JP2011249226A - Method of manufacturing fuel cell catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a fuel cell catalyst which improves both an alloying degree of an alloy carried by a carrier and a percentage content of a metal to be alloyed for platinum.SOLUTION: The present invention is characterized in implementing: a first step for adsorbing a platinum ion by performing ion-exchange reaction between a first mixture containing carbon and cation exchange resin and the platinum ion; a second step for hydrogen-reducing a second mixture obtained by implementing the first step; a third step for adsorbing a cation of a transition metal to the cation-exchange resin contained in a third mixture by performing ion-exchange reaction between the third mixture obtained by implementing the second step, an anion selected from a chlorine ion, a sulfate ion and a nitrate ion and a solution containing the cation of the transition metal selected from cobalt, nickel and iron; and a fourth step for hydrogen-reducing the cation of the transition metal in a fourth mixture obtained by implementing the third step.

Description

  The present invention relates to a method for producing a fuel cell catalyst.

  Fuel cells are attracting attention as power generation systems that have little negative impact on the environment. In particular, a polymer electrolyte fuel cell using an ion exchange membrane as an electrolyte is considered promising as an in-vehicle power source because it has a low operating temperature and can be miniaturized.

Such an electrode used for a fuel cell includes a conductive material, an ion exchange resin, and a catalyst metal. When a gas containing hydrogen and a gas containing oxygen are respectively supplied to the anode and cathode of a fuel cell equipped with this electrode on both sides of the polymer electrolyte membrane, the following reaction proceeds at the anode and cathode to generate power. can get.
Anode: 2H ₂ → 4H ⁺ + 4e ⁻
Cathode: O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O
Overall: 2H ₂ + O ₂ → 2H ₂ O

  The above reaction takes place at a reaction site formed on the contact surface between the proton conduction path called the cluster part of the ion exchange resin and the conductive material. In order to promote the above reaction, it is necessary to efficiently support a catalytic metal having high catalytic activity at the reaction site. As such a catalytic metal, platinum having a good catalytic activity is widely used.

  Many studies have been made on a method for further improving the catalytic activity of the catalytic metal. For example, Patent Document 1 discloses a catalytic metal obtained by alloying platinum and an auxiliary metal such as cobalt at a predetermined mixing ratio. A fuel cell catalyst in which particles are supported on carbon powder (support) has been proposed.

  The catalyst proposed in Patent Document 1 is manufactured through a supporting process for supporting platinum (Pt) and an auxiliary metal (M) on a support, and an alloying process for alloying platinum and auxiliary metal supported on the support. Is done. The supporting step is performed by impregnating a support with a platinum salt solution and a metal salt solution containing an auxiliary metal, and the alloying step includes a support supporting platinum and an auxiliary metal obtained by the supporting step. Performed by reducing at high temperature.

Japanese Patent No. 3643552

In the method for producing a catalyst proposed in Patent Document 1, in order to obtain a platinum-auxiliary metal alloy (Pt-M alloy) having a high degree of alloying, a support on which platinum and an auxiliary metal are supported is 800 ° C. or higher. It is necessary to reduce at a high temperature (see paragraph [0016] of Patent Document 1). Here, the degree of alloying refers to the ratio of the auxiliary metal alloyed with platinum among the auxiliary metals contained in the catalyst.
However, when a support carrying platinum and an auxiliary metal is reduced at a high temperature of 800 ° C. or higher, Pt-M alloy particles tend to aggregate, and when Pt-M alloy particles agglomerate, the particle diameter of the alloy particles increases. Therefore, there was a problem that the catalytic activity was lowered.

Further, in the above production method, when the reduction temperature of the carrier supporting platinum and the auxiliary metal is set to a low temperature of, for example, 250 ° C. or lower and alloyed, the reduction rate of the auxiliary metal ions is reduced and The solid solution amount of the auxiliary metal is reduced, thereby reducing both the degree of alloying of the Pt-M alloy and the content of the auxiliary metal in the catalyst. Thus, there is a concern that sufficient catalytic activity cannot be obtained with an electrode using a catalyst having a low degree of alloying of the Pt-M alloy supported on the support and a low auxiliary metal content.
The present invention has been completed based on the above circumstances, and has a high degree of alloying of an alloy supported on a carrier and a high content of transition metal alloyed with platinum. It aims at providing the manufacturing method of.

  As a result of intensive studies to solve the above-mentioned problems, a mixture containing carbon and a cation exchange resin and platinum ions are subjected to an ion exchange reaction, and then hydrogen reduction is performed, whereby the cluster part of the cation exchange resin and the carbon are mixed. A platinum-supporting carbon having platinum supported on the contact surface was prepared, and then the platinum-supporting carbon and the transition metal compound were subjected to an ion exchange reaction to adsorb the cation of the transition metal, followed by hydrogen reduction to obtain a cation exchange resin. By alloying platinum and transition metal supported on the contact surface between the cluster part and carbon, both the degree of alloying of the alloy supported on the carrier and the content of transition metal alloyed with platinum are both obtained. It has been found that a method for producing a high fuel cell catalyst can be provided. In the production method of the present invention, even if the alloying of platinum and the transition metal is performed at a low temperature (for example, 250 ° C. or less), the alloying degree of the alloy supported on the carrier and the transition alloyed with platinum The knowledge that the metal content can be increased was also obtained.

That is, the present invention includes a first step of adsorbing the platinum ions on the cation exchange resin by causing an ion exchange reaction between the first mixture containing carbon and a cation exchange resin and platinum ions, A second step of hydrogen reducing platinum ions in the second mixture obtained by executing the first step, a third mixture obtained by executing the second step, and chlorine ions An anion selected from sulfate ions and nitrate ions and a cation in a solution containing a cation of a transition metal selected from cobalt, nickel and iron to cause an ion exchange reaction of the cation of the transition metal. A third step of adsorbing the cation exchange resin contained in the mixture of No. 3 in the fourth mixture obtained by executing the third step; A method for producing a catalyst for a fuel cell and executes a fourth step of hydrogen reduction cations of the transition metals.
In the present invention, the degree of alloying refers to the proportion of transition metals alloyed with platinum among all transition metals (selected from cobalt, nickel and iron) contained in the catalyst.

The reason why both the degree of alloying of the alloy supported on the carrier and the content of the transition metal alloyed with platinum can be increased by the production method of the present invention is presumed as follows.
In the production method of the present invention, when a platinum-supported carbon in which platinum is supported on the contact surface between a cluster part of cation exchange resin and carbon and an ion-exchange reaction between the transition metal compound, that is, a proton and a cation of the transition metal A strong acid (hydrochloric acid, sulfuric acid, nitric acid) is generated by an ion exchange reaction with (the execution of the third step). The strong acid generated at this time slightly dissolves the surface of the nanosized platinum particles previously supported on the contact surface between the cluster portion of the cation exchange resin and the carbon, and the platinum ions and the transition metal cations coexist on the platinum surface. Analysis is considered to occur. As a result, the amount of transition metal alloyed with platinum increases. A strong acid accompanying the ion exchange reaction is generated in the cluster part of the cation exchange resin. In the case of the production method of the present invention, platinum is supported on the contact surface between the cluster part and carbon in advance, so that the strong acid easily comes into contact with the platinum surface. As a result, in the production method of the present invention, eutectoid of platinum ions and transition metal ions occurs effectively following the dissolution of platinum.
From the above, in the present invention, even when alloying is performed at a lower temperature than the conventional method (for example, hydrogen reduction at 250 ° C. or lower), the alloying degree of the platinum-transition metal alloy and the transition metal in the catalyst It is considered that a catalyst having a high content can be obtained.

And in the manufacturing method of this invention, as above-mentioned, since alloying can be accelerated | stimulated even at low temperature (250 degrees C or less), aggregation of an alloy particle can be suppressed, and, thereby, the particle diameter of an alloy particle An increase can be prevented.
Therefore, according to the present invention, it is possible to increase both the degree of alloying of the alloy supported on the carrier and the content of the metal alloyed with respect to platinum, and to prevent an increase in the particle diameter of the alloy particles. The catalytic activity of the catalytic metal can be increased.

In the production method of the present invention, the platinum ions are chemically reduced with hydrogen after the ion exchange reaction between the platinum ions and the cation exchange resin, so that the platinum ions are in contact with the cluster portion of the cation exchange resin and the carbon. Then, the nano-sized platinum particles previously supported on the contact surface between the cluster part of the cation exchange resin and the carbon by reducing with hydrogen after the ion exchange reaction between the cation of the transition metal and the cation exchange resin. And the transition metal are alloyed. As a result, according to the present invention, the catalytic metal having high catalytic activity can be efficiently supported on the reaction site formed on the contact surface between the proton conduction path called the cluster part of the ion exchange resin and the conductive material. It can be expected to produce a catalyst having a high utilization rate of the catalyst metal.
As described above, according to the present invention, there is provided a method for producing a fuel cell catalyst having a high degree of alloying of an alloy supported on a carrier and a high content of a metal alloyed with platinum, and having a high catalytic activity. Can be provided.

In the present invention, the following configuration is preferable.
If at least one of the temperature at which platinum ions are reduced with hydrogen and the temperature at which transition metal cations are reduced with hydrogen is 250 ° C. or less, the decomposition of the cation exchange resin due to high temperature exposure and the suppression of the aggregation of alloy particles are suppressed. Is preferable.
It is particularly preferable that both the hydrogen reduction temperature of the platinum ion (second step) and the hydrogen reduction temperature of the cation of the transition metal (fourth step) are 250 ° C. or lower.
The solution containing the anion and the cation of the transition metal is obtained by dissolving a transition metal compound in a solvent, and the transition metal compound is selected from chloride ion, sulfate ion and nitrate ion in the solvent. When the compound is dissolved in the solvent, the transition metal cation and the anion can be obtained at the same time. In addition, although water, alcohol, etc. can be used as a solvent, it is preferable from a viewpoint of safety to use water.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the catalyst for fuel cells with which both the alloying degree of the alloy carry | supported by a support | carrier and the content rate of the transition metal alloyed with respect to platinum can be provided.

A graph showing the relationship between the number of cobalt loadings and the degree of alloying of the platinum-cobalt alloy, and the relationship between the number of cobalt loadings and the content of cobalt alloyed with the catalyst (platinum). Graph showing the relationship between the number of cobalt loadings and the particle size of platinum-cobalt alloy particles Graph showing the relationship between cobalt reduction temperature and degree of alloying Graph showing the relationship between cobalt reduction temperature and cell voltage

The method for producing a catalyst for a fuel cell according to the present invention comprises subjecting platinum ions to cation exchange by subjecting protons of cation exchange resins and platinum ions in a first mixture containing carbon and a cation exchange resin to an ion exchange reaction. A step of adsorbing the resin (first step), a step of hydrogen reduction of the second mixture obtained by the execution of the first step (second step), and a third of the step obtained by the execution of the second step The cation exchange resin contained in the third mixture is converted into a cation exchange resin contained in the third mixture by causing an ion exchange reaction between the proton of the cation exchange resin in the mixture and the cation in the solution containing the anion and the transition metal cation. And performing a step of adsorbing (third step) and a step of reducing the fourth mixture obtained by performing the third step (fourth step) with hydrogen. That.
Hereinafter, the first to fourth steps will be described in order.

(First step)
In the first step, platinum ions are adsorbed on the cation exchange resin by performing an ion exchange reaction between protons of the cation exchange resin and platinum ions in the first mixture containing carbon and the cation exchange resin.
The first mixture can be prepared by, for example, mixing carbon, an aqueous solution of a cation exchange resin, and a solvent (such as ethanol) by a known method and then removing the solvent.

  As a method of removing the solvent from the first mixture, for example, there is a method of drying the solvent. As a method for drying the solvent, for example, standing drying, blow drying, warm air drying, infrared heating drying, far infrared heating drying, reduced pressure drying, spray drying, and the like can be used. Among these methods, it is preferable to perform spray drying from the viewpoint that particles can be obtained immediately from the state of the mixture, the particle diameter can be easily adjusted, and the treatment can be performed in a large amount.

  As the carbon used as the material of the first mixture, a material having high electron conductivity and high corrosion resistance can be used. As such carbon, for example, carbon black such as acetylene black and furnace black, activated carbon, and the like can be used. Preferably, Denka Black [manufactured by Denki Kagaku Kogyo Co., Ltd.], Vulcan XC-72 (manufactured by Cabot Corp.), Black Peal 2000 (manufactured by Cabot Corp.) or Ketjenblack EC [manufactured by Ketjenblack International Co., Ltd.] Use carbon black. In addition to this, carbon having a high graphitization degree can be used. As carbon having a high degree of graphitization, Tokablack # 3855 [manufactured by Tokai Carbon Co., Ltd.], Denkaback FX-35 [Electrochemical Industry Co., Ltd.] or carbon black was baked at a high temperature to increase the degree of graphitization. Things can be used.

Examples of the cation exchange resin used as the material of the first mixture include cation exchange resins having proton conductivity, such as perfluorocarbon sulfonic acid type or styrene-divinylbenzene sulfonic acid type cation exchange resin, or carboxylic acid of those resins. It is preferable to use the acid form.
In the present invention, a cation exchange resin solution in which a cation exchange resin is dissolved in a solvent can be used as the cation exchange resin. As the solvent, a liquid containing water and alcohol (ethanol or the like) in an arbitrary ratio is preferably used, but only a solvent contained in the cation exchange resin solution may be used.

A method of adsorbing platinum ions on the cation exchange resin (Pt adsorption step) by an ion exchange reaction between protons of the cation exchange resin in the first mixture and platinum ions will be described.
A solution containing platinum is prepared by dissolving a compound containing platinum (for example, [Pt (NH ₃ ) ₄ ] Cl ₂ or the like) in a liquid containing water. Subsequently, by immersing the first mixture in a solution containing platinum, platinum ions are adsorbed on the cation exchange resin contained in the first mixture. The adsorption of platinum ions is due to an ion exchange reaction between protons of the cation exchange resin and platinum ions.

In the present invention, platinum ions are difficult to adsorb on the surface of the carbon (conductive material) exposed without being coated with the cation exchange resin, and are exchanged with protons of the cation exchange resin. What adsorb | sucks preferentially to the cluster part (proton conduction path | route) of a cation exchange resin is preferable.
Pt ²⁺ or platinum complex ions can be used as platinum ions having such adsorption characteristics. Examples of platinum complex ions include platinum complex ions that can be represented by [Pt (NH ₃ ) ₄ ] ²⁺ , [Pt (NH ₃ ) ₆ ] ⁴⁺ , platinum complex ions coordinated with nitrate groups or nitroso groups, and the like. Can be used.

(Second step: Pt carrying step)
In the second step, the mixture (second mixture) containing the compound in which platinum ions are adsorbed on the contact surface between the cluster portion of the cation exchange resin and the carbon obtained by executing the first step is reduced with hydrogen. In this way, platinum-supported carbon in which platinum is supported on the contact surface between the cluster portion of the cation exchange resin and the carbon is produced.

Examples of the hydrogen reduction method include a gas phase reduction method using hydrogen or a gas containing hydrogen. The gas containing hydrogen is preferably a mixed gas of an inert gas such as nitrogen, helium or argon and hydrogen, and a mixed gas having a hydrogen content of 10 vol% or more is particularly preferable.
The reduction temperature for hydrogen reduction of the second mixture is preferably 250 ° C. or lower, and more preferably 100 ° C. or higher and 250 ° C. or lower. When the hydrogen reduction temperature of the second mixture is less than 100 ° C., the supported amount of platinum becomes insufficient. When the hydrogen reduction temperature exceeds 250 ° C., the platinum particle diameter of the platinum-supported carbon obtained in the second step may increase, or decomposition may occur due to high-temperature exposure of the cation exchange resin.

  The platinum-supporting carbon obtained by executing the second step may be further immersed in a solution containing platinum ions to adsorb platinum ions, and then hydrogen reduction may be performed. By repeating this adsorption of platinum ions and hydrogen reduction, the amount of platinum supported on the platinum supported carbon can be increased. That is, the amount of platinum supported can be controlled by increasing or decreasing the number of platinum ion adsorption and hydrogen reduction.

(Third step: transition metal cation adsorption step)
In the third step, a transition is made to platinum-supported carbon by causing an ion exchange reaction between the proton of the cation exchange resin and the cation of the transition metal in the third mixture containing the platinum-supported carbon obtained in the second step. Adsorb metal cations. Specifically, by immersing the third mixture in a solution containing an anion selected from chlorine ion, sulfate ion and nitrate ion, and a cation of a transition metal selected from cobalt, nickel and iron, platinum-supported carbon The cation of the cation exchange resin can adsorb the cation of the transition metal.

  In the present invention, the solution containing an anion selected from chloride ion, sulfate ion and nitrate ion and a transition metal cation selected from cobalt, nickel and iron is obtained by dissolving a transition metal compound in a solvent. When a transition metal compound that produces a cation of a transition metal selected from a chloride ion, a sulfate ion and a nitrate ion and a transition metal selected from cobalt, nickel and iron in a solvent such as water is used, It is preferable to dissolve in a solvent since a cation and an anion of a transition metal can be obtained simultaneously. Specific examples of the transition metal compound include cobalt chloride, nickel chloride, iron chloride, cobalt sulfate, nickel sulfate, iron sulfate, cobalt nitrate, nickel nitrate, and iron nitrate. The transition metal cation is preferably cobalt, which is less affected by the deterioration of the cation exchange resin, and the anion is preferably a chloride ion that reacts with the proton to produce hydrochloric acid with high platinum solubility. Moreover, although water, alcohol, etc. can be used for a solvent, it is preferable from a viewpoint of safety to use water.

(Fourth process: alloying process)
In the fourth step, the fourth mixture containing platinum-carrying carbon on which the cation of the transition metal obtained by performing the third step is adsorbed is reduced by hydrogen, whereby the cluster part of the cation exchange resin and the carbon The platinum and the transition metal supported on the contact surface are alloyed. Examples of the hydrogen reduction method include the same method as in the second step.

  The hydrogen reduction of the fourth mixture is preferably performed at 250 ° C. or lower. When the temperature of hydrogen reduction of the fourth mixture exceeds 250 ° C., the cation exchange resin in the fourth mixture may be decomposed. When the hydrogen reduction temperature of the fourth mixture is 150 ° C. or higher and 200 ° C. or lower, the degree of alloying of the platinum-transition metal alloy supported on carbon can be increased, and the decomposition of the cation exchange resin can be achieved. Is also more preferable because When the hydrogen reduction temperature of the fourth mixture is lower than 150 ° C., the degree of alloying of the platinum-transition metal alloy supported on carbon is lowered, and sufficient catalytic activity may not be obtained.

  The platinum-transition metal alloy-supported carbon obtained by executing the fourth step is further immersed in an aqueous solution of a transition metal compound to adsorb a cation of the transition metal, and then subjected to hydrogen reduction. May be. By repeating this adsorption of transition metal cations and hydrogen reduction, the transition metal content in the catalyst is increased. That is, the transition metal content can be controlled by increasing or decreasing the number of adsorption and hydrogen reduction of transition metal cations.

(Catalyst obtained by the production method of the present invention)
The catalyst obtained by the production method of the present invention described above is, for example, dispersed in N-methyl-2-pyrrolidone or the like, applied to a substrate as a slurry, and dried to form a sheet-like catalyst layer. Depending on the method, it can be used as a catalyst for an electrode for a fuel cell.

<Examples and Comparative Examples>
The invention is further illustrated by examples embodying the invention.
1. Comparison of degree of alloying of Pt—Co alloy supported on carbon supported on Pt—Co alloy obtained by the method of the present invention and comparative method and Co content alloyed with Pt

Example 1
(1) Preparation of mixture containing platinum (Pt) -supporting carbon 10 g of carbon powder (Cabot, Vulcan XC-72), 50 g of cation exchange resin solution (Aldrich, Nafion 5 mass% solution), and ethanol Is made into a slurry, and this slurry is dried by spray drying (120 ° C.) to produce a powder containing a cation exchange resin and a carbon powder (solid mixture, cation exchange resin blending ratio 20 mass%). did.
A powder containing a cation exchange resin and carbon powder is impregnated in an aqueous solution [Pt (NH ₃ ) ₄ ] Cl ₂ for 2 hours, and [Pt (NH ₃ ) ₄ ] ²⁺ ions are ionized in the cluster part of the cation exchange resin. It was adsorbed by an exchange reaction (Pt adsorption step).
The mixture obtained through the Pt adsorption step is subjected to hydrogen reduction (reduction temperature 150 ° C., reduction time 8 hours), thereby supporting Pt nanoparticles on the contact surface between the cluster portion of the cation exchange resin and the carbon. A mixture containing carbon (Pt-supported carbon) was obtained (Pt-supporting step).
The mixture obtained through this Pt supporting step was further impregnated with [Pt (NH ₃ ) ₄ ] Cl ₂ aqueous solution for 2 hours, and then subjected to hydrogen reduction (reduction temperature 150 ° C., reduction time 8 hours). A mixture containing carbon carrying mass% of Pt was obtained.
The amount of Pt supported (% by mass) is based on the total mass of Pt and carbon.

(2) Preparation of mixture containing platinum-cobalt alloy (Pt-Co alloy) -carrying carbon The mixture containing carbon carrying 5% by mass of Pt obtained in (1) is immersed in an aqueous CoCl ₂ (cobalt chloride) solution. Thus, cobalt ions were adsorbed (Co adsorption step).
The mixture obtained through the Co adsorption step was subjected to hydrogen reduction (reduction temperature 200 ° C., reduction time 24 hours) to obtain a mixture containing Pt—Co alloy-supported carbon (Pt—Co alloy support step). This mixture is a mixture in which cobalt (Co) is supported once.

(3) The mixture obtained in (2) was immersed in a CoCl 2 aqueous solution to adsorb Co ²⁺ ions, and then hydrogen reduction (reduction temperature 200 ° C., reduction time 24 hours) supported Co ( Co adsorption / support process). The obtained mixture containing Pt—Co alloy-supported carbon has Co supported twice.
(4) Co was supported on the mixture obtained in (3) by executing the Co adsorption / supporting step of (3). The mixture containing the Pt—Co alloy-supported carbon obtained here is supported three times.
(5) Co was supported on the mixture obtained in (4) by executing the Co adsorption / supporting step of (3). The mixture containing the Pt—Co alloy-supported carbon obtained here has a support count of four.
(6) Co was supported on the mixture obtained in (5) by executing the Co adsorption / supporting step of (3). The mixture containing the Pt—Co alloy-supported carbon obtained here is supported five times.

(7) Calculation of alloying degree of Pt—Co alloy and Co content in catalyst (2), (3), (4), (5), (6) For the sample from which the unreduced metal was removed by washing with sulfuric acid (mol / liter), an ICP emission analyzer [manufactured by Nippon Jarrell-Ash Co., Ltd. (current Thermo Fisher Scientific), model: IRIS / AP] was used. Thus, the total Co content (Co _tot ) was determined, and XRD analysis was performed using an X-ray diffractometer [manufactured by Rigaku Corporation, model: RINT TTR-III] to obtain an X-ray diffraction pattern. The obtained X-ray patterns are respectively the X-ray diffraction pattern of (2), the X-ray diffraction pattern of (3), the X-ray diffraction pattern of (4), the X-ray diffraction pattern of (5), and the X of (6). A line diffraction pattern is used.
The Fe content _(Co al _/ Co _tot), was calculated using the following equation (1).

Wherein (1), _{Co al} denotes the content of Co was platinum alloyed (atom%). Co _tot (total Co content) and Pt _tot (total Pt content) in the formula (1) are obtained from ICP analysis, and X _Co (atomic fraction of Co in the Pt—Co alloy) is expressed by the following formula (2) (Vegard equation).

Wherein (2), a is the lattice constant determined from the peak position in the Pt in the X-ray pattern of the resulting samples, _{a 0} is the lattice constant of bulk Pt (0.3925nm), _{a s} the _Pt 3 Co The lattice constant (0.3831 nm) and Xs is the atomic fraction of Pt ₃ Co (0.25).
The alloyed and Co al of each mixture _(content of platinum and alloyed Co) was calculated by the following method.
(2) using a lattice constant calculated from the peak position of the X-ray diffraction pattern after calculating the X _Co of, Co _al contained in alloyed and mixtures from all Co content obtained from ICP (2) (Content ratio of Co alloyed with Pt) was calculated.
Using a lattice constant calculated from the peak position of the X-ray diffraction pattern of (3) in then calculating the X _Co, Co _al contained in alloyed and mixtures from all Co content obtained from ICP (3) (Content ratio of Co alloyed with Pt) was calculated.
Using a lattice constant calculated from the peak position of the X-ray diffraction pattern of (4) After calculating the X _Co, Co _al contained in alloyed and mixtures from all Co content obtained from ICP (4) (Content ratio of Co alloyed with Pt) was calculated.
Using a lattice constant calculated from the peak position of the X-ray diffraction pattern after calculating the X _Co of (5), Co _al contained in alloyed and mixtures from all Co content obtained from ICP (5) (Content ratio of Co alloyed with Pt) was calculated.
Using a lattice constant calculated from the peak position of the X-ray diffraction pattern of (6) after calculating the X _Co, Co _al contained in alloyed and mixtures from all Co content obtained from ICP (6) (Content ratio of Co alloyed with Pt) was calculated.

  Table 1 shows the degree of alloying of the Pt—Co alloy contained in each mixture and the content (atom%) of Co alloyed with Pt. FIG. 1 shows the number of times of loading Co (times) and alloying. And the relationship between the number of times of loading Co (times) and the content of Co alloyed with Pt (atom%).

(Comparative Example 1)
(1) A step of impregnating carbon powder in an aqueous solution of H ₂ PtCl ₆ for 2 hours and physically adsorbing PtCl ₆ ^2- ions to the carbon powder, followed by hydrogen reduction (reduction temperature 150 ° C., reduction time 8 hours). This was repeated a predetermined number of times to produce Pt-supported carbon in which the amount of Pt supported relative to the total amount of Pt and carbon was 5% by mass.
(2) Co ²⁺ ions were physically adsorbed in the same manner by immersing this Pt-supported carbon in a CoCl ₂ aqueous solution (Co adsorption step).
The mixture obtained through the Co adsorption step was subjected to hydrogen reduction (reduction temperature 200 ° C., reduction time 24 hours) to obtain a mixture containing Pt—Co alloy-supported carbon (Pt—Co alloy support step). For this mixture, the degree of alloying and the Co content alloyed with Pt were calculated in the same manner as in Example 1. In addition, the mixture obtained here has one loading of Co.

(3) The mixture obtained in (2) was immersed in a CoCl ₂ aqueous solution to adsorb Co ²⁺ ions, and then hydrogen reduction (reduction temperature 200 ° C., reduction time 24 hours) supported Co. (Co adsorption / support process). The obtained mixture containing Pt—Co alloy-supported carbon has Co supported twice.
(4) The Co adsorption / supporting step (3) was performed on the mixture obtained in (3) to support Co. The mixture containing the Pt—Co alloy-supported carbon obtained here is supported three times.
(5) The Co adsorption / supporting step (3) was performed on the mixture obtained in (4) to support Co. The mixture containing the Pt—Co alloy-supported carbon obtained here has a support count of four.
(6) The Co adsorption / supporting step (3) was performed on the mixture obtained in (5) to support Co. The mixture containing the Pt—Co alloy-supported carbon obtained here is supported five times.

(7) Calculation of degree of alloying and Co content rate Each mixture obtained in (2), (3), (4), (5), (6) was washed with 0.5 M sulfuric acid to obtain unreduced The sample from which the metal was removed was subjected to ICP analysis and XRD analysis in the same manner as in (7) of Example 1, and the alloying degree of the Pt—Co alloy and the Co content alloyed with Pt were calculated. It was shown in FIG.
In FIG. 1, the horizontal axis indicates the number of times of loading (times), the vertical axis (left side) indicates the degree of alloying, and the vertical axis (right side) indicates the Co content (atom%) alloyed with Pt.
1, ◯ indicates the degree of alloying of the mixture obtained by the method of Example 1, Δ indicates the degree of alloying of the mixture obtained by the method of Comparative Example 1, and ● indicates the degree of alloying of the mixture obtained by the method of Example 1. The alloyed Co content of the obtained mixture is shown, and the triangle indicates the alloyed Co content of the mixture obtained by the method of Comparative Example 1.

(Results and discussion)
In the mixture obtained by the method of Example 1, the degree of alloying of the Pt—Co alloy is higher than that of the mixture obtained by the conventional method of Comparative Example 1, and the number of loadings of Example 1 is two times or more. Then, Co content rate was higher than the mixture obtained by the comparative example 1. In addition, when the number of loadings of Example 1 is one and the number of loadings of Comparative Example 1 are one, the mixture obtained by the method of Example 1 is obtained by the method of Comparative Example 1. The Co content was higher than that of the mixture.
From these results, it was found that according to the method of the present invention, a catalyst having a high alloying degree of the Pt—Co alloy supported on carbon and a high content of Co alloyed with Pt can be obtained.

2. Comparison of oxygen reduction activity of the catalyst obtained by the method of the example and the method of the comparative example The catalyst obtained in (3) of Example 1 (the number of times of loading Co is 2 times) and the comparative example by the following method The oxygen reduction activities of the catalysts obtained in (2) of 1 (with two Co loadings) were compared.

  A catalyst layer slurry was prepared by ultrasonically dispersing 10 mg of the catalyst in 1000 μl of an ethanol solution (EtOH 80 vol%). After 25 μl of this catalyst layer slurry was dropped onto a rotating disk electrode [outer diameter (Teflon (registered trademark) rod) 15 mm, carbon electrode portion diameter 3 mm], it was dried at room temperature saturated ethanol vapor pressure. Next, after performing pressure press at 130 ° C. and 0.2 MPa, the catalyst layer other than the carbon electrode portion was removed. Subsequently, 20 μl of a 0.2% by mass Nafion solution (manufactured by Aldrich, diluted with 5% Nafion solution by ethanol) was added dropwise, followed by drying at room temperature saturated ethanol vapor pressure. Next, this electrode was heat-treated at 130 ° C. in an air atmosphere for 30 minutes to produce a rotating disk electrode for activity evaluation against oxygen reduction reaction.

  Convection voltammetry was performed using the rotating disk electrode using the catalyst prepared by the method of Example 1 and the rotating disk electrode using the catalyst prepared by the method of Comparative Example 1 manufactured as described above. After obtaining a convection voltammogram with the rotation speed changed from 400 to 2000 rpm, a Koutechy-Levich plot was created from the current value at 0.7 V, and the activity-dominated current density serving as an index of oxygen reduction activity was determined.

As a result of convective voltammetry, it was found that the catalyst produced by the method of Example 1 had a higher activity-dominated current density than the catalyst produced by the method of Comparative Example 1. From this result, it became clear that the catalyst produced by the present invention has higher oxygen reduction activity than the catalyst produced by the comparative method.
The catalyst produced by the production method of the present invention has a high oxygen reduction activity because, in the catalyst produced by the production method of the present invention, in addition to the high degree of alloying, in the catalyst metal (Pt—Co alloy). This is probably due to the high content of alloyed Co.

3. Comparison of particle diameters of Pt-Co alloy particles supported on Pt-Co alloy-supported carbon obtained by the production method of the present invention and the comparative production method The mixture obtained by the production method of Example 1 is shown below. About the mixture obtained by the manufacturing method of the comparative example 2, the particle diameter of the Pt-Co alloy particle contained in each mixture was compared.

(Comparative Example 2)
A mixture containing Pt—Co alloy-supported carbon (mixture with only one Co support) was obtained in the same manner as in Comparative Example 1 except that the mixture obtained through the Co adsorption step was reduced with hydrogen at a reduction temperature of 1000 ° C. It was. Similarly to Comparative Example 1, Comparative Example 2 was a mixture with two Co loadings, a mixture with three Co loadings, a mixture with four Co loadings, and a five Co loading. A mixture was made.

(Calculation of particle size)
The mixture obtained in (1) of Example 1 (mixture before Co loading), each mixture obtained by the production method of Example 1 with 1 to 5 times of loading of Co, and Comparative Example 2 shown below. Analytical samples were prepared by washing each mixture having 0 to 5 Co loadings obtained by the production method with 0.5 M sulfuric acid to remove unreduced metal. Each sample was subjected to XRD analysis using an X-ray diffractometer [manufactured by Rigaku Corporation, model: RINT TTR-III] to obtain an X-ray diffraction pattern.
The particle diameter (nm) of the Pt—Co alloy was calculated from the half width of the Pt (220) peak of the X-ray diffraction pattern using the Scherrer equation, and is shown in Table 2 and FIG. Table 2 also shows the particle diameters of those with 0 Co loadings (Pt particles), but FIG. 2 shows the particle diameters of Pt-Co particles with 1 or more Co loadings. It was.

FIG. 2 shows the relationship between the number of supported Cos and the particle diameter of Pt—Co alloy particles. The horizontal axis in FIG. 2 is the number of times supported (times), and the vertical axis is the particle diameter of Pt—Co alloy particles ( nm).
In FIG. 2, ◯ indicates the particle diameter of Pt—Co alloy particles in the mixture obtained by the method of Example 1, and Δ indicates particles of Pt—Co alloy particles in the mixture obtained by the method of Comparative Example 2. Indicates the diameter.

(Results and discussion)
The particle diameter of the Pt—Co alloy particles contained in the mixture obtained by the production method of the present invention (Example 1) is larger than the particle diameter of the Pt—Co alloy particles contained in the mixture obtained by the method of Comparative Example 2. I found it small.
In the method of Example 1, the Co reduction temperature is a low temperature (200 ° C.), whereas in the method of Comparative Example 2, the Co reduction temperature is a high temperature of 1000 ° C .. Seems to have grown.
Further, it was found that both the method of the present invention and the method of Comparative Example 2 show that the particle diameter of the Pt—Co alloy particles increases as the number of Co loadings increases.

4). Examination of Co reduction temperature Co reduction temperatures of 150 ° C., 250 ° C., and 270 ° C. (Example 2, Example 3, and Example 4) in the production method of the present invention were examined.

(Example 2)
A mixture containing Pt—Co alloy-supported carbon was prepared in the same manner as in Example 1 except that the hydrogen reduction temperature of Co was set to 150 ° C. The degree of alloying of the Pt—Co alloy was calculated by the same method as in (7) of 1 and shown in Table 3 and FIG.

(Example 3)
A mixture containing Pt—Co alloy-supported carbon (Co was supported once) was prepared in the same manner as in Example 1 except that the hydrogen reduction temperature of Co was set to 250 ° C. Similarly, the degree of alloying of the Pt—Co alloy was calculated and shown in Table 3 and FIG.

Example 4
A mixture containing Pt—Co alloy-supported carbon (Co was supported once) was prepared in the same manner as in Example 1 except that the hydrogen reduction temperature of Co was set to 270 ° C. (7) of Example 1 Similarly, the degree of alloying of the Pt—Co alloy was calculated and shown in Table 3 and FIG.
FIG. 3 shows the relationship between the Co hydrogen reduction temperature and the degree of alloying of the Pt—Co alloy. The horizontal axis in FIG. 3 is the Co hydrogen reduction temperature (° C.), and the vertical axis is the Pt—Co alloy. Indicates the degree of alloying.
The results of Example 1 are also shown in the corresponding places in Table 3.

Furthermore, a polymer electrolyte fuel cell single cell using a mixture containing Pt—Co alloy-supported carbon of Example 1, Example 2, Example 3, and Example 4 (Co supported 5 times) as a cathode catalyst. Then, the power generation performance of the single fuel cell was compared. Hydrogen was supplied to the anode of the single cell and air was supplied to the cathode, and the cell voltage at a current density of 300 mAcm ⁻² was examined. Table 3 shows the ratio of the cell voltage values of Example 2, Example 3, and Example 4 with respect to the case where the cell voltage value of Example 1 is 1, and the ratio of each cell voltage value is represented by the reduction temperature. The results plotted against (° C.) are shown in FIG.

(Results and discussion)
From the results shown in Table 3 and FIG. 3, it was found that the degree of alloying increases as the reduction temperature increases. This seems to be due to the fact that the diffusion of Co atoms to Pt was promoted as the reduction temperature increased.

  On the other hand, from the results shown in Table 3 and FIG. 4, it was found that the cell voltage increased as the reduction temperature increased, and decreased when the temperature exceeded 250 ° C. This is because, as the degree of alloying increases, the activity of the catalyst is improved, and as a result, the value of the voltage increases, but when it exceeds 250 ° C., decomposition of the cation exchange resin begins to occur. The voltage is thought to have dropped.

5). Examination of anions produced by transition metal compound in solvent In the production method of the present invention, cobalt chloride, cobalt acetate, and cobalt formate (Example 1, Comparative Example 3, and Comparative Example 4) are used as the transition metal compound. In this case, the degree of alloying of the catalyst and the content of Co alloyed with Pt in the catalyst were investigated.

(Comparative Example 3)
Except that cobalt acetate [Co (CH ₃ COO) ₂ ] was used as the transition metal compound, in the same manner as in (6) of Example 1, a mixture containing Pt—Co alloy-supported carbon (the number of times Co was supported) 5 times), and for this mixture, the alloying degree of the Pt—Co alloy and the alloyed Co content (atom%) in the catalyst were determined by the same method as in (7) of Example 1, It is shown in Table 4.

(Comparative Example 4)
Except that cobalt formate [Co (HCOO) ₂ ] was used as the transition metal compound, the mixture containing Pt—Co alloy-supported carbon (Co supported number of times was 5) in the same manner as in (6) of Example 1. For this mixture, the degree of alloying of the Pt—Co alloy and the alloyed Co content (atom%) in the catalyst were determined by the same method as in Example 7 (7). This is shown in FIG.
In the corresponding part of Table 4, the results of Example 1 are also shown.

(Results and Discussion)
The results shown in Table 4 show that the degree of alloying of the catalyst of Example 1 and the Co content alloyed with Pt in the catalyst are higher than those of Comparative Examples 3 and 4. This factor can be considered as follows. The acid produced by ion exchange between cobalt ions and protons is hydrochloric acid (CoCl ₂ ⇔2H ⁺ → Co ²⁺ + 2HCl) in Example 1, and acetic acid [Co (CH ₃ COO) ₂ ⇔2H ⁺ → Co in Comparative Example 3. ²⁺ + 2CH ₃ COOH], Comparative Example 4 is formic acid [Co (HCOO) ₂ ⇔2H ⁺ → Co ²⁺ + 2HCOOH].
In the case of Example 1, the surface layer of platinum nanoparticles supported in advance is slightly dissolved by hydrochloric acid, and eutectoid of platinum ions and cobalt ions tends to occur.
On the other hand, in the case of Comparative Examples 3 and 4, since the acid produced is a weak acid, the platinum nanoparticles supported in advance do not dissolve. Therefore, it is considered that Example 1 has a higher degree of alloying and a Co content alloyed with Pt than Comparative Examples 3 and 4. Based on the above, transitions with counter anions that can generate strong acids with relatively high platinum solubility (sulfuric acid and nitric acid are effective in addition to hydrochloric acid) by ion exchange with protons. It has been found suitable to use a metal compound.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention.
(1) Although cobalt chloride is used as the transition metal compound in the above embodiment, nickel chloride, iron chloride, cobalt sulfate, cobalt nitrate, or the like may be used instead.

Claims (3)

A first step of adsorbing the platinum ions on the cation exchange resin by causing an ion exchange reaction between the first mixture containing carbon and a cation exchange resin and platinum ions;
A second step of hydrogen reducing platinum ions in the second mixture obtained by performing the first step;
In a solution comprising a third mixture obtained by performing the second step, an anion selected from chloride ion, sulfate ion and nitrate ion, and a cation of a transition metal selected from cobalt, nickel and iron A third step of adsorbing the cation of the transition metal on the cation exchange resin contained in the third mixture by causing an ion exchange reaction with the cation;
And a fourth step of performing hydrogen reduction of the cation of the transition metal in the fourth mixture obtained by executing the third step. A method for producing a fuel cell catalyst, comprising: 2. The method for producing a fuel cell catalyst according to claim 1, wherein at least one of a temperature at which platinum ions are reduced by hydrogen and a temperature at which a cation of a transition metal is reduced by hydrogen is 250 ° C. or lower. The solution containing the anion and the cation of the transition metal is obtained by dissolving a transition metal compound in a solvent, and the transition metal compound is selected from chloride ion, sulfate ion and nitrate ion in the solvent. 3. The method for producing a fuel cell catalyst according to claim 1, wherein an anion is produced and a cation of a transition metal selected from cobalt, nickel and iron. 4.

Images