JP2007313423A - Metal catalyst, its manufacturing method and fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a metal catalyst manufactured by depositing a metal fine particle having the particle size smaller than that of the metal fine particle available in the market on the surface of carbon black being a carrier particle, a method for manufacturing the metal catalyst and a fuel cell which is formed by using the metal catalyst and has excellent cell characteristics. <P>SOLUTION: The metal catalyst is manufactured by depositing many metal fine particles on the surface of the carbon black irradiated with ionizing radiation. The method for manufacturing the metal catalyst comprises the steps of: irradiating carbon black with ionizing radiation; dispersing the carbon black irradiated thus in a reaction system having a liquid phase containing a metal ion being a raw material of the metal fine particle; and reducing the metal ion by the action of a reducing agent to precipitate the reduced metal ion as a fine particle and deposit the precipitated metal fine particle on the surface of the dispersed carbon black. The fuel cell comprises the manufactured metal catalyst. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  In particular, the present invention relates to a metal catalyst that can be suitably used as a catalyst for a fuel cell, a method for producing the same, and a fuel cell formed using the metal catalyst.

  As a catalyst for a fuel cell or the like, a metal catalyst, particularly a metal catalyst using a noble metal, is used, but since noble metal elements are present in a limited amount on the earth, the amount used is reduced as much as possible. Moreover, it is required to improve the action as a catalyst as much as possible. Therefore, in order to meet this requirement, as the metal catalyst, for example, a catalyst having a composite structure in which metal fine particles are supported on the surface of carrier particles made of carbon black or an inorganic compound is widely used. Yes.

  Since the catalytic action is mainly exerted on the surface of the metal, in the metal catalyst having the composite structure, in order to minimize the amount of metal used while maintaining good catalytic action, It is effective to make the metal fine particles supported on the surface as small as possible and have a large specific surface area.

  As a method for supporting the metal fine particles on the surface of the carrier particles, there are a high temperature treatment method called an impregnation method, a liquid phase reduction method, a gas phase method, and the like. Reduction method, that is, by reducing the metal ions by the action of a reducing agent in a state where the carrier particles are dispersed in a liquid phase reaction system containing metal ions that form the metal fine particles. In addition, a method of precipitating in the form of fine particles and supporting it on the surface of carrier particles is becoming widespread.

In order to reduce the particle size of the metal fine particles formed by the liquid phase reduction method, it is considered effective to increase the specific surface area of the carrier particles. Therefore, for example, Patent Document 1, as the carrier particles, specific surface area of 250 to 350 ² / g, particle size and non-graphitizable carbon black is 150～350A, specific surface area of 900~1500m ² / g, particle It has been proposed to use easily graphitizable carbon black having a diameter of 200 to 400 mm.

In Patent Document 2, as carrier particles, the specific surface area is 70 to 800 m ² / g, the average lattice spacing d _{002 of} [002] plane is 0.337 to 0.348 nm, and the crystallite size Lc ₍₀₀₂₎ It has been proposed to use a carbon support having a thickness of 3 to 18 nm. Further, Patent Document 3 proposes to use carbon powder having a specific surface area of 600 to 1200 m ² / g as carrier particles.

In order to reduce the particle size of the metal fine particles, an impurity element such as boron, silicon, phosphorus, sulfur, nitrogen or the like is included in the liquid phase reaction system when the metal fine particles are deposited by the liquid phase reduction method. In addition, a method of co-depositing these with a metal element is also effective. For example, in Non-Patent Document 1, metal fine particles are obtained by co-depositing platinum and ruthenium as metal elements with phosphorus as impurity elements. It has been reported that the particle size of can be reduced to 2 nm.
JP-A-8-117598 JP 2000-268828 A JP 2003-308849 A Proceedings of the 72nd Annual Meeting of the Electrochemical Society of Japan 1G09 (issued April 1, 2005)

  However, there is a limit to the effect of reducing the particle size of the metal fine particles by these methods, and by reducing the particle size of the metal fine particles further than this limit and improving the catalyst efficiency, for example, a battery of a fuel cell In order to make the characteristics even better than the current situation, it is necessary to take new measures. An object of the present invention is to provide a metal catalyst in which metal fine particles having a particle diameter smaller than that of the current state are supported on the surface of carbon black as carrier particles, a manufacturing method thereof, and a battery formed using the metal catalyst. The object is to provide a fuel cell having excellent characteristics.

  In order to solve the above-mentioned problem, the invention according to claim 1 includes a metal catalyst comprising carbon black irradiated with ionizing radiation and a large number of metal fine particles supported on the surface of the carbon black. It is.

Conventionally, in the initial stage of deposition and loading of metal fine particles by the liquid phase reduction method, it contains elements with high electronegativity (oxygen, nitrogen, fluorine, sulfur, phosphorus, etc.) present on the surface of carbon black, for example A metal that is a source of the metal fine particles, in which a chemical species such as —COOH, —C═O, —C≡N, —CF ₃ , —C—SO ₃ , —C—PO ₃ is contained in the reaction system. The metal ions adsorbed on the adsorption sites are reduced by the action of a reducing agent to function as growth nuclei, and metal fine particles grow from the nuclei as a starting point.

  Therefore, as described above, as the specific surface area of carbon black is increased, the number of chemical species that function as adsorption sites on the surface of the carbon black increases, so that the metal adsorbed on the adsorption site. The number of metal microparticles generated with the ions as nuclei also increases. Therefore, for example, when the amount of metal ions present in the liquid phase reaction system (≈the amount of supported metal fine particles) is constant, the particle size of each metal fine particle is small.

  However, the number of the above-mentioned chemical species per unit area on the surface of carbon black is almost constant depending on the type of carbon black classified mainly based on the production method, and the specific surface area of carbon black is Since there is a limit in the range that can be increased, the conventional technique has a limit in the effect of reducing the particle size of the metal fine particles.

  On the other hand, in the invention of claim 1, by irradiating the carbon black before supporting the metal fine particles with ionizing radiation, the C = C bond etc. existing on the surface of the carbon black is cut, In addition to the above-described chemical species, an oxygen-containing functional group or a hydrophilic group that functions as an adsorption site can be generated on the surface.

  Therefore, according to the first aspect of the present invention, the number of adsorption sites per unit area on the surface of carbon black is increased, and the number of metal fine particles supported on the surface is increased more than ever. Therefore, it is possible to improve the catalyst efficiency of the metal catalyst by making the particle size of each metal fine particle smaller than the current particle size. As a result, when the metal catalyst is used in, for example, a fuel cell, the battery characteristics can be made better than the current state.

  The invention according to claim 2 is the metal catalyst according to claim 1, wherein the ionizing radiation is an electron beam, and the carbon black is irradiated with an electron beam so that the absorbed dose is 0.1 to 1.5 MGy. is there. The electron beam belongs to ionizing radiation that can directly ionize atoms and molecules by being irradiated.

  Therefore, according to the invention described in claim 2, the energy efficiency for generating the oxygen-containing functional group or hydrophilic group functioning as an adsorption site on the surface of the carbon black described above can be improved, and the electron beam Can be generated by accelerating electrons with an accelerator, regardless of nuclear decay, like other ionizing radiations, thus improving safety.

  In the invention according to claim 2, the reason why the absorbed dose of the electron beam is in the range of 0.1 to 1.5 MGy is as follows. That is, when the absorbed dose is less than the above range, the effect of generating an oxygen-containing functional group or a hydrophilic group that functions as an adsorption site on the surface of carbon black, as described above, by sufficiently irradiating with an electron beam is sufficient. Since it cannot be obtained, there is a possibility that the number of adsorption sites and the number of metal fine particles generated using the metal ions adsorbed on the adsorption sites as nuclei cannot be increased sufficiently.

  In addition, when the above range is exceeded, the surface of the carbon black is excessively damaged by the electron beam irradiation, and on the other hand, the effect of generating oxygen-containing functional groups and hydrophilic groups that function as adsorption sites on the surface. Therefore, there is a possibility that the number of adsorption sites and the number of metal fine particles generated using metal ions adsorbed on the adsorption sites as nuclei cannot be sufficiently increased. Therefore, in any of the above cases, the effect of reducing the particle size of the supported metal fine particles may not be sufficiently obtained.

Invention of Claim 3 is a metal catalyst of Claim 1 or 2 whose BET specific surface area of carbon black before irradiating ionizing radiation is 100-1500 m < ² > / g. In the invention according to claim 3, the reason why the BET specific surface area of carbon black before irradiation with ionizing radiation is within the range of 100 to 1500 m ² / g is as follows.

  That is, if the BET specific surface area is less than the above range, the adjacent metal fine particles generated on the surface of the carbon black are too close to each other, and the adjacent metal fine particles are fused in the growth process. The opportunity to increase the particle size increases. Therefore, even if ionizing radiation is applied to generate oxygen-containing functional groups and hydrophilic groups that function as adsorption sites on the surface, the effect of reducing the particle size of the supported metal fine particles is sufficient. May not be obtained.

  In addition, when the above range is exceeded, the adjacent spacing of a large number of metal fine particles is too wide, so that oxygen gas or hydrogen gas diffuses between the metal fine particles when using a metal catalyst, particularly for a fuel cell. This process becomes the rate-determining step of the catalytic reaction, and there is a possibility that the battery characteristics of the fuel cell cannot be made better than the current state.

  The invention according to claim 4 is characterized in that the metal fine particles are at least selected from the group consisting of platinum, palladium, gold, silver, rhodium, iridium, ruthenium, osmium, cobalt, manganese, nickel, iron, chromium, molybdenum, and titanium. The metal catalyst according to claim 1, wherein the metal catalyst is a fine particle containing one kind of metal. According to invention of Claim 4, since all the said metals have favorable catalytic activity, the catalyst efficiency of a metal catalyst can be improved further.

  According to a fifth aspect of the present invention, there is provided the step of irradiating carbon black with ionizing radiation, and the carbon black dispersed in a liquid phase reaction system containing metal ions that form the metal fine particles. A method for producing a metal catalyst, comprising the step of reducing metal ions by the action of a reducing agent and precipitating them in the form of fine particles and supporting them on the surface of carbon black.

  According to the invention of claim 5, as described above, the step of irradiating carbon black with ionizing radiation on the metal catalyst having a small particle size of the metal fine particles and excellent catalyst efficiency, and the carbon after the irradiation By passing through a step of supporting a large number of metal fine particles on the surface of black by a liquid phase reduction method, it can be efficiently produced.

  The invention according to claim 6 is a fuel cell comprising the metal catalyst according to any one of claims 1 to 4. According to the sixth aspect of the present invention, as described above, by using a metal catalyst having a small particle size of the metal fine particles and excellent catalyst efficiency, the cell characteristics of the fuel cell can be further improved compared to the current state. It can be good.

  According to the present invention, a metal catalyst in which metal fine particles having a particle size smaller than that of the current particle are supported on the surface of carbon black as support particles, an efficient production method thereof, and the metal catalyst are used. Therefore, it is possible to provide a fuel cell having excellent battery characteristics.

《Metal catalyst》
The metal catalyst of the present invention is characterized by comprising carbon black irradiated with ionizing radiation and a large number of metal fine particles supported on the surface of the carbon black. Various carbon blacks that can be used as metal catalyst supports can be used as the carbon black used as a single particle by irradiating with ionizing radiation. In particular, measurement was performed before irradiating with ionizing radiation. Carbon black having a BET specific surface area of 100 to 1500 m ² / g is preferred.

  That is, if the BET specific surface area of the carbon black is less than the above range, the adjacent metal fine particles generated on the surface of the carbon black are too close to each other. Opportunities to increase particle size through fusion increase. Therefore, even if ionizing radiation is applied to generate oxygen-containing functional groups and hydrophilic groups that function as adsorption sites on the surface, the effect of reducing the particle size of the supported metal fine particles is sufficient. May not be obtained.

In addition, when the above range is exceeded, the adjacent spacing of a large number of metal fine particles is too wide, so that oxygen gas or hydrogen gas diffuses between the metal fine particles when using a metal catalyst, particularly for a fuel cell. This process becomes the rate-determining step of the catalytic reaction, and there is a possibility that the battery characteristics of the fuel cell cannot be made better than the current state. In consideration of these problems, the BET specific surface area of carbon black is particularly preferably 200 to 1400 m ² / g even within the above range.

  Platinum, palladium, gold, silver, and rhodium that can further improve the catalytic efficiency of the metal catalyst because it has good catalytic activity as the metal fine particles supported on the surface of carbon black irradiated with ionizing radiation , Iridium, ruthenium, osmium, cobalt, manganese, nickel, iron, chromium, molybdenum, and fine particles containing at least one metal selected from the group consisting of titanium and titanium.

  In consideration of improving the catalyst efficiency, the particle size of the metal fine particles may be smaller than the current state, that is, 2 nm described in Non-Patent Document 1 described above, but is particularly 1.1 nm or less. preferable. When the particle size exceeds 1.1 nm, there is a possibility that the effect of improving the catalyst efficiency of the metal catalyst by reducing the particle size of the metal fine particles may not be sufficiently obtained. The lower limit of the particle size of the metal fine particles is not particularly limited, but is practically preferably 0.5 nm or more.

  The supported amount of the metal fine particles is preferably 15 to 60% by weight of the total amount of the metal catalyst. If the loading amount is less than the above range, the adjacent spacing of a large number of metal fine particles generated on the surface of the carbon black is excessively widened, whereas if the loading amount exceeds the above range, The adjacent interval becomes too close, and in the growth process, adjacent metal fine particles are fused to increase the chance of increasing the particle size. Therefore, in any of the above cases, there is a possibility that the cell characteristics of the fuel cell cannot be made even better than the current situation. In consideration of these problems, the supported amount of the metal fine particles is preferably 17 to 26% by weight of the total amount of the metal catalyst, even within the above range.

  The metal catalyst of the present invention is supported on the surface of carbon black irradiated with ionizing radiation, and has metal fine particles having a smaller particle size and a larger specific surface area than before, particularly for fuel cells. When used as a battery, the battery characteristics can be made even better than the current situation. In addition, the metal catalyst of this invention can be used conveniently also as a catalyst for gas purifications.

<< Production Method of Metal Catalyst >>
The method for producing a metal catalyst according to the present invention comprises a step of irradiating carbon black with ionizing radiation, and the carbon black is dispersed in a liquid phase reaction system containing metal ions that form metal fine particles. And a step of reducing the metal ions by the action of a reducing agent to deposit them in the form of fine particles and supporting them on the surface of carbon black.

When ionizing radiation is irradiated to the carbon black before supporting the metal fine particles, the C = C bond and the like existing on the surface of the carbon black are cut, and the electron black inherently present on the surface has high electronegativity. Including chemical elements such as —COOH, —C═O, —C≡N, —CF ₃ , —C—SO ₃ , —C—PO ₃ , including elements (oxygen, nitrogen, fluorine, sulfur, phosphorus, etc.) In addition, similarly to the chemical species, oxygen-containing functional groups and hydrophilic groups that function as adsorption sites can be generated.

  Therefore, the number of adsorption sites per unit area on the surface of carbon black can be increased, and the number of metal fine particles supported on the surface can be increased more than before. It is possible to improve the catalyst efficiency of the metal catalyst by making the particle size smaller than the current state. As a result, when the metal catalyst is used in, for example, a fuel cell, the battery characteristics can be made better than the current state.

  The step of irradiating carbon black with ionizing radiation can be carried out by any method and procedure. For example, as the ionizing radiation applied to the carbon black, as described above, the C = C bond existing on the surface of the carbon black is cut as it is irradiated and functions as an adsorption site on the surface. Any of various ionizing radiations capable of generating an oxygen-containing functional group or a hydrophilic group can be used, and an electron beam is particularly preferable.

  Since the electron beam belongs to the direct ionizing radiation that can directly ionize atoms and molecules by being irradiated among the ionizing radiation, it functions as an adsorption site described above on the surface of the carbon black. In addition to improving energy efficiency for generating oxygen-containing functional groups and hydrophilic groups, electron beams can be generated by accelerating electrons with an accelerator, unlike other ionizing radiations, regardless of nuclear decay. Therefore, safety can be improved.

  In order to irradiate the carbon black with an electron beam, for example, using a conveyor type electron beam irradiation device, the carbon black contained in an arbitrary container such as a plastic bag is conveyed continuously by the conveyor of the device. It is preferable to irradiate the electron beam with respect to the processing efficiency of carbon black.

  The amount of electron beam irradiation is preferably 0.1 to 1.5 MGy in terms of absorbed dose. If the absorbed dose of the electron beam is less than the above range, the effect of generating an oxygen-containing functional group or a hydrophilic group that functions as an adsorption site on the surface of the carbon black described above by irradiation with the electron beam is sufficient. Therefore, the number of adsorption sites and the number of metal fine particles generated with the metal ions adsorbed on the adsorption sites as nuclei cannot be increased sufficiently. The effect of reducing the diameter may not be sufficiently obtained.

  In addition, when the above range is exceeded, the surface of the carbon black is excessively damaged by the electron beam irradiation, and on the other hand, the effect of generating oxygen-containing functional groups and hydrophilic groups that function as adsorption sites on the surface. Is not sufficiently obtained, the number of adsorption sites and the number of metal fine particles generated using the metal ions adsorbed on the adsorption sites as nuclei cannot be increased sufficiently, and the supported metal fine particles There is a possibility that the effect of reducing the particle size of the material may not be sufficiently obtained.

  In order to adjust the absorbed dose of the electron beam, for example, in the case of the conveyor type electron beam irradiation apparatus described above, the electron beam irradiation time is adjusted by adjusting the transport speed of the carbon black. A method such as adjusting the acceleration voltage of the wire can be adopted. Moreover, the absorbed dose of an electron beam can be calculated | required with a film dosimeter, for example. In the film dosimeter, when an electron beam is irradiated onto the dose measuring film to obtain absorption energy, the spectral characteristics change, and the change amount of the spectral characteristics and the absorbed dose of the electron beam are correlated. It is something that uses that.

  Next, in the present invention, metal fine particles are supported on the surface of carbon black by a liquid phase reduction method. That is, a reducing agent and a metal compound serving as a metal ion source are respectively prepared at a predetermined concentration, and a liquid phase reaction system is prepared in which a predetermined amount of carbon black irradiated with ionizing radiation is dispersed, By reacting the reaction system for a certain time under a predetermined temperature condition, the reduced metal can be precipitated and supported on the surface of carbon black dispersed in the liquid phase.

  At this time, the temperature and viscosity of the liquid, the presence or absence of stirring, and when stirring, the particle diameter of the formed metal fine particles can be adjusted by changing the stirring speed and the like. That is, as the temperature of the liquid is lower and the viscosity is higher, and when stirring is performed, the particle size of the formed metal fine particles tends to be smaller as the stirring speed is lower. Therefore, it is preferable to set the liquid temperature, viscosity, stirring conditions, and the like while considering the type and particle size of the metal fine particles to be formed, the type of reducing agent to be used, and other conditions.

  As the metal ion source that is the basis of the metal fine particles, any of various metal compounds containing a metal element and soluble in a liquid phase reaction system can be used. Moreover, it is preferable that the metal compound does not contain an impurity element such as a halogen element, which may cause abnormal nucleus growth as a starting point of nucleus growth, if possible. However, even when using a metal compound containing an impurity element, by adjusting the reaction conditions, etc., it is possible to suppress abnormal nucleation and to support fine metal particles with a small particle size on the surface of carbon black. is there.

As a metal compound suitable as a metal ion source, for example, in the case of platinum, dinitrodiammine platinum (II) (Pt (NO ₂ ) ₂ (NH ₃ ) ₂ ), hexachloroplatinum (IV) acid hexahydrate (H ₂ [PtCl ₆ ] · 6H ₂ O) and the like, and dinitrodiammine platinum (II) is particularly preferable. In the case of palladium, palladium chloride (II) solution (PdCl ₂ ), and in the case of gold, tetrachloroauric (III) acid tetrahydrate (HAuCl ₄ .4H ₂ O) and the like can be mentioned.

In the case of silver, silver nitrate (I) (AgNO ₃ ), silver methanesulfonate (CH ₃ SO ₃ Ag) and the like can be mentioned, and silver (I) nitrate is particularly preferable. For rhodium, rhodium (III) chloride solution (RhCl ₃ .3H ₂ O), for iridium, hexachloroiridium (III) hexahydrate (2 (IrCl ₆ ) · 6H ₂ O), for ruthenium These include ruthenium (III) nitrate solution (Ru (NO ₃ ) ₃ ) and, in the case of osmium, osmium oxide (VIII) (OsO ₄ ).

In the case of cobalt, cobalt nitrate (II) hexahydrate (Co (NO ₃ ) ₂ .6H ₂ O), cobalt sulfate (II) heptahydrate (CoSO ₄ .7H ₂ O), basic cobalt carbonate ( II) (xCoCO ₃ · yCo (OH) ₂ · zH ₂ O, x, y, and z vary depending on the production method, usually x = 2, y = 3, z = 1), cobalt chloride (II) hexahydrate (CoCl ₂ · 6H ₂ O), acetylacetone cobalt (II) (Co [CH (COCH ₃ ) ₂ ] ₂ ), cobalt acetate (II) tetrahydrate (Co (CH ₃ COO) ₂ · 4H ₂ O), etc. Is mentioned.

In the case of manganese, manganese nitrate (II) hydrate (Mn (NO ₃ ) ₂ .nH ₂ O, n = 4 to 6), manganese chloride (II) tetrahydrate (MnCl ₂ .4H ₂ O), And ammonium manganese sulfate (II) hexahydrate (Mn (NH ₄ ) ₂ (SO ₄ ) ₂ .6H ₂ O). In the case of iron, iron nitrate (III) hexahydrate, nonahydrate (Fe (NO ₃ ) ₃ · 6H ₂ O, 9H ₂ O), iron (II) chloride tetrahydrate (FeCl ₂ · 4H) ₂ O), iron sulfate (II) heptahydrate (FeSO ₄ .7H ₂ O), acetylacetone iron (III) (Fe [CH (COCH ₃ ) ₂ ] ₃ ) and the like.

In the case of nickel, nickel nitrate (II) hexahydrate (Ni (NO ₃ ) ₂ .6H ₂ O), nickel chloride (II) hexahydrate (NiCl ₂ .6H ₂ O), nickel sulfate (II) Heptahydrate (NiSO ₄ .7H ₂ O), acetylacetone nickel (II) (Ni [CH (COCH ₃ ) ₂ ] ₂ ), basic nickel carbonate (II) (aNiCO ₃ · bNi (OH) ₂ · cH ₂ O, a, b and c differ depending on the production method, usually a = 2, b = 3, c = 4), nickel acetate (II) tetrahydrate (Ni (CH ₃ COO) ₂ .4H ₂ O), etc. Is mentioned.

In the case of chromium, acetylacetone chromium (III) (Cr [CH (COCH ₃ ) ₂ ] ₃ ), chromium chloride (II) (CrCl ₂ ), chromium nitrate (III) nonahydrate (Cr (NO ₃ ) _3. 9H ₂ O) and the like. In the case of molybdenum, molybdenum chloride (V) (MoCl ₅ ), and in the case of titanium, titanium chloride (IV) solution (TiCl ₄ ) and the like can be mentioned.

  As the reducing agent, considering that the particle size of the metal fine particles should be as small as possible, a reducing agent having a weakest reducing power is preferably used. Examples of the reducing agent having a weak reducing power include alcohols such as methanol, ethanol and isopropyl alcohol, ascorbic acid and the like, as well as ethylene glycol, glutathione and organic acids (citric acid, malic acid, tartaric acid, etc.). , Reducing sugars (glucose, galactose, mannose, fructose, sucrose, maltose, raffinose, stachyose, etc.) and sugar alcohols (sorbitol, etc.). Especially, reducing sugars and sugars as derivatives thereof Alcohols or alcohols are preferred. Further, when alcohols and other reducing agents are used in combination as the reducing agent, the amount of metal fine particles supported on the surface of carbon black can be increased.

  Further, in the liquid phase reaction system, for example, a pH adjusting agent for adjusting the pH of the reaction system to a range suitable for reduction precipitation of metal ions, a dispersing agent for dispersing carbon black, a liquid phase Various additives such as a viscosity modifier for adjusting the viscosity of the resin may be added.

  Among them, various acids and alkalis can be used as the pH adjuster, and in particular, acids and alkalis that do not contain an impurity element that may cause abnormal nucleus growth as a starting point of nucleus growth. Is preferably used. Nitric acid etc. can be mentioned as an acid which does not contain an impurity element, Ammonia water etc. can be mentioned as an alkali.

  The preferred range of the pH of the liquid phase varies depending on the type of metal to be deposited and the type of metal compound as the source of the metal ion, and the lower the pH within the preferred range. The particle diameter of the formed metal fine particles tends to be small. Therefore, whether or not to add a pH adjuster, taking into account the type and particle size of the fine metal particles to be formed, the type of reducing agent to be used, and other conditions, and how much should be added Are preferably selected.

  Moreover, as a dispersing agent and a viscosity modifier, conventionally well-known various compounds can be used, However, It is preferable to use the polymer dispersing agent which has both these functions. Examples of the polymer dispersant include amine polymer dispersants such as polyethyleneimine and polyvinylpyrrolidone, hydrocarbon polymer dispersants having a carboxylic acid group in the molecule such as carboxymethylcellulose, or 1 Examples thereof include a copolymer having a polyethyleneimine moiety and a polyethylene oxide moiety in the molecule (hereinafter referred to as “PEI-PO copolymer”).

  The amount of addition of the polymer dispersant is not particularly limited, but as the addition amount increases, the viscosity of the liquid phase increases and the particle size of the formed metal fine particles tends to decrease, so the metal to be produced It is preferable to set a suitable addition amount range in consideration of the particle size of the fine particles, the type of reducing agent used, and other conditions.

"Fuel cell"
The fuel cell of the present invention includes the metal catalyst of the present invention. As is well known, a fuel cell is a battery that takes out an electric current obtained when an oxygen and hydrogen are reacted electrochemically in the presence of a catalyst, and can use the metal catalyst of the present invention. Specific examples of the battery include a polymer electrolyte fuel cell (PEFC) and a direct methanol fuel cell (DMFC).

  The PEFC and DMFC are layered porous on both sides of an ion exchange membrane made of a solid polymer, such as a perfluorosulfonic acid polymer membrane represented by DuPont's registered trademark Nafion. Membrane Electrode Assembly (MEA) obtained by joining electrodes (gas diffusion electrode, oxygen electrode and hydrogen electrode), and a conductive carrier and gas flow path provided outside thereof Usually, a plurality of cells including a bipolar plate are stacked (cell stack) in accordance with the output required for the fuel cell.

  The difference between PEFC and DMFC is that, in the former, hydrogen gas is brought into contact with the hydrogen electrode, whereas in the latter, methanol or an aqueous methanol solution is brought into contact in place of hydrogen gas.

  The metal catalyst of this invention is used in order to form a porous electrode among each part which comprises the said PEFC and DMFC. For example, the metal catalyst of the present invention in which platinum fine particles are supported on the surface of carbon black is mixed with a liquid containing a fluorine-based ion exchange resin (Nafion solution) to prepare a mixture. By applying and drying on both sides of the perfluorosulfonic acid polymer membrane as an ion exchange membrane as described, an MEA in which a porous electrode to be a hydrogen electrode and an oxygen electrode is formed on both sides of the membrane is configured. The

  In particular, in DMFC, carbon monoxide generated when methanol decomposes is adsorbed on the surface of platinum that forms platinum fine particles of the metal catalyst that constitutes the hydrogen electrode, and the catalytic activity is significantly reduced. Are known. Therefore, as a metal catalyst constituting the hydrogen electrode, a catalyst having platinum fine particles in which a small amount of ruthenium is mixed or alloyed with platinum in order to increase the catalytic activity, that is, to improve the catalytic activity per unit weight. It is preferable to use it.

  Also, as the metal catalyst constituting the oxygen electrode, in order to enhance the catalytic activity, a catalyst having platinum fine particles in which a small amount of palladium, nickel, or iron is mixed or alloyed with a small amount of palladium, nickel or iron is used. Is preferred.

  In order to produce a metal catalyst having platinum fine particles mixed with or alloyed with ruthenium, palladium, nickel, iron or the like, for example, in the production method of the present invention described above, Into a reaction system containing platinum ions, a metal element ion to be mixed or alloyed is added together with the platinum ion, and the metal element ion is combined with the platinum ion by the action of a reducing agent. It may be deposited in the form of fine particles and supported on the surface of carbon black.

  A PEFC-type fuel cell, after humidifying at a predetermined temperature, is brought into contact with oxygen while controlling its passing amount to the oxygen electrode, and is brought into contact with hydrogen while controlling its passing amount. It can generate electricity. Further, the DMFC type fuel cell generates power by bringing oxygen into contact with the oxygen electrode while controlling its passing amount, and bringing methanol or its aqueous solution into contact with the hydrogen electrode while controlling its passing amount. be able to.

<Example 1>
A large number of carbon blacks (Ketjen Black EC-600JD manufactured by Lion Co., Ltd., BET specific surface area: about 1300 m ² / g) stored in 5 g portions each in a plastic bag are prepared. Using a conveyor type electron beam irradiation apparatus [manufactured by Nissin Electric Co., Ltd., output 800 kV, 35 mA], the electron beam was continuously irradiated while being conveyed by the conveyor of the apparatus. The irradiation amount of the electron beam was expressed as an absorbed dose and set to 0.12 MGy.

  Next, carbon black irradiated with the electron beam, ethanol, and a dinitrodiammine platinum nitrate solution having a platinum concentration of 50 g / liter were added to pure water to prepare a liquid phase reaction system. The concentration of each component was carbon black: 10 g / liter, ethanol: 500 ml / liter, dinitrodiammine platinum nitrate solution: 38 g / liter.

  Next, the reaction system is reacted at about 80 ° C. for 6 hours under reflux with stirring at 500 rpm using a magnetic stirrer to thereby form the surface of the carrier particles dispersed in the liquid phase. A platinum catalyst was produced by depositing the reduced platinum into fine particles and supporting them.

<Examples 2 and 3>
The platinum catalyst was treated in the same manner as in Example 1 except that the amount of electron beam irradiated to carbon black was 0.48MGy (Example 2) and 1.44MGy (Example 3) in terms of absorbed dose. Manufactured.

<Comparative example 1>
A platinum catalyst was produced in the same manner as in Example 1 except that carbon black was not irradiated with an electron beam.

<Comparative example 2>
A platinum catalyst was produced in the same manner as in Example 1 except that the amount of electron beam applied to the carbon black was 2.40 MGy in terms of absorbed dose.

<Examples 4 to 6>
A platinum catalyst was produced in the same manner as in Examples 1 to 3 except that carbon black having a BET specific surface area of about 220 m ² / g [Vulcan XC-72R manufactured by Cabot Corporation] was used. The irradiation amount of the electron beam applied to the carbon black was 0.12 MGy (Example 4), 0.48 MGy (Example 5), and 1.44 MGy (Example 6) in terms of absorbed dose.

<Comparative Example 3>
A platinum catalyst was produced in the same manner as in Examples 4 to 6 except that carbon black was not irradiated with an electron beam.

<Comparative example 4>
Platinum catalysts were produced in the same manner as in Examples 4 to 6 except that the amount of electron beam irradiated to carbon black was 2.40 MGy in terms of absorbed dose.

<Measurement of supported amount, particle size and catalytic activity of platinum fine particles>
The supported amount, particle size and catalytic activity of the platinum fine particles were determined by the following procedure. That is, by the CO adsorption method, the produced platinum catalyst is treated under conditions of a pretreatment temperature of 120 ° C. and an adsorption temperature of 50 ° C. to determine the amount of CO adsorption. From the result, the surface area of the platinum fine particles supported on the surface of carbon black And catalytic activity (m ² / g-cat) were calculated. Further, the supported amount (% by weight) of platinum fine particles in the produced platinum catalyst was measured by ICP (Inductively Coupled Plasma) spectroscopy. Further, the particle size (nm) of platinum fine particles supported on the surface of carbon black was calculated from the supported amount and the previous surface area.

The above results are shown in Table 1 and FIGS. In each figure, -o- indicates the result of a system using carbon black having a specific surface area of about 1300 m ² / g, and-■-indicates the result of a system using carbon black having a specific surface area of about 220 m ² / g. Is shown.

From Table 1 and FIGS. 1 to 3, carbon black is used in both the system using carbon black having a specific surface area of about 1300 m ² / g and the system using carbon black having a specific surface area of about 220 m ² / g. In addition, by irradiating an electron beam in the range of 0.1 to 1.5 MGy in terms of absorbed dose, platinum deposited and supported on the surface of the carbon black by a reduction deposition method in a subsequent step. It has been found that the catalyst activity can be improved by reducing the particle size of the fine particles and increasing the loading.

In Examples and Comparative Examples of the present invention, the absorbed dose (MGy) of the electron beam irradiated to the carbon black, and the particle size (nm) of platinum fine particles deposited and supported on the surface of the carbon black by the reduction precipitation method It is a graph which shows the relationship. In Examples and Comparative Examples of the present invention, the absorbed dose (MGy) of the electron beam irradiated to the carbon black, and the supported amount (% by weight) of platinum fine particles deposited and supported on the surface of the carbon black by the reduction deposition method. It is a graph which shows the relationship. In Examples and Comparative Examples of the present invention, the absorbed dose (MGy) of the electron beam irradiated to the carbon black and the catalytic activity of the platinum catalyst supported by depositing platinum fine particles on the surface of the carbon black by the reduction deposition method ( It is a graph which shows the relationship with m < ² > / g-cat).

Claims (6)

  A metal catalyst comprising carbon black irradiated with ionizing radiation and a large number of metal fine particles supported on the surface of the carbon black.   The metal catalyst according to claim 1, wherein the ionizing radiation is an electron beam, and the electron beam is applied to the carbon black so that the absorbed dose is 0.1 to 1.5 MGy. The metal catalyst according to claim 1 or 2, wherein the carbon black before irradiation with ionizing radiation has a BET specific surface area of 100 to 1500 m ² / g.   The fine metal particles are fine particles containing at least one metal selected from the group consisting of platinum, palladium, gold, silver, rhodium, iridium, ruthenium, osmium, cobalt, manganese, nickel, iron, chromium, molybdenum, and titanium. The metal catalyst according to any one of claims 1 to 3.   The step of irradiating the carbon black with ionizing radiation, and the carbon black dispersed in a liquid phase reaction system containing metal ions that are the basis of the metal fine particles, A method of producing a metal catalyst, comprising a step of reducing by action and precipitating in the form of fine particles and supporting the fine particles on the surface of carbon black. A fuel cell comprising the metal catalyst according to claim 1.

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