Supported metal catalyst

Abstract

Provided is a supported metal catalyst that has an electrical conductivity permissible as a catalyst and that is capable of inhibiting generation of hydrogen peroxide.　The present invention provides a supported metal catalyst comprising: a carrier powder; and fine metal particles that are supported by the carrier powder. The carrier powder is an aggregate of fine carrier particles. The fine carrier particles each have a chain portion that is formed by fusion bonding of a plurality of crystallites into the form of a chain. The fine carrier particles are formed of a metal oxide. The metal oxide includes cerium. The supported metal catalyst has an electrical conductivity of at least 10-4 S / cm.

Description

[Technical Field]

[0001] This invention relates to a supported metal catalyst. [Background technology]

[0002] Patent Document 1 discloses an electrode catalyst obtained by supporting platinum on Nb-SnO2 produced by the flame method, and a polymer electrolyte fuel cell using the same. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] WO2015 / 050046 [Overview of the project] [Problems that the invention aims to solve]

[0004] In polymer electrolyte fuel cells like the one described in Patent Document 1, oxygen is reduced during power generation, generating hydrogen peroxide, which can degrade the electrolyte membrane. Therefore, technologies to suppress the generation of hydrogen peroxide are desired.

[0005] This invention has been made in view of these circumstances, and provides a supported metal catalyst that has an acceptable electrical conductivity as a catalyst and can suppress the generation of hydrogen peroxide. [Means for solving the problem]

[0006] According to the present invention, a supported metal catalyst comprises a carrier powder and metal nanoparticles supported on the carrier powder, wherein the carrier powder is an aggregate of carrier nanoparticles, the carrier nanoparticles have a chain-like portion formed by the fusion bonding of a plurality of crystallites in a chain, the carrier nanoparticles are composed of a metal oxide, the metal oxide contains cerium, and the supported metal catalyst has an electrical conductivity of 10 -4 A supported metal catalyst with a density of S / cm or higher is provided.

[0007] As a result of intensive studies by the present inventors, it has been found that when the metal oxide constituting the carrier fine particles contains cerium, the generation of hydrogen peroxide is suppressed. Cerium oxide has extremely low electrical conductivity and is usually not selected as a material constituting the carrier fine particles. However, in the present invention, by increasing the loading amount of the metal fine particles, the electrical conductivity of the supported metal catalyst is increased to a value acceptable as a catalyst of 10 -4 S / cm or more. As a result, a supported metal catalyst having an electrical conductivity acceptable as a catalyst and capable of suppressing the generation of hydrogen peroxide was obtained, and the present invention was completed.

Brief Description of Drawings

[0008] [Figure 1] It is a model diagram of the catalyst structure of the supported metal catalyst 100. [Figure 2] It is a view obtained by extracting the carrier fine particles 150 from FIG. 1. [Figure 3] It is a view showing the state of the branch 160 of the carrier fine particles 150 in FIG. 1. [Figure 4] It is a view showing the gas diffusion path in FIG. 1. [Figure 5] It is a cross-sectional view passing through the center of the burner 2 of the production apparatus 1 for producing the carrier powder. [Figure 6] It is an enlarged view of the region X in FIG. 5. [Figure 7] It is a cross-sectional view taken along the line A-A in FIG. 5. [Figure 8] It is an enlarged view of the region Y in FIG. 7. [Figure 9] It shows a TEM image of the supported metal catalyst 100 of Example 1. [Figure 10] It is a configuration diagram of an apparatus for measuring the mass activity of the oxygen reduction reaction. [Figure 11] It is a graph showing the measurement results of the mass activity of the oxygen reduction reaction. [Figure 12] It is a configuration diagram of an apparatus for measuring the amount of hydrogen peroxide generated. [Figure 13]Figure 13A is a graph showing the measured amount of hydrogen peroxide generated for Example 4 and Comparative Example 1, and Figure 13B is a graph showing the measured amount of hydrogen peroxide generated for Example 3, Example 5, Example 6, and Comparative Example 2. [Figure 14] This graph shows the measurement results of the current-voltage characteristics. [Figure 15] This graph shows the change in open-circuit voltage over time. [Figure 16] This is a schematic diagram showing the state in which Ce3+ is induced in the vicinity of metal nanoparticles 130. [Figure 17] Figure 17A shows the TEM image of the measurement region for the supported metal catalyst 100 of Example 1, and Figure 17B shows the graph of the measurement results. [Figure 18] Figure 18A shows the TEM image of the measurement area for the supported metal catalyst 100 of Example 1, and Figures 18B and 18C are graphs showing the measurement results. [Modes for carrying out the invention]

[0009] Embodiments of the present invention will be described below with reference to the drawings. The various features shown in the embodiments below can be combined with each other. Furthermore, each feature constitutes an independent invention.

[0010] 1. Supported metal catalyst 100 As shown in Figures 1 to 4, the supported metal catalyst 100 comprises a carrier powder, which is an aggregate of carrier nanoparticles 150 having chain-like portions formed by the fusion bonding of multiple crystallites 120 in a chain-like manner, and metal nanoparticles 130 supported on the carrier powder. The following describes each component.

[0011] 1-1. Carrier fine particles 150 and carrier powder As shown in Figures 1 to 3, the carrier nanoparticles 150 have three-dimensional voids 110 surrounded by their branches 160 and pores existing between the multiple branches. The branches 160 are portions where the chain-like part, formed by the fusion bonding of multiple crystallites 120 constituting the carrier nanoparticles 150 in a chain-like manner, branches off. The three-dimensional arrangement of the carrier nanoparticles 150 described above forms a gas diffusion pathway for diffusing oxygen, which is an oxidizing agent, and / or hydrogen, which is a fuel, and transporting them onto the supported metal catalyst 100.

[0012] As shown in Figures 1 to 3 as examples of structural models of supported metal catalysts, the carrier nanoparticle 150 has a total of four pores: a first pore surrounded by points where branches connect (branching points, hereafter sometimes simply referred to as branches) b1, b2, b5, b4, b1; a second pore surrounded by branching points b1, b2, b3, b1; a third pore surrounded by branching points b2, b3, b6, b7, b5, b2; and a fourth pore surrounded by branching points b1, b3, b6, b7, b5, b4, b1. If the surface surrounded by the branching points of each pore (the first to fourth pores) is called the pore surface, then the void 110 is a three-dimensional space surrounded by these four pore surfaces. The carrier nanoparticle 150 thus has multiple pores surrounded by multiple branching points where multiple branches connect. The structure is such that three-dimensional spaces (voids) surrounded by multiple pores are continuously provided. Therefore, this void becomes a gas diffusion path for gases such as oxygen and hydrogen. Figure 4 is a diagram showing the gas diffusion path in Figure 1. Figure 4 shows an example of a gas diffusion path in the void 110. The flow (gas diffusion path) 170 of oxidizer (gas), fuel gas, etc. can flow in the desired direction through the void 110 as shown in Figure 4. In other words, this void 110 becomes a gas diffusion path.

[0013] Furthermore, a simple configuration of the carrier nanoparticles 150 may be one in which there is simply one pore (for example, a first pore surrounded by branching points b1, b2, b5, b4, b1). In this case, there will be a void 110 equal to the thickness of the crystallite grains of the crystallite 120. In an even simpler configuration, the carrier nanoparticles 150 may have one or more branches. Even in this case, because there are branches between the carrier nanoparticles 150, they cannot adhere tightly to each other, and a void 110 can be provided between them.

[0014] The term "pore" used above can also be rephrased as "closed curve (closed loop)." Alternatively, it can be rephrased as having a void 110 surrounded by a closed surface containing multiple branching points (for example, branching points b1 to b7). The branching points b1 to b7 can be considered as the centroids of the crystallites of the metal oxide constituting the carrier nanoparticles 150 to which the branches are connected, or they can be any single point on these crystallites.

[0015] The size of the crystallites 120 is preferably 10 to 30 nm, and more preferably 10 to 15 nm. Specifically, this size is, for example, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, and 30 nm, and may also be within the range of any two of the values ​​exemplified here. The size of the crystallites 120 (crystallite diameter) can be determined from the full width at half maximum of the peaks in the XRD pattern based on the Scherrer formula. If the crystallites 120 are too small, oxides may be more easily dissolved, which may reduce the durability of the catalyst. If the crystallites 120 are too large, the secondary pore volume may be reduced, which may increase the likelihood of flooding.

[0016] The aggregate of the carrier particles 150 is in powder form. Such an aggregate is called "carrier powder".

[0017] The average particle size of the carrier fine particles 150 in the carrier powder is preferably 0.1 μm to 4 μm, and more preferably 0.5 μm to 2 μm. The average particle size of the carrier fine particles 150 can be measured by a laser diffraction / scattering particle size distribution analyzer.

[0018] The BET specific surface area of ​​the carrier powder is 12 m². 2 Preferably 25m / g or more. 2 A value of 1 / g or more is even more preferable. This BET specific surface area is, for example, 12 to 100 m². 2 The values ​​are per gram, specifically, for example, 12, 15, 20, 25, 30, 35, 40, 45, 50, and 100 ml. 2 The value is / g, and may be within the range of any two of the values ​​exemplified here.

[0019] The carrier powder preferably has a porosity of 50% or more, and more preferably 60% or more. The porosity is, for example, 50 to 80%, specifically, for example, 50, 55, 60, 65, 70, 75, and 80%, and may be within the range of any two of the values ​​exemplified here. The porosity can be determined by mercury intrusion or FIB-SEM.

[0020] The carrier powder preferably has an angle of repose of 50 degrees or less, and more preferably 45 degrees or less. In this case, the carrier powder has a fluidity similar to that of wheat flour and is easy to handle. This angle of repose is, for example, 20 to 50 degrees, specifically, for example, 20, 25, 30, 35, 40, 45, and 50 degrees, and may be within the range of any two of the values ​​exemplified here. The angle of repose can be determined by the drop volume method.

[0021] The carrier nanoparticles 150 are composed of a metal oxide. The metal oxide contains cerium. Cerium oxide has the property of suppressing the generation of hydrogen peroxide, so the inclusion of cerium in the metal oxide suppresses the generation of hydrogen peroxide during power generation in the fuel cell. It is thought that cerium oxide suppresses the generation of hydrogen peroxide by utilizing oxygen vacancies on its surface to capture OH radicals generated by the reduction of O2.

[0022] The atomic ratio of cerium to the total metal contained in the metal oxide is preferably 0.3 to 1. This is because in this case, the effect of suppressing the generation of hydrogen peroxide is remarkable. Specifically, this atomic ratio is, for example, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.00, and it may also be within the range between any two of the values exemplified here.

[0023] The metal oxide may contain other elements besides cerium. Examples of other elements include at least one selected from rare earth elements typified by gadolinium and yttrium, group 5 elements typified by niobium and tantalum, group 6 elements typified by tungsten, and group 15 elements typified by antimony. By incorporating rare earth elements (trivalent), oxygen defects can be generated, and the capture and decomposition of hydrogen peroxide decomposition intermediates (such as OH) can be promoted. By incorporating group 5 elements typified by Nb, Ta, etc. and group 6 elements typified by tungsten (elements with a valence of +5 or higher), electron conductivity (promotion of electron supply) can be induced, and the reduction reaction of hydrogen peroxide can be promoted.

[0024] As shown in FIG. 16, the supported metal catalyst 100 is considered to exhibit high electrical conductivity by inducing Ce with a high conductivity in the vicinity of the site where the metal fine particles 130 are supported due to the reduction action of the metal fine particles 130. From this perspective, as other elements, it is preferable that they are rare earth elements or trivalent metals of other elements. In this case, Ce is easily induced by the principle of electrical neutrality. 3+ 3+

[0025] Also, from another perspective, when the ionic radius of the tetravalent cation of cerium (Ce 4+ ) is R1, and the ionic radius of the cation of the valence of other elements (in the case of the n-valent metal M, M n+ , in the case of tetravalent Zr, Zr 4+ ) is R2, it is preferable that the other elements are those for which R2 / R1 is 0.99 or less. Ce 3+ is Ce4+ Because its ionic radius is larger than that of Ce 4+ Ce is used to alleviate the lattice distortion caused by adding elements with smaller ionic radii. 3+ Ce 4+ The ionic radius of is 0.97 Å, and Zr 4+ Since the ionic radius of is 0.84 Å, if the other element is Zr, then R2 / R1 = 0.87.

[0026] R2 / R1 is, for example, 0.70 to 0.99, specifically, for example, 0.70, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, and may be within the range between any two of the values ​​exemplified here, or less than or equal to any two of them.

[0027] Since cerium oxide has very low electrical conductivity, the presence of cerium in the metal oxide constituting the carrier nanoparticles 150 also lowers the electrical conductivity of the carrier powder. The electrical conductivity of the carrier powder is, for example, 10 3 S / cm or less, 10 -15 ~10 3 It is preferable that the electrical conductivity is (S / cm). Specifically, this electrical conductivity is, for example, 10 α S / cm (where α is specifically, for example, -15, -10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3), and may be within the range between any two of the values ​​exemplified here, or less than or equal to any one of them. Electrical conductivity can be measured according to the JIS standard (JIS K 7194).

[0028] 1-2. Metal fine particles 130 The metal nanoparticles 130 are fine particles of metal that can function as a catalyst. Preferably, the metal nanoparticles 130 are composed of platinum alone, or an alloy of platinum and another metal (e.g., a transition metal). Cobalt (Co) or nickel (Ni) are preferred as the transition element, with cobalt being particularly preferred. In addition, the metal (cerium or other elements) of the metal oxide constituting the carrier nanoparticles may diffuse into the metal nanoparticles 130, and even in this case, the metal nanoparticles 130 can still exhibit catalytic function. Therefore, the metal nanoparticles 130 may also contain cerium or other elements.

[0029] The proportion of platinum in the metal nanoparticles 130 is preferably 80 atomic percent or more. Since metals other than platinum tend to dissolve during operation, the higher the proportion of platinum, the better the durability of the catalyst. Specifically, this proportion may be, for example, 80, 85, 90, 95, or 100 atomic percent, or it may be within the range of any two of the values ​​exemplified here.

[0030] The metal nanoparticles 130 have a crystallite size of 2 to 10 nm, as determined from the XRD pattern. If the crystallite size is too small, it dissolves easily as the electrode reaction progresses, and if the crystallite size is too large, the electrochemically active surface area becomes small, making it difficult to obtain the desired electrode performance. Specifically, this crystallite size is, for example, 2, 3, 4, 5, 6, 7, 8, 9, and 10 nm, and may also be within the range of any two of the values ​​exemplified here. This crystallite size can be determined from the full width at half maximum of the peaks in the XRD pattern based on the Scherrer formula.

[0031] The ratio of metal nanoparticles 130 to the total of carrier powder and metal nanoparticles 130 is preferably 20 to 70% by mass, and more preferably 30 to 60% by mass. The higher this ratio, the easier it is for adjacent metal nanoparticles to partially fuse together and form a continuum of metal nanoparticles. On the other hand, if this ratio is too high, the voids in the carrier nanoparticles 150 may be blocked by the metal nanoparticles 130, resulting in poor material diffusion. Specifically, this ratio may be, for example, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, or 70% by mass, and may also be within the range of any two of the values ​​exemplified here.

[0032] In this way, by increasing the proportion of metal nanoparticles 130, adjacent metal nanoparticles 130 fuse together on the carrier nanoparticles 150, forming a wire-like continuum of metal nanoparticles 130. This continuum forms a conductive path, thereby increasing the electrical conductivity of the supported metal catalyst 100. Cerium oxide has extremely low electrical conductivity and is therefore not usually selected as a material to constitute the carrier nanoparticles 150. However, in this invention, by increasing the amount of supported metal nanoparticles 130, the electrical conductivity of the supported metal catalyst 100 is increased by 10 -4 The value has been increased to an acceptable level for a catalyst, which is S / cm or higher.

[0033] This electrical conductivity is, for example, 10 -4 ~10 2 It is S / cm, specifically for example 10 -4 , 10 -3 , 10 -2 , 10 -1 The values ​​are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 50, and 100 S / cm, and may be within the range of any two of the values ​​exemplified here, or greater than or equal to any one of them.

[0034] For metal nanoparticles 130, if I1 is the peak intensity of the (111) plane in the XRD pattern and I2 is the peak intensity of the (200) plane, then I1 / I2 ≥ 1.9 is preferred, I1 / I2 ≥ 2.2 is more preferred, and I1 / I2 ≥ 2.5 is even more preferred. Since the (111) plane of platinum has higher catalytic activity than the (200) plane, for metal nanoparticles 130 containing platinum, the larger the I1 / I2 value, the higher the catalytic activity. The larger the amount of metal nanoparticles 130 supported, the larger the I1 / I2 value becomes, so the I1 / I2 value can be increased by increasing the amount of metal nanoparticles 130 supported. The I1 / I2 value is, for example, between 1.9 and 7.0, specifically, for example, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, and 7.0, and may also be within the range of any two of the values ​​exemplified here. In this specification, the peak intensity ratio refers to the intensity ratio at the peak top.

[0035] 2. Applications of the supported metal catalyst 100 The supported metal catalyst 100 is suitably used as an electrode catalyst in an electrochemical cell. An electrochemical cell refers to a cell that generates an electrochemical reaction, and examples include fuel cells that generate electricity using fuels such as hydrogen or methanol by electrochemical reactions, hydrogen purification and boosting devices that produce high-pressure, high-purity purified hydrogen gas from hydrogen-containing gases by electrochemical reactions, redox flow batteries that perform charging and discharging by redox reactions, and water electrolysis cells that decompose water into hydrogen and oxygen by electrochemical reactions.

[0036] From the perspective of suppressing the generation of hydrogen peroxide during power generation in a fuel cell, the supported metal catalyst 100 It is particularly preferable to use it as an electrode catalyst for a polymer electrolyte fuel cell equipped with an electrolyte membrane between the anode and cathode. In this case, the degradation of the electrolyte membrane is suppressed by suppressing the generation of hydrogen peroxide. It is also thought that the generation of hydrogen peroxide can be suppressed by adding a cerium compound to the electrolyte membrane, but in that case there is a problem that the cerium compound migrates. On the other hand, in this embodiment, since the metal oxide constituting the carrier nanoparticles contains cerium, the migration of cerium is suppressed.

[0037] Furthermore, since cerium oxide is not easily dissolved even under acidic conditions, the supported metal catalyst 100 can be used as either the anode or the cathode of the fuel cell.

[0038] Furthermore, the supported metal catalyst 100 has high heat resistance because the metal oxide constituting the carrier nanoparticles contains cerium. Therefore, fuel cells constructed using the supported metal catalyst 100 can operate at temperatures of 120°C or higher.

[0039] 3. Method for producing carrier powder First, a manufacturing apparatus 1 that can be used for the production of carrier powder will be described using Figures 5 to 8. The manufacturing apparatus 1 comprises a burner 2, a raw material supply unit 3, a reaction cylinder 4, a recovery unit 5, and a gas storage unit 6. The raw material supply unit 3 comprises an outer cylinder 13 and a raw material flow cylinder 23.

[0040] The burner 2 is cylindrical, and the raw material supply unit 3 is located inside the burner 2. Burner gas 2a flows between the burner 2 and the outer cylinder 13. The burner gas 2a is used to form a flame 7 at the tip of the burner 2 upon ignition. The flame 7 forms a high-temperature region of 1000°C or higher. The burner gas 2a preferably contains a flammable gas such as propane, methane, acetylene, hydrogen, or nitrous oxide. In one example, a mixed gas of oxygen and propane can be used as the burner gas 2a. The temperature of the high-temperature region is, for example, 1000 to 2000°C, specifically, for example, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, and 2000°C, and may be within the range of any two of the values ​​exemplified here.

[0041] A raw material solution 23a for producing carrier powder is passed through the raw material distribution tube 23. The raw material solution 23a used contains a metal compound. Examples of metal compounds include fatty acid metal (Ce, Gd, etc.) salts. The number of carbon atoms in the fatty acid is, for example, 2 to 20, preferably 4 to 15, and more preferably 6 to 12. As fatty acid metal salts, octoyl acid metal salts (cerium octoylate, gadolinium octoylate, etc.) are preferred. In the raw material solution 23a, the metal compound is preferably dissolved or dispersed in a non-aqueous solvent.

[0042] A misting gas 13a, used to atomize the raw material solution 23a, flows between the outer cylinder 13 and the raw material flow cylinder 23. When the misting gas 13a and the raw material solution 23a are ejected together from the tip of the raw material supply unit 3, the raw material solution 23a is atomized. The mist 23b of the raw material solution 23a is sprayed into the flame 7, and the metal compounds in the raw material solution 23a undergo a thermal decomposition reaction in the flame 7, generating a carrier powder which is an aggregate of carrier fine particles 150 having chain-like portions formed by the fusion bonding of crystallites 120 in a chain-like manner. In one example, the misting gas 13a is oxygen.

[0043] The reaction cylinder 4 is located between the recovery unit 5 and the gas storage unit 6. A flame 7 is formed inside the reaction cylinder 4. The recovery unit 5 is equipped with a filter 5a and a gas discharge unit 5b. Negative pressure is applied to the gas discharge unit 5b. As a result, an airflow directed towards the gas discharge unit 5b is generated inside the recovery unit 5 and the reaction cylinder 4.

[0044] The gas storage section 6 is cylindrical and includes a cooling gas introduction section 6a and a slit 6b. Cooling gas 6g is introduced into the gas storage section 6 from the cooling gas introduction section 6a. Since the cooling gas introduction section 6a is oriented along the tangent to the inner circumferential wall 6c of the gas storage section 6, the cooling gas 6g introduced into the gas storage section 6 through the cooling gas introduction section 6a swirls along the inner circumferential wall 6c. A burner insertion hole 6d is provided in the center of the gas storage section 6. A burner 2 is inserted through the burner insertion hole 6d. The slit 6b is provided adjacent to the burner insertion hole 6d and surrounds the burner insertion hole 6d. Therefore, when the burner 2 is inserted through the burner insertion hole 6d, the slit 6b surrounds the burner 2. The cooling gas 6g in the gas storage section 6 is driven by the negative pressure applied to the gas discharge section 5b and discharged from the slit 6b toward the reaction cylinder 4. The cooling gas 6g can be any gas capable of cooling the generated oxides, and an inert gas is preferred, such as air. The flow rate of the cooling gas 6g is preferably at least twice the flow rate of the burner gas 2a. There is no particular upper limit to the flow rate of the cooling gas 6g, but for example, it is 1000 times the flow rate of the burner gas 2a. The flow rate of cooling gas 6g / flow rate of burner gas 2a is, for example, 2 to 1000, specifically, for example, 2, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 100, 200, 500, 1000, and may be within the range of any two of the values ​​exemplified here. In this embodiment, the cooling gas 6g is flowed by applying negative pressure to the gas discharge section 5b, but the cooling gas 6g may be flowed by applying positive pressure to the cooling gas inlet section 6a.

[0045] In this embodiment, cooling gas 6g is supplied around the flame 7 through the slit 6b, so the cooling gas 6g flows laminarly around the flame 7. As a result, the mist 23b, crystallites 120, and carrier particles 150 are not disturbed by the cooling gas 6g, and are sufficiently heated by the flame 7 as they move along the flame 7, allowing the reaction to proceed. Furthermore, after the carrier particles 150 leave the flame 7, they are immediately cooled by the cooling gas 6g, so the structure with chain-like portions is maintained. The cooled carrier particles 150 are captured and collected by the filter 5a.

[0046] In this embodiment, the carrier powder, which is an aggregate of carrier fine particles 150, can be produced using a manufacturing apparatus 1. A high-temperature region of 1000°C or higher is formed at the tip of the burner 2 by a flame 7, and a cooling gas 6g is supplied around the high-temperature region through a slit 6b while a thermal decomposition reaction of a metal compound is carried out in this high-temperature region. The high-temperature region may be formed by means other than the flame 7, such as plasma.

[0047] 4. Method for producing the supported metal catalyst 100 The method for producing the supported metal catalyst 100 comprises a carrier powder production step, a supporting step, a heat treatment step, and a reduction step.

[0048] <Carrier powder production process> In the carrier powder production process, the carrier powder is produced using the method described above.

[0049] <Supporting process> In the loading process, metal nanoparticles 130 are loaded onto a carrier powder. This loading can be performed using methods such as the reverse micelle method, the colloid method, or the impregnation method, but the colloid method is preferred because even when a large amount of metal nanoparticles 130 are loaded, the metal nanoparticles 130 do not overlap easily.

[0050] In the colloid method, metal colloid particles are adsorbed onto a carrier powder. More specifically, a dispersion is prepared by dispersing metal colloid particles produced by the colloid method in an aqueous solution, and the metal colloid particles are added to and mixed into the dispersion to adsorb the colloid particles onto the surface of the carrier powder. The carrier powder with the adsorbed colloid particles can be separated from the dispersion medium by filtration and drying. In one example, metal colloid particles can be produced by adding a reducing agent to a liquid containing a metal-containing colloid precursor to reduce the precursor, but the metal-containing colloid precursor may also be used directly as metal colloid particles.

[0051] <Heat treatment process> In the heat treatment process, heat treatment is performed after the adsorption process to transform the metal colloid particles into metal nanoparticles 130. The heat treatment temperature is, for example, 150 to 750°C, specifically, for example, 500, 550, 600, 650, 700, and 750°C, and may be within the range of any two of the values ​​exemplified here. During this heat treatment process, crystallites grow, but if the heat treatment temperature is too low, the crystallites 120 of the carrier nanoparticles 150 will not grow sufficiently and will be easily dissolved. On the other hand, the higher the heat treatment temperature, the smaller the secondary pore volume becomes, so if the heat treatment temperature is too high, the secondary pore volume becomes too small, and the flooding phenomenon is likely to occur.

[0052] The heat treatment time is, for example, 0.1 to 20 hours, with 0.5 to 5 hours being preferred. Specifically, this time may be, for example, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, or 20 hours, and may be within the range of any two of the values ​​exemplified here.

[0053] The heat treatment can be carried out under an inert gas atmosphere such as nitrogen or an inert gas atmosphere containing 1-4% hydrogen.

[0054] <Restoration Process> In the reduction process, the metal nanoparticles 130 are reduced after the heat treatment process. The reduction process can be carried out by performing heat treatment in a reducing atmosphere containing a reducing gas such as hydrogen. The reduction process can be omitted if it is not necessary.

[0055] The temperature for this heat treatment is, for example, 70 to 300°C, preferably 100 to 200°C. Specifically, this temperature may be, for example, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 250, or 300°C, and may be within the range of any two of the values ​​exemplified here.

[0056] The duration of this heat treatment is, for example, 0.01 to 20 hours, with 0.1 to 5 hours being preferred. Specifically, this duration may be, for example, 0.01, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, or 20 hours, and may be within the range of any two of the values ​​exemplified here.

[0057] When the reducing gas is hydrogen, its concentration is, for example, 0.1 to 100% by volume, preferably 0.2 to 10% by volume, and more preferably 0.5 to 3% by volume. Specifically, this concentration may be, for example, 0.1, 0.2, 0.5, 1, 1.5, 2, 2.5, 3, 10, or 100% by volume, and may be within the range of any two of the values ​​exemplified here.

[0058] In the heat treatment process, the metal nanoparticles 130 may be in an oxidized state after heat treatment, in which case the metal nanoparticles 130 may not exhibit catalytic activity. In this case, the catalytic activity of the metal nanoparticles 130 can be increased by reducing them. [Examples]

[0059] Supported metal catalysts were manufactured using the method described below, and various evaluations were performed.

[0060] 1. Manufacturing of supported metal catalyst 100 <Example 1> (Manufacturing of carrier powder) The carrier powder was manufactured using the manufacturing apparatus 1 shown in Figures 5 to 8. A mixture of oxygen (5 L / min) and propane gas (1 L / min) was used as the burner gas 2a. This gas was ignited to form a flame (chemical flame) 7 of over 1600°C at the tip of the burner 2. The raw material solution 23a was prepared by mixing cerium octoate and gadolinium octoate in a molar ratio of 0.8:0.2 in mineral split turpentine and dissolving them. Oxygen was used as the atomizing gas 13a. A mixture of 9 L / min of atomizing gas 13a and 3 g / min of raw material solution 23a was sprayed from the tip of the raw material supply unit 3 (a spray nozzle / atomizer) into the center of the flame and burned to produce carrier powder, which is an aggregate of carrier fine particles 150. At that time, by creating a negative pressure at the gas discharge section 5b, air was drawn in from the slit 6b at a flow rate of 170 L / min, and the generated carrier powder was recovered into the recovery unit 5 (with filter 5a). The raw material supply section 3 has a double-pipe structure (total length 322.3 mm), with oxygen gas supplied from the outer cylinder 13 and raw material solution 23a supplied to the raw material flow tube 23. The tip of the raw material flow tube 23 has a fluid nozzle and an air nozzle, where the raw material solution 23a was converted into mist 23b. The amount of carrier powder recovered was 10 g or more after 60 minutes of operation.

[0061] (Supporting, heat treatment, and reduction of metal nanoparticles 130) Next, the metal nanoparticles 130 were supported on a carrier powder and subjected to heat treatment and reduction.

[0062] <Supporting process> First, 1.875 mL of an aqueous solution of hexahydrate chloroplatinic acid was dissolved in 120 mL of ultrapure water, and then 5.9 g of sodium sulfite was added and the mixture was stirred.

[0063] The solution was diluted with 450 ml of water, and the pH of the solution was adjusted to 5 using NaOH. Then, 60 ml of hydrogen peroxide was added, and the pH was readjusted to 5 with NaOH.

[0064] To the obtained dispersion, a dispersion prepared by dispersing 0.50 g of carrier powder in 100 mL of ultrapure water was added, and the mixture was stirred at 90°C for 3 hours. After cooling to room temperature, the mixture was filtered and washed with ultrapure water and alcohol, then dried overnight at 80°C to support the metal nanoparticles 130 on the carrier powder.

[0065] <Heat treatment process> In the heat treatment process, the sample after the loading process was subjected to heat treatment at 400°C for 2 hours in a nitrogen atmosphere.

[0066] <Restoration Process> In the reduction process, the metal nanoparticles 130 were reduced by heat treatment of the sample from the heat treatment process at 150°C for 2 hours in 1% hydrogen.

[0067] Through the above process, a supported metal catalyst 100 was obtained in which metal nanoparticles 130 were supported on a carrier powder.

[0068] <Examples 2-4> Supported metal catalyst 100 was manufactured in the same manner as in Example 1, except that the amount of carrier powder added in the supporting process was changed so that the proportion of metal nanoparticles was as shown in Table 1.

[0069] [Table 1]

[0070] <Example 5> The supported metal catalyst 100 was prepared in the same manner as in Example 3, except that only cerium octylate was used as the raw material solution 23a.

[0071] <Example 6> The supported metal catalyst 100 was prepared in the same manner as in Example 3, except that the raw material solution 23a used cerium octoate and zirconium octoate in a molar ratio of 0.5:0.5.

[0072] <Comparative Example 1> As the catalyst of Comparative Example 1, a commercially available Pt / C catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., type TEC10E50E, supported amount 50% by mass) was used.

[0073] <Comparative Example 2> As the catalyst of Comparative Example 2, a commercially available Pt / C catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., type TEC10E30E, supported amount 30% by mass) was used.

[0074] 2. Various Measurements and Evaluations

[0075] The following various measurements and evaluations were carried out.

[0076] <Taking TEM Images> The TEM image of the supported metal catalyst 100 of Example 1 is shown in Fig. 9. As shown in Fig. 9, it can be seen that the adjacent metal fine particles 130 are partially fused and bonded to form a wire-like continuum and extend along the surface of the carrier powder.

[0077] <STM-EELS Measurement> The results of STM-EELS measurement on the supported metal catalyst 100 of Example 1 are shown in Fig. 17. Fig. 17A shows the measurement regions (ROI) 12 and 00. ROI00 is a region closer to the metal fine particles 130 than ROI12. As shown in Fig. 17B, a change was observed in the peak position and intensity near 885 eV from ROI12 to ROI00. This change indicates that the valence of Ce changes from tetravalent to trivalent from ROI12 to ROI00.

[0078] <STEM-EDX Measurement> Figure 18 shows the results of STEM-EDX measurement performed on the supported metal catalyst 100 of Example 1. Figure 18A shows the area where the measurement was performed. This measurement was performed in a region that crossed the metal nanoparticles 130. Figures 18B and 18C show the same measurement results, with Figure 18B highlighting the measurement results for Pt and Ce, and Figure 18C highlighting the measurement results for Gd. This result indicates that Ce and Gd are diffused within the metal nanoparticles 130. Note that the vertical axis in Figures 18B and 18C does not reflect the intensity ratio between elements, and the abundance of Ce and Gd cannot be determined from these graphs.

[0079] <Measurement of electrical conductivity> Each catalyst was packed into a uniaxial press and compressed at a pressure of 16 MPa, and its resistance was measured. Four different levels of catalyst were measured, and the electrical resistance of the catalyst was calculated from the slope of the straight line obtained from the correlation between the obtained electrical resistance and the sample thickness or weight, and then converted to electrical conductivity. The results are shown in Table 1.

[0080] <Measurement of peak intensity ratio and crystallite size of metal nanoparticles> XRD measurements were performed on the supported metal catalyst 100, and the peak intensity ratio of the metal nanoparticles (=((111) peak intensity I1 / (200) peak intensity I2)) was calculated from the XRD pattern. In addition, the crystallite size of the metal nanoparticles was calculated from the full width at half maximum of the (200) peak based on the Scherrer formula. The results are shown in Table 1. As shown in Table 1, it can be seen that the higher the proportion of metal nanoparticles 130, the larger the peak intensity ratio.

[0081] <Measurement of mass activity of oxygen reduction reaction> The oxygen reduction activity was measured using the electrochemical measuring device 15 shown in Figure 10.

[0082] The apparatus 15 comprises a glass cell 15a, a working electrode 15b, a counter electrode 15c, and a reference electrode 15d. The potential of the working electrode 15b relative to the reference electrode 15d is adjustable by a potentiostat (not shown). The glass cell 15a contains an electrolyte (0.1M-HClO4) 15e. Nitrogen or oxygen can be blown into the electrolyte 15e. The working electrode 15b is made of glassy carbon (GC), is cylindrical, and has a supported metal catalyst 100 coated on its underside. The underside of the working electrode 15b and the counter electrode 15c are immersed in water 15e. The reference electrode 15d is liquid-junctioned with the electrolyte 15e by a salt bridge 15f.

[0083] The supported metal catalyst 100 was dispersed in a mixed solution of 80 wt% water and 20 wt% ethanol, coated onto the underside of the working electrode 15b, and dried. Measurements were performed while blowing oxygen into the electrolyte 15e. During the measurement, the working electrode 15b was rotated around its central axis, and the current value was measured with the potential of the working electrode 15b relative to the reference electrode 15d (Potential / V vs RHE) set to 0.85 or 0.9V.

[0084] The results obtained are shown in Figure 11. As shown in Figure 11, the catalyst of Example 1 has a higher mass activity in the oxygen reduction reaction than the catalyst of Comparative Example 1.

[0085] <Measurement of hydrogen peroxide generation amount> The amount of hydrogen peroxide generated was measured using the electrochemical measuring device 15 shown in Figure 12. The device 15 in Figure 12 is the same as the device 15 in Figure 10, except that it is equipped with an annular electrode 15g surrounding the working electrode 15b.

[0086] The amount of hydrogen peroxide generated was measured using the following method. First, while blowing nitrogen into the electrolyte 15e, the working electrode 15b was rotated around its central axis. In this state, the potential (Potential / V vs RHE) of the working electrode 15b and the annular electrode 15g relative to the reference electrode 15d was changed between 0 and 0.10, and the current flowing through the working electrode 15b and the annular electrode 15g was measured.

[0087] The currents flowing through the working electrode 15b and the annular electrode 15g are respectively I R and I D Based on the following formula, p(H2O2) was calculated. In the formula below, N represents the experimentally determined hydrogen peroxide capture rate. p(H2O2)=2I R / ( I D ×N+I R )

[0088] Figure 13A shows the results obtained from measurements for Example 4 and Comparative Example 1. As shown in Figure 13A, it can be seen that the catalyst of Example 4 generates less hydrogen peroxide than the catalyst of Comparative Example 1.

[0089] Similarly, the results obtained from measurements for Example 3, Example 5, Example 6, and Comparative Example 2 are shown in Figure 13B. As shown in Figure 13B, the order of hydrogen peroxide generation, from least to most, was Example 6, Example 3, Example 5, and Comparative Example 2.

[0090] <Measurement of current-voltage characteristics> In a standard cell developed by the Japan Automobile Research Institute (JARI), a single cell was constructed using the supported metal catalyst 100 from Example 1 as the anode catalyst and the same Pt / C catalyst as in Comparative Example 1 as the cathode electrode catalyst. The amount of electrode catalyst used was 0.10 mg / cm³. 2 The current-voltage characteristics of this single cell were measured when it was operated at 120°C, cathode humidity 95%RH, cathode oxygen gas pressure 200kPaG, and anode hydrogen gas pressure 200kPaG.

[0091] The results are shown in Figure 14. Generally, in automotive fuel cells, the current is 1.0 A / cm at 0.6 V. 2 It is desirable to have an output of 0.6V, but as shown in Figure 14, the fuel cell configured using the catalyst of Example 1 has an output of 1.0A / cm² at 0.6V. 2 It was found that an output exceeding [a certain value] could be obtained.

[0092] <Hydrogen peroxide resistance test> Furthermore, to conduct hydrogen peroxide resistance testing, a holding test was performed under the open-circuit voltage state where the amount of hydrogen peroxide generated was highest. In Example 2, the open-circuit voltage was measured when the standard cell was operated under the same conditions as in "Measurement of Current-Voltage Characteristics," except that the humidity at both electrodes was set to 80% RH.

[0093] In Comparative Example 2, the test was conducted under the same conditions as in Example 1, except that the anode electrode catalyst was the same Pt / C catalyst as in Comparative Example 1. In Comparative Example 3, Ce 3+ The test was conducted under the same conditions as in Comparative Example 2, except that the polymer film was impregnated with ions.

[0094] The results are shown in Figure 15. Generally, in automotive fuel cells, as in Comparative Example 3, Ce 3+ Although hydrogen peroxide resistance is impregnated by impregnating the polymer film with ions, in the fuel cell of Example 2, Ce 3+ Even without impregnation of the polymer film, the potential remained stable for over 350 hours, longer than in Comparative Example 3, confirming improved hydrogen peroxide resistance. [Explanation of Symbols]

[0095] 1: Manufacturing equipment, 2: Burner, 2a: Burner gas, 3: Raw material supply section, 4: Reaction cylinder, 5: Recovery unit, 5a: Filter, 5b: Gas discharge section, 6: Gas storage section, 6a: Cooling gas introduction section, 6b: Slit, 6c: Inner peripheral wall, 6d: Burner insertion hole, 6g: Cooling gas, 7: Flame, 13: Outer cylinder, 13a: Misted gas, 15: Electrochemical measuring device, 15a: Glass cell, 15b: Working electrode, 15c: Counter electrode, 15d: Reference electrode, 15e: Electrolyte, 15f: Salt bridge, 15g: Annular electrode, 23: Raw material flow cylinder, 23a: Raw material solution, 23b: Mist, 100: Supported metal catalyst, 110: Void, 120: Crystallite, 130: Metal nanoparticles, 150: Carrier nanoparticles, 160: Branching

Claims

1. A fuel cell electrode catalyst comprising a carrier powder and metal nanoparticles supported on the carrier powder, The aforementioned carrier powder is an aggregate of carrier fine particles, The carrier microparticles have a chain-like portion formed by the fusion bonding of multiple crystallites in a chain-like manner, The carrier fine particles are composed of a metal oxide. The aforementioned metal oxide includes cerium, The aforementioned electrode catalyst for fuel cell has an electrical conductivity of 10 －４ It is S / cm or higher, The atomic ratio of cerium to the total metal contained in the aforementioned metal oxide is 0.3 to 1. An electrode catalyst for a fuel cell, wherein the ratio of the metal fine particles to the total amount of the carrier powder and the metal fine particles is 35 to 45% by mass.

2. An electrode catalyst for a fuel cell comprising a carrier powder and metal fine particles supported on the carrier powder, The aforementioned carrier powder is an aggregate of carrier fine particles, The carrier microparticles have a chain-like portion formed by the fusion bonding of multiple crystallites in a chain-like manner, The carrier fine particles are composed of a metal oxide. The aforementioned metal oxide includes cerium and other elements other than cerium. The aforementioned electrode catalyst for fuel cell has an electrical conductivity of 10⁻⁴ S / cm or higher. The ratio of the metal fine particles to the total amount of the carrier powder and the metal fine particles is 20 to 70% by mass. The atomic ratio of cerium to the total metal contained in the aforementioned metal oxide is 0.3 to 1. The aforementioned other elements are trivalent metals, which are electrode catalysts for fuel cells.

3. An electrode catalyst for a fuel cell comprising a carrier powder and metal fine particles supported on the carrier powder, The aforementioned carrier powder is an aggregate of carrier fine particles, The carrier microparticles have a chain-like portion formed by the fusion bonding of multiple crystallites in a chain-like manner, The carrier fine particles are composed of a metal oxide. The aforementioned metal oxide includes cerium and other elements other than cerium. The aforementioned electrode catalyst for fuel cell has an electrical conductivity of 10⁻⁴ S / cm or higher. The ratio of the metal fine particles to the total amount of the carrier powder and the metal fine particles is 20 to 70% by mass. The atomic ratio of cerium to the total metal contained in the aforementioned metal oxide is 0.3 to 1. A fuel cell electrode catalyst in which, if R1 is the ionic radius of a tetravalent cerium cation and R2 is the ionic radius of a valence cation of the other element, then R2 / R1 is 0.99 or less.

4. An electrode catalyst for a fuel cell according to any one of claims 1 to 3, The aforementioned carrier powder has an electrical conductivity of 10 ３ A fuel cell electrode catalyst with a density of S / cm or less.

5. An electrode catalyst for a fuel cell according to any one of claims 1 to 4, An electrode catalyst for fuel cells, wherein the proportion of platinum contained in the aforementioned metal nanoparticles is 80 atomic percent or more.

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