JP2008153192A - Precious metal containing catalyst, method for manufacturing same, membrane/electrode assembly, fuel cell and fuel cell electric power generation system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a precious metal containing catalyst which has excellent catalyst characteristics while reducing amount of usage of precious metal elements as much as possible and a method for manufacturing the same and, also, to provide a membrane/electrode assembly using the precious metal containing catalyst, a fuel cell and a fuel cell electric power generation system. <P>SOLUTION: The precious metal containing catalyst is composed of a catalyst metal and a carrier carrying the catalyst metal. The catalyst metal is constituted of core particles and a precious metal element existing on surfaces of the core particles, has a particle size of 1 to 30 nm, and satisfies that Ec is larger than Es, where Ec is a variation amount of a free energy before and after absorption of oxygen by the element existing on the core particles and Es is a variation amount of a free energy before and after absorption of oxygen by the precious metal element. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a noble metal-containing catalyst suitable as an oxygen reduction catalyst applied to a fuel cell, an air zinc battery, an oxygen sensor, and the like, a membrane / electrode structure using the noble metal-containing catalyst, a fuel cell, and a fuel cell power generation system. It is.

  As an oxygen reduction catalyst applied to a fuel cell or the like, for example, a noble metal catalyst such as platinum is dispersed as finely as possible to obtain high catalytic activity.

  In recent years, a technique has been proposed in which the catalyst activity is increased by making the catalyst have a structure in which the catalyst is attached to the surface of a relatively large core, for example, in a shell shape, such as a core-shell structure.

  For example, Patent Document 1 discloses a structure in which platinum black or an alloy of platinum and a transition metal is formed into fine particles having an average particle diameter of 1 to 5 μm, and an aggregate of these fine particles is formed as a surface layer on the core. Fuel cell catalysts have been proposed. When the electron micrograph attached to patent document 1 is seen, the magnitude | size of a catalyst is about 100 nm and the thickness of a surface layer is about 10 nm. In Patent Document 1, by using fine particles having a catalytic function as an aggregate, the effective surface area required for the catalytic reaction is remarkably increased, thereby enhancing the catalytic activity.

  Patent Document 2 discloses a catalyst supported on a conductive substrate, which is composed of a core portion made of a noble metal alloy and a shell portion made of a noble metal-containing layer having a composition different from that of the core portion formed on the outer periphery thereof. A fuel cell catalyst having a shell structure is disclosed. In Patent Document 2, it is said that the catalyst activity and the catalyst durability can be improved by adopting the above configuration. Patent Document 2 compares the power generation performance of 36 types of catalysts having a platinum alloy core combined with a shell having a predetermined composition. Various voltage characteristics are obtained by adjusting the alloy core composition and the shell composition. Is obtained.

  Furthermore, Non-Patent Document 1 discloses a catalyst in which a platinum monoatomic layer is coated with palladium or the like. In Non-Patent Document 1, the coating is performed using underpotential deposition.

JP 2006-128117 A JP-A-2005-135900 "The Journal of Physical Chemistry B, 2004, Vol. 108, pp. 10955-10964.

  However, as described above, the catalyst disclosed in Patent Document 1 has a particle size as large as about 100 nm and does not sufficiently bring out catalyst characteristics. In the technique disclosed in Patent Document 1, it is difficult to produce a finer catalyst than that disclosed in Patent Document 1 because the formation of the surface layer is complicated.

  In addition, as described above, in the oxygen reduction catalyst, it is usual to use a noble metal element such as platinum as a catalyst metal. However, since such noble metal element is expensive, the cost of the oxygen reduction catalyst is reduced. In order to improve the productivity, it is preferable to significantly reduce the amount of the noble metal element used in constituting the catalyst or not to use the noble metal element. However, the catalysts disclosed in Patent Document 2 and Non-Patent Document 1 are based on the premise that a precious metal element or an alloy thereof is a main component of the catalyst, and the amount of the precious metal element used is reduced or stopped. Is virtually impossible.

  The present invention has been made in view of the above circumstances, and its purpose is to reduce the amount of noble metal element used as much as possible, while having a noble metal-containing catalyst having excellent catalytic characteristics, a method for producing the same, and the noble metal-containing catalyst. An object of the present invention is to provide a membrane / electrode assembly, a fuel cell, and a fuel cell power generation system using a catalyst.

  The noble metal-containing catalyst of the present invention that has achieved the above object is a noble metal-containing catalyst composed of a catalyst metal and a carrier supporting the catalyst metal, the catalyst metal comprising core particles and the surface of the core particles And the particle size is 1 to 30 nm, the free energy change amount before and after oxygen adsorption in the element of the core particle is Ec, and before and after oxygen adsorption in the noble metal element When the amount of change in free energy at Es is Es, Ec> Es.

  In addition, the production method of the present invention is characterized in that, in producing the noble metal-containing catalyst of the present invention, a noble metal element is adhered to the surface of the core particles supported on the support by a displacement plating method.

  Further, a membrane / electrode structure having a catalyst electrode using the noble metal-containing catalyst of the present invention, a fuel cell using the membrane / electrode structure, and a fuel cell power generation system using the fuel cell are also included in the present invention. Is done.

  According to the present invention, a noble metal-containing catalyst having excellent catalytic characteristics while reducing the amount of noble metal element used as much as possible, a method for producing the same, a membrane / electrode assembly using the noble metal-containing catalyst, and a fuel cell In addition, a fuel cell power generation system can be provided.

<Precious metal-containing catalyst>
1 and 2 are schematic views of the noble metal-containing catalyst of the present invention. 1 and 2 show a cross section of the main part of the noble metal-containing catalyst, but some constituent elements are not hatched indicating that they are cross sections. Reference numeral 1 denotes a catalyst metal, and a noble metal element 10 is present on the surface of the core particle 20. In FIG. 1, the noble metal element 10 is present on the surface of the core particle 20 in a state of a layer having a uniform thickness. However, the thickness of the layer composed of the noble metal element 10 may not be uniform, As long as the catalyst performance can be secured, the noble metal element 10 may be present on the surface of the core particle 20 in an island shape as shown in FIG. Further, the portion of the core particle that is in contact with the carrier may not be coated or may be coated.

  Reference numeral 30 denotes a carrier that supports a noble metal-containing catalyst. For example, when the noble metal-containing catalyst is used for a catalyst electrode (such as an electrode for a fuel cell), the carrier 30 has a conductivity (carbon such as carbon black). Etc.).

  Although the core particles 20 are shown in a hemispherical shape in FIG. 1 and a semi-ellipsoidal shape in FIG. 2, some of these core particles 20 enter the pores of the carrier 30. The shape of the core particle 20 is not particularly limited, and may be any shape such as a substantially spherical shape (including a spherical shape) or a substantially ellipsoidal shape (including an elliptical shape).

  In the noble metal-containing catalyst of the present invention, when the amount of change in free energy before and after oxygen adsorption in the element of the core particle is Ec, and the amount of change in free energy before and after oxygen adsorption in the noble metal element is Es, Ec> Es It is.

  FIG. 3 shows a change in free energy in an intermediate state when water is generated from hydrogen ions and oxygen by a catalytic reaction. The data shown in FIG. 3 is based on the results of the first principle computer simulation. In FIG. 3, □ is the result for the nickel catalyst, ● is the result for the platinum catalyst, ○ is the result for the noble metal-containing catalyst of the present invention in which platinum is present in layers on the surface of the core particles composed of nickel. The results are shown respectively.

  FIG. 3 shows that the energy difference in each intermediate state is smaller in the noble metal-containing catalyst of the present invention than in the case of the nickel catalyst and the platinum catalyst. Although the potential barrier itself is not shown in FIG. 3, the energy between the intermediate states is close to each other as described above, suggesting that the transition between the intermediate states is likely to occur. ing.

  In the noble metal-containing catalyst of the present invention, the energy change between the intermediate states as described above is caused by the small amount of change Ec in the free energy before and after oxygen adsorption, according to the analysis by computer simulation. A noble metal element having a large change amount Es of free energy before and after oxygen adsorption present on the surface of the core particle, while attracting oxygen involved in the catalytic reaction with the core particle having an element having a strong adsorptive force. This is probably because the oxygen attracting force is moderately suppressed. Attracting the oxygen involved in the catalytic reaction is important for the initiation of the catalytic reaction, while it is important that the oxygen attracting force is not too strong so that the catalytic reaction proceeds without delay. That is, in the noble metal-containing catalyst of the present invention, the core particles and the noble metal elements present on the surface thereof are combined so that the relationship of Ec> Es can be satisfied, so that the core particles can adsorb oxygen. The degree of oxygen adsorption is controlled by the noble metal element present on the surface of the particles to ensure good catalyst characteristics, and deterioration of the catalyst characteristics over time can be suppressed.

  In addition, in the noble metal-containing catalyst of the present invention, an element having a change amount Ec of the free energy larger than the change amount Es of the free energy related to the noble metal element is used for the core particle that is the main component of the catalyst metal. Is present only on the surface of the core particles, while ensuring good catalytic properties, the amount of noble metal elements used in the catalyst is reduced as much as possible, thereby reducing the cost of the catalyst. Productivity has been improved.

  Examples of the noble metal element present on the surface of the core particle according to the noble metal-containing catalyst of the present invention include platinum, palladium, gold, rhodium, iridium, ruthenium, silver and osmium.

  Further, it is preferable that the noble metal element present on the surface of the core particle is substantially only a single element. By adopting the above-described configuration, the composition and configuration of the surface portion of the catalyst metal according to the noble metal-containing catalyst of the present invention can be simplified, so that reproducibility and stability in catalyst production are enhanced. In the present specification, “the noble metal element present on the surface of the core particle is substantially only a single element” means that, for example, in the production process of the noble metal-containing catalyst, the core is unavoidable. This means that the element that is positively present on the surface of the core particle is only a single noble metal element, excluding the element that has adhered to the surface of the particle.

  As an element which the core particle which concerns on the noble metal containing catalyst of this invention has, transition metal elements other than a noble metal element are mentioned, Specifically, cobalt, iron, nickel, and tungsten are preferable.

  The core particles are preferably substantially composed of a single transition metal element. By adopting the above-described configuration, the composition and configuration of the core portion of the catalyst metal according to the noble metal-containing catalyst of the present invention can be simplified, so that reproducibility and stability in catalyst production are further increased. As used herein, “the core particle is substantially composed of only a single transition metal element” means that, for example, in the production process of a noble metal-containing catalyst, it is unavoidably contained in the core particle. This means that the element that is positively used for the construction of the core particle is only a single transition metal element, except for the element that has been formed.

  Table 1 shows change amounts Es and Ec of free energy before and after oxygen adsorption for typical noble metal elements present on the surface of the core particles and transition metal elements constituting the core particles.

  Es and Ec shown in Table 1 are free energies generated before and after the reaction of the following formula (1) according to the description of “The Journal of Physical Chemistry B, 2004, Vol. 108, pp. 17886-17892”. The amount of change in free energy in the presence of a potential of 1.23 V was obtained. However, in the calculation of Es and Ec, since the positive and negative are defined so that the value of strongly adsorbing oxygen becomes a larger value, the sign is opposite to that of the above document.

2 (H ⁺ + e ⁻ ) + 1 / 2O ₂ → 2 (H ⁺ + e ⁻ ) + O ^* (1)

In addition, “O ^* ” in the formula (1) means a state in which oxygen atoms are adsorbed on the constituent surface when the fcc (111) surface is configured only by each element, and “ ^* ” indicates the surface. Shows the site. In the calculation of each numerical value shown in Table 1, a calculation method called “Projector Augmented Wave Method” is applied, and the numerical value is rounded for easy understanding. Since the calculation method is different from the results shown in the above document, the detailed numerical values are different, but the results are almost the same as the size.

  As a combination of the transition metal element of the core particle and the noble metal element present on the surface of the core particle, the combination of the transition metal element: cobalt, nickel or tungsten and the noble metal element: platinum, palladium, gold, rhodium or iridium In addition, a combination of cobalt and iridium and a combination of tungsten and gold is preferable. More specifically, the combination of “transition metal element of core particle / noble metal element present on the surface of core particle” includes cobalt / platinum, nickel / platinum, tungsten / platinum, cobalt / palladium, nickel / palladium, Tungsten / palladium, cobalt / gold, nickel / gold, cobalt / rhodium, nickel / rhodium, tungsten / rhodium, nickel / iridium and tungsten / iridium are preferred. When the transition metal element included in the core particle and the noble metal element present on the surface of the core particle are in the above-described combination, platinum is present in layers on the surface of the core particle composed of nickel shown in FIG. As in the case of noble metal-containing catalysts, transitions between the intermediate states are more likely to occur when computer-simulated changes in free energy in each intermediate state when water is generated from hydrogen ions and oxygen by a catalytic reaction. It is because the result which suggests was obtained.

  In the computer simulation, which has been found that a noble metal-containing catalyst having better oxygen reduction activity can be constituted by a combination of the transition metal element included in the core particle exemplified above and the noble metal element present on the surface of the core particle, According to the description of Journal of Physical Chemistry B, 2004, Vol. 108, pp. 17886-17892, the free energy in a state where a potential of 1.23 V is present after the fcc (111) plane is formed. Calculated.

  In the calculation model, as shown in FIG. 4, the surface portion (outermost surface layer) was composed of a noble metal element, and the second and third layers were composed of transition metal elements as core particles. Since the calculation uses a periodic calculation means called a supercell, a model in which the structure shown in FIG. 4 is repeated infinitely in the plane direction and the vertical direction is calculated. In the vertical direction, calculation was performed with a vacuum of about 10 mm above and below. In calculating the free energy, first-principles calculations were performed for the state in which oxygen atoms were adsorbed on the outermost surface and the state in which OH was adsorbed on the outermost surface. As a first-principles calculation method, unlike the above-mentioned document, a calculation method called Projector Augmented Wave Method is applied in the same manner as shown in FIG. 3, but the fcc (111) plane is composed of three layers and the second layer is calculated. The third-layer nuclear position is fixed and the outermost nuclear position is optimized in accordance with the above document. The Projector Augmented Wave Method is a technique that has been attracting attention in recent years as a technique capable of performing first-principles calculations with higher reliability than in the past.

FIG. 5 shows a potential of 1.23 V in the case of “cobalt / gold” and “nickel / gold” in the combination of “transition metal element of core particle / noble metal element present on the surface of core particle”. Shows the change in free energy in the presence of. In FIG. 5, “O ^* ” indicates a state in which oxygen atoms are adsorbed on the outermost surface, “OH ^* ” indicates a state in which OH is adsorbed on the outermost surface, and “ ^* ” indicates a site on the surface.

According to the theoretical study by the present inventors, the oxygen reduction reaction of pure platinum has a large free energy change in the process of OH adsorption after oxygen atoms are adsorbed, and there is a free energy difference of about 0.84 eV. Considering the result shown in FIG. 5 with that in mind, the combination of “transition metal element of core particle / noble metal element present on the surface of core particle” is “cobalt / gold” and “nickel / gold” It can be seen that the free energy change is smaller than 0.84 eV in both the process of OH adsorption after the adsorption of oxygen atoms and the process of generating H ₂ O after OH adsorption. Therefore, when the combination of “the transition metal element of the core particle / the noble metal element present on the surface of the core particle” is “cobalt / gold” and “nickel / gold”, it is suitable for the oxygen reduction reaction.

FIG. 6 shows a potential of 1.23 V when the combination of “transition metal element of core particle / noble metal element present on the surface of core particle” is “nickel / iridium” and “tungsten / iridium”. Shows the change in free energy in the presence of. FIG. 6 shows that the combination of “nickel / iridium” is suitable for the oxygen reduction reaction although there is a somewhat large change in free energy in the process of generating H ₂ O after OH adsorption. In particular, the combination of “tungsten / iridium” has a relatively small change in free energy in each process and is suitable for an oxygen reduction reaction.

FIG. 7 shows that the combination of “transition metal element of core particle / noble metal element present on the surface of core particle” is “cobalt / rhodium”, “nickel / rhodium”, and “tungsten / rhodium”. For the case, the change in free energy in the presence of a potential of 1.23 V is shown. FIG. 7 shows that the combination of “cobalt / rhodium” and “nickel / rhodium” tends to have a large change in free energy in the process of OH adsorption after adsorption of oxygen atoms, but is suitable for oxygen reduction reaction. is there. Further, the combination of “tungsten / rhodium” is suitable for the oxygen reduction reaction although there is a somewhat large change in free energy in the process of generating H ₂ O after OH adsorption.

In FIG. 8, the combinations of “transition metal element of core particle / noble metal element present on the surface of core particle” are “cobalt / palladium”, “nickel / palladium”, and “tungsten / palladium”. For the case, the change in free energy in the presence of a potential of 1.23 V is shown. Referring to FIG. 8, in the case of a combination of “cobalt / palladium”, “nickel / palladium”, and “tungsten / palladium”, a process of OH adsorption after oxygen adsorption and a process of generating H ₂ O after OH adsorption. It can be seen that the change in free energy is smaller than 0.84 eV. Therefore, these “cobalt / palladium”, “nickel / palladium” and “tungsten / palladium” combinations are suitable for the oxygen reduction reaction.

In FIG. 9, the combinations of “transition metal element of core particle / noble metal element present on the surface of core particle” are “cobalt / platinum”, “nickel / platinum”, and “tungsten / platinum”. For the case, the change in free energy in the presence of a potential of 1.23 V is shown. Referring to FIG. 8, in the case of a combination of “cobalt / platinum”, “nickel / platinum”, and “tungsten / platinum”, a process of OH adsorption after oxygen adsorption and a process of generating H ₂ O after OH adsorption. It can be seen that the change in free energy is smaller than 0.84 eV. Therefore, these “cobalt / platinum”, “nickel / platinum” and “tungsten / platinum” combinations are suitable for the oxygen reduction reaction.

  In the noble metal-containing catalyst of the present invention, the particle size of the catalyst metal (particle size including the core particle and the noble metal element present on the surface of the core particle) is 30 nm or less from the viewpoint of securing excellent catalyst characteristics. It is preferable that it is 10 nm or less. On the other hand, if the particle diameter of the catalyst metal related to the noble metal-containing catalyst is too small, the effect of suppressing deterioration with time of the catalyst characteristics may be reduced. Therefore, the particle diameter of the catalyst metal is 1 nm or more, and 3 nm or more. Preferably there is.

The particle size of the catalyst metal can be measured from direct observation with a transmission electron microscope, and the average particle size can be calculated by measuring the particle size of a large number of catalyst metal particles. The acceleration voltage at the time of measurement may be about 200 kV. It can also be determined from the width of the diffraction line obtained by the X-ray diffraction method. In this case, the obtained value is the value of the crystallite diameter, but since the catalyst metal particles are almost single crystals, the crystallite diameter can be regarded as the particle diameter. Here, the relationship between the half width β (rad) of the diffraction line and the crystallite diameter D is expressed by the following equation.
D = λ / βcos θ
In the above formula, λ is the X-ray wavelength (Å), and θ is the diffraction angle (°). For example, Cu can be used as the X-ray source, and the X-ray output may be about 50 kV and 250 mA.

  In the noble metal-containing catalyst of the present invention, the thickness of the noble metal element present on the surface of the core particle is preferably 0.5 nm or more, more preferably 0.8 nm or more, and 8 nm or less. It is preferably 4 nm or less, more preferably 2 nm or less. This is because the noble metal element present on the surface of the core particle has the above thickness, so that the oxygen adsorption force by the core particle can be controlled better, but this is based on the following computer simulation. It is obtained from examination.

  In FIG. 10, the combination of “transition metal element of core particle / noble metal element present on the surface of core particle” is composed of “cobalt / palladium” noble metal-containing catalyst, catalyst composed only of palladium, and cobalt only. The change of the free energy in each intermediate state when water is produced by a catalytic reaction from hydrogen ions and oxygen using a catalyst. The result shown in FIG. 10 is similar to the result shown in FIG. 5 and the like, but the calculation method called Projector Augmented Wave Method is applied, but the fcc (111) plane is composed of three layers, and the nuclear positions of the second and third layers are shown. , The position of the nucleus on the outermost surface is optimized, and the free energy in the state where a potential of 1.23 V exists is calculated using the result.

Note that “O ^* ”, “OH ^* ”, and “ ^* ” in FIG. 10 are the same as those in FIG. Further, in FIG. 10, the results for the noble metal-containing catalyst in which palladium is present in a layered manner on the surface of the core particles composed of cobalt, and the results for the catalyst composed of only palladium (pure Pd). And □ show the results with a catalyst composed of only cobalt (pure Co). For comparison, the results with a catalyst composed of only platinum (pure Pt) are indicated by ◇.

  FIG. 10 shows that the difference in free energy before and after oxygen atom adsorption is smaller in the case of a noble metal-containing catalyst composed of a combination of “cobalt / palladium” than in the case of pure Co catalyst and pure Pd catalyst. I understand that. FIG. 10 does not show the potential barrier itself of the state transition, but suggests that transition between the intermediate states is likely to occur because the energy between the intermediate states is close to each other. That is, based on the result with the pure Co catalyst (の) and the result with the pure Pd catalyst (●) in FIG. 12, the amount of free energy Ec generated before and after the oxygen adsorption on the cobalt of the core particles and the surface of the core particles It can be seen that there is a relationship of Ec> Es between the amount of free energy Es that occurs before and after oxygen adsorption for the palladium present, and that the pure Co catalyst adsorbs oxygen atoms more strongly than the pure Pd catalyst.

  On the other hand, paying attention to the result (◯) of the noble metal-containing catalyst composed of the combination of “cobalt / palladium” in FIG. 10, the model of this catalyst is the surface of the core particles (second layer and third layer) made of cobalt. This corresponds to the one in which a monoatomic layer (outermost surface layer) of palladium is formed. If the amount of change in free energy generated before and after oxygen adsorption is Ecs1 for this catalyst, Ecs1 <Es. A noble metal-containing catalyst in which a monoatomic layer of palladium is formed on the surface of the core particles made of is weaker in oxygen adsorption than a pure Pd catalyst. Here, when the surface of the core particle made of cobalt is covered with a thick palladium layer instead of a monoatomic layer, if the thickness of the palladium layer is sufficiently large, it is almost equal to a pure Pd catalyst (● in FIG. 10). It should adsorb oxygen with strength. From these, by controlling the thickness of the noble metal element present on the surface of the core particle, the oxygen adsorption force of the catalyst is moderately weakened compared to the case of the core particle alone, and as a result, it is composed of the noble metal element alone. The present inventors considered that it is possible to promote the progress of the entire oxygen reduction reaction than a catalyst (for example, the pure Pd catalyst described above). That is, attracting oxygen in the catalytic reaction is important for the initiation of the catalytic reaction, and at the same time, the fact that the oxygen attracting force is not too strong is a weight for the catalytic reaction to proceed smoothly until the end. In the noble metal-containing catalyst, by adjusting the thickness of the noble metal element present on the surface of the core particle, it is possible to appropriately control such oxygen adsorption force to ensure better catalyst characteristics, and to prevent deterioration of the catalyst characteristics over time. More highly controlled.

  Further, FIG. 11 shows a combination of a “noble metal element present on the surface of the core particle / the noble metal element present on the surface of the core particle” in which the combination is “nickel / palladium” (◯ in the figure), and a catalyst composed only of palladium (Pure Pd, in the figure), when using a catalyst composed only of cobalt (pure Co) and a catalyst composed solely of platinum (pure Pt), water is produced by catalytic reaction from hydrogen ions and oxygen. The change of the free energy in each intermediate state when generating is shown. The results in FIG. 11 were obtained in the same manner as in FIG. 10 except that the noble metal-containing catalyst composed of the combination of “nickel / palladium” was nickel in the second and third layers of the model. . The results shown in FIG. 11 are the same as those in FIG. 10, and the relationship of Ecs1 <Es is assumed, where Ecs1 is the amount of change in free energy that occurs before and after oxygen adsorption for a noble metal-containing catalyst composed of a combination of “nickel / palladium”. Therefore, the noble metal-containing catalyst in which the palladium monoatomic layer is formed on the surface of the core particle made of nickel has weaker oxygen adsorption than the pure Pd catalyst. By controlling the thickness of the precious metal element present, the oxygen adsorption capacity of the catalyst is moderately weakened compared to the case of the core particle alone, and as a result, the overall oxygen reduction reaction proceeds more than the catalyst composed of the precious metal element alone. It is suggested that it is possible to promote.

  As a result of further investigations from these results, it was found that the oxygen adsorption force of the core particles can be better controlled when the thickness of the noble metal element present on the surface of the core particles is the above-mentioned preferred value.

Note that the thickness of the noble metal element in the present specification can be determined from, for example, the particle size of the core particle and the molar ratio of the element contained in the core particle to the noble metal element. The particle diameter of the core particle can be measured by using the X-ray diffraction method, and the molar ratio between the element of the core particle and the noble metal element present on the surface can be measured by high frequency inductively coupled plasma emission spectrometry. . From these values and the density and atomic weight of the element and the noble metal element of the core particle, the number of atoms of the noble metal element per noble metal-containing catalyst particle is calculated. Since the surface of the core particle having a certain particle diameter is coated with the obtained noble metal element having the number of atoms, the thickness of the noble metal element can be obtained. The particle size of the core particle is d _C , the molar ratio of the element and the noble metal element of the core particle is S / C, the density of the core element is ρ _C , the density of the noble metal element is ρ _s , and the atomic weight of the core element is M _C , When the atomic weight of the noble metal element is M _S , the thickness d _{S of the} noble metal element can be expressed by the following formula.
d _S = (d _C / 2) × [{(S / C) × (M _S / M _C ) × (ρ _C / ρ _S ) +1} ^1/3 −1] Substituting each numerical value into this equation Can calculate the thickness of the noble metal element.

  10 and FIG. 11, the theoretical calculation results (◇ in the figure) of the pure Pt catalyst indicate that the oxygen reduction reaction with the pure Pt catalyst is a rate-determining step in which oxygen is adsorbed and then OH is adsorbed. Has a free energy difference of about 0.84 eV, but for this magnitude, a noble metal-containing catalyst (◯ in the figure) composed of a combination of “cobalt / palladium” in FIG. 10 and “nickel / palladium” in FIG. In the noble metal-containing catalyst (○ in the figure) consisting of the above combinations, the difference in free energy in each process is smaller, and in the noble metal-containing catalyst consisting of the combination of “cobalt / palladium”, the free energy difference in each process is smaller. From these results, the element having the core particle is made of cobalt or nickel, and the noble metal element existing on the surface of the core particle is made of palladium, so that it has an oxygen reduction activity equivalent to or better than that of the pure Pt catalyst. The present inventors thought that there was a possibility that a noble metal-containing catalyst could be constructed.

  Based on the above idea, the present inventors actually produced a noble metal-containing catalyst and evaluated its catalytic properties. The details will be described in the examples described later. From the evaluation results, in the noble metal-containing catalyst of the present invention, the element contained in the core particle is cobalt or nickel, and the noble metal element present on the surface of the core particle is palladium. More preferably, the element contained in the core particle is cobalt, and the noble metal element present on the surface of the core particle is further preferably palladium.

  Next, the manufacturing method of the noble metal containing catalyst of this invention is demonstrated. Examples of the method for producing the noble metal-containing catalyst of the present invention include the following three methods.

  The first method is a method in which a noble metal element is attached to the surface of core particles supported on a carrier by a displacement plating method.

  The second method is a method by sputtering. When a noble metal element is deposited on the surface of the core particles supported on the carrier by sputtering, a uniform surface layer (noble metal element layer) having a thickness of about two atomic layers or more is formed. In this case, the thickness of the surface layer is four times or more than the effective radius of the noble metal element constituting the surface layer, specifically 0.5 nm to 0.8 nm or more.

  The third method is a method in which a noble metal element is attached to the surface of the core particle supported on the carrier by vacuum deposition.

When the core particle is composed of cobalt and the noble metal-containing catalyst of the present invention is configured using palladium as the noble metal element present on the surface of the core particle, the diameter d _Co (nm) of the core particle and the surface of the core particle The relationship between the molar ratio Pd / Co of palladium present and cobalt constituting the core particles is 0.5 <(d _Co / 2) × [{(Pd / Co) × (M _Pd / M _Co ) × ( It is preferable to adjust so that ρ _Co / ρ _Pd ) +1} ^1/3 −1] <6.1. Here, M _Pd : Atomic weight of palladium (106.4), M _Co : Atomic weight of cobalt (58.93), ρ _Co : Cobalt density (8.90 g / cm ³ ), ρ _Pd : Palladium density (12 0.02 g / cm ³ ).

When the above numerical values are substituted into M _Pd , M _Co , ρ _Co, and ρ _Pd in the relational expression, 0.5 <(d _Co /2)×[{(1.3369×Pd/Co)+1} ^1/3 -1] <6.1.

Here, when the thickness of palladium existing on the surface of the core particle is d _Pd (nm), d _Pd is obtained by the following formula (2).

Therefore, when the noble metal-containing catalyst of the present invention is constituted with the core particle made of cobalt and the noble metal element present on the surface of the core particle as palladium, the molar ratio Pd of the core particle diameter d _Co and palladium to cobalt is used. It is preferable to adjust so that the relationship with / Co is 0.5 <(d _Co /2)×[{(1.3369×Pd/Co)+1} ^1/3 −1] <6.1. . That is, by controlling the thickness of palladium on the surface of the core particles so that the oxygen adsorption force can be adjusted better by producing a noble metal-containing catalyst while adjusting so that _dCo and Pd / Co satisfy the above relationship. it can.

<Membrane / electrode structure>
FIG. 12 schematically shows an example (MEA for fuel cell) of the membrane / electrode structure (MEA) of the present invention. FIG. 12 is a cross-sectional view of the MEA, but some constituent elements are not hatched to indicate that they are cross sections. In FIG. 12, 50 is an integrated MEA to which the noble metal-containing catalyst of the present invention is applied as a catalyst electrode. The integrated MEA 50 is configured by integrating an electrolyte membrane 51, a catalyst electrode 52 applied to the electrolyte membrane 51, a gas diffusion layer 53 disposed so as to be in contact with the catalyst electrode 52, and a gasket 54 which is a sealing means. . Reference numeral 55 denotes a manifold for supplying fuel or the like to the integrated MEA 50. Reference numeral 56 denotes a power generation surface of the integrated MEA 50. Electricity is generated by supplying a fuel such as hydrogen or a methanol aqueous solution to one of the power generation surfaces and an oxidant such as air to the other.

  As the electrolyte membrane 51, a fluorine-based or hydrocarbon-based electrolyte membrane can be applied. Further, as described above, the noble metal-containing catalyst of the present invention is applied to the catalyst electrode 52. The noble metal-containing catalyst of the present invention may be applied to one or both of the cathode electrode and the anode electrode, but particularly when applied to the cathode electrode, the efficiency is remarkably improved.

  The integrated MEA 50 is an MEA and its peripheral members integrated for ease of handling, but may be configured as an MEA in which only the electrolyte membrane 51 and the catalyst electrode 52 are integrated.

  Since the MEA of the present invention is applied with the noble metal-containing catalyst of the present invention having good catalytic properties, excellent durability, and suppressed deterioration of the catalytic properties with time, the MEA has high performance at low cost. It becomes.

<Fuel cell>
FIG. 13 schematically shows an example of the fuel cell of the present invention. In FIG. 13, reference numeral 60 denotes a fuel cell (fuel cell stack) using an integrated MEA 50 (MEA of the present invention) to which the noble metal-containing catalyst of the present invention is applied as a catalyst electrode. The fuel cell stack 60 is configured by stacking a current collector plate 61, a separator (bipolar plate) 62, and an integrated MEA 50. In FIG. 13, in order to facilitate understanding of the configuration of the fuel cell of the present invention, the separators and fixing means at the ends, the cooling means of the fuel cell stack, etc. are omitted, and the repeated structure of the stack is shown in a simplified manner. Yes.

  For example, in a household 1 kW class fuel cell stack, several tens of MEAs are used in one stack.

  The current collecting plate 61 is a means for taking out in series the electrical output generated by each power generating surface 56, and a metal plate can be applied. The separator 62 has a groove for uniformly supplying a fuel such as hydrogen or a methanol aqueous solution or an oxidizing agent such as air to each power generation surface 56, and is a means for simultaneously achieving electrical conduction. It can be made of a metal or metal material. It is desirable that the current collector 61 and the separator 62 take into account the corrosion resistance in the power generation environment of the fuel cell from the viewpoint of long-term stable power generation.

  The fuel cell of the present invention may be a fuel cell stack configured by stacking a plurality of cells as shown in FIG. 13, or in a simple case, may be a single cell configured by one cell. .

  Since the fuel cell of the present invention is applied with the noble metal-containing catalyst of the present invention having good catalytic characteristics, excellent durability, and suppressed deterioration of the catalytic characteristics with time, it is low in cost and highly functional. It becomes a fuel cell. In particular, when a fuel cell stack is formed by stacking several tens of MEAs in series, the cost reduction effect of the noble metal-containing catalyst of the present invention is great.

  In addition, since the noble metal-containing catalyst of the present invention has a simple composition, it has good reproducibility in catalyst production and can suppress variation in characteristics. Therefore, even in a fuel cell stack using several tens of MEAs, the characteristics for each MEA Uniformization is easy.

<Fuel cell power generation system>
FIG. 14 schematically shows an example of the fuel cell power generation system of the present invention. In FIG. 14, reference numeral 70 denotes a fuel cell power generation system in which the fuel cell stack 60 using the integrated MEA 50 to which the noble metal-containing catalyst of the present invention is applied as a catalyst electrode is used as a power generation unit. In a household fuel cell system, power is generated from a fuel cell stack using city gas or the like as fuel and air as an oxidant. The rated output of a household fuel cell system is generally 1 kW class. When the fuel cell power generation system is used as a cogeneration system, the heat recovered by cooling the fuel cell stack is simultaneously used as a heat source for the home.

  A household fuel cell power generation system is required to have a life span of 40,000 hours to 90,000 hours or more. In addition, an efficiency of about 35% LHV or more is required as the power generation efficiency.

  According to the fuel cell to which the noble metal-containing catalyst of the present invention is applied, and the fuel cell power generation system using the fuel cell, the catalyst electrode related to power generation has good catalytic characteristics and excellent durability, and catalytic characteristics. Since the noble metal-containing catalyst of the present invention in which deterioration with time is suppressed is applied, it is possible to provide a system with both reliability and efficiency at a practical price.

  Moreover, the fuel cell of the present invention to which the noble metal-containing catalyst of the present invention is applied and the fuel cell power generation system of the present invention using the fuel cell can also be applied to a fuel cell power source for moving bodies such as automobiles and motorcycles. The rated output of the fuel cell power source for moving bodies is generally several kW class to 100 kW class, and the amount of catalyst to be used increases. In these cases as well, as in the case of home use, a fuel cell power generation system having both reliability and efficiency can be provided at a practical price.

  Furthermore, the fuel cell of the present invention to which the noble metal-containing catalyst of the present invention is applied and the fuel cell power generation system of the present invention using the fuel cell can also be applied to a small fuel cell power source for portable use and the like. The rated output of a small salt battery power supply is generally several to several tens of watts and the amount of catalyst used is small, but a practical fuel cell power generation system can be provided in terms of reliability and ease of manufacture.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

Example 1
First, Co (NO ₃ ) ₂ · 6H ₂ O: 2.47 g was dissolved in 50% by weight ethanol aqueous solution: 10 ml, and this solution and conductive carbon [“Vulcan XC72R (trade name)” manufactured by CABOT]]: 2.00 g was mixed with a mortar. The mixture was dried at 50 ° C. in the atmosphere for 12 hours and then calcined at 500 ° C. for 3 hours in an argon gas atmosphere containing 3% by volume of hydrogen to obtain conductive carbon carrying cobalt. The amount of cobalt supported on the conductive carbon supporting cobalt was measured by high-frequency induction plasma emission analysis and found to be 20.0% by mass. Moreover, as a result of calculating the crystallite diameter of cobalt from the diffraction spectrum obtained by the X-ray diffraction method, it was 25 nm. Therefore, the average particle diameter of cobalt was estimated to be 25 nm.

Next, 0.50 g of the conductive carbon carrying cobalt was added to 100 ml of an aqueous solution containing 0.12 g of PdCl ₂ and stirred for 1 hour while maintaining at 50 ° C. In this step, palladium is deposited on cobalt as shown in the following formula (3) by displacement plating.
Pd ²⁺ + Co → Pd + Co ²⁺ (3)

  After displacement plating, filtration, washing with pure water were performed, and drying was performed at 50 ° C. for 12 hours to obtain a noble metal-containing catalyst in which palladium was present in layers on the surface of core particles composed of cobalt. The obtained noble metal-containing catalyst was measured for the amount of palladium and cobalt supported by high-frequency induction plasma emission analysis. As a result, palladium was 12.6% by mass, cobalt was 11.6% by mass, and the molar ratio of palladium to cobalt Pd / Co was 0.6.

  Further, a transmission electron micrograph of the obtained noble metal-containing catalyst is shown in FIG. It can be seen that the contrast of the outer periphery of the particles is different, and the cobalt core is covered with palladium.

In the noble metal-containing catalyst of Example 1, 30% by mass of core cobalt was eluted by displacement plating. Therefore, the diameter of the cobalt core is considered to be reduced to 22 nm. Further, the palladium thickness d _Pd can be calculated from the cobalt core diameter d _Co (nm) and the molar ratio Pd / Co of palladium and cobalt by the above relational expression, and is 2.4 nm.

Example 2
A noble metal-containing catalyst was obtained in the same manner as in Example 1 except that 0.50 g of conductive carbon carrying the same cobalt as used in Example 1 was added to 100 ml of a solution containing 0.03 g of PdCl ₂ . . With respect to the obtained noble metal-containing catalyst, the supported amounts of palladium and cobalt were measured by high frequency induction plasma emission analysis. As a result, palladium was 3.3% by mass, cobalt was 16.9% by mass, and the molar ratio of palladium to cobalt Pd / Co was 0.1. The cobalt core diameter and palladium thickness in the noble metal-containing catalyst of Example 2 were calculated in the same manner as in Example 1. As a result, the cobalt core diameter was 24 nm and the palladium thickness was 0.5 nm.

Comparative Example 1
The catalyst of Soekawa Rikagaku Co., Ltd. was used as the catalyst of Comparative Example 1.

Comparative Example 2
A catalyst in which platinum was supported on conductive carbon produced as follows was used as the catalyst of Comparative Example 2. First, conductive carbon (“Ketjen Black EC” manufactured by Lion): 0.42 g, aqueous solution containing 28.2% by mass of H ₂ PtCl ₆ : 2.2 g, aqueous solution containing 37% by mass of HCHO: 0.52 g , And water: 500 ml was mixed, and the pH was adjusted to 8 with 1 mol / l NaOH aqueous solution. Then, the temperature was raised to 70 ° C., and stirring was continued for 3 hours while maintaining the pH at 8 with the NaOH aqueous solution. Thereafter, the solution was filtered, washed with pure water, and dried at 80 ° C. for 12 hours or more to obtain a catalyst having platinum supported on conductive carbon. With respect to the obtained catalyst, the amount of platinum supported was measured by high frequency induction plasma emission spectrometry. As a result, it was 56% by mass.

About the catalyst of Examples 1-2 and Comparative Examples 1-2, oxygen reduction activity was evaluated. The evaluation method was electrochemical evaluation using a rotating disk electrode. To the working electrode, a solution in which each catalyst is dispersed in pure water is dropped on a disk electrode and dried under reduced pressure. Then, a 0.1 mass% Nafion (registered trademark) solution is dropped from above the catalyst and dried under reduced pressure. What was done was used. The amount of catalyst on the working electrode was 0.2 mg / cm ² and the amount of Nafion (registered trademark) was 0.05 mg / cm ² . A platinum wire was used for the counter electrode and Ag / AgCl (saturated KCl) was used for the reference electrode. A 0.5 mol / l sulfuric acid aqueous solution was used as the electrolyte, and the temperature was 35 ° C. Before the oxygen reduction activity evaluation, 0.03 to 1.2 V [vs. Potential scrambling was performed in the range of NHE (Normal Hydrogen Electrode)] to determine the amount of hydrogen desorption. Thereafter, potential scavenging was performed in the range of 0.2 to 1.1 V (vs. NHE) while bubbling with nitrogen, and the obtained current was used as the background current. Next, potential scrambling was performed in the range of 0.2 to 1.1 V (vs. NHE) while bubbling with oxygen, and the oxygen current was obtained by subtracting the background current from the obtained current. When measuring the oxygen reduction current, the working electrode disk electrode was rotated at various rotational speeds. The number of revolutions is 400, 625, 900, 1600, and 2500 rpm. The oxygen reduction current increases as the number of reactants supplied increases as the rotational speed increases. The relationship between the reciprocal of the measured oxygen reduction current value i (mA) at the predetermined potential and the -1/2 power of the angular velocity ω (rad / s) of the electrode is expressed by the Koutecky-Levich equation shown in the following equation (4). Is done.

Where i _K : activity-dominated current (mA), n: number of reaction electrons, F: Faraday constant (C / mol), A: geometric area (cm ² ) of disk electrode, c: activity of reactant (mol / ml) ), D: diffusion coefficient of the reactant (cm ² / s), v: kinematic viscosity coefficient (cm ² / s) of the solution, and the relationship between the rotation speed f (rpm) of the electrode and the angular velocity ω (rad / s) Is ω = 2πf / 60. In the equation (4), from the intercept of ω ^−1/2 = 0 (ω = ∞, that is, the supply amount of the reactant is infinite) of the approximate line of the oxygen reduction current value i obtained at various angular velocities ω. the reciprocal of i _K can be obtained. Therefore, i _K obtained becomes net activity there is no influence of the diffusion of reactants the catalyst. The oxygen reduction activity was compared with the value obtained by dividing the hydrogen desorption electricity quantity obtained the i _K in advance. Here, since the amount of electricity desorbed from hydrogen is a value proportional to the reaction surface area of the catalyst, the oxygen reduction activity means the oxygen reduction current per unit reaction surface area.

  Table 2 shows the oxygen reduction activity of each catalyst at 0.7 V (vs. NHE).

  As is apparent from Table 2, the noble metal-containing catalysts of Examples 1 and 2 show higher oxygen reduction activity than the catalysts of Comparative Examples 1 and 2.

FIG. 16 shows the relationship between the reciprocal of the Pd thickness of the catalysts of Examples 1 and 2 and Comparative Example 1 and the oxygen reduction activity at 0.7 V (vs. NHE). Since the oxygen reduction activity at 0.7 V (vs. NHE) of the platinum catalyst of Comparative Example 2 is 1.63 A / C, in order to obtain an oxygen reduction activity higher than this, from the result of FIG. reciprocal of d _Pd is sufficient if the range of ^{_{0.16 <d Pd -1 <2.05 (}} nm -1). Accordingly, the range of the palladium thickness d _Pd is preferably 0.5 <d _Pd <6.1 (nm). Therefore, from the above formula (2), the relationship between the diameter dCo (nm) of the core particles composed of cobalt and the molar ratio Pd / Co of palladium and cobalt is 0.5 <(d _Co / 2) × It is preferable that [{(Pd / Co) × (M _Pd / M _Co ) × (ρ _Co / ρ _Pd ) +1} ^1/3 −1] <6.1 is satisfied.

It is a schematic diagram which shows the structural example of the noble metal containing catalyst of this invention. It is a schematic diagram which shows the other structural example of the noble metal containing catalyst of this invention. It is a graph which shows the free energy change between each intermediate state at the time of producing | generating water from hydrogen ion and oxygen using the noble metal containing catalyst of this invention. It is a figure which shows the calculation model of the noble metal containing catalyst used for the theoretical calculation. Using the noble metal-containing catalyst of the present invention in which the combination of the transition metal element constituting the core particle and the noble metal element present on the surface of the core particle is “cobalt / gold” and “nickel / gold”, It is a graph which shows the free energy change between each intermediate state in the case of producing | generating water. Using the noble metal-containing catalyst of the present invention in which the combination of the transition metal element constituting the core particle and the noble metal element existing on the surface of the core particle is “nickel / iridium” and “tungsten / iridium”, It is a graph which shows the free energy change between each intermediate state in the case of producing | generating water. Using the noble metal-containing catalyst of the present invention in which the combination of the transition metal element constituting the core particle and the noble metal element existing on the surface of the core particle is “cobalt / rhodium”, “nickel / rhodium” and “tungsten / rhodium” It is a graph which shows the free energy change between each intermediate state in the case of producing | generating water from hydrogen ion and oxygen. Using the noble metal-containing catalyst of the present invention in which the combination of the transition metal element constituting the core particle and the noble metal element present on the surface of the core particle is “cobalt / palladium”, “nickel / palladium”, and “tungsten / palladium” It is a graph which shows the free energy change between each intermediate state in the case of producing | generating water from hydrogen ion and oxygen. Using the noble metal-containing catalyst of the present invention in which the combination of the transition metal element constituting the core particle and the noble metal element present on the surface of the core particle is “cobalt / platinum”, “nickel / platinum” and “tungsten / platinum” It is a graph which shows the free energy change between each intermediate state in the case of producing | generating water from hydrogen ion and oxygen. It is a graph which shows the free energy change between each intermediate state at the time of producing | generating water from hydrogen ion and oxygen using the noble metal containing catalyst which has the monoatomic layer of palladium on the surface of the core particle comprised with cobalt. It is a graph which shows the free energy change between each intermediate state at the time of producing | generating water from hydrogen ion and oxygen using the noble metal containing catalyst which has the monoatomic layer of palladium on the surface of the core particle comprised with nickel. It is a schematic diagram which shows an example of the film | membrane and electrode structure of this invention. It is a schematic diagram which shows an example of the fuel cell of this invention. It is a schematic diagram which shows an example of the fuel cell power generation system of this invention. 2 is a transmission electron micrograph of the noble metal-containing catalyst of Example 1. It is a graph which shows the relationship between the reciprocal number of the thickness of palladium, and oxygen reduction activity of the noble metal containing catalyst of Examples 1-2 and the catalyst of Comparative Example 1.

Explanation of symbols

1 catalytic metal 10 noble metal element 20 core particle 30 support

Claims (16)

A noble metal-containing catalyst composed of a catalyst metal and a carrier supporting the catalyst metal,
The catalyst metal is composed of a core particle and a noble metal element present on the surface of the core particle, and has a particle size of 1 to 30 nm.
Ec> Es, where Ec is the amount of change in free energy before and after oxygen adsorption in the element of the core particle, and Es is the amount of change in free energy before and after oxygen adsorption in the noble metal element. Precious metal-containing catalyst.   The noble metal-containing catalyst according to claim 1, wherein the core particles have a transition metal element other than the noble metal element.   The noble metal-containing catalyst according to claim 1 or 2, wherein the thickness of the noble metal element present on the surface of the core is 0.5 to 8 nm.   The noble metal-containing catalyst according to any one of claims 1 to 3, wherein the noble metal element present on the surface of the core particle is substantially only a single element.   The noble metal-containing catalyst according to any one of claims 1 to 4, wherein the noble metal element present on the surface of the core particle is platinum, palladium, gold, rhodium, iridium, ruthenium, silver or osmium.   The noble metal-containing catalyst according to any one of claims 2 to 5, wherein the core particles are substantially composed of a single transition metal element.   The noble metal-containing catalyst according to any one of claims 2 to 6, wherein the transition metal element contained in the core particles is cobalt, iron, nickel, or tungsten.   The combination of the transition metal element of the core particle and the noble metal element present on the surface of the core particle is a combination of the transition metal element: cobalt, nickel or tungsten and the noble metal element: platinum, palladium, gold, rhodium or iridium. The noble metal-containing catalyst according to any one of claims 2 to 7, which is other than a combination of cobalt and iridium and a combination of tungsten and gold.   The noble metal-containing catalyst according to claim 8, wherein the element contained in the core particle is cobalt or nickel, and the noble metal element present on the surface of the core particle is palladium.   The noble metal-containing catalyst according to claim 9, wherein the element contained in the core particle is cobalt. The relationship between the diameter d _Co (nm) of the core particle composed of cobalt and the molar ratio Pd / Co of palladium existing on the surface of the core particle and cobalt constituting the core particle is 0.5 <(d _Co The noble metal-containing catalyst according to claim 10, wherein /2)×[{(1.3369×Pd/Co)+1} ^1/3 -1] <6.1. A method for producing a noble metal-containing catalyst according to any one of claims 1 to 11,
A method for producing a noble metal-containing catalyst, comprising depositing a noble metal element on a surface of a core particle supported on a carrier by a displacement plating method. The core particle is composed of cobalt, and the noble metal element present on the surface of the core particle is palladium, and the diameter d _Co (nm) of the core particle, and the palladium present on the surface of the core particle and cobalt constituting the core particle The relationship with the molar ratio Pd / Co is adjusted to 0.5 <(d _Co /2)×[{(1.3369×Pd/Co)+1} ^1/3 −1] <6.1. The manufacturing method of the noble metal containing catalyst as described in any one of.   It has a catalyst electrode using the noble metal containing catalyst in any one of Claims 1-11, The film | membrane and electrode structure characterized by the above-mentioned.   A fuel cell comprising the membrane / electrode structure according to claim 14.   A fuel cell power generation system using the fuel cell according to claim 15.

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