JP5792514B2 - Catalyst production method - Google Patents

Description

  The present invention relates to a method for producing a catalyst comprising platinum particles and a conductive carrier carrying the platinum particles, and more particularly to a method for producing a catalyst capable of suitably covering a surface of platinum particles with a copper layer.

  Conventionally, surface treatments such as plating, physical vapor deposition (PVD), and chemical vapor deposition (CVD) are generally performed by coating the surface of a substrate such as particles with a different material, while maintaining the properties of the substrate. This may be done to modify only the properties of the substrate surface. On the other hand, this type of surface treatment is used for reducing the amount of expensive metal material used as a coating material, improving the utilization rate, or suppressing characteristic deterioration.

  For example, an electrode for a fuel cell is a catalyst in which platinum particles are supported on a conductive carrier such as granular carbon, and a platinum particle itself is used as a catalyst for an oxygen sensor or the like. Such platinum particles are particles of several nanometers made of platinum or a platinum alloy. In such a catalyst, the surface of the platinum particles and the vicinity thereof function as a catalyst. Therefore, improvement in the utilization rate (reduction in the amount of use) of expensive platinum or a platinum alloy is desired.

  Furthermore, it is known that when such a catalyst is used in an environment where a fuel cell is used, platinum particles may be oxidized and eluted, and may be coarsened by reprecipitation, resulting in a decrease in cell performance. Therefore, a technique has been proposed in which a thin gold layer is formed on a part of the surface of the platinum particles (gold is modified on the surface of the platinum particles) to suppress the oxidation and elution of the platinum particles.

For example, (1) by fixing platinum particles on an electrode (working electrode), immersing the electrode in an aqueous copper sulfate solution in a nitrogen atmosphere, and applying an appropriate reduction potential to the electrode, a copper monoatomic layer Is deposited on the surface of the platinum particles to produce copper-coated platinum particles, (2) the copper-coated platinum particles are rinsed with purified water together with the electrode, copper ions present in the solution are removed, and (3) the copper-coated platinum particles Has been proposed in which a monoatomic layer of copper is replaced with gold by immersing the substrate in a HAuCl ₄ aqueous solution (see, for example, Patent Document 1).

  Thus, according to the manufacturing apparatus described in Patent Document 1, the surface of platinum particles is coated with a copper monoatomic layer using an underpotential deposition (UPD) method, and the copper monoatomic layer is galvanically replaced with gold. By doing so, a catalyst in which gold is modified on the surface of the platinum particles can be obtained.

Special table 2009-510705 gazette

  Here, as shown in Patent Document 1, as a step before obtaining a catalyst in which gold is modified on the surface of platinum particles, the precipitation of a copper monoatomic layer (copper layer) using the UPD method should normally be coated. A working electrode (WE) in which particles are fixed, a counter electrode (CE) and a reference electrode (RE) with respect to the working electrode are immersed in a solution containing copper ions, and the working electrode is set to a predetermined position using an external power source. This is done by holding the potential.

  However, since an apparatus such as that shown in Patent Document 1 uses an external power supply, the apparatus becomes large, and the external power supply must adjust the potential of the working electrode with respect to the reference electrode, which is complicated. Furthermore, platinum particles must be applied to the surface of the working electrode and fixed, which is not suitable for mass production of the catalyst.

  The present invention has been made in view of such problems, and its object is to easily mass-produce a catalyst including at least platinum particles coated with a copper layer on its surface without using an external power source. Another object is to provide a method for producing a catalyst.

  As a result of intensive studies, the inventors have obtained the following new findings. Specifically, when at least a part of a copper material (electrode) is immersed in an equilibrium acid aqueous solution containing copper ions, the potential of the copper material is equal to the copper standard electrode potential (about 0.35 V) regardless of the location. ). When this copper material and platinum particles (or a conductive carrier carrying platinum particles) are brought into contact, the potential of the platinum particles becomes the same as that of the copper material. When the platinum particles are brought into contact with the acid aqueous solution in this state, UPD occurs almost instantaneously, and the surface of the platinum particles is covered with a copper layer for one atomic layer. In this way, the copper material itself in contact with the acid aqueous solution utilizes an unprecedented principle that it functions as a power source of about 0.35 V without an external power source.

  However, when such a principle is used, since platinum particles or conductive carriers are suspended in the acid aqueous solution (suspension), all the platinum particles come into contact with the copper material, and the surface of the platinum particles is contacted. It is not easy to determine whether a copper layer has been coated. Therefore, the inventors use a working electrode made of platinum or a platinum alloy, and contact the platinum particles on which the copper layer is formed with this working electrode, so that the above-described principle and the time of the working electrode with respect to the reference electrode are changed. New knowledge that it was possible to judge whether or not all the platinum particles were covered with the copper layer was confirmed by confirming a change in potential.

  The present invention is based on the inventors' new knowledge, and the method for producing a catalyst according to the present invention includes the surface of platinum particles made of platinum or a platinum alloy, or the conductive carrier carrying platinum particles. In this method, the surface of platinum particles is coated with a copper layer. In the production method, in the state where the copper material is immersed in a suspension obtained by suspending an acid aqueous solution containing copper ions with the platinum particles or the conductive carrier, the suspension is stirred and the copper material is mixed with the copper material. A deposition step of depositing a copper layer on the surface of the platinum particles by contacting the platinum particles or the conductive support, and a deposition state of the copper layer on the surface of the platinum particles in the deposition step according to the deposition step And an evaluation process to be evaluated. The evaluation step includes an immersion step in which a reference electrode and a working electrode made of platinum or a platinum alloy are immersed in the suspension. By stirring the suspension, the platinum particles on which the copper layer is formed are obtained. A measuring step of measuring a time until the potential of the working electrode becomes a constant potential with respect to the reference electrode while depositing a copper layer on the surface of the working electrode by contacting the working electrode; and after the measuring step The removal step of removing the copper layer deposited on the working electrode is repeatedly performed, and the deposition step is completed based on the amount of change in the measurement time in the repeated measurement step. It is characterized by that.

  According to the present invention, an acid aqueous solution containing copper ions is suspended by stirring the suspension while the copper material is immersed in the suspension suspended in the platinum particles or the conductive carrier. The platinum particles or conductive carrier contained in the liquid comes into contact with the copper material in the acid aqueous solution containing copper ions. Thereby, the platinum particle | grains by which the copper layer (copper monoatomic layer) for 1 atomic layer was coat | covered on the surface, or the electroconductive support | carrier carrying this can be obtained without an external power supply.

That is, without an external power source, a Cu dissolution reaction (Cu → Cu ²⁺ + 2e ⁻ ) occurs as an oxidation reaction on the surface of the copper material (copper electrode), and a Cu precipitation reaction (Pt + Cu ²⁺ + 2e) occurs on the platinum particle surface. ^- → Cu-Pt) occurs. And since generation | occurrence | production of oxygen gas does not arise in the surface of a copper material, a copper monoatomic layer (copper layer) is hard to peel from a platinum particle.

  During stirring, some of the platinum particles whose platinum particles do not come into contact with the copper material are not coated with a copper layer (not precipitated), so the surface of all these platinum particles is covered with a copper layer. In order to confirm whether or not it has been done, an evaluation step is performed in parallel with the precipitation step to evaluate the precipitation state of the copper layer on the surface of the platinum particles in the precipitation step.

  In an evaluation process, a reference electrode, a working electrode, and a counter electrode are immersed as needed. For the working electrode, an electrode made of platinum or a platinum alloy is used. By stirring the suspension, platinum particles that are not coated with a copper layer come into contact with the copper material, and a copper layer (copper monoatomic layer) is deposited on the surface thereof. On the other hand, at the working electrode, platinum particles on which a copper layer (copper monoatomic layer) is deposited come into contact, and a copper layer is deposited on the surface thereof.

  Here, as the suspension is stirred, the proportion of the platinum particles whose surface is coated with the copper layer increases, and accordingly, the proportion of the copper layer deposited on the surface of the working electrode also increases. When the copper layer is deposited on the entire surface where the working electrode is immersed, the potential of the working electrode with respect to the reference electrode becomes a constant potential regardless of the stirring of the suspension. In the state where the copper layer is deposited on all platinum particles, the time until the potential of the working electrode becomes a constant potential (that is, the deposition of the copper layer on the entire surface of the working electrode from the start of the deposition of the copper layer of the working electrode) The time until this is approximately constant. Using this point, the completion of the deposition of the copper layer on the surface of all platinum particles is judged.

  That is, in the present invention, a series of steps consisting of a measurement step and a removal step is repeated, and the precipitation step is terminated based on the amount of change for each measurement time measured in the repeated measurement step. That is, when there is almost no change amount per measurement time (nearly zero), in other words, when the previous measurement time and the current measurement time in the repeated measurement process are almost the same, all the platinum particles It can be considered that the deposition of the surface copper layer has been completed. At this time, for example, the agitation of the suspension is finished, the copper material is taken out of the suspension, or the suspension itself is taken out, thereby completing the precipitation step.

  Thus, in addition, in the conventional UPD using an external power source, platinum particles or conductive carriers were fixed on the surface of the working electrode. By simply contacting the copper material, the surface of the platinum particles can be coated with the copper layer, and furthermore, it can be easily determined that the surface of all the platinum particles is coated with the copper layer. Thereby, the platinum particle with which the copper layer was coat | covered, or the electroconductive support | carrier which carry | supported this can be mass-produced easily.

  Here, the platinum particles referred to in the present invention are particles containing platinum atoms as a main material, and are particles made of platinum or an alloy thereof. Also. The copper material is a member made of a material that acts as a copper electrode (copper is ionized), and is a member made of copper or an alloy thereof. Moreover, the catalyst as used in the field of this invention means the catalyst which consists only of platinum particles, or the catalyst which consists of an electroconductive support | carrier with which platinum particles were carry | supported, and means what contains at least platinum particles.

  In addition, in the evaluation process, by comparing the measurement time of the measurement process that is repeatedly performed, in order to determine the end of precipitation of the copper layer on the surface of the platinum particles, It is preferable that the area of the working electrode immersed in is always constant regardless of the repeated immersion process.

  Therefore, as a more preferred embodiment, as the working electrode, a working electrode coated with a glass material on the base end side so that at least the tip side is exposed is used, and in the dipping step, all the exposed portions on the tip side are used. However, the working electrode is immersed so as to be immersed in the suspension. By setting it as such an aspect, the fixed area of a working electrode can be easily immersed in suspension in the immersion process performed repeatedly.

  Further, the material used for the reference electrode is not particularly limited, as long as it is chemically stable with respect to the suspension, and is not particularly limited, but in a preferred embodiment, A reference electrode made of copper is used as the reference electrode, and in the immersion step, the acid aqueous solution in the suspension is in contact with the reference electrode, and the platinum particles or the conductive carrier in the suspension The reference electrode is immersed so as not to come into contact.

  According to this aspect, by using copper for the reference electrode, when the working electrode is coated with a copper layer, the potential of the working electrode with respect to the reference electrode becomes the standard electrode potential of copper (about 0.35 V). It can be easily determined that the working electrode is coated with the copper layer. In addition, since the reference electrode is in contact with the acid aqueous solution in the suspension so as not to come into contact with the platinum particles or the conductive carrier, the surface of the reference electrode is stabilized (the potential of the reference reference electrode does not change). ). Thereby, the potential of the working electrode with respect to the reference electrode can be stably measured.

  If the copper layer attached to the surface of the working electrode can be removed in the removal step, the removal method is not particularly limited, such as mechanical removal, chemical removal, or electrochemical removal. Absent. However, as a more preferred embodiment, in the removing step, the potential of the working electrode with respect to the reference electrode is controlled to a potential higher than the standard electrode potential of copper, whereby the copper layer coated on the surface of the working electrode Remove.

  According to this aspect, the potential of the working electrode with respect to the reference electrode is controlled to a potential higher than the standard electrode potential of copper, so that the copper formed on the surface of the working electrode is immersed in the suspension. The layer elutes in the suspension (in the aqueous acid solution), and the copper layer can be quickly removed from the surface of the working electrode. Thereby, the copper surface of a working electrode can be made into the state again immersed in suspension, without taking out a working electrode.

  According to the present invention, a catalyst containing at least platinum particles having a copper layer coated on the surface can be easily mass-produced without using an external power source.

The schematic schematic of the apparatus used for the manufacturing method of the catalyst which concerns on embodiment of this invention. The flowchart for demonstrating the manufacturing method of the catalyst which concerns on this invention. It is a schematic diagram for demonstrating the state of the catalyst in the flow shown in FIG. 2, (a) is sectional drawing of the catalyst before a precipitation process, (b) is a partial expansion of the catalyst after a precipitation process. Sectional drawing. It is a schematic diagram for demonstrating the state of the surface of the working electrode in the evaluation process shown in FIG. 2, (a) is the figure which showed the state before a copper layer coating platinum catalyst contacts a working electrode, (b) is a figure for demonstrating the state immediately after the contact, (c) is a figure for demonstrating the state which the copper layer deposited on the working electrode. The figure which showed the relationship between the electric potential of the working electrode at the time of the 1st copper layer deposition in an Example, and deposition time. The figure which showed the relationship of the electric potential of the working electrode at the time of the 2nd-5th copper layer deposition in an Example, and deposition time. The figure which showed the result of the cyclic voltammetry for confirming the precipitation state of a copper layer.

Embodiments of the catalyst production method according to the present embodiment will be described below.
FIG. 1 is a schematic diagram of an apparatus used in a method for producing a catalyst according to an embodiment of the present invention. FIG. 2 is a flowchart for explaining the method for producing a catalyst according to the present invention. FIG. 3 is a schematic diagram for explaining the state of the catalyst in the flow shown in FIG. 2, (a) is a sectional view of the catalyst before the deposition step, and (b) is a view of the catalyst after the deposition step. It is a partial expanded sectional view.

  FIG. 4 is a schematic diagram for explaining the state of the surface of the working electrode in the evaluation step shown in FIG. 2, and (a) is a diagram showing the state before the copper layer-covered platinum catalyst contacts the working electrode. (B) is a figure for demonstrating the state immediately after the contact, (c) is a figure for demonstrating the state which the copper layer deposited on the working electrode.

  The method for producing a catalyst according to the present invention is a method for producing a catalyst containing platinum particles made of platinum alloy or platinum with a copper layer coated on the surface, and a deposition step of depositing a copper layer on the surface of the platinum particles shown below. The catalyst can be manufactured through an evaluation step for evaluating the deposition state of the copper layer in the deposition step, and a gold substitution step for replacing the copper layer with gold after the evaluation (after completion of the deposition step).

1. Precipitation process As shown in Drawing 2, deposition process S21 which precipitates a copper layer on the surface of platinum particles is performed. Specifically, platinum particles or conductive carriers carrying platinum particles are prepared. In the present embodiment, a platinum-carrying conductor 3a in which platinum particles 32 are carried on a conductive carrier 31 shown in FIG. The platinum particles are made of a platinum alloy or platinum, and examples of the platinum alloy include a PtFe alloy, a PtMo alloy, a PtCu alloy, a PtRu alloy, a PtSn alloy, a PtW alloy, a PtCo alloy, a PtNi alloy, a PtIr alloy, and a PtAu alloy. The platinum alloy is not particularly limited as long as it is a platinum alloy that can act as a catalyst. Further, the size of the platinum particles is not particularly limited. For example, when used for an electrode catalyst for a fuel cell, 2 to 10 nm is preferable.

Examples of the conductive carrier material supporting platinum particles include carbon, titanium oxide (TiO ₂ , Ti ₄ O _7, etc.), molybdenum oxide, tantalum oxide, and the like. , Plate shape, rod shape, or net shape.

  A copper monoatomic layer (copper layer) is deposited on the surface of such platinum particles (or platinum particles supported on a conductive carrier) by the UPD method, and the surface of the platinum particles is coated with the copper layer. Specifically, a suspension of an acid aqueous solution containing copper ions suspended in the above-described platinum-supported conductor 3a in a copper deposition tank 18 shown in FIG. 1 under an inert gas (nitrogen gas, argon gas, etc.) atmosphere. The turbid liquid 11 is introduced, and at least a part of the flat copper material 10 is immersed in the suspension 11. In this state, the suspension 11 is stirred by the stirrer 17 to bring the copper material 10 and the conductive carrier (or platinum particles) into contact.

  At this time, the copper material 10 in contact with the acid aqueous solution (suspension 11) becomes equal to the standard electrode potential (about 0.35 V) of copper, and further, the platinum particles (or the conductive carrier in contact with the copper material 10 are removed). The potential of the interposed platinum particles is the same as that of the copper material 10. In such a state, the acid aqueous solution is in contact with the platinum particles in the state where the acid aqueous solution and the copper material 10 and between the platinum particles and the copper material 10 are in contact with each other, as shown in FIG. As described above, the copper layer-covered platinum-supported conductor 3b in which the copper monoatomic layer (copper layer) 33 is deposited on the surface of at least a part of the platinum particles 32 can be obtained almost instantaneously.

That is, without an external power source, a Cu dissolution reaction (Cu → Cu ²⁺ + 2e ⁻ ) occurs as an oxidation reaction on the surface of the copper material 10, and on the surface of a part of the platinum particles, Cu ions from the aqueous acid solution form Cu ions. A precipitation reaction (Pt + Cu ²⁺ + 2e ⁻ → Cu—Pt) occurs.

Thus, when the copper material 10 is brought into contact with the acid aqueous solution (suspension 11), copper ions are eluted. In the acid aqueous solution in which copper ions are present, dissolution of copper and precipitation of copper ions occur on the surface of the copper material 10. When this reaches an equilibrium state, the surface potential of the copper material at this time becomes the standard electrode potential of copper. When platinum particles (which may be provided with a conductive carrier) are brought into contact with the copper material 10 in such a state, the potential of the platinum particles becomes the same potential. At this potential (equilibrium potential), a copper monoatomic layer is deposited on the surface of the platinum particles.

  Here, the copper material 10 is a member having at least a surface thereof made of copper or a copper alloy (not necessarily entirely made of copper or a copper alloy), and a direct connection between the copper material, the platinum particles, and the aqueous acid solution. Or indirectly, it can conduct ionically.

  Specifically, the copper material 10 is preferably high-purity copper, and low-purity copper containing inevitable impurities as long as the potential of the platinum particles can be the standard electrode potential of copper (about 0.35 V). Or a copper alloy containing an alloy element. For example, a copper alloy containing a metal (for example, Zn, Sn, etc.) whose oxidation-reduction potential is lower than copper is not desirable in this embodiment. This is because the base metal elutes preferentially over copper (or the electrode has a potential lower than the standard electrode potential of copper during elution). Therefore, a copper alloy containing a metal (for example, Pt, Ag, Au, Pd, Ir, etc.) nobler than copper can be suitably used for the copper material 10.

  In the case where a copper alloy is used for the copper material 10, the copper elution rate becomes slower as the copper content decreases. Moreover, as copper elution progresses, the ratio of copper on the surface of the copper material decreases, and direct or indirect contact between copper and platinum particles hardly occurs. Therefore, when using a copper alloy, the copper content of the copper material 10 is preferably as much as possible. In addition, although the flat copper material was used as the copper material 10, the granular piece which consists of copper or a copper alloy, a stick | rod, a thin wire | line, a net | network (mesh), a lump etc. can be mentioned.

Moreover, the acid aqueous solution which comprises the suspension 11 is an acid solution in which UPD is possible for platinum particles, and is obtained by dissolving a soluble copper salt in an acid aqueous solution. Examples of the soluble copper salt include copper sulfate (CuSO ₄ ), copper chloride (CuCl ₂ ), copper acetate (Cu ₂ (CH ₃ COO) ₄ ), and copper nitrate (Cu (NO ₃ ) ₂ ). it can.

Here, the concentration of copper ions in the acid aqueous solution is not particularly limited, and an optimum concentration can be selected according to the purpose. In the present embodiment, copper is eluted from the copper material by the amount of copper deposited on the surface of the platinum particles by UPD. Therefore, the reaction proceeds even when the copper ion concentration is at the μM level. However, industrially, it is preferable to proceed the reaction promptly, and in that case, the copper ion concentration is preferably 1 mM or more. On the other hand, when the copper ion concentration is excessive, the solute is likely to precipitate. Accordingly, the copper ion concentration is preferably not more than the saturation concentration, and more preferably not more than 90% of the saturation concentration.

  The acid contained in the aqueous solution is not particularly limited as long as it can dissolve Cu. Examples of such acids include nitric acid, sulfuric acid, perchloric acid, hydrochloric acid, and boric acid. The combination of the Cu salt and the acid is not particularly limited, and can be arbitrarily selected according to the purpose.

  Furthermore, if the pH of the acid aqueous solution becomes too high, it is generally difficult to bring a copper material described later to a predetermined potential. Moreover, the reaction rate decreases as the pH in the aqueous solution increases. Therefore, from such a viewpoint, the pH of the aqueous solution is preferably 4 or less. On the other hand, if the pH is too low, the surface of the conductive carrier (for example, carbon) may be oxidized. Accordingly, the pH of the aqueous solution is preferably 1 or more.

  The acid aqueous solution may further contain components other than copper ions, acid, and water as long as the platinum particles or the platinum alloy is not poisoned. As other components, for example, an anion derived from a soluble copper salt, an organic solvent (for example, higher alcohols) having a low reducing property, and the like may further be included. The aqueous acid solution is preferably sufficiently bubbled with an inert gas (for example, nitrogen gas or argon gas) during or before the reaction in order to remove dissolved oxygen.

  As described above, since the copper material 10 in contact with the suspension 11 functions as a power source of 0.35 V, it is not necessary to prepare an external power source or a reference electrode in order to deposit a monoatomic layer of copper. . Moreover, since it is not necessary to apply platinum particles to the working electrode and fix them, it is suitable for mass production. Further, although the copper material also acts as a counter electrode, since only the copper dissolution reaction occurs as an oxidation reaction on the surface of the copper material, oxygen gas is not generated and the peeling of the copper layer can be suppressed.

2. Evaluation process During stirring, some platinum particles whose platinum particles do not contact the copper material are not coated with a copper layer on the surface (not precipitated), so the copper layer on the surface of all these platinum particles As shown in FIG. 2, an evaluation step S20 is performed in parallel with the precipitation step S21. The evaluation step S20 is a step of evaluating the deposition state of the copper layer on the surface of the platinum particles in the precipitation step S21. The evaluation step S20 includes an immersion step S22, a measurement step S23, a removal step S24, and a determination step S25.

  First, in the immersion step S22, as shown in FIG. 1, a working electrode 14, a counter electrode 15, and a reference electrode 16 are prepared. The working electrode 14 uses a platinum wire made of platinum or a platinum alloy coated with the glass material 12, and preferably an electrode made of the same material as the platinum particles. As the counter electrode 15, an electrode coated with a glass material 12 and having a platinum plate made of platinum or a platinum alloy connected to the front end side of a platinum wire is used. Further, a copper wire is used for the reference electrode 16, and this copper wire is inserted into a glass tube 16a having a glass frit (glass filter) 16b attached to the bottom surface. The glass frit 16b does not allow the platinum-supported conductor 3a in the suspension to pass through the glass tube 16a but allows only the aqueous acid solution to pass therethrough. And as shown in FIG. 1, the working electrode 14, the counter electrode 15, and the reference electrode 16 are immersed in the suspension 11 mentioned above.

  Next, the measurement step S23 is performed in accordance with the stirring in the precipitation step S21 described above. Specifically, as shown in FIG. 4A, in the precipitation step, a part of the platinum-supported conductor 3a becomes a copper layer-covered platinum-supported conductor 3b, which is in the suspension around the working electrode 14. Exists. By further stirring the suspension, as shown in FIG. 4B, copper 33 ′ is deposited on the surface of the working electrode 14 by bringing the copper layer-covered platinum-supported conductor 3 b into contact with the working electrode 14. This is the same as the principle of depositing a copper layer on the surface of the platinum particles described above. On the other hand, the copper layer-covered platinum-supported conductor 3b in contact with the working electrode 14 returns to the platinum-supported conductor 3a (see FIG. 3A).

  Here, in the measurement step S23, the time is measured with the time when the deposition step S21 and the dipping step S22 are started together (that is, when the copper layer starts to deposit on the working electrode) as the start time. Since copper is used for the reference electrode 16 as the copper layer in which the working electrode 14 is immersed is deposited, the potential of the working electrode 14 with respect to the reference electrode 16 approaches the standard electrode potential of copper. Finally, as shown in FIG. 4 (c), when the copper layer is deposited on the entire surface immersed in the working electrode 14, the potential of the working electrode 14 with respect to the reference electrode 16 is increased regardless of the stirring of the suspension. Since the potential is constant (copper standard electrode potential), the time t1 from the start time to the end time is calculated with this time as the end time of the measurement.

  Next, as shown in FIG. 2, in the removal step S24, the copper layer attached to the surface of the working electrode is removed. In the present embodiment, in the removing process, for example, each electrode is connected to a potentiostat, and the potential of the working electrode 14 with respect to the reference electrode 16 is controlled to a potential (for example, 0.7 V) higher than the standard electrode potential of copper. As a result, the copper layer coated on the surface of the working electrode 14 is removed. Here, since the portion in which the counter electrode 15 is immersed is a platinum plate, the removal efficiency of the copper layer on the surface of the working electrode 14 can be further increased as compared with the platinum wire. In this way, without removing the working electrode 14 from the suspension 11, the copper layer is removed from the surface, and the working electrode 14 with platinum exposed on the surface is immersed again in the suspension 11. it can.

  Further, the glass tube 16a to which the glass frit 16b is attached contacts the reference electrode 16 with the acid aqueous solution in the suspension 11, but does not contact the platinum-supported conductor 3a in the suspension. Thereby, the potential of the reference electrode 16 serving as a reference can be stabilized during the measurement.

  Here, when the series of steps including the measurement step S23 and the removal step S24 is the first time, the determination step S25 is not performed, and the series of steps including the measurement step S23 and the removal step S24 is repeated. Then, when the series of steps is repeated, the precipitation step is completed based on the amount of change for each measurement time (T1, T2,... Tn-1, Tn (n = number of repetitions)) in the measurement step S23 performed repeatedly. decide.

  Specifically, as shown in FIG. 2, the (n-1) th measurement time Tn-1 is compared with the nth measurement time Tn, and the difference (that is, the amount of change) is a predetermined time t (however, t When ≈0) or less, it is determined that the copper layer is deposited on all platinum particles, and the completion of the deposition process is determined (the deposition process is terminated). This is because when the copper layer is deposited on all platinum particles, the time until the potential of the working electrode 14 becomes a constant potential (that is, the copper on the entire surface of the working electrode from the start time of the deposition of copper on the working electrode). This is because the amount of change is substantially zero. The precipitation process is completed by removing the copper material 10 or the suspension 11 from the copper precipitation tank 18 or stopping the stirrer 17. On the other hand, when the amount of change is greater than the predetermined time t, it is determined that a part of the platinum particles have not yet deposited the copper layer, and the series of steps is repeated without terminating the precipitation step.

3. Gold Substitution Step In the gold substitution step S26, the copper layer-covered platinum-supported conductor (suspension) obtained in the precipitation step S21 is stirred in a gold ion aqueous solution in an inert gas, whereby the copper in the copper layer is agitated. Replace with gold. Thereby, the gold particles are modified on the surface of the platinum particles 32. Here, as the gold ion aqueous solution, an aqueous solution capable of replacing copper with gold, such as a salt containing AuCl ₄ ^— , may be used, and hydrochloric acid or perchloric acid may be further added to this aqueous solution.

Hereinafter, the present invention will be described by way of examples.
Copper sulfate aqueous solution (50 mM CuSO ₄ /0.1 MH ₂ SO ₄ ) dipped in copper material, 90 ml of platinum-supported carbon (30 mass% Pt / C) with 30% by mass of platinum particles, sulfuric acid solution Platinum-supported carbon was added and stirred so as to be 200 ml. As a result, a suspension containing platinum-supported carbon having a copper layer coated on the surface of platinum particles was obtained. This suspension was put into a storage tank, and 4 copper rods (φ5.5 mm) for performing the precipitation step were immersed therein. On the other hand, in order to perform the evaluation process, a working electrode made of a platinum wire (powder potential monitor) partially coated with a glass material on a potentiostat, and a copper wire in which CuSO ₄ / H ₂ SO ₄ is liquid-coupled by a glass frit And a counter electrode covered with a glass coating so as to expose the platinum plate. Then, the potential of the working electrode with respect to the reference electrode was monitored.

  Next, when the potential of the working electrode becomes 0.81 V, the suspension is stirred under a stirring condition of 300 rpm, and the potential of the working electrode with respect to the reference electrode is measured. 35V) and the time was monitored. The result is shown in FIG. At this time, the working electrode was covered with a copper layer. Thereafter, the potential of the working electrode with respect to the reference electrode was controlled to about 0.7 V, and the copper layer coated on the surface of the working electrode was removed.

  Similarly, the suspension is stirred again, and the time until the potential of the working electrode with respect to the reference electrode becomes the standard electrode potential of copper (about 0.35 V) is measured, and then the surface of the working electrode is coated. The step of removing the copper layer was further performed 4 times (5 times in total). The result is shown in FIG.

The obtained suspension was applied to a glassy carbon electrode, an electrolyte solution: 0.1M HClO ₄ , a reference electrode: a reversible hydrogen electrode, a counter electrode: a platinum plate, and a sweep rate of 100 mV / S, as shown in FIG. Click voltammetry (CV) was performed, and the amount of copper layer deposited on the surface of the platinum particles was measured from the amount of electricity from which copper was desorbed. Further, CV was repeated until the hydrogen adsorption amount on the surface of the platinum particles became constant, and the hydrogen adsorption amount was calculated. Thereby, the atomic ratio of copper and hydrogen was calculated. Such an operation was performed by collecting the suspension four times. The results are shown in Table 1.

(Results and Discussion)
As shown in FIGS. 5 and 6, the measurement time (time to reach the standard electrode potential of copper) was shortened as the number of times progressed from the first measurement time. This is considered to be because the ratio of the platinum particles on which the copper layer is deposited is increased. Furthermore, as shown in FIG. 6, the measurement times were almost the same at the fourth time and the fifth time. That is, the amount of change in the fourth and fifth measurement times is almost zero. This is presumably because the copper layer was deposited on almost all platinum particles. As shown in Table 1, the atomic weight ratio of hydrogen and copper is close to 1 regardless of the number of times, and from this, it was confirmed that the surface of all platinum particles was coated with copper. . Therefore, it can be seen that the completion of the deposition step may be determined based on the amount of change in the measurement time that is repeatedly performed.

  As mentioned above, although it explained in full detail using embodiment of this invention, a concrete structure is not limited to this embodiment and an Example, There exists a design change in the range which does not deviate from the summary of this invention. They are also included in the present invention.

  3a: platinum carrying conductor, 3b: copper layer coated platinum carrying conductor, 10 ... copper material, 11 ... suspension, 12 ... glass material, 14 ... working electrode, 15 ... counter electrode, 16 ... reference electrode, 18: copper Deposition tank, 16a: glass tube, 16b: glass frit, S20: evaluation process, S21: precipitation process, S22: dipping process, S23: measurement process, S24: removal process, S25: determination process, S26: gold replacement process, 31 : Conductive carrier, 32: platinum particles, 33: copper layer, 33 ': copper

Claims (4)

A method for producing a catalyst in which a surface of platinum particles made of platinum or a platinum alloy or a surface of the platinum particles of a conductive carrier supporting platinum particles is coated with a copper layer,
In the production method, in the state where the copper material is immersed in a suspension obtained by suspending an acid aqueous solution containing copper ions with the platinum particles or the conductive carrier, the suspension is stirred and the copper material is mixed with the copper material. A deposition step of depositing a copper layer on the surface of the platinum particles by contacting the platinum particles or the conductive support, and a deposition state of the copper layer on the surface of the platinum particles in the deposition step according to the deposition step And an evaluation process to evaluate,
The evaluation step includes a dipping step of dipping a reference electrode and a working electrode made of platinum or a platinum alloy in the suspension,
By stirring the suspension, platinum particles having the copper layer formed thereon are brought into contact with the working electrode, so that the potential of the working electrode with respect to the reference electrode is reduced while the copper layer is deposited on the surface of the working electrode. A series of steps consisting of a measurement step for measuring the time until a constant potential is reached and a removal step for removing the copper layer deposited on the working electrode after the measurement step are repeated.
The method for producing a catalyst, characterized in that the precipitation step is terminated based on a change amount for each measurement time in the measurement step that is repeatedly performed. As the working electrode, using a working electrode covered with a glass material on the base end side so that at least the tip side is exposed,
2. The method for producing a catalyst according to claim 1, wherein in the dipping step, the working electrode is dipped so that all exposed portions on the tip side are dipped in the suspension. Using a reference electrode made of copper as the reference electrode,
In the immersion step, the reference electrode is immersed so that the acid aqueous solution in the suspension is in contact with the reference electrode and the platinum particles or the conductive carrier in the suspension is not in contact. The method for producing a catalyst according to claim 1 or 2, wherein:   In the removing step, the copper layer coated on the surface of the working electrode is removed by controlling the potential of the working electrode with respect to the reference electrode to a potential higher than a standard electrode potential of copper. A method for producing the catalyst according to claim 3.

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