JP5162404B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

Description

  The present invention relates to an exhaust gas purifying catalyst and a method for producing the same.

Conventionally, porous metals have been used in the field of exhaust gas purification catalysts. As such a porous metal, Japanese Unexamined Patent Application Publication No. 2007-63598 (Patent Document 1) discloses a porous metal thin film including a hole in which one or more holes are further formed inside the hole. Yes. Further, in Japanese Patent Application Laid-Open No. 2004-143497 (Patent Document 2), a porous material having a pore size of 1 μm or less formed by aggregating a plurality of metal fine particles having a primary particle size of 200 nm or less and fusing a contact portion. Metal particles are disclosed. Furthermore, in Japanese Patent Application Laid-Open No. 2006-22367 (Patent Document 3), a hydrophilic polymer having a linear polyethyleneimine skeleton and an aqueous solvent are mixed, the mixture is heated and then cooled, or linear A step (I) of obtaining a hydrophilic polymer hydrogel by mixing a hydrophilic polymer having a polyethylene imine skeleton and an organic solvent, and adding water to the mixed solution; and an aqueous solvent solution of the hydrogel and metal ions And step (II) of obtaining a complex of the hydrophilic polymer and the metal by spontaneously reducing metal ions or reducing with a reducing agent, and washing the complex with a water-soluble organic solvent And the manufacturing method of the porous metal which has the process (III) and sintering, and the porous metal obtained by implementing the method are disclosed. However, conventional porous metals as described in Patent Documents 1 to 3 do not have sufficient catalytic activity when used as exhaust gas purifying catalysts.
JP 2007-63598 A JP 2004-143497 A JP 2006-22367 A

  The present invention has been made in view of the above-described problems of the prior art, and aims to provide an exhaust gas purifying catalyst having excellent CO oxidation performance and sufficiently high catalytic activity, and a method for producing the catalyst. And

  As a result of intensive studies to achieve the above object, the present inventors obtained an alloy (preferably an intermetallic compound) containing a specific metal and Al, and then obtained the alloy (preferably an intermetallic compound). It is found that a porous metal having a pore with a central pore diameter of 1 to 15 nm can be obtained by eluting Al from the catalyst, and an exhaust gas purifying catalyst comprising the porous metal having such a central pore diameter Was found to have a sufficiently high catalytic activity, and the present invention was completed.

That is, the exhaust gas purifying catalyst of the present invention comprises a porous metal having pores with a central pore diameter of 1 to 15 nm , and
The porous metal contains Au and Fe .

Furthermore, in the exhaust gas purifying catalyst of the present invention, the porous metal preferably has a specific surface area of 1 to 30 m ² / g.

In the method of manufacturing the exhaust gas purifying catalyst of the present invention, a dissolved Au and Al and Fe, to obtain an alloy containing said Au, Al and Fe step,
Elution of Al from the alloy to obtain a porous metal having pores with a center pore diameter of 1 to 15 nm;
It is the method characterized by including.

  In the method for producing an exhaust gas purifying catalyst of the present invention, it is preferable that 70 at% or more of the total Al in the alloy is eluted from the alloy in the step of eluting Al from the alloy.

  Furthermore, in the method for producing an exhaust gas purifying catalyst of the present invention, the alloy is preferably an intermetallic compound.

  The reason why the above object is achieved by the exhaust gas purifying catalyst and the exhaust gas purifying catalyst manufacturing method of the present invention is not necessarily clear, but the present inventors speculate as follows. That is, in the method for producing an exhaust gas purifying catalyst of the present invention, an alloy containing at least one metal selected from the group consisting of Ru, Os, Rh, Ir, Pd, Pt, Ag, and Au and Al (preferably After obtaining an intermetallic compound, the alloy (preferably intermetallic compound) is subjected to a so-called leaching treatment to elute Al from the alloy (preferably intermetallic compound). Thus, by eluting Al from the alloy (preferably an intermetallic compound), the remaining metal becomes a porous metal. In addition, the central pore diameter of the porous metal obtained by such leaching treatment is such that the central pore diameter of the conventional porous metal as described in Patent Documents 1 to 3 is either micrometer or submicrometer. Although it is a scale, it becomes a very fine size of 1 nm to 15 nm. Furthermore, by eluting Al in this way, it is presumed that various crystal planes are exposed on the surface of the porous metal and so-called steps and kinks are formed in large quantities. Since such steps and kinks are considered to be energetically unstable and more likely to adsorb molecules than terraces, it is assumed that the resulting porous metal has a sufficiently high catalytic activity. That is, the porous metal according to the present invention has sufficiently fine pores as compared with the conventional porous metal, and its surface state becomes irregular, so that it is sufficiently high to easily adsorb molecules. It is presumed that the catalyst activity can be exhibited. Further, in the porous metal according to the present invention, since a large number of steps and kinks are formed as described above, those that have been considered to be metals (inert metals) that do not exhibit sufficient catalytic activity ( For example, Au) can also be used as a catalytically active species. Furthermore, since the porous metal according to the present invention exhibits sufficient catalytic activity even by itself, it is not always necessary to use a carrier (for example, an inorganic oxide) that has been considered to be essential for an exhaust gas purification catalyst. This eliminates the need for latent heat of vaporization of the adsorbed water during use, and it is assumed that the catalytic activity at low temperatures is sufficiently improved. Therefore, it is presumed that the exhaust gas purifying catalyst comprising such a porous metal exhibits a sufficiently high catalytic activity.

  According to the present invention, it is possible to provide an exhaust gas purifying catalyst having excellent CO oxidation performance and sufficiently high catalytic activity, and a method for producing the catalyst.

  Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.

  First, the exhaust gas purifying catalyst of the present invention will be described. That is, the exhaust gas purifying catalyst of the present invention is characterized by comprising a porous metal having pores with a central pore diameter of 1 to 15 nm.

  The “central pore diameter” as used herein is a curve (pore diameter) in which the value (dV / dD) obtained by differentiating the pore volume (V) with respect to the pore diameter (D) is plotted against the pore diameter (D). This is the pore diameter at the maximum peak of the distribution curve) and can be determined by the method described below. That is, first, the porous metal is cooled to a liquid nitrogen temperature (−196 ° C.), nitrogen gas is introduced, and the amount of adsorption is determined. Next, the pressure of nitrogen gas to be introduced is gradually increased, and the amount of nitrogen gas adsorbed with respect to each equilibrium pressure is plotted to obtain an adsorption isotherm. Then, the pore diameter distribution curve is obtained using the adsorption isotherm. The adsorption amount of the nitrogen gas can be determined by a constant volume method or a weight method, but in the present invention, it is determined by adopting the BET method. Moreover, as a method for obtaining the pore size distribution curve from the adsorption isotherm, a calculation method such as Cranston-Inklay method, Pollimore-Heal method, BJH method can be used. In the present invention, the BJH method is used. adopt.

  The central pore diameter of such a porous metal needs to be 1 to 15 nm, more preferably 2 to 12 nm, and particularly preferably 3 to 10 nm. When the center pore diameter of such a porous metal is less than the lower limit, its production tends to be difficult, and when it exceeds the upper limit, the catalytic activity tends to be insufficient.

  Moreover, as such a porous metal, from the viewpoint of achieving both activity and durability, at least one metal selected from the group consisting of Ru, Os, Rh, Ir, Pd, Pt, Ag, and Au ( Hereinafter, it is preferable to contain “metal (A)”. Among such metals (A), Rh, Ir, Pd, Pt, and Au are more preferable, and Rh, Pd, Pt, and Au are particularly preferable. Further, such a porous metal may be composed of a single element of the metal (A) or may contain two or more of the metal (A). It may contain one or more of the metal (A) and one or more of other metals. Moreover, as such a porous metal, it is preferable to consist of a metal (A) simple substance or an alloy containing the metal (A). A porous metal containing such a metal (A) tends to exhibit a sufficiently high catalytic activity.

  Further, the porous metal contains the metal (A) from the viewpoint of improving the catalytic activity itself, and at least one metal selected from the group consisting of Fe, Cu, Co, and Ni (hereinafter, “ It is preferable to further contain a metal (B) ”. In such a porous metal, among the metals (B), from the viewpoint of catalytic activity, Fe, Cu, and Co are more preferable, Fe and Cu are further preferable, and Fe is particularly preferable.

  Moreover, as a content rate of the said metal (A) in the said porous metal, or a content rate of the total amount of a metal (A) and a metal (B), it is 60-100 mass% with respect to the total mass of a porous metal. It is preferable that it is 70-100 mass%. When the content ratio is less than the lower limit, sufficient catalytic activity tends to be not obtained.

  Further, when the porous metal contains the metal (A) and the metal (B), the content ratio of the metal (A) and the metal (B) is a molar ratio (metal (A): metal. (B)) is preferably 5: 1 to 1: 5. When the content ratio of such a metal (A) is less than the lower limit, the activity tends to be insufficient. On the other hand, when the content exceeds the upper limit, the binary effect tends to be lost.

  Further, the shape of such a porous metal is not particularly limited, and may be a film or a powder, but is preferably a powder from the viewpoint of obtaining a sufficient specific surface area. Moreover, it is preferable that it is 100-200 micrometers as an average primary particle diameter of such a particulate porous metal. If the particle size is less than the lower limit, the production tends to be difficult. On the other hand, if the particle size exceeds the upper limit, the specific surface area tends to decrease and the catalyst performance and the like tend to decrease.

The specific surface area of the porous metal is not particularly limited, is preferably from 1-30 m ² / g, more preferably 3~30m ² / g. When the specific surface area is less than the lower limit, sufficient catalytic activity tends to be not obtained. On the other hand, when the upper limit is exceeded, the structure of the porous metal may be collapsed.

  In addition, the exhaust gas purifying catalyst of the present invention may be provided with the porous metal, and may be composed of the porous metal alone or a mixture with other catalysts. Further, in such an exhaust gas purifying catalyst, other components (for example, NOx occlusion material) that can be used for the exhaust gas purifying catalyst within a range not impairing the effects of the present invention are appropriately supported on the porous metal. May be. Moreover, as a method for producing the exhaust gas purifying catalyst of the present invention, the method for producing an exhaust gas purifying catalyst of the present invention described later can be suitably employed.

  Further, the form of the exhaust gas purifying catalyst of the present invention is not particularly limited, and may be a honeycomb-shaped monolith catalyst, a pellet-shaped pellet catalyst, or the like. The method for producing the exhaust gas purifying catalyst in such a form is not particularly limited, and a known method can be appropriately employed. For example, the exhaust gas purifying catalyst is formed by molding the porous metal powder into a pellet shape. And a method of obtaining a catalyst for exhaust gas purification by coating a catalyst substrate with a slurry containing the porous metal powder. Further, such a catalyst base is not particularly limited and is appropriately selected depending on the use of the obtained exhaust gas purification catalyst, etc., but is not limited to a DPF base, a monolithic base, a pellet base, a plate base. A material or the like is preferably employed. The material of such a catalyst substrate is not particularly limited, but a substrate made of a ceramic such as cordierite, silicon carbide, mullite, or a substrate made of a metal such as stainless steel including chromium and aluminum is preferable. Adopted.

  Next, the manufacturing method of the exhaust gas purifying catalyst of the present invention will be described.

The method for producing an exhaust gas purifying catalyst of the present invention comprises dissolving at least one metal selected from the group consisting of Ru, Os, Rh, Ir, Pd, Pt, Ag, and Au with Al. A step (first step) of obtaining an alloy (preferably an intermetallic compound) containing
A step of eluting Al from the alloy (preferably an intermetallic compound) to obtain a porous metal having pores having a central pore diameter of 1 to 15 nm (second step);
It is the method characterized by including. Hereafter, it demonstrates for every process.

  First, the first step will be described. In the first step, at least one metal selected from the group consisting of Ru, Os, Rh, Ir, Pd, Pt, Ag, and Au (hereinafter referred to as “metal (A)”) and Al are dissolved, In this step, an alloy (preferably an intermetallic compound) containing the metal and Al is obtained.

  The metal suitable as such a metal (A) is the same as that described for the metal (A) used for the exhaust gas purifying catalyst of the present invention. By using such a metal (A) as one of the raw material metals, it becomes possible to produce a porous metal containing the metal (A) that is preferably used in the above-described exhaust gas purifying catalyst of the present invention. .

  In the present invention, Al is used as one of the essential raw materials for producing an alloy (preferably an intermetallic compound). By using such Al, it is possible to produce a porous metal having a sufficiently fine center pore diameter when the obtained alloy (preferably intermetallic compound) is subjected to a leaching treatment. Become.

  In such a melting step, other metals may be contained in addition to the metal (A) and Al according to the design of the target porous metal. As such other metals, from the viewpoint of the catalytic activity of the finally obtained catalyst, at least one metal selected from the group consisting of Fe, Cu, Co and Ni (hereinafter referred to as “metal (B)”) .) Is preferred. That is, in the present invention, it is preferable to dissolve the metal (B) together with the metal (A) and Al. The kind of metal suitable as such a metal (B) is the same as what was demonstrated in the metal (B) used for the exhaust gas purification catalyst of the above-mentioned this invention. By using such a metal (B) as one of the raw metals together with the metal (A) and Al, the metal (A) suitable as a porous metal used in the exhaust gas purifying catalyst of the present invention described above. And a porous metal containing the metal (B) can be produced.

  Moreover, the shape of the metal (metal (A), metal (B), Al) used as such a raw material metal is not particularly limited, but is preferably in the form of powder from the viewpoint of easy handling. The particle diameter of the metal used as such raw material metal is not particularly limited.

  Furthermore, the method for dissolving such a raw material metal is not particularly limited as long as it is a method capable of dissolving these metals, and a known method can be appropriately employed. For example, an arc melting furnace You may employ | adopt the arc melting method using. Further, in such a melting step, the various metals of the raw metal may be dissolved separately, or the various metals of the raw metal may be simultaneously dissolved and mixed in the same furnace. In addition, in the case where the various metals of the raw metal are separately dissolved, in order to obtain an alloy containing the various metals of the raw metal (preferably an intermetallic compound), the melt of the various metals remains in a dissolved state. Need to mix.

  Furthermore, after melting the raw material metal, an alloy (preferably an intermetallic compound) containing the metal and Al can be obtained by solidifying the molten mixture of the various metals of the raw material metal. In addition, the method for such solidification is not particularly limited, and a known method can be appropriately used. The alloy (preferably an intermetallic compound) thus obtained is preferably in a state in which various elements of a metal used as a raw metal are mixed at an atomic level. The state of such an alloy (preferably an intermetallic compound) can be confirmed by determining whether or not a diffraction line belonging to a single compound is shown by XRD measurement.

  In the case where the metal (B) is not included in the alloy (preferably intermetallic compound), the ratio of the metal (A) and Al in the alloy (preferably intermetallic compound) is a molar ratio ( Metal (A): Al) is preferably 2: 3 to 1: 5, more preferably 1: 2 to 1: 3. When the content ratio of the metal (A) is less than the lower limit, it is difficult to elute Al. On the other hand, when the content exceeds the upper limit, it is difficult to form a stable alloy (preferably an intermetallic compound). Tend to be.

  In the case where the metal (B) is contained in the alloy (preferably an intermetallic compound), the ratio of the total amount of the metal (A) and metal (B) in the alloy to Al is the metal (A ) And (B) and the design of the intended porous metal, etc., and cannot be generally stated, but the molar ratio (total molar amount of metals (A) and (B): molar amount of Al ) Is preferably 1: 5 to 2: 3, more preferably 1: 2 to 1: 3. Further, the ratio of the metal (A) to the metal (B) in the alloy (preferably intermetallic compound) depends on the type of the metal (A) and (B) to be used, the design of the intended porous metal, etc. Although it is different and cannot be generally stated, the molar ratio (metal (A): metal (B)) is preferably 5: 1 to 1: 5, more preferably 4: 1 to 1: 4. preferable. When the content ratio of Al is less than the lower limit, elution of Al at the time of leaching tends to be difficult. On the other hand, when the upper limit is exceeded, it is difficult to form a stable alloy (preferably an intermetallic compound). Tend to be. Further, when the content ratio of the metal (B) is less than the lower limit, it tends to be difficult to obtain a sufficient binarization effect, and when the content exceeds the upper limit, the binarization effect tends to be lost. .

  In addition, the shape of such an alloy (preferably an intermetallic compound) is not particularly limited, and may be various shapes such as a film shape, a rod shape, and a powder shape. In addition, the method in particular which shape | molds in such a shape is not restrict | limited, A well-known method can be employ | adopted suitably, For example, after melt | dissolving raw material metal, after obtaining an ingot, this is grind | pulverized and is made into a powder form May be adopted. Further, from the viewpoint of obtaining a more uniform composition, it is preferable to anneal the alloy (preferably an intermetallic compound) after the solidification. The conditions for such annealing treatment are not particularly limited, and known conditions employed in known annealing methods can be appropriately employed.

  Next, the second step will be described. The second step is a step of eluting Al from the alloy (preferably an intermetallic compound) to obtain a porous metal having pores with a central pore diameter of 1 to 15 nm.

  The method for eluting Al from the alloy (preferably intermetallic compound) is not particularly limited as long as it is a method capable of eluting Al from the alloy (preferably intermetallic compound). Alternatively, a method (acid / base leaching treatment) in which a base is brought into contact with the alloy (preferably an intermetallic compound) is preferable. Such an acid is not particularly limited, but nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid and carboxylic acids are preferred from the viewpoint of sufficiently dissolving Al and being relatively easy to handle, and nitric acid, sulfuric acid and hydrochloric acid are more preferred. preferable. The base is not particularly limited, but from the viewpoint of sufficiently dissolving Al and being relatively easy to handle, ammonia, an alkali metal hydroxide, and an alkaline earth metal hydroxide are preferable. Metal hydroxides are more preferred. Further, when such an acid or base is brought into contact with the alloy (preferably an intermetallic compound), the acid or base may be brought into contact as an aqueous solution from the viewpoint of rapid and easy reaction with Al. preferable. In addition, when eluting Al in this way, Al does not simply elute, but other metals (metals that finally form a porous metal) in the alloy (preferably intermetallic compound) are also present. Although it elutes, it is speculated that such other metals recrystallize very early to form part of the porous metal.

  In the step of eluting Al from the alloy (preferably intermetallic compound), 70 at% or more (more preferably 75 at%) of all Al atoms in the alloy (preferably intermetallic compound) from the alloy (preferably intermetallic compound). % Or more, more preferably 80 at% or more). If such an elution amount of Al is less than the lower limit, a metal having sufficient porosity tends not to be formed. In addition, as a method for eluting 70 at% or more of Al in the alloy (preferably intermetallic compound) from the alloy (preferably intermetallic compound), the alloy (preferably intermetallic compound) is used. Since it varies depending on the type, the type of acid / base used, its concentration, etc., it is not a general rule and is not particularly limited, and Al is eluted from the alloy (preferably an intermetallic compound) by 70 at% or more. Various elution methods can be appropriately employed by appropriately adjusting the elution conditions such as the type and concentration of acid / base, elution time, etc. When leaching treatment is adopted as the elution method, for example, the amount of total Al in the alloy (preferably intermetallic compound) is obtained from the charged amount, and the amount of Al eluted in the solution used for the leaching treatment is calculated. A method of performing a leaching process until it is confirmed to be 70 at% or more of the total Al in the alloy (preferably an intermetallic compound) may be adopted. A method of confirming the amount of Al and performing a leaching process until the amount of remaining Al becomes 30 at% or less of the total amount of Al in the alloy (preferably intermetallic compound) before leaching may be adopted.

  Further, by eluting Al from the alloy (preferably an intermetallic compound) as described above, a porous metal having pores with a central pore diameter of 1 to 15 nm can be obtained. The porous metal thus obtained may be appropriately subjected to washing treatment or drying treatment after elution of Al from the alloy (preferably intermetallic compound). And the exhaust gas purification catalyst can be obtained by using the porous metal obtained in this way. The exhaust gas purifying catalyst thus obtained is the same as that described in the exhaust gas purifying catalyst of the present invention.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

( Reference Example 1)
<Synthesis process of intermetallic compound>
First, Au (purity: 99.95% by mass) and Al (purity: 99.9% by mass) were weighed and used for each 3.92 g (Au) and 1.08 g (Al), and these were used in an arc melting furnace apparatus. (GES Co., Ltd., trade name “GMAC-1100”) was introduced into a dissolution chamber, dissolved in the dissolution chamber under the conditions of Ar pressure: 500 Torr, arc electrode current: 120 A, and then allowed to cool naturally. An ingot of an intermetallic compound containing Au and Al was produced by solidification. Next, the ingot was vacuum sealed (1 × 10 ⁻⁵ Torr) in a quartz reaction vessel, and the ingot was annealed (800 ° C., 100 hours). Next, the ingot of the intermetallic compound after the annealing treatment is pulverized to be finely powdered, and finally fined to 200 to 100 μm using a sieve to obtain a finely powdered intermetallic compound (composition: Al ₂ Au). It was. In addition, when the intermetallic compound obtained in this way was measured by XRD, it was confirmed that the intermetallic compound was in another alloy state in which Au and Al were mixed at the atomic level and neither Al nor Au. It was.

<Al selective leaching step>
First, 5 g of finely powdered intermetallic compound obtained as described above was added to 250 g of 10% NaOH aqueous solution and stirred (room temperature, 4 hours) to elute Al from the intermetallic compound. Next, the intermetallic compound residue was taken out of the aqueous solution, washed with water and dried (50 ° C., 12 hours) to produce a porous metal containing Au, which was used as it was as an exhaust gas purification catalyst. The elution amount of Al was 99 at% with respect to the total amount of Al in the intermetallic compound.

( Reference Example 2)
In the selective elution step of Al, an exhaust gas purifying catalyst made of a porous metal was prepared in the same manner as in Reference Example 1 except that 10% HNO ₃ was used instead of the 10% NaOH aqueous solution and the stirring time was changed to 12 hours. Obtained. The elution amount of Al was 70.8 at% with respect to the total amount of Al in the intermetallic compound.

( Reference Example 3)
Exhaust gas purifying catalyst made of porous metal was obtained in the same manner as in Reference Example 1 except that 10% HCl was used instead of 10% NaOH aqueous solution in the selective elution step of Al and the stirring time was changed to 4 hours. It was. The elution amount of Al was 100 at% with respect to the total amount of Al in the intermetallic compound.

(Example 1 )
In the intermetallic compound synthesis step, Au (purity: 99.95% by mass), Al (purity: 99.9% by mass), and Fe (purity: 99.9% by mass) each 2.63 g (Au), 1. Exhaust gas purification composed of porous metal in the same manner as in Reference Example 1 except that 1.66 g (Al) and 0.74 g (Fe) were weighed and the composition of the intermetallic compound was Al ₇₀ Au ₁₅ Fe ₁₅ A catalyst was obtained. The elution amount of Al was 100 at% with respect to the total amount of Al in the intermetallic compound.

(Example 2 )
In the intermetallic compound synthesis step, Au (purity: 99.95% by mass), Al (purity: 99.9% by mass), and Fe (purity: 99.9% by mass) are each 3.08 g (Au), 1. For exhaust gas purification comprising porous metal in the same manner as in Reference Example 1 except that 1.48 g (Al) and 0.44 g (Fe) were weighed and the composition of the intermetallic compound was Al ₇₀ Au ₂₀ Fe ₁₀ . A catalyst was obtained. The elution amount of Al was 100 at% with respect to the total amount of Al in the intermetallic compound.

( Reference Example 4 )
In the intermetallic compound synthesis step, Au (purity: 99.95% by mass), Al (purity: 99.9% by mass), and Co (purity: 99.9% by mass) each 2.61 g (Au), 1.62 g (Al), 0.78 g (Co) Weighed and used for purification of exhaust gas made of porous metal in the same manner as in Reference Example 1 except that the composition of the intermetallic compound was Al ₇₀ Au ₁₅ Co ₁₅ A catalyst was obtained. The elution amount of Al was 100 at% with respect to the total amount of Al in the intermetallic compound.

( Reference Example 5 )
In the intermetallic compound synthesis step, Au (purity: 99.95% by mass), Al (purity: 99.9% by mass), and Cu (purity: 99.9% by mass) are each 2.67 g (Au), 1. For exhaust gas purification made of porous metal in the same manner as in Reference Example 1 except that 1.48 g (Al) and 0.88 g (Cu) were weighed and the composition of the intermetallic compound was Al ₇₀ Au ₁₅ Cu ₁₅ A catalyst was obtained. The elution amount of Al was 97.7 at% with respect to the total amount of Al in the intermetallic compound.

( Reference Example 6 )
In the intermetallic compound synthesis step, Au (purity: 99.95% by mass), Al (purity: 99.9% by mass), and Ni (purity: 99.9% by mass) were each 2.81 g (Au), 1. Exhaust gas purification made of porous metal in the same manner as in Reference Example 1 except that 1.36 g (Al) and 0.86 g (Ni) were weighed and the composition of the intermetallic compound was Al ₇₀ Au ₁₅ Ni ₁₅ A catalyst was obtained. The elution amount of Al was 71.8 at% with respect to the total amount of Al in the intermetallic compound.

(Comparative Example 1)
A fine powdery intermetallic compound (composition: Al ₂ Au) was produced in the same manner as in Reference Example 1 except that the selective elution step of Al was not performed, and the intermetallic compound was made porous for comparison as it was. This was used as a metal and used as an exhaust gas purification catalyst for comparison.

(Comparative Example 2)
After weighing 5.25 g of Au (purity: 99.95% by mass), this was molded into a ribbon using a single-roll liquid quenching device (trade name “RQM-T-20” manufactured by Makabe Giken Co., Ltd.). A porous metal for comparison made of Au in the form of a ribbon was manufactured, and the porous metal was used as an exhaust gas purification catalyst for comparison.

(Comparative Example 3)
A Na ₂ CO ₃ aqueous solution is dropped into a mixed aqueous solution of a predetermined amount of HAuCl ₄ and a predetermined amount of Fe (NO ₃ ) ₃ to form a coprecipitate (coprecipitation method), and the obtained precipitate is washed with water, After drying, firing in the air (400 ° C., 5 hours) produced an exhaust gas purification catalyst for comparison consisting of 5 mass% Au and Fe ₂ O ₃ .

(Comparative Example 4)
In the step of synthesizing the intermetallic compound, Al (purity: 99.9% by mass) and Fe (purity: 99.9% by mass) were weighed and used at 2.46 g (Al) and 2.54 g (Fe), respectively. A porous metal for comparison was produced in the same manner as in Reference Example 1 except that the composition of the intermetallic compound was Al ₂ Fe, and the porous metal was directly used as an exhaust gas purification catalyst for comparison. The elution amount of Al was 77.9 at% with respect to the total amount of Al in the intermetallic compound.

[Characteristic Evaluation of Catalysts Obtained in Examples 1-2, Reference Examples 1-6, and Comparative Examples 1-4]
<Measurement of central pore diameter>
The center pore diameter of each porous metal obtained in Examples 1-2, Reference Examples 1-6 and Comparative Examples 1-4 was measured. In such measurement, first, each porous metal is cooled to a liquid nitrogen temperature (−196 ° C.) using a gas adsorption method pore distribution measuring machine (BELSORP-mini) manufactured by Nippon Bell Co., Ltd. The adsorption isotherm was measured by introducing nitrogen gas. Next, the pore distribution curve of each porous metal was determined by the BJH method using the adsorption isotherm. And the center pore diameter of each porous metal was calculated | required from the obtained pore distribution curve, respectively. The results are shown in Table 1.

<Measurement of specific surface area>
The specific surface area of each porous metal obtained in Examples 1-2, Reference Examples 1-6 and Comparative Examples 1-4 was measured. For measurement of such a specific surface area, a gas adsorption method pore distribution measuring machine (BELSORP-mini) manufactured by Nippon Bell Co., Ltd. was used. The specific surface area was measured by N ₂ adsorption (BET 1-point method). The results are shown in Table 1.

<Measurement with a scanning electron microscope (SEM)>
Each porous metal obtained in Reference Examples 1 to 3 and Comparative Example 1 was observed with a scanning electron microscope (SEM). Scanning electron microscope (SEM) photographs of each porous metal are shown in FIG. 1 ( Reference Example 1), FIG. 2 ( Reference Example 2), FIG. 3 ( Reference Example 3), and FIG. 4 (Comparative Example 1), respectively. .

<Catalyst activity evaluation test>
Using each of the exhaust gas purifying catalysts obtained in Examples 1 and 2, Reference Examples 1 to 6 and Comparative Examples 1 to 4, a test for evaluating the oxidation activity of CO was performed. In such a test, first, a sample in which 50 mg of each exhaust gas purifying catalyst was filled in a quartz reaction tube (inner diameter: 4 mm) was manufactured, and then the sample was placed in an atmospheric pressure fixed bed flow reactor. installed. Next, components contained in the outgas after contacting the exhaust gas purification catalyst with an incoming gas composed of CO (4% by volume), O ₂ (2% by volume), and He (remainder). Was measured. In addition, the flow conditions of the entered gas were 50 ml / min and SV = 120,000 h ⁻¹ . In addition, the components in the outgas include online gas chromatography [detector: TCD (trade name “GC-8A” manufactured by Shimadzu), column: MS 5A (CO, O ₂ ), and Porapak Q (CO ₂ )]. Was analyzed. The CO conversion rate by the CO oxidation reaction (CO + 1 / 2O ₂ = CO ₂ ) is expressed by the following formula (1):
[CO conversion rate (%)] = {([CO concentration in input gas] − [CO concentration in output gas]) / [CO concentration in input gas]} × 100 (1)
It was obtained by calculating. In addition, this test is performed several times for each catalyst, and the temperature condition during the test is 60 ° C, 80 ° C, 100 ° C, 150 ° C, 200 ° C, 250 ° C, 300 ° C, 350 ° C for each time. Either temperature was set. The CO conversion rate of each catalyst at each temperature measured in this way is shown in FIGS.

From the SEM photograph (FIG. 1) of the porous metal obtained in Table 1 and Reference Example 1 and the SEM photograph (FIG. 4) of the porous metal obtained in Comparative Example 1, the intermetallic compound was subjected to leaching treatment. It was found that sufficiently fine pores were formed. Further, as is apparent from the results shown in Table 1, the case of using a binary intermetallic compound ( Reference Examples 1 to 3) and the case of using a ternary intermetallic compound (Examples 1 to 2). In both of Reference Examples 4 to 6 ), the pore diameter of the porous metal is in the range of 1 to 15 nm, and it was confirmed that the pores exist in the meso region. That is, from the results shown in Table 1 and FIGS. 1 to 4, in the porous metals obtained in Examples 1 and 2 and Reference Examples 1 to 6 , the intermetallic compound was leached with an aqueous solution of an acid or a base. It was confirmed that the center pore diameter of the pores was in the range of 1 to 15 nm, and the pores were present in the meso region. Also, from these results, fine pores can be formed (porous) by eluting Al from an intermetallic compound containing Al and a specific metal (so-called leaching treatment). It was found that the center pore diameter of the pores of the obtained porous metal can be controlled in the region of 15 nm or less.

  On the other hand, as is clear from the results shown in Table 1, in the porous metal for comparison obtained in Comparative Examples 1 to 4, the measured central pore diameters are all on the order of μm, It was found that fine pores on the order of nm were not formed. In general, the metal on the metal-supported oxide catalyst prepared by impregnation or coprecipitation is known to be fine particles but not to have a pore structure. In the porous metal, the measured central pore diameter is not derived from Au, but derived from iron oxide. That is, the central pore diameter of the porous metal obtained in Comparative Example 2 corresponds to the central pore diameter of iron oxide in the porous metal.

Further, as is apparent from the results shown in FIG. 5, the exhaust gas purifying catalysts (systems not containing Fe, Cu, Co and Ni) obtained in Reference Examples 1 to 3 and Comparative Examples 1 and 2 were used. When the activities were compared, it was confirmed that the porous metals obtained in Reference Examples 1 to 3 had a higher CO conversion rate than the porous metals obtained in Comparative Examples 1 and 2. Further, as is clear from the results shown in FIG. 6, when the catalytic activities of the exhaust gas purifying catalysts (systems containing Fe) obtained in Examples 1-2 and Comparative Examples 3-4 are compared, Example 1 is compared. porous metal obtained in ~ 2, rather than porous metals obtained in Comparative example 3 to 4, CO conversion is high, it was confirmed that a sufficient catalytic activity, especially from a low temperature. Further, in a system containing Au and Fe, a catalyst (Examples 1 to 2 ) having a porous metal having a pore having a central pore diameter of 1 to 15 nm and containing Au and Fe. It can be seen that the catalyst activity is higher than that of the catalyst in which Au is supported on the Fe ₂ O ₃ support (Comparative Example 3), and the difference becomes more remarkable as the temperature becomes lower. From these results, it was confirmed that a porous metal having a sufficiently high catalytic activity can be produced by applying a leaching treatment to make it porous.

Further, as is clear from the results shown in FIGS. 5 to 7, the exhaust gas purifying catalysts (Au formed a porous metal formed in Examples 1-2, Reference Examples 1-6 and Comparative Example 2). The exhaust gas purification catalysts obtained in Examples 1-2 and Reference Examples 1-6 show a sufficiently higher catalytic activity than the exhaust gas purification catalyst obtained in Comparative Example 2. confirmed. From these results, it can be seen that sufficiently high catalytic activity can be obtained by setting the central pore diameter of the porous metal in the range of 1 to 15 nm.

Further, as is apparent from the results shown in FIG. 7, the porous metal obtained is made to contain other metals than Au by adding intermetallic compounds other than Au and Al (Fe, Co, Cu, Ni) during the production. It was confirmed that inclusion of a metal was effective from the viewpoint of catalytic activity. Furthermore, when the exhaust gas purifying catalysts obtained in Example 1 and Reference Examples 4 to 6 were compared, it was found that the effectiveness of the elements was in the order of Fe>Cu>Co> Ni from the viewpoint of catalytic activity.

  As described above, according to the present invention, it is possible to provide an exhaust gas purifying catalyst having excellent CO oxidation performance and sufficiently high catalytic activity, and a method for producing the catalyst. Such an exhaust gas purifying catalyst of the present invention is particularly useful for applications such as an oxidation catalyst when purifying exhaust gas because it has excellent oxidation activity at low temperatures.

2 is a scanning electron microscope (SEM) photograph of the porous metal produced in Reference Example 1. 4 is a scanning electron microscope (SEM) photograph of the porous metal produced in Reference Example 2. 4 is a scanning electron microscope (SEM) photograph of the porous metal produced in Reference Example 3. 2 is a scanning electron microscope (SEM) photograph of the intermetallic compound produced in Comparative Example 1. It is a graph which shows the relationship between CO conversion rate and temperature of the catalyst for exhaust gas purification obtained in Reference Examples 1 to 3 and Comparative Example 1. It is a graph which shows the relationship between CO conversion rate and temperature of the catalyst for exhaust gas purification obtained in Examples 1-2 and Comparative Examples 3-4. It is a graph which shows the relationship between CO conversion rate of the catalyst for exhaust gas purification obtained in Reference Example 1, Example 1, and Reference Examples 4-6 , and temperature.

Claims (5)

Comprising a porous metal having pores with a central pore diameter of 1 to 15 nm , and
The exhaust gas purifying catalyst, wherein the porous metal contains Au and Fe . ^2. The exhaust gas purifying catalyst according to claim 1, wherein the porous metal has a specific surface area of 1 to 30 m ² / g. By dissolving Au and Al and Fe, and obtaining an alloy containing said Au, Al and Fe,
Elution of Al from the alloy to obtain a porous metal having pores with a center pore diameter of 1 to 15 nm;
A method for producing an exhaust gas purifying catalyst, comprising: 4. The method for producing an exhaust gas purifying catalyst according to claim 3 , wherein in the step of eluting Al from the alloy, 70 at% or more of the total Al in the alloy is eluted from the alloy. The method for producing an exhaust gas purifying catalyst according to claim 3 or 4, wherein the alloy is an intermetallic compound.