WO2018131206A1 - コアシェル触媒および酸素還元方法 - Google Patents
コアシェル触媒および酸素還元方法 Download PDFInfo
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- WO2018131206A1 WO2018131206A1 PCT/JP2017/030982 JP2017030982W WO2018131206A1 WO 2018131206 A1 WO2018131206 A1 WO 2018131206A1 JP 2017030982 W JP2017030982 W JP 2017030982W WO 2018131206 A1 WO2018131206 A1 WO 2018131206A1
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- 239000003054 catalyst Substances 0.000 title claims abstract description 197
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 title claims abstract description 107
- 239000001301 oxygen Substances 0.000 title claims abstract description 107
- 229910052760 oxygen Inorganic materials 0.000 title claims abstract description 107
- 239000011258 core-shell material Substances 0.000 title claims abstract description 53
- 238000000034 method Methods 0.000 title claims abstract description 39
- 230000009467 reduction Effects 0.000 title claims abstract description 12
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 claims abstract description 246
- 229910052697 platinum Inorganic materials 0.000 claims abstract description 98
- 238000006722 reduction reaction Methods 0.000 claims abstract description 49
- 229910052709 silver Inorganic materials 0.000 claims abstract description 20
- 239000004332 silver Substances 0.000 claims abstract description 15
- 125000004430 oxygen atom Chemical group O* 0.000 claims description 102
- KDLHZDBZIXYQEI-UHFFFAOYSA-N Palladium Chemical compound [Pd] KDLHZDBZIXYQEI-UHFFFAOYSA-N 0.000 claims description 73
- 229910052763 palladium Inorganic materials 0.000 claims description 30
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 13
- 238000006243 chemical reaction Methods 0.000 abstract description 49
- 239000011257 shell material Substances 0.000 abstract description 42
- 239000011162 core material Substances 0.000 abstract description 25
- 239000002245 particle Substances 0.000 abstract description 16
- 239000000446 fuel Substances 0.000 abstract description 14
- 239000000463 material Substances 0.000 abstract description 10
- 230000000694 effects Effects 0.000 abstract description 7
- 238000001179 sorption measurement Methods 0.000 description 95
- 230000004888 barrier function Effects 0.000 description 44
- 238000005755 formation reaction Methods 0.000 description 44
- 230000004913 activation Effects 0.000 description 42
- 230000015572 biosynthetic process Effects 0.000 description 40
- MYMOFIZGZYHOMD-UHFFFAOYSA-N Dioxygen Chemical compound O=O MYMOFIZGZYHOMD-UHFFFAOYSA-N 0.000 description 36
- 238000005381 potential energy Methods 0.000 description 36
- 238000004364 calculation method Methods 0.000 description 34
- 238000010494 dissociation reaction Methods 0.000 description 26
- 230000005593 dissociations Effects 0.000 description 23
- 230000008569 process Effects 0.000 description 22
- 238000004088 simulation Methods 0.000 description 21
- XLYOFNOQVPJJNP-UHFFFAOYSA-O oxonium Chemical compound [OH3+] XLYOFNOQVPJJNP-UHFFFAOYSA-O 0.000 description 19
- 230000003197 catalytic effect Effects 0.000 description 18
- 125000004429 atom Chemical group 0.000 description 17
- 238000013459 approach Methods 0.000 description 15
- 238000010586 diagram Methods 0.000 description 15
- 229910001882 dioxygen Inorganic materials 0.000 description 14
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 12
- MHAJPDPJQMAIIY-UHFFFAOYSA-N Hydrogen peroxide Chemical compound OO MHAJPDPJQMAIIY-UHFFFAOYSA-N 0.000 description 11
- 230000037361 pathway Effects 0.000 description 9
- -1 platinum ions Chemical class 0.000 description 9
- 239000007771 core particle Substances 0.000 description 6
- 229910052759 nickel Inorganic materials 0.000 description 6
- 239000000126 substance Substances 0.000 description 6
- 230000007704 transition Effects 0.000 description 6
- KJTLSVCANCCWHF-UHFFFAOYSA-N Ruthenium Chemical compound [Ru] KJTLSVCANCCWHF-UHFFFAOYSA-N 0.000 description 5
- 230000008859 change Effects 0.000 description 5
- 230000007423 decrease Effects 0.000 description 5
- 229910052707 ruthenium Inorganic materials 0.000 description 5
- 238000003775 Density Functional Theory Methods 0.000 description 4
- 239000003792 electrolyte Substances 0.000 description 4
- 230000005484 gravity Effects 0.000 description 4
- 125000000864 peroxy group Chemical group O(O*)* 0.000 description 4
- 230000005588 protonation Effects 0.000 description 4
- 208000018459 dissociative disease Diseases 0.000 description 3
- 238000005182 potential energy surface Methods 0.000 description 3
- 208000001408 Carbon monoxide poisoning Diseases 0.000 description 2
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 2
- 230000010757 Reduction Activity Effects 0.000 description 2
- BQCADISMDOOEFD-UHFFFAOYSA-N Silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 description 2
- 229910052802 copper Inorganic materials 0.000 description 2
- 239000010949 copper Substances 0.000 description 2
- 230000002596 correlated effect Effects 0.000 description 2
- 238000000151 deposition Methods 0.000 description 2
- 238000013461 design Methods 0.000 description 2
- 238000009826 distribution Methods 0.000 description 2
- 238000002848 electrochemical method Methods 0.000 description 2
- 229910052739 hydrogen Inorganic materials 0.000 description 2
- 239000001257 hydrogen Substances 0.000 description 2
- 125000004435 hydrogen atom Chemical class [H]* 0.000 description 2
- 239000007791 liquid phase Substances 0.000 description 2
- 239000002105 nanoparticle Substances 0.000 description 2
- 238000005457 optimization Methods 0.000 description 2
- 238000007254 oxidation reaction Methods 0.000 description 2
- 150000003839 salts Chemical class 0.000 description 2
- 238000004904 shortening Methods 0.000 description 2
- 229910052723 transition metal Inorganic materials 0.000 description 2
- 150000003624 transition metals Chemical class 0.000 description 2
- JSMRMEYFZHIPJV-UHFFFAOYSA-N C1C2CCC1C2 Chemical compound C1C2CCC1C2 JSMRMEYFZHIPJV-UHFFFAOYSA-N 0.000 description 1
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 239000011149 active material Substances 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 238000006555 catalytic reaction Methods 0.000 description 1
- 239000003638 chemical reducing agent Substances 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 230000001276 controlling effect Effects 0.000 description 1
- 230000008021 deposition Effects 0.000 description 1
- 238000003795 desorption Methods 0.000 description 1
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- 238000011156 evaluation Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 230000005283 ground state Effects 0.000 description 1
- 238000000731 high angular annular dark-field scanning transmission electron microscopy Methods 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 229940097156 peroxyl Drugs 0.000 description 1
- 238000010248 power generation Methods 0.000 description 1
- 238000009789 rate limiting process Methods 0.000 description 1
- 238000009790 rate-determining step (RDS) Methods 0.000 description 1
- 230000008707 rearrangement Effects 0.000 description 1
- 239000004065 semiconductor Substances 0.000 description 1
- 239000012279 sodium borohydride Substances 0.000 description 1
- 229910000033 sodium borohydride Inorganic materials 0.000 description 1
- 238000004758 underpotential deposition Methods 0.000 description 1
Images
Classifications
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- 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
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B13/00—Oxygen; Ozone; Oxides or hydroxides in general
- C01B13/02—Preparation of oxygen
- C01B13/0229—Purification or separation processes
- C01B13/0233—Chemical processing only
- C01B13/024—Chemical processing only by reduction
-
- 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
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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
- 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
- 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
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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
- B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
- B01J35/396—Distribution of the active metal ingredient
- B01J35/398—Egg yolk like
-
- 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
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- 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
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- 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
- H01M4/88—Processes of manufacture
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- 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
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- 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
- H01M4/921—Alloys or mixtures with metallic elements
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- 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
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- 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 an oxygen reduction method using the core-shell catalyst.
- platinum materials having high activity are mainly used as electrode catalysts for fuel cells.
- platinum is a rare metal and is also expensive, so there is a need to reduce its use.
- the core-shell structure is a structure using platinum, which is a highly active material only on the surface (shell) of catalyst particles, and using other materials for the inside (core) which do not contribute to catalytic reaction.
- Patent Document 1 has, as a core, at least one transition metal selected from the group consisting of nickel, copper, palladium, silver, ruthenium and the like, and is selected from the group consisting of platinum, nickel, copper and the like.
- a core-shell catalyst shelled with at least one transition metal is described. The core-shell catalyst achieves high catalytic activity by reducing carbon monoxide poisoning.
- Patent Document 1 describes, as a specific example, a core-shell catalyst having ruthenium as a core and platinum as a shell among the above-mentioned candidate substances, and the catalytic activity can be determined from the HAADF-STEM image. It is shown to be high in the (111) and (002) planes that make up the observed cuboctahedral particles.
- this core-shell catalyst is provided for the purpose of showing high catalytic activity in the hydrogen oxidation reaction which is the anode reaction of the fuel cell by reducing carbon monoxide poisoning, and it is preferable to use the cathode reaction of the fuel cell.
- the catalytic activity in certain oxygen reduction reactions There is no consideration regarding the catalytic activity in certain oxygen reduction reactions.
- Patent Document 1 describes, as another specific example, a core-shell catalyst having a core of nickel and a shell of platinum.
- the core-shell catalyst is described to have high catalytic activity when used in an oxygen reduction reaction as compared to a platinum nanoparticle catalyst.
- no specific description is given as to the active side of the reaction in this core-shell catalyst.
- ruthenium and nickel are electrochemically unstable due to their relatively low oxidation potential (large negative value). Therefore, a catalyst using ruthenium or nickel has a problem that it easily dissolves when used under severe conditions (strong acidity, high potential) in the cathode of a fuel cell.
- the present invention has been made in view of the above problems, and an object thereof is a catalyst used for an oxygen reduction reaction which is a cathode reaction of a fuel cell, which has a high electrochemical stability and is relatively inexpensive.
- the present invention is to provide a catalyst having a core-shell structure, which has higher cost performance of catalytic activity than when platinum particles are used as a catalyst while reducing the amount of platinum used as a core, and an oxygen reduction method using the catalyst. .
- the inventor of the present invention uses the first principle using CMD (Computational Material Design) (see Computer Material Design Introduction (see Hideaki Sakurai et al., Published by Osaka University Press, October 20, 2005) to achieve the above object. Calculations were carried out, and as a result of intensive studies, we focused on silver and palladium as core materials and came to the present invention.
- CMD Computer Material Design Introduction
- the core-shell catalyst according to the present invention is a core-shell catalyst which contains silver in its core and contains platinum in its shell layer and is used in an oxygen reduction reaction, wherein a plurality of platinum atoms constituting the shell layer A 111) or (001) plane is formed, and in the above-mentioned shell layer, the distance between the closest platinum atoms is 2.81 ⁇ to 2.95 ⁇ .
- the core-shell catalyst according to the present invention is a core-shell catalyst used in an oxygen reduction reaction which contains palladium in its core and contains platinum in its shell layer, and it has a face-centered cubic lattice structure by a plurality of platinum atoms constituting the shell layer. A 111) or (001) plane is formed, and in the above-mentioned shell layer, the distance between the closest platinum atoms is 2.783 ⁇ to 2.81 ⁇ .
- the core-shell catalyst in one aspect of the present invention contains silver in the core and platinum in the shell layer, and the (111) or (001) face of the face-centered cubic lattice is formed by a plurality of platinum atoms constituting the shell layer. While being formed, the distance between the closest platinum atoms in the shell layer is 2.81 ⁇ to 2.95 ⁇ .
- the core-shell catalyst in one aspect of the present invention comprises palladium in the core and platinum in the shell layer, and the (111) face or (001) of the face-centered cubic lattice by the plurality of platinum atoms constituting the shell layer. The face is formed, and the distance between the closest platinum atoms in the shell layer is 2.783 ⁇ to 2.81 ⁇ .
- an activation barrier exists only in the dissociative adsorption step in which oxygen molecules are dissociated into oxygen atoms and adsorbed on the catalyst surface, and then protons are attached to the oxygen atoms.
- the step of reacting to form water and the step of desorbing water from the catalyst surface can be a catalyst having no activation barrier.
- such characteristics of the core-shell catalyst in one aspect of the present invention are at the same level as catalyst particles consisting only of platinum.
- silver and palladium are more electrochemically stable than ruthenium and nickel and relatively less expensive than platinum.
- the activity of the catalyst surface is at the same level as that of catalyst particles consisting only of platinum, but the cost (material cost) of the entire catalyst can be reduced. That is, the cost performance as an index indicating the size of the catalyst activity relative to the cost of the entire catalyst is improved. Therefore, a catalyst for use in an oxygen reduction reaction which is a cathode reaction of a fuel cell, which uses platinum particles as a catalyst while reducing the amount of platinum using an electrochemically stable, relatively inexpensive material as a core It is possible to provide a catalyst having a core-shell structure, which has a cost performance of catalytic activity higher than in the case where it was, and an oxygen reduction method using the catalyst.
- FIG. 1 It is a figure which shows the adsorption site of an oxygen molecule at the time of seeing the (111) side of Pt ML Ag from the normal direction of (111) side, and (a)-(c) are HBF site, HTF site, respectively. And, it is a figure showing an F-NT-F site. It is a figure which shows the potential energy surface of the adsorption reaction of an oxygen molecule in (111) plane of Pt ML Ag, and (a)-(c) is respectively in HBF site, HTF site, and F-NT-F site It is a figure which shows a potential energy curved surface.
- FIG. 8 is a diagram showing the transition of the positional relationship between Pt and an oxygen atom when the dissociative adsorption reaction of oxygen molecules proceeds in the route I shown in (a) of FIG. 7, (a) is a molecular adsorption state, (B) shows an activated state, (c) shows a dissociated state, and (d) shows a stabilized state.
- FIG. 8 is a diagram showing the transition of the positional relationship between Pt and an oxygen atom when the dissociative adsorption reaction of oxygen molecules proceeds in a route J shown in (b) of FIG. 7, (a) is a molecular adsorption state, (B) is an activated state, (c) is a figure which shows a dissociative adsorption state.
- FIG. 1 It is a figure which shows (111) surface of ML Ag and Pt ML Pd.
- Pt, the Pt ML Pd, and Pt ML Ag (111) plane a graph showing the calculation result of the potential energy of the H 2 O formed.
- Pt, in (111) plane of Pt ML Pd, and Pt ML Ag is a diagram illustrating the magnitude of the oxygen dissociation step, OH formation process, and activation barrier of H 2 O forming step.
- (A) is a diagram showing a state density on the surface of the (111) plane of platinum, a diagram showing a state density in (b) the surface of the (111) plane of Pt ML Ag.
- HTH site in (001) plane of Pt ML Ag, HBH site BB sites, and, for TBT site is a graph showing changes in adsorption energy when oxygen molecules each is adsorbed to dissociate into oxygen atoms.
- (001) plane of Pt ML Ag it is a graph showing the calculation result of the potential energy of OH formation.
- Pt, and at (001) plane of Pt ML Ag it is a graph showing the calculation result of the potential energy of H 2 O formed.
- Pt, in (001) plane of Pt ML Ag, and Pt ML Pd it shows the magnitude of the oxygen dissociation step, OH formation process, and activation barrier of H 2 O forming step.
- a core-shell catalyst using silver or palladium as a core material and platinum as a shell material which is an embodiment of the present invention, has the same level of catalytic activity as platinum catalyst particles in an oxygen reduction reaction that occurs at the cathode of a fuel cell. And a catalyst capable of reducing the amount of platinum used.
- the (111) plane or the (001) plane of the face-centered cubic lattice is formed by a plurality of platinum atoms constituting the shell layer.
- the distance between the nearest platinum atoms in the shell layer is 2.81 ⁇ to 2.95 ⁇
- palladium the core material
- the distance between the nearest platinum atoms in the shell layer is 2. 783 ⁇ to 2.81 ⁇ .
- the distance between the closest platinum atoms in the shell layer may be the average of the distances between the closest platinum atoms in the plurality of platinum atoms present in the shell layer.
- the average value of the distance between such closest platinum atoms can be measured, for example, based on the X-ray absorption fine structure (XAFS) method.
- the method for producing the core-shell catalyst in one embodiment of the present invention is not particularly limited, and a chemical method such as a liquid phase reduction method or an electrochemical method such as an underpotential deposition method (UPD method) may be used.
- a chemical method such as a liquid phase reduction method or an electrochemical method such as an underpotential deposition method (UPD method) may be used.
- UPD method underpotential deposition method
- a salt containing platinum constituting the shell is added to a solution in which silver or palladium core particles are dispersed in a solution, or a solution in which a carrier supporting the core particles is suspended.
- a core-shell catalyst can be obtained by reducing platinum ions in a solution using hydrogen, a reducing agent such as sodium borohydride or alcohol, and depositing platinum on the core particles.
- a salt containing platinum constituting the shell is added to a solution in which silver or palladium core particles are dispersed in a liquid, or a solution in which a carrier supporting the core particles is suspended.
- the deposition rate of platinum on the core nanoparticle surface can be controlled to produce a core-shell catalyst.
- the distance between the closest platinum atoms in the shell layer varies with the thickness of the platinum layer.
- the core-shell catalyst in one embodiment of the present invention exhibits the same level of catalytic activity as the catalyst particles of platinum in the oxygen reduction reaction on the catalyst surface by being the distance between the nearest platinum atoms of the width described above.
- the present inventors conducted simulations using first principle calculations based on density functional theory to evaluate the catalytic activity of a core-shell catalyst using silver or palladium as a core material and platinum as a shell material.
- the first principle calculation is a calculation method based on the density functional theory that indicates that the energy of the ground state of the interacting multielectron system is determined by the density distribution of electrons (P. Hohenberg and W. Kohn, Phys. Rev. 136, B864 (1964), W. Kohn and LJ Sham, Phys. Rev. 140, A1133 (1965), or "Semiconductor Electronic Structure", Asakura Shoten, published by Akio Fujiwara, 3rd See chapter).
- First-principles calculations allow us to quantitatively discuss the electronic structure of matter without empirical parameters, and in fact many demonstrations show comparable effectiveness to experiments. In this simulation, calculation was performed using the general density gradient approximation method, which is currently the most accurate among the first principle calculations.
- a core-shell catalyst using silver as a core material and platinum as a shell material (hereinafter Pt ML Ag) and palladium as a core material and platinum as a shell material is used as a core-shell catalyst (hereinafter Pt ML Pd)
- Pt ML Pd a core-shell catalyst
- Pt a catalyst composed of only platinum
- each catalyst was a catalyst consisting of 6 atomic layers unless otherwise specified.
- a monoatomic layer of Pt was stacked on five atomic layers of Ag and Pd.
- the thickness of the platinum layer which is the shell layer is not limited to the monoatomic layer.
- Reactions progress in three pathways: (1) oxygen dissociation, (2) peroxyl dissociation, and (3) hydrogen peroxide dissociation as reaction models for the oxygen reduction reaction.
- the model is known.
- FIG. 1 is a view showing a reaction model of a pathway of molecular oxygen dissociation in an oxygen reduction reaction.
- oxygen molecules are adsorbed on the catalyst surface (O 2 + * ⁇ O 2 * ).
- * Represents the catalyst surface
- O 2 * represents that the oxygen molecule is adsorbed on the catalyst surface.
- oxygen molecules adsorbed on the catalyst surface are dissociated into oxygen atoms (O 2 * + * ⁇ O * + O * ).
- FIG. 2 is a view showing a reaction model of the pathway of peroxy dissociation in the oxygen reduction reaction.
- oxygen molecules are adsorbed on the catalyst surface (O 2 + * ⁇ O 2 * ).
- protons transferred from the anode side through the electrolyte react with oxygen molecules on the catalyst surface to form OOH on the catalyst surface (O 2 * + H + + + e ⁇ ⁇ OOH * ).
- OOH dissociates into an oxygen atom and OH (OOH * ⁇ O * + OH * ).
- an oxygen atom and protons catalyst surface react to form OH (O * + H + + e - ⁇ OH *).
- OH on the catalyst surface reacts with protons to form and desorb water (OH * + H + + e ⁇ ⁇ H 2 O).
- FIG. 3 is a view showing a reaction model of the pathway of hydrogen peroxide dissociation in the oxygen reduction reaction.
- oxygen molecules are adsorbed on the catalyst surface (O 2 + * ⁇ O 2 * ).
- protons transferred from the anode side through the electrolyte react with oxygen molecules on the catalyst surface to form OOH (O 2 * + H + + + e ⁇ ⁇ OOH * ).
- the OOH on the catalyst surface reacts with the proton to form H 2 O 2 on the catalyst surface (OOH * + H + + + e ⁇ ⁇ H 2 O 2 * ).
- H 2 O 2 on the catalyst surface is dissociated into OH (H 2 O 2 * ⁇ OH * + OH * ), OH on the catalyst surface reacts with protons, and water is generated and released (OH * + H + + e - ⁇ H 2 O ).
- adsorption of molecular oxygen to the catalyst surface first occurs in any reaction model of molecular oxygen dissociation, peroxy dissociation, and hydrogen peroxide dissociation.
- the (110) plane in the FCC structure has a lower in-plane atomic density as compared to other planes (eg, (111) plane, (001) plane). Therefore, in the (110) plane, it is predicted that adsorption of molecular oxygen, which is the first step of the oxygen reduction reaction, is likely to occur, as compared with other planes.
- (110) plane> (111) plane> (001) plane with respect to the reaction activity in the orientation plane of platinum.
- the (110) plane has a large surface energy as compared to other planes and is chemically unstable. Therefore, it is known that the (110) plane is difficult to form on the Pt particle surface as compared to other planes.
- the inventors of the present invention investigated the oxygen reduction reaction on the (110) plane of Pt ML Ag and Pt ML Pd, and obtained the following findings. That is, under the power generation conditions of the fuel cell (in the state where a voltage is applied), the (110) plane of Pt ML Ag has higher catalytic activity than the (110) plane of the catalyst (Pt) consisting only of platinum. It is also shown that the (110) face of Pt ML Pd exhibits similar catalytic activity.
- the (110) planes of Pt, Pt ML Ag, and Pt ML Pd all release generated water from the catalyst surface as the final step of the oxygen reduction reaction when the electrode potential is present. It has been found that there is an activation barrier in the process, which is the process of rate limiting the reaction.
- FIG. 4 is a view of the (111) plane in the FCC structure as viewed from the normal direction ([111] direction) of the (111) plane.
- a catalyst having a (111) face of the FCC structure on the surface On-top (hereinafter, Top) site, Bridge (hereinafter, B) site, HCP as adsorption sites of oxygen atoms.
- B Bridge
- HCP as adsorption sites of oxygen atoms.
- a total of four sites can be considered: the Hollow (hereinafter HH) site and the FCC Hollow (hereinafter FH) site.
- the Top site is an adsorption site present on the atoms of the first layer of the catalyst surface.
- the interatomic distance is equal between the [ ⁇ 110] direction which is the in-plane direction and the [0-11] direction which is the in-plane direction.
- the B site is an adsorption site existing between atoms.
- the HH site is an adsorption site present in the Hollow located above the atoms of the second layer below the first layer when viewed from the [111] direction.
- the FH site is an adsorption site present in Hollow located above the position (Hollow) surrounded by the atoms of the second layer below the first layer when viewed from the [111] direction.
- the HH site means that, when atoms are stacked on the first layer to form the ⁇ layer, the ⁇ layer, the first layer, and the second layer have a hexagonal close-packed structure (HCP). It is an adsorption site that exists in Hollow at the position where the atoms of the alpha layer are stacked when atoms are stacked to form.
- the FH site refers to the case where atoms are stacked so that the alpha layer, the first layer, and the second layer form a face-centered cubic structure (FCC) in the above case (the layer is ABCABC in the [111] direction in the FCC) ... periodically repeated), which are adsorption sites present in the hollow of the position where the atoms of the ⁇ layer are stacked.
- FCC face-centered cubic structure
- the symbol "-" should originally be added on the numeral representing the direction, but in the present specification, for convenience of notation, it is attached in front of the numeral.
- the difference between the energy (E 0 ) when the oxygen atom exists at an infinite distance from the catalyst surface and the energy (E) when the oxygen atom is adsorbed on the catalyst surface was the adsorption energy for each of the adsorption sites were determined adsorption energy of oxygen atoms.
- FIG. 5 is a view showing calculation results of adsorption energy of oxygen atom to each adsorption site on (111) plane of FCC structure for Pt, Pt ML Ag and Pt ML Pd.
- unstable indicates that the state in which the oxygen atom is adsorbed to the adsorption site is not stable.
- the adsorption energy was minimized at the FH site. From this, when oxygen atoms are adsorbed on the catalyst surface in the (111) plane of the FCC structure, it is stable to be adsorbed to the FH site in any of Pt, Pt ML Ag and Pt ML Pd. I know that there is.
- Top site, B site and NT site can be considered as adsorption sites when oxygen molecules are adsorbed on the catalyst surface.
- the NT site means a near top site, which is an adsorption site present in the vicinity of the top site.
- the state in which the oxygen molecule is adsorbed at the Top site means that the center of gravity of the oxygen molecule is located at the Top site, that is, the center of gravity of each of the two oxygen atoms.
- the middle point is in the state of being located at the Top site.
- the difference between the energy (E 0 ) when the oxygen molecule is present at a distance from the catalyst surface at infinity and the energy (E) when the oxygen molecule is adsorbed on the catalyst surface was the adsorption energy for each of the adsorption sites were determined adsorption energy of oxygen molecules.
- the arrangement of oxygen molecules (three-dimensional angle of oxygen molecules) can be considered innumerably, but in the following, as a result of calculation, the adsorption energy is low (the adsorption state is stable) oxygen molecules The same applies to the following figures.
- FIG. 6 is a view showing adsorption sites of oxygen molecules when the (111) plane of Pt ML Ag is viewed from the normal direction ([111] direction) of the (111) plane, and (a) to (c) ) Indicate the HBF site, the HTF site, and the F-NT-F site, respectively.
- the state where the oxygen molecule is adsorbed to the HBF site means that the center of gravity of the oxygen molecule is located at the B site and two oxygen atoms are arranged on the line connecting the HH site, the B site, and the FH site It means that it is in the state of being
- the state of being adsorbed to the HTF site means the similar state at the HH site, the Top site, and the FH site
- the state of being adsorbed to the F-NT-F site is the FH site, NT It means the same situation at the site, and another FH site.
- the oxygen molecule had an adsorption height (distance from oxygen molecule to the catalyst surface) of about 4.5 ⁇ , and an adsorption energy of about ⁇ 0.07 eV. That is, it can be seen that oxygen molecules are in a state of being gently adsorbed at a position slightly away from the catalyst surface (molecular adsorption state).
- oxygen molecules were brought close to the catalyst surface, and a simulation was conducted on the change in potential energy when the oxygen molecules are dissociated into oxygen atoms and adsorbed.
- FIG. 7 is a diagram showing the potential energy surface of the dissociative adsorption reaction of oxygen molecules on the (111) plane of Pt ML Ag, where (a) to (c) are the HBF site, the HTF site, and the F ⁇ , respectively.
- the potential energy surface at the NT-F site is shown.
- the horizontal axis represents the distance r between oxygen atoms
- the vertical axis represents the distance z between the oxygen molecule and the catalyst surface, and in the figure, contour lines of potential energy are shown for every 0.2 eV.
- the dissociative adsorption reaction of oxygen molecules at the HBF site on the (111) plane of Pt ML Ag is considered to proceed along a path along dotted line I shown in the figure.
- the point ⁇ in the figure indicates the molecular adsorption state in which oxygen molecules are adsorbed on the catalyst surface.
- the distance z decreases and the distance r increases as oxygen molecules approach the catalyst surface. Then, through the state of point ⁇ in the figure, the distance r is further increased, and it is understood that the oxygen molecule is dissociated and adsorbed to the oxygen atom.
- the potential energy at the point ⁇ is 1.4 eV larger than the molecular adsorption state (point ⁇ ), and it can be seen that there is an activation barrier in the path along the dotted line I.
- FIG. 8 is a diagram showing the transition of the positional relationship between Pt and an oxygen atom when the dissociative adsorption reaction of the oxygen molecule proceeds in the route I shown in (a) of FIG. 7, (a) is the molecule The adsorbed state, (b) shows an activated state, (c) shows a dissociated state, and (d) shows a stabilized state.
- the oxygen molecules are adsorbed at a position about 4.5 ⁇ away from the catalyst surface, and in this case, the distance between oxygen atoms does not change.
- the distance between oxygen atoms becomes slightly longer (activated state).
- an oxygen molecule dissociates to an oxygen atom and an oxygen atom adsorb
- the oxygen atoms move from the HH site to the FH site and rearrange. The results were obtained.
- the arrangement of oxygen atoms after this rearrangement is the same as the dissociative adsorption state at the F-NT-F site described later.
- FIG. 9 is a diagram showing the transition of the positional relationship between Pt and an oxygen atom when the dissociative adsorption reaction of the oxygen molecule proceeds in the route J shown in FIG. 7B, and FIG. (B) shows the activated state, (c) shows the dissociative adsorption state.
- oxygen molecules disassociate into oxygen atoms through an activated state in which the distance between oxygen atoms becomes slightly longer as they approach the catalyst surface from the molecular adsorption state. Then, it can be seen that oxygen atoms are adsorbed to the HH site and the FH site. In this case, the adsorbed oxygen atoms were not rearranged by structural relaxation.
- FIG. 10 shows the positional relationship between Pt and oxygen atoms when the dissociative adsorption reaction proceeds from the state where molecular oxygen is adsorbed to the F-NT-F site as shown in FIG. 7C. It is a figure which shows transition, (a) is a molecular-like adsorption state, (b) is an activation state, (c) has shown the dissociative adsorption state.
- oxygen molecules disassociate into oxygen atoms through an activated state in which the distance between oxygen atoms becomes slightly longer as they approach the catalyst surface from the molecular adsorption state. Then, it can be seen that oxygen atoms are adsorbed at two FH sites.
- FIG. 11 shows adsorption energy, activation barrier, and oxygen energy when oxygen molecules dissociate into oxygen atoms and are adsorbed to the HBF site, the HTF site, and the F-NT-F site on the (111) plane of Pt ML Ag, respectively. It is a figure which shows the calculation result of oxygen atomic distance.
- oxygen molecules molecularly adsorbed to the HBF site are dissociated into oxygen atoms and adsorbed, they are described as unstable because they become the arrangement of the F-NT-F site by structural relaxation.
- the distance between oxygen atoms is 3.40 ⁇ for the HTF site and 2.95 ⁇ for the F-NT-F site.
- the F-NT-F site has a lower activation barrier and a larger absolute value of the adsorption energy of the dissociatively adsorbed oxygen atoms than the HTF site. This indicates that in the (111) plane of Pt ML Ag, a reaction in which oxygen molecules dissociate into oxygen atoms and adsorb to two FH sites is relatively easy to occur.
- the dissociative adsorption of the F-NT-F site includes the case where the structural relaxation occurs from the HBF site to the F-NT-F site.
- oxygen molecules dissociate into oxygen atoms through the activated state in which the distance between oxygen atoms becomes a little longer as they approach the catalyst surface from the molecular adsorption state, and oxygen atoms are adsorbed to the two FH sites.
- state of adsorption the state of adsorption to the F-NT-F site
- the value of this activation barrier was 1.2 eV.
- the distance between the oxygen atoms adsorbed to the two FH sites was 2.78 ⁇ .
- protons (H + ) transferred from the anode side through the electrolyte react with oxygen atoms on the catalyst surface, and the catalyst surface to form the OH in the (O * + H + + + e - ⁇ OH *).
- protons (H + ) are transferred from hydronium ions (H 3 O + ) to oxygen atoms (O * ) adsorbed on the catalyst surface to form OH on the catalyst surface (O * + H 3 O + ⁇ OH * + H 2 O), simulation was performed.
- the (111) plane of Pt ML Ag shows the results of the simulation for the formation reaction of OH.
- FIG. 12 is a view showing a process in which protons are transferred from the hydronium ion to the oxygen atom adsorbed on the catalyst surface.
- the distance z2 was fixed, and the distance z1 was changed to simulate changes in the potential energy of OH formation.
- FIG. 13 is a diagram showing calculation results of potential energy of OH formation (first OH formation) in the (111) plane of Pt ML Ag.
- FIG. 14 is a graph showing calculation results of potential energy of OH formation (second OH formation) for dissociatively adsorbed oxygen atoms present in the vicinity of the formed OH in the (111) plane of Pt ML Ag is there.
- the abscissa represents the distance z1 between the oxygen atom and the proton on the catalyst surface
- the ordinate represents the potential energy
- the distance z2 between the oxygen atom in the catalyst surface and the oxygen atom contained in water molecules.
- the distance z3 is first shortened because the hydronium ion approaches the oxygen atom adsorbed on the catalyst surface. Then, next, protons are transferred from the hydronium ion to the oxygen atom adsorbed on the catalyst surface, the distance z1 becomes short, and OH is formed on the catalyst surface.
- the potential energy is lowest at a distance z1 of about 1.0 ⁇ , and the potential energy at a distance z1 of about 1.0 ⁇ is the distance z2 of 4.0 ⁇ , 3 It becomes lower as shorter as .5 ⁇ , 3.0 ⁇ and 2.5 ⁇ .
- the distance z2 is 2.0 ⁇
- the potential energy at a distance z1 of about 1.0 ⁇ is large because the hydronium ion is too close to the catalyst surface, but the same as the other distances z2 There is a valley of potential energy at a distance z1 of about 1.0 ⁇ .
- FIG. 15 is a view in which the (111) plane in the FCC structure is viewed from the normal direction of the (111) plane when OH is formed on the catalyst surface, and (a) to (c) are each Pt. , Pt ML Ag, and Pt ML Pd (111) plane is shown.
- OH formed on the (111) plane of Pt, Pt ML Ag, and Pt ML Pd is Top site, Top site and B site, and Top site, respectively. It is found that the adsorption to the From the fact that the stable adsorption site of the oxygen atom was the FH site in the (111) plane of Pt, Pt ML Ag, and Pt ML Pd (see FIG. 5), the hydronium was attached to the oxygen atom adsorbed on the catalyst surface After the process of transferring protons from ions, it is understood that OH adsorbed on the catalyst surface relaxes and moves (rearranges). Here, in the simulation, it is calculated that OH adsorbed on the catalyst surface moves after OH formation, but in the actual reaction, it moves during OH formation (in parallel with OH formation) Is also conceivable.
- FIG. 16 is a diagram showing calculation results of potential energy of H 2 O formation in (111) plane of Pt, Pt ML Pd, and Pt ML Ag.
- the potential energy of the state in which the generated H 2 O was adsorbed on the catalyst surface on the (111) plane of Pt ML Pd and Pt ML Ag was determined to calculate adsorption energy.
- the activation barrier has a negligible value throughout the reaction even in the step of desorbing generated H 2 O from the catalyst surface. I understood.
- FIG. 17 is a diagram showing the sizes of activation barriers in the oxygen dissociation step, the OH formation step, and the H 2 O formation step on the (111) plane of Pt, Pt ML Pd, and Pt ML Ag.
- the activation barrier has a negligible value throughout the reaction.
- the inventors of the present application simulated the density of states in the surface (face-centered cubic lattice (111) surface) of Pt ML Ag according to the present embodiment using the first principle calculation based on density functional theory. . The results are shown in FIG.
- FIG. 18 is a figure which shows the density of states in the surface ((111) surface of a face-centered cubic lattice) of platinum.
- B) in FIG. 18 is a diagram showing a state density in ((111) plane of a face-centered cubic lattice) surface Pt of Pt ML Ag.
- the density of states of one 5s orbital, three 5p orbitals, and five 5d orbitals are shown, and 0 on the horizontal axis indicates the Fermi level.
- the five s orbitals (dxy, dyz, dzz, dxz, dx2-y2) to be focused on are emphasized because the density of states of the s orbital and p orbital is small. Is shown.
- the present inventors have conducted density functional theory on the distance between the nearest platinum atoms including the (111) face of the face-centered cubic lattice of the core-shell particles (Pt ML Ag and Pt ML Pd) in the present embodiment.
- the simulation was performed using first principle calculation based on.
- the calculation results show that in the structure in which Pt of a monoatomic layer is stacked on Ag, the distance between the nearest platinum atoms is 2.95 ⁇ , and in the structure in which Pt of a monoatomic layer is stacked on Pd, The distance between the closest platinum atoms was 2.783 ⁇ .
- the distance between the closest platinum atoms in the Pt layer as a shell layer laminated on the surface of the core particle (Ag or Pd). That is, as the number of stacked layers increases, the distance between the closest platinum atoms in the Pt layer approaches the interatomic distance in Pt of bulk (for example, inside of a mass of Pt of micrometer order or more). As a result of calculation using the first principle calculation, the distance between the nearest platinum atoms in bulk Pt is 2.81 ⁇ .
- the distance between the closest platinum atoms in the shell layer of Pt ML Ag (the (111) face of the face-centered cubic lattice) in the present embodiment is 2.81 ⁇ to 2.95 ⁇ .
- the distance between the closest platinum atoms in the shell layer of Pt ML Pd (the (111) face of the face-centered cubic lattice) is 2.783 ⁇ to 2.81 ⁇ .
- Each Pt ML Ag and Pt ML Pd in this embodiment by the distance between the closest Pt atoms is in the range calculated above, bonding force of the oxygen atoms and the Pt becomes optimum, generation of H 2 O ⁇ There is no activation barrier for the dissociation reaction. If the distance between the nearest platinum atoms is narrower than this, the activation barrier of the adsorption reaction of oxygen molecules becomes large, and the oxygen reduction activity is lowered. On the other hand, when it becomes wider, an activation barrier for H 2 O generation / dissociation reaction appears, and a plurality of activation barriers exist in the oxygen reduction reaction, so the oxygen reduction activity is reduced.
- the shell layer is preferably one to three atomic layers.
- FIG. 19 is a view of the (001) plane in the FCC structure as viewed from the normal direction ([001] direction) of the (001) plane.
- a total of three sites of Top site, Bridge site and Hollow site can be considered as adsorption sites of oxygen atoms.
- the Top site is an adsorption site present on the atoms of the first layer of the catalyst surface.
- the Bridge site is an adsorption site that is present between the closest atoms.
- the hollow site is an adsorption site present at a position surrounded by four atoms.
- FIG. 20 is a graph showing changes in adsorption energy when oxygen molecules dissociate and adsorb to oxygen atoms for HTH site, HBH site, BB site and TBT site on the (001) plane of Pt ML Ag. is there.
- the energy in the initial state in which oxygen molecules exist at infinity from the catalyst surface was taken as a reference (0 eV).
- the initial state, the state of molecular adsorption on the catalyst surface, the activated state, and the state of oxygen molecules being dissociated into oxygen atoms and adsorbed on the catalyst surface are shown.
- the value of the activation barrier in the activated state is 0.28 eV, and the adsorption energy of the oxygen atom is as large as -1.4 eV in the BB site I understand. That is, in the (001) plane of Pt ML Ag, oxygen molecules dissociate into oxygen atoms through an activated state in which the distance between oxygen atoms becomes slightly longer as they approach the catalyst surface from the molecular adsorption state. It was found that the state in which oxygen atoms are adsorbed to two Bridge sites (the state adsorbed to BB sites) has the smallest activation barrier and a large absolute value of adsorption energy.
- FIG. 21 is a diagram showing the calculation results of the potential energy of OH formation in the (001) plane of Pt ML Ag.
- FIG. 22 is a diagram showing calculation results of potential energy of H 2 O formation in Pt and Pt ML Ag (001) plane.
- the potential energy of the state in which the generated H 2 O was adsorbed on the catalyst surface on the (001) plane of Pt ML Ag was determined to calculate the adsorption energy.
- the activation barrier had a negligible value in the entire reaction also in the step of desorbing generated H 2 O from the catalyst surface in the (001) plane of Pt ML Ag.
- FIG. 23 is a diagram showing the sizes of activation barriers in the oxygen dissociation step, the OH formation step, and the H 2 O formation step on the (001) plane of Pt, Pt ML Ag, and Pt ML Pd.
- the (001) plane of Pt ML Ag has an activation barrier of the same size as the (001) plane of Pt. doing.
- the OH formation step and the H 2 O formation step proceed without an activation barrier on any of Pt and Pt ML Ag (001).
- OH formation process occurs in each dissociative oxygen atom, OH formation by protonation of only one side and second OH formation in case of both protonation were evaluated, but activation is also possible in any OH formation process. It confirmed that there was no barrier. And it turned out that an activation barrier does not exist also in the process of desorbing generated H 2 O from the catalyst surface.
- the distance between the closest atoms in the (111) plane and the distance between the closest atoms in the (001) plane are the same with each other, so “2.5. The same is true for (001) as described for platinum interatomic distance and density of states.
- the reaction path of dissociative adsorption is dominant in the oxygen reduction reaction, and there is an activation barrier in the dissociative adsorption step of molecular oxygen, but in the subsequent steps It was found that there was no active barrier. Therefore, the dissociative adsorption process of oxygen molecules is the rate-limiting step.
- such (111) planes and (001) planes can be predominantly present.
- the core-shell catalyst can be used in place of catalyst particles consisting only of platinum to be used for the oxygen reduction reaction, and the amount of platinum used can be reduced.
- Ag and Pd as core materials are materials having high electrochemical stability and low cost compared to platinum, and can improve the catalytic activity at material cost.
- a catalyst for use in an oxygen reduction reaction which is a cathode reaction of a fuel cell, which uses platinum particles as a catalyst while reducing the amount of platinum using an electrochemically stable, relatively inexpensive material as a core It is possible to provide a catalyst having a core-shell structure, which has a cost performance of catalytic activity higher than in the case where it was. And the oxygen reduction method using the said catalyst can be provided.
- the core-shell catalyst according to the present invention is a core-shell catalyst used in an oxygen reduction reaction that contains silver in the core and contains platinum in the shell layer, and is face-centered cubic by the plurality of platinum atoms that constitute the shell layer.
- the (111) or (001) plane of the lattice is formed, and in the above-mentioned shell layer, the distance between the closest platinum atoms is 2.81 ⁇ to 2.95 ⁇ .
- the core-shell catalyst according to the present invention is a core-shell catalyst used in an oxygen reduction reaction which contains palladium in its core and contains platinum in its shell layer, and it has a face-centered cubic lattice structure by a plurality of platinum atoms constituting the shell layer. A 111) or (001) plane is formed, and in the above-mentioned shell layer, the distance between the closest platinum atoms is 2.783 ⁇ to 2.81 ⁇ .
- the shell layer is preferably a 1 to 3 atomic layer.
- an oxygen reduction method is an oxygen reduction method using the above-mentioned core-shell catalyst, wherein oxygen molecules are dissociated into oxygen atoms and adsorbed onto the (111) plane or the (001) plane; A process of forming a water molecule by reacting an oxygen atom adsorbed on the (111) plane or the (001) plane with a proton, and a process of removing the water molecule from the (111) plane or the (001) plane Including.
- the present invention can be suitably used as a catalyst for an oxygen reduction reaction, in particular, a cathode electrode catalyst of a fuel cell.
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Abstract
Description
触媒活性の評価の前提として、まず、燃料電池のカソード反応である酸素還元反応について説明する。
(2.1.酸素原子の吸着)
まず、酸素還元反応の第1段階である酸素分子の吸着を考える前に、酸素原子が触媒表面に吸着する場合について考える。
次に、酸素分子が触媒表面に吸着する場合を考える。表面にFCC構造の(111)面を有する触媒では、酸素分子が触媒表面に吸着する場合に、吸着サイトとして、Topサイト、Bサイト、NTサイトが考えられる。NTサイトとは、Near Topサイトを意味しており、Topサイトの近傍に存在する吸着サイトである。ここで、例えば、酸素分子がTopサイトに吸着している状態とは、酸素分子の重心がTopサイトに位置する状態であることを意味しており、すなわち、二つの酸素原子のそれぞれの重心の中点がTopサイトに位置する状態であるという事である。
PtMLAg、およびPtMLPdの(111)面において、酸素分子の触媒表面への分子状吸着は、触媒表面から少し離れた位置にて緩やかに吸着する状態であること、また、酸素分子が、活性化障壁を超えて酸素原子に解離して触媒表面に吸着した状態が安定であることが分かった。そのため、以下では、酸素還元反応の反応経路の1つである解離吸着の経路において、解離吸着の次段階の反応であるOHの形成について検討を行う。
PtMLAg、およびPtMLPdの(111)面において、活性化障壁無しで解離吸着した2個の酸素原子から2個のOHが形成されることが分かった。次に、酸素還元反応の反応経路の1つである解離吸着の経路において、OHの形成の次段階の反応であるH2Oの形成について検討を行う。H2Oの形成では、触媒表面のOHと、プロトンとが反応し、H2Oが生成する(OH*+H++e-→H2O)。ここでは、ヒドロニウムイオンから、触媒表面のOHに対してプロトンが受け渡され、H2Oが形成されるものとし(OH*+H3O+→H2O*+H2O)、シミュレーションを行った。
ここで、従来、下記(i)および(ii)の知見が知られている。
上記したことと同様のシミュレーションを、FCC構造における(001)面について行った結果を以下に示す。
このように、本発明の一実施形態におけるコアシェル触媒は、酸素還元反応のうち解離吸着の反応経路が支配的であり、酸素分子の解離吸着工程に活性化障壁が存在するが、その後の工程では活性障壁が存在しないことがわかった。そのため、酸素分子の解離吸着工程が律速段階となっている。
Claims (4)
- 銀をコアに含み、白金をシェル層に含む酸素還元反応に用いるコアシェル触媒であって、
上記シェル層を構成する複数の白金原子によって面心立方格子の(111)面または(001)面が形成されているとともに、
上記シェル層において、最近接白金原子間の距離が2.81Å~2.95Åであることを特徴とするコアシェル触媒。 - パラジウムをコアに含み、白金をシェル層に含む酸素還元反応に用いるコアシェル触媒であって、
上記シェル層を構成する複数の白金原子によって面心立方格子の(111)面または(001)面が形成されているとともに、
上記シェル層において、最近接白金原子間の距離が2.783Å~2.81Åであることを特徴とするコアシェル触媒。 - 上記シェル層は1~3原子層であることを特徴とする請求項1または2に記載のコアシェル触媒。
- 請求項1から3のいずれか1項に記載のコアシェル触媒を用いる酸素還元方法であって、
上記(111)面または(001)面に、酸素分子が酸素原子に解離して吸着する工程と、
上記(111)面または(001)面に吸着した酸素原子と、プロトンとを反応させ、水分子を形成する工程と、
上記(111)面または(001)面から水分子を脱離する工程とを含むことを特徴とする酸素還元方法。
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WO2022138270A1 (ja) | 2020-12-22 | 2022-06-30 | 田中貴金属工業株式会社 | 酸素還元反応用のコアシェル触媒及び触媒の設計方法 |
CN114914459A (zh) * | 2022-06-20 | 2022-08-16 | 燕山大学 | 一种筛选锰基单原子催化剂催化活性的阴离子表面修饰方法 |
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KR102256751B1 (ko) | 2021-05-25 |
JP7103604B2 (ja) | 2022-07-20 |
US20200122123A1 (en) | 2020-04-23 |
KR20190103392A (ko) | 2019-09-04 |
TWI660777B (zh) | 2019-06-01 |
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