Metal powder slurry

Abstract

This technology provides a method for more efficiently supporting a target metal, which functions as a catalyst, on a porous material than conventional impregnation methods using metal salt solutions. [Solution] A metal powder slurry comprising metal powder, a solvent, and an additive, wherein the metal powder has a number average particle size of 40 nm to 200 nm and is present in an amount of 1% to 30% by mass relative to the metal powder slurry, the solvent has a boiling point of 50°C to 150°C, and the additive is an organic polymer containing a carboxylic acid and having a comb-like structure, with a mass average molecular weight of 1,000 to 10,000, and is present in an amount of 1 to 5 parts by mass per 100 parts by mass of the metal powder.

Description

[Technical Field]

[0001] This invention relates to a metal powder slurry used by impregnating a porous material. [Background technology]

[0002] Porous materials, formed from ceramics such as alumina, silica, titania, zirconia, and magnesia, as well as inorganic materials such as metals, carbon, and glass, have countless microscopic pores within them, resulting in an extremely large specific surface area compared to solid structures. Taking advantage of this characteristic, porous materials are used as catalyst supports for exhaust gas catalysts in internal combustion engines such as vehicles, catalysts for treating exhaust gases and wastewater in factories, catalyst supports for reaction catalysts in various manufacturing processes for chemicals, food, and pharmaceuticals, as well as catalyst supports and electrodes in fuel cells and water electrolysis cells.

[0003] When using the above-mentioned porous material as a catalyst support, it is necessary to support the catalytic substance within the pores of the porous material (hereinafter also referred to as the porous support). The impregnation method is a known method for supporting catalysts. In the impregnation method, a salt solution of the target metal that functions as a catalyst, such as a nitrate or acetate, is impregnated into the porous material by immersion, coating, spraying, or dropping, and then dried. After that, heat treatment is performed to support the target metal, for example, with a particle size of about 20 nm.

[0004] For example, Non-Patent Document 1 discloses a technique for supporting a catalyst made of metal particles on a porous body by an impregnation step of impregnating a porous body with a metal salt solution such as a nitrate of metal ions as a catalyst precursor, a thermal decomposition step of heat-treating the impregnated porous body under predetermined heat treatment conditions to convert the catalyst precursor into a predetermined catalyst composition, and a cooling step of cooling the heat-treated porous body.

[0005] Furthermore, Patent Document 1 discloses a manufacturing method for a Ni / SiO2 catalyst in which Ni is supported on binary-pore silica having two types of pores: macropores having pore diameters in the micrometer range and mesopores having pore diameters in the nanometer range. This manufacturing method involves impregnating the binary-pore silica with a solution consisting of a nickel compound such as nickel nitrate, a carboxylic acid compound, and a polar solvent such as water or alcohol, and then heat-treating it at about 500 to 1000°C to volatilize the solvent and decompose organic matter. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] MC Tucker, et al, J. Power Sources, 489, 229439 (2021) [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2005-254091 [Overview of the project] [Problems that the invention aims to solve]

[0008] In the conventional impregnation method for catalyst loading described above, it is necessary to repeat the series of steps consisting of the impregnation and thermal decomposition steps multiple times in order to load a large amount of catalyst. As a result, catalyst production takes a long time, and productivity has been a challenge. Therefore, in order to reduce the number of repetitions of the above series of steps and improve productivity, a method of impregnating a porous material with a high-concentration metal salt solution is sometimes employed.

[0009] However, metal salt solutions cannot contain metal species dissolved in concentrations exceeding the saturation concentration of the metal salt used. Furthermore, increasing the concentration of the metal salt solution makes it easier for solid components to precipitate, requiring strict temperature control of the metal salt solution. In addition, as the concentration of the metal salt solution increases, its viscosity increases, and its viscosity changes significantly with temperature, making it difficult to handle during preparation and impregnation, which can result in an inability to stably maintain the quality of the catalyst.

[0010] This invention has been made in view of the problems with the conventional impregnation method for catalyst support described above, and aims to provide a technology that can support a target metal that functions as a catalyst on a porous body more efficiently than the conventional impregnation method using a metal salt solution. [Means for solving the problem]

[0011] To achieve the above objective, the metal powder slurry according to the present invention is a metal powder slurry having a metal powder, a solvent, and an additive, wherein the metal powder has a number average particle size of 40 nm to 200 nm and is contained in an amount of 1% to 30% by mass relative to the metal powder slurry, the solvent has a boiling point of 50°C to 150°C, and the additive is an organic polymer containing a carboxylic acid and having a comb-like structure, with a mass average molecular weight of 1,000 to 10,000, and is contained in an amount of 1 to 5 parts by mass per 100 parts by mass of the metal powder. [Effects of the Invention]

[0012] According to the present invention, the target metal that functions as a catalyst can be supported on a porous body more efficiently than in conventional impregnation methods using metal salt solutions. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram of a comb-shaped organic polymer contained in an additive that constitutes a metal powder slurry according to an embodiment of the present invention. [Figure 2]The structural formula of acrylic acid (a) which is the main raw material of the comb-shaped organic polymer in FIG. 1 and polyethylene glycol methacrylate (b) which is an example of polyalkylene glycol methacrylate. [Figure 3] It is a block flow diagram of the method for producing a metal powder slurry according to an embodiment of the present invention. [Figure 4] It is a SEM image of a cross-section when the metal-supported porous body obtained by impregnating the metal powder slurry prepared in Example 1 by dropping is cut in the vertical direction. [Figure 5] Among the cross-sections of FIG. 4, it is an enlarged SEM image near the dropping surface. [Figure 6] Among the cross-sections of FIG. 4, it is an enlarged SEM image of a portion about 100 μm deep from the dropping surface. [Figure 7] Among the cross-sections of FIG. 4, it is an enlarged SEM image of a portion about 200 μm deep from the dropping surface. [Figure 8] Among the cross-sections of FIG. 4, it is an enlarged SEM image of a portion about 300 μm deep from the dropping surface. [Figure 9] Among the cross-sections of FIG. 4, it is an enlarged SEM image of a portion about 400 μm deep from the dropping surface.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the metal powder slurry according to the present invention will be described in detail. Note that the present invention is not limited to the following embodiments, and various modification examples and alternative examples can be included without departing from the gist of the present invention. That is, the scope of rights of the present invention extends to the scope of the claims and their equivalent scope.

[0015] 1. Metal Powder Slurry The metal powder slurry of the embodiment of the present invention comprises a metal powder having a number average particle size of 40 nm to 200 nm, a solvent having a boiling point of 50°C to 150°C, and an additive consisting of an organic polymer having a comb-like structure with a mass average molecular weight of 1,000 (hereinafter also referred to as 1k) to 10,000 (hereinafter also referred to as 10k) including a carboxylic acid. This metal powder slurry contains 1% to 30% by mass of the metal powder relative to the metal powder slurry, and contains 1 to 5 parts by mass of the additive per 100 parts by mass of the metal powder.

[0016] The metal powder slurry of the embodiment of the present invention allows for the impregnation of a porous support in the form of a slurry in which the target metal, in the form of a fine powder that functions as a catalyst, is dispersed in a solvent containing additives, as described above. This makes it possible to adjust the concentration of the target metal in the slurry to a higher level, and thus reduces the number of repetitions of the series of steps, including the impregnation and drying steps, compared to conventional methods of impregnation with a metal salt solution. In other words, in the case of conventional metal salt solutions, the substance supported on the support consists of metal ions that function as a catalyst and their counterions, so the concentration of metal atoms in the metal salt solution for one batch of impregnation is significantly low, which sometimes poses a problem in terms of productivity. For example, comparing 58.7 g of metallic nickel particles with 58.7 g of nickel nitrate (Ni(NO3)2), the amount of nickel is 1.0 mol and 0.32 mol, respectively, and when using nitrate, the nickel content is reduced to about one-third compared to metallic nickel. Furthermore, in conventional impregnation methods where nickel is supported on a porous body using nitrate, for example, when the impregnation and drying steps are repeated about 5 times, the particle size of nickel supported on the porous body is about 20 nm.

[0017] In contrast, the metal powder slurry of the embodiment of the present invention, by constructing the slurry with the above-mentioned elements, can adjust the content of the target metal in the slurry to a higher level as described above, and also with a shear speed of 1000 s. -1The upper limit of the viscosity of the metal powder slurry can be preferably kept below 130 mPa·s, more preferably below 110 mPa·s, and most preferably below 100 mPa·s. By keeping the viscosity below 100 mPa·s in this way, it becomes easier to penetrate the porous body with the metal powder slurry during impregnation. There is no particular limit to the lower limit of the viscosity of the metal powder slurry, but the viscosity will be about 1 to 2 mPa·s when the content of the metal powder in the metal powder slurry is 1% by mass. The elements constituting the metal powder slurry of the embodiment of the present invention will be described below.

[0018] (1) Metal powder The metal powder, which is a component of the metal powder slurry in the embodiment of the present invention, has a number-average particle size of 40 nm to 200 nm. This allows the metal powder to be efficiently supported within the fine pores of the porous carrier during impregnation. In other words, since the pore size of porous materials generally used as catalyst carriers is about 2 to 4 μm, the particle size of the metal powder is one order of magnitude smaller than the pore size. Therefore, obstacles are less likely to occur during the introduction and diffusion of the metal powder into the pores of the porous carrier during impregnation. The above number-average particle size can be determined, for example, by measuring the particle size of 100 to 200 arbitrarily selected metal powders in an SEM image taken using a scanning electron microscope with a magnification of 40,000x, and taking the arithmetic mean of these values.

[0019] As described above, by setting the lower limit of the number-average particle size of the metal powder to 40 nm or more, preferably 50 nm or more, even when the catalyst is exposed to high-temperature reaction conditions during operation, the metal particles acting as catalysts become less likely to sinter with each other. This reduces the likelihood of problems such as a decrease in the surface area of ​​the catalyst due to sintering, which can lead to a decrease in reaction efficiency. In other words, the sintering temperature of particles generally depends on their particle size; the smaller the particle size, the lower the sintering temperature. The particle size of catalyst particles obtained by conventional loading methods involving impregnation with metal salt solutions is generally less than 40 nm. Depending on the application of the catalyst, for example, when used as a catalyst for high-temperature reactions, there is a risk of sintering at a temperature lower than the reaction temperature.

[0020] In contrast, the metal powder used in the metal powder slurry of the embodiment of the present invention has a lower limit of particle size of 40 nm or more, so as described above, it can be easily impregnated into the pores of the porous body and sintering can be suppressed. The upper limit of the number average particle size of the metal powder is preferably 150 nm or less, and more preferably 100 nm or less, from the viewpoint of increasing the amount that can be supported.

[0021] In the metal powder slurry of the embodiment of the present invention, the content of the metal powder in the metal powder slurry is 1% by mass or more and 30% by mass or less. If the content of the metal powder in the metal powder slurry is less than 1% by mass, the amount of metal that can be attached to the inside of the pores of the porous body will be insufficient, and the catalytic function will not be fully expressed. However, from the viewpoint of supporting a larger amount of metal in a single impregnation step, this content is preferably 5% by mass or more. On the other hand, if the content of the metal powder exceeds 30% by mass, the viscosity of the metal powder slurry will increase excessively, making it impossible to impregnate the metal powder to a sufficient depth inside the pores of the porous body.

[0022] The metal powder used in the metal powder slurry of the embodiments of the present invention is preferably one or more of nickel (Ni), cobalt (Co), and copper (Cu). Since all of these metal species have high catalytic activity, they have been conventionally used as heterogeneous catalysts in various processes. Furthermore, because these metal species are relatively abundant elements on Earth, they are attracting attention as substitutes for precious metals such as Pt, Pd, Rh, and Ru, which are scarce and expensive.

[0023] (2) Additives The additive, which is a component of the metal powder slurry in the embodiment of the present invention, consists of an organic polymer containing a carboxylic acid and having a comb-like structure, with a mass-average molecular weight of 1,000 to 10,000. This improves the dispersibility of the metal powder in the metal powder slurry and also improves the wettability to porous materials, making impregnation easier. The mass-average molecular weight of this organic polymer is preferably 8,000 or less, more preferably 5,000 or less, and most preferably around 3,000. If the mass-average molecular weight exceeds 10,000, the viscosity of the metal powder slurry increases, making impregnation of porous materials difficult. Conversely, if it is less than 1,000, the metal powder slurry tends to foam easily, which also makes impregnation of porous materials difficult.

[0024] As mentioned above, organic polymers used as additives have a comb-like structure. Organic polymers can be categorized into comb-like polymers, star-shaped polymers, ladder-shaped polymers, dendritic polymers, etc., depending on the geometric structure of the molecular chains branching from the main chain, and each can exhibit actions and effects derived from its unique structure. Among these, in the metal powder slurry of the embodiment of the present invention, by using an additive containing an organic polymer that contains a carboxylic acid and has a comb-like structure, as shown in Figure 1, high dispersibility and wettability can be achieved. These effects are obtained by having the ionic functional groups such as carboxylic acids in the organic polymer play a role in adsorption with metal powders, while the branched chains of the comb-like structure play a role in controlling solubility in the solvent and acting as graft chains that have a steric repulsion effect.

[0025] Incidentally, when using the metal powder slurry of the embodiment of the present invention as a catalyst by impregnating a porous body, the impregnation is followed by a drying treatment and then a calcination treatment under predetermined heat treatment conditions. Since the above-mentioned additive is thermally decomposed by this calcination treatment, it is preferable that the additive does not generate harmful decomposition gases such as nitrogen oxides or sulfur oxides, or thermal decomposition gases that reduce catalytic activity, through this thermal decomposition. Furthermore, it is preferable that the additive leaves no ash residue after thermal decomposition, or if any ash residue remains, it is very small.

[0026] Organic polymers that satisfy the above requirements include those composed of polyalkylene glycol methacrylate and polymers of acrylic acid (CH2=CHCOOH) and / or methacrylic acid (CH2=C(CH3)COOH). Figure 2(a) shows the molecular structure of acrylic acid, and Figure 2(b) shows the molecular structure of polyethylene glycol methacrylate as an example of polyalkylene glycol methacrylate. In this case, it is preferable that the degree of polymerization of the acrylic acid or methacrylic acid constituting the organic polymer is between 5 and 70. When the degree of polymerization is between 5 and 70, the metal powder is easily dispersed, and the viscosity of the metal powder slurry is also at an appropriate value.

[0027] In the metal powder slurry of the embodiment of the present invention, the above-mentioned additive is contained in an amount of 1 to 5 parts by mass per 100 parts by mass of metal powder. If the content exceeds 5 parts by mass, the additive is more likely to remain in the form of organic matter even after calcination treatment, reducing catalytic activity, which necessitates increasing the treatment temperature or lengthening the treatment time in the thermal decomposition process. Furthermore, if the content exceeds 5 parts by mass, foaming is more likely to occur, making it difficult for the metal powder to penetrate into the carrier pores during impregnation. As a countermeasure, it becomes necessary to remove foam during impregnation or to operate carefully to prevent foaming, which reduces productivity and workability. Conversely, if the content is less than 1 part by mass, the effect of the above-mentioned additive is almost negligible. As described above, the additive is added to improve the dispersibility of the metal powder slurry and ensure ease of impregnation, so it is preferable that it does not remain in the catalyst carrier in the end, and ideally, the catalyst can be supported simply by drying after impregnation, which is preferable for simplifying the manufacturing process.

[0028] This organic polymer is preferably one whose pH is between 6 and 8 when dissolved in water at a concentration of 5% by mass. If the pH is below 6, the metal powder will dissolve more easily in the metal powder slurry, and the particle shape may not be maintained. Conversely, if the pH is greater than 8, the dispersibility of the metal powder in the metal powder slurry will decrease, and it will be more likely to settle.

[0029] (3) Dispersion medium The solvent that serves as the dispersion medium in the metal powder slurry of the embodiments of the present invention has a boiling point of 50°C to 150°C. If the boiling point of the solvent is between 50°C and 150°C, it is easier to maintain the concentration of the metal powder slurry and it dries easily in the drying process, resulting in less residual solvent. On the other hand, if the boiling point of the solvent is below 50°C, the solvent volatilizes easily, causing excessive evaporation of the solvent from the metal powder slurry, which tends to increase the concentration of the metal powder and makes it difficult to stabilize the quality of the metal powder slurry. Furthermore, if the boiling point of the solvent is too low, there are safety concerns such as ignition due to static electricity. Conversely, if the boiling point of the solvent is higher than 150°C, it is difficult to dry the solvent in the drying process after impregnating the porous body with metal powder, and there is a higher possibility that solvent will remain in the subsequent thermal decomposition process. If residual solvent is present in the thermal decomposition process, the solvent may evaporate rapidly, potentially damaging the porous body.

[0030] The specific type of solvent is not particularly limited as long as it can maintain the dispersion of the catalytic metal powder and penetrate into the pores of the porous material. It will be appropriately selected depending on the characteristics of the catalyst powder and the surface of the porous material. However, if neither the catalyst powder nor the porous material has been surface-treated, their surfaces are considered to be in a highly polar state, so it is preferable to use a highly polar solvent as the dispersion medium. For example, water, a polar organic solvent, or a mixture of water and a polar organic solvent is preferred.

[0031] For the polar organic solvents mentioned above, it is preferable to use an organic solvent whose polarity value, which is the sum of the polarity term δP and the hydrogen bonding term δH of the Hansen solubility parameter of the organic solvent divided by the molecular weight, is 0.18 or higher. If the polarity value of this organic solvent is 0.18 or higher, the polarity is sufficiently high, making it easier to maintain the dispersion state of the metal powder and to wet and impregnate porous materials. Conversely, if the polarity value of this organic solvent is less than 0.18, it becomes difficult to maintain the dispersion state of the metal powder, and it becomes difficult to wet and impregnate porous materials. The values ​​of the polarity term δP and the hydrogen bonding term δH of the Hansen solubility parameter can be determined using commercially available software (HSPiP 6th edition, ver6.0.03).

[0032] Examples of the polar organic solvents mentioned above include methanol (boiling point 65°C, polarity value 1.080), ethanol (boiling point 78°C, polarity value 0.612), propanol (boiling point 97°C, polarity value 0.403), isopropanol (boiling point 82°C, polarity value 0.374), 1-butanol (boiling point 117°C, polarity value 0.290), sec-butyl alcohol (boiling point 99°C, polarity value 0.273), isobutyl alcohol (boiling point 108°C, polarity value 0.291), tert-butyl alcohol (boiling point 82.4°C, polarity value 0.267), acetone (boiling point 56°C, polarity value 0.300), acetonitrile (boiling point 82°C, polarity value 0.587), tetrahydrofuran (boiling point 66°C, polarity value 0.190), or mixtures of two or more of these. When using a mixture of water and a polar organic solvent, there are no particular limitations on the mixing ratio as long as they mix without separating. Similarly, when using a mixture of two or more polar organic solvents, there are no particular limitations on the mixing ratio as long as they mix without separating. It is preferable that none of the above additives or solvents contain sulfur, halogens, or cyanide, and it is especially desirable that sulfur, like carbon, does not remain, as it can become a catalyst poison if it remains. In addition to the above considerations, ethanol is particularly preferred as the polar organic solvent from the standpoint of availability and safety.

[0033] 2. Method for producing metal powder slurry Next, the metal powder slurry manufacturing method of the embodiment of the present invention described above will be explained using the case where the metal powder is nickel powder as an example. As shown in Figure 3, the manufacturing method of the metal powder slurry of this embodiment of the present invention consists of a reaction solution preparation step of preparing a reaction solution containing a nickel salt that acts as a catalyst, a crystallization step of reducing the nickel salt contained in the reaction solution to obtain nickel crystallization powder, a crushing step of crushing aggregates of the obtained nickel crystallization powder, and a mixing and dispersion step of mixing the crushed nickel crystallization powder with a solvent or the like to make a nickel powder slurry. Each of these steps will be explained below.

[0034] 2-1 Reaction solution preparation process First, in the reaction solution preparation step, a water-soluble nickel salt, a metal salt of a metal nobler than nickel, a complexing agent, hydrazine as a reducing agent, and alkali hydroxide as a pH adjuster are added to water as a solvent in predetermined proportions, and an amine compound is added and mixed as needed to prepare the reaction solution.

[0035] In this reaction solution preparation step, the reaction solution may be prepared by mixing a first solution prepared using a water-soluble nickel salt, a metal salt of a metal nobler than nickel, and a complexing agent with a second solution prepared using hydrazine and alkali hydroxide immediately before the crystallization step, or by mixing a third solution prepared using a water-soluble nickel salt, a metal salt of a metal nobler than nickel, a complexing agent, and hydrazine with alkali hydroxide immediately before the crystallization step.

[0036] As described above, the reason for preparing the reaction solution in two stages is that if water-soluble nickel salt, hydrazine, and alkali hydroxide are present in the same solution, a reduction reaction may occur during the reaction solution preparation stage, depending on the temperature and pH, potentially causing nickel crystallization. On the other hand, by preparing the reaction solution in two stages as described above, the timing of the onset of nickel crystallization can be controlled, making it possible to stably produce a fine nickel powder with a narrow particle size distribution.

[0037] The temperature of the reaction solution described above should preferably be adjusted within the range of 10 to 30°C. If the temperature is below 10°C, the cost and time required to cool the raw materials may be excessive, while if the temperature is above 30°C, some of the metal salts may crystallize during the preparation of the reaction solution, resulting in a broader particle size distribution. The various raw materials used in the preparation of the reaction solution are described in detail below.

[0038] (1) Solvent From the viewpoint of reducing the impurity content in the resulting nickel powder, high-purity water such as ultrapure water with an conductivity of 0.06 μS / cm or less, or pure water with an conductivity of 1 μS / cm or less, is preferred as the solvent, and among these, pure water is preferred because it is inexpensive and readily available.

[0039] (2) Water-soluble nickel salts The water-soluble nickel salt is not particularly limited as long as it is readily soluble in water; for example, one or more selected from nickel chloride, nickel sulfate, and nickel nitrate can be used. Among these nickel salts, nickel chloride, nickel sulfate, or a mixture thereof is more preferable.

[0040] (3) Salts of metals nobler than nickel Metals nobler than nickel have a lower ionization tendency than nickel, and can therefore act as nucleating agents. Specifically, by including a metal salt of a metal nobler than nickel in the reaction solution, the metal nobler than nickel can be preferentially reduced and precipitated over nickel during the crystallization process, thus serving as an initial nucleus for nickel particle crystallization. By growing particles from this initial nucleus, finer nickel crystallization powder (nickel powder) can be produced.

[0041] When preparing the reaction solution described above, the number-average particle size of the nickel powder can be adjusted by appropriately adjusting the molar ratio of the metal salt of a metal nobler than nickel to the nickel. Depending on the application of the catalyst, the pore size of the porous material used as a support may differ, so the number-average particle size of the nickel powder introduced into the porous material can be determined according to the pore size of the porous material. For example, chemical catalysts that promote various chemical reactions, and electrode catalysts used to convert chemical energy into electrical energy or to cause chemical changes using electrical energy, often have pore sizes of several μm, so it is sufficient to ensure that the number-average particle size of the nickel powder is 200 nm or less.

[0042] As for metal salts of metals nobler than nickel, there are no particular limitations as long as they are water-soluble and have a lower ionization tendency than nickel. Examples include water-soluble precious metal salts such as copper salts, gold salts, silver salts, platinum salts, palladium salts, rhodium salts, and iridium salts. More specifically, examples of water-soluble copper salts include copper sulfate, examples of water-soluble silver salts include silver nitrate, and examples of water-soluble palladium salts include sodium palladium(II) chloride, ammonium palladium(II) chloride, palladium(II) nitrate, and palladium(II) sulfate. Among these, palladium salts are preferred because, although the particle size distribution is somewhat broader, the particle size of the resulting nickel powder can be controlled to be finer.

[0043] (4) Complexing agent As a complexing agent, it is preferable to use at least one of nitrilotriacetic acid and its salts, which are compounds containing three carboxyl groups in one molecule. Examples of salts of nitrilotriacetic acid include sodium salt and potassium salt of nitrilotriacetic acid. By using at least one of nitrilotriacetic acid and its salts as a complexing agent, the carboxyl groups and nickel form a three-dimensional network, thereby reducing the consumption of hydrazine as a reducing agent and allowing adjustment of the viscosity of the reaction solution. The amount of this complexing agent added is preferably in the range of 0.05 to 5 moles per mole of nickel atoms, more preferably in the range of 0.05 to 1 mole, even more preferably in the range of 0.05 to 0.8 moles, and most preferably in the range of 0.05 to 0.5 moles.

[0044] (5) Hydrazine For the hydrazine (N2H4, molecular weight: 32.05) used as a reducing agent, either anhydrous hydrazine or hydrated hydrazine (N2H4·H2O, molecular weight: 50.06) may be used. In the latter case, for example, commercially available industrial-grade 60% by mass hydrated hydrazine can be used. Hydrazine is suitable as a reducing agent because it has high reducing power in alkaline conditions, the by-products of the reduction reaction are nitrogen gas and water, so impurities are less likely to be generated in the reaction solution, and it has the advantages of having a low impurity content and being readily available.

[0045] It is preferable to add hydrazine in an amount of 0.6 to 1.8 moles per mole of nickel. If the amount added is less than 0.6 moles, there is a risk that the nickel in the reaction solution will not be completely reduced, and if the amount added exceeds 1.8 moles, the effect will not be further enhanced, and in fact, the consumption of hydrazine will be excessive, which is undesirable from an economic standpoint. Note that the entire amount of hydrazine may not be added in the reaction solution preparation step, but rather in installments during the reduction reaction process in the subsequent crystallization step.

[0046] (6) Alkali hydroxide Since the reducing power of hydrazine increases with the alkalinity of the reaction solution, alkali hydroxide is added as a pH adjuster to increase alkalinity. There are no particular limitations on the type of alkali hydroxide, but due to ease of availability and cost, alkali metal hydroxides are preferred, and sodium hydroxide or potassium hydroxide are particularly preferred. The amount of alkali hydroxide added is preferably such that the pH of the reaction solution at the reaction temperature is preferably 9.5 or higher, more preferably 10 or higher, and even more preferably 10.5 or higher, so that the reducing power of hydrazine as a reducing agent is sufficiently increased.

[0047] (7) Amine compounds The amine compound added to the reaction solution as needed acts as an inhibitor of hydrazine autolysis, an accelerator of the reduction reaction, and an inhibitor of the linkage between nickel particles. Preferably, it is a compound containing two or more primary amino groups (-NH2) in its molecule, or a compound containing one primary amino group (-NH2) and one or more secondary amino groups (-NH-) in its molecule. Examples include alkyleneamines and their derivatives.

[0048] Specifically, examples of alkyleneamines include one or more selected from ethylenediamine (H2NC2H4NH2), diethylenetriamine (H2NC2H4NHC2H4NH2), triethylenetetramine (H2N(C2H4NH)2C2H4NH2), tetraethylenepentamine (H2N(C2H4NH)3C2H4NH2), and pentaethylenehexamine (H2N(C2H4NH)4C2H4NH2). Examples of alkyleneamine derivatives include one or more selected from tris(2-aminoethyl)amine (N(C2H4NH2)3) and (2-aminoethyl)-2-aminoethanol (H2NC2H4NHC2H4OH). These alkyleneamines and alkyleneamine derivatives are water-soluble, and among them, ethylenediamine and diethylenetriamine are particularly preferred because they are readily available and inexpensive.

[0049] The action of the above amine compound as a reducing reaction accelerator is due to nickel ions (Ni) in the reaction solution. 2+ This is thought to be due to its function as a complexing agent, which complexes ) to form nickel complex ions. Furthermore, its effects as an inhibitor of hydrazine self-decomposition and an inhibitor of linkage between nickel particles are presumed to be due to the interaction between the primary amino group (-NH2) and secondary amino group (-NH-) within the amine compound molecule and the surface of hydrazine or nickel crystallization powder.

[0050] The ratio of moles of amine compound to moles of nickel atoms in the reaction solution ((moles of amine compound / moles of nickel atoms) × 100) is preferably in the range of 0.01 to 5 mol%, and more preferably in the range of 0.03 to 2 mol%. If this ratio is less than 0.01 mol%, the amount of amine compound is too small, and it may not function as an inhibitor of hydrazine self-decomposition, an accelerator of the reduction reaction, or an inhibitor of the bonding of nickel particles. Conversely, if the above ratio exceeds 5 mol%, the complexing agent's function in forming nickel complex ions becomes too strong, which may result in unstable particle growth of the nickel crystallized powder, deterioration of the granularity and sphericity of the nickel powder, resulting in irregular shapes, or the formation of many coarse particles where nickel particles aggregate with each other, making it difficult to efficiently introduce the nickel powder into the pores of the porous material. In addition, to suppress the involvement of the amine compound in nucleation, the amine compound may be added to the reaction solution immediately after the start of the crystallization process.

[0051] (8) Other contents The above reaction solution may also contain various additives such as dispersants, complexing agents other than nitrilotriacetic acid and its salts, and defoamers. These dispersants and complexing agents improve the granularity (sphericity) and surface smoothness of the nickel crystallized powder, and reduce the generation of coarse particles. In addition, defoamers can suppress foaming mainly caused by nitrogen gas generated during the crystallization reaction in the crystallization process.

[0052] Known substances can be used as dispersants, such as alanine (CH3CH(COOH)NH2), glycine (H2NCH2COOH), triethanolamine (N(C2H4OH)3), and diethanolamine (also known as iminodiethanol) (NH(C2H4OH)2).

[0053] Known substances can be used as complexing agents, for example, hydroxycarboxylic acids, carboxylic acids (organic acids containing at least one carboxyl group), hydroxycarboxylic acid salts and hydroxycarboxylic acid derivatives, carboxylate salts and carboxylic acid derivatives, specifically tartaric acid, citric acid, malic acid, ascorbic acid, formic acid, acetic acid, pyruvic acid, and their salts and derivatives.

[0054] The defoaming agent is not particularly limited as long as it has excellent antifoaming properties under alkaline conditions, but oil-type or solvent-type silicone-based or non-silicone-based defoaming agents can be suitably used. Furthermore, methionine may be added, which can act as an auxiliary agent for inhibiting the self-decomposition of hydrazine and as an inhibitor of the formation of linked coarse particles, and can also contribute to the spheroidization (surface smoothing) of nickel particles.

[0055] 2-2 Crystallization process The crystallization step is a process in which the water-soluble nickel salt in the reaction solution prepared in the above reaction solution preparation step is crystallized by a reduction reaction with hydrazine in the presence of alkali hydroxide to produce nickel crystallization powder. The crystallization reaction of metallic nickel (Ni) from nickel ions by this reduction reaction is a two-electron reaction shown in Equation 1 below, and the reaction with hydrazine (N2H4) is a four-electron reaction shown in Equation 2 below. As mentioned above, when nickel chloride (NiCl2) is used as the nickel salt and sodium hydroxide (NaOH) is used as the alkali hydroxide, the reduction reaction as a whole is represented by the reaction in which nickel hydroxide (Ni(OH)2) produced by the neutralization reaction of nickel chloride and sodium hydroxide is reduced by hydrazine, as shown in Equation 3 below. Stoichiometrically, this reaction requires 0.5 moles of hydrazine (N2H4) for the reduction of 1 mole of nickel (Ni). As can be seen from the reaction equation 2 below, the stronger the alkalinity, the greater the reducing power of hydrazine. Therefore, alkali hydroxide is used as a pH adjuster to increase alkalinity, thereby accelerating the reduction reaction of hydrazine.

[0056] [Formula 1] Ni 2+ +2e- →Ni↓ (2-electron reaction) [Formula 2] N2H4→N2↑+4H + +4e - (4-electron reaction) [Formula 3] 2NiCl2+N2H4+4NaOH→2Ni(OH)2+N2H4+4NaCl→2Ni↓+N2↑+4NaCl+4H2O

[0057] As mentioned above, by adding an amine compound to the reaction solution, the autodecomposition reaction of hydrazine shown in Equation 4 below can be suppressed. This reduces the consumption of expensive hydrazine and suppresses the production of large amounts of ammonia as a by-product due to the autodecomposition of hydrazine, which is a factor that increases the cost of treating wastewater.

[0058] [Formula 4] 3N2H4→N2↑+4NH3

[0059] Thus, the reason why amine compounds can suppress the self-decomposition of hydrazine is thought to be that the molecules of the amine compound are adsorbed onto the surface of the nickel crystallization powder in the reaction solution, inhibiting contact between the active surface of the nickel crystallization powder and the hydrazine molecules, or that the molecules of the amine compound act on the surface of the nickel crystallization powder, inactivating its catalytic activity and suppressing the self-decomposition of hydrazine that is produced by the active surface of the nickel crystallization powder acting as a catalyst.

[0060] The crystallization reaction described above is preferably carried out under conditions where the temperature is controlled to 40-90°C using a water bath or the like. By adjusting the temperature within this range, the reaction rate of the reduction reaction and the crystallinity of the nickel crystallization powder (i.e., the crystallite size of the nickel particles) can be controlled. If the liquid temperature exceeds 90°C, the autodecomposition reaction of hydrazine may be excessively accelerated, increasing the consumption of hydrazine and causing severe foaming of the reaction solution, which may make it difficult to carry out the crystallization reaction stably. Conversely, if the liquid temperature is below 40°C, the crystallinity of the nickel crystallization powder may decrease significantly, or the reduction reaction may slow down, resulting in excessive crystallization time and reduced productivity.

[0061] The nickel crystallized powder produced by the reduction reaction in the crystallization process can be recovered by a general method consisting of solid-liquid separation of the slurry obtained after the reaction, washing of the solid portion obtained by solid separation, and drying. After this drying process, the nickel powder may be further heat-treated in an inert gas or reducing gas atmosphere at, for example, about 200-300°C, if necessary.

[0062] For solid-liquid separation of the nickel crystallization powder from the slurry described above, solid-liquid separation means such as a Denver filter, filter press, centrifuge, or decanter can be used. Furthermore, the washing described above is preferably carried out by a water washing method in which repulping and dewatering are performed one or more times using high-purity water such as pure water with an conductivity of 1 μS / cm or less. In addition, for the drying treatment described above, it is preferable to dry the product by holding it at an atmospheric temperature of preferably 50 to 300°C, more preferably 80 to 150°C, for a predetermined time using a general-purpose drying apparatus such as an air dryer, hot air dryer, inert gas atmosphere dryer, or vacuum dryer.

[0063] The heat treatment, if necessary, may be performed using a heating furnace such as an electric furnace, or it may be performed in a drying apparatus such as an inert gas atmosphere dryer or vacuum dryer used in the drying process described above, by performing heat treatment at an atmosphere of around 200-300°C under an inert gas, reducing gas, or vacuum atmosphere, thereby performing heat treatment immediately following the drying process. In this way, by performing heat treatment after the drying process, the properties of the surface of the nickel powder, such as the ratio of nickel metal, nickel oxide, and nickel hydroxide, can be changed. Specifically, the proportion of nickel oxide can be increased, or the proportion of nickel hydroxide can be decreased. Furthermore, since crystal growth is promoted by the heat treatment, the crystallite size can be increased by increasing the heat treatment temperature.

[0064] 2-3 Crushing process When an amine compound is added to the reaction solution, as mentioned above, the amine compound acts as an inhibitor of nickel particle aggregation during crystallization, thereby suppressing the aggregation of nickel particles into coarse particles during the reduction and precipitation process. However, depending on the crystallization conditions, it is difficult to completely suppress the formation of coarse particles, and if an amine compound is not added, the proportion of coarse particles increases. Therefore, it is preferable to provide a crushing step as a next step for the nickel crystallized powder (nickel powder) obtained in the above crystallization step, as needed. This crushes the coarse particles contained in the nickel crystallized powder, making it finer. There are no particular limitations on the crushing method, but dry crushing methods such as spiral jet crushing and counter-jet mill crushing, wet crushing methods such as high-pressure fluid impact crushing, and other general-purpose crushing methods can be used.

[0065] 2-4 Mixing and dispersion process The mixing and dispersion step involves preparing a nickel powder slurry by mixing the nickel crystallization powder (nickel powder) produced in the above-mentioned series of steps with a solvent having a boiling point of 50°C to 150°C and a polarity of 0.18 or higher, and an additive consisting of an organic polymer with a predetermined molecular weight that contains a carboxylic acid and has a comb-like structure, in a predetermined mixing ratio. General mixing equipment can be used to mix and disperse these nickel crystallization powders, solvents, and additives. Examples include a container with a stirrer, a thin-film swirling mixer, an ultrasonic cleaner, a ball mill, and a bead mill.

[0066] 3. Method for producing a catalyst by impregnation using a metal powder slurry 3-1 Impregnation process Next, a method for producing a catalyst by impregnation using the metal powder slurry of the embodiment of the present invention described above will be explained. First, the porous body to be used as a catalyst carrier is impregnated with the metal powder slurry of the embodiment of the present invention described above, preferably under normal or reduced pressure, by methods such as immersion, spraying, coating, or dropping. When impregnating under reduced pressure, a commercially available vacuum impregnation apparatus can be used. Areas on the carrier where metal powder should not be supported can be masked with an adhesive or paint that does not dissolve in the dispersion medium constituting the metal powder slurry.

[0067] 3-2 Drying process Next, by drying the porous body impregnated as described above under predetermined heat treatment conditions, a catalyst can be produced in which the metal powder is supported within the pores of the porous body.

[0068] 3-3 Pyrolysis process Depending on the application of the catalyst, after drying, a process is performed to thermally decompose the additive made of organic polymers by holding it in an air or nitrogen atmosphere at a temperature of 500°C for a predetermined time.

[0069] 3-4 Firing process Depending on the application of the catalyst, a calcination process may be performed after the above-mentioned thermal decomposition process to impart catalytic activity. For example, by heating in a reducing atmosphere at 500°C containing hydrogen gas, the oxide film on the surface of the supported metal powder can be reduced, thereby increasing the catalytic activity. [Examples]

[0070] Next, the metal powder slurry of the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited in any way by the following examples. In the following examples and comparative examples, samples of metal powder slurry or metal salt solution were first prepared using the raw materials shown in Table 1 below, and after measuring the viscosity of each, they were supported on a porous carrier by impregnation, and each sample was evaluated by measuring the impregnation depth and the amount of metal supported in the resulting metal-supported porous body.

[0071] [Table 1]

[0072] [Example 1] (Preparation of metal powder slurry) Nickel powder (Ni1) with a number-average particle size of 60 nm, prepared by the method shown in Figure 3, ethanol (S2) as a solvent, and a comb-shaped polymer (degree of polymerization of acrylic acid: 20, mass-average molecular weight: 3k) (D1) consisting of polyethylene glycol methacrylate and acrylic acid as an additive were prepared. The nickel powder (Ni1) content in the metal powder slurry was 5% by mass, and the additive (D1) was weighed out so that it was contained in a ratio of 3 parts by mass per 100 parts by mass of nickel powder (Ni1). These were loaded into a thin-film swirling mixer (Filmix® 30-L type) manufactured by Primix Corporation, and dispersed for 5 minutes at a rotation speed of 22,000 rpm to prepare the metal powder slurry of Sample 1. This metal powder slurry had an additive (D1) content of 0.15% by mass and an ethanol (S2) content of 94.85% by mass.

[0073] (Viscosity measurement of metal powder slurry) The viscosity of the metal powder slurry of Sample 1, prepared as described above, was measured using an Anton Paar rheometer (MCR302e) fitted with a cone plate (25 mm in diameter, 0.5° cone angle). In this measurement, the metal powder slurry of Sample 1 was poured onto a stage set to 25°C, and the shear rate was set to 1000 s. -1 The shear viscosity (mPa·s) was measured.

[0074] (Fabrication of porous materials) Next, a porous body made of alumina was prepared as a porous carrier for supporting the metal powder slurry. Specifically, commercially available alumina powder was mixed with polyvinyl butyral (Eslec BH-6) manufactured by Sekisui Chemical Co., Ltd. as a binder resin, polymethyl methacrylate particles (MX500L) manufactured by Soken Chemical Co., Ltd. as a pore-forming agent, and dihydroterpineol as a solvent, weighed in the predetermined proportions and mixed using a rotary-orbit mixer.

[0075] The resulting mixture was coated onto a polyethylene terephthalate film to a thickness of approximately 0.25 mm and dried at an ambient temperature of 80°C for 5 hours. The resulting dried film was crushed into a powder using a mortar and pestle, and this powder was loaded into a tablet molding machine and formed into a roughly cylindrical shape. The resulting molded body was degreased by raising the temperature to 525°C at a rate of 2°C / min in an air atmosphere and holding it at the target temperature of 525°C for 1 hour. Furthermore, it was fired by raising the temperature to 1400°C at a rate of 5°C / min in a nitrogen atmosphere and holding it at the target temperature of 1400°C for 1 hour. This produced cylindrical pellets (outer diameter 3.7 mm, height 2 mm) made of alumina porous material with a porosity of approximately 70%. The sides (curved parts) of these pellets were coated with a commercially available hydrophobic paint to prevent impregnation from the sides.

[0076] (Supporting metal onto a porous carrier) The porous pellets prepared as described above were impregnated with the metal powder slurry of Sample 1 mentioned earlier. Specifically, the metal powder slurry was placed in a glass bottle with an inner diameter of 21 mm to a height of about 2 cm from the bottom, and the bottle was shaken by hand in an up-and-down motion with a stroke of about 15 cm at a rate of about 4 back-and-forth movements per second for 10 seconds to uniformly disperse the metal powder. Immediately after shaking, 20 μL of the metal powder slurry was taken from the metal powder slurry using a micropipette and dropped onto the circular upper surface of a cylindrical pellet placed on a horizontal stand with its central axis facing up and down. After dropping, the pellet was left to stand for 30 minutes, and then placed in a convection oven and dried at an ambient temperature of 80°C for 5 hours. After this drying process, any excess metal powder adhering to the surface of the pellet was scraped off with a spatula. In this way, a metal-supported porous body was prepared in which metal was supported inside the pores of the porous body.

[0077] To confirm the state of metal powder support within the pores of the cylindrical metal-supported porous body fabricated in this manner, the metal-supported porous body was cut along a plane containing its central axis and photographed. Figures 4 to 9 show the overall SEM image of this cut surface, as well as SEM images at various depths from the top surface (hereinafter also referred to as the dropping surface) where the metal powder slurry was dropped.

[0078] Figure 4 is an SEM image of the almost entire fracture surface viewed from directly in front. The black area above the dashed line in the figure is the area where the metal powder is supported, and it can be confirmed that the metal powder is impregnated to a depth of about 400-500 μm from the dropping surface of the porous material. Figure 5 is an SEM image of the fracture surface near the dropping surface of the porous material. Numerous granular nickel particles can be seen adhering to the inner wall surface of the pores within the four circular and elliptical dashed lines. Figure 6 is an SEM image of the fracture surface at a depth of approximately 100 μm from the dropping surface of the porous material. Numerous granular nickel particles can be seen adhering to the inner wall surface of the pores within the three elliptical dashed lines. Figure 7 is an SEM image of the fracture surface at a depth of approximately 200 μm from the dropping surface of the porous material. As indicated by the arrows within the three dashed lines, a certain number of nickel particles can be seen adhering together to the inner wall surface of the pores. Figure 8 shows an SEM image of a cross-section of the porous material at a depth of approximately 300 μm from the drop surface. A small amount of nickel particles can be seen adhering to the inner wall surface of the pores in the areas indicated by the arrows. Figure 9 shows an SEM image of a cross-section of the porous material at a depth of approximately 400 μm from the drop surface. No nickel powder can be observed at a depth of approximately 400 μm from the surface of the porous material, suggesting that the nickel powder did not penetrate to this depth.

[0079] [Example 2] Metal powder slurries for samples 2 to 5 were prepared in the same manner as in Example 1, except that the content of the metal powder in the metal powder slurry was varied. After measuring their viscosity, metal-supported porous bodies were prepared by impregnating each of them into porous pellets, as in Example 1.

[0080] [Example 3] Sample 6 was prepared in the same manner as Sample 2 of Example 2, except that copper powder with a number-average particle size of 90 nm was used instead of nickel powder. After measuring its viscosity, a metal-supported porous body was fabricated by impregnating porous pellets with the slurry in the same manner as in Example 1.

[0081] [Example 4] Except for using metal powders with varying number-average particle sizes, metal powder slurries for samples 7 to 11 were prepared in the same manner as for sample 2 in Example 2. After measuring their viscosity, metal-supported porous bodies were fabricated by impregnating porous pellets with the slurry, as in Example 1.

[0082] [Example 5] Metal powder slurries for samples 12-15 were prepared in the same manner as for sample 2 in Example 2, except that various types of solvents were used instead of ethanol (S2). After measuring the viscosity of the slurries, metal-supported porous bodies were fabricated by impregnating porous pellets with the slurries in the same manner as in Example 1.

[0083] [Example 6] Metal powder slurries for samples 16-18 were prepared in the same manner as for sample 2 in Example 2, except that the content of additive (D1) per 100 parts by mass of nickel powder (Ni1) was varied. After measuring the viscosity of the slurries, metal-supported porous bodies were produced by impregnating porous pellets with the slurries in the same manner as in Example 1.

[0084] [Example 7] Except for using additives with various molecular weights and degrees of polymerization, metal powder slurries for samples 19-21 were prepared in the same manner as for sample 2 in Example 2. After measuring their viscosity, metal-supported porous bodies were fabricated by impregnating them into porous pellets in the same manner as in Example 1.

[0085] [Example 8] Except for changing the molecular weight of the additive and varying its content relative to 100 parts by mass of nickel powder (Ni1), metal powder slurries for samples 22 and 23 were prepared in the same manner as for sample 2 in Example 2. After measuring their viscosity, metal-supported porous bodies were fabricated by impregnating porous pellets with the slurry, in the same manner as in Example 1.

[0086] [Comparative Example 1] 20 μL of a nickel nitrate aqueous solution with a nickel concentration of 3.5 mol / L was taken using a micropipette and dropped onto the circular upper surface of a cylindrical porous pellet, which had been prepared in the same manner as in Example 1 and placed on a horizontal stand with its central axis facing vertically. After dropping, the pellet was left to stand for 30 minutes, and then placed in a convection oven and dried at an ambient temperature of 80°C for 5 hours. After this drying treatment, it was further subjected to thermal decomposition in air at an ambient temperature of 500°C. In this way, a metal-supported porous body was prepared in which metal was supported inside the pores of the porous body.

[0087] [evaluation] <Metal impregnation depth> The cylindrical metal-supported porous bodies prepared in Examples 1-8 and Comparative Example 1 were cut vertically so that the plane containing the central axis was exposed. Platinum was then coated onto the cut surface using an Auto Fine Coater (JEC-3000FC) manufactured by JEOL Ltd. The impregnation depth was measured using a scanning electron microscope (JSM-7200F) manufactured by JEOL Ltd., determining the distance from the drop surface during impregnation to the position where the metal particles penetrated most deeply on the cut surface. The obtained measurement results were evaluated according to the following evaluation criteria. ○: Impregnation depth of 300 μm or more. △: Impregnation depth is 200 μm or more and less than 300 μm. ×: Impregnation depth is less than 200 μm.

[0088] <Metal load amount> The amount of metal supported inside the pores of the porous material was evaluated based on the amount of metal supported up to 200 μm from the dropping surface of the porous material (hereinafter also referred to as the metal supported amount (200 μm)). The metal supported amount (200 μm) was calculated using the following formula 5. Here, W1 is the weight of the alumina porous material before impregnation with the metal powder slurry (unit: mg), W2 is the weight of the metal-supported porous material after impregnation with the metal powder slurry (unit: mg), H1 is the impregnation depth of the metal powder from the dropping surface of the porous material (unit: mm), and S1 is the volume of pellets from the dropping surface of the porous material to the impregnation depth (unit: mm 3 ) [Formula 5] Metal loading (200 μm) = (W2 - W1) / S1 × 0.2 / H1

[0089] The metal loading calculated by the above formula 5 was evaluated according to the following evaluation criteria. 〇: Metal loading (200 μm) is 0.1 mg / mm 3 or more. △: Metal loading (200 μm) is 0.015 mg / mm 3 or more and less than 0.1 mg / mm 3 ×: Metal loading (200 μm) is less than 0.015 mg / mm 3

[0090] In the thermal decomposition treatment of nickel nitrate in Comparative Example 1, since nickel oxide (NiO) as a thermal decomposition product adheres to the inside of the pores of the porous body, in the evaluation of the metal loading of the above metal-supported porous body, nickel oxide was converted to metallic Ni. That is, in Comparative Example 1, the value of (W2 - W1) used in the calculation of the metal loading in the above formula 5 was multiplied by (Ni atomic weight 58.71 / NiO formula weight 74.69) to obtain the loading of metallic Ni.

[0091] <Overall evaluation> The overall evaluation was performed according to the following criteria using the evaluation results of the above "depth of metal impregnation" and "metal loading". 〇: All evaluation results are "〇". △: At least one of the evaluation results is "△", but none of them is "×". ×: At least one of the evaluation results is "×".

[0092] The above evaluation results when the metal powder slurry samples and metal salt solution samples prepared in Examples 1 to 8 and Comparative Example 1 were impregnated into the porous body are summarized in Table 2 below together with the types and contents of the raw materials of each sample and the viscosities of each sample.

[0093] [Table 2]

[0094] The results in Table 2 above show the following: For all samples 1-4, the nickel powder content in the nickel powder slurry was within the range of 1-30% by mass, so the overall evaluation was "○" except for sample 3. For sample 3, the nickel powder content was at the lower limit, allowing for a lower viscosity of the nickel powder slurry compared to samples 1-2 and 4, but the metal loading evaluation was "△". In samples 1-4, the metal loading increased in proportion to the nickel powder content in the nickel powder slurry used.

[0095] On the other hand, sample 5 received a "×" rating for impregnation depth. This is likely because sample 5 had a nickel powder content exceeding the upper limit, resulting in excessively high viscosity of the nickel powder slurry. Consequently, nickel particles were not properly introduced and diffused into the pores of the porous material.

[0096] Sample 6 was subjected to the same conditions as Sample 2, except that copper powder with a particle size 1.5 times larger was used instead of nickel powder. Although the viscosity of the metal powder slurry was slightly reduced compared to Sample 2, likely due to the effect of particle size, the evaluation results for "metal impregnation depth" and "metal load" were almost the same as those for Sample 2. In other words, when metal is loaded onto a porous material by impregnation with a metal powder slurry, it is considered that the "metal impregnation depth" and "metal load" will hardly change even if the type of metal is changed, as long as the metal powder is within the specified particle size range. Therefore, it is considered that good results similar to those of Sample 2 and Sample 6 can be obtained even when cobalt powder is used instead of nickel powder.

[0097] For samples 7-10, the number-average particle size of the nickel powder used in the nickel powder slurry was within the range of 40-200 nm, so the overall evaluation was "○". On the other hand, for sample 11, the number-average particle size of the nickel powder exceeded the above upper limit, so the evaluation of the impregnation depth was "×". The reason for this is that, as can be seen from the fact that the viscosity decreases with increasing the particle size of the nickel powder in samples 7-11, while it is possible to lower the viscosity of the nickel powder slurry by increasing the particle size of the nickel powder, if the particle size becomes too large, it is thought that physical obstacles arise in the introduction and diffusion of nickel powder into the pores of the porous material.

[0098] For samples 12-14, the boiling point of the solvent used in the nickel powder slurry was between 50°C and 150°C, and the polarity value was within the range of 0.18 or higher, so the overall evaluation was "○". On the other hand, for sample 15, the polarity value of the solvent was less than 0.18, so the evaluation of the impregnation depth was "×". The reason for this is thought to be that if the polarity of the solvent is too low, the dispersibility of the metal powder slurry decreases and the wettability of the slurry on the surface of the porous material decreases, which in turn makes it difficult to promote the introduction of metal powder into the pores of the porous material.

[0099] Samples 16 and 17 all contained additives within the range of 1 to 5 parts by mass per 100 parts by mass of nickel powder, resulting in an overall evaluation of "○". On the other hand, sample 18 exceeded the above upper limit for additive content, resulting in an evaluation of "×" for impregnation depth. The reason for this is thought to be that although additives play a role in improving dispersibility, if the content in the nickel powder slurry becomes too high, problems such as foaming occur, and as a result, it becomes difficult to promote the introduction and diffusion of nickel powder into the pores of the porous material.

[0100] Sample 19 received an overall evaluation of "○" because the mass-average molecular weight of the organic polymer used as an additive was within the range of 1k to 10k and had a comb-like structure. On the other hand, in Sample 20, the mass-average molecular weight of the organic polymer exceeded the above upper limit, and in Sample 21, the organic polymer did not contain a carboxylic acid, so both received an evaluation of "×" for impregnation depth. The reason for this is thought to be that if the molecular weight of the organic polymer becomes too high, the viscosity of the nickel powder slurry containing it increases, resulting in poor introduction and diffusion of nickel powder into the pores of the porous material. Also, if the organic polymer does not contain a carboxylic acid, the dispersibility and wettability effects provided by it are not achieved, so in this case as well, it is thought that the introduction and diffusion of nickel powder into the pores of the porous material will not proceed well.

[0101] Sample 22 received an overall evaluation of "○" because the organic polymer used as an additive had a comb-like structure and a mass-average molecular weight within the range of 1k to 10k, and its content per 100 parts by mass of nickel powder was within the range of 1 to 5 parts by mass. On the other hand, although the organic polymer used as an additive in Sample 23 had a comb-like structure and a mass-average molecular weight within the range of 1k to 10k, its content per 100 parts by mass of nickel powder was at the upper limit, resulting in a relatively high viscosity and an evaluation of the impregnation depth of "△".

[0102] In sample 24, by using an aqueous nickel nitrate solution instead of a metal powder slurry, the viscosity was significantly reduced, allowing the solution to impregnate almost the entire porous body. As a result, the amount of metal supported up to 200 μm from the dropping surface was significantly lower compared to samples 1-23, which used a metal powder slurry.

Claims

1. A metal powder slurry comprising metal powder, a solvent, and an additive, The metal powder has a number-average particle size of 40 nm to 200 nm and is present in an amount of 1% to 30% by mass relative to the metal powder slurry; the solvent has a boiling point of 50°C to 150°C; and the additive is an organic polymer containing a carboxylic acid and having a comb-like structure, with a mass-average molecular weight of 1,000 to 10,000, and is present in an amount of 1 to 5 parts by mass per 100 parts by mass of the metal powder.

2. The metal powder slurry according to claim 1, wherein the additive comprises a polymer of polyalkylene glycol methacrylate and acrylic acid and / or methacrylic acid, and the degree of polymerization of the acrylic acid and / or methacrylic acid is 5 or more and 70 or less.

3. The metal powder slurry according to claim 1, wherein the metal powder is contained in an amount of 5% by mass or more and 30% by mass or less relative to the metal powder slurry.

4. The metal powder slurry according to claim 1, wherein the solvent is water and / or a polar organic solvent.

5. The metal powder slurry according to claim 4, wherein the polar organic solvent is a solvent in which the value obtained by dividing the sum of the polar term δP and the hydrogen bonding term δH of the Hansen solubility parameter by the molecular weight is 0.18 or more.

6. The metal powder slurry according to claim 4, wherein the polar organic solvent is ethanol.

7. The metal powder slurry has a shear speed of 1000 s. －１ The metal powder slurry according to claim 1, wherein the viscosity in the solution is 1 mPa·s or more and less than 130 mPa·s.

8. The metal powder slurry according to claim 1, wherein the metal powder comprises at least one of Ni, Co, and Cu.

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