Coating processes – Electrical product produced – Carbon base
Reexamination Certificate
1999-08-20
2001-07-17
Parker, Fred J. (Department: 1762)
Coating processes
Electrical product produced
Carbon base
C427S115000, C427S125000, C427S304000, C205S205000, C204S294000, C429S047000
Reexamination Certificate
active
06261632
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for improving the adhesion of metal particles to a carbon substrate.
BACKGROUND OF THE INVENTION
More specifically, the invention relates to a process for improving the adhesion of particles of metals, such as platinum, to a carbon substrate, the supported metals being used, in particular, for manufacturing the electrodes of a membrane electrolysis cell having a cathode for the reduction of oxygen, which produces a solution of an alkali metal hydroxide and of chlorine by the electrolysis of an aqueous alkali-metal chloride (NaCl or KCl) solution.
Such an electrolysis cell generally consists of a cation exchange membrane which divides the cell into an anode compartment and at least one cathode compartment in which the cathode is placed, the compartment being supplied with a gas containing oxygen.
The anode and cathode reactions in such electrolytic processes using a cathode for the reduction of oxygen are:
at the anode: 2Cl
−
→Cl
2
+2e
−
at the cathode: H
2
O+½O
2
+2e
−
→2OH
−
In order to speed up the reaction occurring at the cathode and lower the absolute value of the reaction overpotential these are employed, catalysts, generally noble metals such as platinum, silver or gold deposited on electrically conducting supports, such as in particular carbon materials having a high specific surface area.
Preferably, platinum or silver, and more particularly platinum, is used since, although this is an expensive metal, it is the one for which the cathodic overpotential is lowest in absolute value.
It has been found in tests on the stability of catalysts based on platinum deposited on carbon in hot concentrated sodium hydroxide under oxygen for several hours that there is a significant loss of platinum.
This significant loss of platinum results in a deterioration in the electrochemical performance, such as an increase in the cell voltage.
This loss of platinum from the catalyst used in the electrodes of membrane electrolysis cells having a cathode for the reduction of oxygen has been mentioned by various authors.
Thus, S. Kohda et al. in Soda to Enso, 1995, Volume 46(10), pages 402 to 419 (Absc. Number: 124-129655) report that, after a continuous operation of 175 days, analysis of the electrodes indicates a considerable loss of platinum. This loss, according to these authors, is influenced by the operating conditions and in particular by the electric current.
In order to limit the loss of platinum, these authors have proposed using platinum-iridium as the catalyst.
Kagaku Koggakukai in Soda to Enso, 1996, Volume 47(1), pages 16 to 31 points out that, in the case of air cathodes in a basic medium (NaOH solution containing 9 mol/l of NaOH) at 85° C., there is a loss of platinum and a deterioration of the carbon support.
SUMMARY OF THE INVENTION
A process for improving the adhesion of metal particles to a carbon substrate has now been discovered, this being characterized in that, before the said metal particles are deposited on the carbon substrate, the said carbon substrate is treated in an alkaline medium at a temperature of between 50° C. and 100° C., and preferably close to 90° C., in a stream of a gas containing oxygen for a period of at least 2 hours and preferably of greater than 24 hours.
According to the present invention, the carbon substrate is introduced into a highly concentrated solution of an alkali metal hydroxide.
Preferably, sodium hydroxide (NaOH) solutions will be used. The concentration by weight of the alkali metal hydroxide solutions is at least 30% and preferably between 30% and 60%.
The amount by weight of treated substrate may vary over a wide range. Generally, from 0.1 g to 10 g per 100 ml of alkali metal hydroxide solution are treated.
According to the present invention, the gas containing oxygen is bubbled into the concentrated alkali metal hydroxide solution, the solution being heated to the treatment temperature and containing the carbon substrate to be treated.
Having completed the treatment, the substrate is filtered under reduced pressure and then rinsed several times with deionized water. Next, it is dried at 80° C./90° C. at atmospheric pressure and then under reduced pressure at approximately 100° C.
According to the present invention, the gas containing oxygen may be air, oxygen-enriched air or oxygen.
Preferably, oxygen will be used.
The volume concentration of oxygen in the gas is at least 20% and preferably at least 50%.
The oxygen-enriched gases are preferably decarbonated beforehand.
The metal particles comprising one or more metals may be deposited on the carbon substrate after treatment in an alkaline medium according to a preferred so-called impregnation-reduction method differing little from the protocol described by Bartholomev and Boudard in Journal of Catalysis, 1972, Volume 25, pages 173 to 181. However, according to the present invention, the first step of the protocol of partially oxidizing the carbon substrate at 600° C. is optional not mandatory, prior to subjecting the substrate treatment in the alkaline medium, in order to obtain an improvement in adhesion. Furthermore, whereas the fourth step of the protocol of reducing the impregnated substrate is advantageously carried out at 500° C. in a stream of hydrogen, it is even more advantageous to slowly raise the temperature to 500° C. during the reduction step.
By way of illustration of carbon substrates that can be used according to the present invention, mention may be made of the graphite called TIMCAL HSAG-® 300 having a specific surface area of 360 m
2
/g; SIBUNIT 5®, which is a carbon having a specific surface area of 360 m
2
/g; VULCAN XC-72R® carbon which is a furnace black having a specific surface area of 300 m
2
/g.
The process of the invention applies most particularly to any metal whose crystallites deposited on a carbon substrate have an average diameter of less than 10 nm.
By way of illustration of metals forming the metal particles deposited on the carbon substrate treated according to the process of the present invention, mention may be made of platinum, silver, gold, ruthenium, iridium or a mixture of two or more of the aforementioned metals. The invention relates most particularly to platinum or a platinum-silver mixture.
The stability test consists of introducing the material comprising metal particles deposited on the carbon substrate having undergone the alkaline treatment, hereinafter called material, in a highly concentrated solution of an alkali metal hydroxide.
Preferably, sodium hydroxide (NaOH) solutions will be used. The concentration by weight of the alkali metal hydroxide solutions is at least 30% and preferably between 30% and 60%.
The amount by weight of material tested may vary over a wide range. In general, it varies from 0.1 g to 10 g per 100 ml of alkali metal hydroxide solution.
According to the present invention, the gas containing oxygen is bubbled into the concentrated alkali metal hydroxide solution, the solution being heated to the treatment temperature and containing the material to be tested.
Having completed the stability test, the material is filtered under reduced pressure and then rinsed several times with deionized water. Next, it is dried at 80° C./90° C. at atmospheric pressure and then under reduced pressure at approximately 100° C.
According to the present invention, the gas containing oxygen may be air, oxygen-enriched air or oxygen.
Preferably, oxygen will be used.
The volume concentration of oxygen in the gas is at least 20% and preferably at least 50%.
The oxygen-enriched gases are preferably decarbonated beforehand.
The conditions of the stability test may be different from the substrate treatment conditions.
The process according to the present invention makes it possible to increase the adhesion of the metal particles to the carbon substrate and therefore limit the loss of metal particles.
Another subject of the invention relates also to the materials comprising metal particles preferably deposited by imp
Andolfatto Franåoise
Miachon Sylvain
Atofina
Millen White Zelano & Branigan
Parker Fred J.
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