Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Making catalytic electrode – process only
Reexamination Certificate
1999-03-02
2001-02-27
Bell, Bruce F. (Department: 1741)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Making catalytic electrode, process only
C502S182000, C502S184000, C502S185000, C502S325000, C502S330000, C502S334000, C502S339000, C502S344000, C502S347000, C427S181000, C427S191000, C204S242000, C204S292000
Reexamination Certificate
active
06194338
ABSTRACT:
SUMMARY OF THE INVENTION
The present invention relates to a stabilized bimetal supported catalyst based on platinum or silver, used in the electrodes for electrochemical cells.
More specifically, the invention relates to a bimetal supported catalyst based on platinum or silver, used for the electrodes of an electrolysis cell having a membrane and an oxygen-reducing cathode which produces a solution of alkali metal hydroxide and of chlorine by the electrolysis of an aqueous solution of an alkali metal chloride (NaCl, KCl).
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 said cathode is placed, the said compartment being supplied with an oxygen-containing gas.
The anode and cathode reactions in such electrolytic processes using an oxygen-reducing cathode are:
at the anode:
2Cl
−
→Cl
2
+2
e
at the cathode:
H
2
O+½O
2
+2
e→
2OH
−
In order to speed up the reactions occurring at the cathodes and to lower the reaction overvoltage in absolute value, catalysts are used, generally noble metals such as platinum, silver or gold, which are deposited on electrically conductive supports such as, especially, carbonaceous materials having a high specific surface area or metal substrates.
Preferably, platinum or silver is used, most particularly platinum since, although this is an expensive metal, it is the one which has the lowest value of cathode overpotential in absolute value.
The Applicant has observed, especially during periods when electrolysis cells like those mentioned above have stopped operating, that there is an agglomeration of platinum or silver particles on the support used. This has the effect of decreasing the “active” specific surface area of the catalyst, resulting in a loss of performance. In particular, an increase in the cell voltage is observed, resulting in increasing consumption of energy.
Tomantscheger et al. (J. Power Sources 1992, vol. 39 (1), pages 21-41) have observed when catalysts consisting of platinum particles supported on carbon are exposed to potassium that there is a significant agglomeration of the said platinum particles. The diameter of the Pt particles goes from 50 Å to 200 Å after approximately 48 hours' exposure to potassium at 80° C.
Pataki et al. (Electrochemical Soc. Meet., Toronto, Ont., May 12-17, 1985, Ext. Abstr. No. 659, pages 924-925) have shown that by pretreating a catalyst consisting of platinum particles supported on carbon with carbon monoxide (CO) there was a much smaller agglomeration of the said Pt particles when, as previously, the said catalyst was exposed to potassium.
The Applicant has now discovered that it is possible to decrease, or even eliminate the agglomeration of platinum or silver particles of supported catalysts by simultaneously depositing, on the support, when preparing the said catalyst, a first metal M1 chosen from platinum and silver and a second metal M2, different from the first and chosen from the group consisting of platinum, silver, gold, ruthenium, iridium, rhodium and osmium.
SUMMARY OF THE INVENTION
One subject of the present invention is therefore a stabilized bimetal catalyst comprising an electrically conductive support and a metallic deposited coating consisting of a first metal M1 chosen from a platinum and silver and a second metal M2, different from the first and chosen from the group consisting of platinum, silver, gold, ruthenium, iridium, rhodium and osmium.
Preferably, M2 is platinum, silver or ruthenium.
According to the present invention, the mass ratio M1/M2 of the metals M1 and M2 of the metallic deposited coating is at least equal to 1 and preferably between 1 and 20.
According to the present invention, carbonaceous materials, such as graphite, furnace black, carbon black or carbon powder, having specific surface areas of at least 50 m
2
/g and preferably between 100 and 600 m
2
/g, will be used as the electrically conductive support. These carbonaceous materials may be pretreated, especially in order to create functional groups on the surface of the carbonaceous material.
By way of illustration of such carbonaceous supports, 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, and the carbon VULCAN XC-72R which is a furnace black having a specific surface area of 300 m
2
/g.
The bimetal catalyst of the present invention may be prepared by a co-reduction of mixtures of reducible metal salts using a process which consists in simultaneously impregnating, in a solvent medium, the electrically conductive support with a solution of a metal salt of the first metal M1, chosen from platinum and silver, and with a solution of a metal salt of the second metal M2, different from the first, chosen from the group consisting of platinum, silver, gold, ruthenium, iridium, rhodium and osmium; in slowly evaporating to dryness, with stirring and with an inert-gas sparge, the suspension consisting of the solutions of the metal salts of the metals M1 and M2 and the carbonaceous support; in drying the powder obtained, under atmospheric pressure, at a temperature of between 60° C. and 80° C. and then, under a reduced pressure, at a temperature of between 90° C. and 110° C.; in subjecting the dried powder obtained to a stream of hydrogen, at temperatures ranging from 300° C. to 600° C., achieved at a rate of temperature rise of between 0.2° C. and 1.5° C. per minute and preferably ranging from 0.2° C. to 1° C. per minute, and then in stopping the heating or else in holding the temperature obtained for a time of at most 20 hours and preferably for a time of between 2 hours and 16 hours.
Preferably, alcoholic or aqueous/alcoholic solutions of the metal salts of the metals M1 and M2 are used. The preferred alcohol is ethanol. The concentrations by weight of the metal salts of the metals M1 and M2 of the said solutions may vary over a wide range. They may range from a few grams to a few tens of grams of salts per liter. They are calculated in such a way that, when the said solutions containing the salts of metals M1 and M2 are mixed, a mass ratio M1/M2, of the metals M1 and M2 to be simultaneously deposited on the support, at least equal to 1, and preferably between 1 and 20, is obtained.
Next, having produced the mixture of the said solutions, a carbonaceous support and an impregnation solvent, such as benzene or toluene, in an amount of at least 10 ml of solvent per gram of support, are introduced into the said mixture and then the suspension obtained is held for a few days at ambient temperature with stirring and with a nitrogen sparge. This allows the solvents to evaporate and the support to be perfectly impregnated by the metal salts.
Next, the powder obtained is firstly dried at atmospheric pressure at a temperature of between 60° C. and 80° C., preferably at a temperature close to 70° C., and then at a reduced pressure at a temperature of between 90° C. and 110° C., preferably at a temperature close to 100° C.
At this stage, the support is impregnated by the salt of M1 and the salt of M2. Next, the cations are reduced in a stream of hydrogen.
The stability of the bimetal catalysts of the present invention which are thus obtained is tested. For this purpose, they are put into suspension in alkaline solutions at temperatures of between 600C and 90° C. and exposed to a stream of oxygen for several hours, or even several days.
The change in the size of the particles of M1 and in the metallic deposited coating consisting of M1 and M2 is monitored, before and after the treatment, by X-ray diffraction.
This method of analysis makes it possible to state that a stabilized bimetal supported catalyst is obtained.
This is because the particles of metal M1 no longer build up when they are deposited on a support simultaneously with particles of a metal M2, different from metal M1.
The bimetal supported catalysts, sta
Andolfatto Francoise
Miachon Sylvain
Bell Bruce F.
Elf Atochem S.A.
Millen White Zelano & Branigan P.C.
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