U.S. patent application number 14/767267 was filed with the patent office on 2016-01-14 for method for synthesizing bimetal catalyst particles made of platinum and of another metal and use thereof in an electrochemical hydrogen production method.
The applicant listed for this patent is COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES. Invention is credited to Nicolas GUILLET, Joseph NGAMENI JIEMBOU.
Application Number | 20160010229 14/767267 |
Document ID | / |
Family ID | 48140086 |
Filed Date | 2016-01-14 |
United States Patent
Application |
20160010229 |
Kind Code |
A1 |
GUILLET; Nicolas ; et
al. |
January 14, 2016 |
METHOD FOR SYNTHESIZING BIMETAL CATALYST PARTICLES MADE OF PLATINUM
AND OF ANOTHER METAL AND USE THEREOF IN AN ELECTROCHEMICAL HYDROGEN
PRODUCTION METHOD
Abstract
A process for the synthesis of particles of bimetal catalyst
based on platinum and on at least one second metal comprises the
chemical reduction of a first platinum-based salt or complex and of
at least one second salt or complex based on the second metal, the
chemical reduction comprising the following stages: the preparation
of a mixture comprising the first platinum-based salt or complex
and the second salt or complex based on the second metal, in the
presence of a pure reducing agent in the liquid form under ambient
temperature and pressure conditions, the conditions being
respectively defined as equal to 25.degree. C. and 100 kPa;
bringing the mixture to a temperature between approximately the
freezing temperature of water and the freezing temperature of the
reducing agent.
Inventors: |
GUILLET; Nicolas; (Pizancon,
FR) ; NGAMENI JIEMBOU; Joseph; (Grenoble,
FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
COMMISSARIAT A L'ENERGIE ATOMIQUE ET AUX ENERGIES
ALTERNATIVES |
Paris |
|
FR |
|
|
Family ID: |
48140086 |
Appl. No.: |
14/767267 |
Filed: |
February 24, 2014 |
PCT Filed: |
February 24, 2014 |
PCT NO: |
PCT/EP2014/053535 |
371 Date: |
August 11, 2015 |
Current U.S.
Class: |
205/638 ;
502/185; 502/339; 502/5 |
Current CPC
Class: |
B01J 37/035 20130101;
B82Y 40/00 20130101; C01B 2203/1041 20130101; C25B 1/02 20130101;
Y02P 20/141 20151101; Y02P 20/52 20151101; B01J 35/006 20130101;
B01J 37/345 20130101; B82Y 30/00 20130101; B01J 23/626 20130101;
C01B 2203/0238 20130101; Y02P 20/142 20151101; C01B 2203/1241
20130101; C25B 11/0484 20130101; B01J 35/0013 20130101; C01B 3/40
20130101; C01B 2203/107 20130101; B01J 21/18 20130101; B01J 37/16
20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C25B 1/02 20060101 C25B001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 2013 |
FR |
1351679 |
Claims
1. A process for the synthesis of particles of bimetal catalyst
based on platinum and on at least one second metal, comprising
chemical reduction of a first platinum-based salt or complex and of
at least one second salt or complex based on said second metal,
said chemical reduction comprising the following stages: preparing
a mixture comprising said first platinum-based salt or complex and
said second salt or complex based on said second metal, in the
presence of a pure reducing agent in the liquid form under ambient
temperature and pressure conditions, said conditions being
respectively defined as equal to 25.degree. C. and 100 kPa; and
bringing said mixture to a temperature between approximately the
freezing temperature of water and the freezing temperature of the
reducing agent.
2. The process for the synthesis of particles of bimetal catalyst
as claimed in claim 1, wherein the reducing agent is formic acid,
the bringing to a temperature being carried out between
approximately 0.degree. C. and 8.degree. C.
3. The process for the synthesis of particles of bimetal catalyst
as claimed in claim 1, wherein the reducing agent is hydrazine
(N.sub.2H.sub.4), the bringing to a temperature being carried out
between approximately 0.degree. C. and 2.degree. C.
4. The process for the synthesis of particles of bimetal catalyst
as claimed in claim 1, further comprising the mixing of platinum
salt or complex and of salt or complex of said second metal, in the
presence of particles of carbon black or of metal oxide or of metal
nitride or of metal carbide.
5. The process for the synthesis of particles of bimetal catalyst
as claimed in claim 1, wherein the amount of reducing agent is
greater than or equal to the amount necessary to carry out the
chemical reduction of all of the platinum salts or complexes and of
the salts or complexes of the second metal.
6. The process for the synthesis of particles of bimetal catalyst
as claimed in claim 1, wherein the reaction is carried out in the
presence of an additional energy source which makes it possible to
accelerate the chemical reduction operation without promoting the
growth of nanoparticles.
7. The process for the synthesis of particles of bimetal catalyst
as claimed in claim 6, wherein the additional energy source is an
ultraviolet radiation source.
8. The process for the synthesis of particles of bimetal catalyst
as claimed in claim 7, wherein the ultraviolet radiation source
emits in a wavelength range of between approximately 200 nm and 300
nm.
9. The process for the synthesis of particles of bimetal catalyst
as claimed in claim 1, wherein the second metal is tin or ruthenium
or molybdenum or cobalt.
10. The process for the synthesis of particles of bimetal catalyst
as claimed in claim 9, wherein the size distribution of said
particles exhibits a median size of 4 nm and a standard deviation
of 1.1.
11. The use of the process for the synthesis of particles of
bimetal catalyst as claimed in claim 1, in a method for the
electrochemical production of hydrogen comprising a catalytic
reforming reaction in the presence of said catalyst particles and
of a gas mixture comprising hydrocarbon compounds.
12. The use of the process for the synthesis of particles of
bimetal catalyst as claimed in claim 11, wherein the gas mixture
comprises carbon monoxide, carbon dioxide and methane.
Description
[0001] The field of the invention is that of H.sub.2/O.sub.2 fuel
cells. The use of this type of fuel cell in the automobile industry
instead of and in place of internal combustion engines is still
encountering problems related to the storage of the hydrogen:
pressurized hydrogen tanks (gaseous storage) are potentially
dangerous and metal hydrides (storage in solid form) are
inappropriate due to their low energy density, as described in the
paper by Schlapbach L. and Zuttel A., Hydrogen-storage materials
for mobile applications, Nature, 2001, 414, 353-8.
[0002] The production of hydrogen onboard the vehicle by catalytic
reforming offers an alternative solution to the direct storage of
hydrogen, as described in the paper by Basile A., Galluci F. and
Paturzo L., "Hydrogen production from methanol by oxidative steam
reforming carried out in a membrane reactor", Catalysis Today,
2005, 104, 251-9, but a purification stage is necessary in order to
feed the fuel cell.
[0003] For this type of application, gaseous biofuels or
hydrocarbons, such as natural gas, or liquid biofuels and
hydrocarbons, such as alcohol, gasoline or diesel oil, are
potentially sources of hydrogen and an electrochemical cell
connected to a low-power electrical supply can be used to extract,
at low temperature, hydrogen from the gas mixture comprising carbon
monoxide (CO), carbon dioxide (CO.sub.2), methane (CH.sub.4) and
other gases.
[0004] By applying a sufficient electric voltage to the terminals
of the cell, the electro-oxidation of hydrogen and of the carbon
monoxide take place at the anode:
[0005] H.sub.2.fwdarw.2H.sup.++2e.sup.-(E.degree.
(H.sup.+/H.sub.2)=0 vs. SHE, electrochemical equilibrium standard
potential, SHE being the potential of the standard hydrogen
electrode)
CO.sub.ads+H.sub.2O.fwdarw.CO.sub.2+2H.sup.++2e.sup.-
as described in the paper by M. Ciureanu et al., "Electrochemical
Impedance Study of Electrode-Membrane Assemblies in PEM Fuel Cells
I. Electro-oxidation of H.sub.2 and H.sub.2/CO Mixtures on Pt-Based
Gas-Diffusion Electrodes", Journal of the Electrochemical Society,
1999, 146, 4031-4040.
[0006] Platinum is very often used as reaction catalyst in
electrochemical systems but its use in a purification application
as anode material presents a problem, although it is the best
material used at low temperature for the electro-oxidation reaction
of hydrogen (H.sub.2.fwdarw.2H.sup.++2e.sup.-).
[0007] This is because platinum (Pt) is expensive, rare and less
effective at low potential because of the ability which carbon
monoxide (present in the reformed hydrogen) has to poison it. This
poisoning takes place by irreversible adsorption of the CO at the
surface of the Pt, blocking the adsorption sites available, thus
preventing the adsorption of the hydrogen and its oxidation.
[0008] In point of fact, the electro-oxidation of CO at the surface
of Pt supported on carbon, that is to say of CO to CO.sub.2
(CO+H.sub.2O.fwdarw.CO.sub.2+2H.sup.++2e.sup.-) takes place at
relatively high potentials located around from 0.7 to 0.8 V vs.
SHE, which requires a not insignificant energy contribution.
[0009] In order to overcome the difficulties encountered with
platinum Pt related essentially to its poor tolerance toward carbon
monoxide CO, novel anode catalysts are being looked for.
[0010] In order to result in these novel types of catalysts, the
preparation of an alloy of Pt with a transition metal can be
envisaged or the combination of highly divided Pt with a phase of
the metal oxide type. Several platinum alloys, such as
platinum-ruthenium (Pt--Ru), platinum-tin (Pt--Sn),
platinum-molybdenum (Pt--Mo) or platinum-cobalt (Pt--Co), supported
on carbon black with a high specific surface, are forming the
subject of studies with the aim of finding the best platinum-based
alloy which has good tolerance toward carbon monoxide CO and which
makes it possible, moreover, to oxidize said carbon monoxide CO at
low potential.
[0011] Furthermore, it is known that the performance of these
anodic catalysts with respect to an electro-oxidation of H.sub.2/CO
depends on their structure, on their chemical composition, on the
nanometric size of the particles and on the nature of their
support. Command of the technique for elaboration and of the method
for the preparation of the catalysts makes it possible to control
the physicochemical properties. The metal nanoparticles are
generally synthesized by chemical reduction of metal salts or
complexes in the presence of a reducing agent. During the
synthesis, the mechanism of development of the nanoparticles takes
place by germination and growth. This stage is important as it
determines the size of the particles and consequently impacts the
electroactive surface of the catalyst.
[0012] The production of catalyst nanoparticles by chemical
reduction of metal salts or complexes in the presence of a reducing
agent is a method which is simple to carry out but the command and
the control of the phases of germination and growth of the
nanoparticles remain a major issue.
[0013] For this reason and in this context, the subject matter of
the present invention is a method based on the optimization of the
operating conditions of the synthesis in order to limit the phase
of growth of the nanoparticles and to promote that of the
germination, making it possible to increase the number of
seeds.
[0014] In comparison with a spontaneous development during the
synthesis of particles by chemical reduction (without modifying the
operating parameters), the limitation of the growth phase makes it
possible to obtain nanoparticles with small sizes, typically of the
order of 2 to 5 nm, and a greater number of particles.
[0015] Uniting these two aspects offers a microstructural
configuration in which the electroactive surface of the catalyst is
optimal.
[0016] Modifying the temperature at which the chemical reduction of
the metal salts and complexes takes place represents the most
effective means for reducing the rate of growth of the particles as
the two parameters develop in the same direction.
[0017] More specifically, a subject matter of the present invention
is a process for the synthesis of particles of bimetal catalyst
based on platinum and on at least one second metal, characterized
in that it comprises the chemical reduction of a first
platinum-based salt or complex and of at least one second salt or
complex based on said second metal, said chemical reduction
comprising the following stages: [0018] the preparation of a
mixture comprising said first platinum-based salt or complex and
said second salt or complex based on said second metal, in the
presence of a pure reducing agent in the liquid form under ambient
temperature and pressure conditions (ATPCs), said conditions being
defined at 25.degree. C. and 100 kPa; [0019] bringing said mixture
to a temperature between approximately the freezing temperature of
water and the freezing temperature of the reducing agent.
[0020] According to an alternative form of the invention, the
reducing agent is formic acid, the temperature of said chemical
reduction being carried out at a temperature of between
approximately 0.degree. C. and 8.degree. C., which can
advantageously be of the order of 4.degree. C.
[0021] According to an alternative form of the invention, the
reducing agent is hydrazine, the temperature of said reduction
being carried out at a temperature of between 0.degree. C. and
2.degree. C.
[0022] According to an alternative form of the invention, the
reducing agent is formaldehyde, the temperature of said reduction
being carried out at a temperature of between -19.degree. C. and
0.degree. C.
[0023] According to an alternative form of the invention, the
process comprises the mixing of platinum salt or complex and of
salt or complex of said second metal, in the presence of particles
of carbon black or of metal oxide or of metal nitride or of metal
carbide.
[0024] According to an alternative form of the invention, the
amount of reducing agent is greater than or equal to the amount
necessary to carry out the chemical reduction of all of the
platinum salts or complexes and of the salts or complexes of the
second metal.
[0025] According to an alternative form of the invention, the
reaction is carried out in the presence of an additional energy
source which makes it possible to accelerate the chemical reduction
operation without promoting the growth of nanoparticles.
[0026] According to an alternative form of the invention, the
additional energy source is an ultraviolet radiation source.
[0027] According to an alternative form of the invention, the
ultraviolet radiation source emits in a wavelength range of between
approximately 200 nm and 300 nm.
[0028] According to alternative form of the invention, the second
metal is tin or ruthenium or molybdenum or cobalt.
[0029] According to an alternative form of the invention, the
particles of bimetal catalyst are based on platinum and tin and
their size distribution exhibits a median value of 4 nm with a low
dispersion: the standard deviation is of the order of 1.1.
[0030] Another subject matter of the invention is the use of the
process for the synthesis of particles of bimetal catalyst
according to the invention, in a method for the electrochemical
production of hydrogen comprising a catalytic reforming reaction in
the presence of said catalyst particles and of a gas mixture
comprising hydrocarbon compounds. This is because the metal
particles obtained by the method of synthesis of the invention
appear to be less sensitive to contamination by the hydrocarbon
compounds present in the gas mixture than the particles obtained
according to the methods described in the state of the art.
[0031] According to an alternative form, the gas mixture comprises
carbon monoxide, carbon dioxide and methane.
[0032] A better understanding of the invention will be obtained and
other advantages will become apparent on reading the description
which will follow, given without implied limitation, and by virtue
of the appended figures, among which:
[0033] FIG. 1 illustrates the UV absorption spectra of the
PtCl.sub.6.sup.2- ions;
[0034] FIG. 2 illustrates the distribution in a size of particles
of the Pt.sub.3Sn/C catalysts synthesized at ambient temperature
and at 4.degree. C., according to the process of the invention;
[0035] FIG. 3 illustrates the cyclic voltammogram obtained with
regard to Pt.sub.3Sn/C synthesized at ambient temperature in an
aqueous H.sub.2SO.sub.4 solution with a concentration of 0.5M at
25.degree. C.;
[0036] FIG. 4 illustrates the cyclic voltammogram obtained with
regard to Pt.sub.3Sn/C synthesized at 4.degree. C. in an aqueous
H.sub.2SO.sub.4 solution with a concentration of 0.5M at 25.degree.
C.;
[0037] FIG. 5 illustrates the change in the current for the
electro-oxidation of H.sub.2 over Pt.sub.3Sn/C synthesized at
ambient temperature and at 4.degree. C. in aqueous H.sub.2SO.sub.4
solution with a concentration of 0.5M after having subjected the
catalyst to a gas mixture composed of 50 ppm of CO in H.sub.2 and
at 0.24 V vs. RHE and reached a virtually stationary state of the
change in the hydrogen oxidation current over time.
[0038] For the synthesis of a platinum-metal (Pt--M) alloy
catalyst, the Applicant uses the FAM (Formic Acid Method) method as
described in the paper by E. I. Santiago et al., "CO tolerance on
PtMo/C electrocatalysts prepared by the formic acid method",
Electrochimica Acta, 48 (2003), 3527-3534.
[0039] According to the present invention, it is proposed to use
solutions of salts or complexes of Pt and of a metal ally as
precursors of Pt--M catalysts supported or not on carbon black with
a high specific surface, a metal oxide (TiO.sub.2, ZrO.sub.2,
Al.sub.2O.sub.3, and the like), metal nitrides (TiN, TaN, BN) or
metal carbides (TiC, WC, W.sub.2C, Mo.sub.2C, and the like).
[0040] In the case of a carbon support, the solutions of salts or
complexes of Pt and of the allied metal element M are mixed with
the carbon support and the combination is vigorously stirred with
ultrasound for at least half an hour, the time necessary in order
to obtain a homogeneous mixture.
[0041] The volume of the solutions is determined from the
concentration of the solutions and so as to obtain the desired
atomic composition of the metal alloy. The weight of support of
metal nanoparticles of use in the synthesis is determined so that
it represents 50% of the weight of the catalyst synthesized. Formic
acid is used as reducing agent and is added to the mixture
described above.
[0042] The volume of formic acid must be in excess in order for the
chemical reduction of the metal salts or complexes to be
complete.
[0043] The whole of the mixture is subsequently brought to a
temperature between the freezing temperature of water and that of
formic acid. After from 12 to 72 hours, the chemical reduction is
complete and a metal powder representing the catalyst is
obtained.
[0044] Lowering the temperature promotes the decrease in the rate
of growth of the nanoparticles and consequently lengthens the
duration of chemical reduction of the metal salts. In order to
overcome this lengthening of duration, it is advisable to
accelerate the chemical reduction reaction without, however,
increasing the rate of growth.
[0045] The flask containing the mixture can advantageously be
exposed to an energy source, for example a source of ultraviolet
(UV) rays, which makes it possible, by virtue of this energy
contribution, to accelerate the reaction (the germination of the
particles), thus promoting the multiplicity of seeds in the
nanoparticulate state without, however, promoting their growth. The
wavelength of the UV rays is preferably chosen between 200 and 300
nm, corresponding to the absorption region of the platinum Pt
complex, as shown in FIG. 1.
[0046] Implementational Example
[0047] Pt--Sn/C catalysts with the molar composition 3:1 were
synthesized by chemical reduction with formic acid.
[0048] Solutions of K.sub.2PtCl.sub.6.6H.sub.2O and
SnCl.sub.2.2H.sub.2O from Sigma-Aldrich were used as precursors of
the catalysts formed of Pt--Sn supported on carbon black with a
high specific surface (Vulcan XC-72R, Cabot Corp., 250
m.sup.2/g).
[0049] The aqueous solutions of platinum and tin salts with a
concentration of 0.01 M were mixed in the presence of carbon black
and stirred vigorously under ultrasound for a period of time of
approximately 1 hour. The volumes of the
K.sub.2PtCl.sub.6.6H.sub.2O and SnCl.sub.2.2H.sub.2O solutions
mixed are respectively 15 ml and 5 ml, so as to obtain an atomic
ratio of 3:1.
[0050] A large amount of formic acid HCOOH (ACS reagent, greater
than or equal to 98%, Sigma-Aldrich), with a molar ratio of the
order of 1000:1 between the formic acid and the metal salts, used
as reducing agent, is added to the mixture in order to make
possible simultaneous reduction of the platinum and tin salts.
[0051] Two examples of synthesis operating conditions were carried
out: [0052] the first example at ambient temperature for 24 hours;
[0053] the second example at 4.degree. C. for 72 hours. At the end
of the period of time specified above, a metal powder was obtained.
The weight of Pt+Sn represents 50% by weight of the catalyst. The
temperature of 4.degree. C. was chosen so that it is between 0 and
8.degree. C.
[0054] The objective of carrying out the synthesis at this
temperature is to reduce, during the synthesis, the rate of growth
of the nanoparticles, which has a growth with temperature
dependence.
[0055] The results obtained are listed in the table below and
relate to the physicochemical properties of the Pt.sub.3Sn/C
catalysts synthesized at ambient temperature and at 4.degree.
C.
TABLE-US-00001 Electroactive Mean size of surface Electro-oxidation
the particles (Hupd) potential of CO in Catalyst (nm)
(cm.sup.2.sub.Pt. cm.sup.-2.sub.geo) V (volts) Pt.sub.3Sn/C 5.9 266
0.38 (Ambient temperature) Pt.sub.3Sn/C at 4.degree. C. 3.9 379
0.28
[0056] The electroactive surface corresponds more specifically to
the surface which is electrochemically active for the reactions
under consideration, which it is desired to increase.
[0057] FIG. 2 illustrates the size distributions of particles of
Pt.sub.3Sn/C catalysts synthesized at ambient temperature
(25.degree. C.) and at 4.degree. C. and demonstrates the high
percentage of particles of small size, typically from 3 to 4 nm,
with the synthesis process of the invention.
[0058] More specifically, the size parameters listed in the table
below are obtained:
TABLE-US-00002 Pt.sub.3Sn/C (25.degree. C.) Pt.sub.3Sn/C (4.degree.
C.) Mean size 5.89 3.92 Median size 6.00 4.00 Standard deviation
1.45 1.10
[0059] Measurements carried out by energy dispersive X-ray
spectrometry (EDS) analysis also provide the results below:
TABLE-US-00003 EDS composition Unit cell parameter Catalyst (Pt:Sn
at. %) (A) Pt.sub.3Sn/C (25.degree. C.) 85:15 3.99231 Pt.sub.3Sn/C
(4.degree. C.) 74:26 3.99231
[0060] The electroactive surface of the catalyst prepared at
4.degree. C. is thus much greater than that of the same catalyst
synthesized at 25.degree. C.
[0061] This means that the amount of catalyst necessary, for
example for the satisfactory operation of a system for the
electrochemical production of hydrogen comprising a catalytic
reforming reaction in the presence of particles of catalyst
obtained according to the present invention and of a gas mixture
comprising hydrocarbon compounds, can thus advantageously be lower
than that used with a catalyst obtained with a process of the prior
art.
[0062] The Applicant has produced electrochemical half-cell cyclic
voltammograms at 25.degree. C. with a scan rate of 10 mV/s,
relating to the different catalysts prepared.
[0063] It should be remembered that cyclic voltammetry is an
electrochemical analysis method based on the measurement of the
current resulting from the reduction or the oxidation of the
compounds which come into contact with a working electrode (the
sample studied) under the effect of a controlled variation in the
difference in potential with an electrode with the set potential,
known as reference electrode. It makes it possible to identify and
to quantitatively measure a large number of compounds and also to
study the chemical reactions including these compounds.
[0064] The high absorption power can be characterized by the
absence of oxidation peaks (positive current) on the voltammogram
exhibiting the current density measured as a function of the
potential applied E at the working electrode. The absence of these
peaks on the voltammogram reflects the blocking of the adsorption
sites of the catalyst by another entity.
[0065] FIG. 3 relates to the results obtained with the Pt.sub.3Sn/C
particles prepared at 25.degree. C. The curve 3a relates to the
voltammetry curve (under N.sub.2, in 0.5M H.sub.2SO.sub.4 at
25.degree. C. and 10 mV.s.sup.-1) produced after contamination of
the catalyst by adsorption of CO.
[0066] The comparison may be established with the curve 3b produced
after desorption of the CO (voltammetry cycle: VC).
[0067] The catalyst is completely contaminated: no oxidation
current is observed on the curve 3a in the potential range
corresponding to the desorption of electro-adsorbed hydrogen (Hupd:
underpotentially deposited H): between 0.1 and 0.4 V vs. RHE. The
oxidation of the CO begins when the potential of the electrode
exceeds 0.4 V vs. RHE.
[0068] FIG. 4 relates to the results obtained with the Pt.sub.3Sn/C
particles prepared at 4.degree. C. For the catalyst prepared at
4.degree. C., the same cyclic voltammetry curve after contamination
of the catalyst by the adsorption of CO (curve 4a) shows that the
catalyst is not totally contaminated: the peaks for desorption of
the hydrogen are still observed on the curve 4a between 0.1 and 0.4
V vs. RHE and the oxidation of the CO begins at a potential of the
electrode of 0.3 V vs. RHE.
[0069] The comparison can be established with the curve 4b produced
after desorption of the CO (voltammetry cycle: VC).
[0070] The gain, in energy terms, for the oxidation of the hydrogen
in the presence of traces of contaminating gases is close to 30%
(0.72 kWh/Sm.sup.3.sub.H2 with the catalyst synthesized at
4.degree. C., versus 0.96 kWh//Sm.sup.3.sub.H2 with the catalyst
synthesized at 25.degree. C.).
[0071] The Applicant has also monitored the change in the current
for the electro-oxidation of H.sub.2 over Pt.sub.3Sn/C synthesized
at ambient temperature and at 4.degree. C. in an electrochemical
half-cell device, when the electrode is supplied with a gas mixture
composed of 50 ppm of CO in H.sub.2 and subjected to a potential of
0.24 V vs. RHE.
[0072] FIG. 5 shows, via the curves 5a and 5b, the difference in
behavior with a catalyst produced according to the present
invention at a temperature of 4.degree. C., in comparison with a
catalyst produced at ambient temperature.
[0073] The current density measured over time (j) is relative to
that measured when the sample is supplied with pure hydrogen
(j.sub.max). Initially, the catalyst is not contaminated; thus
j=j.sub.max and j.sub.jmax=1.
[0074] In the case of the catalyst synthesized at 25.degree. C.
(curve 5a), the current density measured over time rapidly
decreases to reach 71% of the initial current after 1 hour of
poisoning with CO. For the catalyst synthesized at 4.degree. C.,
the current measured after 1 hour of poisoning is 93% of the
initial current. The catalyst synthesized at 4.degree. C. is thus
much more tolerant toward CO than that synthesized at 25.degree. C.
(FIG. 5b).
* * * * *