Method for manufacturing catalyst having supported catalyst particles of core/shell structure

Abstract

A method for forming catalyst particles, each of which has a core/shell structure, by a Cu-UPD method. Namely, a method of manufacturing a catalyst wherein catalyst particles, each of which has a core/shell structure composed of a shell layer that is formed of platinum and a core particle that is covered with the shell layer and is formed of a metal other than platinum, are supported on a carrier. This method is characterized by comprising: an electrolysis step wherein the carrier supporting the core particles is electrolyzed in an electrolytic solution containing copper ions, so that copper is precipitated on the surfaces of the core particles; and a substitution reaction step wherein a platinum compound solution is brought into contact with the core particles, on which copper has been precipitated, so that the copper on the surface of each core particle is substituted by platinum, thereby forming a shell layer that is formed of platinum. This method is further characterized in that the platinum compound solution in the substitution reaction step contains citric acid.

Claims

1. A method of manufacturing a catalyst comprising a catalytic particle supported on a carrier, the carrier comprising electrically-conductive carbon powder or electrically-conductive ceramic powder, the catalytic particle having a core/shell structure comprising: a shell layer; and a core particle covered with the shell layer and comprising a metal other than platinum, the method comprising the steps of: subjecting said catalytic-particle-supported carrier to electrolysis in a copper-ion-containing electrolytic solution, thereby depositing copper on a surface of the core particle, as an electrolytic treating process; and bringing a platinum compound solution into contact with the copper-deposited core particle to substitute the copper on the surface of the core particle with platinum, thereby forming a shell layer comprising platinum, as a substitution reaction process; wherein the platinum compound solution in the substitution reaction process contains citric acid, prior to the electrolytic treating process, an electrolytic apparatus is placed within a closed space where an oxygen concentration has been reduced before introduction of the electrolytic apparatus, and bubbling of inert gas is performed into the electrolytic solution for 4 hours or more and 48 hours or less, and an amount of dissolved oxygen in the electrolytic solution in the electrolytic treating process is controlled to 1 ppm or lower.

2. The method of manufacturing a catalyst according to claim 1, wherein a content of the citric acid in the platinum compound solution is 40-fold or less on the basis of the number of moles of the platinum compound.

3. The method of manufacturing a catalyst according to claim 1, wherein the electrolytic treating process is an electrolytic treatment, which is conducted by accumulating the core-particle-supported carrier on a working electrode.

4. The method of manufacturing a catalyst according to any of claims 1, 2 and 3, wherein a metal which constitutes the core particle is palladium, iridium, rhodium, gold, or an alloy of these metals.

5. The method of manufacturing a catalyst according to claim 2, wherein the electrolytic treating process is an electrolytic treatment, which is conducted by accumulating the core-particle-supported carrier on a working electrode.

6. The method of manufacturing a catalyst according to claim 2, wherein a metal which constitutes the core particle is palladium, iridium, rhodium, gold, or an alloy of these metals.

7. The method of manufacturing a catalyst according to claim 3, wherein a metal which constitutes the core particle is palladium, iridium, rhodium, gold, or an alloy of these metals.

8. The method of manufacturing a catalyst according to claim 1, wherein a dissolved oxygen content in the platinum compound solution in the substitution reaction step is 1 ppm or less.

9. The method of manufacturing a catalyst according to claim 1, wherein an amount of the platinum compound in the platinum compound solution in the substitution reaction step is from an equivalent quantity to quadruple quantity on a basis of a necessary number of moles of platinum compound calculated from Cu-UPD.

10. The method of manufacturing a catalyst according to claim 2, wherein a dissolved oxygen content in the platinum compound solution in the substitution reaction step is 1 ppm or less.

11. The method of manufacturing a catalyst according to claim 2, wherein an amount of the platinum compound in the platinum compound solution in the substitution reaction step is from an equivalent quantity to quadruple quantity on a basis of a necessary number of moles of platinum compound calculated from Cu-UPD.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an illustrative drawing of the constitution of the electrolysis apparatus used in the present embodiment.

(2) FIG. 2 is an illustrative drawing of the conventional method of manufacturing a catalytic particle having a core/shell structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) The best mode of the embodiment in the present invention will be explained below.

First Embodiment

(4) In this embodiment, a catalyst containing supported catalytic particle having a core/shell structure where palladium is employed as the core particle was manufactured, and then activity thereof was evaluated. At first, 35 g of carbon powder (trade name: Ketjen Black EC, specific surface area: 800 g/m.sup.3), which was to be the carrier of the catalyst, was immersed in a palladium chloride solution (amount of Pd, 15 g (0.028 mol)), then the solution was neutralized with sodium carbonate. The resultant material was subjected to a reduction treatment with sodium formate to produce carbon powder supporting a palladium particle which was to be the core particle.

(5) Subsequently, the palladium particle surface was covered with a copper layer. The electrolysis apparatus used in the present embodiment is shown in FIG. 1. The electrolysis apparatus shown in FIG. 1 is equipped with a counter electrode tube having platinum mesh as a counter electrode and reference electrode inserted in an electrolysis vessel housing an electrolytic solution. The bottom part of the electrolysis vessel is composed of carbon blocks which act as a working electrode. The counter electrode, reference electrode and working electrode are connected to a potential control device.

(6) In the electrolytic treatment of the palladium particles, 6 L of a sulfuric acid solution (0.05 M) was introduced into the electrolysis vessel at first, then 50 g (0.32 mol) of copper sulfate was dissolved therein, followed by pretreatment for reducing a dissolved oxygen content. In this pretreatment, nitrogen was blown into a glove box so as to make the oxygen concentration therein about 0 ppm at first, then nitrogen blowing into the glove box and nitrogen bubbling into the electrolytic solution were performed for 12 hours, while the electrolysis apparatus was placed therein. Then, it was confirmed that the dissolved oxygen content in the electrolytic solution was 1 ppm or less prior to the electrolytic treatment. Ten grams of the carbon powder supporting the palladium particles in the manner described above were immersed to the bottom part of the electrolysis vessel, then copper is electrolytically deposited while the potential was controlled with the potential control device. The electrolysis conditions in this electrolytic treatment are described below. In addition, the nitrogen blowing into the glove box and nitrogen bubbling into the electrolytic solution were continued also during the electrolytic treatment.

(7) Electrolysis Conditions

(8) Potential: fixed potential at 0.39 V (vs. RHE)

(9) Potential fixing time: 3 hours

(10) After the electrolytic treatment step, 3.4 g (0.0083 mol) of potassium chloroplatinate was dissolved in the electrolysis vessel as a platinum compound solution. Also, 48 g of citric acid was added simultaneously. Herewith, the displacement reaction between the copper on the palladium core particle surface and platinum. The reaction period of time in this displacement reaction step was set to 1 hour. After forming the platinum shell layer, the carbon powder was filtered and recovered, then washed with pure water followed by drying at 60 C. to give the catalyst.

(11) The amount of the catalyst obtained in the manufacturing step described above is 10 g. This amount manufactured shows that 100000-fold or more amount of the catalyst can be manufactured in one step on the basis of that in the conventional Cu-UPD technique (g order).

Comparative Examples 1, 2

(12) Commercially available catalysts containing a platinum particle and platinum alloy particle, respectively, were provided for the comparison with the catalyst containing the supported catalytic particle having the core/shell structure. The provided catalysts are a platinum catalyst (trade name: TEC10E50E) and platinum-cobalt catalyst (trade name: TEC36E52).

(13) Then, the activities (Mass Activities) were determined for the catalysts of First Embodiment and Comparative Examples 1 and 2, respectively. The evaluation method employed is to investigate an oxygen reduction activity while rotating a rotary disk electrode with 8 g of a catalyst applied thereto in an electrolytic solution. Flowing oxygen reduction currents were determined for a range of 0.1 V-1.0 V at a sweeping rate of 5 mV/s, while this electrode was rotated at each steady rate (1000 rpm, 1250 rpm, 1500 rpm, 1750 rpm, 2000 rpm, 2250 rpm, 2500 rpm) in an electrolytic solution saturated with oxygen. After the measurement, the mass activities of platinum were calculated in a manner where the current value at 0.9 V for each rotational rate was approximated by the Koutecky-Levich equation, followed by calculation of the mass activity from the kinetically controlled current. These results are shown in Table 1.

(14) TABLE-US-00001 TABLE 1 Mass Activity (at 0.90 V A/g.sub.Pt) First Embodiment 598 Comparative 185 Example 1 Comparative 315 Example 2

(15) Table 1 illustrates that the catalyst manufactured in the present embodiment exhibits an extremely higher oxygen reduction activity in comparison with the platinum catalyst and platinum-cobalt catalyst of Comparative Examples. The manufacturing method of the present embodiment can be confirmed to be satisfactory also from a characteristic view point of the manufactured catalyst.

Second Embodiment

(16) In this embodiment, catalysts were manufactured while the amount of citric acid added in the displacement reaction step following the electrolytic treatment step was varied. Palladium-nickel alloy was employed as the core particle. The same carbon powder as used in the First Embodiment which was to be a carrier was immersed in a solution of palladium nitrate (amount of Pd, 53 g (0.50 mol)) and nickel nitrate (amount of Ni, 176 g (3.0 mol)), then the solution was neutralized with sodium hydroxide. Then, a particle comprising a palladium-nickel alloy was formed on the carbon powder by means of a heat treatment. Subsequently, this carrier was immersed in 0.5 M sulfuric acid (80 C.) to remove the nickel. Such formation of the palladium-nickel alloy particle and removal of the nickel are performed in order to form pores on the alloy particle surface due to elution of the nickel, thereby enhancing the surface area and activity thereof.

(17) Then, copper was electrolytically deposited on the core particle surface in the apparatus and under the conditions same as those employed in the First Embodiment. Furthermore, potassium chloroplatinate and citric acid were added to the electrolytic solution after the electrolysis, followed by formation of the platinum shell in the same way as First Embodiment. In this embodiment, plural catalysts were manufactured while the amount of citric acid added was varied. Additionally, a catalyst treated without addition of citric acid was also manufactured. Then, activities of the catalysts were evaluated in the same way as First Embodiment. These results are shown in Table 2.

(18) TABLE-US-00002 TABLE 2 Amount of citric acid added Amount Mass Activity added Molar ratio to Pt (at 0.90 V A/g.sub.Pt) 0 g zero-fold 287 16 g tenfold 384 32 g 20-fold 370 48 g 30-fold 684 64 g 40-fold 635

(19) It can be confirmed that the existence of citric acid addition results in difference of the catalytic activity from Table 2. The amount of citric acid added is preferably a tenfold to 40-fold amount.

INDUSTRIAL APPLICABILITY

(20) The present invention is to optimize the platinum displacement treatment step in a method of manufacturing a catalyst according to the Cu-UPD technique, thereby achieving an enhanced amount of the catalyst produced. According to the present invention, a catalyst having a core/shell structure which exhibits a satisfactory activity can be efficiently manufactured, and also a cost reduction effect due to reduction in the amount of platinum used can be expected.