifically, the gel can be prepared by the following method.

  For example, in the case of using an alumina sol having a pH of 4.0 and stable at pH 3 to 5, a mixed aqueous solution of a palladium source (including a solution form) and a magnesium source (including a solution form) is added to the alumina sol to have a pH of 1 to 2. By adding in this manner, gelation proceeds. At this time, it is confirmed that the viscosity is within the above range. The mixture is stirred until the change in viscosity becomes ± 100 mPa · s (cP) / second, and a lanthanum source (including solution form) is further added, so that the palladium source and the magnesium source are three-dimensional with the lanthanum-containing alumina gel. A complex incorporated in the network structure is obtained. If necessary, this composite is wash-coated on a three-dimensional structure together with a refractory inorganic oxide or the like to obtain a catalyst in which Pd / Pd oxide and Mg / Mg oxide are coated with lanthanum-containing alumina. It is done.

(Slurry process)
The slurrying step is a step of forming a slurry of the gel in which a complex composed of a palladium source and a magnesium source is coated with a lanthanum-containing alumina gel. Specifically, it can be prepared by mixing the gel with other components such as a refractory inorganic oxide and performing wet pulverization.

  At this time, the content of the composite in the mixture of the composite and other components such as a refractory inorganic oxide is preferably 2 to 40% by mass, and more preferably 5 to 20% by mass. If it is 2 mass% or more, since the ratio of the composite is sufficiently high, the effect of improving heat resistance can be sufficiently exhibited. Moreover, if it is 40 mass% or less, since the ratio of a composite_body | complex will not become high too much, it will be easy to adjust a pore diameter to the predetermined range.

  The “wet pulverization” refers to pulverization of a mixed solution of the composite and other components such as a refractory inorganic oxide using a pulverizer such as a ball mill.

  The particle diameter of the mixed component contained in the slurry measured by a dynamic light scattering method is preferably 2 to 10 μm, more preferably 3 to 8 μm. If the particle diameter is within such a range, the adhesion strength to the three-dimensional structure can be increased, and the particles can be supported well with a predetermined pore size distribution. Moreover, the solid content concentration in the slurry is preferably in the range of 10 to 60% by mass.

(Wash coat process)
The washcoat step is a step of supporting the slurry obtained in the slurrying step on a three-dimensional structure.

(Dry sintering process)
The dry sintering step is a step of drying and firing the three-dimensional structure obtained by wash-coating the slurry in the wash coat step. Conditions such as temperature and time required for drying and firing are not particularly limited, but are preferably performed until the mass change of the three-dimensional structure disappears.

  The drying is performed, for example, in air at a temperature of 50 to 200 ° C., more preferably 80 to 180 ° C., preferably 1 minute to 10 hours, more preferably 3 minutes to 8 hours, and particularly preferably 5 to 60 minutes. Can be done under conditions.

  Moreover, the said baking is 300-1000 degreeC, for example, Preferably it is 400-600 degreeC for 10 minutes-10 hours, Preferably it is 30 minutes-6 hours, Especially preferably, it can carry out on the conditions for 1-3 hours. If the calcination temperature is 300 ° C. or higher, hydrocarbons and the like contained in the raw material can be removed by burning, and if it is 1000 ° C. or lower, pore shrinkage can be suppressed.

(III) Exhaust gas purification method The catalyst of the present invention as described above or the catalyst produced by the method as described above is only exhaust gas from an internal combustion engine having an A / F value of the theoretical air-fuel ratio (14.7). In other words, it has a high purification capacity for exhaust gas from an internal combustion engine that is less than the stoichiometric air-fuel ratio. Therefore, the catalyst according to the present invention can be suitably used for treating exhaust gas containing reducing gas in the exhaust gas of the internal combustion engine, and particularly has high reducibility during acceleration from an internal combustion engine such as a gasoline engine. It has an excellent effect on purification of nitrogen oxides and carbon monoxide contained in the exhaust gas. Preferably, the catalyst according to the present invention can be suitably used for treating exhaust gas having an A / F value of less than 14.7 in exhaust gas of an internal combustion engine.

  Accordingly, the present invention provides a method for purifying exhaust gas, that is, exhaust gas, characterized by treating nitrogen oxide and carbon monoxide in exhaust gas of an internal combustion engine including a reducing gas using the catalyst of the present invention. There is also provided a method for purifying nitrogen oxides and carbon monoxide in exhaust gas having an air-fuel ratio of less than 14.7 in the exhaust gas of an internal combustion engine using a purification catalyst. The method includes the step of exposing the catalyst of the present invention to an exhaust gas of an internal combustion engine containing a reducing gas. Here, “exposing to gas” in this specification refers to bringing the exhaust gas purifying catalyst into contact with the gas, and not only bringing the entire catalyst surface into contact with the gas, but also a part of the catalyst surface into the gas. It is also included in the case of contact.

  The method of exposing the exhaust gas purification catalyst to the exhaust gas is not particularly limited. For example, the exhaust gas purification catalyst is installed in the exhaust flow path of the exhaust port of the internal combustion engine, and the exhaust gas is discharged to the exhaust flow path. This can be done by letting it flow into.

The gas is not particularly limited as long as it is an exhaust gas of an internal combustion engine. For example, nitrogen oxide (for example, NO, NO ₂ , N ₂ O), carbon monoxide, carbon dioxide, oxygen, hydrogen, Ammonia, water, sulfur dioxide, hydrocarbons in general.

  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.

  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, within the temperature range of the exhaust gas during normal operation. Here, the air-fuel ratio in the exhaust gas of the internal combustion engine having a temperature of 0 ° C. to 750 ° C. is less than 10 to 30, preferably 11 or more and less than 14.7.

  The catalyst of the present invention as described above or the catalyst produced by the method as described above may be exposed to an exhaust gas having a temperature of 750 to 1200 ° C. Here, the air-fuel ratio of the exhaust gas having a temperature of 750 to 1200 ° C. is preferably 10 to 18.6. Further, the time for which the exhaust gas purifying catalyst is exposed to the oxygen-excess exhaust gas having a temperature of 750 to 1200 ° C. is not particularly limited, and may be exposed for 5 to 100 hours, for example.

  The temperature of the exhaust gas is preferably the exhaust gas at the catalyst inlet. Here, 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 purifying catalyst is installed to the 20 cm internal combustion engine side. The “catalyst bed” is the center of the distance from the end surface on the exhaust gas inflow side to the end surface on the exhaust gas outflow side and the center in the vertical direction of the exhaust pipe (if the exhaust pipe is not circular, The center of gravity of the cross section in the vertical direction of the tube.

  Further, a similar or different exhaust gas purifying catalyst may be arranged at the front stage (inflow side) or the rear stage (outflow side) of the catalyst of the present invention.

  The effects of the present invention will be described using the following examples and comparative examples. However, the technical scope of the present invention is not limited only to the following examples.

Example 1
A palladium nitrate aqueous solution as a palladium raw material, magnesium hydroxide as a magnesium raw material, NO ³ -as a raw material for alumina as a coating material, and a stable pH 4 alumina sol at pH ³ to 5, and lanthanum acetate as a lanthanum raw material Each raw material was weighed so that the mass ratio of coated palladium: magnesium oxide MgO (magnesium Mg): alumina: lanthanum oxide was 0.5: 1 (0.6): 3.85: 0.4.

  Next, palladium nitrate and magnesium hydroxide were mixed, and the mixed solution was added to the alumina sol. Immediately after the addition, the alumina sol was gelled to obtain a gel in which palladium nitrate and magnesium hydroxide were coated with the alumina gel. After stirring this gel for 5 minutes, lanthanum acetate was added and further stirred for 10 minutes to obtain gel Ga in which palladium nitrate and magnesium hydroxide were coated on an alumina layer containing lanthanum acetate. The viscosity of the gel Ga at this time exceeded 5,000 cP, and gelation was confirmed.

  Next, each raw material was weighed so that the mass ratio of impregnated palladium: alumina: ceria-zirconia composite oxide: barium oxide was 2: 127.75: 50: 7 to obtain a solution Sa. The gel Ga and the solution Sa were mixed so that the mass ratio of the coated palladium in the gel Ga: palladium in the solution Sa was 0.5: 2, and wet-pulverized to form a slurry. This slurry was coated on 0.7 L of cordierite carrier, dried at 150 ° C. for 10 minutes, and then air calcined at 500 ° C. for 1 hour to obtain a catalyst of 192.5 g (Pd converted to Pd, Mg converted to MgO) Catalyst A coated with the components was obtained.

Example 2
Using the same raw materials as in Example 1, the mass ratio of coated palladium: magnesium oxide MgO (magnesium Mg): alumina: lanthanum oxide was 0.5: 0.5 (0.3): 3.85: 0.4. Each raw material was weighed so that the gel Gb was obtained by the same procedure as in Example 1.
The viscosity of the gel Gb at this time exceeded 5,000 cP, and gelation was confirmed.

  Next, each raw material was weighed so that the mass ratio of impregnated palladium: alumina: ceria-zirconia composite oxide: barium oxide was 2: 128.25: 50: 7 to obtain a solution Sa. The gel Gb and the solution Sb were mixed so that the mass ratio of the covered palladium in the gel Gb: palladium in the solution Sa was 0.5: 2, and wet-pulverized to form a slurry. This slurry was coated on 0.7 L of cordierite carrier, dried at 150 ° C. for 10 minutes, and then air calcined at 500 ° C. for 1 hour to obtain a catalyst of 192.5 g (Pd converted to Pd, Mg converted to MgO) Catalyst B coated with the components was obtained.

Comparative Example 1
Using the same raw materials as in Example 1, each raw material was weighed so that the mass ratio of coated palladium: alumina: lanthanum oxide was 0.5: 3.85: 0.4. Gel Gc was obtained by the procedure. The viscosity of the gel Gc at this time exceeded 5,000 cP, and gelation was confirmed.

  Next, each raw material was weighed so that the mass ratio of impregnated palladium: alumina: ceria-zirconia composite oxide: barium oxide was 2: 128.75: 50: 7 to obtain a solution Sc. The gel Gb and the solution Sb were mixed so that the mass ratio of the covered palladium in the gel Gc: palladium in the solution Sc was 0.5: 2, and wet-pulverized to form a slurry. This slurry was wash-coated on a 0.7 L cordierite carrier, dried at 150 ° C. for 10 minutes, and then air baked at 500 ° C. for 1 hour to coat 192.5 g (Pd is converted to Pd) of the catalyst component. Catalyst C was obtained.

Comparative Example 2
Using the same raw materials as in Example 1, each raw material was adjusted so that the mass ratio of impregnated palladium: impregnated magnesium oxide: alumina: ceria-zirconia composite oxide: barium oxide was 2.5: 1: 132: 50: 7. Were weighed to obtain a solution Sd. The solution Sd was wet pulverized to form a slurry. This slurry was coated on 0.7 L of cordierite carrier, dried at 150 ° C. for 10 minutes, and then air calcined at 500 ° C. for 1 hour to obtain a catalyst of 192.5 g (Pd converted to Pd, Mg converted to MgO) Catalyst D coated with the components was obtained.

Comparative Example 3
Using the same raw materials as in Example 1, each raw material was weighed so that the mass ratio of impregnated palladium: alumina: ceria-zirconia composite oxide: barium oxide was 2.5: 133: 50: 7, and a solution Se was prepared. Obtained. The solution Se was wet pulverized to form a slurry. This slurry was wash-coated on a 0.7 L cordierite carrier, dried at 150 ° C. for 10 minutes, and then air calcined at 500 ° C. for 1 hour to coat a catalyst component of 192.5 g (Pd is converted to Pd). Catalyst E was obtained.

  The compositions of Catalysts A and B obtained in Examples 1 and 2 and Catalysts C, D and E obtained in Comparative Examples 1 to 3 are summarized in Table 1 below.

  Here, coated palladium and coated magnesium oxide mean palladium and magnesium oxide prepared in a state of being coated with lanthanum-containing alumina. On the other hand, impregnation such as impregnated palladium and impregnated magnesium oxide means palladium and magnesium oxide supported without being covered with lanthanum-containing alumina.

<Durability treatment>
Catalysts A and B obtained in Examples 1 and 2 and Catalysts C, D and E obtained in Comparative Examples 1 to 3 were respectively 40 cm downstream from the exhaust ports of the inline 6-cylinder and 3.0 L engines. It was installed and the temperature of the catalyst bed was 980 ° C. The catalyst inlet part A / F was operated at 14.6 for 20 seconds, and then a cycle of stopping the fuel supply and operating for 5 seconds was repeated for a total of 50 hours to perform the durability treatment.

<Performance evaluation of exhaust gas purification catalyst>
Each catalyst after the endurance treatment is installed 30 cm downstream from the in-line 6-cylinder, 2.4-L engine exhaust port, the catalyst bed temperature is set to 400 ° C., and the A / F is 14.1, 14. 3, the gas discharged from the catalyst outlet was sampled while varying the A / F amplitude ± 0.5 at a frequency of 0.5 Hz, and the CO, THC, and NOx purification rates were calculated. The results are shown in Tables 2 and 3. Similarly, the catalyst bed temperature was set to 500 ° C., and A / F was measured at 14.1. The results are shown in Table 4.

  As shown in Tables 2 to 4, it can be confirmed that the catalysts A and B of Examples 1 and 2 show a higher NOx purification rate than the catalysts C, D and E of Comparative Examples 1 to 3. .

Example 3
Using the same raw materials as in Example 1, the mass ratio of coated palladium: magnesium oxide MgO (magnesium Mg): alumina: lanthanum oxide was 3: 6 (3.6): 63.1 = 1.9. Each raw material was weighed, and a gel Gf was obtained in the same procedure as in Example 1. The viscosity of the gel Gf at this time exceeded 5,000 cP, and gelation was confirmed.

  Next, each raw material was weighed so that the mass ratio of ceria-zirconia composite oxide: barium oxide was 50: 9 to obtain a solution Sf. The gel Gf and the solution Sf were mixed so that the mass ratio of the coated palladium in the gel Gf: ceria-zirconia composite oxide in the solution Sf was 3:50, and wet-pulverized to form a slurry. This slurry was wash-coated on 0.875 L cordierite carrier, dried at 150 ° C. for 10 minutes, and then air calcined at 500 ° C. for 1 hour. Coated catalyst F was obtained.

Comparative Example 4
Using the same raw materials as in Example 1, each raw material was weighed so that the mass ratio of coated palladium: alumina: lanthanum oxide was 3: 63.1: 1.9, and the same procedure as in Example 1 was performed. A gel Gg was obtained. The viscosity of the gel Gg at this time exceeded 5,000 cP, and gelation was confirmed.

  Next, each raw material was weighed so that the mass ratio of ceria-zirconia composite oxide: barium oxide was 56: 9 to obtain a solution Sg. The gel Gg and the solution Sg were mixed so that the mass ratio of the coated palladium in the gel Gg: ceria-zirconia composite oxide in the solution Sg was 3:56, and wet-pulverized to form a slurry. This slurry was washed with 0.875 L of cordierite carrier, dried at 150 ° C. for 10 minutes, and then air calcined at 500 ° C. for 1 hour to obtain 133 g (Pd is converted to Pd, Mg is converted to MgO) Catalyst G coated with

<Durability treatment>
The catalyst F of Example 3 and the catalyst G of Comparative Example 4 were installed 40 cm downstream from the exhaust ports of the inline 6-cylinder and 3.0 L engines, respectively, and the temperature of the catalyst bed was 980 ° C. The catalyst inlet part A / F was operated at 14.6 for 20 seconds, and then a cycle of stopping the fuel supply and operating for 5 seconds was repeated for a total of 50 hours to perform the durability treatment.

<Performance evaluation of exhaust gas purification catalyst>
Each catalyst after the endurance treatment is installed 30 cm downstream from the inline 6 cylinder, 2.4 L engine exhaust port, the catalyst bed temperature is set to 400 ° C., and the A / F is 14.1, 14.3, The gas discharged from the catalyst outlet was sampled while varying the A / F amplitude ± 0.5 at 14.5 and the frequency 0.5 Hz, and the purification rates of CO, THC, and NOx were calculated. The results are shown in Table 5. Similarly, the catalyst bed temperature was set to 500 ° C., and A / F was measured at 14.1, 14.3, 14.5 (A / F amplitude ± 0.5). The results are shown in Table 6. Further, similarly, the catalyst bed temperature was set to 500 ° C., and A / F was measured at 14.1, 14.3, 14.5 (A / F amplitude ± 1.0). The results are shown in Table 7.

  As shown in Tables 5 to 7, at 400 ° C. and 500 ° C., the catalyst of the example was compared with the catalyst of the comparative example under the fuel excess condition of A / F value of 14.1 to 14.5. High purification performance is confirmed.

<TEM-EDS analysis>
Only the catalytically active component of each catalyst after performance evaluation of the durability treatment and exhaust gas purification catalyst was peeled off from the cordierite carrier, and TEM-EDS analysis was performed. As shown in FIG. 4 and FIG. 5, the catalysts A and B in the examples were detected in the same particle as Pd and Mg, and were supported in close proximity as a composite. Moreover, when the EDS spectrum intensity was plotted with respect to the distance on the white dotted line, the results of the line analysis performed on the white dotted line region with respect to Pd and Mg, the catalysts A and B of the examples are shown in FIGS. As shown in FIG. 5, since the intensity distributions of the Pd particles and the Mg particles are similar, it can be seen that a composite is formed.

  On the other hand, as shown in FIG. 6, in the catalyst D of the comparative example, Mg is detected regardless of Pd, indicating that Pd and Mg do not form a complex.

  Furthermore, since Al and La are detected so as to cover the composite particles, it is indicated that the composite is covered with lanthanum-containing alumina. In addition, the gel is pulverized by drying and firing, and the cordierite carrier is coated with the slurry containing the powder, and the slurry containing the gel is directly coated instead of performing the second drying and firing. It is shown that Pd / Pd oxide and Mg / Mg oxide can be coated with lanthanum-containing alumina even in one firing process.

  The catalyst of the present invention can efficiently purify exhaust gas from an internal combustion engine even at high temperatures. Therefore, it can be widely applied to all industries using an internal combustion engine, for example, automobiles, railways, ships, aircrafts, various industrial machines and the like.

1 ... lanthanum-containing alumina,
2 ... complex of palladium or palladium oxide and magnesium or magnesium oxide,
3 ... Palladium or palladium oxide 4 ... Magnesium or magnesium oxide.

Claims (9)

Complexes of palladium or palladium oxide and magnesium also containing magnesium oxide (a), arranged in at least a portion Gala lanthanum-containing alumina (b) Ru exhaust gas purification name is covered with the said complex (a) Catalyst. The catalyst according to claim 1, wherein the composite has a Pd (Pd equivalent) / Mg (Mg equivalent) mass ratio of 10/1 to 1/5. The mass ratio of Pd to lanthanum-containing alumina in the composite is 4: 1 to 1: 100, and the mass ratio of Mg to lanthanum-containing alumina is 10: 1 to 1: 5. The catalyst according to 1. The content of lanthanum (in terms of La2O3) in the lanthanum-containing alumina is lanthanum (La
The catalyst according to any one of claims 1 to 3, which is 0.2 to 30% by mass relative to the total amount of 2O3 and alumina (Al2O3). The exhaust gas purification catalyst according to any one of claims 1 to 4, further comprising cerium oxide or a ceria-zirconia composite oxide. An exhaust gas purifying catalyst comprising the active component of the catalyst according to any one of claims 1 to 5 supported on a fire-resistant three-dimensional structure. At least a portion of the complex is a palladium or palladium oxide and magnesium also comprising magnesium oxide, having to obtain a complex coated with lanthanum-containing alumina, according to any one of claims 1 to 6 A method for producing an exhaust gas purifying catalyst. A gel containing a composite coated with the lanthanum-containing alumina is obtained, the gel is further made into a slurry, the slurry is washcoated on the three-dimensional structure, and the three-dimensional structure on which the slurry is coated is dried and fired. The manufacturing method according to claim 7, further comprising: Exhaust gas from an internal combustion engine using the exhaust gas purification catalyst according to any one of claims 1 to 5 or the exhaust gas purification catalyst produced by the method according to any one of claims 6 to 8. A method for purifying hydrocarbons or nitrogen oxides, comprising purifying nitrogen oxides and carbon monoxide in exhaust gases having an air-fuel ratio of less than 14.7.