JP6778472B2 - Platinum alloy powder and its manufacturing method - Google Patents

Description

The present invention relates to a platinum alloy powder and a method for producing the same, and more particularly to a platinum alloy powder used for an electrode catalyst of an alkaline fuel cell and a method for producing the same.

A fuel cell is a cell that generates electricity by oxidizing fuel such as hydrogen or methanol and reducing oxygen, and is a very useful power source from the viewpoint of protecting the global environment because of its high theoretical energy conversion efficiency.

Among fuel cells, all-solid-state alkaline fuel cells use an anionic conductive polymer as an electrolyte and the electrode atmosphere becomes alkaline, so transition metals other than platinum do not elute, and various metal catalysts can be used. It has superior properties to other fuel cells in that it can use various fuels.

JP-A-2010-131581

However, since platinum is expensive, a catalyst containing only platinum is not preferable because it increases the price of an alkaline fuel cell. On the other hand, a non-platinum catalyst has lower activity than platinum, and systematic catalyst development has been carried out. Absent.

Therefore, a catalyst consisting only of nickel or cobalt other than platinum does not have a sufficient catalytic function. Therefore, while containing platinum, the amount of expensive platinum used can be reduced, and platinum and other metals such as nickel and cobalt can be used. There is a demand for a highly active catalyst in the above alloy.

Therefore, an object of the present invention is to provide a method for producing a platinum alloy powder that can be used as an electrode catalyst having a high catalytic function, and a platinum alloy powder obtained by the method for producing the platinum alloy powder.

The method for producing a platinum alloy powder according to the present invention that achieves the above-mentioned object is a platinum alloy powder in which an alloy powder of platinum and a transition metal other than iron is acid-treated to expose platinum on the surface of the alloy powder. In the production method, the transition metal is nickel or cobalt, and the content of platinum in the platinum alloy powder is 0.99 mol% or more and 6.25 mol% or less.

Further, the platinum alloy powder according to the present invention that achieves the above-mentioned object is a platinum alloy powder composed of platinum and a transition metal other than iron, and the platinum is exposed on the surface. It is nickel or cobalt, and the content of the platinum is 0.99 mol% or more and 6.25 mol% or less.

In the present invention, by exposing platinum to the surface, it is possible to more effectively utilize the catalytic function of platinum and obtain a platinum alloy powder having a high catalytic function.

The CV measurement result of the PtNi ₃ catalyst subjected to the acid treatment of this invention is shown. The CV measurement result of the PtCo ₃ catalyst subjected to the acid treatment of this invention is shown. The LSV measurement results before and after the acid treatment of the PtNi ₃ catalyst are shown. The LSV measurement results before and after the acid treatment of the PtCo ₃ catalyst are shown. The mass activity of the PtNi ₃ catalyst and the PtCo ₃ catalyst before and after the acid treatment is shown. The mass activity per total metal weight of Samples 1 to 6 and the mass activity per platinum weight are shown. The mass activity per total metal weight and the mass activity per platinum weight due to the difference in the nickel ratio to platinum are shown. FIG. 7 (A) shows the mass activity of PtNi ₇ , and FIG. 7 (B) shows the mass activity of PtNi ₁₅ . Shows mass activity. The mass activity per platinum weight before and after the acid treatment of PtNi _x (x = 0, 3, 7, 15) is shown. The X-ray diffraction result by the difference of the acid treatment condition of PtNi ₃ is shown. The mass activity per total metal weight and the mass activity per platinum weight due to the difference in acid treatment conditions of PtNi ₃ are shown.

Hereinafter, a method for producing a platinum alloy powder to which the present invention is applied and the platinum alloy powder obtained by the production method will be described in detail with reference to the drawings. The present invention is not limited to the following detailed description unless otherwise specified. Hereinafter, a case where the platinum alloy powder is applied to the electrode catalyst (hereinafter, referred to as a platinum alloy catalyst) will be described.
1. 1. Platinum alloy catalyst 2. Manufacturing method of platinum alloy catalyst

<1. Platinum alloy catalyst ＞
Platinum alloy catalysts are used, for example, as catalysts for cathodes and anodes of alkaline fuel cells. The platinum alloy catalyst is composed of platinum and a transition metal excluding iron (hereinafter, simply referred to as a transition metal). In this platinum alloy catalyst, platinum is exposed on the surface by eluting the transition metal existing on the surface of the alloy powder of platinum and the transition metal by an acid treatment described later. The exposure of platinum is part or all of the surface of the platinum alloy catalyst.

The transition metal is not particularly limited, but chromium, manganese, cobalt, nickel, copper, molybdenum, ruthenium, rhodium, palladium, silver, tungsten, iridium, gold, these metal compounds, and two or more of these metals are used. It is a fine particle made of an alloy containing the mixture, and is not limited to a binary system but may be a ternary system or more. Among these, cobalt, inexpensive nickel, and the like are preferable as the transition metal. Therefore, the platinum alloy catalyst is not limited to the binary system, and may be a ternary system or more.

The platinum content is 25 mol% or less based on the total platinum alloy catalyst obtained. The platinum alloy catalyst can sufficiently exert its catalytic function even if the platinum content is as small as 25 mol% or less. That is, since this platinum alloy catalyst uses a small amount of platinum, the cost of the catalyst can be reduced, and the cost of the alkaline fuel cell can also be reduced.

The particle size of the platinum alloy catalyst is 20 nm or less, preferably 10 nm or less, and more preferably 5 nm or less.

This platinum alloy catalyst can be adsorbed to a general catalyst carrier such as carbon black with high dispersibility like a conventional catalyst.

The platinum alloy catalyst as described above is an alloy of platinum and a transition metal in which platinum and a transition metal are not eluted in an alkaline solution, and platinum is exposed on the surface, so that platinum alone or platinum is exposed. It has higher activity than the platinum alloy catalyst that has not been used, and has an excellent function as a catalyst. Further, since the platinum alloy catalyst has high activity even if the platinum content is small, the battery characteristics of the alkaline fuel cell can be improved at low cost.

<2. Platinum alloy catalyst manufacturing method>
The method for producing a platinum alloy catalyst consists of platinum and a transition metal, and the surface of the alloy powder is covered with the transition metal by eluting the transition metal existing on the surface of the alloy powder of the platinum and the transition metal. A platinum alloy catalyst can be produced by exposing the platinum.

(Alloy powder)
The alloy powder is an alloy of platinum and a transition metal. In this alloy powder, a part of platinum is covered with a transition metal, and the other part is exposed and alloyed with platinum. The alloy powder is obtained by mixing platinum and a transition metal in a predetermined ratio, raising the temperature to a predetermined temperature under an inert atmosphere using an electric furnace, heating for a predetermined time, and then allowing to cool. To obtain the alloy powder adsorbed on the catalyst carrier, platinum, the transition metal, and the catalyst carrier are mixed and similarly heated.

An alcohol such as isopropyl alcohol may be added when the platinum and the transition metal are mixed, or when the platinum, the transition metal and the catalyst carrier are mixed. By adding alcohol to make a paste, it can be mixed uniformly. The inert atmosphere is created by blowing an inert gas such as hydrogen, nitrogen, or a mixed gas thereof into the reaction vessel. The heating temperature of the mixture of platinum and the transition metal or the mixture of platinum, the transition metal and the catalyst carrier is about 500 ° C. to 1000 ° C. The heating time varies depending on the mixing ratio of platinum and the transition metal, etc., but is, for example, about 10 minutes to 12 hours.

The mixing ratio of platinum and the transition metal is not particularly limited, but is 1: 3 to 1: 100. By setting the mixing ratio of platinum and the transition metal in the range of 1: 3 to 1: 100, the function as a catalyst can be exhibited while suppressing the content of platinum.

(Acid treatment)
The purpose of the acid treatment is to elute the transition metal covering the platinum. The acid treatment exposes platinum on the surface of the alloy powder by eluting the transition metal existing on the surface of the alloy powder of platinum and the transition metal, effectively utilizing the catalytic function of platinum, and platinum by alloying. The catalytic function of the obtained platinum alloy catalyst is improved by changing the distance and / or the electronic state. Since the exposure of platinum differs depending on the acid treatment conditions and the state of the alloy powder described later, the proportion of platinum exposed after the acid treatment differs depending on these conditions and conditions. The platinum alloy catalyst is not limited to the case where platinum is exposed on the entire surface of the platinum alloy catalyst, but also includes a case where a part of the surface is covered with a transition metal and platinum is exposed on other parts.

The acid treatment is a method in which the alloy powder is immersed alone or in an acidic solution with the alloy powder adsorbed on a catalyst carrier such as carbon, the alloy powder alone or the alloy powder is adsorbed on a catalyst carrier such as carbon. After that, there is a method of applying it to an electrode such as a glassy carbon electrode, immersing it in an acidic solution in a state of being set in an RDE (Rotating Disk Electrode) device, performing potential sweep, and performing an electrochemical treatment.

The acidic solution is not particularly limited, but a general acidic solution such as a perchloric acid solution, a sulfuric acid solution, a nitric acid solution, a hydrochloric acid solution, or an acetic acid solution can be used. The pH of the acidic solution is preferably in the range of 1 to 3, and more preferably about pH 1.

When the acid treatment is carried out by immersing in an acidic solution, the treatment time is not particularly limited because it varies depending on the pH of the acidic solution, stirring conditions and the like.

The conditions for the potential sweep are not particularly limited, but are, for example, performed in the range of 0.05 V to 1.2 V with respect to the reversible hydrogen electrode at an appropriately determined sweep rate and number of cycles.

In the acid treatment, it is preferable to stir the acidic solution. As the stirring method, a stirrer may be used, or the electrode itself may be rotated to stir. The rotation speed is not particularly limited. In the acid treatment, the dissolution rate of the transition metal can be controlled by stirring.

The acid treatment is preferably carried out in an inert gas atmosphere by aerating hydrogen gas, nitrogen gas or the like. The acid treatment method and conditions are not limited to the above-mentioned methods as long as the transition metal covering platinum can be eluted.

In the above method for producing a platinum alloy catalyst, an alloy powder of platinum and a transition metal or an alloy powder is immersed in an acidic solution while being supported on a catalyst carrier, or an electrochemical treatment is performed in a state of being immersed in the acidic solution. This makes it possible to obtain a platinum alloy catalyst in which a part of the transition metal on the surface of the alloy powder is eluted and the platinum inside is exposed. The obtained platinum alloy catalyst is an alloy of platinum and a transition metal, and since platinum is exposed on the surface, a catalyst of platinum contained is compared with platinum alone or a platinum alloy catalyst in which platinum is not exposed. It can effectively utilize its function and has high catalytic activity due to changes in the distance between platinum due to alloying and / or changes in the electronic state. Therefore, by using the obtained platinum alloy catalyst as a catalyst for an alkaline fuel cell, it is possible to obtain an alkaline fuel cell having good battery characteristics at low cost.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

[Example 1]
<Exposure of platinum by acid treatment>
Regarding the platinum-nickel alloy (PtNi ₃ ) and the platinum-cobalt alloy (PtCo ₃ ), the one before the acid treatment and the one after the acid treatment are compared, and the exposure of platinum by the acid treatment gives catalytic activity. The presence or absence of impact was evaluated.

First, carbon black particles supporting a platinum-nickel alloy powder and carbon black particles supporting a platinum-cobalt alloy powder were produced as follows. In the following, carbon black particles carrying platinum-nickel alloy powder or platinum-cobalt alloy powder will be referred to as a catalyst.

The method for producing the catalyst is as follows: First, the precursor and carbon black particles so that the amount of catalyst is 0.5 g (inner metal 0.2 g, carbon black 0.3 g) and Pt: M (nickel or cobalt) = 1: 3. Was mixed in an agate mortar. On the way, 1.5 mL of isopropyl alcohol (IPA) was added and further mixed. Next, in an electric furnace (furness), H ₂ gas and N ₂ gas were supplied and the temperature was raised to 800 ° C. over 2 hours in an inert atmosphere. The temperature was kept constant for 2 hours. Then, the H ₂ gas was stopped and the mixture was allowed to cool to room temperature while supplying the N ₂ gas to obtain a catalyst.

When the obtained catalyst was evaluated by a transmission electron microscope (TEM), it was confirmed that metal particles having a diameter of about 3 nm were highly dispersed and supported on carbon black.

Next, acid treatment was performed. First, the catalyst is applied to the glassy carbon electrode of the RDE apparatus to obtain an electrode in which the catalyst is supported on the glassy carbon. Specifically, a glassy carbon (geometric region: 0.196 cm ² ) electrode polished with aluminum was used. The catalyst dispersion was prepared by mixing 10 mg of the catalyst in 12.5 mL of a 24% isopropyl alcohol solution and 50 μL of a perfluorocarbon material (trade name Nafion manufactured by SIGMA-ALDRICH, 5 wt%). did. Then, 10 μL of the dispersion liquid of the catalyst was applied to the surface of the glassy carbon electrode and dried at room temperature. As a result, an electrode in which a catalyst was supported on glassy carbon was obtained.

Next, an electrode in which a catalyst was supported on glassy carbon was set in the RDE apparatus. The electrode was immersed in a 0.1 M perchloric acid solution (HClO ₄ aq) in which N ₂ was aerated while being set in the RDE device, and swept in the range of 0.05 V to 1.2 V with respect to the reversible hydrogen electrode. Acid treatment was performed by CV measurement at a speed of 100 mV / sec, 50 mV / sec, and 20 mV / sec. Next, after aerating N ₂ into a 0.1 M sodium hydroxide aqueous solution (NaOHaq), the electrode after acid treatment is immersed in the sodium hydroxide aqueous solution, and CV measurement is performed in the sodium hydroxide aqueous solution at a sweep rate of 20 mV / sec. went.

The CV measurement result of PtNi ₃ is shown in FIG. 1, and the CV measurement result of PtCo ₃ is shown in FIG. From the results shown in FIGS. 1 and 2, since the peak area around 0.1 to 0.3 V increases after the acid treatment, the surface area of platinum exposed by the acid treatment increases. I understand. That is, it can be seen that a part of platinum is exposed on the surface of the platinum alloy catalyst by the acid treatment.

Further, the electrode on which the catalyst is supported on glassy carbon is swept at a rotation speed of 1600 rpm and the Hg / HgO (mercury / mercury oxide) electrode is swept from −0.85 V to 0.4 V at 20 mV / s. LSV measurement was performed. The LSV measurement result of PtNi ₃ at a rotation speed of 1600 rpm is shown in FIG. 3, and the LSV measurement result of PtCo ₃ is shown in FIG. In the LSV measurement, the region where the reduction current density increases around 0.8 to 1V with respect to the reversible hydrogen electrode is related to the reaction activity, and the higher the absolute value of the current density at the noble potential, the higher the oxygen reduction activity. It will be expensive. From the results shown in FIGS. 3 and 4, both PtNi ₃ and PtCo ₃ have an oxygen reduction current rising from a noble potential due to acid treatment, and at a specific potential in the region near 0.8 to 1.0 V. By comparison, the current density was higher after the acid treatment, indicating that the acid treatment increased the oxygen reduction activity.

FIG. 5 shows the results of determining the mass activity per platinum weight from the LSV measurement results for PtNi ₃ and PtCo ₃ . From the results shown in FIG. 5, PtNi ₃ and PtCo ₃ have almost the same or slightly higher mass activity before the acid treatment than the catalyst in which Pt is adsorbed on carbon black (manufactured by Tanaka Kikinzoku Co., Ltd .: TKK), but the acid. It can be seen that it becomes very high after processing.

[Example 2]
<Differences in acid treatment conditions (relationship with electrochemical treatment and stirring)>
The platinum-nickel alloy (PtNi ₃ ) was subjected to acid treatment under the acid treatment conditions shown in Table 1, and the mass activity of the obtained platinum alloy catalyst was evaluated. Note that sample 1 is not subjected to acid treatment. Acid solution impregnation means that a catalyst (carbon black particles carrying alloy powder) is applied to a glassy carbon electrode and dried, and the electrode on which the catalyst is supported on glassy carbon is coated with a 0.1 M perchloric acid aqueous solution (HClO ₄ aq). ), The catalyst was acid-treated under the conditions shown in Table 1. In the electrochemical treatment of sample 2, the potential sweep was performed in the order of 20 cycles at a sweep rate of 100 mV / s, 5 cycles at 50 mV / s, and 5 cycles at 20 mV / s.

Further, FIG. 6 shows the mass activity of Samples 1 to 6 per total metal weight and the mass activity per platinum weight. The mass activity was calculated from the LSV measurement results by performing LSV measurement on each sample. The LSV measurement conditions are the same as those of the LSV measurement performed in <Exposure of platinum by acid treatment> of Example 1 described above.

From the results shown in FIG. 6, the mass activity of the acid-treated samples 2, 4 to 6 is increased as compared with the acid-treated sample 1. In particular, in Sample 5, the same activity as that of Sample 2 subjected to the electrochemical treatment was obtained. Therefore, from the results shown in FIG. 6, it can be seen that high activity can be obtained if a structure in which platinum is exposed can be formed regardless of the acid treatment method.

[Example 3]
<Difference in Ni ratio to Pt>
The mass activity due to the difference in the nickel ratio to platinum was evaluated. FIG 7 (A), shows the PtNi _7, which the acid treatment was conducted under the same conditions as sample 2 and 5 of Table 1, and the comparison of the mass activity of PtNi ₇ not subjected to acid treatment (Sample 1) .. The FIG. 7 (B), the show for PtNi _15, having been subjected to the acid treatment under the same conditions as sample 2 and 5 of Table 1, and PtNi ₁₅ not subjected to acid treatment comparisons of the mass activity (Sample 1) .. The mass activity was calculated from the LSV measurement results by performing LSV measurement on each sample. The LSV measurement conditions are the same as those of the LSV measurement performed in <Exposure of platinum by acid treatment> of Example 1 described above.

From the results shown in FIG. 7, the mass activity was increased by the acid treatment regardless of the nickel ratio, and in PtNi ₁₅ , the ones subjected to the same acid treatment as the samples 2 and 5 had high mass activity. It turns out that it can be obtained.

From the results shown in FIGS. 6 and 7, the mass activity per platinum weight of PtNi _x (x = 0, 3, 7, 15) before and after the acid treatment is summarized in FIG. The acid treatment is the same as in Sample 5.

From the results shown in FIG. 8, it can be seen that the platinum alloy catalyst after the acid treatment has high activity regardless of the Ni ratio. Further, it can be seen that the acid-treated platinum alloy catalyst has higher activity than platinum alone before and after acid treatment, and has higher catalytic function than platinum, which is said to have high activity.

[Example 4]
<Differences in acid treatment conditions (acid treatment of alloy powder only)>
Acid treatment is performed by stirring carbon black particles carrying an alloy powder of platinum and nickel (PtNi ₃ ) in an acid solution (0.1 M HClO ₄ aq) for 1 hour or 3 hours at room temperature or 60 ° C. went. Then, X-ray diffraction (XRD: X-ray diffraction) was performed on the alloy powder not treated with acid and the alloy powder treated with acid. The result is shown in FIG. From the results shown in FIG. 9, the peak intensities in the surface indices (111) and (200) derived from nickel alone are lower in those after acid treatment than those without acid treatment, and the acid treatment time. It can be seen that as the temperature increases, the amount of decrease in peak intensity increases and the amount of transition metal elution increases.

Further, LSV measurement was performed on the one not subjected to the acid treatment and the one subjected to the acid treatment, and the result of determining the mass activity from the measurement result is shown in FIG. The LSV measurement conditions are the same as those of the LSV measurement performed in <Exposure of platinum by acid treatment> of Example 1 described above. From the results shown in FIG. 10, in the state of the alloy powder, the mass activity is also increased by the acid treatment, and the conditions are more severe than the acid treatment at 1 hour / room temperature, that is, 3 hours / room temperature or 1 hour / 60. It can be seen that under the condition of ℃, the mass activity decreases, but the mass activity becomes higher than that without the acid treatment. In FIG. 9, h is time and rt is room temperature.

From the results of Examples 1 to 3, the alloy powder of platinum and the transition metal was acid-treated to expose a part of platinum to the surface, as compared with platinum alone or the alloy powder not subjected to acid treatment. It can be seen that the activity is increased and the catalytic function can be improved.

Claims (6)

A method for producing a platinum alloy powder in which an alloy powder of platinum and a transition metal other than iron is acid-treated to expose platinum on the surface of the alloy powder.
The transition metal is nickel or cobalt.
A method for producing a platinum alloy powder, wherein the content of platinum in the platinum alloy powder is 0.99 mol% or more and 6.25 mol% or less. The method for producing a platinum alloy powder according to claim 1, wherein the acid treatment is performed by immersing the alloy powder in an acidic solution. The method for producing a platinum alloy powder according to claim 2, wherein the acid treatment further performs potential sweep. A platinum alloy powder composed of platinum and a transition metal other than iron, with the platinum exposed on the surface.
The transition metal is nickel or cobalt.
The platinum alloy powder has a platinum content of 0.99 mol% or more and 6.25 mol% or less. The platinum alloy powder according to claim 4, wherein the platinum is exposed on a part of the surface. The platinum alloy powder according to claim 4 or 5, which is an electrode catalyst for an alkaline fuel cell.