Method for producing metal nanowire catalyst

Abstract

SOLUTION: A method for producing a metal nanowire catalyst 10 comprises a supporting step (S20) of supporting metal nanowires 14 on carbon supports 12, and a washing step (S30) of removing impurities from supports obtained by supporting the metal nanowires 14 on the carbon supports 12, wherein the washing step (S30) comprises a first mixing step (S31) of bringing a solution containing a hydride reducing agent into contact with the supports, and a first separation step of removing first impurities separated from the supports from a first mixture obtained in the first mixing step (S31).SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a metal nanowire catalyst. [Background technology]

[0002] Metal nanowires are expected to be a catalyst material with excellent catalytic activity due to their large surface area relative to their mass (specific surface area), and excellent durability due to their relatively stable surface. Such metal nanowires are synthesized by reducing metal salts in a solvent to which a dispersant and a protective agent have been added. Examples of metal nanowires that have been synthesized include gold, silver, copper, platinum, and nickel.

[0003] Non-Patent Document 1 describes an example of synthesis of metal nanowires made of platinum-nickel alloy and their catalytic activity. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] "Synthesis of Platinum Alloy Nanowire Catalyst and Its Oxygen Reduction Reaction Activity", Yoshimi Iguchi et al., Battery Symposium PDF Abstracts (CD-ROM), Vol. 61, ROMBUNNO.3G02, 2020 Summary of the Invention [Problem to be solved by the invention]

[0005] The metal nanowires synthesized as described above are mixed with a carbon support in a solvent to be supported on the carbon support, yielding a metal nanowire catalyst. A dispersant is used when supporting the metal nanowires on the carbon support. The dispersant is adsorbed on the surface of the metal nanowires to prevent aggregation of the metal nanowires in the solvent. However, if the metal nanowires are not washed sufficiently, the surface of the metal nanowires will be covered with impurities, resulting in reduced catalytic activity.

[0006] In order to remove impurities from the metal nanowires, the inventors attempted to wash them using organic solvents and water, but found that conventional washing methods were unable to sufficiently remove the impurities and did not improve catalytic activity as much as expected.

[0007] Furthermore, the inventors of the present application noticed that organic substances were used as dispersants and protective agents, and attempted to remove impurities by heating the metal nanowire catalyst to temperatures of several hundred degrees Celsius and volatilizing or oxidizing (burning). However, they found that treating the metal nanowire catalyst at high temperatures caused sintering (aggregation due to sintering) of the carbon support and metal nanowire catalyst, reducing the surface area and, in turn, reducing catalytic activity.

[0008] An object of the present invention is to solve the above-mentioned problems. [Means for solving the problem]

[0009] An aspect of the present disclosure is a method for producing a metal nanowire catalyst, comprising: a supporting step of supporting metal nanowires on a carbon support; and a washing step after the supporting step of removing impurities from the support having the metal nanowires supported on the carbon support, wherein the washing step comprises a first mixing step of contacting the support with a solution containing a hydride reducing agent; and a first separation step of removing, from the first mixture obtained in the first mixing step, a first impurity that was separated from the support in the first mixing step. [Effects of the Invention]

[0010] The above metal nanowires catalyst According to the manufacturing method of (1), it is possible to remove impurities attached to the metal nanowires while maintaining the shape of the metal nanowires, thereby improving catalytic activity. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is an explanatory diagram of a method for producing a metal nanowire catalyst. [Figure 2] FIG. 2 is an explanatory diagram of the synthesis process for synthesizing metal nanowires. [Figure 3] FIG. 3 is an explanatory diagram of the supporting step of supporting metal nanowires on a carbon support. [Figure 4] FIG. 4 is an explanatory diagram of a cleaning process for a carbon support (support) carrying metal nanowires. [Figure 5] Figure 5 is a graph showing the results of measuring the weight change using a thermobalance method for a metal nanowire catalyst (Comparative Example 1) that was not subjected to the cleaning process of Figure 4 and a metal nanowire catalyst (Example) that was subjected to the cleaning process of Figure 4. [Figure 6] Figure 6A is a transmission electron microscope photograph of the metal nanowire catalyst of Comparative Example 1, Figure 6B is a transmission electron microscope photograph of the metal nanowire catalyst of Comparative Example 2 that was heat-treated at 450°C, and Figure 6C is a transmission electron microscope photograph of the metal nanowire catalyst of the Example that was washed with a cleaning solution containing sodium borohydride. [Figure 7] FIG. 7 is a table showing the results of measuring the surface atomic concentrations of the metal nanowire catalysts according to Comparative Example 1, Comparative Example 2, and Example by X-ray photoelectron spectroscopy (XPS). [Figure 8] FIG. 8 is a graph showing the results of measuring the mass activity ratios of the metal nanowire catalysts according to Comparative Example 1, Comparative Example 2, and Example by a rotating disk electrode method (RDE). DETAILED DESCRIPTION OF THE INVENTION

[0012] 6A to 6C, metal nanowire catalyst 10 has a structure in which short metal nanowires 14 are dispersed and supported on the surface of fine granular carbon support 12 (carbon particles). Such metal nanowire catalyst 10 is used, for example, in electrodes of electrochemical cells such as fuel cells or water electrolysis devices.

[0013] The carbon support 12 is not particularly limited as long as it can support the metal nanowires 14. For example, CA250 (trade name) manufactured by Denka Company Limited, OSAB (trade name) manufactured by Denka Company Limited, Vulcan (trade name) manufactured by Cabot Corporation, Ketjenblack (registered trademark) manufactured by Ketjenblack International, Norit (trade name) manufactured by Norit Corporation, Black Pearl manufactured by Cabot Corporation, Acetylene Black (trade name) manufactured by Chevron Corporation, and VGCF (registered trademark) manufactured by Resonac Corporation can be used as the carbon support 12. Furthermore, the carbon support 12 is not limited to being granular, and may be one-dimensional or two-dimensional, such as a carbon nanotube, a carbon nanohorn, a carbon nanowall, or a carbon nanofiber.

[0014] The carbon support 12 is dispersed in a particulate form in a solvent in a supporting step described later. In the solvent, the carbon support 12 may form aggregates in which a plurality of particles are aggregated.

[0015] The metal element constituting the metal nanowire 14 can be appropriately selected from any metal element depending on the purpose. The metal element may be, for example, a base metal element such as nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), tin (Sn), aluminum (Al), zinc (Zn), titanium (Ti), niobium (Nb), tungsten (W), molybdenum (Mo), chromium (Cr), or vanadium (V). The metal element may also be a noble metal element such as platinum (Pt), silver (Ag), gold (Au), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), or osmium (Os). The above elements may be present alone or in combination with other elements, or the metal nanowire 14 may be composed of a plurality of the above-listed elements. The metal nanowire 14 may also be composed of an alloy combining several of the above elements. As an electrode catalyst for a fuel cell, metal nanowires 14 made of platinum or a platinum alloy are suitable.

[0016] Generally, the metal nanowires 14 have a diameter of about 1 nm to 10 nm. The length of the metal nanowires 14 varies depending on the type of material and the manufacturing method, but is generally about several hundred nm to several μm. The metal nanowires 14 have a long, thin wire shape because their total length is longer than their diameter. The metal nanowires 14 may be used in the metal nanowire catalyst 10 in a state where they are shortened and cut into short fibers, for example, compared to immediately after synthesis.

[0017] In one embodiment, the metal nanowires 14 may be intentionally shortened to a length that allows them to be supported on the carbon support 12. If the average length of the metal nanowires 14 in the metal nanowire catalyst 10 is equal to or less than the particle size of the carbon support 12 or its aggregates (secondary particles), dispersibility is improved and they can be favorably supported on the carbon support 12. Note that if the length of the metal nanowires 14 immediately after synthesis is sufficiently short, the fiber shortening step is not necessary.

[0018] The metal nanowire catalyst 10 of this embodiment is produced by the following method.

[0019] As shown in FIG. 1, the method for producing the metal nanowire catalyst 10 of this embodiment includes, in order of steps, a synthesis step (S10), a supporting step (S20), and a washing step (S30).

[0020] The method for producing the metal nanowire catalyst 10 begins with a synthesis step (S10). The synthesis step (S10) is a step for synthesizing metal nanowires 14 and includes the steps shown in Figure 2. The synthesis step (S10) begins with a metal salt solution preparation step (S11). The metal salt solution preparation step (S11) includes a step of mixing a metal salt as a precursor with glucose, octadecene, and oleylamine.

[0021] The precursor used in the metal salt solution preparation step (S11) is a salt of the above metal element. The precursor is selected from substances that can be reduced to a metal by reaction with the mixture in the synthesis step described below. For example, when synthesizing platinum nanowires, platinum(II) acetylacetonate can be used as the precursor. Octadecene is the solvent. Other solvents can also be used instead of octadecene.

[0022] Oleylamine functions as a reducing agent and stabilizer in subsequent processes. That is, oleylamine acts as an electron donor at high temperatures, reducing the precursor metal salt and depositing the metal. Oleylamine also suppresses the growth of the deposited metal particles by covering their surfaces, stopping particle growth at nanosize and allowing more metal salt to be used for growing metal nanowires 14.

[0023] Glucose also functions as a reducing agent for the precursor. Instead of glucose, a polyol can be used. The polyol can be hexadecanediol, tetraethylene glycol, propylene glycol, trimethylene glycol, diethylene glycol, ethylene glycol, or stearyl glycol.

[0024] In the metal salt solution preparation step (S11), the mixture is mixed with a stirrer or the like, and then further dispersed uniformly with an ultrasonic homogenizer or the like.

[0025] Next, the synthesis step (S10) proceeds to a catalyst addition step (S12) in which a protective agent and a catalyst are added. The protective agent is, for example, polyvinylpyrrolidone (PVP). Polyvinylpyrrolidone has multiple carbonyl groups (C=O bond groups) in its molecule. The carbonyl groups of polyvinylpyrrolidone adsorb to the deposited metal, causing chain-like polyvinylpyrrolidone molecules to surround the deposited metal. This prevents the metal from growing spherically, and the deposited metal grows one-dimensionally (in a wire-like shape). The catalyst is added to promote the growth of metal nanowires 14. The catalyst may be, for example, a tungsten (W) complex, a chromium (Cr) complex, or a molybdenum (Mo) complex. Tungsten hexacarbonyl can be suitably used when synthesizing platinum nanowires.

[0026] Next, the synthesis step (S10) proceeds to a heat treatment step (S13). In the heat treatment step (S13), the mixed solution prepared in the catalyst addition step (S12) is heated to a predetermined reaction temperature (for example, 110°C to 140°C) to grow metal nanowires 14. The heat treatment step (S13) is carried out for several tens of minutes to several tens of hours. The heat treatment step (S13) includes a temperature increase step of heating from room temperature to a predetermined temperature, and a cooling step of cooling from the predetermined temperature to room temperature. The temperature increase step and the cooling step are carried out at a predetermined temperature increase rate or temperature decrease rate, respectively. The heat treatment step may be carried out while stirring the mixed solution.

[0027] Next, the synthesis step (S10) proceeds to a filtration step (S14). In the filtration step (S14), the metal nanowires 14 are extracted from the mixed solution by a method such as filtration.

[0028] Next, the synthesis step (S10) proceeds to a separation step (S15). In the separation step (S15), a washing solvent is added to the metal nanowires 14 recovered by filtration. The washing solvent may be, for example, a mixture of acetone, cyclohexane, and ethanol. Adding the washing solvent removes some of the impurities adhering to the metal nanowires 14. The washing solvent and the metal nanowires 14 are mixed by stirring and applying ultrasonic waves.

[0029] Next, in the separation step (S15), centrifugation is performed to separate the mixture containing the washing solvent and the metal nanowires 14 into a precipitate layer and a supernatant layer. The precipitate layer contains the metal nanowires 14, and the supernatant layer is composed of the washing solvent. Thereafter, the supernatant layer is removed, and the metal nanowires 14 are recovered as the precipitate layer.

[0030] Thereafter, the separation step (S15) may be repeated multiple times by adding more washing solvent to the recovered precipitate layer and performing centrifugation.

[0031] The metal nanowires 14 obtained in the separation step (S15) are surrounded by impurities containing polyvinylpyrrolidone and oleylamine. The impurities surrounding the multiple metal nanowires 14 tend to form hydrophobic bonds due to interactions between nonpolar groups. Therefore, drying the metal nanowires 14 promotes aggregation of the metal nanowires 14, making it difficult to disperse the metal nanowires 14 in the carbon support 12. Therefore, after the separation step (S15), a storage solvent addition step (S16) is performed in which a storage solvent is added to the metal nanowires 14. For example, ethanol (alcohol) can be used as the storage solvent. In the storage solvent addition step (S16), ultrasonic waves may be applied to disperse the metal nanowires 14 in the storage solvent. The storage solvent addition step (S16) produces a metal nanowire dispersion in which the metal nanowires 14 are dispersed in the storage solvent.

[0032] This completes the synthesis step (S10). The metal nanowires 14 synthesized in the synthesis step (S10) are used in the supporting step (S20) as a metal nanowire dispersion.

[0033] Next, the method for producing the metal nanowire catalyst 10 proceeds to the supporting step (S20). As shown in FIG. 3, the supporting step (S20) proceeds to the support mixing step (S21) in which the carbon support 12 and the metal nanowires 14 are mixed. In the support mixing step (S21), the carbon support 12, the metal nanowire dispersion, and the dispersion solvent are mixed. The dispersion solvent is a mixture of a polar solvent and a non-polar solvent. For example, an organic solvent mixture of ethanol and cyclohexane can be used as the dispersion solvent. Furthermore, in the support mixing step (S21), oleylamine may be further added as a dispersant. Oleylamine can efficiently disperse the metal nanowires 14, which have been aggregated into bundles by polyvinylpyrrolidone, in the solvent.

[0034] Next, the supporting step (S20) proceeds to an ultrasonic treatment step (S22). In the ultrasonic treatment step (S22), preparation Ultrasonic waves are applied to the resulting mixture using an ultrasonic homogenizer. The ultrasonic treatment step (S22) is performed for, for example, several tens of minutes to several hours. The ultrasonic treatment step (S22) separates the metal nanowires 14 that have aggregated into bundles, and the separated metal nanowires 14 are shortened into short fibers. The length of the metal nanowires 14 depends on the frequency and power of the applied ultrasonic waves. The shortened metal nanowires 14 adhere to the surface of the carbon support 12 in a dispersed state, and a support in which the metal nanowires 14 are supported on the carbon support 12 is obtained.

[0035] Next, the supporting step (S20) proceeds to a concentration step (S23). In the concentration step (S23), the mixture that has been subjected to the ultrasonic treatment step (S22) is separated by centrifugation into a precipitate layer containing the support and a supernatant layer containing the dispersion solvent. In the concentration step (S23), the supernatant layer is removed and the precipitate layer in which the support is concentrated is recovered.

[0036] Next, the supporting step (S20) proceeds to a drying step (S24). In the drying step (S24), the precipitate layer (suspension) obtained in the concentration step (S23) is dried. The drying step (S24) is performed, for example, by drying under reduced pressure. In the drying step (S24), the solvent is removed from the support, and a solid support is obtained.

[0037] Thereafter, the supporting step (S20) proceeds to a firing step (S25). The firing step (S25) is performed by heating the support to a firing temperature of 200°C to 300°C in an inert gas atmosphere containing, for example, about 3% hydrogen gas. For example, if the firing temperature in the firing step (S25) is set to about 250°C, this is preferable because it promotes the removal of impurities attached to the surfaces of the metal nanowires 14 while preventing shrinkage due to sintering.

[0038] Next, the method for producing the metal nanowire catalyst 10 proceeds to the cleaning step (S30). As shown in FIG. 4, the cleaning step (S30) includes the steps shown in the figure. The cleaning step (S30) begins with a first mixing step (S31). The first mixing step (S31) includes a process of mixing the support with a first cleaning solution to form a first mixture. The first mixing step (S31) separates first impurities from the support. The first cleaning solution is a solution containing a hydride reducing agent and a solvent. The solvent used for the first cleaning solution contains, for example, alcohol and water.

[0039] The hydride reducing agent used in the first cleaning solution hydrogenates the carbon-carbon double bonds of the organic matter remaining on the metal nanowire support. The hydride reducing agent also reduces carbonyl compounds, such as ketones and aldehydes, in the organic matter to alcohols. The hydride reducing agent chemically alters the organic matter, thereby reducing its adsorptivity to the metal nanowires 14. The hydride reducing agent also chemically alters the organic matter as impurities, thereby improving its affinity for the solvent.

[0040] The hydride reducing agent may be at least one of sodium borohydride, lithium aluminum hydride, lithium borohydride, borane, sodium cyanoborohydride, and sodium triacetoxyborohydride. Sodium borohydride is safe to handle and is suitable in terms of ease of handling. However, the cleaning effect of the hydride reducing agent is not limited to sodium borohydride.

[0041] In the first mixing step (S31), impurities such as oleylamine and polyvinylpyrrolidone used in the supporting step are believed to adhere to the surface of the metal nanowires 14. Oleylamine accepts hydride ions from the hydride reducing agent to become octadecylamine. For example, when sodium borohydride is used as the hydride reducing agent, oleylamine and sodium borohydride react as follows: C 18 H 37 N + NaBH4 → C 18 H 37 NH2 + NaBO2 + H2 (Reaction 1) C 18 H 37 N+2NaBH4→C 18 H 37 NH2 + 2NaB(OH)2 + H2 (Reaction 2)

[0042] In addition, some of the sodium borohydride also reacts with water to produce sodium tetrahydroborate and hydrogen (Reaction Scheme 2). The above reaction is an example of a reaction called reductive amination, which utilizes the reducing power of a hydride reducing agent. This reaction converts unsaturated functional groups such as carbonyl compounds and imines into amines.

[0043] The first cleaning liquid is prepared in advance of the first mixing step (S31). Since the decomposition of the hydride reducing agent also proceeds in the presence of water, it is preferable that the first cleaning liquid be prepared immediately before the first mixing step (S31).

[0044] Next, the washing step (S30) proceeds to a first centrifugation step (S32). In the first centrifugation step, the first mixture obtained in the first mixing step (S31) is centrifuged. In the first centrifugation step (S32), the first mixture is separated into a first precipitate layer containing the carrier and a first supernatant layer containing impurities separated from the carrier.

[0045] Next, the washing step (S30) proceeds to a first supernatant removal step (S33). In the first supernatant removal step (S33), the first supernatant layer separated in the first centrifugation step (S32) is removed. As a result, a first precipitate layer containing the support is recovered.

[0046] If necessary, the process may return to the first mixing step (S31), and the steps from the first mixing step (S31) to the first supernatant removal step (S33) may be repeated on the first precipitate layer.

[0047] Thereafter, the washing step (S30) proceeds to a second mixing step (S34). In the second mixing step (S34), the first precipitate layer is mixed with a second washing liquid to prepare a second mixture. Pure water (ion-exchanged water) is used as the second washing liquid. The second washing liquid removes second impurities consisting of ionic substances from the support. Examples of the second impurities include calcium ions, magnesium ions, sodium ions, potassium ions, chloride ions, bicarbonate ions, sulfate ions, and nitrate ions. In this embodiment, the second mixing step (S34) is also effective in removing nitrate ions derived from the oxidation of oleylamine.

[0048] Next, the washing step (S30) proceeds to a second centrifugation step (S35). In the second centrifugation step (S35), the second mixture obtained in the second mixing step (S34) is centrifuged. By the second centrifugation step (S35), the second mixture is separated into a second precipitate layer containing the support and a second supernatant layer containing second impurities separated from the support.

[0049] Next, the washing step (S30) proceeds to a second supernatant removal step (S36), in which the second supernatant layer is removed and the second precipitate layer is collected.

[0050] If necessary, the second mixing step (S34) to the second supernatant removal step (S36) may be repeated once or multiple times.

[0051] Thereafter, the washing step (S30) proceeds to a third mixing step (S37). In the third mixing step (S37), a third mixture is prepared by mixing a third washing liquid with the second precipitate layer. The third washing liquid is an organic solvent, and for example, a polar solvent can be used. For example, acetone, methanol, ethanol, acetic acid, ethyl acetate, chloroform, toluene, xylene, methyl ethyl ketone, or a mixture thereof can be used as the third washing liquid. Furthermore, using acetone, a polar solvent with high affinity for water, as the third washing liquid is preferable because it can remove not only fat-soluble impurities but also water-soluble impurities.

[0052] The third mixing step (S37) removes third impurities from the support. The third impurities are, for example, fats, oils, waxes, resins, rubbers, polymers, etc. The third impurities may include the protective agent used in the synthesis step (S10) and the dispersant used in the support step (S20).

[0053] Next, the washing step (S30) proceeds to a third centrifugation step (S38), which separates the third mixture into a third supernatant layer and a third precipitate layer.

[0054] Next, the washing step (S30) proceeds to a third supernatant removal step (S39). In the third supernatant removal step (S39), the third supernatant layer is removed. The support is recovered together with the third precipitate layer.

[0055] If necessary, the third mixing step (S37) to the third supernatant removal step (S39) may be repeated.

[0056] Next, the washing step (S30) proceeds to the drying step (S40). The drying step (S40) is carried out by subjecting the third precipitate layer to reduced pressure drying. The reduced pressure drying is carried out by maintaining heating at 50°C for 72 hours in a reduced pressure atmosphere.

[0057] This completes the cleaning step (S30), and the metal nanowire catalyst 10 of this embodiment is completed.

[0058] Below, as an example of the metal nanowire catalyst 10, a platinum nanowire catalyst was actually produced and evaluated, and the results will be described.

[0059] (Synthesis of platinum nanowires) A metal salt solution was prepared by mixing 18.75 mmol of platinum(II) acetylacetonate (Pt(acac)2), 22.5 mmol of glucose, 30 ml of 1-octadecene, and 45 ml of oleylamine with a stirrer (S11). Ultrasonic waves were then applied to the metal salt solution for 10 minutes, and further stirring and mixing were performed.

[0060] Next, in the catalyst addition step (S12), 0.225 mmol of polyvinylpyrrolidone and 0.15 mmol of hexacarbonyltungsten (W(CO)6) were added to the metal salt solution. Then, in the heat treatment step (S13), the metal salt solution was subjected to heat treatment. The heat treatment step (S13) was carried out at 120°C for 3 hours while stirring the metal salt solution.

[0061] Next, the metal salt solution after the heat treatment was subjected to a filtration step (S14), and the platinum nanowires were recovered. Next, a separation step (S15) was performed. That is, a mixture of acetone, cyclohexane, and ethanol was added to the recovered platinum nanowires. Thereafter, the suspension containing the platinum nanowires was centrifuged, and the supernatant layer was removed, and the platinum nanowires were recovered together with the precipitate layer.

[0062] Ethanol was added as a storage solution to the precipitate layer obtained by centrifugation, and ultrasonic waves were applied for 15 minutes. This process resulted in the preparation of a platinum nanowire dispersion in which platinum nanowires were dispersed in ethanol. The platinum nanowire dispersion thus prepared was transferred to a bottle and stored.

[0063] (Supporting process) Next, a supporting step (S20) was carried out in which the synthesized platinum nanowires were supported on the carbon support 12. 9630 mg of carbon support 12 was collected, and a platinum nanowire dispersion containing 107 mg of platinum nanowires, 200 ml of ethanol, 200 ml of cyclohexane, and 20 ml of oleylamine were added to the carbon support 12 and mixed, thereby carrying out a support mixing step (S21).

[0064] Next, as an ultrasonic treatment step (S22), the carrier mixing step (S21) preparation Ultrasonic waves were applied to the resulting mixture using an ultrasonic homogenizer. The ultrasonic treatment step (S22) was carried out for 60 minutes. Next, in the concentration step (S23), the mixture that had undergone the ultrasonic treatment step (S22) was centrifuged. Thereafter, the supernatant layer of the mixture was removed, and the support was recovered as a precipitate layer. Thereafter, in the drying step (S24), the support was dried under reduced pressure. Thereafter, in the calcination step (S25), the support was calcined at 250°C for 12 hours in an inert gas atmosphere containing hydrogen gas at a concentration of 3%.

[0065] (Comparative Example 1) The metal nanowire catalyst 10 of Comparative Example 1 is a support obtained in the above-mentioned calcination step (S25).

[0066] (Comparative Example 2) The metal nanowire catalyst 10 of Comparative Example 2 was prepared by carrying out an additional calcination treatment at 450° C. on the support obtained in the calcination step (S25) described above in order to remove organic substances.

[0067] (Example) The metal nanowire catalyst 10 of the example was prepared by carrying out the following washing step on the support obtained in the above-mentioned calcination step (S25). First, in the first mixing step (S31), 3.33 g of sodium borohydride was mixed with the first washing liquid to form a first mixture. preparation The first washing liquid was a mixture of 83 ml of water and 83 ml of ethanol. Next, 250 mg of the support obtained in the baking step (S25) and 166 ml of the first washing liquid were placed in a specified container and mixed. This operation caused foaming from the support. After the foaming from the support subsided, centrifugation was performed as a first centrifugation step (S32), and the supernatant layer of the first mixture was removed as a first supernatant removal step (S33). The addition of the first washing liquid (first mixing step (S31)), the first centrifugation step (S32), and the first supernatant removal step (S33) were repeated five times. The first washing liquid was prepared and used as needed.

[0068] Next, in the second mixing step (S34), the first precipitate layer was transferred to a 50 ml centrifuge tube, 40 ml of pure water (ion-exchanged water) was added, and the mixture was mixed by hand for 1 minute. This was followed by a second centrifugation step (S35) and a second supernatant removal step (S36). The second mixing step (S34), second centrifugation step (S35), and second supernatant removal step (S36) were repeated three times.

[0069] Next, in the third mixing step (S37), 40 ml of acetone was added to the second precipitate layer as a third washing solution, and the mixture was mixed by hand for 1 minute. This was followed by the third centrifugation step (S38) and the third supernatant removal step (S39). The third mixing step (S37), the third centrifugation step (S38), and the third supernatant removal step (S39) were repeated three times.

[0070] Thereafter, in the drying step (S40), the suspension of the third precipitate layer was dried under reduced pressure at 50° C. for 72 hours, thereby obtaining the metal nanowire catalyst 10 according to the example.

[0071] Next, the metal nanowire catalysts 10 according to Comparative Example 1, Comparative Example 2, and Example were evaluated.

[0072] (Evaluation results by TG) As shown in FIG. 5, weight changes were measured by thermogravimetry (TG) for metal nanowire catalyst 10 of Comparative Example 1 and metal nanowire catalyst 10 of the Example. The measurements were performed in a nitrogen gas atmosphere at a temperature increase rate of 10°C / min. As shown in the figure, as the temperature increases, impurities adhering to metal nanowire catalyst 10 are volatilized and removed, resulting in a decrease in weight. The weight change for metal nanowire catalyst 10 of the Example was approximately 0.3% smaller than the weight change for metal nanowire catalyst 10 of Comparative Example 1. This result indicates that impurities are reduced in metal nanowire catalyst 10 of the Example, which was subjected to the cleaning step (S30).

[0073] (TEM observation results) Next, as shown in Figures 6A to 6C, the metal nanowire catalysts 10 of Comparative Example 1, Comparative Example 2, and Example were observed using a transmission electron microscope. As shown in Figure 6A (Comparative Example 1) and Figure 6C (Example), the carbon support 12 and platinum nanowires are dispersed in the metal nanowire catalysts 10 of Comparative Example 1 and Example, maintaining a high surface area. In contrast, as shown in Figure 6B, the carbon support 12 and platinum nanowires in the metal nanowire catalyst 10 of Comparative Example 2 have undergone sintering due to heat treatment at 450°C. In Comparative Example 2, the porosity of the carbon support 12 has decreased, and the platinum nanowires have aggregated into clumps.

[0074] (Atomic concentration measurement by XPS) As shown in Figure 7, the surface atomic concentration of the metal nanowire catalyst 10 of each of Comparative Example 1, Comparative Example 2, and Example was measured by X-ray photoelectron spectroscopy. The nitrogen atomic concentration is derived from the dispersant and protective agent (oleylamine). oxygenThe atomic concentration of platinum reflects the reduction state of the platinum nanowire surface, and the lower this value, the more reduction has progressed and the better the catalytic activity. The platinum atomic concentration indicates that more platinum nanowires are present on the surface of the metal nanowire catalyst 10.

[0075] It was confirmed that the metal nanowire catalyst 10 of Comparative Example 1, which was not subjected to a cleaning process, had higher atomic concentrations of nitrogen and oxygen than those of Comparative Example 2 and the Example, and that more impurities were attached to the surface. It was also confirmed that the amount of platinum nanowires exposed on the surface of the metal nanowire catalyst 10 of Comparative Example 1 was smaller than those of Comparative Example 2 and the Example.

[0076] In the case of metal nanowire catalyst 10 of Comparative Example 2 (heat-treated at 450°C), the atomic concentration of nitrogen derived from the dispersant and protective agent was reduced, resulting in the lowest remaining impurities. The atomic concentration of oxygen was also reduced, confirming that the reduction of the surface of the platinum nanowires was progressing. Furthermore, the atomic concentration of platinum was also increased, confirming that more platinum nanowires were exposed on the surface compared to Comparative Example 1.

[0077] In the case of metal nanowire catalyst 10 of the example, the atomic concentration of nitrogen was reduced compared to Comparative Example 1, confirming that the impurity concentration was reduced by the cleaning process. The atomic concentration of oxygen in metal nanowire catalyst 10 of the example was reduced compared to Comparative Example 1, confirming that the reduction of the surface of the platinum nanowires was progressing. Furthermore, the atomic concentration of platinum was increased in the example compared to Comparative Example 1, confirming that more platinum nanowires were exposed on the surface.

[0078] (Mass activity ratio by RDE method) Next, as shown in Figure 8, the mass activity ratio was measured by the RDE (Rotating Disc Electrode) method. The Pt / C catalyst was used for comparison, and the measurement results were for a normal platinum catalyst in which platinum particles were supported on a carbon support 12. The mass activity ratio in Figure 8 is a value (relative value) that expresses the catalytic activity per 1g of platinum in the catalyst to be measured as a ratio to the catalytic activity per 1g of platinum in the Pt / C catalyst. The mass activity ratio of the Pt / C catalyst is 1.

[0079] The metal nanowire catalyst 10 of Comparative Example 1 had a mass activity ratio of 1.7, and thus exhibited higher catalytic activity than when platinum particles were supported.

[0080] The metal nanowire catalyst 10 of Comparative Example 2 had a mass activity ratio of 0.4, which was lower than that of the Pt / C catalyst. Although the metal nanowire catalyst 10 of Comparative Example 2 had a reduced impurity concentration as shown in Figure 7, it is believed that sintering of the platinum nanowires occurred, reducing the surface area and decreasing the catalytic activity as shown in Figure 6B.

[0081] In contrast, the metal nanowire catalyst 10 according to the example exhibited a mass activity ratio twice that of the Pt / C catalyst, and thus exhibited an improved mass activity ratio compared to Comparative Example 1. The metal nanowire catalyst 10 according to the example was washed using a hydride reducing agent, thereby effectively removing impurities. Furthermore, the metal nanowire catalyst 10 according to the example can prevent sintering. Furthermore, the catalytic activity of the metal nanowire catalyst 10 according to the example is improved by reducing the surface of the platinum nanowires with the hydride reducing agent. As a result, the metal nanowire catalyst 10 according to the example exhibits excellent catalytic activity.

[0082] The following additional notes are further disclosed regarding the above embodiment.

[0083] (Appendix 1) The method for producing a metal nanowire catalyst (10) of the present disclosure includes a supporting step (S20) of supporting metal nanowires (14) on a carbon support (12), and a washing step (S30) of removing impurities from the support having the metal nanowires supported on the carbon support after the supporting step. The washing step includes a first mixing step (S31) of contacting the support with a solution containing a hydride reducing agent, and a first separation step of removing, from the first mixture obtained in the first mixing step, a first impurity that was separated from the support in the first mixing step.

[0084] (Appendix 2) In the method for manufacturing a metal nanowire catalyst described in Appendix 1, the first separation step may include a first centrifugation step (S32) of centrifuging the first mixture to separate it into a first precipitate layer containing the support and a first supernatant layer containing the first impurity, and a first supernatant removal step (S33) of removing the first supernatant layer.

[0085] (Appendix 3) In the method for producing a metal nanowire catalyst described in Appendix 2, the cleaning process may further include, after the first separation process, a second mixing process (S34) in which pure water is added to the first precipitate layer and mixed, and a second separation process in which a second impurity separated from the support in the second mixing process is removed from the second mixture obtained in the second mixing process.

[0086] (Appendix 4) In the method for manufacturing a metal nanowire catalyst described in Appendix 3, the second separation step may include a second centrifugation step (S35) of centrifuging the second mixture to separate it into a second precipitate layer containing the support and a second supernatant layer containing the second impurity, and a second supernatant removal step (S36) of removing the second supernatant layer.

[0087] (Appendix 5) In the method for producing a metal nanowire catalyst described in Appendix 4, the washing step may further include, after the second separation step, a third mixing step (S37) in which an organic solvent is added to the second precipitate layer and mixed, and a third separation step in which a third impurity separated from the support in the third mixing step is removed from the third mixture obtained in the third mixing step.

[0088] (Appendix 6) In the method for producing a metal nanowire catalyst described in Appendix 5, the third separation step may include a third centrifugation step (S38) of centrifuging the third mixture to separate it into a third precipitate layer containing the support and a third supernatant layer containing the third impurity, and a third supernatant removal step (S39) of removing the third supernatant layer.

[0089] (Appendix 7) The method for producing a metal nanowire catalyst according to Supplementary Note 6 may include, after the third separation step, a drying step (S40) of drying the third precipitate layer.

[0090] (Appendix 8) In the method for producing a metal nanowire catalyst described in any one of Appendixes 1 to 7, the hydride reducing agent may include at least one of sodium borohydride, lithium aluminum hydride, lithium borohydride, borane, sodium cyanoborohydride, and sodium triacetoxyborohydride.

[0091] (Appendix 9) In the method for producing a metal nanowire catalyst according to any one of Supplementary Notes 5 to 7, the organic solvent may include at least one of acetone, toluene, xylene, methyl ethyl ketone, ethyl acetate, ethanol, methanol, acetic acid, and chloroform, which can efficiently remove fat-soluble impurities from the metal nanowire catalyst.

[0092] Although the present disclosure has been described in detail, the present disclosure is not limited to the individual embodiments described above. Various additions, substitutions, modifications, partial deletions, etc. are possible in these embodiments without departing from the gist of the present disclosure or the spirit of the present disclosure derived from the content of the claims and their equivalents. These embodiments can also be implemented in combination. For example, in the above-described embodiments, the order of each operation and the order of each process are shown as examples and are not limited to these. The same applies when numerical values ​​or mathematical expressions are used in the description of the above-described embodiments. [Explanation of symbols]

[0093] 10...metal nanowire catalyst 12...carbon support 14...Metal nanowire

Claims

1. A method for producing a metal nanowire catalyst, comprising: a supporting step of supporting metal nanowires on a carbon support; After the supporting step, a cleaning step is performed to remove impurities that react with at least a hydride reducing agent from the support in which the metal nanowires are supported on the carbon support, and the cleaning step includes: a first mixing step of contacting the support with a solution containing the hydride reducing agent; A method for producing a metal nanowire catalyst, comprising: a first separation step of removing, from the first mixture obtained in the first mixing step, a first impurity that has been separated from the support in the first mixing step.

2. 2. The method for producing a metal nanowire catalyst according to claim 1, wherein the first separation step comprises: a first centrifugation step of centrifuging the first mixture to separate it into a first precipitate layer containing the support and a first supernatant layer containing the first impurities; a first supernatant removal step of removing the first supernatant layer.

3. 3. The method for producing a metal nanowire catalyst according to claim 2, wherein the washing step further comprises, after the first separation step: a second mixing step of adding pure water to the first precipitate layer and mixing the mixture; A method for producing a metal nanowire catalyst, comprising: a second separation step of removing, from the second mixture obtained in the second mixing step, the second impurities separated from the support in the second mixing step.

4. 4. The method for producing a metal nanowire catalyst according to claim 3, wherein the second separation step comprises: a second centrifugation step of centrifuging the second mixture to separate it into a second precipitate layer containing the support and a second supernatant layer containing the second impurities; a second supernatant removal step of removing the second supernatant layer.

5. 5. The method for producing a metal nanowire catalyst according to claim 4, wherein the washing step further comprises, after the second separation step: a third mixing step of adding an organic solvent to the second precipitate layer and mixing the organic solvent; A method for producing a metal nanowire catalyst, comprising: a third separation step of removing third impurities separated from the support in the third mixing step from the third mixture obtained in the third mixing step.

6. 6. The method for producing a metal nanowire catalyst according to claim 5, wherein the third separation step comprises: a third centrifugation step of centrifuging the third mixture to separate it into a third precipitate layer containing the support and a third supernatant layer containing the third impurities; a third supernatant removal step of removing the third supernatant layer.

7. 7. The method for producing a metal nanowire catalyst according to claim 6, further comprising, after the third separation step, a drying step of drying the third precipitate layer.

8. 8. The method for producing a metal nanowire catalyst according to claim 1, wherein the hydride reducing agent comprises at least one of sodium borohydride, lithium aluminum hydride, lithium borohydride, borane, sodium cyanoborohydride, and sodium triacetoxyborohydride.

9. 8. A method for producing a metal nanowire catalyst according to any one of claims 5 to 7, wherein the organic solvent includes at least one of acetone, toluene, xylene, methyl ethyl ketone, ethyl acetate, ethanol, methanol, acetic acid, and chloroform.

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