U.S. patent application number 10/521567 was filed with the patent office on 2006-07-06 for method for preparing colloidal solution and carrier having colloidal particles fixed on surface thereof, fuel cell cathode, fuel cell anode and method for preparing the same and fuel cell using the same, and low temperature oxidation catalyst, method for preparing the same and fuel cell fuel modifyi.
This patent application is currently assigned to Nippon Sheet Glass Co.. Invention is credited to Akihiro Hishinuma, Yohei Iseki, Kiyoshi Miyashita, Toyo Okubo, Tsutomu Sakai, Hiroshi Shingu.
Application Number | 20060144189 10/521567 |
Document ID | / |
Family ID | 30119239 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060144189 |
Kind Code |
A1 |
Miyashita; Kiyoshi ; et
al. |
July 6, 2006 |
Method for preparing colloidal solution and carrier having
colloidal particles fixed on surface thereof, fuel cell cathode,
fuel cell anode and method for preparing the same and fuel cell
using the same, and low temperature oxidation catalyst, method for
preparing the same and fuel cell fuel modifying device using the
same
Abstract
A method for forming colloidal particles by boiling a solution
containing a metal salt and a reducing agent; and a method for
preparing a colloidal solution wherein the concentration of the
metal salt in the solution is 1.times.10.sup.-4 mol/L or more and
less than 4.times.10.sup.-4 mol/L; the equivalent concentration of
the reducing agent is four times or more and 20 times or less the
equivalent concentration of the metal salt; and the reaction time
is 60 minutes or more and 300 minutes or less. A carrier wherein
colloidal particles are fixed on the surface of a substrate by
applying the colloidal solution prepared by the above-described
method. Methods for manufacturing a fuel cell cathode, a fuel cell
anode, and a low temperature oxidation catalyst wherein a colloidal
solution prepared in the state wherein a solution containing a
metal salt and a reducing agent is boiled to remove dissolved
oxygen is applied to a substrate and colloidal particles are fixed
on the substrate.
Inventors: |
Miyashita; Kiyoshi; (Osaka,
JP) ; Hishinuma; Akihiro; (Osaka, JP) ; Sakai;
Tsutomu; (Kyoto, JP) ; Okubo; Toyo; (Kyoto,
JP) ; Shingu; Hiroshi; (Kyoto, JP) ; Iseki;
Yohei; (Kyoto, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Nippon Sheet Glass Co.
Osaka-shi
JP
JP
|
Family ID: |
30119239 |
Appl. No.: |
10/521567 |
Filed: |
June 16, 2003 |
PCT Filed: |
June 16, 2003 |
PCT NO: |
PCT/JP03/07607 |
371 Date: |
August 29, 2005 |
Current U.S.
Class: |
75/371 ;
204/290.01; 204/290.14; 428/558 |
Current CPC
Class: |
B01J 35/0013 20130101;
B01J 37/16 20130101; B22F 9/24 20130101; H01M 4/926 20130101; B01J
13/0008 20130101; B22F 1/0022 20130101; H01M 8/0668 20130101; Y02P
70/50 20151101; H01M 2004/8689 20130101; H01M 4/92 20130101; Y02E
60/50 20130101; H01M 4/885 20130101; B82Y 30/00 20130101; Y10T
428/12097 20150115; B01J 13/0043 20130101; H01M 4/8882
20130101 |
Class at
Publication: |
075/371 ;
428/558; 204/290.01; 204/290.14 |
International
Class: |
C22B 3/44 20060101
C22B003/44; B22F 9/24 20060101 B22F009/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 16, 2002 |
JP |
2002-207248 |
Dec 3, 2002 |
JP |
2002-351837 |
Dec 3, 2002 |
JP |
2002-351838 |
Dec 3, 2002 |
JP |
2002-351839 |
Claims
1. A colloidal solution preparing method for forming colloidal
particles by boiling a solution containing a metal salt and a
reducing agent, wherein the concentration of the metal salt in said
solution is 1.times.10.sup.-4 mol/L or more and less than
4.times.10.sup.-4 mol/L; the equivalent concentration of the
reducing agent is four times or more and 20 times or less the
equivalent concentration of the metal salt; and the reaction time
is 60 minutes or more and 300 minutes or less.
2. A colloidal solution preparing method for forming colloidal
particles by boiling a solution containing a metal salt and a
reducing agent, wherein the concentration of the metal salt in said
solution is 4.times.10.sup.-4 mol/L or more and less than
6.times.10.sup.-4 mol/L; the equivalent concentration of the
reducing agent in four times or more and 20 times or less the
equivalent concentration of the metal salt; and the reaction time
is 30 minutes or more and 150 minutes or less.
3. A colloidal solution preparing method for forming colloidal
particles by boiling a solution containing a metal salt and a
reducing agent, wherein the concentration of the metal salt in said
solution is 6.times.10.sup.-4 mol/L or more and 15.times.10.sup.-4
mol/L or less; the equivalent concentration of the reducing agent
in four times or more and 20 times or less the equivalent
concentration of the metal salt; and the reaction time is 30
minutes or more and 90 minutes or less.
4. A colloidal solution preparing method for forming colloidal
particles by boiling a solution containing a metal salt and a
reducing agent, wherein the concentration of the metal salt in said
solution is 4.times.10.sup.-4 mol/L or more and less than
6.times.10.sup.-4 mol/L; the equivalent concentration of the
reducing agent in twice or more and less than four times the
equivalent concentration of the metal salt; and the reaction time
is 60 minutes or more and 120 minutes or less.
5. A colloidal solution preparing method for forming colloidal
particles by boiling a solution containing a metal salt and a
reducing agent, wherein the concentration of the metal salt in said
solution is 6.times.10.sup.-4 mol/L or more and 15.times.10.sup.-4
mol/L or less; the equivalent concentration of the reducing agent
in twice or more and less than four times the equivalent
concentration of the metal salt; and the reaction time is 30
minutes or more and 240 minutes or less.
6. A colloidal solution preparing method for forming colloidal
particles by boiling a solution containing a metal salt and a
reducing agent, wherein the concentration of the metal salt in said
solution is 4.times.10 4 mol/L or more and less than
6.times.10.sup.-4 mol/L; the equivalent concentration of the
reducing agent is once or more and less than twice the equivalent
concentration of the metal salt; and the reaction time is 60
minutes or more and 120 minutes or less.
7. A colloidal solution preparing method for forming colloidal
particles by boiling a solution containing a metal salt and a
reducing agent, wherein the concentration of the metal salt in said
solution is 6.times.10.sup.-4 mol/L or more and 15.times.10.sup.-4
mol/L or less; the equivalent concentration of the reducing agent
in once or more and less than twice the equivalent concentration of
the metal salt; and the reaction time is 30 minutes or more and 120
minutes or less.
8. The colloidal solution preparing method according to claim 1
wherein said reducing agent is a citrate.
9. The colloidal solution preparing method according to claim 1
wherein the average particle diameter of said colloidal particles
is 1.6 to 5 nm.
10. A carrier wherein colloidal particles are fixed on the surface
of a substrate by applying the colloidal solution prepared by the
method according to claim 1.
11. The carrier according to claim 10 wherein said substrate is
glass fiber or scale-like glass.
12. The carrier according to claim 10 wherein said substrate is
porous.
13. A method for manufacturing a fuel cell cathode wherein a
colloidal solution prepared in the state wherein a solution
containing a metal salt and a reducing agent is boiled to remove
dissolved oxygen is applied to a substrate, and colloidal particles
are fixed on said substrate.
14. The method for manufacturing a fuel cell cathode according to
claim 13, wherein said metal salt is chloroplatinic acid.
15. The method for manufacturing a fuel cell cathode according to
claim 13, wherein said reducing agent is sodium citrate.
16. The method for manufacturing a fuel cell cathode according to
claim 13, wherein the average particle diameter of said colloidal
particles is 1.6 to 5 nm.
17. A fuel cell cathode manufactured using the method according to
claim 13.
18. A fuel cell using the cathode according to claim 17.
19. A method for manufacturing a fuel cell anode wherein a
colloidal solution prepared in the state wherein a solution
containing a metal salt and a reducing agent is boiled to remove
dissolved oxygen is applied to a substrate, and colloidal particles
are fixed on said substrate.
20. The method for manufacturing a fuel cell anode according to
claim 19, wherein said metal salt is chloroplatinic acid.
21. The method for manufacturing a fuel cell anode according to
claim 19, wherein said reducing agent is sodium citrate.
22. The method for manufacturing a fuel cell anode according to
claim 19, wherein the average particle diameter of said colloidal
particles is 1.6 to 5 nm.
23. A fuel cell anode manufactured using the method according to
claim 19.
24. A fuel cell using the anode according to claim 23.
25. A method for preparing a low-temperature oxidation catalyst
wherein a colloidal solution prepared in the state wherein a
solution containing a metal salt and a reducing agent is boiled to
remove dissolved oxygen is applied to a substrate, and colloidal
particles are fixed on said substrate.
26. The method for preparing a low-temperature oxidation catalyst
according to claim 25 wherein said metal salt is chloroplatinic
acid.
27. The method for preparing a low-temperature oxidation catalyst
according to claim 25 wherein said reducing agent is sodium
citrate.
28. The method for preparing a low-temperature oxidation catalyst
according to claim 25, wherein the average particle diameter of
said colloidal particles is 1.6 to 5 nm.
29. A low-temperature oxidation catalyst prepared using the method
according to claim 25.
30. A fuel modifying device for a fuel cell using the
low-temperature oxidation catalyst according to claim 29.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for paring a
colloidal solution containing colloidal particles that can exert
catalytic functions and the like; and to a carrier wherein the
colloidal particles are fixed on various substrates. The present
invention also relates to a fuel cell cathode that promotes oxygen
reduction reaction; a method for the manufacture thereof, and a
fuel cell using the same. The present invention also relates to a
fuel cell anode that promotes hydrogen oxidation reaction; a method
for the manufacture thereof; and a fuel cell using the same.
Furthermore, the present invention relates to a low temperature
oxidation catalyst the exhibits excellent catalytic activity under
a service temperature of 300.degree. C. or below, a method for the
manufacture thereof and a fuel modifying device for a fuel cell
using the low-temperature oxidation catalyst.
BACKGROUND ART
[0002] It is broadly known that fine particles of a metal, such as
platinum, fictions as a catalyst. When fine metal particles are
used as a catalyst, in order to widen the contact area with a
reaction material, or to easily handle the catalyst, the fine metal
particles are nominally fixed on the surfaces of substrates, such
as fibers, woven fabrics, non-woven fabrics, films and powdered
bodies. Although there are various means to fix fine metal
particles in the surfaces of substrates, a method for applying a
colloidal solution containing fine metal particles onto the surface
of the substrate is the most convenient method that can evenly fix
the particles.
[0003] As a method for preparing a colloidal solution containing
fine metal particles, a dispersion method wherein bulk metal is
pulverized using a pulverizer, such as a colloid mill, and the
pulverized fine particles are dispersed in a solvent. However, fine
metal particles obtained using a dispersion method have large
average particle diameters, and are relatively difficult to fix on
the surface of a substrate. For example, when glass fiber or the
like is treated with acid and alkali to make the surface thereof
porous, the average pore diameter of the formed pores in the order
of 1 to 10 nm; therefore, fine metal particles cannot enter into
the pores, and cannot be well fixed on the surface of the glass
river. Also in the dispersion method, since the width of the
particle size distribution of fine metal particles is large, it is
difficult to evenly fix fine metal particles on the surface of the
substrate.
[0004] As a preparing method other than the dispersion method, a
method whereas a reducing agent is added to a solution of a metal
chloride to reduce the metal ions and to form fine metal particles
has been known (Seitaro Namba and Ichiro Ohkura, "How to Prepare
and Use Platinum Colloid", journal name: "Surface", Vol, 21, No.
8(1983), pp. 450-456). In this publication, there is the following
description.
"2. How to Prepare Platinum Colloid
2.1 Platinum Colloid
[0005] For preparing platinum colloid, a 2-L round bottom flask
with a condenser is used, and 960 ml of distilled water is poured
therein and well boiled using a mantle heat. To this 60 ml of an
aqueous solution of chloroplatinic acid (1 g-Pt/L) is added, the
solution is brought to boiling again, 120 ml of an aqueous solution
of sodium citrate (1% by weight) is added, and boiling is
continued. Although the solution is initially light yellow in color
due to the presence of chloroplatinic acid, it is gradually
darkened, and becomes dark brown in 30 minutes after the addition
of sodium citrate. When refluxing is further continued, the color
of the solution changed to black in one hour, and thereafter,
change in color is no longer observed. To stop the reaction, the
reaction solution is placed in an ice water bath. The platinum
colloid thus obtained is very stable, and if stared in a
refrigerator, no aggregation is observed for several months.
[0006] Although this preparing method is very simple, care must be
taken to the follow three aspects in preparing: 1) the vessel is
carefully cleaned, and is used after immersing in aqua regia to be
used; 2) special care must be taken to water to be used, and
ion-exchanged water distilled twice is used; and 3) heating is
consistently continued during reaction to maintain the state of
vigorous reaction. By taking these cares, platinum colloid can be
prepared at high reproducibility.
[0007] The reason of vigorous boiling during reaction is because
oxygen in the air interferes with the reaction. The platinum
colloid must be eared in the state wherein dissolved oxygen is
removed, and if the colloid is prepared in the state wherein the
solution is not vigorously boiled, reproducible results cannot be
obtained because a long time is consumed for the synthesis, or
aggregation occurs. In the state wherein dissolved oxygen is
removed by blowing an inert gas, such as nitrogen gas, the platinum
colloid can be prepared at a temperature as low as 70.degree.
C.
[0008] Chloroplatinic acid and sodium citrate that have not reacted
can be removed by passing the solution through a column packed with
the ion-exchange resin, Amberlite MB-1. Although the degree of
removal can be defined by measuring the electrical conductivity of
the solution, 6 ml of the ion-exchange resin is sufficient for 100
ml of the colloidal solution. At tis time, the quantity of the
platinum colloid adsorbed on the ion-exchange resin is extremely
small."
[0009] Furthermore, this publication describes that in the
above-described means, the average particle diameter of the
platinum colloid increases with increase in the refluxing time
(reaction time) of the reacting solution, and when the reaction
time is about 5 hours, the average particle diameter reaches about
32 angstroms and thereat it is kept constant This publication also
describes that platinum exerts a significant catalytic activity
after the particle diameter has exceeded about 16 angstroms.
[0010] As means for fixing fine metal particles of the surface of a
subs Japanese Patent Application Laid-Open No. 07-256112 describes
a method for fixing metal platinum on zeolite, wherein zeolite is
fed in an aqueous solution of divalent platinum ammine salt
suction-filtered, washed with water and heat-dried, heated to
500.degree. C. and maintained at this temperature, exposed to
oxygen for a predetermined time, and then exposed to hydrogen to
fix metal platinum on zeolite. According to this method, fine
platinum particles are fixed in the pores of zeolite of an average
diameter of 0.4 to 2 nm, and a catalyst carrier that can adsorb and
oxidize bipolar molecules, such as carbon monoxide can be
obtained.
[0011] The technique for preparing a material for fuel cells using
carbon black or graphite having conductivity as a substrate in lieu
of the above-described zeolite has also been known. For example,
Japanese Patent Application Laid-Open No. 2002-222655 describes a
method for preparing a cathode catalyst, wherein a commercially
available carbon powder is added in a carrier solution prepared by
adding an aqueous solution of ruthenium nitrate to a solution of a
platinum ethoxide complex, and dried and further exposed to
hydrogen while heating in an electric furnace to form a
platinum-ruthenium alloy on above-described carbon powder.
Furthermore, Japanese Patent Application Laid-Open No. 2001-357857
describes a method for manufacturing a fuel cell anode wherein
carbon black fired at a high temperature is immersed in a solution
containing chloroplatinic acid and formalin and cooled to
-10.degree. C., and then sodium hydroxide is dropped to fix
ultra-fine platinum particles to the carbon black.
[0012] When the present inventors prepared a colloidal solution of
platinum according to the description of the above-described
publications, it was confirmed that the average particle diameter
of colloidal particles reached about 35 angstroms at the time when
60 minutes elapsed from the starting of the reaction, and about 70
to 80% of fed chloroplatinic acid was converted to colloidal
particles. After 60 minutes had elapsed from the starting of the
reaction, eve if boiling was continued until 5 hours elapsed, the
average particle diameter of colloidal particles, and the ratio of
the weight of the colloidal particles to the weight of fed platinum
(hereafter referred to as Metal recovery rated showed little
change.
[0013] This method for preparing a colloidal solution can be said
to be an excellent method, because the uniformity of the particle
diameters of the colloidal particles is high, and the metal
recovery rate is as relatively high as 70 to 80%. However, if the
average particle diameter of the colloidal particles can be closer
to 16 angstroms, the catalytic activity can be further elevated,
because the specific surface area of the colloidal particles can be
increased that much. The room for the improvement of the metal
recovery rate is also left.
[0014] Also the method described in Japanese Patent Application
Laid-Open No. 07-256112 had a problem in that since ultra-fine
platinum particles that exhibited catalytic activity were formed in
the pores of zeolite, impurities contained in the materials of
ultra-fine platinum particles or residue formed during the forming
process could not be removed even if heating or firing were
performed, and coated the surfaces of the ultra-fine platinum
particles, or remained in the pores, lowering the catalytic
activity in spite of the quantity of the fixed ultra-fine platinum
particle&
[0015] Also the method described in Japanese Patent Application
Laid-Open No. 2002-222655 had a problem in that impurities in the
solution were adhered, or residue formed during the forming process
remained on the surfaces of the carbon powder and the
platinum-ruthenium alloy.
[0016] Furthermore, the method described in Japanese Patent
Application Laid-Open No. 2001-357857 had a problem in that
impurities in the solution were adhered, or residue formed during
deposition remained on the surfaces of deposited ultra-fine
platinum particles.
[0017] Therefore, the present invention was completed taking
particular note of such problems. It is an object of the present
invention is to provide a method for easily preparing a colloidal
solution that can prepare colloidal particles having smaller
particle diameters while elevating the metal recovery rate. Another
object of the present invention is to provide a carrier that
exhibits functions, such as catalyzing efficiently by fixing the
colloidal particles on a substrate.
[0018] Another object of the present invention is to provide a fuel
cell cathode that can efficiently perform an oxygen reduction
reaction, even if the adhesion quantity of the colloidal particles,
which is a catalyst, is small. Also another object of the present
invention is to provide a method for easily manufacturing the fuel
cell cathode. A further object of the present invention is to
provide a fuel cell having power generation efficiency using the
fuel cell cathode.
[0019] Another object of the present invention is to provide a fuel
cell anode that can efficiently perform a hydrogen oxidation
reaction, even if the adhesion quantity of the colloidal particles,
which is a catalyst, is small. Also another object of the present
invention is to provide a method for easily manufacture the fuel
cell anode. A further object of the present invention is to provide
a fuel cell having power generation efficiency using the fuel cell
anode.
[0020] Another object of the present invention is to provide a
low-temperature oxidation catalyst that exhibits a high catalytic
activity, even if the adhesion quantity of the colloidal particles,
which is a catalyst is small. Also another object of the present
invention is to provide a method for easily manufacturing the
low-temperature oxidation catalyst. A further object of the present
invention is to provide a fuel modifying device for a fuel cell
that causes a CO shifting reaction even if the service temperate is
300.degree. C. or below, particularly 200.degree. C. or below,
using the low-temperature oxidation catalyst having a big catalytic
activity.
DISCLOSURE OF THE INVENTION
[0021] The present invention is characterized, in a method for
preparing a colloidal solution described in the above described
publications, in that the parameters of the concentration of a
metal salt, the ratio of equivalent concentration of a reducing
agent to the equivalent concentration of the metal salt, and the
reaction time are sophisticatedly combined and adjusted.
Specifically, the optimal conditions for preparing in the method
for preparing a colloidal solution are provided.
[0022] Since the colloidal particles prepared using the method of
the present invention have particle diameters of the 10-nm order,
and have extremely high uniformity, they can be easily and strongly
fixed only be applying the colloidal solution onto porous glass
fibers or the like using means well known to the art. Therefore,
according to the present invention, a carrier having extremely high
catalytic activity and durability can be easily manufactured.
[0023] The methods for manufacturing a fuel cell cathode, fuel cell
anode, and a low-temperature oxidation catalyst according to the
present invention are characterized in that colloidal particles are
fixed on a substrate, using a colloidal solution pared in a state
wherein a solution containing a metal salt and a reducing agent is
boiled to remove dissolved oxygen. According to this method, since
the colloidal solution having extremely high stability even without
containing a stabilizer or the like is applied onto a substrate, no
heating or firing of the substrate for forming the colloidal
particles are required, the colloidal particles can be easily fixed
on the substrate, and there is no lowering of the catalytic
activity due to the formation of residue. Also since the above
described colloidal solution can be prepared from an inexpensive
metal salt as a starting material using a simple process, the
paring costs are extremely low. Also since the above described
colloidal solution has a uniform particle diameter, and Is
difficult to aggregate even in high concentration, no stabilizers
for preventing precipitation are required, and the colloidal
particles can be evenly fixed on the substrate.
[0024] Also according to a fuel cell cathode according to the
present invention, the oxygen reducing reaction can be efficiently
started even if the quantity of adhered colloidal particles is
small. Furthermore, according to a fuel cell according to the
present invention, the efficiency of power generation can be
improved.
[0025] Also according to a fuel cell anode according to the present
invention, he hydrogen oxidizing reaction can be efficiently
started even if the quantity of adhered colloidal particles is
small. Furthermore, according to a fuel cell according to the
present invention, the efficiency of power generation can be
improved.
[0026] Also according to a low-temperature oxidation catalyst
according to the present invention, a high catalytic activity can
be exhibited even if the fixing rate of the catalyst is low.
Furthermore, according to a fuel modifying device for a fuel cell
according to the present invention, the CO shifting reaction can be
started at a high efficiency even at a low temperature of
300.degree. C. or below, in particular, 150 to 200.degree. C.
Therefore, since this device can contribute to the improvement of
the power generating efficiency of fuel cells, and enables the use
of members having not so high heat resistance, the option of member
selection is widened.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a diagram showing the results of measuring CO
conversion rates of the Examples.
BEST MODE FOR CARRYING OUT THE INVENTION
[0028] The best mode of the present invention will be described in
detail below. The present invention follows basically means
described in the above-described publication (Seitaro Namba and
Ichiro Ohkura, "How to Prepare and Use Platinum Colloid", journal
name: "Surface", Vol. 21, No. 8 (1983), pp. 450-456). Therefore,
only items different from this publication will be described.
[0029] The reducing agent is not specifically limited as long as it
dissolves in water, and alcohols, citric acids, marboxylic acids,
ketones, ethers, aldehydes or esters can be exemplified. The
combination of two or more of these can also be used As alcohols,
methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol or
glycerin can be exemplified. As citric acids, citric acid, and
citrate, such as sodium acetate, potassium cite and ammonium
citrate cam be exemplified As carboxylic acids, formic acid, acetic
acid, fumaric acid, malic acid, succinic acid, aspartic acid or
carboxylates thereof can be exemplified. As ketones, acetone or
methylethyl ketone can be can be exemplified. As ethers, diethyl
ether cam be exemplified. As aldehydes, formalin or acetaldehyde
can be exemplified. As esters, methyl formate, methyl acetate or
ethyl acetate can be exemplified.
[0030] From the point of view of form stable colloidal particles of
a metal having a particle diameter of 1 to 20 nm, alcohols, citric
acids or carboxylic acids are preferable as the reducing agent. In
particular, citric acids are preferable for forming stable
colloidal particles of a metal having a particle diameter of 1 to 5
nm. However, since the catalytic activity or the like of colloidal
particles is sharply lowered if the particle diameter thereof is
less than 1.6 nm, the particle diameter thereof is preferably 1.6
nm or more.
[0031] The solvent composing the reaction solution is not
specifically limited as long as it can dissolve the reducing agent
and the metal salt, and water, alcohols, ketones or ethers can be
exemplified. The combination of two or more of these can also be
used. As alcohols, methanol, ethanol, 1-propanol or 2-propanol can
be exemplified. As ketones, methyl formate, methyl acetate or ethyl
acetate can be exemplified. As ethers, methyl ethyl ether or
diethyl ether can be exemplified. From point of view of completely
dissolving the metal salts, the solvent is preferably water or
alcohols.
[0032] The metal salt is not specifically limited as long as it is
dissolved in the solvent and reduced by a reducing agent, For
example, chlorides, nitrates, sulfites or metal complex compounds
of platinum (Pt), ruthenium (Ru), gold (Au), lead (Pb), rhodium
(Rh), iridium (Ir), cobalt (Co), iron (Fe), nickel (Ni), copper
(Cu) or tin (Sn) can be listed. The combination of two or more of
these can also be used. If the combination of two or more of metal
salts is used, the colloidal particles of an alloy can be prepared
When a platinum salt is used as the metal salt, the particle
diameter of colloidal particles becomes particularly small, and
stable colloidal particles of 1 to 5 nm can be obtained.
[0033] Boiling a reaction solution containing a metal salt and a
reducing agent accelerates the reducing reaction of metal ions
while elating dissolved oxygen. In the initial stags of the
reaction, it is considered that the metal ions in the reaction
solution are concurrently reduced to form an innumerable number or
meal atoms are formed, and these drift in the reaction solution.
This can be estimated from the fact that although the colloidal
solution 30 minutes after the start of tie reaction exhibit little
catalytic activity in Example 1 described below, the platinum
concentration thereof (the concentration of the residual metal
after removing metal ions, that is, the concentration of colloidal
particles) becomes relatively high. Specifically, it is considered
that metal atoms and the clusters thereof that cannot exhibit
catalytic activity are present because the particle diameter has
not reached 1.6 nm, although the metal ions are reduced and no
longer present as ions, and are not caught by ion-exchange resin.
It is considered that since these meal atoms attract metal ions and
cause the reducing reaction to occur, or metal atoms and clusters
aggregate, the colloidal particles grow gradually to catalytic
activity. From these, it is estimated that the growth of colloidal
particles in the reaction solution is closely related to
"equivalent concentration of the reducing agent to the equivalent
concentration of the metal salt" directly related to the generation
of metal atoms; the "concentration of the metal salt" directly
related to the frequencies of the collision of the metal atoms, the
clusters thereof and metal ions; and the "reaction time" directly
related to the reducing reaction and the collision of metal atoms.
Therefore, by sophisticatedly changing and combining these three
parameter; more advantageous preparing conditions than in the
method for preparing a colloidal solution described in the above
publication are found and specified.
[0034] As a first embodiment thereof, the reaction time is 60
minutes or more and 300 minutes or less when the concentration of
the metal salt in the reaction solution is 1.times.10.sup.-4 mol/L
or more and less than 4.times.10.sup.-4 mol/L, and the equivalent
concentration of the reducing agent is four times or more and 20
times or less the equivalent concentration of the metal salt.
According to the first embodiment, the meal recovery rate can be 80
to 100%, and the average particle diameter of metal colloidal
particles can be 2.3 nm or smaller.
[0035] As a second embodiment thereof, the reaction time is 30
minutes or more and 150 minutes or less when the concentration of
the metal salt in the reaction solution is 4.times.10.sup.-4 mol/L
or more and less tan 6.times.10.sup.-4 mol/L, and the equivalent
concentration of the reducing agent is four times or more and 20
times or less the equivalent concentration of the metal salt
According to the second embodiment, the average particle diameter
of metal colloidal particles can be 2 nm or smaller without
precipitating the metal colloidal particles.
[0036] As a third embodiment thereof, the reaction time is 30
minutes or more and 90 minutes or less when the concentration of
the metal salt in the reaction solution is 6.times.10.sup.-4 mol/L
or more and 15.times.10.sup.-4 mol/L or less, and the equivalent
concentration of the reducing agent is four times or more and 20
times or less the equivalent concentration of the metal salt
According to the third embodiment, the average particle diameter of
metal colloidal particles can be reduced without precipitating the
metal colloidal particles.
[0037] As a fourth embodiment thereof, the reaction time is 60
minutes or more and 120 minutes or less when the concentration of
the metal salt in the reaction solution is 4.times.10.sup.-4 mol/L
or more and less than 6.times.10.sup.-4 mol/L, and the equivalent
concentration of the reducing agent is twice or more and less than
four times the equivalent concentration of the metal salt.
According to the fourth embodiment the average particle diameter of
metal colloidal particles can be 23 nm or smaller without
precipitating the metal colloidal particles.
[0038] As a fifth embodiment thereof, the reaction time is 30
minutes or more and 240 minutes or less when the concentration of
the metal salt in the reaction solution is 6.times.10.sup.-4 mol/L
or more and 15.times.10.sup.-4 mol/L or less, and the equivalent
concentration of the reducing agent is twice or more and less than
four times the equivalent concentration of the metal salt According
to the fifth embodiment the average particle diameter of metal
colloidal particles can be 1.8 nm or smaller without precipitating
the metal colloidal particles.
[0039] As a sixth embodiment thereof, the reaction time is 60
minutes or more and 120 minutes or less when the concentration of
the metal salt in the reaction solution is 4.times.10.sup.-4 mol/L
or more and less than 6.times.10.sup.-4 mol/L, and the equivalent
concentration of the reducing agent is equal to one or more and
less than twice the equivalent concentration of the metal salt
According to the sixth embodiment, the average particle diameter of
metal colloidal particles can be 2.3 nm or smaller without
precipitating the metal colloidal particles.
[0040] As a seventh embodiment thereof, the reaction time is 30
minutes or more and 120 minutes or less when the concentration of
the metal salt in the reaction solution is 6.times.10.sup.-4 mol/L
or more and 15.times.10-4 mol/L or less, and the equivalent
concentration of the reducing agent is equal to one or more and
less than twice the equivalent concentration of the metal salt
According to the seventh embodiment, the average particle diameter
of metal colloidal particles can be 1.8 nm or smaller without
precipitating the metal colloidal particles.
[0041] If the concentration of the metal salt is less than
1.times.10.sup.-4 mol/L, the reaction solution is unsuitable for
industrial applications, because the reducing reaction of metal
ions is difficult to occur, and a long time is required until the
colloidal particles grow to a predetermined particle diameter. On
the other hand, if the concentration of the metal salt exceeds
15.times.10.sup.-4 mol/L, colloidal particles are easily
aggregated, and precipitation occurs in the early stage aft the
start of reaction. If the equivalent concentration of the reducing
agent in the reaction solution is less than the equivalent
concentration of the metal salt, some of the metal salt cannot be
reduced and the metal recovery rate lowers naturally. On the other
hand, if the equivalent concentration of the reducing agent exceeds
20 times the equivalent concentration of the metal salt, since the
reducing agent itself aggregates and precipitates, the metal salt
is also entangled in the precipitation, and the metal recovery rate
lowers.
[0042] The colloidal solution prepared under the conditions of the
above embodiments can be applied to various substrates using means
well known to the art such as a dip method and a spray method. The
kind of the substrate is not specifically limited as long as
colloidal particles can be fixed. For example, carbons, inorganic
ceramics or organic polymers can be used. Two or more of these
substrates can also be used in combination. As carbons, activated
charcoal, charcoal and carbon fibers can be exemplified. As
inorganic ceramics, alumina, titania, magnesia, silica or zeolite
can be exemplified. As organic polymers, polyethylene,
polypropylene, polystyrene, polyimide, polysulfone, polysilicone,
Nafion or polycellulose can be exemplified. Among these, glass
fibers and scale-shaped glass are suited. When chemical treatment
is performed using an acid or alkali solution, the surface of glass
fibers and scale-shaped glass become porous easily. Since the
average diameter of pores in these porous bodies is normally in the
order of 1 to 10 nm, the colloidal particles of a metal prepared
under the above conditions of the embodiments can well enter in the
pores of the porous body, and can be strongly fixed there.
Therefore, since a carrier using porous gas fiber or scale-shaped
glass as the substrate has a large specific surface area of the
substrate, the carrier can exhibit highly efficient catalytic
activity, and excels in durability. This carrier is processed into
textiles, non-woven fabrics, and utilized, for example, as an
exhaust-gas cleaning filter in an engine muffler.
EXAMPLES
[0043] The invention of a method for preparing a colloidal
solution, and a carrier having colloidal particles fixed on the
guru thereof will be more specifically described below, using
examples, a reference example and comparative examples.
Reference Example 1
[0044] First, a colloidal solution was prepared according to the
description of the above-described publication. Referring to the
colloidal solution of Reference Example 1, examples, the reference
example and comparative examples will be sequentially described. A
1,500-ml flask, a 100-ml conical flask a 200-ml conical fask a
reflux condenser and a stirrer were immersed in aqua regia for a
day and night, and the apparatuses were well cleaned using pure
water ion-exchanged and ultra-filtered Into the 1,500 ml flask 850
ml of pure water ion-exchanged and ultra-filtered and the stirrer
are charged, the reflux condenser is installed on the flask and the
pure water was heated to a temperature of 100.degree. C. In order
to remove dissolved oxygen, the pure water was boiled as it was for
one hour. On the other hand, 0.1328 g of tetratachloroplanic acid,
hexahydrate (50 mg as platinum) was weighed and charged into the
100-ml conical flask and pure water ion-exchanged and
ultra-filtered was added to make 50 ml. One gram of sodium citrate
was weighed and charged in the 200-ml conical flask, and pure water
ion-exchanged and ultra-filtered was added to make 100 ml. After
removing dissolved oxygen of the pure water, the aqueous solution
of tetratachloroplanic acid was charged from the 100-ml conical
flask to the 1,500-ml flask, and heated to 100.degree. C. again.
Furthermore, in order to remove dissolved oxygen, boiling was
performed for 30 minutes. Then, the aqueous solution of sodium
citrate slowly added from the 200-ml flask so that the boiling
state could be maintained. In this reaction solution, the
concentration of platinum is 50 mg/L=2.6.times.10.sup.-4
mol/L=1.0.times.10.sup.-3 N, and the ratio of the mol concentration
of sodium citrate to the mol concentration of platinum is 13.2.
Since sodium citrate function as monatomic donor, the equivalent
concentration of sodium citrate to the equivalent concentration of
platinum is 3.3.
[0045] After the aqueous solution of sodium citrate was completely
added in the 1,500-ml flask, the reducing reaction was continued in
the boiling state, and reaction was stopped 30 minutes, 60 minutes,
90 minutes, 120 minutes and 240 minutes after the state of
reaction, and each reaction solution was rapidly cooled to room
temperature. The cooled reaction solution was passed through a
column packed with ion-exchanged resin Amberlite MB-1 (manufactured
by Organo Corporation), and metal ions and the reducing agent left
in the reaction solution were removed to obtain a stable colloidal
solution. The concentration of the colloidal particles in the
colloidal solution was measured using plasma emission speetometry,
and an adequate amount of the colloidal particles were further
sampled to determine the state of the catalytic activity thereof,
that is, the particle diameter of the colloidal particles utilizing
the hydrogen peroxide decomposing reaction. The results of these
measurements are shown in Table 1 below together with the
concentration of the reaction solution and the like. Also in order
to confirm the accuracy of the measurement of the colloidal
particles utilizing the hydrogen peroxide decomposing reaction, the
average particle diameter of the colloidal particles of platinum in
the reaction solution rapidly cooled to room temperature 120
minutes aft the start of the reaction was measured using a
transmission electron microscope. As a result, the average particle
diameter of the colloidal particles was 3.5 nm, and it was
confirmed to agree with the result of measurement utilizing the
hydrogen peroxide decomposing reaction. The recovery rate of
platinum can be obtained by dividing the platinum concentration in
the product (colloidal particles) column in Table 1 by the platinum
concentration and converting the quotient to percentage.
(Example 1) to (Example 7) and (Comparative Example 1)
[0046] A colloidal solution of platinum was prepared in the same
manner as in the above-described Reference Example 1 except that
the added quantity of platinum and the added quantity of sodium
citrate were changed to those shown in Table 1, and the properties
thereof were studied. The results are as in Table 1 below. In Table
1, the Product (colloidal particles) columns where the values of
platinum concentration or catalytic activity lower with the lapse
of time show the occurrence of precipitation. When precipitation
occurred, the platinum concentration and catalytic activity of the
supernatant excluding precipitations were measured. Example 1 is
the example according to the first embodiment, that is, claim 1,
and the Example 2 and the examples that follow are examples
corresponding to the number of each claim.
Comparative Example 2
[0047] A colloidal solution of platinum was prepared in the same
manner as in Reference Example 1 except that the ratio of the
equivalent concentration of sodium citrate to the equivalent
concentration of platinum in the reaction solution is 33 times. As
a result the occurrence of precipitation started in the reaction
solution before 60 elapsed after the start of the reaction.
TABLE-US-00001 TABLE 1 Product Reaction solution (colloidal
particles) Sodium Catalytic Platinum citrate (*1) Reaction Platinum
activity concentration (mol/ time concentration (mol-O.sub.2/
(mg/L) (mol/L) (N) mol-Pt) (N/N-Pt) (min) (mg/L) mg-Pt/min) Example
1 50 0.00026 0.00103 26.4 6.6 30 14.94 0.00 60 44.23 0.41 90 42.28
0.98 120 40.00 1.44 240 52.82 2.04 Example 2 100 0.00051 0.00205
26.4 6.6 30 65.66 1.34 60 77.96 0.87 90 74.87 1.27 120 79.39 2.70
240 23.44 1.28 Example 3 150 0.00077 0.00308 26.4 6.6 30 90.47 1.00
60 84.21 0.89 90 88.10 0.93 120 36.90 0.43 240 1.24 0.00 Example 4
100 0.00051 0.00205 13.2 3.3 30 64.17 0.44 60 67.49 1.62 90 68.27
2.12 120 69.98 1.73 240 1.36 1.78 Example 5 150 0.00077 0.00308
13.2 3.3 30 108.09 1.19 60 115.29 1.10 90 109.24 0.49 120 120.82
2.58 240 109.96 3.84 Reference 50 0.00026 0.00103 13.2 3.3 30 23.97
0.20 Example 1 60 34.91 0.91 90 24.99 0.81 120 31.25 0.79 240 39.05
0.82 Example 6 100 0.00051 0.00205 4.4 1.1 30 69.86 0.59 60 73.32
1.96 90 70.08 1.86 120 76.32 1.51 240 78.08 0.73 Example 7 150
0.00077 0.00308 4.4 1.1 30 114.08 1.73 60 109.85 2.46 90 122.18
3.25 120 115.44 3.02 240 4.87 1.04 Compara- 50 0.00026 0.00103 4.4
1.1 30 25.30 0.62 tive 60 26.50 0.67 Example 1 90 20.70 0.57 120
27.51 0.45 240 1.21 0.00 Note: (*1) In the reaction solution,
sodium citrate functions as a mono-electron donor (reducing
agent).
[0048] By comparing the examples and the reference example in Table
1, the followings are known.
[0049] In the above Examples 1 to 7, if the reaction tine is
adequately adjusted, the metal recovery rate can be improved, or
the catalytic activity can be elevated than the preparing method
described in the above publications without causing the
precipitation of colloidal particles.
[0050] The colloidal particles prepared using the above described
method have particle diameters in the order of 1 nm, have extremely
high uniformity, requires no stabilizers even in a
high-concentration state and causes little aggregation. Therefore,
by using me well known to the art such as immersing the substrates
in the colloidal solution, the colloidal particles can be easily
and strongly fixed on the substrate. Also by using the colloidal
solution, no heating or firing together with the substrate for
forming colloidal particles is required, and not only the preparing
process can be simplified, but also the formation of residues and
impurities byproducts) due to above described formation can be
avoided. As a result, according to the method of the preset
invention, a fuel cell cathode, a fuel cell anode with extremely
nigh catalytic activity and durability, and a low-temperature
oxidation catalyst can be easily and conveniently obtained
[0051] Means for applying the colloidal solution prepared to the
substrate is not specifically limited, but means well known to the
art, such as a dip method and a spray method, can be exemplified.
The kind of the substrate is not specifically limited as long as
colloidal particles can be fixed, and the substrate has required
functions as a file cell cathode and a fuel cell anode, such as
electric conductivity and heat resistance. For example, graphite or
carbon black can be used. The form and shape of the substrate are
also not specifically limited, but rod shape, fiber shape, plate
shape, woven fabrics or bulk (aggregated body) can be
exemplified.
[0052] The kind of the substrate for a low-temperature oxidation
catalyst is not specifically limited as long as the substrate can
fix colloidal particles, and has required functions as the carrier
of the low-temperature oxidation catalyst such as oxidation
resistance and beat resistance. For example, carbon inorganic
ceramics or organic polymers can be use& Two or more kinds can
also be used in combination. As carbon, activated charcoal,
charcoal, carbon fibers and carbon black can be exemplified. As
inorganic ceramics, alumina, titania, magnesia, silica or zeolite
can be exemplified. As organic polymers, polyethylene,
polypropylene, polystyrene, polyimide, polysulfone, polysilicone,
Nafion or polycellulose can be exemplified. The form and shape of
the substrate are also not specifically limited, but fibers, woven
fabrics, non-woven fabrics, films or powders can be exemplified.
Among these, glass fibers and scale-shaped glass are suited. When
chemical treatment is performed using an acid or alkali solution,
the surfaces of glass fibers and scale-shaped glass become porous
easily. Since the average diameter of pores in these porous bodies
is normally in the order of 1 to 10 nm, the colloidal particles of
a metal prepared under the above embodiments can well enter in the
pores of the porous body, and can be strongly fixed there.
[0053] The reducing agent is not specifically limited as long as it
dissolves in water, and alcohols, citric acids, carboxylic acids,
ketones, ethers, aldehydes or esters can be exemplified. The
combination of two or more of these can also be used. As alcohols,
methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol or
glycerin can be exemplified. As citric acids, citric acid, and
citrates, such as sodium citrate, potassium citrate and ammonium
citrate can be exemplified. As carboxylic acids, formic acid,
acetic acid, fumaric acid, malic acid, succinte acid, aspic acid or
carboxylates thereof can be exemplified. As ketones, acetone or
methylethyl ketone can be can be exemplified. As ethers, diethyl
ether can be exemplified. As aldehydes, formalin or acetaldehyde
can be-exemplified. As esters, methyl formate, methyl acetate or
ethyl acetate can be exemplified. Among these, sodium citrate is
particularly preferable because of its high reducibility and easy
handing.
[0054] From the point of view of forming stable colloidal particles
of a metal having a particle diameter of 1 to 20 nm alcohols,
citric acids or carboxylic acids are pebble as the reducing agent.
In particular, citric acids are preferable for forming stable
colloidal particles of a meal having a particle diameter of 1 to 5
nm. However, since the catalytic activity of colloidal particles at
a low temperature (e.g., at 100.degree. C. or below) is sharply
lowered if the particle diameter thereof is less than 1.6 nm, the
particle diameter thereof is preferably 1.6 nm or more.
[0055] The solvent composing the reaction solution is not
specifically limited as long as it can dissolve the reducing agent
and the metal salt, and water, alcohols, ketones or ethers can be
exemplified. The combination of two or more of these can also be
used. As alcohols, meal, ethanol, 1-propanol or 2-propanol can be
exemplified. As ketones, methyl formate, methyl acetate or ethyl
acetate can be exemplified. As ethers, methyl ethyl ether or
diethyl ether can be exemplified. From point of view of completely
dissolving the metal salts, the solvent is preferably water or
alcohols.
[0056] The meal salt is not specifically limited as long as it is
dissolved in the solvent, is reduced by a reducing agent, and can
function as a catalyst when it becomes colloidal particles. For
example, chlorides, nitrates, sulfates or metal complex compounds
of platinum (Pt), ruthenium (Ru), gold (Au), lead (Pb), rhodium
(Rh), iridium (Ir), cobalt (Co), iron (Fe), nickel (Ni), copper
(Cu) or tin (Sn) can be listed. The combination of two or more of
these can also be used. If the combination of two or more of metal
salts is used, the colloidal particles of an alloy can be prepared.
When a platinum salt is used as the metal salt, the particle
diameter of colloidal particles becomes particularly small, and
stable colloidal particles of 1 to 5 nm can be obtained. In
particular, if chloroplatinic acid is used, the particle diameter
of the colloidal particles becomes more uniformed.
[0057] If colloidal particles are fixed on a conductive substrate,
a fuel cell cathode and a fuel cell anode can be obtained only by
processing them using means well known to the art. Since colloidal
particles are not coated with impurities or residues in the fuel
cell cathode, the catalytic activity, that is, oxygen-reducing
reactivity thereof is extremely high. Therefore, since the
oxygen-reducing reaction occurs at high efficiency in the fuel cell
using the cathode according to the present invention, the
power-generation efficiency thereof is improved. Also since
colloidal particles are not coated with impurities or residues in
the fuel cell anode, the catalytic activity, that is,
hydrogen-oxidizing reactivity thereof is extremely high. Therefore,
since the hydrogen-oxidizing reaction occurs at high efficiency in
the fuel cell using the anode according to the present invention,
the power generation efficiency thereof is improved. The method for
manufacturing the fuel cell is not specifically limited, but means
well known to the art can be used as it is.
[0058] Also if colloidal particles are fed on a substrate, this can
be utilized a low-temperature oxidation catalyst. Here, a
"low-temperature oxidation catalyst" means an oxidation catalyst
that is used at 300.degree. C. or below, especially in the
temperature region between 100 and 300.degree. C. Since colloidal
particles are not coated with impurities or residues in the
low-temperature oxidation catalyst, the catalytic activity is
extremely high. Especially in the low-temperature region between
150 and 200.degree. C., the catalytic activity is two to three
times the catalytic activity of the catalyst using colloidal
particles prepared by existing methods, specifically by heating or
firing on the surface of a carrier, such as zeolite. This aspect
will be described later in examples.
[0059] Therefore, if the low-temperature oxidation catalyst is
utilized in a fuel modifying device for a fuel cell, the
CO-shifting reaction (CO+H.sub.2O.fwdarw.CO.sub.2+H.sub.2) can be
efficiently caused at 300.degree. C. or below, especially in the
low-temperature region between 150 and 200.degree. C. Therefore,
since conventional manufacturing methods and members can be used
without modification in the fuel modifying device for a fuel cell
according to the present invention, the power-generation efficiency
of a fuel cell can be easily and conveniently improved at low
costs. The method for manufacturing the fuel modifying device for a
fuel cell is not specifically limited, but means well known to the
art can be used as it is.
[0060] The invention of a fuel cell cathode, a fuel cell anode; a
method for preparing the same, and a fuel cell will be specifically
described below using examples and comparative examples.
Example 8
[0061] A 1,500-ml flask, a 100-ml conical flask, a 200-ml conical
flask, a reflux condenser and a stirrer were immersed in aqua regia
for a day and night, and the apparatuses were well cleaned using
pure water ion-exchanged and ultra-filtered. Into the 1,500-ml
flask, 850 ml of pure water ion-exchanged and ultra-filtered and
the stirrer were charged, the reflux condenser is installed on the
flask, and the pure water was heated to a temperature of
100.degree. C. In order to remove dissolved oxygen, the pure water
was boiled as it was for one hour. On the other hand, 0.1328 g of
tetratachloroplanic acid, hexahydrate (50 mg as platinum) was
weighed and charged into the 100 ml conical and pure water
ion-exchanged and ultra-fitted was added to make 50 ml. One grain
of sodium citrate was weighed and charged in the 200 ml conical
flask, and pure water ion-exchanged and ultra-filtered was added to
make 100 ml. After removing dissolved oxygen of the pure water The
aqueous solution of tetratachloroplanic acid was charged from the
100-ml conical flask to the 1,500-ml flask, and heated to
100.degree. C. again. Furthermore, in order to remove dissolved
oxygen, boiling was performed for 30 minutes. Then, the aqueous
solution of sodium citrate was slowly added from the 200-ml flask
so that the boiling state could be maintained. In this reaction
solution, the concentration of platinum is 50
mg/L=2.6.times.10.sup.-4 mol/L=1.0.times.10.sup.-3 N, and the ratio
of the mol concentration of sodium citrate to the metal
concentration of platinum is 13.2. Since sodium citrate function as
monatomic donor, the equivalent concentration of sodium citrate to
the equivalent concentration of platinum is 3.3.
[0062] After the aqueous solution of sodium citrate was completely
added in the 1,500-ml flask, tie reducing reaction was continued in
the boiling state, and reaction was stopped 120 minutes aft the
start of reaction, and each reaction solution was rapidly cooled to
room temperature, The cooled reaction solution was passed through a
column packed with ion-exchange resin Amberlite MB-1 (manufactured
by Organo Corporation), and metal ions and the reducing agent left
in the reaction solution were removed to obtain a stable colloidal
solution. The concentration of the colloidal particles in the
colloidal solution was measured using plasma emission spectrometry,
and an adequate amount of the colloidal particles were further
sampled to determine the state of the catalytic activity thereof,
tat is, the particle diameter of the colloidal particles using the
hydrogen peroxide decomposing reaction. As a result the
concentration of the colloidal particles of platinum was 31.25
mg/L, and tee catalytic activity was 0.79 mol-O.sub.2mg-Pt/min Also
in order to confirm the accuracy of the measurement of the
colloidal particles utilizing the hydrogen peroxide decomposing
reaction, the average particle diameter of the colloidal particles
of platinum was measured using a transmission electron microscope.
As a result, the average particle diameter of the colloidal
particles was 3.5 nm, and it was confined to agree with the result
of measurement utilizing the hydrogen peroxide decomposing
reaction.
[0063] After immersing a commercially available graphite electrode
(thin disc of an outer diameter of 3 mm) in a colloidal solution
prepared using the above-described means for a predetermined time,
it was pulled up and naturally dried. The quantity of the colloidal
particles of platinum fixed on the surface of the graphite
electrode was 10 .mu.g-Pt/cm.sup.2. The graphite electrode carrying
the colloidal particles of platinum fixed thereon was closely
adhered on the end face of a working electrode (rod-like gate of an
outer diameter of 6 mm), and immerse in an aqueous solution of
sulfuric acid of a concentration of 0.5 M so that the colloidal
particles of platinum penetrate. Then, oxygen gas was introduced
into the aqueous solution of sulfuric acid, and bubbled for a while
to saturate oxygen in the solution. While continuing bubbling, a
reference electrode was connected to the working electrode, and the
magnitude of the cathode current was measured. As a result, the
cathode current value was i (O.sub.2)=-2.31 A/g-Pt
Example 9
[0064] A colloidal solution and a fuel cell cathode were prepared
in the same manner as in Example 8, except that the time from the
start of the reaction to the stop of the reaction in the state
wherein the aqueous solution of sodium citrate had been completely
added in the 1,500 ml flask, and the reducing reaction was
continued in the boiling state. The average particle diameter of
the colloidal particles of platinum in the colloidal solution
measured using a transmission electron microscope was known to be
1.1 nm. The quantity of the colloidal particles of platinum fixed
on the surface of the graphite electrode was 10 .mu.g-Pt/cm.sup.2
same as in Example 8. The cathode current value measured under the
same conditions as in Example 8 was i (O.sub.2)=-235 A/g-Pt.
Comparative Example 3
[0065] The means for fixing the colloidal particles of platinum on
a graphite electrode in Example 8 was changed as follows: A
graphite electrode was immersed in a solution containing bisacetyl
acetonate platonic acid, pulled up after allowing to stand for a
while, and heated and fired in the presence of hydrogen to form the
colloidal particles of platinum on the graphite electrode. The
quantity of the colloidal particles of platinum fixed on the
surface of the graphite electrode was 10 .mu.l-P/cm.sup.2 same as
in Example 8. A fuel cell cathode was fabricated in the same manner
as in Example 8, and the cathode current value was measured. As a
result, i (O.sub.2)=-2.03 A/g-Pt.
Example 10
[0066] A 1,500 ml flask, a 100-ml conical fask, a 200-ml conical
fask, a reflux condenser and a stirrer were immersed in aqua regia
for a day and night, and the apparatuses were well cleaned using
pure water ion-exchanged and ultra-filtered. Into the 1,500-ml
flask, 850 ml of pure water ion-exchanged and ultra-filtered and
the stirrer are charged, the reflux condenser is installed on the
fask and the pure water was heated to a temperature of 100.degree.
C. In order to remove dissolved oxygen, the pure water was boiled
as it was for one hour. On the other hand, 0.1328 g of
tetachloroplatinic acid, hexahydrate (50 mg as platinum) was
weighed and charged into the 100-ml conical flask, and pure water
ion-exchanged and ultra-filtered was added to make 50 ml. One gram
of sodium citrate was weighed and charged in the 200 ml conical
flask, and pure water ion-exchanged and ultra-filtered was added to
make 100 ml. After removing dissolved oxygen of the pure water, the
aqueous solution of to tetachloroplatinic acid was charged from the
100 nm conical flask to the 1,500-ml flask, and heated to
100.degree. C. again. Furthermore, in order to remove dissolved
oxygen, boiling was performed for 30 minutes. Then, the aqueous
solution of sodium citrate was slowly added from the 200 ml flask
so that the boiling state could be maintained. In this reaction
solution, the concentration of platinum is 50
mg/L=2.6.times.10.sup.-4 mol/L=1.0.times.10.sup.-3 N, and the ratio
of the mol concentration of sodium citrate to the mol concentration
of platinum is 13.2. Since sodium citrate function as monatomic
donor, the equivalent concentration of sodium citrate to the
equivalent concentration of platinum is 3.3.
[0067] After the aqueous solution of sodium citrate was completely
added in the 1,500-ml flak, the reducing reaction was continued in
the boiling state, and reaction was stopped 120 minutes after the
start of reaction, and each reaction solution was rapidly cooled to
roam temperature. The cooled reaction solution was passed through a
column packed with ion-exchange resin Amberlite MB-1 (manufactured
by Organo Corporation), and metal ions and the reducing agent left
in the reaction solution were removed to obtain a stable colloidal
solution. The concentration of the colloidal particles in the
colloidal solution was measured plasma emission spectrometry, and
an adequate amount of the colloidal particles were fierier sampled
to determine the state of the catalytic activity thereof that is,
the particle diameter of the colloidal particles utilizing the
hydrogen peroxide decomposing reaction. As a result, the
concentration of the colloidal particles of platinum was 31.25
mg/L, and the catalytic activity was 0.79 mol-O.sub.2/mg-Pt/mm.
Also in order to confirm the accuracy of the same measurement of
the colloidal particles utilizing the hydrogen peroxide decomposing
reaction, the average particle diameter of the colloidal particles
of platinum was measured using a transmission electron microscope.
As a result the average particle diameter of the colloidal
particles was 3.5 nm, and it was confirmed to agree with the result
of measurement utilizing the hydrogen peroxide decomposing
reaction.
[0068] After immersing a commercially available graphite electrode
(thin disc of an outer diameter of 3 mm) in a colloidal solution
prepared using the above-described means for a predetermined time,
it was pulled up and naturally dried. The quantity of the colloidal
particles of platinum fixed on the surface of the graphite
electrode was 10 .mu.g-Pt/cm.sup.2. The graphite electrode carrying
the colloidal particles of platinum fixed thereon was closely
adhered on the end face of a work electrode (rod-like graphite of
an outer diameter of 6 mm), and immersed in an aqueous solution of
sulfuric acid of a concentration of 0.5 M so that the colloidal
particles of platinum penetrate. Then, hydrogen gas was introduced
into the aqueous solution of sulfuric acid, and bubbled for a while
to saturate hydrogen in the solution. While continuing bubbling, a
reference electrode was connected to the working electrode, and the
magnitude of the anode current was measured. As a result, the anode
current value was i (H.sub.2)=1.28 A/g-Pt.
Comparative Example 4
[0069] The means for fixing the colloidal particles of platinum on
a graphite electrode in Example 10 was changed as follows: A
graphite electrode was immersed in a solution containing bisacetyl
acetate platonic acid, pulled up after allowing to stand for a
while, and heated and fired in the presence of hydrogen to form the
colloidal particles of platinum on the graphite electrode. The
quantity of the colloidal particles of platinum fixed on the
sulfite of the graphite electrode was 10 .mu.g-Pt/cm.sup.2 same as
in Example 10. A fuel cell cathode was fabricated in the same
manner as in Example 10, and the anode current value was measured.
As a result, i (H.sub.2)=1.08 A/g-Pt.
Example 11
[0070] A 1,500-ml flask, a 100-ml conical flask, a 200-ml conical
flask, a reflux condenser and as were immersed in aqua regia for a
day and night, and the apparatuses were well cleaned using pure
water ion-exchanged and ultra-filtered. Into the 1,500-ml fask, 850
ml of pure water ion-exchanged and ultra-filtered and the stirrer
are charged, the reflux condenser is installed on the flask, and
the pure water was heated to a temperature of 100.degree. C. In
order to remove dissolved oxygen, the pure water was boiled as it
was for one hour. On the other hand, 0.1328 g of tetachloroplatinic
acid, hexahydrate (50 mg as platinum) was weighed and charged into
the 100-ml conical flask, and pure water ion-exchanged and
ultra-filtered was added to make 50 ml. One gram of sodium citrate
was weighed and charged in the 200-ml conical flask and pure water
ion-exchanged and ultra-filtered was added to make 100 ml. After
removing dissolved oxygen of the pure water, the aqueous solution
of tetrachloroplatinic acid was charged from the 100-ml conical
flask to the 1,500-ml flask, and heated to 100.degree. C. again.
Furthermore, in order to remove dissolved oxygen, boiling was
performed for 30 minutes. Then, the aqueous solution of sodium
citrate was slowly added from the 200-ml flask so that the boiling
state could be maintained. In this reaction solution, the
concentration of platinum is 50 mg/L=2.6.times.10.sup.-4
mol/L=1.0.times.10.sup.-3 N, and the ratio of the mol concentration
of sodium citrate to the mol concentration of platinum is 13.2.
Since sodium citrate function as monatomic donor, the equivalent
concentration of sodium citrate to the equivalent concentration of
platinum is 3.3.
[0071] After the aqueous solution of sodium citrate was completely
added in the 1,500-ml flask the reducing reaction was continued in
the boiling state, and reaction was sa ed 120 minutes after the
start of reaction, and each reaction solution was rapidly cooled to
room temperature. The cooled reaction solution was passed through a
column packed with ions resin Amberlite MB-1 (manufactured by
Organo Corporation), and metal ions and the reducing agent left in
the reaction solution were removed to obtain a stable colloidal
solution. The cooled reaction of the colloidal particles in the
colloidal solution was measured using plasma emission spectrometry,
and an adequate amount of the colloidal particles were further
sampled to determine the state of the catalytic activity thereof,
that is, the particle diameter of the colloidal particles utilizing
the hydrogen peroxide decomposing reaction As a remit, the
concentration of the colloidal particles of platinum was 31.25
mg/L, and the catalytic activity was 0.79 mol-O.sub.2/mg-Pt/min.
Also in order to confirm the accuracy of the measurement of the
colloidal particles utilizing the hydrogen peroxide decomposing
reaction, the average particle diameter of the colloidal particles
of platinum was measured using a transmission electron microscope.
As a result the average particle diameter of the colloidal
particles was 3.5 nm, and it was confirmed to agree with the result
of measurement utilizing the hydrogen peroxide decomposing
reaction.
[0072] After adding the fine alumina particles in the colloidal
solution prepared using the above-described means and immersing for
a predetermined time, the fine particles were pulled up and
naturally died. Thereafter, the fine alumina particles were stored
in a desiccator for several days to completely remove the solvent
of the colloidal solution Thereafter, the fixation rate of the
colloidal particles of platinum in the fine alumina particles was
measured (calculated from the mass difference between before and
after the fixation of the colloidal particles of platinum). The
fixation rats of the colloidal particles of platinum was 0.27% by
mass.
[0073] In order to measure the catalytic activity of the fine
alumina particle to which the colloidal particles have adhered,
that is, a low-temperature oxidation catalyst the CO conversion
rate was measured using the following means: As the me conditions,
a mixed gas consisting of the gas composition (ratio by volume) of
CO=0.80%, Co.sub.2=20.2%, H.sub.2=38.5%, O.sub.2=0.8%, and He=39.7%
was used; and each CO conversion rate was measured when the spatial
velocity (SV) was 11,000 h.sup.-1 (catalyst 2 ml/gas flow rate 22
L/H); and the temperature of the low-temperature oxidation catalyst
and the mixed gas was 50, 100, 150, 200, 250 or 300.degree. C. The
results are shown in FIG. 1.
Example 12
[0074] A colloidal solution and a low temperature oxidation
catalyst were prepared in the same manner as in Example 11, except
that tie time from the start of the reaction to the stop of the
reaction was shorten in the state wherein the aqueous solution of
sodium citrate had been completely added in the 1,500 ml flask, and
the reducing reaction was continued in the boiling state. The
average particle diameter of the colloidal particles of platinum in
the colloidal solution measured using a transmission electron
microscope was known to be 1.1 nm. The fixing rate of the colloidal
particles of platinum in the low-temperature oxidation catalyst
(fine alumina particles having the colloidal particles of platinum
fixed thereon) was 0.20% by mass.
[0075] Furthermore, the CO conversion rate of the low-temperature
oxidation catalyst was measured Under the same conditions as in
Example 1. The results ate shown in FIG. 1.
Comparative Example 5
[0076] In Example 11, the low-temperature oxidation catalyst was
not prepared, but commercially available fine alumna particles
having the colloidal particles of platinum fixed hereon (N-220,
manufactured by Sud-Chemie Catalysts Japan, Inc.) were used. The
fixing rate of the colloidal particles of platinum in the fine
alumina particles was 0.20% by mass, and since the measured
specific surface area of the colloidal particles of platinum was
about 160 m.sup.2/g, the average particle diameter thereof is
considered to be about 1 nm, substantially equal to the colloidal
particles of platinum prepared in Example 12. Further, the CO
conversion rate was measured under the same conditions as in
Example 11. The results are shown in FIG. 1.
[0077] By comparing these Examples 11, 12 and Comparative Example
5, the low-temperature oxidation catalyst according to the present
invention has higher catalytic activity tan the catalyst prepared
by existing methods. In particular, within the range between 150
and 200.degree. C., the catalytic activity is as high as two to
three times the catalyst prepared by existing methods.
[0078] By comparing Example 11 and Example 12, it is known that the
larger the average particle diameter of the colloidal particles of
platinum, the lower temperature region the catalytic activity
starts elevating. However, since the larger the average particle
diameter, the easier the colloidal particles of platinum
precipitate, it is preferable that the average particle diameter is
5 nm or less. Specifically, the preferable average particle
diameter of the colloidal particles of platinum is 3.5 to 5 nm.
INDUSTRIAL APPLICABILITY
[0079] Since the present invention is constituted as described
above, it exerts the following effects. In a method for forming the
colloidal particles of a metal by boiling a solution containing a
metal salt and a reducing agent, the average particle diameter of
the colloidal particles can be further reduced and uniformize, and
various properties thereof, such as catalytic activity, can be
improved by combining and adjusting three preparing conditions
(parameters) directly affecting the formation, association and
aggregation of the colloidal particles. The metal recovery rate of
the metal salt can also be improved. Furthermore, by fixing the
colloidal particles on a porous substrate or the like, a carrier
having a high catalytic activity, as well as durability, can be
obtained.
[0080] In addition, since the present invention is constituted as
described above, it exerts the following effects. According to the
methods for preparing a cathode, a fuel cell anode, and a
low-temperature oxidation catalyst of the present invention, since
an extremely stable colloidal solution contains no impurities, such
as a stabilizer, is applied onto the substrate, no beating or
firing of the substrate for obtaining the colloidal particles
becomes required, and concurrently, the lowering of the catalytic
activity due to the occurrence of residues or the like can be
avoided. Also since the colloidal solution can be prepared in a
simple process using an inexpensive metal salt as the starting
material, the preparation costs can be suppressed. Also since the
colloidal solution has a uniform particle diameter, and hardly
aggregates even in a high concentration, no stabilizers for
preventing precipitation are required.
[0081] In addition, according to a fuel cell cathode of the present
invention, water can be efficiently formed from hydrogen ions and
oxygen. Furthermore, according to a fuel cell of the present
invention, power-generating efficiency can be improved. In
addition, according to a fuel cell anode of the present invention,
hydrogen ions can be efficiently formed. Furthermore, according to
a fuel cell of the present invention, power-generating efficiency
can be improved.
[0082] Moreover, according to a low-temperature oxidation catalyst
of the present invention, catalytic functions can be exerted at a
high efficiency in an environment of 300.degree. C. or below,
particularly 150 to 200.degree. C. Furthermore, according to fuel
modifying device for a fuel cell of the present invention, the CO
shifting reaction can be efficiently caused even at low
temperatures. Therefore, the device can contribute to the
improvement of the power-generating efficiency of the fuel cell,
and by utilizing members having not so high heat resistance, the
manufacturing costs can be reduced.
* * * * *