JP2022158480A - Electrode catalyst - Google Patents

Abstract

To provide a novel electrode catalyst including tin oxide-based particles with Pt-based fine particles supported on its surface.SOLUTION: An electrode catalyst comprises tin oxide-based particles of 30 m2/g or more in a specific surface area, and Pt-based fine particles supported on the tin oxide-based particles. The electrode catalyst is 150 A/gPt or more in oxygen reductive reaction mass activity at 0.9 V, and/or 650 A/gPt or more in oxygen reductive reaction mass activity at 0.85 V. The tin oxide-based particles preferably have pores of 50 nm or smaller in a pore size. The tin oxide-based particles are preferably made of SnO2 doped with Sb, Nb, Ta and/or W.SELECTED DRAWING: Figure 4

Description

TECHNICAL FIELD The present invention relates to an electrode catalyst, and more particularly to an electrode catalyst in which Pt-based fine particles are supported on the surface of tin oxide-based particles.

A polymer electrolyte fuel cell (PEFC) includes a membrane electrode assembly (MEA) in which catalyst layers are bonded to both sides of an electrolyte membrane. A gas diffusion layer is usually arranged on the outside of the catalyst layer. Furthermore, a current collector (separator) having a gas flow path is arranged outside the gas diffusion layer. A PEFC usually has a structure (fuel cell stack) in which a plurality of single cells composed of such an MEA, gas diffusion layers and current collectors are stacked.

In a PEFC, the catalyst layer generally consists of a mixture of an electrode catalyst in which catalyst metal fine particles such as platinum are supported on the surface of a carrier, and a catalyst layer ionomer. Carbon materials such as carbon black and acetylene black are usually used for the catalyst carrier. However, it is known that the carbon support is oxidized and corroded when exposed to a high potential, and the catalyst metal fine particles supported on the support drop off, thereby degrading the electrode performance. For this reason, it has been proposed to use a conductive metal oxide that is stable at a high potential as a support material.

For example, in Non-Patent Document 1,
(a) Among various conductive metal oxides, non-stoichiometric titanium oxide (TiO _x ) or tin oxide doped with foreign elements (Nb, Sb, etc.) is promising as a catalyst carrier;
(b) In particular, it is described that tin oxide, which is stable in strongly acidic and high-potential environments, is promising as a catalyst carrier for PEFC cathodes.

In Non-Patent Document 2,
(a) a catalyst in which Pt is supported on a carrier made of Sn _0.96 Nb _0.04 O _2-δ (the BET specific surface area of the carrier is 37 m ² /g, the amount of Pt supported is 10.0 wt%);
(b) A catalyst in which Pt is supported on a carrier made of Sn _0.96 Sb _0.04 O _2-δ (the BET specific surface area of the carrier is 33 m ² /g, and the amount of Pt supported is 12.3 wt%).
is disclosed.
The same document states that the oxygen reduction reaction (ORR) mass activity at 0.85 V for these catalysts is 515-608 A/g _PT .

Non-Patent Document 3 discloses a catalyst in which Pt is supported on a carrier made of Sb—SnO ₂ xerogel.
The same document states that the ORR mass activity at 0.9 V of this catalyst is 92 A/g _Pt .

Non-Patent Document 4 discloses a catalyst in which Pt is supported on a carrier made of Sb—SnO ₂ xerogel.
The same document states that the ORR mass activity at 0.9 V of this catalyst is 32 A/g _Pt .

Non-Patent Document 5 discloses a catalyst in which Pt is supported on a carrier made of Nb—SnO ₂ and further supported on carbon (VGCF).
The same document states that the ORR mass activity at 0.9 V of this catalyst is 141 A/g _Pt and the ORR mass activity at 0.85 V is 541 A/g _Pt .

Furthermore, Non-Patent Document 6 discloses a catalyst in which Pt is supported on a carrier made of Ta—SnO ₂ nanofibers (the BET specific surface area of the carrier is 27 m ² /g).
The same document states that the ORR mass activity at 0.9 V of this catalyst is 307-465 A/g _Pt .

A cathode electrode catalyst for PEFC is required to have high ORR activity. In addition to this, the higher the support rate of Pt, which is the active species, the better. This is because when a catalyst layer with a low Pt loading rate is used to prepare a catalyst layer, the thickness of the catalyst layer is the same as when a catalyst layer with the same Pt basis weight (amount of Pt per electrode area) is prepared using a catalyst with a high Pt loading rate. This is because it is disadvantageous in terms of electron resistance, proton transfer resistance, and gas diffusion resistance in the catalyst layer.

In order to increase the loading of Pt, it is necessary to increase the surface area of the carrier. When Pt is supported at a high loading rate on the surface of a carrier having a small specific surface area, fine Pt particles aggregate on the carrier, or Pt is carried as coarse particles on the carrier. As a result, the specific surface area of Pt is decreased and the ORR mass activity is decreased.

However, in the case of oxide supports such as tin oxide supports, there is a trade-off between specific surface area and electrical conductivity. In general, it is necessary to reduce the particle size of the oxide support particles in order to increase the specific surface area. conductivity decreases. If the conductivity of the support is low, the utilization of the supported Pt is low, or the ORR activity (specific activity) per surface area of Pt is low, resulting in a low ORR mass activity.

Among the Pt/M-SnO ₂ described in Non-Patent Documents 1 to 6, the Pt/Ta-SnO ₂ nanofibers of Non-Patent Document 6 are reported to have the highest ORR mass activity. However, since the Ta—SnO ₂ nanofiber support has a small specific surface area of 27 m ² /g, when the Pt loading rate is increased from 7 mass% to 34 mass%, the electrochemical specific surface area (ECSA) of Pt becomes 73 m ² /g _Pt . from 48 m ² /g _Pt and the ORR mass activity at 0.9 V also drops from 465 A/g _Pt to 307 A/g _Pt .

On the other hand, the Sb—SnO ₂ airgel carrier of Non-Patent Document 4 has a large specific surface area of 85 m ² /g, but the ORR specific activity and mass activity at 0.9 V are 100 μA/cm ² _Pt and 32 A/g _Pt , respectively. and low.
As described above, there are no examples of Pt/M-SnO ₂ catalysts reported in the past that achieve both a high specific surface area and a high ORR mass activity.

T. Arai et al., SAE Int. J. Alt. Power., 2017, 6, 145 K. Kakinuma et al., Electrochim. Acta, 2013, 110, 316 V. K. Ramani, DOE 2020 Annual Merit Review, #FC145 G. Ozouf et al., J. Electrochem. Soc., 2018, 165, F3036 S. Matsumoto et al., J. Electrochem. Soc., 2018, 165, F1164 I. Jimenez-Morales et al., ACS Catal., 2020, 10, 10399

The problem to be solved by the present invention is to provide a novel electrode catalyst in which Pt-based fine particles are supported on the surface of tin oxide-based particles.
Another problem to be solved by the present invention is to achieve both a high specific surface area and a high ORR mass activity in an electrode catalyst in which Pt-based fine particles are supported on the surface of tin oxide-based particles.

In order to solve the above problems, an electrode catalyst according to the present invention has the following configuration.
(1) the electrode catalyst,
tin oxide-based particles having a specific surface area of 30 m ² /g or more;
and Pt-based fine particles supported on the tin oxide-based particles.
(2) the electrode catalyst,
The oxygen reduction reaction mass activity at 0.9 V is 150 A/g _Pt or more, and/or
The oxygen reduction reaction mass activity at 0.85 V is 650 A/g _Pt or more.

In an electrode catalyst in which Pt-based particles are supported on the surfaces of tin oxide-based particles, optimizing the microstructure and/or composition of the tin oxide-based particles can achieve both a high specific surface area and a high ORR mass activity. In particular, when the tin oxide-based particles have mesopores, they exhibit a high ORR mass activity while having a high specific surface area. It is considered that this is because ionomer poisoning is reduced by supporting Pt-based particles in the mesopores.

1 is an SEM image (backscattered electron image) of Pt/Sb—SnO ₂ obtained in Example 1 (ALD method). 2 is an SEM image (backscattered electron image) of Pt/Sb—SnO ₂ obtained in Example 2 (colloid method). 1 shows XRD patterns of Pt/Sb—SnO ₂ obtained in Example 1 (ALD method) and Example 3 (colloid method). 4 is a cyclic voltammogram of Pt/Sb—SnO ₂ obtained in Examples 2 and 3. FIG.

An embodiment of the present invention will be described in detail below.
[1. Electrocatalyst]
The electrode catalyst according to the present invention is
tin oxide-based particles having a specific surface area of 30 m ² /g or more;
and Pt-based fine particles supported on the tin oxide-based particles.

[1.1. Tin oxide particles]
[1.1.1. composition]
"Tin oxide _- based particles" refer to SnO2 particles or _SnO2 particles containing a dopant.
In the present invention, the type of dopant is not particularly limited. Dopants include, for example, Nb, Sb, W, Ta, and Al. SnO ₂ may contain any one of these dopants, or may contain two or more of them.

Among these, SnO ₂ doped with Sb, Nb, Ta, and/or W is preferable as the tin oxide-based particles.
In particular, the tin oxide-based particles are preferably SnO ₂ doped with Sb. SnO ₂ doped with Sb has a higher electrical conductivity than SnO ₂ containing other dopants, and is suitable as a catalyst carrier for supporting Pt-based fine particles.

In SnO ₂ doped with Sb, the conductivity increases as the doping amount of Sb increases. In order to obtain such an effect, the doping amount of Sb is preferably 2.5 at % or more. The doping amount of Sb is more preferably 5.0 at % or more.
On the other hand, if the doping amount of Sb is excessive, the carrier concentration becomes excessive, and the conductivity may decrease. Therefore, the doping amount of Sb is preferably 15.0 atomic % or less. The doping amount of Sb is more preferably 10.0 atomic % or less.

[1.1.2. Specific surface area]
In general, as the specific surface area of the tin oxide-based particles increases, the Pt-based fine particles can be supported in a highly dispersed state, so the ORR mass activity is improved. Therefore, the larger the specific surface area of the tin oxide-based particles, the better. In order to obtain a high ORR mass activity, the specific surface area of the tin oxide-based particles should be 30 m ² /g or more. The specific surface area is preferably 50 m ² /g or more, more preferably 100 m ² /g or more.

[1.1.3. Pore diameter]
The tin oxide-based particles preferably have pores with a pore diameter of 50 nm or less (hereinafter also referred to as "mesopores").
Here, "mesopores" generally refer to pores having a diameter of 2 nm or more and 50 nm or less. Also included are pores with a diameter of less than 2 nm (so-called “micropores”).
"Pore size" refers to the average diameter of mesopores.
The pore diameter is obtained by analyzing the adsorption side data of the nitrogen adsorption isotherm of the tin oxide-based particles by the BJH method and obtaining the pore diameter (most frequent peak value or mode pore diameter) when the pore volume is maximized. can get.

When the tin oxide-based particles have mesopores, and the Pt-based fine particles are supported on the tin oxide-based particles, the proportion of the Pt-based fine particles present in the mesopores increases. Therefore, when Pt-based fine particles are supported in the mesopores of tin oxide-based particles to form an electrode catalyst, and a catalyst layer is produced using such an electrode catalyst and an ionomer, the Pt-based fine particles are poisoned by the ionomer, and Performance degradation caused by this can be suppressed.

When the tin oxide-based particles have mesopores, the pore size (size of mesopores) affects the performance of the electrode catalyst. In general, if the pore diameter becomes too small, it becomes difficult to support the Pt-based fine particles in the mesopores. As a result, when a catalyst layer is produced using the electrode catalyst and ionomer according to the present invention, the Pt-based fine particles may be poisoned by the ionomer. Therefore, the pore diameter is preferably 1 nm or more. The pore diameter is more preferably 2 nm or more.
On the other hand, if the pore diameter is too large, the ionomer may enter the mesopores and poison the Pt-based fine particles supported in the mesopores by the ionomer. Therefore, the pore diameter is preferably 20 nm or less. The pore diameter is more preferably 10 nm or less, more preferably 5 nm or less.

[1.1.4. shape]
In the present invention, the shape of the tin oxide-based particles is not particularly limited as long as the above conditions are satisfied. The tin oxide-based particles may be isolated particles, or may be particles having a beaded structure in which porous primary particles are fused together.
Here, the term “beaded structure” refers to a structure in which primary particles are fused together in a beaded shape.

By using the method described later, tin oxide-based particles having a bead-like structure in which porous primary particles are fused can be obtained. Particles with a beaded structure (that is, secondary particles) have relatively large voids between the primary particles because the primary particles are loosely connected to each other. Therefore, when an electrode catalyst is produced using tin oxide-based particles having a beaded structure and an ionomer is used to produce a catalyst layer, appropriate voids are formed in the catalyst layer. As a result, the gas diffusion resistance of the catalyst layer is lowered.
In addition, since the primary particles are aggregates of fine crystallites, there are relatively fine voids (mesopores) inside the primary particles. Therefore, when this is used as a catalyst carrier, poisoning of the Pt-based fine particles by the ionomer can be suppressed.

The shape of the primary particles is not particularly limited. When the tin oxide-based particles are produced by the method described later, the primary particles usually do not have a perfect spherical shape, and have an irregular shape with an aspect ratio of about 1.1 to 3.

As will be described later, the tin oxide particles according to the present invention are produced using mesoporous carbon as a template. Mesoporous carbon is produced using mesoporous silica as a mold. Mesoporous silica is usually synthesized by polycondensing a silica source in a reaction solution containing a silica source, a surfactant and a catalyst.

At this time, if the concentration of the surfactant and the concentration of the silica source in the reaction solution are each limited to a specific range, a mesoporous structure having a renella structure and a specific surface area, pore diameter, etc. within a specific range can be obtained. Silica is obtained.
When mesoporous silica having such a beaded structure is used for the first mold, mesoporous carbon having a beaded structure can be obtained. Further, when mesoporous carbon having a beaded structure is used for the second mold, tin oxide particles having a beaded structure can be obtained.

[1.1.5. Average particle size of primary particles]
"Average particle size of primary particles" refers to the average value of the maximum dimensions of primary particles measured by scanning electron microscope (SEM) observation.
When the tin oxide-based particles are particles having a bead-like structure in which porous primary particles are fused together, the average particle size of the primary particles is not particularly limited, and an optimum value can be selected according to the purpose. can be selected.

In general, when the average particle size of the primary particles is too small, it becomes difficult to support the Pt-based fine particles. Therefore, the average particle size of primary particles is preferably 0.05 μm or more. The average particle size is more preferably 0.06 μm or more, more preferably 0.07 μm or more.
On the other hand, if the average particle size of the primary particles is too large, the thickness of the catalyst layer will increase, and the ionic resistance and electronic resistance in the catalyst layer will increase. Therefore, the average particle size of the primary particles is preferably 2 μm or less. The average particle size is more preferably 1 μm or less, more preferably 0.5 μm or less.

[1.1.6. Conductivity of powder compact]
"Conductivity of powder compact" means
(a) forming tin oxide-based particles using two stainless steel discs and a plastic jig with a cylindrical hole;
(b) A value obtained by measuring the voltage while applying a pressure of 2.4 MPa to the obtained powder compact while applying a constant current.
The electrical conductivity of the compact (ie, tin oxide-based particles) depends primarily on the type and amount of dopant. By optimizing the composition of the tin oxide-based particles, the electrical conductivity of the green compact becomes 1×10 ⁻³ S/cm or more. If the manufacturing conditions are optimized, the electrical conductivity will be 1×10 ⁻² S/cm or more.
By using the method described later, it is possible to synthesize even tin oxide-based particles whose compact has an electrical conductivity of about 10 S/cm.

[1.1.7. Pore capacity]
"Pore volume" refers to the volume of mesopores contained in the primary particles and does not include the volume of voids between primary particles.
The pore capacity is obtained by analyzing the adsorption data of the nitrogen adsorption isotherm of the tin oxide-based particles by the BJH method, and calculating with a value of P/P ₀ =0.03 to 0.99.

When the tin oxide-based particles according to the present invention are used as a catalyst carrier for PEFC, if the pore volume becomes too small, the ratio of the catalyst particles supported in the pores becomes small. Therefore, the pore volume is preferably 0.1 mL/g or more. The pore volume is preferably 0.15 mL/g or more, more preferably 0.2 mL/g or more.
On the other hand, if the pore volume becomes too large, the ratio of the pore walls of the tin oxide-based particles becomes small, resulting in low electronic conductivity. In addition, the amount of ionomer intrusion increases, and the activity may decrease due to catalyst poisoning. Therefore, the pore volume is preferably 1 mL/g or less. The pore volume is preferably 0.7 mL/g or less, more preferably 0.5 mL/g or less.

[1.1.8. tap density]
"Tap density" refers to a value measured according to JIS Z 2512.
When the tin oxide-based particles of the present invention are used in the catalyst layer of PEFC, if the tap density of the tin oxide-based particles is too low, the resulting catalyst layer becomes too thick and the proton conductivity decreases. Therefore, the tap density is preferably 0.005 g/cm ³ or more. The tap density is preferably 0.01 g/cm ³ or more, more preferably 0.05 g/cm ³ or more.
On the other hand, if the tap density is too high, it becomes difficult to secure voids that can suppress flooding in the catalyst layer when the catalyst layer is produced using this. Therefore, the tap density is preferably 1.0 g/cm ³ or less. The tap density is preferably 0.75 g/cm ³ or less.

[1.2. Pt-based fine particles]
[1.2.1. composition]
"Pt-based microparticles" refer to microparticles made of Pt or a Pt alloy. The Pt-based fine particles are carried on the outer surface of the tin oxide-based particles or the inner surface of the mesopores.
When the Pt-based fine particles are composed of a Pt alloy, the composition of the Pt alloy (that is, the type and content of alloying elements) is not particularly limited, and an optimum composition can be selected according to the purpose. As a Pt alloy, for example,
(a) an alloy containing Pt and one or more noble metal elements other than Pt (for example, Pt--Pd alloy, Pt--Ru alloy, Pt--Ir alloy, etc.);
(b) an alloy containing Pt and one or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.) (e.g., Pt--Fe alloy, Pt--Co alloy, Pt-- Ni chest collar, Pt-Cr alloy, Pt-V alloy, Pt-Ti alloy, etc.),
and so on.

[1.2.2. Average particle size]
"Average particle size of Pt-based fine particles" refers to the average value of the maximum dimension of Pt-based fine particles measured by scanning electron microscope (SEM) observation.
The average particle size of Pt-based fine particles affects the mass activity. In general, when the average particle size of the Pt-based fine particles becomes too large, the mass activity of the Pt-based fine particles decreases. Therefore, the average particle size of the Pt-based fine particles is preferably 5 nm or less. The average particle size is more preferably 4 nm or less.
On the other hand, if the average particle size of the Pt-based fine particles is too small, the components that form the fine particles, such as Pt, tend to be eluted. Therefore, the average particle size of the Pt-based fine particles is preferably 1 nm or more. The average particle size is more preferably 2 nm or more.

[1.2.3. Load]
The amount of the Pt-based fine particles to be supported is not particularly limited, and an optimum amount to be supported can be selected according to the purpose. In general, when the amount of Pt-based fine particles supported becomes too small, the thickness of the catalyst layer required to obtain a predetermined basis weight increases, and the electronic resistance, proton transfer resistance, and/or gas diffusion resistance of the catalyst layer increases. increases. Therefore, the supported amount of Pt-based fine particles is preferably 5 mass % or more. The supported amount is more preferably 10 mass % or more, more preferably 15 mass % or more.
On the other hand, when the amount of Pt-based fine particles carried is excessive, the Pt-based fine particles agglomerate on the surface of the carrier, which rather reduces the activity of the electrode catalyst. Therefore, the supported amount of Pt-based fine particles is preferably 60 mass % or less. The supported amount is more preferably 50 mass % or less, more preferably 40 mass % or less.

[1.3. Characteristic]
[1.3.1. Oxygen reduction reaction (ORR) mass activity at 0.9 V]
By optimizing the composition and/or structure (in particular, the composition and/or structure of the tin oxide-based particles) in the electrode catalyst according to the present invention, the ORR mass activity at 0.9 V becomes 150 A/g _Pt or more. By further optimizing the composition and/or structure of the electrocatalyst, the ORR mass activity at 0.9 V is 300 A/g _Pt or higher, alternatively 400 A/g _Pt or higher.

[1.3.2. Oxygen reduction reaction (ORR) mass activity at 0.85 V]
In the electrode catalyst according to the present invention, optimizing the composition and/or structure (in particular, the composition and/or structure of the tin oxide-based particles) results in the ORR mass activity at 0.9 V falling within the above range, Alternatively, the ORR mass activity at 0.85V is greater than or equal to 650 A/g _Pt . By further optimizing the electrocatalyst composition and/or structure, the ORR mass activity at 0.85 V is 1500 A/g _Pt or higher, or 2000 A/g _Pt or higher.

[2. Method for producing mesoporous silica (first template)]
Tin oxide-based particles can be produced by various methods. Of these, in order to produce tin oxide particles having a beaded structure, it is first necessary to produce mesoporous silica (first template) having a beaded structure. Such mesoporous silica is
(a) condensation polymerization of the silica source in a reaction solution containing a silica source, a surfactant and a catalyst to prepare precursor particles;
(b) separating and drying the precursor particles from the reaction solution;
(c) if necessary, the dried precursor particles are subjected to a diameter-expanding treatment;
(d) obtained by firing the precursor particles;

[2.1. Condensation polymerization step]
First, the silica source is polycondensed in a reaction solution containing a silica source, a surfactant and a catalyst to obtain precursor particles (condensation polymerization step).

[2.1.1. silica source]
In the present invention, the type of silica source is not particularly limited. Examples of silica sources include
(a) tetraalkoxysilanes such as tetramethoxysilane, tetraethoxysilane, tetraisopropoxysilane, tetrabutoxysilane, dimethoxydiethoxysilane, tetraethyleneglycooxysilane;
(b) trialkoxysilanes such as 3-mercaptopropyltrimethoxysilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethyl)aminopropyltrimethoxysilane;
and so on. Any one of these may be used as the silica source, or two or more may be used in combination.

[2.1.2. Surfactant]
When the silica source is polycondensed in the reaction solution, if a surfactant is added to the reaction solution, the surfactant forms micelles in the reaction solution. Since hydrophilic groups are gathered around the micelles, the silica source is adsorbed on the surfaces of the micelles. Further, the micelles to which the silica source is adsorbed self-organize in the reaction solution, and the silica source undergoes polycondensation. As a result, mesopores caused by micelles are formed inside the primary particles. The size of the mesopores can be controlled (from 1 to 50 nm) mainly by the molecular length of the surfactant.

In the present invention, an alkyl quaternary ammonium salt is used as the surfactant. An alkyl quaternary ammonium salt refers to a compound represented by the following formula (a).
CH ₃ —(CH ₂ ) _n —N ⁺ (R ₁ )(R ₂ )(R ₃ )X ^— (a)

(a) In the formula, R ₁ , R ₂ and R ₃ each represent an alkyl group having 1 to 3 carbon atoms. R ₁ , R ₂ and R ₃ may be the same or different. R ₁ , R ₂ and R ₃ are all preferably the same in order to facilitate aggregation (micelle formation) between alkyl quaternary ammonium salts. Furthermore, at least one of R ₁ , R ₂ and R ₃ is preferably a methyl group, and all are preferably methyl groups.
(a) In formula, X represents a halogen atom. Although the type of halogen atom is not particularly limited, X is preferably Cl or Br in terms of availability.

(a) In the formula, n represents an integer of 7-21. In general, the smaller n is, the smaller the central pore size of the mesopores is and the more spherical mesoporous material is obtained. On the other hand, the larger the value of n, the larger the central pore size. As a result, a layered compound is produced and a mesoporous body cannot be obtained. n is preferably 9-17, more preferably 13-17.

Among those represented by formula (a), alkyltrimethylammonium halide is preferred. Examples of alkyltrimethylammonium halides include hexadecyltrimethylammonium halide, octadecyltrimethylammonium halide, nonyltrimethylammonium halide, decyltrimethylammonium halide, undecyltrimethylammonium halide and dodecyltrimethylammonium halide.
Among these, alkyltrimethylammonium bromide and alkyltrimethylammonium chloride are particularly preferred.

When synthesizing mesoporous silica, one type of alkyl quaternary ammonium salt may be used, or two or more types may be used. However, since the alkyl quaternary ammonium salt serves as a template for forming mesopores in the primary particles, its type has a great influence on the shape of the mesopores. In order to synthesize silica particles with more uniform mesopores, it is preferable to use one type of alkyl quaternary ammonium salt.

[2.1.3. catalyst]
When polycondensing a silica source, a catalyst is usually added to the reaction solution. When synthesizing particulate mesoporous silica, it is preferable to use an alkali such as sodium hydroxide or aqueous ammonia as the catalyst.

[2.1.4. solvent]
As the solvent, water, an organic solvent such as alcohol, a mixed solvent of water and an organic solvent, or the like is used.
alcohol is
(1) monohydric alcohols such as methanol, ethanol and propanol;
(2) dihydric alcohols such as ethylene glycol;
(3) trihydric alcohols such as glycerin;
Either one is fine.
When a mixed solvent of water and an organic solvent is used, the content of the organic solvent in the mixed solvent can be arbitrarily selected depending on the purpose. In general, adding an appropriate amount of organic solvent to the solvent facilitates control of particle size and particle size distribution.

[2.1.5. Composition of Reaction Solution]
The composition of the reaction solution affects the external shape and pore structure of mesoporous silica to be synthesized. In particular, the concentration of the surfactant and the concentration of the silica source in the reaction solution have a great effect on the average primary particle size, pore size, pore volume and tap density of the mesoporous silica particles.

[A. Surfactant concentration]
If the concentration of the surfactant is too low, the particle deposition rate will be slow and a structure in which the primary particles are connected cannot be obtained. Therefore, the surfactant concentration should be 0.03 mol/L or more. The surfactant concentration is preferably 0.035 mol/L or more, more preferably 0.04 mol/L or more.

On the other hand, if the concentration of the surfactant is too high, the precipitation rate of the particles becomes too fast, and the primary particle diameter easily exceeds 300 nm. Therefore, the surfactant concentration should be 1.0 mol/L or less. The surfactant concentration is preferably 0.95 mol/L or less, more preferably 0.90 mol/L or less.

[B. Concentration of silica source]
If the concentration of the silica source is too low, the deposition rate of particles will be slow, and a structure in which primary particles are connected cannot be obtained. Alternatively, the surfactant may be excessive and uniform mesopores may not be obtained. Therefore, the silica source concentration should be 0.05 mol/L or more. The silica source concentration is preferably 0.06 mol/L or more, more preferably 0.07 mol/L or more.

On the other hand, if the concentration of the silica source is too high, the precipitation rate of the particles becomes too fast, and the primary particle diameter easily exceeds 300 nm. Alternatively, sheet-like particles may be obtained instead of spherical particles. Therefore, the silica source concentration should be 1.0 mol/L or less. The silica source concentration is preferably 0.95 mol/L or less, more preferably 0.9 mol/L or less.

[C. Concentration of catalyst]
In the present invention, the catalyst concentration is not particularly limited. In general, too low a concentration of catalyst slows down the deposition rate of the particles. On the other hand, if the catalyst concentration is too high, the particle precipitation rate will increase. The optimum concentration of the catalyst is preferably selected according to the type of silica source, the type of surfactant, target physical properties, and the like.

[2.1.6 Reaction conditions]
A silica source is added in a solvent containing a predetermined amount of surfactant for hydrolysis and polycondensation. Thereby, the surfactant functions as a template, and precursor particles containing silica and surfactant are obtained.
As for the reaction conditions, optimum conditions are selected according to the type of silica source, the particle size of the precursor particles, and the like. In general, the preferred reaction temperature is -20 to 100°C. The reaction temperature is more preferably 0 to 90°C, more preferably 10 to 80°C.

[2.2. Drying process]
Next, the precursor particles are separated from the reaction solution and dried (drying step).
Drying is performed to remove the solvent remaining in the precursor particles. Drying conditions are not particularly limited as long as the solvent can be removed.

[2.3. diameter expansion]
Next, if necessary, the dried precursor particles may be subjected to diameter-expanding treatment (diameter-expanding step). "Diameter-enlarging treatment" refers to treatment for enlarging the diameter of mesopores in primary particles.
Specifically, the diameter-expanding treatment is performed by subjecting the synthesized precursor particles (from which the surfactant has not been removed) to a hydrothermal treatment in a solution containing a diameter-expanding agent. This treatment can enlarge the pore size of the precursor particles.

Examples of diameter-enlarging agents include
(a) hydrocarbons such as trimethylbenzene, triethylbenzene, benzene, cyclohexane, triisopropylbenzene, naphthalene, hexane, heptane, octane, nonane, decane, undecane, dodecane;
(b) acids such as hydrochloric acid, sulfuric acid, nitric acid;
and so on.

The reason why the pore diameter is enlarged by hydrothermal treatment in the presence of hydrocarbons is that silica rearrangement occurs when the pore-enlarging agent is introduced from the solvent into the pores of the more hydrophobic precursor particles. It is considered to be for
Further, the reason why the pore size is enlarged by the hydrothermal treatment in the coexistence of an acid such as hydrochloric acid is considered to be the progress of dissolution and reprecipitation of silica inside the primary particles. Optimizing the manufacturing conditions results in the formation of radial pores inside the silica. When this is hydrothermally treated in the presence of an acid, dissolution and reprecipitation of silica occur, and radial pores are converted into continuous pores.

Conditions for the diameter-expanding treatment are not particularly limited as long as the desired pore diameter can be obtained. Generally, it is preferable to add a diameter-enlarging agent to the reaction solution in an amount of about 0.05 mol/L to 10 mol/L, and to perform hydrothermal treatment at 60 to 150°C.

[2.4. Firing process]
Next, after performing a diameter-expanding treatment as necessary, the precursor particles are sintered (sintering step). As a result, mesoporous silica particles having a beaded structure are obtained.
Firing is performed to dehydrate and crystallize the precursor particles in which OH groups remain, and to thermally decompose the surfactant remaining in the mesopores. The baking conditions are not particularly limited as long as dehydration/crystallization and thermal decomposition of the surfactant are possible. Firing is usually carried out by heating at 400° C. to 700° C. for 1 hour to 10 hours in the atmosphere.

[3. Method for producing mesoporous carbon (second mold)]
Next, mesoporous silica having a beaded structure is used as a mold to produce mesoporous carbon having a beaded structure (second mold). Such mesoporous carbon is
(a) preparing mesoporous silica as a first template,
(b) depositing carbon in the mesopores of the mesoporous silica to prepare a silica/carbon composite;
(c) obtained by removing silica from the composite;
Moreover, in order to promote graphitization of the obtained mesoporous carbon, the mesoporous carbon may be heat-treated at a temperature higher than 1500° C. after removing the silica.

[3.1. First mold preparation step]
First, mesoporous silica to be the first template is prepared (first template preparation step). The details of the method for producing mesoporous silica are as described above, so the description is omitted.

[3.2. Carbon precipitation process]
Next, carbon is deposited in the mesopores of the mesoporous silica to produce a silica/carbon composite (carbon deposition step).
Specifically, the deposition of carbon into the mesopores is
(a) introducing a carbon precursor into the mesopores;
(b) by polymerizing and carbonizing a carbon precursor within the mesopores;

[3.2.1. Introduction of Carbon Precursor]
"Carbon precursor" refers to a substance capable of producing carbon by thermal decomposition. Specifically, such carbon precursors include:
(1) a liquid at room temperature and a thermally polymerizable polymer precursor (e.g., furfuryl alcohol, aniline, etc.),
(2) mixtures of aqueous solutions of carbohydrates and acids (e.g., monosaccharides such as sucrose, xylose, glucose, disaccharides, polysaccharides, sulfuric acid, hydrochloric acid, nitric acid, phosphorus mixtures with acids such as acids),
(3) mixtures of two-component curing type polymer precursors (for example, phenol and formalin, etc.);
and so on.
Among these, the polymer precursor can be impregnated into the mesopores without being diluted with a solvent, so a relatively small number of impregnation times produces a relatively large amount of carbon in the mesopores. can be made Moreover, there is an advantage that a polymerization initiator is unnecessary and handling is easy.

When a liquid or solution carbon precursor is used, the larger the amount of liquid or solution to be adsorbed per time, the better.
Also, when a mixture of an aqueous carbohydrate solution and an acid is used as the carbon precursor, the amount of acid is preferably the minimum amount that allows the organic matter to be polymerized.
Furthermore, when a mixture of two-liquid curing type polymer precursors is used as the carbon precursor, the optimum ratio is selected according to the type of the polymer precursor.

[3.2.2. Polymerization and carbonization of carbon precursor]

The polymerized carbon precursor is then carbonized within the mesopores.
Carbonization of the carbon precursor is performed by heating mesoporous silica containing the carbon precursor to a predetermined temperature in a non-oxidizing atmosphere (for example, in an inert atmosphere or in vacuum). Specifically, the heating temperature is preferably 500° C. or higher and 1200° C. or lower. If the heating temperature is less than 500°C, carbonization of the carbon precursor will be insufficient. On the other hand, if the heating temperature exceeds 1200° C., silica reacts with carbon, which is not preferable. The optimum heating time is selected according to the heating temperature.

The amount of carbon generated in the mesopores should be at least the amount that allows the carbon particles to maintain their shape when the mesoporous silica is removed. Therefore, when the amount of carbon produced by one filling, polymerization and carbonization is relatively small, it is preferable to repeat these steps multiple times. In this case, the conditions of each repeated step may be the same or different.
In addition, when each step of filling, polymerization and carbonization is repeated multiple times, each carbonization step performs carbonization at a relatively low temperature, and after the final carbonization is completed, carbonization is performed again at a higher temperature. may be processed. If the final carbonization treatment is carried out at a temperature higher than that of the previous carbonization treatments, the carbon that has been introduced into the pores in multiple steps can be easily integrated.

[3.3. First mold removal step]
Next, the mesoporous silica, which is the first template, is removed from the composite (first template removing step). As a result, a mesoporous carbon (second mold) having a beaded structure is obtained.
Specifically, as a method for removing mesoporous silica,
(1) a method of heating the composite in an alkaline aqueous solution such as sodium hydroxide;
(2) a method of etching the composite with an aqueous hydrofluoric acid solution;
and so on.

[3.4. Graphitization process]
Next, if necessary, the mesoporous carbon is heat-treated at a temperature higher than 1500° C. (graphitization step). When the carbon source is carbonized in the mesopores of mesoporous silica, the heat treatment temperature must be lowered in order to suppress the reaction between silica and carbon. Therefore, the degree of graphitization of carbon after carbonization is low. In order to obtain a high degree of graphitization, it is preferable to heat-treat the mesoporous carbon at a high temperature after removing the first template.

If the heat treatment temperature is too low, graphitization will be insufficient. Therefore, the heat treatment temperature is preferably higher than 1500°C. The heat treatment temperature is preferably 1700° C. or higher, more preferably 1800° C. or higher.
On the other hand, even if the heat treatment temperature is raised more than necessary, there is no difference in the effect and there is no practical benefit. Therefore, the heat treatment temperature is preferably 2300° C. or lower. The heat treatment temperature is preferably 2200° C. or less.

[4. Method for producing tin oxide particles]
A method for producing tin oxide-based particles having a beaded structure comprises:
a first step of preparing mesoporous carbon having a renister-like structure;
a second step of precipitating tin oxide or tin oxide containing a dopant (hereinafter collectively referred to as "Sn-containing oxide") in the mesopores of the mesoporous carbon to obtain a Sn-containing oxide/carbon composite; ,
and a third step of removing carbon from the Sn-containing oxide/carbon composite.

[4.1. First step]
First, mesoporous carbon having a renister structure is prepared (first step). The details of the method for producing mesoporous carbon are as described above, so the explanation is omitted.

[4.2. Second step]
Next, a Sn-containing oxide is deposited in the mesopores of the mesoporous carbon (second step). Thereby, a Sn-containing oxide/carbon composite is obtained.
Specifically, the deposition of the Sn-containing oxide into the mesopores is performed by introducing a precursor of the Sn-containing oxide into the mesopores and converting the precursor into the Sn-containing oxide.

[4.2.1. precursor]
Specific examples of precursors for forming Sn-containing oxides in mesopores include:
(1) a compound that contains a metal element that constitutes a Sn-containing oxide, is soluble in a solvent, and can be oxidized and precipitated by dissolved oxygen in the solvent;
(2) a compound that contains a metal element that constitutes an Sn-containing oxide and that can form a metal oxide by thermal decomposition or hydrolysis;
and so on.

Compounds that can be oxidized and precipitated by dissolved oxygen include:
(1) salts containing _divalent Sn such as SnCl2;
(2) salts containing Nb, Sb, W, Ta, or Al _, such as _NbCl5 , _SbCl3 , _WCl6 , _TaCl5 , AlCl3;
and so on.

Compounds capable of forming metal oxides by thermal decomposition or hydrolysis include:
( ₁ ) chlorides such as SnCl4, _SnCl2 , _NbCl5 , _{SbCl3 , WCl6} , _TaCl5 , AlCl3;
( _{2) Tungsten ethoxide (W(OC2H5)6), Sn(OC2H5)2 , Sn ( OC} ( _CH3 ) ₃ ) ₄ , Nb _{( OC2H5} ) ₅ , Ta _{( OC2} alkoxides such as _H5 ) _{5 ,} Sb( _OC2H5 ) ₃ , Al _{( OC2H5} ) ₃ ;
( ₃ ) acetylacetonate salts such as tin acetylacetonate (Sn( _CH3COCHCOCH3 ) ₂ ) _, Al( _CH3COCHCOCH3 ) ₃ ;
(4) Acetates such as Sn( _CH3COO ) ₂ , Sb( _{CH3COO)3 ;}
and so on.

[4.2.2. Introduction of precursor into pores]
When the precursor is liquid, it may be adsorbed in the pores of the mesoporous carbon as it is. Alternatively, the precursor may be dissolved in a suitable solvent and this solution may be adsorbed into the pores of the mesoporous carbon. When the precursor is dissolved in a solvent, the type of solvent and the concentration of the precursor are not particularly limited, and are selected optimally according to the purpose.

[4.2.3. Conversion of precursor to oxide]
After adsorbing the precursor, the precursor is converted to a Sn-containing oxide. The conversion method is not particularly limited, and an optimum method is selected according to the type of precursor.
For example, when chloride is used as a precursor, mesoporous carbon is dispersed in a chloride-dissolved solution and stirred in the air. As the stirring is continued, the chlorides are eventually adsorbed into the mesopores of the mesoporous carbon, and the chlorides in the mesopores gradually become Sn-containing oxides due to dissolved oxygen.

Further, for example, when an alkoxide is used as a precursor, the alkoxide or a solution in which the alkoxide is dissolved is added to the mesoporous carbon, and the mesopores are impregnated with the alkoxide or the solution. When this is heated to a predetermined temperature, polycondensation of the alkoxide occurs to form Sn-containing oxides in the mesopores.
If a sufficient amount of Sn-containing oxide cannot be formed in the mesopores by a single adsorption of the precursor and conversion to Sn-containing oxide, the adsorption and conversion may be repeated multiple times. good.

[4.3. Third step]
Next, carbon is removed from the Sn-containing oxide/carbon composite (third step). Thereby, the tin oxide-based particles according to the present invention are obtained.
A method for removing carbon is not particularly limited, and various methods can be used. Examples of carbon removal methods include:
(1) a method of heating a Sn-containing oxide/carbon composite in an oxidizing atmosphere;
(2) a method of oxygen plasma etching a Sn-containing oxide/carbon composite;
and so on.
Removal conditions such as heating temperature and heating time are not particularly limited as long as the carbon is completely removed without coarsening the crystallites of the Sn-containing oxide.

[5. action]
In an electrode catalyst in which Pt-based fine particles are supported on the surfaces of tin oxide-based particles, optimizing the microstructure and/or composition of the tin oxide-based particles can achieve both a high specific surface area and a high ORR mass activity. In particular, when the tin oxide-based particles have mesopores, they exhibit a high ORR mass activity while having a high specific surface area. It is considered that this is because the ionomer poisoning is reduced by supporting the Pt-based fine particles in the mesopores.

In particular, when porous carbon with a beaded structure is used as a mold and the Sn source is deposited in the pores of the mold, porous SnO2 particles with a _high specific surface area and a beaded structure can be obtained. . Also, adding an appropriate amount of dopant M (eg, Sb) to the Sn source results in porous M--SnO ₂ particles with high electrical conductivity. When Pt or a Pt alloy is supported on the surface of the porous M-SnO ₂ particles using a colloidal method, the porous M-SnO ₂ particles have a high specific surface area, resulting in Pt-based fine particles having a particle size of about 2 to 3 nm. can be highly dispersedly supported on its surface.

In the electrode catalyst thus obtained, the Pt-based fine particles have a high ORR area specific activity due to the high conductivity of the carrier, and as a result exhibit a high ORR mass activity. More specifically, an electrocatalyst having an ORR mass activity of 150 A/g _Pt or more at 0.9 V and/or an ORR mass activity of 650 A/g _Pt at 0.85 V is obtained.

(Examples 1 to 4)
[1. Production of Sb—SnO ₂ carrier]
[1.1. Production of bead-shaped mesoporous silica (radial pores)]
To a mixed solvent of methanol (MeOH): 4.6 g and ethylene glycol (EG): 4.6 g, 30 mass% cetyltrimethylammonium chloride aqueous solution: 56.3 g was added and stirred at room temperature. 1M NaOH:8.8g was added to this, and it heated at 50 degreeC. Hereinafter, this solution is referred to as "first solution".
Next, 12.3 g of tetraethoxysilane (TEOS) was dissolved in a mixed solvent of 6.5 g of MeOH and 6.5 g of EG. Hereinafter, this solution is referred to as "second solution".

The second solution was added to the first solution heated to 50°C. After the mixture became cloudy, the heating was stopped, and the mixture was further stirred for 4 hours or longer. After repeating filtration and re-dispersion in purified water twice, it was dried at 45°C. Further, the dried powder was calcined in the air at 550° C. for 6 hours to obtain a bead-shaped mesoporous silica (radial pores, CSS, Connected Starburst Silica).

[1.2. Production of bead-shaped mesoporous carbon (radial pores)]
CSS: 0.5 g was placed in a container made of PFA, and furfuryl alcohol (FA) was added in an amount corresponding to the pore volume of the CSS and permeated into the pores of the CSS. By heat-treating this at 150° C. for 24 hours, FA was polymerized. Furthermore, this was heat-treated at 500° C. for 6 hours in a nitrogen atmosphere to promote carbonization of FA. After repeating this twice, heat treatment was further performed at 900° C. for 6 hours in a nitrogen atmosphere to obtain a CSS/carbon composite.

This composite was immersed in a 12% HF solution for 4 hours to dissolve the silica component. After the dissolution, filtration and washing were repeated, and the mixture was dried at 45° C. to obtain a bead-like mesoporous carbon (radial pores, CSC, Connected Starburst Carbon). The resulting porous body had a BET specific surface area of 2122 m ² /g, a pore volume of 1.3 mL/g and a pore diameter of 2.2 nm.

[1.3. Preparation of renister-shaped mesoporous Sb—SnO ₂ ]
0.03 g of SbCl ₃ was dissolved in 4 mL of concentrated hydrochloric acid (35 mass%), diluted with 36 mL of purified water, and then 5.0 g of SnCl ₂ was further added and dissolved. 0.1 g of bead-like mesoporous carbon (CSC) was added to this solution and dispersed. After this dispersion was stirred in the air at room temperature for 2 hours, 200 mL of purified water was added, and the mixture was further stirred in the air for 4 hours. Subsequently, filtration and re-dispersion in purified water were repeated twice, followed by drying at 45° C. to obtain a beaded Sb—SnO ₂ /carbon composite.
This bead-shaped Sb--SnO ₂ /carbon composite was treated in an air atmosphere at 320° C. for 24 hours to obtain bead-shaped mesoporous Sb--SnO ₂ (Sb doping amount: 2.5 at %).

Furthermore, in the same manner as described above, except that the amount of SbCl ₃ dissolved in concentrated hydrochloric acid was changed to 0.06 g, 0.12 g, or 0.18 g, mesoporous Sb—SnO ₂ with different Sb doping amounts (Sb doping amounts : 3.2 at %, 5.6 at %, or 8.1 at %). In addition, "the amount of Sb doping" means the value obtained by ICP analysis.

[2. Preparation of electrode catalyst]
[2.1. Example 1: Pt/Sb—SnO ₂ by ALD method]
An ALD apparatus was used to deposit Pt on the Sb—SnO ₂ carrier particles. MeCpPtMe ₃ (methylcyclopentadienyltrimethylplatinum) was used as the Pt precursor. Sb—SnO ₂ carrier particles (Sb doping amount: 2.5 at %): A sample tube containing 100 mg was heated to 150° C., and a container containing a Pt precursor was heated to 60° C. with a mantle heater. 1 to 4) were repeated 18 times to obtain Pt/Sb—SnO ₂ (Pt loading rate: 35 mass%).
1. Pt precursor supply: _MeCpPtMe3 /Ar, 50ccm, 20min
2. Purge: Ar, 200ccm, 5min
3. Pt precursor reduction: H2, 100 ccm, ₅ min
4. Purge: Ar, 200ccm, 5min

[2.2. Example 2: Pt/Sb—SnO ₂ (1)] by colloid method
0.4 M NaOH/EG solution: 6 mL and 0.04 mM H ₂ PtCl ₆ /EG solution: 6 mL were mixed. This mixed solution was heated at 160° C. for 3 minutes with stirring in a microwave synthesizer (Monowabe 400, manufactured by Anton Paar) to obtain a Pt nanoparticle colloidal solution.
Next, 94 mg of Sb—SnO ₂ carrier particles (Sb doping amount: 2.5 at %) was added to 6 mL of the Pt nanoparticle colloidal solution, and the mixture was stirred at room temperature for 2 hours. Subsequently, an operation of adding 0.15 mL of 1 M HNO ₃ and stirring at room temperature for 1 h was repeated twice. Further, 0.375 mL of 1M HNO ₃ was added and stirred at room temperature for 1 hour. Thereafter, filtration and redispersion in purified water were repeated twice. The solid was vacuum-dried at 70° C. to obtain Pt/Sb—SnO ₂ (Pt loading rate: 20 mass %).

[2.3. Example 3: Pt/Sb--SnO ₂ by colloidal method (2)]
_{Pt _} /Sb-SnO ₂ (Pt loading rate: 20 mass%) was obtained.

[2.4. Example 4: Pt/Sb—SnO ₂ (3)] by colloid method
Sb—SnO ₂ carrier particles (Sb doping amount: 5.6 at%) were used instead of Sb—SnO ₂ carrier particles (Sb doping amount: 2.5 at%), and the amount of the Pt nanoparticle colloidal solution was 10 mL. Pt/Sb—SnO ₂ (Pt loading rate: 30 mass %) was obtained in the same manner as in Example 2 except for this.

[3. Test method]
[3.1. Evaluation of Renaissance Mesoporous Sb—SnO ₂ ]
[3.1.1. _N2 adsorption measurement]
N ₂ adsorption measurement was carried out on the prepared bead-like mesoporous Sb—SnO ₂ . The pore size distribution was determined from the N ₂ adsorption isotherm by the BJH method, and the mode pore size (mode of pore size) was defined as the pore size of the sample.

[3.1.2. conductivity measurement]
Conductivity measurement was performed on the manufactured bead-like mesoporous Sb—SnO ₂ . The electrical conductivity was measured by preparing a powder compact and measuring the voltage value when a direct current was applied to it while applying a pressure of 2.4 MPa.

[3.2. Evaluation of Pt/Sb—SnO ₂ ]
[3.2.1. SEM observation]
SEM observation was performed on the Pt/Sb--SnO ₂ produced.
[3.2.2. XRD measurement]
XRD measurement was performed on the produced Pt/Sb--SnO ₂ .

[3.2.3. Electrochemical evaluation]
The produced Pt/Sb--SnO ₂ was subjected to electrochemical measurement by the rotating disk electrode (RDE) method. RDE measurements were performed at room temperature in 0.1 M perchloric acid. The electrochemical surface area (ECSA) of Pt was calculated from the amount of charge at the hydrogen adsorption/desorption peak of the cyclic voltammogram recorded in the Ar-purged electrolyte.
Linear sweep voltammetry (20 mV/s, forward sweep) was also performed in the electrolyte saturated with O ₂ . ORR activity at 0.9 V or 0.85 V (reversible hydrogen electrode (RHE) standard) was determined by performing background correction, solution resistance correction, and limiting current correction on the obtained voltammogram. For the ORR activity, activity per Pt mass (mass activity, MA) and activity per Pt surface area (specific activity, SA) were calculated.

[4. result]
[4.1. Evaluation of Renaissance Mesoporous Sb—SnO ₂ ]
Table 1 shows the Sb doping amount, specific surface area, pore size, and electrical conductivity of the renister-shaped mesoporous Sb—SnO ₂ . It was found that the electrical conductivity was highest when the Sb doping amount was 5.6 at %. On the other hand, the particles with the Sb doping amount of 8.1 at % had a slightly larger specific surface area than the other particles, but there was no significant difference in pore size from the other particles.

[4.2. Evaluation of Pt/Sb—SnO ₂ ]
[4.2.1. SEM observation]
FIG. 1 shows an SEM image (backscattered electron image) of the Pt/Sb—SnO ₂ obtained in Example 1 (ALD method). FIG. 2 shows an SEM image (backscattered electron image) of the Pt/Sb—SnO ₂ obtained in Example 2 (colloid method). 1 and 2, Pt nanoparticles of about 2 to 3 nm are highly dispersed on the Sb—SnO ₂ support regardless of whether the ALD method or the colloid method is used. I understand.

[4.2.2. XRD measurement]
FIG. 3 shows the XRD patterns of Pt/Sb—SnO ₂ obtained in Example 1 (ALD method) and Example 3 (colloid method). In the electrode catalyst supporting Pt by the colloidal method (Example 3), a diffraction peak derived from Pt (200) can be confirmed near 2θ=47°. On the other hand, in the electrode catalyst supporting Pt by the ALD method (Example 1), the peak corresponding to this shifts to the lower angle side, and it was found to be a peak attributed to PtSb ₃ . From this, it was found that when the ALD method was used, Pt was supported on the surface of the carrier and alloying of Pt and Sn of the carrier proceeded at the same time.

[3.2.3. Electrochemical evaluation]
FIG. 4 shows cyclic voltammograms of Pt/Sb—SnO ₂ obtained in Examples 2 and 3. FIG. Comparing Example 2 (Sb doping amount: 2.5 at%, colloidal method) and Example 3 (Sb doping amount: 5.6 at%, colloidal method), no difference was observed at 0.6 V or less. rice field. However, the Pt oxidation-reduction peak of 0.6 V or more was smaller in Example 2 (Sb doping amount: 2.5 at %) than in Example 3 (Sb doping amount: 5.6 at %). This indicates that when the Sb doping amount is small, the electron conductivity of the carrier is lowered in the high potential region, and the proportion of electrochemically usable Pt particles is lowered.

Table 2 shows the physical properties of the Pt/Sb--SnO ₂ supports of Examples 1 to 4, the Pt loading ratio, and the electrochemical evaluation results. Example 1 (ALD method) showed the lowest ECSA, SA, and MA values. This is considered to be due to the alloying of Pt with Sn of the support when Pt was supported using the ALD method. It is known that a Pt alloy generally has a smaller amount of hydrogen adsorption than pure Pt, and a low ECSA value is obtained when estimating the ECSA from the hydrogen adsorption/desorption peak of a cyclic voltammogram. Moreover, it is considered that the ORR activity also decreased due to the alloying. Therefore, it was found that the colloid method is more suitable than the ALD method for supporting Pt on the renister-like mesoporous Sb—SnO ₂ .

Among the electrocatalysts supported by the colloidal method, the MA of Examples 3 and 4, in which the Sb-doping amount of the support was 5.6 at%, was higher than that of Example 2, in which the Sb-doping amount of the support was 2.5 at%. was also about 15% higher. It is considered that this is because the electron conductivity of carriers decreases in the region of 0.6 V or higher when the Sb doping amount is small. No difference in MA due to the difference in Pt loading ratio (Examples 3 and 4) was observed.

Table 3 shows the ECSA and ORR of Pt/Sb--SnO ₂ obtained in Example 3 and Pt/M--SnO ₂ (M=Sb, Nb, Ta) reported in Non-Patent Documents 2-6. A comparison of activities is shown. The ORR mass activity (MA) of Example 3 is more than three times higher than the MA of Pt/M-SnO ₂ reported in Non-Patent Documents 2-5.
Also, regarding Pt/Ta--SnO ₂ in Non-Patent Document 6, the MA is reported to be about 10% higher than that of Example 3. However, the specific surface area of the Ta--SnO ₂ support of Non-Patent Document 6 is 1/6 of the specific surface area of the Sb--SnO ₂ support used in Example 3.
Therefore, Pt/M-SnO ₂ in which the M-SnO ₂ support has a specific surface area of 30 m ² /g or more and an ORR mass activity of 150 A/g (@ 0.9 V) or 650 A/g (@ 0.85 V). The catalyst has not been reported so far and has been achieved for the first time by the present invention.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The electrode catalyst according to the present invention can be used as an electrode catalyst for an air electrode catalyst layer or an electrode catalyst for an anode catalyst layer of a solid polymer fuel cell.

Claims (8)

An electrocatalyst having the following composition.
(1) the electrode catalyst,
tin oxide-based particles having a specific surface area of 30 m ² /g or more;
and Pt-based fine particles supported on the tin oxide-based particles.
(2) the electrode catalyst,
The oxygen reduction reaction mass activity at 0.9 V is 150 A/g _Pt or more, and/or
The oxygen reduction reaction mass activity at 0.85 V is 650 A/g _Pt or more. 2. The electrode catalyst according to claim 1, wherein the tin oxide-based particles have pores with a pore diameter of 50 nm or less inside. The electrocatalyst according to claim 1 or ₂ , wherein the tin oxide-based particles are made of SnO2 doped with Sb, Nb, Ta and/or W. The tin oxide-based particles are made of SnO ₂ doped with Sb,
4. The electrode catalyst according to any one of claims 1 to 3, wherein the doping amount of said Sb is 2.5 atomic % or more and 15.0 atomic % or less. 5. The electrode catalyst according to any one of claims 1 to 4, wherein the tin oxide-based particles have an electrical conductivity of 1 x ^10-3 S/cm or more in a powder compact. 6. The electrode catalyst according to any one of claims 1 to 5, wherein the tin oxide-based particles have a structure in which porous primary particles are fused in a beaded shape (a beaded structure). The electrode catalyst according to any one of claims 1 to 6, wherein the tin oxide particles have a pore size of 1 nm or more and 20 nm or less. 8. The electrode catalyst according to any one of claims 1 to 7, wherein the Pt-based particles have an average particle size of 5 nm or less.

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