JP2011240242A - Catalyst, method for production thereof, and use thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst which is not corroded in an acidic electrolyte or at a high voltage, is excellent in durability, and has a high oxygen reduction capacity.SOLUTION: The catalyst includes a niobium-containing metal carbonitride oxide and satisfying the following (i) and (ii): (i) in an extended X-ray absorption fine structure (EXAFS) spectrum, a peak at which the Fourier transform intensity in the range of 0-6 Å from a niobium atom is observed in the range of 3-4 Å from a niobium atom and (ii) in an X-ray absorption near edge structure (XANES) spectrum, the spectrum of the niobium-containing metal carbonitride oxide is identical with that of NbO.

Description

  The present invention relates to a catalyst, a production method thereof, and an application thereof.

  Fuel cells are classified into various types depending on the type of electrolyte and the type of electrode, and representative types include alkali type, phosphoric acid type, molten carbonate type, solid electrolyte type, and solid polymer type. Among them, a polymer electrolyte fuel cell that can operate at a low temperature (about −40 ° C.) to about 120 ° C. attracts attention, and in recent years, development and practical application as a low-pollution power source for automobiles is progressing. As a use of the polymer electrolyte fuel cell, a vehicle driving source and a stationary power source are being studied. However, in order to be applied to these uses, durability over a long period of time is required.

  In this polymer solid fuel cell, a polymer solid electrolyte is sandwiched between an anode and a cathode, fuel is supplied to the anode, oxygen or air is supplied to the cathode, and oxygen is reduced at the cathode to extract electricity. is there. Hydrogen or methanol is mainly used as the fuel.

  Conventionally, in order to increase the reaction rate of the fuel cell and increase the energy conversion efficiency of the fuel cell, the fuel cell cathode (air electrode) surface or anode (fuel electrode) surface has a layer containing a catalyst (hereinafter referred to as “for fuel cell use”). Also referred to as “catalyst layer”).

  As this catalyst, a noble metal is generally used, and platinum which is stable at a high potential and has a high activity among noble metals has been mainly used. However, since platinum is expensive and has limited resources, the development of an alternative catalyst has been sought.

  In addition, the noble metal used for the cathode surface may be dissolved in an acidic atmosphere, and there is a problem that it is not suitable for applications that require long-term durability. Therefore, there has been a strong demand for the development of a catalyst that does not corrode in an acidic atmosphere, has excellent durability, and has a high oxygen reducing ability.

  In recent years, materials containing non-metals such as carbon, nitrogen, and boron have attracted attention as catalysts instead of platinum. These non-metal-containing materials are cheaper than precious metals such as platinum and have abundant resources. Therefore, applications to various fields are being studied by universities and research institutions.

  For example, Patent Document 1 discloses a carbonitride oxide obtained by mixing carbide, oxide, and nitride and performing heat treatment at 500 to 1500 ° C. in a vacuum, an inert or non-oxidizing atmosphere. However, the oxycarbonitride disclosed in Patent Document 1 is a thin film magnetic head ceramic substrate material, and the use of this oxycarbonitride as a catalyst has not been studied.

  Patent Document 2 discloses an oxygen reduction electrode material containing a nitride of one or more elements selected from the group of elements of Group 4, Group 5 and Group 14 of the long periodic table as a platinum substitute material. However, these materials containing non-metals have a problem that practically sufficient oxygen reducing ability is not obtained as a catalyst.

Non-Patent Document 1 discloses a method for producing Nb ₁₂ O ₂₉ by reducing Nb ₂ O ₅ . However, Nb ₁₂ O ₂₉ disclosed in Non-Patent Document 1 is disclosed as a material exhibiting antiferromagnetism and conductivity, and has not been studied as an oxygen reduction catalyst for fuel cells.

  The applicant of the present application discloses useful catalysts in Patent Documents 3 and 4 and the like.

Japanese Patent Laid-Open No. 2003-342058 JP 2007-31781 A International Publication No. WO2009 / 031383 Pamphlet International Publication No. WO2009 / 091043 Pamphlet

R.J.Cava, B.Batlogg, J.J.Krajewski, P.Gammel, H.F.Poulsen, W.F.Peck Jr & L.W.Rupp Jr, `` Antiferromagnetism and metallic conductivity in Nb12O29 '', Nature, 1991, vol. 350, p. 598-600

  An object of the present invention is to solve such problems in the prior art, and an object of the present invention is to provide a catalyst that does not corrode in an acidic electrolyte or at a high potential, has excellent durability, and has a high oxygen reduction ability. There is.

As a result of intensive studies to solve the above-described problems of the prior art, the present inventors have observed a specific spectrum in the measurement of X-ray absorption fine structure (hereinafter referred to as “XAFS”). The catalyst made of niobium-containing metal carbonitride, especially the catalyst made of niobium-containing metal carbonitride obtained by oxidizing niobium-containing metal carbonitride under specific conditions has high oxygen reducing ability. As a result, the present invention has been completed. Note that Nb ₁₂ O ₂₉ obtained by the production method described in Non-Patent Document 1 cannot provide a practically sufficient oxygen reducing ability as an oxygen reduction catalyst for fuel cells.

The present invention relates to the following [1] to [13], for example.
[1]
A catalyst comprising a niobium-containing metal carbonitride that satisfies the following conditions (i) and (ii):
(I) Extended X-ray Absorption Fine Structure (hereinafter referred to as “EXAFS”) spectrum, Fourier transform in the range of 3 to 4 cm from niobium atoms and in the range of 0 to 6 cm from niobium atoms. A peak with maximum intensity is observed,
(Ii) In the X-ray absorption near edge structure (hereinafter referred to as “XANES”) spectrum, the spectrum of niobium-containing metal carbonitride is the same as that of Nb ₁₂ O ₂₉ .

[2]
The catalyst according to [1], which is a fuel cell catalyst.
[3]
In the measurement by the powder X-ray diffraction method (Cu-Kα ray) for the niobium-containing metal carbonitride oxide, the maximum value of the X-ray diffraction intensity between diffraction angles 2θ = 25.55 ° to 25.65 ° is I _1. When the maximum value of the X-ray diffraction intensity between the diffraction angle 2θ = 25.65 ° and 26.0 ° is I ₂ , I ₂ / (I ₁ + I ₂ ) is 0.26 to 1.0. The catalyst according to [1] or [2] above, wherein

[4]
The composition formula of the niobium-containing metal oxycarbonitride is NbC _x N _y O _z (where 0.1 ≦ x ≦ 2, 0.05 ≦ y ≦ 1, 0.1 ≦ z ≦ 2, and x + y + z ≦ 5). The catalyst according to any one of [1] to [3] above, which is represented by:

[5]
A method for producing a catalyst, comprising a step of obtaining a catalyst made of niobium-containing metal carbonitride by heat-treating niobium-containing metal carbonitride in an inert gas containing oxygen gas.

[6]
The temperature range of the heat treatment is 750 to 1050 ° C., and the concentration range of oxygen gas in the inert gas is 0.5 to 2% by volume. .

[7]
The method for producing a catalyst according to the above [5] or [6], wherein the inert gas contains hydrogen gas in a range of 4% by volume or less.

[8]
A catalyst layer for a fuel cell comprising the catalyst according to any one of [1] to [4] above.

[9]
The catalyst layer for a fuel cell according to the above [8], further comprising electron conductive particles.
[10]
An electrode having a fuel cell catalyst layer and a porous support layer, wherein the fuel cell catalyst layer is the fuel cell catalyst layer according to the above [8] or [9].

[11]
A membrane electrode assembly having a cathode, an anode, and an electrolyte membrane disposed between the cathode and the anode, wherein the cathode and / or the anode is the electrode according to the above [10], Membrane electrode assembly.

[12]
A fuel cell comprising the membrane electrode assembly according to [11] above.
[13]
A polymer electrolyte fuel cell comprising the membrane electrode assembly according to [11] above.

  The catalyst of the present invention does not corrode in an acidic electrolyte or at a high potential, is stable, has a high oxygen reducing ability, and is inexpensive compared with platinum. Therefore, the fuel cell including the catalyst is relatively inexpensive and has excellent performance.

2 is a powder X-ray diffraction spectrum of the niobium-containing metal carbonitride (catalyst (1)) obtained in Example 1. FIG. 2 is a powder X-ray diffraction spectrum of the niobium-containing metal carbonitride (catalyst (1)) obtained in Example 1. FIG. 2 is a cross-sectional TEM image of the niobium-containing metal carbonitride (catalyst (1)) obtained in Example 1. FIG. 2 is an X-ray absorption edge fine structure (XANES) spectrum of the niobium-containing metal carbonitride (catalyst (1)) obtained in Example 1. 2 is an extended X-ray absorption fine structure (EXAFS) spectrum of the niobium-containing metal carbonitride (catalyst (1)) obtained in Example 1. It is a graph which shows a time-dependent change of the density | concentration of the metal component eluted when 10 mg of catalysts (Pt / C catalyst, NbCNO catalyst) were immersed in 100 ml and 0.1 mol / L sulfuric acid of 80 degreeC. 2 is a graph showing an evaluation of the oxygen reducing ability of a fuel cell electrode (1) in Example 1. FIG. FIG. 8 shows a detection system for XAFS measurement in the embodiment.

<Catalyst>
The catalyst of the present invention is characterized by comprising a niobium-containing metal oxycarbonitride whose specific spectrum is observed in X-ray absorption fine structure (XAFS) measurement.

Specifically, the niobium-containing metal oxycarbonitride satisfies the following conditions (i) and (ii).
(I) In the extended X-ray absorption fine structure (EXAFS) spectrum, a peak having the maximum Fourier transform intensity in the range of 3 to 4 か ら from the niobium atom is observed in the range of 0 to 6 か ら from the niobium atom.
(Ii) In the X-ray absorption edge fine structure (XANES) spectrum, the spectrum of the niobium-containing metal carbonitride is the same as that of Nb ₁₂ O ₂₉ .

  XAFS is an absorption spectrum structure obtained by X-ray irradiation due to excitation of inner-shell electrons, and information for each element of interest can be obtained. Moreover, it is divided into XANES and EXAFS depending on the difference in energy range and excitation process. XANES originates from excitation into an unoccupied orbit and has a spectral structure that depends on the valence and coordination structure of the element of interest. On the other hand, EXAFS is a vibration structure obtained due to the interaction between excited electrons and scattered electrons from neighboring atoms, and the radial distribution function obtained by Fourier transformation is based on the local structure of the element of interest (the surrounding atomic species, Information on the number of coordination atoms and the distance between atoms).

The details of the conditions for X-ray absorption fine structure (XAFS) measurement in the present invention are as described in the column of Examples described later.
Moreover, in the measurement by X-ray powder diffraction method (Cu-Kα ray) with respect to the niobium-containing metal carbonitride oxide (measurement conditions are as follows), the diffraction angle 2θ is between 25.45 ° and 25.65 °. When the maximum value of the X-ray diffraction intensity is I ₁ and the maximum value of the X-ray diffraction intensity between the diffraction angles 2θ = 25.65 ° to 26.0 ° is I ₂ , I ₂ / (I ₁ + I ₂ ) Is preferably 0.26 to 1.0, more preferably 0.4 to 1.0.

<Measurement conditions>
Measuring device: X'Pert PRO made by PANalytical
X-ray output: 45 kV, 40 mA
Scan axis: 2θ / θ
Measurement range (2θ): 10 ° to 89.98 °
Scan size: 0.017 °
Scan step time: 10.3S
Scan type: Continuous
PSD mode: Scanning
Divergent slit (DS) type: Fixed Irradiation width: 10 mm
Measurement temperature: 25 ° C
Target: Cu
K-Alpha 1: 1.54060
K-Alpha2: 1.54443
K-Beta: 1.39225
K-A2 / K-A1 ratio: 0.5
Goniometer radius: 240mm

A catalyst made of niobium-containing metal carbonitride oxide having I ₂ / (I ₁ + I ₂ ) in the above range does not corrode in an acidic electrolyte or at a high potential, is stable, and has a high oxygen reducing ability.
The diffraction line peak observed near the diffraction angle 2θ = 25.5 ° originates from the Nb ₂ O ₅ skeleton (ICSD card: 00-037-1468) and is observed near the diffraction angle 2θ = 25.55 °. The diffraction line peak is derived from the Nb ₁₂ O ₂₉ skeleton (ICSD card: 01-073-1610). Therefore, it is considered that as “I ₂ / (I ₁ + I ₂ )” increases, the abundance ratio of the Nb ₁₂ O ₂₉ skeleton in the niobium-containing metal carbonitride oxide increases. The present inventors presume that such a catalyst composed of a niobium-containing metal carbonitride having a large proportion of Nb ₁₂ O ₂₉ skeleton has a high oxygen reducing ability.

  The diffraction line peak means a peak obtained with a specific diffraction angle and diffraction intensity when a sample (crystalline) is irradiated with X-rays at various angles. In the present invention, the X-ray diffraction intensity I is defined as a value obtained by subtracting the baseline intensity from the diffraction intensity obtained by the following measurement method (however, in the case of a negative value, the X-ray diffraction intensity I is = 0). As the baseline intensity, the diffraction intensity at a diffraction angle 2θ = 22.0 ° is adopted.

A niobium-containing metal carbonitride that satisfies “I ₂ / (I ₁ + I ₂ )” is obtained by, for example, heat-treating niobium-containing metal carbonitride in an inert gas containing oxygen gas. Can do. By controlling the heat treatment conditions, the abundance ratio of the Nb ₁₂ O ₂₉ skeleton in the niobium-containing metal carbonitride can be increased, and the oxygen reduction ability of the finally obtained catalyst can be increased. Details of the method for producing the niobium-containing metal oxycarbonitride will be described later.
Further, in the measurement by the powder X-ray diffraction method (Cu-Kα ray) for the niobium-containing metal carbonitride oxide (measurement conditions are as described above), the diffraction angle is 2θ = 34.5 ° to 35.5 °. Xb-ray diffraction intensity derived from Nb ₁₂ O ₂₉ and Nb ₂ O ₅ between diffraction angles 2θ = 23.00 ° to 24.00 °, where J ₁ is the maximum value of X-ray diffraction intensity derived from NbC _0.5 N _0.5 If the maximum value of the set to J _2, J / a (J ₁ + J ₂₎ and DOO (Degree of Oxidation). When this value is from 0.30 to 0.95, the catalyst activity is preferably high, and when it is from 0.50 to 0.90, the catalyst exhibits an extremely high activity and also has high durability against acids, which is preferable.

In addition, the niobium-containing metal carbonitride oxide usually has a composition formula of NbC _x N _y O _z (where x, y, z represent the ratio of the number of atoms, and 0.1 ≦ x ≦ 2, 0.05 ≦ y ≦ 1, 0.1 ≦ z ≦ 2, and x + y + z ≦ 5). In the above composition formula, it is preferable that 0.1 ≦ x ≦ 1.5, 0.05 ≦ y ≦ 0.95, 0.2 ≦ z ≦ 1.9, and 2.0 ≦ x + y + z ≦ 4.35. .

  In addition to niobium, the niobium-containing metal oxycarbonitride is tin, indium, platinum, tantalum, zirconium, copper, iron, tungsten, chromium, molybdenum, hafnium, titanium, vanadium, cobalt, manganese, cerium, mercury, plutonium, It may contain at least one metal selected from the group consisting of gold, silver, iridium, palladium, yttrium, ruthenium and nickel (hereinafter also referred to as “metal M” or “M”).

The composition formula of the carbonitrous oxide containing niobium and other metals M is Nb _a M _b C _x N _y O _z (where M is “metal M”, a, b, x, y, z are Represents the ratio of the number of atoms, 0.01 ≦ a ≦ 1, 0 ≦ b ≦ 0.99, 0.1 ≦ x ≦ 2, 0.05 ≦ y ≦ 1, 0.1 ≦ z ≦ 2, a + b = 1 And x + y + z ≦ 5.) In the composition formula, 0.05 ≦ a ≦ 0.99, 0.01 ≦ b ≦ 0.95, 0.1 ≦ x ≦ 1.5, 0.05 ≦ y ≦ 0.95, 0.2 ≦ z It is preferable that ≦ 1.9 and 2.0 ≦ x + y + z ≦ 4.35. When the ratio of the number of atoms is within the above range, the oxygen reduction potential tends to increase.

In the present invention, the “niobium-containing metal oxycarbonitride” is represented by a composition formula of Nb _a M _b C _x N _y O _z when the composition formula is Nb _a M _b C _x N _y O _z. Or niobium-containing metal oxide, niobium-containing metal carbide, niobium-containing metal nitride, niobium-containing metal carbonitride, niobium-containing metal carbonate or niobium-containing metal A mixture represented by Nb _a M _b C _x N _y O _z as _a whole (including a compound represented by Nb _a M _b C _x N _y O _z). It does not have to be)) or both.
The oxygen reduction initiation potential of the catalyst used in the present invention, measured according to the following measurement method (A), is preferably 0.5 V (vs. NHE) or more with respect to the reversible hydrogen electrode.

[Measurement method (A):
The catalyst and carbon are placed in a solvent so that the amount of the catalyst dispersed in the carbon that is the electron conductive particles is 1% by mass, and stirred with ultrasonic waves to obtain a suspension. As a carbon source, carbon black (specific surface area: 100 to 300 m ² / g) (for example, XC-72 manufactured by Cabot Corporation) is used, and the catalyst and carbon are dispersed so that the mass ratio is 95: 5. As the solvent, isopropyl alcohol: water (mass ratio) = 1: 1 is used.

  10 μl of the suspension is collected while applying ultrasonic waves, and is quickly dropped on a glassy carbon electrode (diameter: 5.2 mm) and dried. By drying, a catalyst layer for a fuel cell containing the catalyst is formed on the glassy carbon electrode. The formation of the catalyst layer is repeated until 2 mg of the catalyst layer is deposited on the glassy carbon electrode.

  Next, 10 μl of NAFION (registered trademark) (DuPont 5% NAFION (registered trademark) solution (DE521)) diluted 10-fold with isopropyl alcohol was dropped on the catalyst layer for the fuel cell. For 1 hour.

Using the electrode thus obtained, a reversible hydrogen electrode in a 0.5 mol / L sulfuric acid aqueous solution at a temperature of 30 ° C. in a sulfuric acid aqueous solution of the same concentration was used as a reference electrode in an oxygen atmosphere and a nitrogen atmosphere. There is a difference of 0.2 μA / cm ² or more between the reduction current in the oxygen atmosphere and the reduction current in the nitrogen atmosphere when the current-potential curve is measured by polarization at a potential scanning speed of 5 mV / sec. The potential at which it begins to appear is defined as the oxygen reduction start potential. ]

  When the oxygen reduction starting potential is less than 0.7 V (vs. NHE), hydrogen peroxide may be generated when the catalyst is used as a catalyst for a cathode of a fuel cell. Further, the oxygen reduction starting potential is preferably 0.85 V (vs. NHE) or more in order to suitably reduce oxygen. Further, the oxygen reduction starting potential is preferably as high as possible. Although there is no particular upper limit, the theoretical value is 1.23 V (vs. NHE).

  When this potential is less than 0.4 V (vs. NHE), there is no problem at all from the viewpoint of the stability of the niobium-containing metal carbonitride, but oxygen cannot be suitably reduced and is included in the fuel cell. The usefulness of the membrane electrode assembly as a fuel cell catalyst layer is poor.

<Method for producing catalyst>
The method for producing the catalyst is not particularly limited. For example, a niobium-containing metal carbonitride is heat-treated in an inert gas containing oxygen gas or in an inert gas containing oxygen gas and hydrogen gas. The manufacturing method including the process of obtaining a carbonitrous oxide is mentioned.

(Method for producing niobium-containing metal carbonitride)
As a method for obtaining the niobium-containing metal carbonitride used in the above step, a mixture of niobium oxide and carbon is a kind of niobium-containing metal carbonitride by heat-treating in a nitrogen atmosphere or an inert gas containing nitrogen. A method of producing niobium carbonitride (I), a method of producing niobium carbonitride which is a kind of niobium-containing metal carbonitride by heat-treating a mixture of niobium carbide, niobium oxide and niobium nitride in a nitrogen atmosphere or the like Examples thereof include (II) and a method (III) for producing niobium carbonitride which is a kind of niobium-containing metal carbonitride by heat-treating a mixture of niobium carbide and niobium nitride in a nitrogen atmosphere or the like. Also, a method (IV) for producing a niobium-containing metal carbonitride by heat-treating a mixture of the metal M oxide, niobium oxide and carbon in a nitrogen atmosphere or an inert gas containing nitrogen, the metal Method (V) for producing a niobium-containing metal carbonitride by heat-treating a mixture of M oxide, niobium carbide and niobium nitride in an inert gas such as nitrogen gas, or the metal M oxide, carbonization Method (VI) for producing niobium-containing metal carbonitride by heat-treating a mixture of niobium, niobium nitride and niobium oxide in an inert gas such as nitrogen gas, or a compound (for example, organic acid) containing the metal M Salts, chlorides, carbides, nitrides, complexes, etc.), niobium carbide and niobium nitride mixtures by heat-treating them in an inert gas such as nitrogen gas. A method for producing the carbonitride (VII) can be mentioned. Also, a method (VIII) for producing a niobium-containing metal carbonitride by heat-treating a mixture of the raw materials in the production methods (IV) to (VII) and other raw materials in an inert gas such as nitrogen gas. It may be. In the present invention, there is no particular limitation as long as a niobium-containing metal carbonitride can be obtained, and any production method may be used.

Details of these production methods can be understood from the description of paragraphs [0056] to [0082] of Patent Document 3, paragraphs [0059] to [0105] of Patent Document 4, and the like.
The niobium-containing metal carbonitride obtained by the above production methods (I) to (VIII) is preferably crushed. The catalyst obtained by pulverization can be made into a finer powder, and the catalyst can be suitably dispersed in the catalyst layer containing the catalyst. Moreover, since the catalyst area obtained is large and it is excellent in catalytic ability, it is preferable.

  Examples of the method for pulverizing the niobium-containing metal carbonitride include a roll rolling mill, a ball mill, a medium stirring mill, an airflow crusher, a mortar, a method using a tank disintegrator, and the like. An airflow pulverizer is preferable in terms of being able to make finer particles, and a method using a mortar is preferable in terms of facilitating a small amount of processing.

(Method for producing niobium-containing metal carbonitride oxide)
Next, a process for obtaining a niobium-containing metal carbonitride by heat-treating the niobium-containing metal carbonitride in an inert gas containing oxygen gas will be described.

  Examples of the inert gas include nitrogen gas, helium gas, neon gas, argon gas, krypton gas, xenon gas, and radon gas. Nitrogen gas and argon gas are particularly preferable because they are relatively easily available.

  The heat treatment is preferably performed at a temperature range of 750 to 1050 ° C. and an oxygen gas concentration of 0.5 to 2% by volume. When heat treatment is performed under such conditions, a niobium-containing metal oxycarbonitride satisfying the above (i) and (ii) is obtained in the XAFS measurement, and the catalyst made of this niobium-containing metal oxycarbonitride is contained in an acidic electrolyte. It does not corrode at high potentials, is stable, and has a high oxygen reducing ability.

The inert gas may further contain hydrogen gas.
The hydrogen gas concentration in the inert gas depends on the heat treatment time, the heat treatment temperature, and the oxygen concentration, but is preferably from 0.1% by volume to 5% by volume. More preferably, it is 2 volume% or more and 4 volume% or less. When the hydrogen gas concentration is within the above range, it is preferable in that a uniform carbonitride oxide is formed so that oxidation does not proceed excessively.

By controlling the heat treatment conditions, the abundance ratio of the Nb ₁₂ O ₂₉ skeleton in the niobium-containing metal carbonitride can be increased, and the oxygen reduction ability of the finally obtained catalyst can be increased.

Examples of the heat treatment method in this step include a stationary method, a stirring method, a dropping method, and a powder trapping method.
Details of these heat treatment methods can be understood from the descriptions in paragraphs [0091] to [0095] of Patent Document 3, paragraphs [0111] to [0115] of Patent Document 4, and the like.

  As the catalyst of the present invention, the niobium-containing metal carbonitride oxide obtained by the above-described production method or the like may be used as it is, but the niobium-containing metal carbonitride oxide obtained is further crushed to obtain a finer powder. You may use what was made.

Examples of the method for pulverizing the niobium-containing metal carbonitride oxide include a roll rolling mill, a ball mill, a medium agitation mill, an airflow crusher, a mortar, a tank disintegrator method, and the like. A method using an airflow pulverizer is preferable in that the product can be made finer, and a method using a mortar is preferable in that a small amount of processing is easy.
The catalyst of this invention is obtained by the manufacturing method of the niobium containing metal carbonitrous oxide mentioned above.

<Application>
The catalyst of the present invention can be used as an alternative catalyst for a platinum catalyst.
For example, it can be used as a catalyst for fuel cells, a catalyst for exhaust gas treatment, or a catalyst for organic synthesis.
The fuel cell catalyst layer of the present invention is characterized by containing the catalyst.

  The fuel cell catalyst layer includes an anode catalyst layer and a cathode catalyst layer, and the catalyst can be used for both. Since the catalyst is excellent in durability and has a large oxygen reducing ability, it is preferably used in the cathode catalyst layer.

  The catalyst layer for a fuel cell of the present invention preferably further contains electron conductive particles. When the fuel cell catalyst layer containing the catalyst further contains electron conductive particles, the reduction current can be further increased. The electron conductive particles are considered to increase the reduction current because they generate an electrical contact for inducing an electrochemical reaction in the catalyst.

The electron conductive particles are usually used as a catalyst carrier.
Examples of the electron conductive particles include carbon, conductive polymers, conductive ceramics, metals, and conductive inorganic oxides such as tungsten oxide or iridium oxide, and these can be used alone or in combination. In particular, since carbon has a large specific surface area, carbon alone or a mixture of carbon and other electron conductive particles is preferable. That is, the fuel cell catalyst layer preferably contains the catalyst and carbon.

  As carbon, carbon black, graphite, activated carbon, carbon nanotube, carbon nanofiber, carbon nanohorn, fullerene and the like can be used. If the particle size of the carbon is too small, it becomes difficult to form an electron conduction path, and if it is too large, the gas diffusibility of the catalyst layer for the fuel cell is lowered and the utilization rate of the catalyst tends to be lowered. Is preferable, and the range of 10 to 100 nm is more preferable.

When the electron conductive particles are carbon, the mass ratio of the catalyst to carbon (catalyst: electron conductive particles) is preferably 4: 1 to 1000: 1.
The conductive polymer is not particularly limited. For example, polyacetylene, poly-p-phenylene, polyaniline, polyalkylaniline, polypyrrole, polythiophene, polyindole, poly-1,5-diaminoanthraquinone, polyaminodiphenyl, poly (o- Phenylenediamine), poly (quinolinium) salts, polypyridine, polyquinoxaline, polyphenylquinoxaline and the like. Among these, polypyrrole, polyaniline, and polythiophene are preferable, and polypyrrole is more preferable.

  The polymer electrolyte is not particularly limited as long as it is generally used in a fuel cell catalyst layer. Specifically, a perfluorocarbon polymer having a sulfonic acid group (for example, NAFION (registered trademark) (DuPont 5% NAFION (registered trademark) solution (DE521), etc.)), a hydrocarbon polymer having a sulfonic acid group. Compound, polymer compound doped with inorganic acid such as phosphoric acid, organic / inorganic hybrid polymer partially substituted with proton conductive functional group, proton impregnated with phosphoric acid solution or sulfuric acid solution in polymer matrix A conductor etc. are mentioned. Among these, NAFION (registered trademark) (DuPont 5% NAFION (registered trademark) solution (DE521)) is preferable.

  The catalyst layer for a fuel cell of the present invention can be used for either an anode catalyst layer or a cathode catalyst layer. The catalyst layer for a fuel cell of the present invention includes a catalyst layer (catalyst catalyst for cathode) provided on the cathode of a fuel cell because it contains a catalyst having high oxygen reducing ability and hardly corroded even in a high potential in an acidic electrolyte. Layer). In particular, it is suitably used for a catalyst layer provided on the cathode of a membrane electrode assembly provided in a polymer electrolyte fuel cell.

  Examples of the method for dispersing the catalyst on the electron conductive particles as a support include air flow dispersion and dispersion in liquid. Dispersion in liquid is preferable because a catalyst and electron conductive particles dispersed in a solvent can be used in the fuel cell catalyst layer forming step. Examples of the dispersion in the liquid include a method using an orifice contraction flow, a method using a rotating shear flow, and a method using an ultrasonic wave. The solvent used for dispersion in the liquid is not particularly limited as long as it does not erode the catalyst or electron conductive particles and can be dispersed, but a volatile liquid organic solvent or water is generally used. The

Further, when the catalyst is dispersed on the electron conductive particles, the electrolyte and the dispersing agent may be further dispersed at the same time.
The method for forming the catalyst layer for the fuel cell is not particularly limited. For example, a method of applying a suspension containing the catalyst, the electron conductive particles, and the electrolyte to the electrolyte membrane or the gas diffusion layer to be described later. It is done. Examples of the application method include a dipping method, a screen printing method, a roll coating method, and a spray method. In addition, after forming a catalyst layer for a fuel cell on a base material by a coating method or a filtration method using a suspension containing the catalyst, electron conductive particles, and an electrolyte, the catalyst layer for a fuel cell is formed on the electrolyte membrane by a transfer method. The method of forming is mentioned.

The electrode of the present invention is characterized by having the fuel cell catalyst layer and a porous support layer.
The electrode of the present invention can be used as either a cathode or an anode. Since the electrode of the present invention is excellent in durability and has a large catalytic ability, it is more effective when used for a cathode.

  The porous support layer is a layer that diffuses gas (hereinafter also referred to as “gas diffusion layer”). The gas diffusion layer may be anything as long as it has electron conductivity, high gas diffusibility, and high corrosion resistance. Generally, carbon-based porous materials such as carbon paper and carbon cloth are used. Materials and aluminum foil coated with stainless steel and corrosion-resistant materials for weight reduction are used.

  The membrane electrode assembly of the present invention is a membrane electrode assembly having a cathode, an anode, and an electrolyte membrane disposed between the cathode and the anode, wherein the cathode and / or the anode is the electrode. It is characterized by that.

  As the electrolyte membrane, for example, an electrolyte membrane using a perfluorosulfonic acid system or a hydrocarbon electrolyte membrane is generally used. However, a membrane or porous body in which a polymer microporous membrane is impregnated with a liquid electrolyte is used. A membrane filled with a polymer electrolyte may be used.

The fuel cell according to the present invention includes the membrane electrode assembly.
Fuel cell electrode reactions occur at the so-called three-phase interface (electrolyte-electrode catalyst-reaction gas). Fuel cells are classified into several types depending on the electrolyte used, etc., and include molten carbonate type (MCFC), phosphoric acid type (PAFC), solid oxide type (SOFC), and solid polymer type (PEFC). . Among them, the membrane electrode assembly of the present invention is preferably used for a polymer electrolyte fuel cell.

EXAMPLES The present invention will be described below in more detail with reference to examples, but the present invention is not limited to these examples.
In addition, various measurements in Examples and Comparative Examples were performed by the following methods.

[Analysis method]
1. X-ray absorption fine structure (XAFS) measurement;
Measurements were performed under the following conditions using a large synchrotron radiation facility, commonly known as “SPring-8”, in the Harima Science Park City, Hyogo Prefecture.

(1) Radiation source energy range: 3.8 to 72 keV
Horizontal divergence of beam: 1.0 mrad
Photon quantity: ~ 10 ^ 10 photons / s
Energy resolution: ΔE / E ~ 10 ^ -4
Harmonic rejection: ~ 10 ^ -4
(2) Detection system QXAFS method shown in Fig. 8 (3) Information on EXAFS fitting method Software name: athena, artemis, FEFF
Cited designation: "IFEFFIT: interactive EXAFS analysis and FEFF fitting" M. Newville, J. Synchrotron Rad. 8, pp 322--324 (2001).
(4) Information on the XANES fitting method The absorption edge energy was fitted with the Victoreen equation (μM = Cλ ^ 3-Dλ ^ 4), background processing was performed, and standardization was further performed.

2. Powder X-ray diffraction;
Powder X-ray diffraction of the sample was performed using X'Pert PRO manufactured by PANalytical.

  The number of diffraction line peaks in powder X-ray diffraction of each sample was counted by regarding a signal that can be detected at a ratio (S / N) of signal (S) to noise (N) of 2 or more as one peak. The X-ray diffraction intensity I was defined as a value obtained by subtracting the baseline intensity from the diffraction intensity obtained by the following measurement method (however, in the case of a negative value, it was defined as the X-ray diffraction intensity I = 0). ). As the baseline intensity, the diffraction intensity at a diffraction angle 2θ = 22.0 ° was adopted.

3. Elemental analysis;
Carbon: About 0.1 g of a sample was weighed and measured with Horiba EMIA-110.
Nitrogen / oxygen: About 0.1 g of a sample was weighed and sealed in Ni-Cup, and then measured with an ON analyzer.
Niobium: About 0.1 g of a sample was weighed on a platinum dish, and nitric acid-hydrofluoric acid was added for thermal decomposition. This heat-decomposed product was diluted, diluted, and quantified by ICP-MS.

[Example 1]
1. Preparation of the catalyst;
6.08 g (58 mmol) of niobium carbide and 4.81 g (45 mmol) of niobium nitride were sufficiently pulverized and mixed. This mixed powder was heated in a tube furnace at 1600 ° C. for 3 hours in a nitrogen atmosphere to obtain 10.2 g of niobium carbonitride (1), and then pulverized in a mortar.

  A powder X-ray diffraction spectrum of niobium carbonitride (1) is shown in FIG. 2 (lower), and an enlarged view of the diffraction angle 2θ = 20 ° to 45 ° in the powder X-ray diffraction spectrum is shown in FIG. 2 (upper). . Table 1 shows the elemental analysis results of niobium carbonitride (1).

  Next, in the rotary kiln, 950 g of niobium carbonitride (1) 0.50 g was flowed while flowing argon gas containing 1% by volume oxygen gas and nitrogen gas containing 3% by volume hydrogen gas at a volume ratio of 1: 1. Niobium oxycarbonitride (hereinafter also referred to as “catalyst (1)” or “NbCNO (DOO = 0.81)”) by heat treatment at 0 ° C. for 8 hours. ) Was prepared. The weight of the obtained catalyst was 0.59 g.

Table 1 shows the results of elemental analysis of the catalyst (1).
The powder X-ray diffraction spectrum of the catalyst (1) is shown in FIG. 1 (data described as “NbCNO”) and FIG. 2 (data described as “NbCNO (DOO = 0.81)”).
The X-ray absorption edge fine structure (XANES) spectrum of the catalyst (1) is shown in FIG. 4 (data described as “NbCNO (DOO = 0.81)”).
The extended X-ray absorption fine structure (EXAFS) spectrum of the catalyst (1) is shown in FIG. 5 (data described as “NbCNO (DOO = 0.81)”).

2. Production of fuel cell electrodes;
The measurement of the oxygen reducing ability was performed as follows. Catalyst (1) 0.095 g and carbon (Cabot XC-72) 0.005 g were mixed in 10 g of a mixed solution of isopropyl alcohol: pure water = 2: 1, and stirred and suspended with ultrasonic waves. did. 30 μl of this mixture was applied to a glassy carbon electrode (manufactured by Tokai Carbon Co., Ltd., diameter: 5.2 mm) and dried at 120 ° C. for 1 hour. Furthermore, 10 μl of NAFION (registered trademark) (DuPont 5% NAFION (registered trademark) solution (DE521) diluted 10-fold with isopropyl alcohol was applied, dried at 120 ° C. for 1 hour, and an electrode for a fuel cell ( 1) was obtained.

3. Evaluation of oxygen reducing ability;
The catalytic ability (oxygen reducing ability) of the thus produced fuel cell electrode (1) was evaluated by the following method.

  First, the produced fuel cell electrode (1) was polarized in an oxygen atmosphere and a nitrogen atmosphere in a 0.5 mol / L sulfuric acid aqueous solution at 30 ° C. and a potential scanning speed of 5 mV / second, and a current-potential curve was measured. did. At that time, a reversible hydrogen electrode in an aqueous sulfuric acid solution having the same concentration was used as a reference electrode.

From the above measurement results, the potential at which a difference of 0.2 μA / cm ² or more appears between the reduction current in the oxygen atmosphere and the reduction current in the nitrogen atmosphere was defined as the oxygen reduction start potential, and the difference between the ^two was defined as the oxygen reduction current.

The catalytic ability (oxygen reducing ability) of the fuel cell electrode (1) produced by this oxygen reduction starting potential and oxygen reducing current was evaluated.
That is, the higher the oxygen reduction start potential and the larger the oxygen reduction current, the higher the catalytic ability (oxygen reducing ability) of the fuel cell electrode (1).

FIG. 7 shows a current-potential curve obtained by the above measurement. In FIG. 7, DOO = 0.81 is data of the fuel cell electrode (1).
The fuel cell electrode (1) had an oxygen reduction starting potential of 0.89 V (vs. NHE) and was found to have a high oxygen reducing ability.

[Example 2]
Niobium carbonitride (hereinafter also referred to as “catalyst (2)” or “NbCNO (DOO = 0.97)”) in the same manner as in Example 1 except that the heat treatment time was changed from 8 hours to 10 hours. ) Was prepared. The weight of the obtained catalyst was 0.59 g.
The powder X-ray diffraction spectrum of the catalyst (2) is shown in FIG. 2 (data described as “NbCNO (DOO = 0.97)”).
The X-ray absorption fine structure (XANES) spectrum of the catalyst (2) is shown in FIG. 4 (data described as “NbCNO (DOO = 0.97)”).
The extended X-ray absorption fine structure (EXAFS) spectrum of the catalyst (2) is shown in FIG. 5 (data described as “NbCNO (DOO = 0.97)”).

2. Production of fuel cell electrodes and evaluation of oxygen reduction ability;
A fuel cell electrode (hereinafter referred to as “fuel cell electrode (2)”) was produced in the same manner as in Example 1 except that the catalyst (2) was used instead of the catalyst (1).

The catalytic ability (oxygen reducing ability) of the fuel cell electrode (2) was evaluated in the same manner as in Example 1.
The oxygen reduction starting potential of the fuel cell electrode (2) was 0.86 V (vs. NHE).

[Example 3]
Niobium oxycarbonitride (hereinafter also referred to as “catalyst (3)” or “NbCNO (DOO = 0.49)”) in the same manner as in Example 1 except that the heat treatment time was changed from 8 hours to 4 hours. . ) Was prepared. The weight of the obtained catalyst was 0.59 g.
The powder X-ray diffraction spectrum of the catalyst (3) is shown in FIG.

2. Production of fuel cell electrodes and evaluation of oxygen reduction ability;
A fuel cell electrode (hereinafter referred to as “fuel cell electrode (3)”) was produced in the same manner as in Example 1 except that the catalyst (3) was used instead of the catalyst (1).

The catalytic ability (oxygen reducing ability) of the fuel cell electrode (3) was evaluated in the same manner as in Example 1.
The oxygen reduction starting potential of the fuel cell electrode (3) was 0.87 V (vs. NHE).

[Comparative Example 1]
1. Preparation of the catalyst;
According to the production method described in Non-Patent Document 1, Nb ₂ O ₅ was reduced to prepare Nb ₁₂ O ₂₉ (hereinafter also referred to as “catalyst (4)” or “Nb ₁₂ O ₂₉ ”).

The powder X-ray diffraction spectrum of the catalyst (4) is shown in FIG. In the powder X-ray diffraction spectrum, four diffraction line peaks derived from the Nb ₁₂ O ₂₉ skeleton were observed at a diffraction angle of 2θ = 23 ° to 28 °. The X-ray diffraction intensity at the diffraction angle 2θ = 22 ° which is the intensity of the baseline is 106, and the maximum value I ₁ of the X-ray diffraction intensity between the diffraction angle 2θ = 25.55 ° and 25.65 ° is 38. The maximum value I ₂ of the X-ray diffraction intensity between the diffraction angles of 25.65 ° and 26.0 ° was 552, and I ₂ / (I ₁ + I ₂ ) was 0.94.
The X-ray absorption edge fine structure (XANES) spectrum of the catalyst (4) is shown in FIG. 4 (data described as “Nb ₁₂ O ₂₉ ”).
The extended X-ray absorption fine structure (EXAFS) spectrum of the catalyst (4) is shown in FIG. 5 (data described as “Nb ₁₂ O ₂₉ ”).

2. Production of fuel cell electrodes and evaluation of oxygen reduction ability;
A fuel cell electrode (hereinafter referred to as “fuel cell electrode (4)”) was prepared in the same manner as in Example 1 except that the catalyst (4) was used instead of the catalyst (1).

The catalytic ability (oxygen reducing ability) of the fuel cell electrode (4) was evaluated in the same manner as in Example 1.
It was found that the fuel cell electrode (4) had an oxygen reduction starting potential of 0.60 V (vs. NHE) and a low oxygen reducing ability.

Incidentally, FIGS. 1, 2, 4, in Figure _{5, NbO 2, Nb 2 O} 5, Nb 2 CN or data with a description of the NbCNO (DOO = 0.10), respectively, the following sample data It is.

NbO ₂ : NBO02PBNbO2 (Purity: 3N, Shape: Powder) manufactured by Kojundo Chemical Laboratory Co., Ltd.
Nb ₂ O ₅ : NBO07PBNb2O5 (Purity: 3N, Shape: Powder) manufactured by Kojundo Chemical Laboratory Co., Ltd.
Nb ₂ CN: Niobium carbonitride (1)
NbCNO (DOO = 0.10): Niobium carbonitride prepared by the same method as in Example 1 except that the heat treatment time was changed from 8 hours to 1 hour

  The catalyst of the present invention does not corrode in an acidic electrolyte or at a high potential, is excellent in durability, and has a high oxygen reducing ability. Therefore, it can be used in a fuel cell catalyst layer, an electrode, an electrode assembly, or a fuel cell.

Claims (13)

A catalyst comprising a niobium-containing metal carbonitride that satisfies the following conditions (i) and (ii):
(I) In the extended X-ray absorption fine structure (EXAFS) spectrum, a peak having the maximum Fourier transform intensity in the range of 3 to 4 か ら from the niobium atom is observed in the range of 0 to 6 か ら from the niobium atom.
(Ii) In the X-ray absorption edge fine structure (XANES) spectrum, the spectrum of the niobium-containing metal carbonitride is the same as that of Nb ₁₂ O ₂₉ .   The catalyst according to claim 1, wherein the catalyst is a fuel cell catalyst. In the measurement by the powder X-ray diffraction method (Cu-Kα ray) for the niobium-containing metal carbonitride oxide, the maximum value of the X-ray diffraction intensity between diffraction angles 2θ = 25.55 ° to 25.65 ° is I _1. When the maximum value of the X-ray diffraction intensity between the diffraction angle 2θ = 25.65 ° and 26.0 ° is I ₂ , I ₂ / (I ₁ + I ₂ ) is 0.26 to 1.0. The catalyst according to claim 1 or 2, wherein: The composition formula of the niobium-containing metal oxycarbonitride is NbC _x N _y O _z (where 0.1 ≦ x ≦ 2, 0.05 ≦ y ≦ 1, 0.1 ≦ z ≦ 2, and x + y + z ≦ 5). The catalyst according to any one of claims 1 to 3, which is represented by:   A method for producing a catalyst, comprising a step of obtaining a catalyst made of niobium-containing metal carbonitride by heat-treating niobium-containing metal carbonitride in an inert gas containing oxygen gas.   6. The method for producing a catalyst according to claim 5, wherein a temperature range of the heat treatment is 750 to 1050 ° C., and a concentration range of oxygen gas in the inert gas is 0.5 to 2% by volume.   The method for producing a catalyst according to claim 5 or 6, wherein the inert gas contains hydrogen gas in a range of 4% by volume or less.   A catalyst layer for a fuel cell comprising the catalyst according to claim 1.   The fuel cell catalyst layer according to claim 8, further comprising electron conductive particles.   An electrode having a fuel cell catalyst layer and a porous support layer, wherein the fuel cell catalyst layer is the fuel cell catalyst layer according to claim 8 or 9.   A membrane electrode assembly comprising a cathode, an anode, and an electrolyte membrane disposed between the cathode and the anode, wherein the cathode and / or the anode is an electrode according to claim 10. Membrane electrode assembly.   A fuel cell comprising the membrane electrode assembly according to claim 11.   A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 11.