WO2015029602A1 - コアシェル触媒及びコアシェル触媒の製造方法 - Google Patents
コアシェル触媒及びコアシェル触媒の製造方法 Download PDFInfo
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- WO2015029602A1 WO2015029602A1 PCT/JP2014/068057 JP2014068057W WO2015029602A1 WO 2015029602 A1 WO2015029602 A1 WO 2015029602A1 JP 2014068057 W JP2014068057 W JP 2014068057W WO 2015029602 A1 WO2015029602 A1 WO 2015029602A1
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Images
Classifications
-
- 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
- H01M4/928—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
- H01M4/88—Processes of manufacture
- H01M4/8825—Methods for deposition of the catalytic active composition
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/08—Fuel cells with aqueous electrolytes
-
- 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 method for producing the core-shell catalyst.
- Fuel cells convert chemical energy directly into electrical energy by supplying fuel and oxidant to two electrically connected electrodes and causing the fuel to oxidize electrochemically. Therefore, since the fuel cell is not subject to the Carnot cycle, it exhibits high energy conversion efficiency.
- a fuel cell is usually configured by laminating a plurality of single cells having a basic structure of a membrane electrode assembly (MEA) in which an electrolyte membrane is sandwiched between a pair of electrodes.
- MEA membrane electrode assembly
- platinum catalysts and platinum alloy catalysts having high catalytic activity have been employed as electrode catalysts used in fuel cells.
- platinum is expensive and has a small amount of resources, and a reduction in the amount of platinum is required.
- the catalyst using platinum is very expensive, the catalytic reaction occurs only on the particle surface, and the inside of the particle hardly participates in the catalytic reaction. Therefore, the catalyst activity with respect to material cost in the catalyst using platinum was not necessarily high.
- Patent Document 1 discloses a core-shell catalyst in which particles containing palladium are used as a core and covered with a shell containing platinum.
- Patent Document 1 describes the particle diameter of the core-shell catalyst and the number of shell layers.
- the core-shell catalyst for realizing high battery performance when actually configuring a single cell of a fuel cell is not sufficiently specified only by the above-described index described in Patent Document 1. It was found.
- the present invention has been accomplished in view of the above circumstances, and provides a core-shell catalyst capable of achieving high performance of a single cell of a fuel cell and a method for producing the core-shell catalyst.
- the core-shell catalyst of the present invention is a core-shell catalyst comprising a core containing palladium and a shell containing platinum and covering the core,
- the average particle size is 4.70 nm or less
- the standard deviation is 2.00 nm or less
- the frequency of the particle size is 5.00 nm or less is 55% or more. It is characterized by. According to the present invention, it is possible to improve the power generation performance of a single fuel cell.
- the frequency of the core-shell catalyst of the present invention is preferably 71% or more.
- the average particle size is preferably 4.40 nm or less.
- the standard deviation is preferably 1.60 nm or less.
- the present invention it is also possible to provide a core-shell catalyst in which the average thickness of the shell is 0.20 to 0.35 nm.
- the method for producing the core-shell catalyst of the present invention is a method for producing the core-shell catalyst of the present invention, In the number-based particle size frequency distribution, the average particle size is 4.40 nm or less, the standard deviation is 2.00 nm or less, and the frequency of the particle size is 5.00 nm or less is 65% or more.
- a platinum-containing shell is deposited on the surface of the palladium-containing particles. According to the method for producing a core-shell catalyst of the present invention, it is possible to produce a core-shell catalyst in which the platinum-containing shell has an average thickness of 0.20 to 0.35 nm.
- the core-shell catalyst of the present invention high performance of the fuel cell can be achieved.
- FIG. 3 is a graph showing the particle size distribution of Pd particles used in Example 1 and the core-shell catalyst of Example 1. It is a figure which shows the relationship between the current density and cell voltage of Examples 1-7 and Comparative Examples 1-3.
- FIG. 6 is a graph showing the relationship between the average particle size (nm) and the cell voltage (@ 2.6 A / cm 2 ) (V) in Examples 1 to 7 and Comparative Examples 1 to 3. It is a figure which shows the relationship between the standard deviation (nm) and cell voltage (@ 2.6A / cm ⁇ 2 >) (V) in Examples 1-7 and Comparative Examples 1-3.
- the core containing palladium (hereinafter sometimes referred to as Pd-containing core) is a generic term for a core made of palladium and a core made of palladium alloy.
- palladium-containing particles (hereinafter sometimes referred to as Pd-containing particles) is a general term for palladium particles and palladium alloy particles.
- the palladium alloy include alloys of palladium and metal materials selected from the group consisting of iridium, ruthenium, rhodium, iron, cobalt, nickel, copper, silver, and gold, and metals other than palladium constituting the palladium alloy.
- the palladium alloy preferably has a palladium content of 50 mass% or more and less than 100 mass% when the mass of the entire alloy is 100 mass%. This is because a uniform Pt-containing shell can be formed when the palladium content is 50% by mass or more.
- the platinum-containing shell (hereinafter sometimes referred to as Pt-containing shell) is a general term for a shell made of platinum and a shell made of a platinum alloy.
- the platinum alloy include an alloy with a metal material selected from the group consisting of iridium, ruthenium, rhodium, nickel, and gold.
- the metal other than platinum constituting the platinum alloy may be one type or two or more types.
- the platinum alloy preferably has a platinum content of 50 mass% or more and less than 100 mass% when the mass of the entire alloy is 100 mass%. This is because if the platinum content is less than 50% by mass, sufficient catalytic activity and durability cannot be obtained.
- the shell covering the core means not only a form in which the entire surface of the core is covered with the shell, but also a part of the surface of the core is covered with the shell, and a part of the surface of the core is exposed. The form which is doing is also included.
- the core-shell catalyst of the present invention is a core-shell catalyst comprising a core containing palladium and a shell containing platinum and covering the core,
- the average particle size is 4.70 nm or less
- the standard deviation is 2.00 nm or less
- the frequency of the particle size is 5.00 nm or less is 55% or more. It is characterized by.
- the inventor is a core-shell catalyst in which a Pd-containing core is coated with a Pt-containing shell (hereinafter, sometimes referred to as a Pt / Pd core-shell catalyst), and has high battery performance when a single cell of a fuel cell is configured.
- a Pt / Pd core-shell catalyst in which a Pd-containing core is coated with a Pt-containing shell
- the conventional Pt / Pd core-shell catalyst has a problem that the surface of Pd-containing particles having a large particle diameter of 5.00 nm or more is difficult to be covered with the Pt-containing shell during the production process.
- the present inventor used the TEM-EDS (transmission electron microscope-energy dispersive X-ray analysis) for the conventional Pt / Pd core-shell catalyst to determine the particle size of the Pt / Pd core-shell catalyst and the Pt / Pd core-shell catalyst.
- the relationship with the atomic ratio of platinum and palladium in the catalyst was measured (dots in FIG. 1).
- a fine Pt / Pd core-shell catalyst having a particle size of 5 nm or less has a large Pt ratio and tends to exceed the 1 ML line. This is thought to be because when the Pd-containing particles that are the raw material of the Pt / Pd core-shell catalyst have a large particle size, the Pt-containing shell is difficult to form, and the fine Pd-containing particles are preferentially coated with the Pt-containing shell. It is done.
- FIG. 2 shows reaction coordinates and free energy (Gibbs energy) when a Pt shell is formed on the surface of Pd particles and a Pt / Pd core shell is formed.
- free energy As shown in FIG. 2, first, Pd-containing particles having a small particle diameter have higher free energy (Gibbs energy) than Pd-containing particles having a large particle diameter, and are thermally unstable. it is conceivable that.
- the Pd-containing particles having a small particle size have smaller activation energy until the monolayer Pt layer is formed (E1 ⁇ E3) than the Pd-containing particles having a large particle size, which is advantageous in terms of kinetics. It is believed that there is. Furthermore, the Pd-containing particles having a small particle size are smaller than the Pd-containing particles having a large particle size even after the monolayer Pt layer is formed (about 0.5 nm particle size increase). It is considered that the second layer of Pt precipitates (forms a Pt—Pt bond) and reduces the amount of heat. At this time, the heat of formation ⁇ E for the formation of the Pt—Pt bond is estimated to be large.
- the present inventor has found that, in the number-based particle size frequency distribution, the average particle size is 4.70 nm or less, and the standard deviation is 2.00 nm or less, and In order to complete the present invention, it was found that a Pt / Pd core-shell catalyst having a particle size of 5.00 nm or less (hereinafter sometimes simply referred to as a frequency) having a frequency of 55% or more exhibits excellent power generation performance. It came. Specifically, a single cell using a core-shell catalyst whose average particle diameter, standard deviation, and frequency are all within the above range can obtain a high voltage in a high current density region (under high load conditions). Was found by the inventors.
- the output of the fuel cell can be increased.
- the reason why a high voltage can be obtained in a high current density region by using the core-shell catalyst of the present invention is considered as follows. That is, in the core-shell catalyst of the present invention, first, the Pd-containing core has a high coverage with the Pd-containing shell, so that the exposed Pd is small, and further, the Pt-containing shell is uniformly formed on the surface of the Pd-containing core. Guessed. For this reason, it is considered that the specific surface area of Pt is larger than that in the prior art.
- gas diffusion is more dominant than catalytic activity under high load conditions such as a high current density region, but due to the increase in the specific surface area of Pt, the contact area between Pt and the reaction gas increases. It is considered that a high voltage can be obtained.
- the average particle diameter, standard deviation, and frequency of the core-shell catalyst are values in a number-based particle diameter frequency distribution, and are values of primary particles.
- the average particle size, standard deviation, and frequency are determined by measuring the particle size of 600 or more core-shell catalysts by image analysis using a TEM (transmission electron microscope), and measuring the particle size distribution histogram (FIG. 3). Reference) can be created and requested.
- the particle diameter of the core-shell catalyst is a diameter value calculated by converting the projected area of each particle in TEM image analysis into a circle, and was calculated by regarding the projected area of each particle as being equivalent to a perfect circle.
- the frequency with a particle size of 5.00 nm or less means the ratio which the particle
- the frequency of the core-shell catalyst of the present invention may be 55% or more, but from the viewpoint that the voltage increasing effect in a high current density region is particularly high, 71% or more, particularly 73% or more, and further 75% or more, Of these, 84% or more is preferable. Further, from the viewpoint of obtaining a voltage increasing effect even in a low current density region (under a low load condition), it is preferably 58% or more, particularly 60% or more, more preferably 70% or more, and particularly preferably 84% or more.
- the average particle diameter of the core-shell catalyst of the present invention may be 4.70 nm or less. However, since the effect of increasing voltage in a high current density region is high, when the frequency is 71% or more, 4.40 nm. In particular, it is particularly preferably 4.10 nm or less, more preferably 3.90 nm or less. On the other hand, from the viewpoint of mass activity of the core-shell catalyst, the average particle size of the core-shell catalyst is usually preferably 2.50 nm or more.
- the standard deviation of the core-shell catalyst of the present invention may be 2.00 nm or less, but since the effect of increasing the voltage in the high current density region is high, when the frequency is 71% or more, 1.60 nm or less. In particular, it is preferably 1.10 nm or less, more preferably 0.80 nm or less.
- the Pt-containing shell may be either a shell made of platinum or a shell made of a platinum alloy, but is usually preferably a shell made of platinum. According to the present invention, it is possible to provide a Pt / Pd core-shell catalyst having a Pt-containing shell having an average thickness of 0.50 nm or less, further 0.40 nm or less, and further 0.35 nm or less. In addition, since the thickness of the Pt monolayer is 0.20 nm, the average thickness of the Pt-containing shell is preferably 0.20 nm or more. In the examples described later, it has been confirmed that the average thickness of the Pt-containing shell is 0.21 to 0.33 nm.
- the average particle diameter D ave1 of the Pt / Pd core-shell catalyst can be calculated in the same manner as described above.
- the average particle diameter D ave2 of the Pd-containing core is, for example, the value obtained by measuring and calculating the average particle diameter of the Pd-containing particles (raw material particles) before coating with the Pt-containing shell, Can be considered.
- the average particle diameter of the Pd-containing particles may change depending on the pretreatment performed for the purpose of cleaning or the like before the coating treatment with the Pt-containing shell. Therefore, the average particle diameter of the Pd-containing particles can be measured and calculated after the pretreatment has been performed and the average particle diameter equivalent to that of the Pd-containing core in the Pt / Pd core-shell catalyst can be maintained. preferable.
- the average particle diameter of the Pd particles measured and calculated after the pretreatment is regarded as the average particle diameter of the Pd core, and from the difference between the average particle diameter of the Pd particles and the average particle diameter of the Pt / Pd core-shell catalyst.
- the average thickness of the Pt-containing shell is calculated. Further, in the examples, after forming a Cu layer on the surface of the Pd particles by an under-potential method (UPD method), which will be described later, the Cu layer is replaced with Pt, whereby Pt / Pd in which the Pd core is covered with a Pt shell.
- UPD method under-potential method
- the core-shell catalyst is manufactured, in the process of forming the Pt-containing shell by such Cu-UPD method and Pt substitution, elution of the Pd-containing particles subjected to the above pretreatment does not occur, and before and after the formation of the Pt-containing shell It is considered that the average particle size of the Pd-containing particles does not change.
- examples of the Pd-containing core include a core made of palladium and a core made of a palladium alloy, but a Pd core made of palladium is usually preferable.
- the average particle size of the Pd-containing core is not particularly limited as long as it is less than the average particle size of the Pt / Pd core-shell catalyst.
- the number-based particle size frequency distribution is preferably 4.40 nm or less, and from the viewpoint of effective use of platinum, it is preferably 2.00 nm or more.
- the core-shell catalyst of the present invention may be supported on a conductive carrier.
- the conductive carrier include Ketjen Black (trade name: manufactured by Ketjen Black International Co., Ltd.), Vulcan (trade name: manufactured by Cabot), Norit (trade name: manufactured by Norit), and Black Pearl (trade name). : Manufactured by Cabot) and OSAB (trade name: carbon particles such as acetylene black manufactured by Denki Kagaku Kogyo Co., Ltd. and Chevron, conductive carbon materials such as carbon fibers; metal materials such as metal particles and metal fibers; Can be mentioned.
- the average particle size of the conductive carrier is not particularly limited, but is preferably 0.01 to several hundred ⁇ m, more preferably 0.01 to 1 ⁇ m. If the average particle size of the conductive support is less than the above range, the conductive support may be deteriorated by corrosion, and the Pt / Pd core-shell catalyst supported on the conductive support may fall off over time. . Moreover, when the average particle diameter of the conductive support exceeds the above range, the specific surface area is small, and the dispersibility of the Pt / Pd core-shell catalyst may be reduced.
- the specific surface area of the conductive carrier is not particularly limited, but is preferably 50 to 2000 m 2 / g, more preferably 100 to 1600 m 2 / g. If the specific surface area of the conductive support is less than the above range, the dispersibility of the Pt / Pd core-shell catalyst in the conductive support may be reduced. Moreover, when the specific surface area of an electroconductive support
- the Pt / Pd core-shell catalyst loading ratio [ ⁇ (Pt / Pd core-shell catalyst mass) / (Pt / Pd core-shell catalyst mass + conductive carrier mass) ⁇ ⁇ 100%] by the conductive support is not particularly limited. The range of 20 to 60% is preferable.
- the method for producing the core-shell catalyst of the present invention is not particularly limited. For example, it can be produced by the method for producing the core-shell catalyst of the present invention described below.
- the production method of the core-shell catalyst of the present invention is a production method of the core-shell catalyst of the present invention, In the number-based particle size frequency distribution, the average particle size is 4.40 nm or less, the standard deviation is 2.00 nm or less, and the frequency of the particle size is 5.00 nm or less is 65% or more.
- a platinum-containing shell is deposited on the surface of the palladium-containing particles.
- the average particle size, standard deviation, and frequency of the Pd-containing particles are values in a number-based particle size frequency distribution as well as the core-shell catalyst, and are values of primary particles.
- the average particle size, standard deviation, and frequency of the Pd-containing particles were determined by measuring the particle size of 600 or more Pd-containing particles by TEM (transmission electron microscope) image analysis as in the case of the core-shell catalyst. A histogram (see FIG. 3) can be created and determined.
- the particle diameter of the Pd-containing particles is a diameter value calculated by converting the projected area of each particle in the TEM image analysis into a circle as in the case of the core-shell catalyst, and the projected area of each particle is regarded as being equivalent to a perfect circle.
- the average particle diameter, standard deviation, and frequency of the Pd-containing particles are preferably set to values immediately before the Pt-containing shell is precipitated (coating treatment with the Pt-containing shell).
- the average particle diameter of the Pd-containing particles may be changed by the pretreatment performed for the purpose of cleaning or the like before the coating treatment with the Pt-containing shell.
- the standard deviation and frequency may change. Therefore, it is preferable to measure and calculate these values after the pretreatment in which the average particle diameter, standard deviation, and frequency of the Pd-containing particles are not changed or hardly changed.
- the Pd-containing particles As a specific pretreatment of the Pd-containing particles, a general method can be adopted, and examples thereof include a hydrogen bubbling treatment in pure water or an acidic solution, a potential cycle, and the like. It is also possible to combine hydrogen bubbling and potential cycling. Specific methods, conditions, and the like of the hydrogen bubbling treatment and potential cycle can be set as appropriate.
- the acidic solution includes a solution containing an acid such as sulfuric acid, and the specific pH, treatment time, etc. of the acidic solution can be appropriately set.
- the potential scanning range of the potential cycle is, for example, 0.1 to 1 V (vs. RHE), and the number of cycles, the scanning speed, etc. may be set as appropriate.
- the potential cycle can be performed, for example, in an acidic solution.
- Pd-containing particles include palladium particles and palladium alloy particles, but palladium particles are usually preferred.
- the frequency of the Pd-containing particles may be 65% or more, but is preferably more than 82%, particularly 83% or more because a core-shell catalyst having a particularly high voltage increase effect in a high current density region can be easily obtained. Furthermore, it is preferably 84% or more, more preferably 89% or more. Further, even in a low current density region (under a low load condition), since a core-shell catalyst exhibiting a high voltage is easily obtained, 71% or more, particularly 72% or more, further 82% or more, and particularly 89% or more is preferable.
- the average particle diameter of the Pd-containing particles may be 4.40 nm or less, but since a core-shell catalyst having a high voltage increase effect in a high current density region is easily obtained, when the frequency is greater than 82%, It is preferably 3.80 nm or less, particularly 3.60 nm or less, and more preferably 3.40 nm or less.
- the average particle size of the Pd-containing particles is usually preferably 2.00 nm or more.
- the standard deviation of the Pd-containing particles may be 2.00 nm or less, but a core-shell catalyst having a high voltage increase effect in a high current density region is easily obtained. .40 nm or less, particularly 1.30 nm or less, more preferably 1.20 nm or less.
- the Pd-containing particles may be supported on a conductive carrier. Since the conductive carrier has been described in the item of the core-shell catalyst, description thereof is omitted here. A commercially available product can be used as the Pd particle carrier carried on the conductive carrier, or can be synthesized.
- a method for supporting the Pd-containing particles on the conductive support a conventionally used method can be employed. For example, there may be mentioned a method in which Pd-containing particles are mixed in a conductive carrier dispersion liquid in which a conductive carrier is dispersed, filtered, washed, redispersed in ethanol or the like and then dried with a vacuum pump or the like. After drying, heat treatment may be performed as necessary. In the case where Pd alloy particles are used, the synthesis of Pd alloy particles and the loading of the Pd alloy particles on the carrier may be performed simultaneously.
- the method for depositing the Pt-containing shell on the surface of the Pd-containing particles is not particularly limited, and a known method can be adopted.
- the Pt-containing shell may be deposited on the surface of the Pd-containing particles by a one-step reaction such as electrolytic plating or electroless plating.
- a metal layer for example, a copper layer
- UPD underpotential deposition
- Shells may be deposited.
- a method of depositing a Pt-containing shell on the surface of Pd-containing particles will be described by taking an example of a method using Cu-UPD.
- Cu-UPD is a phenomenon in which a monoatomic layer of Cu in a metallic state is formed on a surface of a dissimilar metal having strong bonding strength with Cu at a potential nobler than the redox potential of Cu.
- the Pd-containing core having a Cu atomic layer formed by Cu-UPD on the surface is immersed in a solution containing Pt ions, and the Cu is replaced with Pt by utilizing the difference in ionization tendency.
- a core-shell catalyst coated with a containing shell can be produced.
- a Cu atom layer can be deposited on the surface of the Pd-containing particles by applying a potential that is nobler than the reduction potential of Cu. Further, since Cu has a higher ionization tendency than Pt, Cu on the surface of Pd-containing particles can be replaced with Pt, and a core-shell catalyst in which the surface of Pd-containing particles is coated with Pt can be produced.
- the step of forming a Cu atomic layer on the surface of the Pd-containing particles and the step of replacing Cu on the Pd-containing particles with Pt will be described in order.
- Step of forming a Cu atomic layer on the surface of Pd-containing particles (Cu-UPD step) By applying a potential nobler than the redox potential (equilibrium potential) of Cu to Pd-containing particles in contact with an electrolytic solution containing Cu ions (for example, immersed in the electrolytic solution), the surface of the Pd-containing particles It is possible to cause precipitation of a copper atomic layer by Cu-UPD.
- a potential nobler than the redox potential (equilibrium potential) of Cu to Pd-containing particles in contact with an electrolytic solution containing Cu ions (for example, immersed in the electrolytic solution), the surface of the Pd-containing particles It is possible to cause precipitation of a copper atomic layer by Cu-UPD.
- the electrolytic solution containing Cu ions (hereinafter sometimes referred to as Cu ion-containing electrolytic solution) is not particularly limited as long as it can deposit copper on the surface of the Pd-containing particles by Cu-UPD.
- the Cu ion-containing electrolytic solution is usually composed of a solution obtained by dissolving a predetermined amount of a copper salt in a solvent, but is not particularly limited to this configuration, and some or all of the Cu ions are dissociated and exist in the liquid. Any electrolyte solution may be used.
- the solvent used for the Cu ion-containing electrolytic solution include water and organic solvents, but water is preferable from the viewpoint of not preventing the precipitation of Cu on the surface of the Pd-containing particles.
- the copper salt used in the Cu ion-containing electrolyte include copper sulfate, copper nitrate, copper chloride, copper chlorite, copper perchlorate, and copper oxalate.
- the Cu ion concentration is not particularly limited, but is preferably 10 to 1000 mM.
- Cu ion containing electrolyte solution may contain an acid etc. other than the above-mentioned solvent and copper salt, for example.
- Specific examples of the acid that can be added to the Cu ion-containing electrolyte include sulfuric acid, nitric acid, hydrochloric acid, chlorous acid, perchloric acid, and oxalic acid.
- the counter anion in Cu ion containing electrolyte solution and the counter anion in an acid may be the same, and may differ.
- an inert gas is bubbled in advance in the electrolytic solution. This is because the oxidation of the Pd-containing particles is suppressed and uniform coating with the Pt-containing shell is possible. Nitrogen gas, argon gas, etc. can be used as the inert gas.
- the Pd-containing particles may be immersed and dispersed in the electrolyte solution by adding them to the electrolyte solution in a powder state, or the Pd-containing particle dispersion solution is prepared by previously dispersing the Pd-containing particles in a solvent. You may immerse and disperse
- the solvent used in the Pd-containing particle dispersion the same solvent as that used in the above-described Cu ion-containing electrolytic solution can be used.
- the Pd-containing particle dispersion may contain the acid that can be added to the Cu ion-containing electrolyte.
- the Pd-containing particles may be fixed on the conductive substrate or the working electrode, and the Pd-containing particle fixing surface of the conductive substrate or the working electrode may be immersed in the electrolytic solution.
- a Pd-containing particle paste is prepared using an electrolyte resin (for example, Nafion (trade name) or the like) and a solvent such as water or alcohol, and the conductive substrate or action The method of apply
- a method of applying a potential nobler than the redox potential of Cu to the Pd-containing particles is not particularly limited, and a general method can be adopted.
- a method of immersing a working electrode, a counter electrode, and a reference electrode in an electrolytic solution containing Cu ions, and applying a potential nobler than the oxidation-reduction potential of Cu to the working electrode can be mentioned.
- the working electrode for example, a metal material such as titanium, platinum mesh, or platinum plate, and a material that can ensure conductivity, such as a conductive carbon material such as glassy carbon or carbon plate, can be used.
- the reaction vessel can be formed of the conductive material and function as a working electrode.
- reaction vessel made of a metal material When using a reaction vessel made of a metal material as a working electrode, it is preferable to coat the inner wall of the reaction vessel with RuO 2 from the viewpoint of suppressing corrosion.
- a carbon material reaction vessel When a carbon material reaction vessel is used as a working electrode, it can be used as it is without coating.
- the counter electrode for example, a platinum mesh plated with platinum black and conductive carbon fiber can be used.
- a reversible hydrogen electrode (RHE), a silver-silver chloride electrode, a silver-silver chloride-potassium chloride electrode, or the like can be used.
- RHE reversible hydrogen electrode
- a silver-silver chloride electrode As the reference electrode, a reversible hydrogen electrode (RHE), a silver-silver chloride electrode, a silver-silver chloride-potassium chloride electrode, or the like can be used.
- the potential to be applied is not particularly limited as long as it is a potential at which Cu can be deposited on the surface of the Pd-containing particles, that is, a potential nobler than the oxidation-reduction potential of Cu, but for example, 0.35 to 0.4 V It is preferably within the range of (vs. RHE).
- the time for applying the potential is not particularly limited, but it is preferably secured for 60 minutes or more, more preferably until the reaction current becomes steady and approaches zero.
- the application of the potential may be performed by sweeping the potential in a range including the potential range. Specifically, the potential range to be swept is preferably 0.3 to 0.8 V (vs. RHE).
- the number of potential sweep cycles is not particularly limited, but is preferably 1 to 20 cycles.
- the potential sweep rate is, for example, 0.01 to 100 mV / sec.
- Cu-UPD is preferably performed in an inert gas atmosphere such as a nitrogen atmosphere from the viewpoint of preventing oxidation of the surface of the Pd-containing particles and preventing oxidation of copper.
- the Cu ion-containing electrolyte is appropriately stirred as necessary.
- each of the Pd-containing particles is added to the reaction vessel serving as the working electrode by stirring the electrolytic solution.
- the surface can be contacted and a potential can be uniformly applied to the Pd-containing particles of each palladium particle carrier.
- stirring may be performed continuously or intermittently during the deposition of the Cu atomic layer.
- Step of replacing Cu on Pd-containing particles with Pt The method of replacing Cu (Cu atomic layer) deposited on the surface of the Pd-containing particles in the Cu-UPD process with Pt is not particularly limited. Usually, by contacting Pd-containing particles having Cu deposited on the surface thereof with a solution containing Pt ions (hereinafter sometimes referred to as a Pt ion-containing solution), Cu and Pt are caused to differ in ionization tendency. Can be replaced.
- a solution containing Pt ions hereinafter sometimes referred to as a Pt ion-containing solution
- platinum salts used in the Pt ion-containing solution include K 2 PtCl 4 , K 2 PtCl 6 , and ammonia complexes such as ([PtCl 4 ] [Pt (NH 3 ) 4 ]). Can also be used.
- the Pt ion concentration in the Pt ion-containing solution is not particularly limited, but is preferably 0.01 to 100 mM.
- the solvent that can be used for the Pt ion-containing solution can be the same as the solvent used for the above-described Cu ion-containing electrolytic solution.
- the Pt ion-containing solution may contain, for example, an acid in addition to the solvent and the platinum salt.
- the acid include sulfuric acid, nitric acid, hydrochloric acid, chlorous acid, perchloric acid, and oxalic acid. It is preferable that the Pt ion-containing solution is sufficiently stirred in advance and nitrogen is bubbled in advance in the solution from the viewpoint of preventing oxidation of the surface of the Pd-containing particles and preventing oxidation of Cu.
- the substitution time is not particularly limited, but it is preferable to ensure 10 minutes or more. As the Pt ion-containing solution is added, the potential of the reaction solution increases. Therefore, it is more preferable to substitute until the monitor potential does not change.
- a Pt ion-containing solution may be added to the electrolytic solution used in the Cu-UPD process.
- the potential control is stopped, and the Pt ion-containing solution is added to the Cu ion-containing electrolyte used in the Cu-UPD process. You may make it contact.
- the Pt / Pd core-shell catalyst may be filtered, washed, dried and pulverized.
- the washing of the core-shell catalyst is not particularly limited as long as it can remove impurities without impairing the core-shell structure of the produced core-shell catalyst.
- a method of suction filtration using water, perchloric acid, dilute sulfuric acid, dilute nitric acid or the like can be mentioned.
- the drying of the core-shell catalyst is not particularly limited as long as it can remove the solvent and the like, and examples thereof include a method of maintaining a temperature of 50 to 100 ° C. for 6 to 12 hours in an inert gas atmosphere.
- the core-shell catalyst may be pulverized as necessary.
- the pulverization method is not particularly limited as long as it is a method capable of pulverizing solids.
- Examples of the pulverization include pulverization using a mortar or the like in an inert gas atmosphere or air, and mechanical milling such as a ball mill and a turbo mill.
- the method for producing a core-shell catalyst of the present invention it is possible to produce a Pt / Pd core-shell catalyst having a Pt-containing shell having an average thickness of 0.50 nm or less, further 0.40 nm or less, and further 0.35 nm or less. Is possible.
- the average thickness of the Pt-containing shell is preferably 0.20 nm or more. In the examples described later, it has been confirmed that the average thickness of the Pt-containing shell is 0.21 to 0.33 nm.
- Pd particles were pretreated as follows. That is, 1 g of carbon particles carrying Pd particles (hereinafter sometimes referred to as Pd / C) is put together with an acidic solution into a carbon material reaction vessel, a hydrogen electrode as a reference electrode, a reaction vessel as a working electrode, platinum Using the mesh as a counter electrode, potential scanning was performed in a potential range of 0.1 V to 1 V (vs. RHE) with respect to the working electrode.
- Pd / C carbon particles carrying Pd particles
- the particle size of 600 or more Pd particles was measured by TEM image analysis, and a histogram of particle size distribution (see FIG. 3) was created. The standard deviation and the frequency of particle size of 5.00 nm or less were calculated.
- TEM image analysis in order to obtain an accurate particle size distribution, Pd particles existing in a primary particle state (independently) were extracted and measured. As shown in FIG. 3, the pre-treated Pd particles have an average particle size of 3.34 nm, a standard deviation of 1.26 nm, and a frequency of 89% or less of the particle size of 5.00 nm or less in the number-based particle size frequency distribution. was.
- a Pt-containing shell was formed on the surface of the pretreated Pd particles as follows. That is, after adding a 50 mM Cu ion-containing electrolytic solution (copper sulfate aqueous solution) to the reaction vessel after the pretreatment, 0.37 V was applied to the working electrode for 3 hours. Thereafter, K 2 PtCl 4 was dropped into the reaction vessel in such an amount that the Pt shell formed on the surface of the Pd particles became 1 ML. Subsequently, the solution in the reaction vessel was filtered to recover powder (Pt / Pd core-shell catalyst). The recovered Pt / Pd core-shell catalyst was washed with warm water (pure water) several times and then dried.
- a 50 mM Cu ion-containing electrolytic solution copper sulfate aqueous solution
- the average particle size, standard deviation, and frequency in the number-based particle size frequency distribution were calculated in the same manner as for the Pd particles. As shown in FIG. The frequency of .90 nm, standard deviation of 1.02 nm, and particle size of 5.00 nm or less was 84%. Further, since the Pd particles after the pretreatment are considered to have no change in the number-based particle size frequency distribution in the process of forming the Pt-containing shell, the pretreated Pd particles and the obtained Pt / The Pd core in the Pd core-shell catalyst is considered to have the same particle size frequency distribution.
- Example 2 In the number-based particle size frequency distribution after pretreatment, except that Pd particles having an average particle size of 3.80 nm, a standard deviation of 1.12 nm, and a frequency of particle size of 5.00 nm or less of 84% are used.
- a Pt / Pd core-shell catalyst was produced.
- the obtained Pt / Pd core-shell catalyst had an average particle size of 4.40 nm, a standard deviation of 0.75 nm, and a frequency of 75% or less in particle size of 5.00 nm in the number-based particle size frequency distribution.
- the average thickness of the Pt-containing shell was 0.30 nm.
- Example 3 In the number-based particle size frequency distribution after pretreatment, except that Pd particles having an average particle size of 3.60 nm, a standard deviation of 1.38 nm, and a frequency of particle size of 5.00 nm or less of 83% are used,
- a Pt / Pd core-shell catalyst was produced.
- the obtained Pt / Pd core-shell catalyst had an average particle size of 4.04 nm, a standard deviation of 1.60 nm, and a frequency of 73% or less in particle size of 5.00 nm in the number-based particle size frequency distribution.
- the average thickness of the Pt-containing shell was 0.22 nm.
- Example 4 In the number-based particle size frequency distribution after pretreatment, except that Pd particles having an average particle size of 3.77 nm, a standard deviation of 1.25 nm, and a frequency of particle size of 5.00 nm or less of 82% are used.
- a Pt / Pd core-shell catalyst was produced.
- the obtained Pt / Pd core-shell catalyst had an average particle size of 4.43 nm, a standard deviation of 1.02 nm, and a frequency of 70% or less in particle size of 5.00 nm in the number-based particle size frequency distribution.
- the average thickness of the Pt-containing shell was 0.33 nm.
- Example 5 In the number-based particle size frequency distribution after pretreatment, except that Pd particles having an average particle size of 3.77 nm, a standard deviation of 1.33 nm, and a particle size of 5.00 nm or less having a frequency of 80% are used.
- a Pt / Pd core-shell catalyst was produced.
- the obtained Pt / Pd core-shell catalyst had an average particle size of 4.34 nm, a standard deviation of 1.57 nm, and a frequency of particle size of 5.00 nm or less in the number-based particle size frequency distribution of 64%.
- the average thickness of the Pt-containing shell was 0.29 nm.
- Example 6 In the number-based particle size frequency distribution after pretreatment, except that Pd particles having an average particle size of 4.10 nm, a standard deviation of 1.55 nm, and a particle size of 5.00 nm or less having a frequency of 72% are used.
- a Pt / Pd core-shell catalyst was produced.
- the obtained Pt / Pd core-shell catalyst had an average particle size of 4.60 nm, a standard deviation of 1.65 nm, and a frequency of particle size of 5.00 nm or less in the number-based particle size frequency distribution of 60%.
- the average thickness of the Pt-containing shell was 0.25 nm.
- Example 7 In the number-based particle size frequency distribution after pretreatment, except that Pd particles having an average particle size of 4.05 nm, a standard deviation of 1.56 nm, and a particle size of 5.00 nm or less having a frequency of 71% are used.
- a Pt / Pd core-shell catalyst was produced.
- the obtained Pt / Pd core-shell catalyst had an average particle size of 4.46 nm, a standard deviation of 1.91 nm, and a frequency of particle size of 5.00 nm or less in the number-based particle size frequency distribution of 58%.
- the average thickness of the Pt-containing shell was 0.21 nm.
- Example 1 In the number-based particle size frequency distribution after pretreatment, except that Pd particles having an average particle size of 4.50 nm, a standard deviation of 1.19 nm, and a frequency of particle size of 5.00 nm or less of 64% are used.
- a Pt / Pd core-shell catalyst was produced.
- the obtained Pt / Pd core-shell catalyst had an average particle size of 4.80 nm, a standard deviation of 1.12 nm, and a frequency of 54% or less in particle size of 5.00 nm or less in the number-based particle size frequency distribution.
- the average thickness of the Pt-containing shell was 0.15 nm.
- Example 2 Pt / Pd was performed in the same manner as in Example 1 except that Pd particles having different average particle size, standard deviation, and particle size of 5.00 nm or less in the number-based particle size frequency distribution after pretreatment were used. A core-shell catalyst was produced. The obtained Pt / Pd core-shell catalyst had an average particle size of 5.00 nm, a standard deviation of 2.19 nm, and a frequency of particle size of 5.00 nm or less in the number-based particle size frequency distribution of 48%.
- Example 3 Pt / Pd was performed in the same manner as in Example 1 except that Pd particles having different average particle size, standard deviation, and particle size of 5.00 nm or less in the number-based particle size frequency distribution after pretreatment were used.
- a core-shell catalyst was produced.
- the obtained Pt / Pd core-shell catalyst had an average particle size of 4.60 nm, a standard deviation of 2.52 nm, and a frequency of particle size of 5.00 nm or less in the number-based particle size frequency distribution of 53%.
- MEA power generation performance evaluation (Production of MEA) Using the Pt / Pd core-shell catalysts of Examples 1 to 7 and Comparative Examples 1 to 3, MEAs were produced as follows. Each core-shell catalyst was mixed with an electrolyte solution (20% by mass Nafion (registered trademark) solution) and a solvent (water, 1-propanol, and ethanol), and dispersed with a bead mill to prepare a cathode catalyst ink. This cathode catalyst ink was applied to one surface of the electrolyte membrane by spray coating and dried to form a cathode catalyst layer (20 cm 2 ).
- electrolyte solution 20% by mass Nafion (registered trademark) solution
- a solvent water, 1-propanol, and ethanol
- an anode catalyst ink was prepared in the same manner as the cathode catalyst ink, and applied to the other surface of the electrolyte membrane by spray coating. Dried to form an anode catalyst layer (20 cm 2 ).
- the amount of catalyst ink applied is calculated in advance by the ICP (inductively coupled plasma) analysis, and the Pt amount in the catalyst ink is calculated in advance so that the surface area of platinum is 0.1 mg / cm on each surface of the electrolyte membrane. It was set to be 2 .
- ICP analysis of each catalyst layer of the produced MEA confirmed that the Pt basis weight of the catalyst layer formed on each surface of the electrolyte membrane was 0.1 mg / cm 2 .
- FIG. 5 shows the relationship between the frequency of the Pt / Pd core-shell catalyst particle size of 5.00 nm or less and the cell voltage at a current density of 2.6 A / cm 2.
- the particle size of the Pt / Pd core-shell catalyst is 5.00 nm or less.
- FIG. 6 shows the relationship between the frequency and the cell voltage when the current density is 0.2 A / cm 2 .
- Pt / Pd of the core-shell catalyst average particle size and the current density 2.6A / the relationship between the cell voltage when the cm 2 FIG 7, Pt / Pd standard deviation of the core-shell catalyst and a current density of 2.6A / cm 2
- FIG. 8 shows the relationship with the cell voltage at the time.
- Examples 1 to 7 showed excellent power generation performance.
- Table 1 and FIG. 6 Examples 1 to 7 showed high power generation performance, particularly in the high current density region (2.6 A / cm 2 ).
- Table 1 and FIG. 5 in the high current density region (2.6 A / cm 2 ), the frequency of the Pt / Pd core-shell catalyst is 71% or more, particularly 73% or more, and further 75% or more.
- a high voltage can be obtained at 84% or more.
- the above-mentioned frequency of the Pt / Pd core-shell catalyst is 58% or more, particularly 60% or more, and further 70% or more.
- a high voltage can be obtained at 84% or more.
- Table 1 FIG. 5, FIG. 7, and FIG. 8, in the high current density region (2.6 A / cm 2 ), when the frequency of the Pt / Pd core-shell catalyst is 71% or more, Pt It can be said that a high voltage is obtained when the average particle size of the / Pd core-shell catalyst is 4.40 nm or less, particularly 4.10 nm or less, and further 3.90 nm or less.
- the standard deviation of the Pt / Pd core-shell catalyst is 1.60 nm or less, particularly 1. It can be said that a high voltage is obtained when the thickness is 10 nm or less, and further 0.80 nm or less. Furthermore, according to the present invention, it was confirmed that a Pt-containing shell having an average thickness of 0.21 to 0.33 nm can be formed.
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Abstract
Description
一方、白金を用いた触媒は非常に高価であるにもかかわらず、触媒反応は粒子表面のみで生じ、粒子内部は触媒反応にほとんど関与しない。したがって、白金を用いた触媒における、材料コストに対する触媒活性は、必ずしも高くなかった。
例えば、特許文献1には、パラジウムを含む粒子をコアとし、白金を含むシェルで被覆したコアシェル触媒が開示されている。
本発明は上記実情を鑑みて成し遂げられたものであり、燃料電池の単セルの高性能化を達成可能なコアシェル触媒、及び、該コアシェル触媒の製造方法を提供することである。
個数基準の粒径頻度分布において、平均粒径が4.70nm以下であり、且つ、標準偏差が2.00nm以下であり、且つ、粒径が5.00nm以下の頻度が55%以上であることを特徴とする。
本発明によれば、燃料電池単セルの発電性能を向上させることが可能である。
また、本発明のコアシェル触媒は、前記平均粒径が4.40nm以下であることが好ましい。
また、本発明のコアシェル触媒は、前記標準偏差が1.60nm以下であることが好ましい。
個数基準の粒径頻度分布において、平均粒径が4.40nm以下であり、且つ、標準偏差が2.00nm以下であり、且つ、粒径が5.00nm以下の頻度が65%以上である、パラジウム含有粒子の表面に、白金含有シェルを析出させることを特徴とする。
本発明のコアシェル触媒の製造方法によれば、前記白金含有シェルの平均厚さが0.20~0.35nmであるコアシェル触媒を製造することが可能である。
尚、本発明において、パラジウムを含むコア(以下、Pd含有コアということがある)とは、パラジウムからなるコア及びパラジウム合金からなるコアの総称である。同様に、パラジウム含有粒子(以下、Pd含有粒子ということがある)とは、パラジウム粒子及びパラジウム合金粒子の総称である。
パラジウム合金としては、イリジウム、ルテニウム、ロジウム、鉄、コバルト、ニッケル、銅、銀、及び、金からなる群から選ばれる金属材料とパラジウムとの合金が挙げられ、パラジウム合金を構成するパラジウム以外の金属は1種でも2種以上でもよい。
パラジウム合金は、合金全体の質量を100質量%としたときのパラジウムの含有割合が、50質量%以上100質量%未満であることが好ましい。パラジウムの含有割合が50質量%以上であることにより、均一なPt含有シェルを形成することができるからである。
白金合金としては、イリジウム、ルテニウム、ロジウム、ニッケル、及び、金からなる群から選ばれる金属材料との合金等が挙げられ、白金合金を構成する白金以外の金属は1種でも2種以上でもよい。
白金合金は、合金全体の質量を100質量%としたときの白金の含有割合が、50質量%以上100質量%未満であることが好ましい。白金の含有割合が50質量%未満であるとすると、十分な触媒活性及び耐久性が得られないからである。
本発明のコアシェル触媒は、パラジウムを含むコアと、白金を含み且つ前記コアを被覆するシェルと、を備えるコアシェル触媒であって、
個数基準の粒径頻度分布において、平均粒径が4.70nm以下であり、且つ、標準偏差が2.00nm以下であり、且つ、粒径が5.00nm以下の頻度が55%以上であることを特徴とする。
すなわち、従来のPt/Pdコアシェル触媒では、その製造過程で、5.00nm以上のような大粒径を有するPd含有粒子の表面が、Pt含有シェルで被覆されにくいという問題があった。Pd含有コアが露出したPt/Pdコアシェル触媒を用いた単セルでは、発電時にPd含有コアからのパラジウム溶出が生じ、溶出したパラジウムがPt含有シェル上で再析出する。その結果、Pt/Pdコアシェル触媒の触媒活性が低下し、低電流密度域でも所望の出力が得られなかった。また、Pd含有コアがPt含有シェルで被覆されにくいことから、白金表面積が不足し、高電流密度域において所望の出力が得られなかった。
図1に示すように、5nmを超えるような粒径の大きなPt/Pdコアシェル触媒では、Ptの割合が小さく、1MLラインを下回り、Pdコアが露出しやすい傾向があることが見出された。一方、5nm以下のような粒径の微小なPt/Pdコアシェル触媒では、Ptの割合が大きく、1MLラインを上回る傾向があることが見出された。これは、Pt/Pdコアシェル触媒の原料であるPd含有粒子が大粒径である場合、Pt含有シェルが形成されにくく、微小なPd含有粒子が優先的にPt含有シェルで被覆されるためと考えられる。
すなわち、図2に示すように、まず、小粒径のPd含有粒子は、大粒径のPd含有粒子よりも、高い自由エネルギー(ギブスエネルギー)を有しており、熱的に不安定であると考えられる。また、小粒径のPd含有粒子は、大粒径のPd含有粒子よりも、モノレイヤーのPt層が形成されるまでの活性化エネルギーが小さく(E1<E3)、速度論的にも有利であると考えられる。さらに、小粒径のPd含有粒子は、モノレイヤーのPt層が形成された後(約0.5nm粒径増加)もなお、大粒径のPd含有粒子よりも粒径が小さいため、Pt上に2層目のPtが析出(Pt-Pt結合が形成)し、熱量を下げると考えられる。このとき、Pt-Pt結合の形成の生成熱ΔEは大きいと推定される。従って、大粒径のPd含有粒子へのモノレイヤーのPt層の形成と、小粒径のPd含有粒子へのPt層の多層化と、が競合して進行すると考えられるが、結果的に、小粒径のPd含有粒子の多層化の方が、系全体の安定化効果が大きく、優先的に進行すると考えられる。
本発明のコアシェル触媒を用いることによって、高電流密度域において高電圧が得られる理由は次のように考えられる。つまり、本発明のコアシェル触媒は、まず、Pt含有シェルによるPd含有コアの被覆率が高いため露出したPdが少なく、さらには、Pt含有シェルがPd含有コアの表面に均一に形成されていると推測される。そのため、従来と比較してPtの比表面積が大きいと考えられる。単セルにおいて、高電流密度域のような高負荷条件下では、触媒活性よりもガス拡散が支配的であるが、Ptの比表面積の増加により、Ptと反応ガスとの接触面積が増加し、高電圧が得られると考えられる。
また、本発明において、上記平均粒径、標準偏差及び頻度は、TEM(透過型電子顕微鏡)の画像解析によって、600個以上のコアシェル触媒の粒径を測定し、粒径分布のヒストグラム(図3参照)を作成し、求めることができる。尚、コアシェル触媒の粒径は、TEMの画像解析における各粒子の投影面積を円換算して算出した直径の値であり、各粒子の投影面積を真円と同等とみなして算出した。TEMの画像解析においては、正確な粒径分布を得るため、一次粒子の状態で(単独で)存在するコアシェル粒子を抽出して測定することが好ましい。また、粒径が5.00nm以下の頻度とは、コアシェル触媒を構成する全粒子を100%とした時に、粒径が5.00nm以下の粒子が占める割合を意味する。
本発明によれば、平均厚さが、0.50nm以下、さらに0.40nm以下、さらには0.35nm以下のPt含有シェルを有するPt/Pdコアシェル触媒を提供することが可能である。また、Ptモノレイヤーの厚さは、0.20nmであることから、Pt含有シェルの平均厚さは、0.20nm以上であることが好ましい。尚、後述の実施例では、Pt含有シェルの平均厚さが0.21~0.33nmであることが確認されている。
t=(Dave1-Dave2)/2
また、Pd含有コアの平均粒径Dave2は、例えば、Pt含有シェルで被覆する前のPd含有粒子(原料粒子)の平均粒径を測定、算出した値を、Pd含有コアの平均粒径とみなすことができる。尚、Pd含有粒子の平均粒径は、Pt含有シェルによる被覆処理の前に、洗浄等を目的として行う前処理によって、変化する場合がある。従って、Pd含有粒子の平均粒径は、前処理を行った後の、Pt/Pdコアシェル触媒におけるPd含有コアと同等の平均粒径を保持しうる状態となってから、測定、算出することが好ましい。後述の実施例では、前処理後に測定、算出したPd粒子の平均粒径をPdコアの平均粒径とみなし、該Pd粒子の平均粒径とPt/Pdコアシェル触媒の平均粒径との差分から、Pt含有シェルの平均厚さを算出している。また、実施例では、後述するアンダーポテンシャル法(UPD法)によりPd粒子表面にCu層を形成した後、該Cu層をPtに置換することで、PdコアがPtシェルで被覆されたPt/Pdコアシェル触媒を製造しているが、このようなCu-UPD法及びPt置換によるPt含有シェルの形成工程では、上記前処理を行ったPd含有粒子の溶出等は生じず、Pt含有シェルの形成前後でPd含有粒子の平均粒径が変化しないと考えられる。
Pd含有コアの平均粒径は、Pt/Pdコアシェル触媒の平均粒径未満であれば、特に限定されない。例えば、個数基準の粒径頻度分布において、4.40nm以下であることが好ましく、白金の有効利用の観点からは、2.00nm以上であることが好ましい。
また、導電性担体の比表面積が上記範囲を超える場合、Pt/Pdコアシェル触媒の有効利用率が低下してしまうおそれがある。
導電性担体によるPt/Pdコアシェル触媒担持率[{(Pt/Pdコアシェル触媒質量)/(Pt/Pdコアシェル触媒質量+導電性担体質量)}×100%]は特に限定されず、一般的には、20~60%の範囲であることが好ましい。
本発明のコアシェル触媒の製造方法は、上記本発明のコアシェル触媒の製造方法であって、
個数基準の粒径頻度分布において、平均粒径が4.40nm以下であり、且つ、標準偏差が2.00nm以下であり、且つ、粒径が5.00nm以下の頻度が65%以上である、パラジウム含有粒子の表面に、白金含有シェルを析出させることを特徴とするものである。
また、Pd含有粒子の平均粒径、標準偏差及び頻度は、Pt含有シェルを析出させる(Pt含有シェルによる被覆処理)直前の値とすることが好ましい。上記したように、Pd含有粒子の平均粒径は、Pt含有シェルによる被覆処理の前に、洗浄等を目的として行う前処理によって、変化する場合がある。同様に、標準偏差及び頻度も変化する場合がある。ゆえに、Pd含有粒子の平均粒径、標準偏差、及び頻度が変化しない乃至は変化しにくい状態となった前処理後に、これらの値を測定、算出することが好ましい。
例えば、酸性溶液としては硫酸等の酸を含む溶液が挙げられ、酸性溶液の具体的なpH、処理時間等は適宜設定することができる。また、電位サイクルの電位走査範囲としては、例えば、0.1~1V(vs.RHE)が挙げられ、サイクル数、走査速度等は適宜設定すればよい。電位サイクルは、例えば、酸性溶液中で行うことができる。
Pd含有粒子の上記頻度は、65%以上であればよいが、高電流密度域における電圧増加効果が特に高いコアシェル触媒が得られやすいことからは、82%より大きいことが好ましく、特に83%以上、さらには84%以上、中でも89%以上であることが好ましい。また、低電流密度域(低負荷条件下)においても、高電圧を示すコアシェル触媒が得られやすいことから、71%以上、特に72%以上、さらには82%以上、中でも89%以上が好ましい。
導電性担体に担持されたPd粒子担持体は、市販品を用いることもできるし、合成することもできる。Pd含有粒子を導電性担体に担持する方法としては、従来から用いられている方法を採用することができる。例えば、導電性担体を分散させた導電性担体分散液に、Pd含有粒子を混合し、濾過、洗浄して、エタノール等に再分散した後、真空ポンプ等で乾燥する方法が挙げられる。乾燥後、必要に応じて、加熱処理してもよい。尚、Pd合金粒子を使用する場合には、Pd合金粒子の合成とPd合金粒子の担体への担持が同時に行われてもよい。
以下、Cu-UPDを利用した方法を例に、Pd含有粒子の表面にPt含有シェルを析出させる方法を説明する。
Cuは、Pd表面上ではエネルギー的に安定なため、Cuの還元電位より貴な電位を印加することで、Pd含有粒子の表面にCu原子層を析出させることができる。また、CuはPtよりもイオン化傾向が大きいため、Pd含有粒子表面のCuをPtで置換することができ、Pd含有粒子の表面がPtで被覆されたコアシェル触媒を製造することができる。
以下、Pd含有粒子表面にCu原子層を形成する工程、Pd含有粒子上のCuをPtで置換する工程について、順に説明する。
Cuイオンを含有する電解液と接触(例えば該電解液に浸漬)した状態のPd含有粒子に、Cuの酸化還元電位(平衡電位)よりも貴な電位を印加することによって、Pd含有粒子の表面にCu-UPDによる銅原子層の析出を生じさせることができる。
Cuイオン含有電解液に用いられる溶媒としては、水、有機溶媒が挙げられるが、Pd含有粒子の表面へのCuの析出を妨げないという観点から、水が好ましい。
Cuイオン含有電解液に用いられる銅塩としては、具体的には、硫酸銅、硝酸銅、塩化銅、亜塩素酸銅、過塩素酸銅、シュウ酸銅等が挙げられる。
電解液中において、Cuイオン濃度は、特に限定されないが、10~1000mMであることが好ましい。
Cuイオン含有電解液は、上記溶媒及び銅塩の他にも、例えば、酸等を含んでいてもよい。Cuイオン含有電解液に添加できる酸としては、具体的には、硫酸、硝酸、塩酸、亜塩素酸、過塩素酸、シュウ酸等が挙げられる。尚、Cuイオン含有電解液中の対アニオンと、酸中の対アニオンとは、同一であってもよく、異なっていてもよい。
また、電解液は、予め、不活性ガスをバブリングしておくことが好ましい。Pd含有粒子の酸化を抑制し、Pt含有シェルによる均一な被覆が可能となるからである。不活性ガスとしては、窒素ガス、アルゴンガス等を用いることができる。
また、導電性基材上や作用極上にPd含有粒子を固定し、導電性基材や作用極のPd含有粒子固定面を、電解液に浸漬してもよい。Pd含有粒子を固定する方法としては、例えば、電解質樹脂(例えばナフィオン(商品名)等)と、水やアルコール等の溶媒とを用いて、Pd含有粒子ペーストを調製し、導電性基材や作用極の表面に塗布する方法が挙げられる。
作用極としては、例えば、チタン、白金メッシュ、白金板等の金属材料、及び、グラッシーカーボン、カーボン板等の導電性炭素材料等の導電性が担保できる材料を用いることができる。尚、反応容器を上記導電性材料で形成し、作用極としても機能させることもできる。金属材料の反応容器を作用極として用いる場合、反応容器の内壁には、腐食を抑制する観点から、RuO2をコーティングすることが好ましい。炭素材料の反応容器を作用極として用いる場合は、コーティング無しでそのまま使用することが可能である。
対極としては、例えば、白金メッシュに白金黒をめっきしたもの及び導電性炭素繊維等を用いることができる。
参照極としては、可逆水素電極(reversible hydrogen electrode;RHE)、銀-塩化銀電極及び銀-塩化銀-塩化カリウム電極等を用いることができる。
電位制御装置としては、ポテンショスタット及びポテンショガルバノスタット等を用いることができる。
電位を印加する時間は、特に限定されないが、60分以上確保することが好ましく、反応電流が定常となり、ゼロに近づくまで行なうことがより好ましい。
電位の印加は、上記電位範囲を含む範囲において電位を掃引することによって行ってもよい。掃引する電位範囲としては、具体的には、0.3~0.8V(vs.RHE)であることが好ましい。
電位掃引のサイクル数は、特に限定されないが、1~20サイクルであることが好ましい。また、電位の掃引速度は、例えば、0.01~100mV/秒である。
また、Cuイオン含有電解液は、必要に応じて適宜攪拌することが好ましい。例えば、作用極を兼ねる反応容器を用い、該反応容器内の電解液にPd含有粒子を浸漬、分散させた場合、電解液を攪拌することで、各Pd含有粒子を作用極である反応容器の表面に接触させ、各パラジウム粒子担持体のPd含有粒子に均一に電位を印加させることができる。
この場合、攪拌は、Cu原子層の析出中、連続的に行ってもよいし、断続的に行ってもよい。
Cu-UPD工程でPd含有粒子表面に析出したCu(Cu原子層)をPtに置換する方法は特に限定されない。通常、Ptイオンを含有する溶液(以下、Ptイオン含有溶液ということがある。)に、表面にCuを析出させたPd含有粒子を接触させることによって、イオン化傾向の違いにより、CuとPtとを置換することができる。
Ptイオン含有溶液中においてPtイオン濃度は特に限定されないが、0.01~100mMであることが好ましい。
Ptイオン含有溶液に用いることができる溶媒は、上述したCuイオン含有電解液に用いられる溶媒と同様とすることができる。また、Ptイオン含有溶液には、上記溶媒及び白金塩の他にも、例えば、酸等を含んでいてもよい。酸としては、具体的には、硫酸、硝酸、塩酸、亜塩素酸、過塩素酸、シュウ酸等が挙げられる。
Ptイオン含有溶液は、事前に十分に攪拌し、Pd含有粒子の表面の酸化防止や、Cuの酸化防止の観点から、当該溶液中には予め窒素をバブリングさせることが好ましい。
置換時間(Ptイオン含有溶液とPd含有粒子との接触時間)は、特に限定されないが、10分以上確保することが好ましく、Ptイオン含有溶液を加えていくと、反応溶液の電位が上昇していくため、そのモニター電位が変化しなくなるまで置換させることがより好ましい。
尚、Cu-UPD工程とPt置換工程とを、同じ反応容器内で行う場合には、Cu-UPD工程に使用した電解液に、Ptイオン含有溶液を加えてもよい。例えば、Cu-UPD工程後、電位制御を停止し、Cu-UPD工程において使用したCuイオン含有電解液に、Ptイオン含有溶液を添加することで、Cuが析出したPd含有粒子をPtイオン含有溶液に接触させてもよい。
Pt置換工程の後にPt/Pdコアシェル触媒の濾過、洗浄、乾燥及び粉砕等が行われてもよい。
コアシェル触媒の洗浄は、製造されたコアシェル触媒のコアシェル構造を損なうことなく、不純物を除去できる方法であれば特に限定されない。当該洗浄の例としては、水、過塩素酸、希硫酸、希硝酸等を用いて吸引濾過をする方法が挙げられる。
コアシェル触媒の乾燥は、溶媒等を除去できる方法であれば特に限定されず、例えば、不活性ガス雰囲気下、50~100℃の温度を6~12時間保持させる方法等が挙げられる。
コアシェル触媒は必要に応じて粉砕してもよい。粉砕方法は、固形物を粉砕できる方法であれば特に限定されない。当該粉砕の例としては、不活性ガス雰囲気下、或いは大気下における乳鉢等を用いた粉砕や、ボールミル、ターボミル等のメカニカルミリングが挙げられる。
(実施例1)
まず、Pd粒子を、次のようにして前処理した。すなわち、Pd粒子を担持した炭素粒子(以下、Pd/Cということがある)1gを、酸性溶液と共に、炭素材料製の反応容器に投入し、水素電極を参照極、反応容器を作用極、白金メッシュを対極として、作用極に対して0.1V~1V(vs.RHE)の電位範囲で電位走査した。
続いて、反応容器内の溶液を濾過し、粉末(Pt/Pdコアシェル触媒)を回収した。
回収したPt/Pdコアシェル触媒は、複数回、温水(純水)で洗浄した後、乾燥した。
また、上記前処理後のPd粒子は、Pt含有シェルの形成過程において、その個数基準の粒径頻度分布に変化がないと考えられることから、前処理後のPd粒子と、得られたPt/Pdコアシェル触媒におけるPdコアは、同じ粒径頻度分布を有していると考えられる。従って、上記前処理後のPd粒子の平均粒径と、Pt/Pdコアシェル触媒の平均粒径との差分が、Pt含有シェルの平均厚さの2倍に相当すると考えられる。ゆえに、Pt含有シェルの平均厚さを、平均厚さ=[(Pt/Pdコアシェル触媒の平均粒径)-(Pd粒子の平均粒径)]/2に基づき算出したところ、0.28nmだった。結果を表1に示す。
前処理後の個数基準の粒径頻度分布において、平均粒径が3.80nm、標準偏差が1.12nm、粒径5.00nm以下の頻度が84%であるPd粒子を用いたこと以外は、実施例1と同様にして、Pt/Pdコアシェル触媒を製造した。
得られたPt/Pdコアシェル触媒は、個数基準の粒径頻度分布において、平均粒径が4.40nm、標準偏差が0.75nm、粒径5.00nm以下の頻度が75%だった。
また、Pt含有シェルの平均厚さは、0.30nmだった。
前処理後の個数基準の粒径頻度分布において、平均粒径が3.60nm、標準偏差が1.38nm、粒径5.00nm以下の頻度が83%であるPd粒子を用いたこと以外は、実施例1と同様にして、Pt/Pdコアシェル触媒を製造した。
得られたPt/Pdコアシェル触媒は、個数基準の粒径頻度分布において、平均粒径が4.04nm、標準偏差が1.60nm、粒径5.00nm以下の頻度が73%だった。
また、Pt含有シェルの平均厚さは、0.22nmだった。
前処理後の個数基準の粒径頻度分布において、平均粒径が3.77nm、標準偏差が1.25nm、粒径5.00nm以下の頻度が82%であるPd粒子を用いたこと以外は、実施例1と同様にして、Pt/Pdコアシェル触媒を製造した。
得られたPt/Pdコアシェル触媒は、個数基準の粒径頻度分布において、平均粒径が4.43nm、標準偏差が1.02nm、粒径5.00nm以下の頻度が70%だった。
また、Pt含有シェルの平均厚さは、0.33nmだった。
前処理後の個数基準の粒径頻度分布において、平均粒径が3.77nm、標準偏差が1.33nm、粒径5.00nm以下の頻度が80%であるPd粒子を用いたこと以外は、実施例1と同様にして、Pt/Pdコアシェル触媒を製造した。
得られたPt/Pdコアシェル触媒は、個数基準の粒径頻度分布において、平均粒径が4.34nm、標準偏差が1.57nm、粒径5.00nm以下の頻度が64%だった。
また、Pt含有シェルの平均厚さは、0.29nmだった。
前処理後の個数基準の粒径頻度分布において、平均粒径が4.10nm、標準偏差が1.55nm、粒径5.00nm以下の頻度が72%であるPd粒子を用いたこと以外は、実施例1と同様にして、Pt/Pdコアシェル触媒を製造した。
得られたPt/Pdコアシェル触媒は、個数基準の粒径頻度分布において、平均粒径が4.60nm、標準偏差が1.65nm、粒径5.00nm以下の頻度が60%だった。
また、Pt含有シェルの平均厚さは、0.25nmだった。
前処理後の個数基準の粒径頻度分布において、平均粒径が4.05nm、標準偏差が1.56nm、粒径5.00nm以下の頻度が71%であるPd粒子を用いたこと以外は、実施例1と同様にして、Pt/Pdコアシェル触媒を製造した。
得られたPt/Pdコアシェル触媒は、個数基準の粒径頻度分布において、平均粒径が4.46nm、標準偏差が1.91nm、粒径5.00nm以下の頻度が58%だった。
また、Pt含有シェルの平均厚さは、0.21nmだった。
前処理後の個数基準の粒径頻度分布において、平均粒径が4.50nm、標準偏差が1.19nm、粒径5.00nm以下の頻度が64%であるPd粒子を用いたこと以外は、実施例1と同様にして、Pt/Pdコアシェル触媒を製造した。
得られたPt/Pdコアシェル触媒は、個数基準の粒径頻度分布において、平均粒径が4.80nm、標準偏差が1.12nm、粒径5.00nm以下の頻度が54%だった。
また、Pt含有シェルの平均厚さは、0.15nmだった。
前処理後の個数基準の粒径頻度分布における平均粒径、標準偏差、及び粒径5.00nm以下の頻度が異なるPd粒子を用いたこと以外は、実施例1と同様にして、Pt/Pdコアシェル触媒を製造した。
得られたPt/Pdコアシェル触媒は、個数基準の粒径頻度分布において、平均粒径が5.00nm、標準偏差が2.19nm、粒径5.00nm以下の頻度が48%であった。
前処理後の個数基準の粒径頻度分布における平均粒径、標準偏差、及び粒径5.00nm以下の頻度が異なるPd粒子を用いたこと以外は、実施例1と同様にして、Pt/Pdコアシェル触媒を製造した。
得られたPt/Pdコアシェル触媒は、個数基準の粒径頻度分布において、平均粒径が4.60nm、標準偏差が2.52nm、粒径5.00nm以下の頻度が53%であった。
(MEAの作製)
実施例1~7、比較例1~3のPt/Pdコアシェル触媒をそれぞれ用いて、以下のようにしてMEAを作製した。
各コアシェル触媒を、電解質溶液(20質量%ナフィオン(登録商標)溶液)、及び溶媒(水、1-プロパノール、及びエタノール)と混合し、ビーズミルで分散させ、カソード触媒インクを作製した。このカソード触媒インクを、スプレー塗布により電解質膜の一方の面に塗布、乾燥し、カソード触媒層(20cm2)を形成した。
また、Pt粒子を担持した炭素粒子(田中貴金属製)を用いたこと以外は、カソード触媒インクと同様して、アノード触媒インクを作製し、スプレー塗布により、上記電解質膜の他方の面に塗布、乾燥して、アノード触媒層(20cm2)を形成した。
尚、触媒インクの塗布量は、ICP(誘導結合プラズマ)分析により、触媒インク中のPt量を予め算出しておき、電解質膜の各面に対して、白金の目付け量が0.1mg/cm2となるようにした。また、作製したMEAの各触媒層をICP分析することにより、電解質膜の各面に形成された触媒層のPt目付け量が、0.1mg/cm2であることを確認した。
上記にて作製した、実施例1~7、比較例1~3のMEAを、下記条件にて発電させ、電流密度-電圧曲線を得た。結果を図4に示す。
・背圧 : アノード、カソード共に140kPa[abs]
・ガス流量 : 電流密度のストイキ比1.2/1.5(アノード流量/カソード流量)相当流量(ガスの加湿なし)
・セル温度(冷却水温度):70℃
また、表1、図5に示すように、高電流密度域(2.6A/cm2)においては、Pt/Pdコアシェル触媒の上記頻度が71%以上、特に73%以上、さらに75%以上、中でも84%以上で、高電圧が得られることがわかる。
また、表1、図6に示すように、低電流密度域(0.2A/cm2)においては、Pt/Pdコアシェル触媒の上記頻度が58%以上、特に60%以上、さらに70%以上、中でも84%以上で、高電圧が得られることがわかる。
また、表1、図5、図7、図8に示すように、高電流密度域(2.6A/cm2)においては、Pt/Pdコアシェル触媒の上記頻度が71%以上である場合、Pt/Pdコアシェル触媒の平均粒径が4.40nm以下、特に4.10nm以下、さらに3.90nm以下であると、高電圧が得られるといえる。また、高電流密度域(2.6A/cm2)においては、Pt/Pdコアシェル触媒の上記頻度が71%以上である場合、Pt/Pdコアシェル触媒の標準偏差が1.60nm以下、特に1.10nm以下、さらに0.80nm以下であると、高電圧が得られるといえる。
さらに、本発明によれば、0.21~0.33nmの平均厚さを有するPt含有シェルを形成できることが確認された。
Claims (7)
- パラジウムを含むコアと、白金を含み且つ前記コアを被覆するシェルと、を備えるコアシェル触媒であって、
個数基準の粒径頻度分布において、平均粒径が4.70nm以下であり、且つ、標準偏差が2.00nm以下であり、且つ、粒径が5.00nm以下の頻度が55%以上であることを特徴とするコアシェル触媒。 - 前記頻度が71%以上である、請求項1に記載のコアシェル触媒。
- 前記平均粒径が4.40nm以下である、請求項2に記載のコアシェル触媒。
- 前記標準偏差が1.60nm以下である、請求項2または3に記載のコアシェル触媒。
- 前記シェルの平均厚さが、0.20~0.35nmである、請求項1乃至4のいずれか1項に記載のコアシェル触媒。
- 請求項1に記載のコアシェル触媒の製造方法であって、
個数基準の粒径頻度分布において、平均粒径が4.40nm以下であり、且つ、標準偏差が2.00nm以下であり、且つ、粒径が5.00nm以下の頻度が65%以上である、パラジウム含有粒子の表面に、白金含有シェルを析出させることを特徴とする、コアシェル触媒の製造方法。 - 前記白金含有シェルの平均厚さが0.20~0.35nmである、請求項6に記載のコアシェル触媒の製造方法。
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