JP4773622B2 - Method for producing catalyst for solid polymer electrolyte fuel cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a catalyst for a solid polymer electrolyte fuel cell. In particular, the present invention relates to a method for producing a catalyst for solid polymer electrolyte fuel cell having platinum and ruthenium supported in a composite and having poisoning resistance against carbon monoxide catalyst.
[0002]
[Prior art]
The solid polymer electrolyte fuel cell is attracting attention as a power source for electric vehicles and spacecrafts because it is compact and can take out a higher current density than a phosphoric acid fuel cell. The application of a catalyst as a measure for promoting the electrode reaction of this fuel cell has been widely used. However, what is a problem here is catalyst poisoning by carbon monoxide contained in a trace amount in the hydrogen gas as the supply fuel.
[0003]
It has been conventionally known that a catalyst in which platinum and ruthenium are supported in combination has excellent carbon monoxide catalyst poisoning resistance. Regarding the carbon monoxide catalyst poisoning resistance of this composite catalyst, ruthenium is a substance having hydrophilicity, and this is achieved by oxidizing the carbon monoxide adsorbed on platinum by OH ⁻ bonded to the ruthenium. It is considered a thing. Therefore, in order to improve the carbon monoxide catalyst poisoning resistance of the catalyst, platinum particles and ruthenium particles are highly dispersed without agglomeration, and both noble metal particles are supported as close as possible. It becomes important.
[0004]
Conventionally, as a method for supporting the metal platinum particles and metal ruthenium particles on a catalyst carrier, an aqueous solution of a platinum compound and an aqueous solution of a ruthenium compound are mixed, and a reducing agent such as carbon powder as a carrier and ethyl alcohol is added. In some cases, platinum ions and ruthenium ions are reduced to deposit platinum and ruthenium particles on the carbon powder.
[0005]
As another method, there is a manufacturing method disclosed in JP-A-9-153366. According to this, forming platinum and ruthenium as separate processes, after preliminarily depositing platinum on the surface of the conductive material, and then depositing ruthenium on the surface of the conductive material by an impregnation method, thereby forming a composite catalyst. It is disclosed that the body can obtain.
[0006]
However, since noble metal particles are angstrom order fine particles, it is difficult to support platinum particles and ruthenium particles in a state where they are always close to each other. In particular, in the method in which platinum and ruthenium are simultaneously precipitated as in the above two conventional methods, or in the method in which ruthenium is precipitated by a simple impregnation method, agglomeration of ruthenium particles may occur, and the carbon monoxide catalyst is poisoned. In view of the above, there is a problem that it is difficult to obtain a catalyst having optimum characteristics.
[0007]
[Problems to be solved by the invention]
An object of the present invention is a polymer solid oxide fuel cell catalyst excellent in poisoning of carbon monoxide catalyst, in which platinum particles and ruthenium particles are supported without agglomeration, and both are supported in close proximity. It is in providing the manufacturing method of.
[0008]
[Means for Solving the Problems]
The inventors of the present invention have come up with the present invention that solves the above-mentioned problems by reexamining the conventional method for producing a solid polymer electrolyte fuel cell catalyst. As already described, in the step of supporting both noble metals of the composite catalyst in which platinum and ruthenium are supported, the process of reducing and precipitating the noble metal from the aqueous solution is important. This can be described in more detail as follows. When reducing the noble metal ions in the aqueous solution with the reducing agent, if the concentration product of the reducing agent concentration and the noble metal ion concentration is too small, no noble metal reduction reaction occurs and no noble metal precipitation occurs. On the other hand, when the concentration of the reducing agent is excessive, the reduction reaction of the noble metal occurs at every part on the support due to the reducing power of the reducing agent itself, and the noble metal particles are precipitated in an aggregated state. That is, conventionally, only the reducing power of the reducing agent dominates the dispersed state of the noble metal and determines the quality of the catalyst.
[0009]
In contrast, the present inventor made platinum and ruthenium supported in separate steps for the composite catalyst in which platinum and ruthenium were supported, and intentionally lowered the reducing agent concentration in the ruthenium reduction step, It has been found that the reduction of ruthenium ions occurs when a platinum catalyst having platinum supported on a carrier is allowed to coexist in a ruthenium reducing atmosphere. And it discovered that ruthenium precipitated in the state which adjoined to the platinum particle at this time. This is because, when a platinum catalyst coexists in the solution, oxidation of the reducing agent due to the oxidation of platinum occurs in the vicinity of the platinum particles, and electrons are emitted, and these emitted electrons are involved in the ruthenium ions, and the ruthenium ions are It is based on the phenomenon that it is reduced to metal ruthenium and deposited on the support. At this time, the electron donating reaction occurs only in the range where the oxidizing power in the vicinity of the platinum particles reaches, whereas in the site where platinum does not exist, ruthenium does not precipitate due to the low reducing agent concentration. As a result, the metal ruthenium is deposited in the state of being close to the platinum particles.
[0010]
That is, the present invention preferentially utilizes the above-described electron donating reaction between the reducing agent and ruthenium mediated by the platinum catalyst while controlling the addition amount (reducing agent concentration) within an appropriate range. Is the basic principle. And in order to produce the above effects, the present invention, as described in claim 1, mixes a platinum catalyst and a ruthenium compound solution to produce a mixed solution, and the mixed solution and a gas or liquid having a reducing action Is taken into account in a static mixer.
[0011]
Hereinafter, the present invention will be described in more detail by clarifying each step of the catalyst production method according to the present invention.
[0012]
In the method for producing a polymer electrolyte fuel cell catalyst according to the present invention, first, platinum fine particles are supported on a carrier to produce a platinum catalyst. Examples of the method for producing the platinum catalyst include a method in which a carbon powder carrier is added to and mixed with an aqueous platinum compound solution, and a reducing agent is added to and mixed therewith to reduce platinum fine particles. Here, as the platinum compound aqueous solution, dinitrodiamine platinum nitric acid aqueous solution, chloroplatinic acid aqueous solution and the like can be applied. Moreover, as a reducing agent, sodium borohydride aqueous solution, alcohol, hydrogen gas, etc. can be applied, but alcohol, particularly ethyl alcohol is preferred. Further, the amount of platinum supported on the platinum catalyst thus produced is not particularly limited, but is preferably 20 to 50% by weight.
[0013]
Next, this platinum catalyst is mixed in an aqueous solution of a ruthenium compound. Here, as a kind of ruthenium compound aqueous solution, there are ruthenium chloride, ruthenium nitrate, an aqueous solution of ruthenium complex, etc., but an aqueous solution of ruthenium chloride (RuCl ₃ ) is preferable. The composition of the mixed solution is preferably such that the concentration of the platinum catalyst is 1 to 20% by weight and the ruthenium concentration is 0.1 to 3% by weight. This is because if the balance between the two is not appropriate, the catalyst finally produced has poor catalytic activity or carbon monoxide catalyst poisoning resistance.
[0014]
Then, this mixed solution and a reducing agent are mixed to precipitate ruthenium, but it is preferable to apply hydrogen gas or an aqueous sodium borohydride solution as described in claim 3 to be used as the reducing agent in the present invention. . This is because these reducing agents have sufficient reducing power, and the catalyst produced using these reducing agents does not produce a residue that causes catalyst poisoning.
[0015]
Here, in the present invention, the mixing ratio of the reducing agent and the mixed solution is important. Regarding the mixing amount of the reducing agent, when hydrogen gas is used as the reducing agent, the flow rate of 100% hydrogen gas is set to 0.1 to 5 L / min, and the flow rate of the mixed solution is set to 0.03 to 0.00. It is preferable to mix both at 5 L / min. Regarding the flow rate of hydrogen gas, with hydrogen gas at a flow rate of 0.1 L / min or less, ruthenium is not easily reduced even with the oxidizing power of the platinum catalyst, and when hydrogen gas of 5 L / min or more is mixed, This is because the reduction reaction causes the reduction reaction of ruthenium to occur outside the vicinity of the platinum particles, and the aggregation of the ruthenium particles occurs. In addition, when the mixed solution is less than 0.03 L / min, the dispersibility of ruthenium supported on the platinum catalyst is lowered, and when it exceeds 0.5 L / min, the reduction of ruthenium is difficult to occur.
[0016]
On the other hand, when using a sodium borohydride aqueous solution as the reducing agent, first, the concentration of the sodium borohydride aqueous solution is preferably 0.01 to 2.0% by weight. A 0.01% by weight or less sodium borohydride solution is too dilute and the ruthenium reduction reaction does not proceed sufficiently, and a 2.0% by weight or more sodium borohydride solution causes agglomeration of ruthenium particles, resulting in coarse catalyst particles. This is because. And as a preferable range of the flow volume of sodium borohydride aqueous solution and a mixed solution, both are 0.1-4 L / min. This is because the sodium borohydride solution cannot cause a sufficient reduction reaction at less than 0.1 L / min. On the other hand, if the mixed solution is less than 0.1 L / min, the dispersibility of ruthenium supported on the platinum catalyst is lowered. And the reason why the upper limit of the flow rate of both solutions is set to 4 L / min or less is that if it exceeds 4 L / min, the internal pressure of the piping of the production apparatus becomes too high, and the operational safety is lowered. In addition, the flow rate of this sodium borohydride aqueous solution and mixed solution should just be in the said range, and does not need to be the same flow rate.
[0017]
Furthermore, in the present invention, it is important to make the mixing condition of the mixed solution and the reducing agent appropriate. This is because, as described above, the reducing agent added in the present invention is dilute, and therefore, whether or not ruthenium is precipitated and the state of ruthenium particle dispersion depend on the contact between the mixed solution and the reducing agent. For this reason, in this invention, it is supposed that a static mixer is applied to mixing with a mixed solution and a reducing agent. The static mixer is used because the mixed solution produced in the present invention has a high viscosity because the fine carbon powder is suspended in the aqueous solution. For mixing a highly viscous solution with a gas or liquid, a static mixer is used. This is because a mixer is most suitable. The length of the static mixer used is preferably 3 to 30 mm in inner diameter and 10 to 50 cm in length (number of elements: 20 to 100) in any reducing agent. Below this length or number of elements, mixing is insufficient and ruthenium precipitation is incomplete, and above this length or number of elements, the contact time between the solution and the reducing agent becomes excessive, and ruthenium precipitation is in the vicinity of the platinum particles. This is because it occurs in other cases.
[0018]
The mixed solution in which the reduction reaction has been completed by the above method contains a catalyst in which platinum and ruthenium are well dispersed. And a catalyst can be obtained by filtering and drying this solution. Then, as described in claim 7, the catalyst can be heat-treated to bring both noble metal particles closer to each other to form an alloy, and the carbon monoxide catalyst poisoning resistance is further improved. The alloying by this heat treatment is preferably performed in the range of 600 ° C to 900 ° C. This is because the alloying of the noble metal particles is incomplete at 600 ° C. or lower, while the aggregation of the catalyst particles proceeds and the particle size becomes excessive at 900 ° C. or higher, which affects the activity of the catalyst.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
In the following, preferred embodiments of the present invention will be described. In the present embodiment, a composite catalyst is manufactured by two kinds of methods using a hydrogen gas and an aqueous sodium borohydride solution as a reducing agent, and as a comparative example, the ruthenium is precipitated by the conventional impregnation method. Catalysts were manufactured and the properties of each catalyst were studied.
[0020]
FIG. 1 schematically shows a catalyst production apparatus used in the following embodiments. In FIG. 1, a catalyst manufacturing apparatus includes a raw material tank 1 that contains a liquid for mixing a platinum catalyst and a ruthenium solution, a reducing agent tank 2 that contains a reducing agent, and a static mixer 3 that mixes the mixed solution and the reducing agent. And a suction filter 4 for collecting the fuel cell catalyst by suction filtration of the solution after the reduction reaction mixed by the static mixer 3, and these are connected by a pipe. Further, the suction filter 4 includes an ion exchange resin tower 6 that supplies pure water for washing the fuel cell catalyst catalyst 5 after filtration. FIG. 1 shows a case where a sodium borohydride aqueous solution is used as a reducing agent and a sodium borohydride solution is accommodated as a reducing agent tank. When hydrogen gas is applied as the reducing agent, the reducing agent is used. A hydrogen cylinder is applied to the tank.
[0021]
First Embodiment : 100 g of carbon powder (trade name Vulcan XC72) was mixed with 4500 g of a dinitrodiamine platinum nitric acid solution containing 15% by weight of platinum. After stirring and mixing, 550 mL of 98% ethyl alcohol was added as a reducing agent. This solution was stirred and mixed at about 95 ° C. for 6 hours, and platinum was supported on carbon powder to obtain a platinum catalyst (platinum supported amount: 5.64 g).
[0022]
Next, 720 mL of water was added to 35.47 g (ruthenium: 2.92 g) of a ruthenium chloride solution containing 8.232 wt% ruthenium, mixed and stirred, and then mixed with 25 g of the platinum catalyst produced by the above method. And stored in the raw material tank 1. The platinum catalyst and ruthenium concentrations of the mixed solution at this time are 3.5 wt% and 0.4 wt%, respectively.
[0023]
Then, the mixed solution produced by the above method and the reducing agent were mixed to precipitate ruthenium. In this case, ruthenium is precipitated using 100% hydrogen gas as the reducing agent, the hydrogen gas flow rate is 1000 mL / min, the mixed solution flow rate is 0.07 L / min, the diameter is 6 mmφ, and the length is 30 cm (number of elements: 60). Were mixed using a static mixer. The reaction temperature was 25 ° C.
[0024]
The solution after completion of the reaction is filtered with a suction filter 4 and then dried at 60 ° C. to form a powder, which is then alloyed by holding at 900 ° C. for 1 hour in 50% hydrogen gas (balance: nitrogen gas). Heat treatment was performed to obtain a fuel cell catalyst.
[0025]
Second Embodiment : In this embodiment, the catalyst was produced using a sodium borohydride aqueous solution instead of hydrogen gas as a reducing agent. Therefore, the basic embodiment is not different from the first embodiment. Therefore, the description which overlaps is avoided and only the difference in the form in the ruthenium precipitation process is described.
[0026]
In this ruthenium precipitation step, a platinum catalyst was prepared in the same manner as in the first embodiment, and 450 mL of water was added to 35.47 g (ruthenium: 2.92 g) of a ruthenium chloride solution containing 8.232 wt% ruthenium. After mixing and stirring, 25 g of platinum catalyst (platinum supported amount: 5.64 g) was immersed. Then, the flow rate of this mixed solution and 0.1% sodium borohydride aqueous solution and the flow rate of the mixed solution were both set to 0.4 L / min. The mixture was reacted and reacted to precipitate ruthenium. Then, the solution after completion of the reaction was filtered, dried and heat-treated in the same manner as in the first embodiment to obtain a fuel cell catalyst.
[0027]
Comparative Example : In order to confirm the effect of the method for producing a catalyst according to the present invention, ruthenium was deposited on a platinum catalyst by an impregnation method as a conventional technique to produce a composite catalyst. After adding 100 mL of water to 35.47 g of ruthenium chloride solution (ruthenium: 2.92 g), mixing and stirring, 25 g of platinum catalyst (platinum: platinum: supported on carbon powder in advance by the same method as in the first embodiment). 5.64 g) was added. This mixed solution was stirred at room temperature for 1 hour to impregnate the ruthenium solution. Thereafter, the solution was dried at 60 ° C., and the dried product was reduced under hydrogen reflux at 250 ° C. for 0.5 hour and further at 900 ° C. for 0.5 hour to obtain a fuel cell catalyst.
[0028]
The platinum / ruthenium catalyst produced by the above production method was observed with an electron microscope on the surface of the fuel cell catalyst, and the dispersion state of platinum and ruthenium was examined. FIG. 2 shows a transmission electron micrograph (TEM) of the surface of the fuel cell catalyst according to the first embodiment. FIG. 3 shows a TEM photograph of the surface of a fuel cell catalyst produced by depositing both noble metals from a platinum and ruthenium compound mixed solution as a comparative example. In the photographs of FIGS. 2 and 3, the degree of aggregation of the catalyst particles is expressed by the density of the black spots, and the darker the color of the black spots, the more the catalyst particles are aggregated. From these comparisons, it can be seen that the fuel cell catalyst produced by the method according to the present invention has platinum and ruthenium particles uniformly dispersed, and further has a good dispersion state in which both noble metal particles are close to each other. On the other hand, on the surface of the fuel cell catalyst produced by the conventional method, it was confirmed that there was some particle aggregation due to the presence of dark black spots everywhere. There was a clear difference from the fuel cell catalyst.
[0029]
FIG. 4 shows a TEM photograph of the surface of the fuel cell catalyst according to the second embodiment. In contrast to the comparative example of FIG. 3, the fuel cell catalyst produced by the method according to the present invention is excellent in the uniform dispersibility of particles and the closeness of both noble metal particles as in the case of hydrogen reduction. It turns out that it is in a distributed state.
[0030]
Next, for the fuel cell catalysts according to the first and second embodiments and the comparative example manufactured by the above manufacturing method, a hydrogen electrode side half cell was prepared, and carbon monoxide catalyst poisoning resistance was confirmed. The examination result is shown in FIG. The measurement was performed in hydrogen gas mixed with 100 ppm of carbon monoxide, and the measurement temperature was 60 ° C. In FIG. 5, the horizontal axis represents the current density, the vertical axis represents the measured polarization value, and the polarization value of the electrode catalyst measured at each current density is plotted. As a result, for example, when the polarization values at a current density of 500 mA / cm ² were compared, the polarization value of the catalyst according to the comparative example was 70 mV, whereas the polarization values according to the first embodiment and the second embodiment were compared. The polarization values of the fuel cell catalysts were all 50 mV or less. Therefore, it was confirmed that the fuel cell catalyst according to both embodiments exhibits higher carbon monoxide catalyst toxicity than the fuel cell catalyst according to the comparative example.
[0031]
【The invention's effect】
According to the present invention, a composite fuel cell catalyst in which platinum and ruthenium particles are highly dispersed in a state in which they are close to each other can be obtained. As a result, a fuel cell catalyst excellent in carbon monoxide catalyst poisoning can be obtained. Further, by subjecting the fuel cell catalyst to a heat treatment, the platinum particles and the ruthenium particles can be brought closer to each other to be alloyed, whereby the poisoning of the carbon monoxide catalyst can be further improved.
[Brief description of the drawings]
FIG. 1 is a schematic view of a fuel cell catalyst production apparatus used in first and second embodiments.
FIG. 2 is a TEM photograph of a catalyst surface produced by the production method according to the present invention using hydrogen gas as a reducing agent.
FIG. 3 is a TEM photograph of a catalyst surface produced according to a comparative example.
FIG. 4 is a TEM photograph of a catalyst surface produced by the production method according to the present invention using an aqueous sodium borohydride solution as a reducing agent.
FIG. 5 is a diagram showing a potential-current density in a hydrogen electrode side half cell test performed on fuel cell catalysts manufactured in the first and second embodiments and the comparative example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Raw material tank 2 Reductant tank 3 Static mixer 4 Suction filter 5 Fuel cell catalyst catalyst 6 Ion exchange resin tower

Claims (4)

A platinum catalyst in which platinum fine particles are supported on a carrier is mixed with a ruthenium compound solution to produce a mixed solution, and the mixed solution is reacted with a gas or liquid having a reducing action in a static mixer to reduce ruthenium. A method for producing a catalyst for a solid polymer electrolyte fuel cell , comprising:
Producing a mixed solution such that the concentration of the platinum catalyst is 1 to 20% by weight and the ruthenium concentration is 0.1 to 3% by weight;
Production of solid polymer electrolyte fuel cell catalyst using hydrogen gas as the reducing agent and reacting the mixed solution having a flow rate of 0.03 to 0.5 L / min with hydrogen gas having a flow rate of 0.1 to 5 L / min Method. Ruthenium is reduced by mixing a platinum catalyst with platinum fine particles supported on a carrier and a ruthenium compound solution to produce a mixed solution, and reacting the mixed solution with a gas or liquid having a reducing action in a static mixer. A method for producing a catalyst for a solid polymer electrolyte fuel cell, comprising:
Producing a mixed solution such that the concentration of the platinum catalyst is 1 to 20% by weight and the ruthenium concentration is 0.1 to 3% by weight;
Using the sodium borohydride aqueous solution having a concentration of 0.01 to 2% by weight as the reducing agent, the mixed solution having a flow rate of 0.1 to 4 L / min and the sodium borohydride aqueous solution having a flow rate of 0.1 to 4 L / min. A method for producing a catalyst for a solid polymer electrolyte fuel cell to be reacted. The method for producing a polymer electrolyte fuel cell catalyst according to claim 1 or 2, wherein a static mixer having an inner diameter of 3 to 30 mm and a length of 10 to 50 cm is used as the static mixer. A polymer solid electrolyte type in which platinum and ruthenium on a support are alloyed by further heat-treating the polymer solid oxide fuel cell catalyst produced by the method according to any one of claims 1 to 3. A method for producing a catalyst for a fuel cell.

Images