JPS6349252A - Production of noble metal-deposited catalyst - Google Patents

Abstract

PURPOSE:To obtain the title catalyst capable of being highly activated by preclassifying a carrier to control the diameter of secondarily flocculated particles within a specified range, and depositing a noble metal on the carrier. CONSTITUTION:Carbon powder is dispersed in a soln., the obtained dispersion is classified into respective products having the same particle diameter by natural settling using a settling tube or centrifugal separation, and the powder having uniform particle diameter can be obtained. The secondarily flocculated particles having 0.15-6mum particle diameter is selected from the classified products, and used to prepare a platinum-deposited catalyst and a platinum and iron-deposited catalyst in the same way as the conventional method. Platinum is uniformly dispersed on the carrier in the form of fine particles, the obtained platinum-deposited catalyst has high activity, and sintering of platinum caused by the operating conditions of a battery can be minimized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field to which the Invention Pertains] The present invention relates to a noble metal-supported catalyst in which a noble metal is supported on secondary agglomerated particles, and particularly to a method for producing an electrode catalyst used in a gas diffusion electrode of a fuel cell.

[Prior art and its problems]

Generally, in alkaline fuel cells and phosphoric acid fuel cells, a catalyst is required for the reaction to proceed smoothly. Examples of this catalyst include noble metals such as platinum and radium and their alloys, nickel and its oxides, and tungsten carbide.

Many cyclic organometallic compounds such as Prussian blue have been discovered. Among these, platinum or a platinum alloy is mainly used for fermentation type fuel cells by supporting it on a conductive carrier material such as carbon bran or glue. When a metal is supported on a carrier, it is possible to make the metal particles into a finer particle state than when the metal is not supported, and by making the metal particles finer, the surface agitation per unit weight of the metal particles increases and the number of reaction sites increases. Therefore, the metal can be used effectively as a catalyst. When a metal is supported on a carrier, it is desirable that the metal be uniformly supported on all the carriers.

Generally, when metal is not uniformly supported on carrier particles, that is, when metal is poorly dispersed, the particle size of the supported metal tends to increase. Furthermore, when heat treatment is applied to alloy a catalyst with poor metal dispersion, metal sintering (sink 1)
ng) is likely to occur.

When this sintering occurs, the particle size of the metal increases and the effective surface area decreases! The output of the pond decreases. In addition, if the metal is poorly dispersed, sintering of the metal tends to occur even under battery operating conditions, resulting in a shortened catalyst life.

Methods for supporting metals on carriers include gas phase reduction,
Many attempts have been made, including liquid phase reduction, thermal decomposition, hydrothermal synthesis, and ion exchange, but it has been difficult to uniformly support metal on all carrier particles in any of these methods. The reason for this is that the ease with which metal particles adhere to each carrier particle differs because the surface condition of the carrier (e.g., degree of graphitization, number of surface functional groups, carboxyl groups, phenol groups, etc.) differs from carrier particle to carrier particle. It is believed that this is the case, but the details have not been disclosed. Furthermore, when conductive carbon black is used as a carrier, there is a tendency that small secondary agglomerated particles of the carrier are difficult to support metal, but the reason for this has not been clarified.

[Purpose of the invention]

An object of the present invention is to provide a method for producing a catalyst supported on a noble metal, which can highly activate the catalyst, prevent sintering of catalyst metal particles (shifting), and prevent deterioration in fuel cell performance. .

[Key points of the invention]

In the present invention, the diameter of the secondary agglomerated particles of the carrier is sorted to be within a certain range by pre-classifying the carrier, and by supporting the noble metal on the selected secondary agglomerated particles, the catalyst component is added to the carrier. The purpose is to produce a noble metal-supported catalyst in which metal is uniformly supported.

[Embodiments of the invention]

Conventionally, when carbon black is used as a carrier when manufacturing a noble metal catalyst, a carrier having a wide range of secondary agglomerated particle sizes has been used as is. When the catalyst obtained using this carrier is observed with a transmission electron microscope, there is a large variation in the number of noble metal particles supported by the carrier particles, and some carrier particles are observed that do not support any noble metal at all. Ta.

Therefore, the present inventors believe that there is a correlation between the ease of adhesion of noble metal particles to the carrier and the diameter of the secondary agglomerated particles of the carrier. By utilizing this method, we attempted to produce a noble metal-supported catalyst in which the metal as a catalyst component was uniformly supported on the carrier.

When carbon powder is dispersed in a solution by mechanical stirring or ultrasonic stirring, it has a wide particle size distribution in the solvent as shown in FIG. Carbon powder with uniform particle sizes can be obtained by separating this dispersion solution into particles of the same size using a natural sedimentation method using a sedimentation tube or centrifugation. The sedimentation rate of particles varies depending on the particle size, and Table 1 shows approximate values of particle size and sedimentation rate obtained by natural sedimentation method or centrifugation method. Note that calculations were made assuming that pure water was used as the solvent.

Table 1 Relationship between particle size and sedimentation speed by centrifugation When the rotation speed is constant, carbon powder of the desired particle size can be separated by changing the rotation time. Further, when the rotation time is kept constant, similar separation can be performed by changing the rotation speed. As is clear from Table 1, the natural sedimentation method requires a long sedimentation time, so
It is suitable for separating carbon particles having a relatively large particle size of μm or more. Since the centrifugal separation method can shorten the separation time by increasing the rotational speed, it is also possible to separate particles as small as Q, l, and cut+. Therefore, using any one of these separation methods, it is possible to obtain secondary agglomerated particles whose particle size is sorted within a certain range. The present invention will be described in detail below based on Examples.

(Example) After dispersing acetylene black 2002 in 90 A of solvent using ultrasonic dispersion, the particle size was reduced to 0 by centrifugation.
．． A product classified into a range of 3 to 4 μm was obtained. Next, a platinum-supported catalyst and a platinum-iron-supported catalyst were prepared using the classified acetylene brazing agent 902 in the same manner as in the conventional method. When the obtained platinum-supported catalyst was observed with a transmission electron microscope, the number of platinum particles attached to one support was 8 to 8.
The number of platinum particles was 15, resulting in a platinum-supported catalyst in which platinum particles were more uniformly dispersed and supported than in the conventional method. Furthermore, no clusters of agglomerated platinum particles, which were observed in the conventional method, were observed. The platinum-iron alloy supported catalyst obtained by alloying this catalyst is thought to be less prone to sintering due to the high dispersibility of the original platinum catalyst; however, as observed using a transmission electron microscope, It was confirmed that sintering (shifter 11 ring) had not progressed very much. That is, the crystallite diameter of the supported die particles is 15 to 30 angstroms, and the distribution thereof is narrower than in the conventional method, which shows that the platinum particles are evenly supported. It was also found that the crystallite diameter of the platinum-iron alloy supported catalyst was 4 to 10 angstroms, and crystal growth due to alloying heat treatment was suppressed compared to conventional methods.

(Conventional example) Oxidation 1. 4. Add 90 tons of finished acetylene brane to deionized water. The slurry was dispersed in ultrasonic water to form a slurry. A chloroplatinic acid aqueous solution containing platinum 101 and a sodium carbonate aqueous solution were subsequently added, and the total amount was made up to 5 ml and stirred for 1 hour. The temperature of the slurry was raised to 40 to 70°C, an appropriate amount of 30% hydrogen peroxide solution was added, and while stirring was continued, an amount of formic acid aqueous solution necessary to reduce platinum ions was added dropwise.

This sura 1! - was filtered and washed with water to obtain a platinum supported catalyst. When the obtained catalyst was observed with a transmission electron microscope, the number of platinum particles attached to one carrier varied widely, ranging from 0 to 20. In addition, some communities where platinum particles were aggregated were also observed. Furthermore, these platinum particles had a crystallite size of approximately 15-60 angstroms.

Next, this supported platinum catalyst was ultrasonically dispersed in deionized water (4-e), a ferric nitrate aqueous solution containing 2.8 tons of iron was added, and the mixture was stirred for 1 hour, then heated to 40-70°C for 1 hour. Stirred.

At this stage, all the iron ions in the aqueous solution are adsorbed onto the platinum,
The color of the liquid changed from clear yellow to clear and colorless. Next, the mixture was cooled to room temperature, and dilute ammonia water was added dropwise to obtain l'H8. As a result, iron ions became ferric hydroxide and adhered to the platinum particles. After filtering and washing with water, dry in a nitrogen stream to 850-1
A platinum-iron supported catalyst was obtained by heat treatment at 000°C for 2 hours.

The obtained catalyst was made into a transmission type 1! ! When observed under a microscope, the particle size of the alloy particles was slightly larger than that of the base platinum particles, and crystal growth was particularly remarkable in areas where platinum particles were thought to have formed colonies.

Therefore, the crystallite diameter of the alloy particles was as large as about 30 to 250 angstroms.

As described above, a porous membrane containing 45% by weight of tetrafluoride ethylene fine powder in a uniformly dispersed state was prepared using the supported platinum iron alloy catalyst obtained in the conventional examples and examples. The powder was bound onto water-repellent carbon paper to form an electrode.

The platinum density of the electrodes is 0.50 tn? )'t
/-, and the porosity was 80 to 82%. A small cell was assembled using these electrodes as oxidizer electrodes and operated at normal pressure. For the fuel electrode, the same weight of acetylene black was mixed with the supported platinum catalyst obtained by the conventional method.
Electrodes were made in the same way. The platinum density of the fuel electrode is 0.23
The trot P t/d void was 81%. The characteristics of the small battery using air and a mixed gas of 80% hydrogen and 20% carbon dioxide were as shown in Table 2.

As can be seen from Table 2 of Table 32, using the classified carrier, ml! L
The platinum-iron alloy supported catalyst, in which catalyst particles were uniformly supported, showed higher activity and better stability than those produced by conventional methods.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, the carrier is classified in advance so that the diameter of the secondary agglomerated particles falls within a certain range, and the noble metal is supported on the selected secondary agglomerated particles. Since the platinum-supported catalyst is made into a platinum-supported catalyst, the platinum becomes fine particles that are uniformly dispersed on the support, and the resulting platinum-supported catalyst is highly active. (2000) can be kept low. Furthermore, during the heat treatment during alloying when preparing an alloy catalyst, sintering of platinum is less likely to occur, so that the properties of the supported platinum alloy catalyst can be improved.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the particle size distribution of carbon powder. 1...Distribution when dispersed by mechanical stirring, 2...Distribution when dispersed by ultrasonic stirring. a4 (o next)

Claims (1)

[Scope of Claims] 1) A method for producing a noble metal supported catalyst, characterized in that a noble metal is supported on secondary agglomerated particles that have been classified in advance to have a particle size within a certain range. 2) In the method according to claim 1, the particle size of the secondary agglomerated particles is 0.15 to 6 μm, more preferably 0.
．． A method for producing a noble metal supported catalyst, characterized in that the catalyst has a particle size in the range of 3 to 4 μm.