US20180138518A1 - Core-shell catalyst and reaction acceleration method - Google Patents

Core-shell catalyst and reaction acceleration method Download PDF

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US20180138518A1
US20180138518A1 US15/575,279 US201615575279A US2018138518A1 US 20180138518 A1 US20180138518 A1 US 20180138518A1 US 201615575279 A US201615575279 A US 201615575279A US 2018138518 A1 US2018138518 A1 US 2018138518A1
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state
oxygen
site
adsorbed onto
reduction reaction
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Koichi Matsutani
Takeshi Kaieda
Yasushi Masahiro
Hideaki Kasai
Hiroshi Nakanishi
Koji Shimizu
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Tanaka Kikinzoku Kogyo KK
Osaka University NUC
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Tanaka Kikinzoku Kogyo KK
Osaka University NUC
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Assigned to OSAKA UNIVERSITY, TANAKA KIKINZOKU KOGYO K.K. reassignment OSAKA UNIVERSITY ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: MASAHIRO, YASUSHI, KAIEDA, TAKESHI, MATSUTANI, KOICHI, KASAI, HIDEAKI, NAKANISHI, HIROSHI, SHIMIZU, KOJI
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
    • B01J35/51—Spheres
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/8647—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites
    • H01M4/8657—Inert electrodes with catalytic activity, e.g. for fuel cells consisting of more than one material, e.g. consisting of composites layered
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
    • B01J23/42—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
    • B01J23/44—Palladium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/48—Silver or gold
    • B01J23/50—Silver
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/20—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state
    • B01J35/23—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state in a colloidal state
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/33—Electric or magnetic properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/396—Distribution of the active metal ingredient
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • B01J35/396—Distribution of the active metal ingredient
    • B01J35/397—Egg shell like
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/9041—Metals or alloys
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H01M4/925—Metals of platinum group supported on carriers, e.g. powder carriers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M4/90—Selection of catalytic material
    • H01M4/92—Metals of platinum group
    • H01M4/928—Unsupported catalytic particles; loose particulate catalytic materials, e.g. in fluidised state
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
    • H01M2004/8678—Inert electrodes with catalytic activity, e.g. for fuel cells characterised by the polarity
    • H01M2004/8689—Positive electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • the present invention relates to a core-shell catalyst and a reaction acceleration method in which the core-shell catalyst is used.
  • a method in which a smaller amount of platinum is used by forming a catalyst particle having a core-shell structure into an electrode catalyst by causing a carrier to support the catalyst particle.
  • the core-shell structure is a structure in which platinum, which is a highly active material, is used only in a surface (shell) of a catalyst particle and a material different from platinum is used in an inner part (core) of the catalyst particle which inner part does not contribute to a catalytic reaction.
  • Patent Literature 1 discloses a core-shell catalyst in which ruthenium is used in a core and platinum is used in a shell.
  • the core-shell catalyst is made highly catalytically active by being less poisoned with carbon monoxide.
  • Patent Literature 1 which has an object to provide a core-shell catalyst that is made highly catalytically active by being less poisoned with carbon monoxide, does not consider an oxygen reduction reaction, which is a cathode reaction of a fuel cell.
  • the present invention has been made in view of the problems, and an object of the present invention is to provide (i) a catalyst that has a core-shell structure and is highly active in an oxygen reduction reaction, which is a cathode reaction of a fuel cell, and (ii) a reaction acceleration method in which the catalyst is used.
  • CMD Computational Material Design
  • a core-shell catalyst in accordance with an embodiment of the present invention for accelerating an oxygen reduction reaction contains: silver or palladium as a core material; and platinum as a shell material, the core-shell catalyst having, on a surface thereof, a (110) surface of a face centered cubic lattice.
  • a reaction acceleration method in accordance with an embodiment of the present invention for accelerating an oxygen reduction reaction by use of a (110) surface that is formed on a surface of a core-shell catalyst containing: silver or palladium as a core material; and platinum as a shell material the reaction acceleration method includes: molecularly adsorbing an oxygen molecule onto the (110) surface; forming a water molecule by causing (i) the oxygen molecule adsorbed onto the (110) surface and (ii) a proton to react with each other; and desorbing the water molecule from the (110) surface.
  • a core-shell catalyst of an embodiment of the present invention silver or palladium is used as a core material, platinum is used as a shell material, and the core-shell catalyst has, on a surface thereof, a (110) surface of a face centered cubic lattice.
  • FIG. 1 is a view illustrating a reaction model of the oxygen reduction reaction which reaction model includes a step of oxygen molecule dissociation.
  • FIG. 2 is a view illustrating a reaction model of the oxygen reduction reaction which reaction model includes a step of peroxyl dissociation.
  • FIG. 3 is a view illustrating a reaction model of the oxygen reduction reaction which reaction model includes a step of hydrogen peroxide dissociation.
  • FIG. 4 is a view of a (110) surface of an FCC structure, the (110) surface being seen from a direction normal to the (110) surface.
  • FIG. 5 is a chart showing adsorption energy with which an oxygen atom is adsorbed onto adsorption sites of each of Pt, Pt ML Ag, and Pt ML Pd.
  • FIG. 6 is a chart showing adsorption energy with which an oxygen molecule is adsorbed onto the adsorption sites of each of Pt, Pt ML Ag, and Pt ML Pd.
  • FIG. 7 has views each illustrating how an oxygen molecule is adsorbed onto a Pt surface with the adsorption energy shown in FIG. 6 .
  • (a) through (d) of FIG. 7 are views illustrating how oxygen molecules are adsorbed onto a Top site, an S-bridge site, an L-bridge site, and a Hollow site, respectively, of the Pt surface.
  • FIG. 8 has views each illustrating how an oxygen molecule is adsorbed onto a Pt ML Ag surface with the adsorption energy shown in FIG. 6 .
  • (a) through (d) of FIG. 8 are views illustrating how oxygen molecules are adsorbed onto the Top site, the S-bridge site, the L-bridge site, and the Hollow site, respectively, of the Pt ML Ag surface.
  • FIG. 9 is a chart showing lattice constants of each of platinum, silver, and palladium.
  • FIG. 10 has charts each showing a state density of a catalytic surface.
  • (a) through (c) of FIG. 10 are charts showing respective state densities of a Pt surface, a Pt ML Ag surface, and a Pt ML Pd surface.
  • FIG. 11 each show a change in free energy obtained in each state in the oxygen reduction reaction which includes the step of oxygen molecule dissociation.
  • FIG. 13 is a chart showing potential energy obtained in a state in which OOH is adsorbed onto a Pt surface.
  • FIG. 14 has views each illustrating a state in which OOH is adsorbed onto a Pt surface.
  • (a) of FIG. 14 illustrates a state in which OOH is adsorbed onto the Top site of the Pt surface.
  • (b) of FIG. 14 illustrates a state in which OOH is adsorbed onto the S-bridge site of the Pt surface.
  • FIG. 15 is a view illustrating a state into which each of the states of (a) and (b) of FIG. 14 has been changed by further adsorption of a proton onto the Pt surface.
  • FIG. 16 is a view illustrating a state into which the state illustrated in FIG. 15 has been changed by further adsorption of a proton onto the Pt surface.
  • FIG. 17 show a change in free energy obtained in each state in the oxygen reduction reaction which includes no step of oxygen molecule dissociation and in which Pt is used as a catalyst.
  • FIG. 18 are views each illustrating a state in which OOH is adsorbed onto a Pt ML Ag surface.
  • FIG. 19 are views each illustrating a state in which H 2 O 2 is adsorbed onto the Pt ML Ag surface.
  • FIG. 20 are views each illustrating a state in which H 3 O 2 is adsorbed onto the Pt ML Ag surface.
  • FIG. 21 is a diagram showing a transition, made by a process of the oxygen reduction reaction which includes no step of oxygen molecule dissociation and in which Pt ML Ag is used, from the states illustrated in (a) through (c) of FIG. 8 to the states illustrated in FIGS. 18 through 20 .
  • FIG. 22 show a change in free energy obtained in the oxygen reduction reaction which includes no step of oxygen molecule dissociation and in which Pt ML Ag is used as a catalyst.
  • a core-shell catalyst which is an embodiment of the present invention and which contains silver or palladium as a core material and contains platinum as a shell material is a catalyst for accelerating an oxygen reduction reaction that is caused in, for example, a cathode of a fuel cell.
  • the core material is preferably silver or palladium.
  • the inventors of the present invention carried out a simulation in which first-principles calculation based on density functional theory was used.
  • the first-principles calculation is a calculation method based on the density functional theory showing that “ground-state energy of many-electron systems that interact with each other is determined in accordance with a density distribution of electrons” (see P. Hohenberg and W. Kohn, Phys. Rev. 136, B864 (1964), W. Kohn and L. J. Sham, Phys. Rev.
  • the first-principles calculation makes it possible to quantitatively discuss an electronic structure of a substance without an empirical parameter. Actually, the first-principles calculation allows effectiveness equivalent to that shown by experiments to be shown by many verifications. In the present simulation, a general density gradient approximation method, which is currently the most accurate one of the first-principles calculations, was used to carry out calculation.
  • Pt ML Ag had a structure in which a single atomic layer of Pt was provided on 5 atomic layers of Ag
  • Pt ML Pd had a structure in which a single atomic layer of Pt was provided on 5 atomic layers of Pd.
  • a core-shell catalyst can include, as shell layer(s), not only a single atomic layer of platinum but also 1 to 3 atomic layers of platinum.
  • reaction model of the oxygen reduction reaction include the following three reaction models: a reaction model in which the oxygen reduction reaction proceeds through the step of (1) oxygen molecule dissociation (oxygen dissociation); a reaction model in which the oxygen reduction reaction proceeds through the step of (2) peroxyl dissociation; and a reaction model in which the oxygen reduction reaction proceeds through the step of (3) hydrogen peroxide dissociation.
  • FIG. 1 is a view illustrating a reaction model of the oxygen reduction reaction which reaction model includes the step of oxygen molecule dissociation.
  • an oxygen molecule is adsorbed onto a catalytic surface first (O 2 +* ⁇ O 2 *).
  • the sign “*” means the catalytic surface
  • O 2 * means that the oxygen molecule is adsorbed onto the catalytic surface.
  • the oxygen molecule which has been adsorbed onto the catalytic surface is dissociated into oxygen atoms (O 2 *+* ⁇ O*+O*).
  • FIG. 2 is a view illustrating a reaction model of the oxygen reduction reaction which reaction model includes the step of peroxyl dissociation.
  • an oxygen molecule is adsorbed onto a catalytic surface first (O 2 +* ⁇ O 2 *) as in the case of the step of oxygen molecule dissociation.
  • a proton having been moved from the anode side through an electrolyte and the oxygen molecule on the catalytic surface react with each other, so that OOH is formed on the catalytic surface (O 2 *+H + +e ⁇ ⁇ OOH*).
  • OOH is dissociated into an oxygen atom and OH (OOH* ⁇ O*+OH*).
  • FIG. 3 is a view illustrating a reaction model of the oxygen reduction reaction which reaction model includes the step of hydrogen peroxide dissociation.
  • an oxygen molecule is adsorbed onto a catalytic surface first (O 2 +* ⁇ O 2 *) as in the case of the step of oxygen molecule dissociation.
  • an oxygen molecule is adsorbed onto a catalytic surface first in any of the reaction models of the oxygen reduction reaction which reaction models include the respective steps of oxygen molecule dissociation, peroxyl dissociation, and hydrogen peroxide dissociation.
  • silver, platinum, and palladium each have a face centered cubic (FCC) structure.
  • a (110) surface of the FCC structure and a surface of the FCC structure which surface is equivalent to the (110) surface have a lower in-plane atom density than the other surfaces (e.g., a (100) surface and a (111) surface) of the FCC structure.
  • oxygen molecule adsorption which is a first stage of the oxygen reduction reaction, is considered to be more likely to occur in the (110) surface than in the other surfaces.
  • the following description discusses the simulation which is carried out while attention is focused on the (110) surface, which is considered to have high reaction activity.
  • FIG. 4 is a view of a (110) surface of an FCC structure, the (110) surface being seen from a direction normal to the (110) surface.
  • a catalyst having, on a surface thereof, the (110) surface of the FCC structure, a total of four sites, which are an On-top (hereinafter, Top) site, a Short bridge (hereinafter, S-bridge) site, a Long bridge (hereinafter, L-bridge) site, and a Hollow site, is assumed as adsorption sites of an oxygen atom.
  • Top On-top
  • S-bridge Short bridge
  • L-bridge Long bridge
  • Hollow site is assumed as adsorption sites of an oxygen atom.
  • the Top site is an adsorption site that is present on an atom of a first layer of a catalytic surface. Note here that according to the (110) surface of the FCC structure, a [ ⁇ 110] direction, which is an in-plane direction, and a [001] direction, which is also an in-plane direction, differ in interatomic distance, and the [ ⁇ 110] direction is shorter in interatomic distance than the [001] direction.
  • the S-bridge site is an adsorption site that is present between atoms located in the direction ([ ⁇ 110] direction), which is shorter in interatomic distance.
  • the L-bridge site is an adsorption site that is present between atoms located in the direction ([001] direction), which is longer in interatomic distance.
  • the Hollow site is an adsorption site that is present so as to be surrounded by four atoms. Note that the sign “ ⁇ ”, which is supposed to be given above a numeral indicative of a direction in writing in crystallography, is to be given before the numeral for convenience in writing.
  • FIG. 5 is a chart showing a result of calculation of adsorption energy with which an oxygen atom is adsorbed onto the adsorption sites of each of Pt, Pt ML Ag, and Pt ML Pd.
  • the S-bridge site of any of Pt, Pt ML Ag, and Pt ML Pd has minimum adsorption energy. This reveals that an oxygen atom that is adsorbed onto a catalytic surface is stably adsorbed onto the S-bridge site in any of the cases of Pt, Pt ML Ag, and Pt ML Pd.
  • the Top site, the S-bridge site, the L-bridge site, and the Hollow site are also assumed as adsorption sites also in the case where an oxygen molecule is adsorbed onto a catalytic surface.
  • a state in which an oxygen molecule is adsorbed onto the Top site means a state in which a center of gravity of the oxygen molecule is located at the Top site, i.e., means a state in which a midpoint of respective centers of gravity of two oxygen atoms is located at the Top site.
  • FIG. 6 is a chart showing a result of calculation of adsorption energy with which an oxygen molecule is adsorbed onto the adsorption sites of each of Pt, Pt ML Ag, and Pt ML Pd.
  • a state in which an oxygen molecule is adsorbed onto a certain site means a state in which a center of gravity of the oxygen molecule is present at the certain site.
  • FIG. 6 shows a location of an oxygen atom that has minimum adsorption energy as a result of calculation (is stably adsorbed), and same applies to the drawings following FIG. 6 .
  • the L-bridge site, the Hollow site, the S-bridge site, and the Top site of each of Pt and Pt ML Pd are arranged in an ascending order of adsorption energy, and Pt and Pt ML Pd show identical tendencies.
  • the Hollow site, the S-bridge site, the L-bridge site, and the Top site of Pt ML Ag are arranged in an ascending order of adsorption energy.
  • the L-bridge site of each of Pt and Pt ML Pd is stable as an adsorption site of an oxygen molecule
  • the Hollow site of Pt ML Ag is stable as an adsorption site of an oxygen molecule.
  • FIG. 7 has views each illustrating how an oxygen molecule is adsorbed onto a Pt surface with the adsorption energy shown in FIG. 6 .
  • (a) through (d) of FIG. 7 are views illustrating how oxygen molecules are adsorbed onto the Top site, the S-bridge site, the L-bridge site, and the Hollow site, respectively, of the Pt surface.
  • the drawing on the left is a top view of the Pt surface which is seen from above
  • the drawing on the right is a cross-sectional view of the drawing on the left which drawing is seen from a corresponding one of an arrow A direction through an arrow D direction.
  • the Top site has a distance between oxygen atoms of 1.37 ⁇
  • the S-bridge site has a distance between oxygen atoms of 1.38 ⁇
  • the L-bridge site has a distance between oxygen atoms of 4.88 ⁇
  • the Hollow site has a distance between oxygen atoms of 4.00 ⁇ .
  • FIG. 8 has views each illustrating how an oxygen molecule is adsorbed onto a Pt ML Ag surface with the adsorption energy shown in FIG. 6 .
  • (a) through (d) of FIG. 8 are views illustrating how oxygen molecules are adsorbed onto the Top site, the S-bridge site, the L-bridge site, and the Hollow site, respectively, of the Pt ML Ag surface.
  • the drawing on the left is a top view of the Pt ML Ag surface which is seen from above
  • the drawing on the right is a cross-sectional view of the drawing on the left which drawing is seen from a corresponding one of an arrow E direction through an arrow H direction.
  • the Top site has a distance between oxygen atoms of 1.33 ⁇
  • the S-bridge site has a distance between oxygen atoms of 1.36 ⁇
  • the L-bridge site has a distance between oxygen atoms of 1.36 ⁇
  • the Hollow site has a distance between oxygen atoms of 4.19 ⁇ .
  • FIG. 9 is a chart showing lattice constants of each of platinum (Pt), silver (Ag), and palladium (Pd).
  • An upper row of FIG. 9 which upper row is indicative of a result of calculation, shows calculated values of lattice constants calculated by use of first-principles calculation based on density functional theory
  • a lower row of FIG. 9 which lower row is indicative of experimental values, shows lattice constants of a bulk material which lattice constants are shown in C. Kittel, Introduction to Solid State Theory (Wiley, New York, 2005).
  • FIG. 9 reveals that a calculated value and an experimental value are highly consistent with each other.
  • Pt has a lattice constant whose calculated value is 3.977 ⁇
  • Ag has a lattice constant whose calculated value is 4.166 ⁇
  • Pd has a lattice constant whose calculated value is 3.957 ⁇ .
  • the calculated value of the lattice constant of Ag is 104.75% of the calculated value of the lattice constant of Pt
  • the calculated value of the lattice constant of Pd is 99.5% of the calculated value of the lattice constant of Pt.
  • a value of a lattice constant of Pt ML Ag in which one atomic layer of Pt is provided on Ag is considered to show a tendency that is identical to a tendency of a value of a lattice constant of Ag
  • a value of a lattice constant of Pt ML Pd in which one atomic layer of Pt is provided on Pd is considered to show a tendency that is identical to a tendency of a value of a lattice constant of Pd.
  • FIG. 10 has charts each showing a state density of a catalytic surface.
  • (a) through (c) of FIG. 10 are charts showing respective state densities of a Pt surface, a Pt ML Ag surface, and a Pt ML Pd surface.
  • the drawing on the left shows a state density (total) of a total of orbitals and a state density (5d) of a 5d orbital
  • the drawing on the right shows respective state densities of five 5d orbitals.
  • 0 (zero) on a transverse axis shows Fermi level. As shown in (a) of FIG.
  • Pt has a high electron density near the Fermi level. This reveals that Pt has high catalytic activity.
  • Pt ML Pd shown in (c) of FIG. 10 has a state density similar to that of Pt. This reveals that Pt ML Pd also has high catalytic activity as in the case of Pt.
  • Pt ML Ag shown in (b) of FIG. 10 has a higher electron density near the Fermi level than Pt and Pt ML Pd.
  • Pt ML Ag has a dz 2 orbital whose state density further deviates in a negative direction as compared with state densities of dz 2 orbitals of Pt and Pt ML Pd.
  • the dz 2 orbital of Pt ML Ag is localized near ⁇ 1 eV. This seems to be because an electronic structure of Pt on the Pt ML Ag surface is unstable by being deformed due to a difference in lattice constant between Pt and Ag (described earlier).
  • Pt ML Ag thus has a higher electron density near the Fermi level than Pt and Pt ML Pd due to a change in electronic structure by the difference in lattice constant between Pt and Ag suggests that Pt ML Ag has higher catalytic activity than Pt and Pt ML Pd.
  • an oxygen molecule is stably adsorbed onto a catalytic surface in both Pt and Pt ML Ag by being dissociated into oxygen atoms.
  • the oxygen reduction reaction which includes the step of oxygen molecule dissociation and in which a reaction at the second stage which reaction follows an oxygen molecule adsorption reaction at the first stage is an oxygen molecule dissociation reaction. This is because reaction(s) subsequent to the reaction at the second stage is/are considered to easily proceed in a case where an oxygen molecule is adsorbed onto a catalytic surface while being dissociated into oxygen atoms.
  • free energy G obtained in each state in the oxygen reduction reaction which includes the step of oxygen molecule dissociation and in which Pt and Pt ML Ag are used as catalysts is calculated, an amount of change ⁇ G in free energy is calculated in each state.
  • a method for calculating free energy G is as described below.
  • the enthalpy H is given by a sum of (i) potential energy resulting from interaction between an atomic nucleus and an electron and (ii) kinetic energy of the atomic nucleus (i.e., energy resulting from thermal motion of a molecule and an atom).
  • Free energy is calculated assuming that (i) the electric potential of 0.7 V is a numerical value based on 0.7 V, which is a drive voltage of a common fuel cell, and (ii) the electric potential of 0.9 V is an upper limit of a drive voltage of a fuel cell. Note that free energy is calculated assuming that the temperature T is 300K.
  • FIG. 11 has charts showing a change in free energy obtained in each state in the oxygen reduction reaction which includes the step of oxygen molecule dissociation.
  • (a) of FIG. 11 shows a result of calculation in a case where Pt is used as a catalyst.
  • (b) of FIG. 11 shows a result of calculation in a case where Pt ML Ag is used as a catalyst. Note that in FIG.
  • O 2 shows a state in which an oxygen molecule is present in a molecular state
  • O* shows a state in which an oxygen atom is adsorbed onto a catalytic surface
  • OH* shows a state in which OH is adsorbed onto the catalytic surface
  • H 2 O* shows a state in which H 2 O is adsorbed onto the catalytic surface
  • H 2 O shows a state in which H 2 O is desorbed from the catalytic surface.
  • FIG. 12 shows, for each electric potential applied, the changes, shown in (a) and (b) of FIG. 11 , in free energy obtained in each state in the oxygen reduction reaction in which Pt and Pt ML Ag are used as catalysts.
  • (a) of FIG. 12 shows a state in which no electric potential is applied.
  • (b) of FIG. 12 shows a state in which an electric potential of 0.7 V is applied.
  • (c) of FIG. 12 shows a state in which an electric potential of 0.9 V is applied. Note that in FIG. 12 , a state in which lowest free energy is obtained is shown by a thick line.
  • the oxygen reduction reaction which includes the step of oxygen molecule dissociation is considered to more easily proceed than the other reaction models of the oxygen reduction reaction. Note, however, that in a state in which an electric potential is applied, as in a cathode electrode of a fuel cell, an activation barrier is great in the oxygen reduction reaction and thus the oxygen reduction reaction is considered to less easily proceed.
  • the oxygen reduction reaction which includes the step of oxygen molecule dissociation is considered to less easily proceed, whereas the oxygen reduction reaction which includes no step of oxygen molecule dissociation and includes the step of, for example, hydrogen peroxide dissociation or peroxyl dissociation is considered to predominantly proceed.
  • a reaction model including no step of oxygen molecule dissociation is used to carry out study of the oxygen reduction reaction with respect to each of Pt and Pt ML Ag.
  • an oxygen molecule is adsorbed onto a catalytic surface in a molecular state without being dissociated into oxygen atoms.
  • two sites which are the Top site and the S-bridge site, are assumed as adsorption sites of the oxygen molecule in the oxygen reduction reaction which includes no step of oxygen molecule dissociation and in which Pt is used.
  • the oxygen reduction reaction which includes no step of oxygen molecule dissociation and includes any of the steps of, for example, hydrogen peroxide dissociation and peroxyl dissociation proceeds such that an oxygen molecule (O 2 ) is adsorbed onto the catalytic surface first (O 2 *) in a molecular state, and then the adsorbed oxygen molecule (O 2 *) reacts with a proton (H + ), so that OOH* is formed.
  • potential energy obtained in a state in which OOH is adsorbed onto a Pt surface is calculated with respect to each of a case where an oxygen molecule is adsorbed onto the Top site and a case where an oxygen molecule is adsorbed onto the S-bridge site.
  • FIG. 13 is a chart showing potential energy obtained in a state in which OOH is adsorbed onto a Pt surface.
  • FIG. 14 has views each illustrating a state in which OOH is adsorbed onto a Pt surface.
  • (a) of FIG. 14 illustrates a state in which OOH is adsorbed onto the Top site of the Pt surface.
  • (b) of FIG. 14 illustrates a state in which OOH is adsorbed onto the S-bridge site of the Pt surface.
  • the drawing on the left is a top view of the Pt surface which is seen from above, and the drawing on the right is a cross-sectional view of the drawing on the left which drawing is seen from a corresponding one of an arrow I direction and an arrow J direction.
  • FIG. 15 is a view illustrating a state into which each of the states of (a) and (b) of FIG. 14 has been changed by further adsorption of a proton onto the Pt surface.
  • the drawing on the left is a top view of the Pt surface which is seen from above
  • the drawing on the right is a cross-sectional view of the drawing on the left which drawing is seen from an arrow K direction. Note that in a case where a proton is further adsorbed onto the catalytic surface in each of the states of (a) and (b) of FIG.
  • FIG. 16 is a view illustrating a state into which the state illustrated in FIG. 15 has been changed by further adsorption of a proton onto the Pt surface.
  • the drawing on the left is a top view of the Pt surface which is seen from above, and the drawing on the right is a cross-sectional view of the drawing on the left which drawing is seen from an arrow L direction.
  • potential energy ⁇ E of ⁇ 5.48 eV is obtained. This reveals that there is no activation barrier to a change from the state (H 2 O 2 *) illustrated in FIG. 15 to the state (H 3 O 2 *) in which the proton is further adsorbed.
  • Pt can be said to have no activation barrier during a period in which an oxygen molecule is adsorbed onto the Pt surface and then the state of H 3 O 2 * arises.
  • oxygen reduction reaction which includes the step of peroxyl dissociation proceeds
  • oxygen atoms are uncoupled in a state in which OOH is adsorbed onto the catalytic surface (see FIG. 2 ).
  • oxygen reduction reaction which includes the step of hydrogen peroxide dissociation proceeds
  • oxygen atoms are uncoupled in a state in which H 2 O 2 is adsorbed onto the catalytic surface (see FIG. 3 ).
  • oxygen interatomic distances obtained in the respective states of FIGS. 14 through 16 are found.
  • the oxygen interatomic distance is 2.29 ⁇ .
  • the oxygen interatomic distance is 2.66 ⁇ .
  • the oxygen interatomic distance is 2.81 ⁇ .
  • the oxygen interatomic distance is 2.34 ⁇ .
  • free energy obtained in each state in the reaction model (described earlier, i.e., the reaction model including no step of oxygen molecule dissociation) in which Pt is used as a catalyst is calculated. Further, as in the case of the reaction model including the step of oxygen dissociation, free energy is calculated with respect to not only the state in which no electric potential (0V) is applied but also the state in which an electric potential of 0.7 V is applied and the state in which an electric potential of 0.9 V is applied.
  • FIG. 17 shows a difference in free energy obtained in each state in the oxygen reduction reaction in which Pt is used as a catalyst.
  • (a) of FIG. 17 shows a difference in free energy obtained in each state in a case where the oxygen reduction reaction proceeds in a state in which an oxygen molecule is adsorbed onto the S-bridge site.
  • (b) of FIG. 17 shows a difference in free energy obtained in each state in a case where the oxygen reduction reaction proceeds in a state in which an oxygen molecule is adsorbed onto the Top site.
  • FIG. 17 shows a difference in free energy obtained in each state in the oxygen reduction reaction in which Pt is used as a catalyst.
  • an oxygen molecule which is adsorbed onto the Top site, the S-bridge site, or the L-bridge site of a Pt ML Ag surface is in a molecular state, whereas an oxygen molecule which is adsorbed onto the Hollow site of the Pt ML Ag surface is dissociated into oxygen atoms.
  • potential energy obtained in a state of adsorption of OOH, which state follows a state of adsorption of an oxygen molecule is calculated with respect to each of a case where the oxygen molecule is adsorbed onto the Top site, a case where the oxygen molecule is adsorbed onto the S-bridge site, and a case where the oxygen molecule is adsorbed onto the L-bridge site.
  • FIG. 18 illustrates a state in which OOH is adsorbed onto a Pt ML Ag surface.
  • (a) of FIG. 18 illustrates a case where an oxygen molecule is adsorbed onto the Top site of the Pt ML Ag surface.
  • (b) and (c) of FIG. 18 each illustrate a case where an oxygen molecule is adsorbed onto the L-bridge site of the Pt ML Ag surface.
  • (d) and (e) of FIG. 18 each illustrate a case where an oxygen molecule is adsorbed onto the S-bridge site of the Pt ML Ag surface.
  • FIG. 18 illustrates a state in which OOH is adsorbed onto a Pt ML Ag surface.
  • the drawing on the left is a top view of the Pt ML Ag surface which is seen from above, and the drawing on the right is a cross-sectional view of the drawing on the left which drawing is seen from a corresponding one of an arrow M direction through an arrow Q direction.
  • OOH* is adsorbed onto the L-bridge site.
  • potential energy of ⁇ 2.52 eV is obtained.
  • OOH* may be adsorbed onto the Hollow site as illustrated in (b) of FIG. 18 , or may be adsorbed onto the L-bridge site as illustrated in (c) of FIG. 18 .
  • a state illustrated in (c) of FIG. 18 is identical to a state illustrated in (a) of FIG. 18 .
  • OOH* may be in a state in which the oxygen molecule is arranged in the [ ⁇ 110] direction as illustrated in (d) of FIG. 18 , or may be in a state in which the oxygen molecule is arranged in the [001] direction as illustrated in (e) of FIG. 18 .
  • potential energy of ⁇ 2.34 eV is obtained in the state in which the oxygen molecule is arranged in the [ ⁇ 110] direction
  • potential energy of ⁇ 2.59 eV is obtained in the state in which the oxygen molecule is arranged in the [001] direction.
  • FIG. 19 illustrates a state in which H 2 O 2 is adsorbed onto the Pt ML Ag surface.
  • (a) of FIG. 19 illustrates a state into which each of the states illustrated in (a) and (c) of FIG. 18 has been changed by further adsorption of a proton.
  • (b) of FIG. 19 illustrates a state into which the state illustrated in (b) of FIG. 18 has been changed by further adsorption of a proton.
  • (c) of FIG. 19 illustrates a state into which the state illustrated in (d) of FIG. 18 has been changed by further adsorption of a proton.
  • (d) of FIG. 19 illustrates a state into which the state illustrated in (e) of FIG.
  • FIG. 19 the drawing on the left is a top view of the Pt ML Ag surface which is seen from above, and the drawing on the right is a cross-sectional view of the drawing on the left which drawing is seen from a corresponding one of an arrow R direction through an arrow U direction.
  • potential energy of ⁇ 5.46 eV is obtained.
  • potential energy of ⁇ 3.56 eV is obtained.
  • potential energy of ⁇ 4.26 eV is obtained.
  • potential energy of ⁇ 4.27 eV is obtained.
  • FIG. 20 illustrates a state in which H 3 O 2 is adsorbed onto the Pt ML Ag surface.
  • (a) of FIG. 20 illustrates a state into which the state illustrated in (a) of FIG. 19 has been changed by further adsorption of a proton.
  • (b) of FIG. 20 illustrates a state into which the state illustrated in (b) of FIG. 19 has been changed by further adsorption of a proton.
  • (c) of FIG. 20 illustrates a state into which the state illustrated in (c) of FIG. 19 has been changed by further adsorption of a proton. Note that in a case where a state into which the state illustrated in (d) of FIG.
  • FIG. 21 is a diagram showing a transition, made by a process of the oxygen reduction reaction which includes no step of oxygen molecule dissociation and in which PtMLAg is used, from the states illustrated in (a) through (c) of FIG. 8 to the states illustrated in FIGS. 18 through 20 .
  • a pathway along which the oxygen reduction reaction which includes no step of oxygen molecule dissociation and in which Pt ML Ag is used proceeds is exemplified by five pathways ⁇ through ⁇ .
  • the oxygen molecule is adsorbed onto the L-bridge site in the state of O 2 *, and the oxygen molecule is adsorbed onto the Hollow site in the state of OOH*, which state follows the state of O 2 *.
  • the oxygen reduction reaction which proceeds along the pathway ⁇ the oxygen molecule is adsorbed onto the L-bridge site in the state of O 2 *.
  • the oxygen reduction reaction which proceeds along the pathway ⁇ subsequently proceeds also in a state in which the oxygen molecule is adsorbed onto the L-bridge site.
  • the oxygen molecule is adsorbed onto the Top site in the state of O 2 *, and the oxygen molecule is adsorbed onto the L-bridge site in the state of OOH*. Then, subsequent reactions of the oxygen reduction reaction which proceeds along the pathway ⁇ are identical to those of the oxygen reduction reaction which proceeds along the pathway ⁇ . According to the oxygen reduction reaction which proceeds along the pathway ⁇ , the oxygen molecule is adsorbed onto the S-bridge site in the state of O 2 * so as to be arranged in the [ ⁇ 110] direction, whereas the oxygen molecule is adsorbed onto the S-bridge site in the state of OOH* so as to be arranged in the [001] direction.
  • the oxygen reduction reaction which proceeds along the pathway ⁇ proceeds to H 2 O 2 *.
  • the oxygen reduction reaction which proceeds along the pathway ⁇ does not proceed to the state of H 3 O 2 *.
  • the oxygen molecule is adsorbed onto the S-bridge site in the state of O 2 * so as to be arranged in the [ ⁇ 110] direction.
  • the oxygen reduction reaction which proceeds along the pathway c subsequently proceeds to the states of OOH*, H 2 O 2 *, and H 3 O 2 * in this order also in a state in which the oxygen molecule is adsorbed onto the S-bridge site.
  • a reaction pathway such as the pathway ⁇ , along which a location at which the oxygen molecule is adsorbed is changed from the L-bridge site to the Hollow site, is referred to an L-bridge to Hollow pathway
  • a reaction pathway such as the pathway c, along which a state of the oxygen molecule which is adsorbed onto the S-bridge site so as to be arranged in the [ ⁇ 110] direction is not changed, is referred to as an S-bridge pathway.
  • oxygen interatomic distances obtained in the states, illustrated in FIG. 21 are calculated.
  • the oxygen interatomic distances obtained in the state of OOH* are 2.46 ⁇ , 2.63 ⁇ , 2.48 ⁇ , and 2.62 ⁇ in this order from the left of FIG. 21 .
  • the oxygen interatomic distances obtained in the state of H 2 O 2 * are 2.66 ⁇ , 3.37 ⁇ , 2.54 ⁇ , and 2.87 ⁇ in this order from the left of FIG. 21 .
  • FIG. 22 shows a change in free energy obtained in the oxygen reduction reaction in which Pt ML Ag is used as a catalyst.
  • (a) of FIG. 22 shows a change in free energy obtained in the oxygen reduction reaction which proceeds along the S-bridge pathway.
  • (b) of FIG. 22 shows a change in free energy obtained in the oxygen reduction reaction which proceeds along the L-bridge to Hollow pathway.
  • FIG. 23 shows, for each electric potential applied, the changes in free energy, which changes are shown in FIGS. 17 and 22 .
  • (a) of FIG. 23 shows the state in which no electric potential is applied.
  • (b) of FIG. 23 shows the state in which an electric potential of 0.7 V is applied.
  • (c) of FIG. 23 shows the state in which an electric potential of 0.9 V is applied.
  • a (110) surface of a core-shell catalyst (Pt ML Ag) containing silver as a core material and containing platinum as a shell material is more catalytically active than a (110) surface of a catalyst (Pt) consisting solely of platinum.
  • a core-shell catalyst (Pt ML Pd) containing palladium as a core material and containing platinum as a shell material is substantially as catalytically active as a catalyst (Pt) consisting solely of platinum.
  • a core-shell catalyst in accordance with an embodiment of the present invention for accelerating an oxygen reduction reaction contains: silver or palladium as a core material; and platinum as a shell material, the core-shell catalyst having, on a surface thereof, a (110) surface of a face centered cubic lattice.
  • the core-shell catalyst is preferably arranged such that the core-shell catalyst has a shell that includes 1 to 3 atomic layers of the platinum.
  • a reaction acceleration method in accordance with an embodiment of the present invention for accelerating an oxygen reduction reaction by use of a (110) surface that is formed on a surface of a core-shell catalyst containing: silver or palladium as a core material; and platinum as a shell material the reaction acceleration method includes: molecularly adsorbing an oxygen molecule onto the (110) surface; forming a water molecule by causing (i) the oxygen molecule adsorbed onto the (110) surface and (ii) a proton to react with each other; and desorbing the water molecule from the (110) surface.
  • the present invention is not limited to the embodiments, but can be altered by a skilled person in the art within the scope of the claims.
  • the present invention also encompasses, in its technical scope, any embodiment derived by combining technical means disclosed in differing embodiments.
  • the present invention can be suitably used for a catalyst for an oxygen reduction reaction, especially for a cathode electrode catalyst of a fuel cell.

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