Molecular sieve abstract
The present invention is related to a mesoporous carbon molecular
sieve, which can be used as a catalyst carrier capable of improving
the activity of a supported catalyst and a method of preparing the
same. Additionally, the invention is related to a supported catalyst
employing the mesoporous carbon molecular sieve as a carrier, and
a fuel cell employing the supported catalyst. The mesoporous carbon
molecular sieve has an average primary particle size of less than
about 500 nm, an average mesopore size in the range of about 3 nm
to about 6 nm, and a surface area in the range of about 500 m.sup.2/g
to about 2000 m.sup.2/g.
Molecular sieve claims
What is claimed is:
1. A mesoporous carbon molecular sieve for a support of a supported
catalyst, wherein the mesoporous carbon molecular sieve has an average
primary particle size of less than about 500 .mu.m, an average mesopore
size in the range of about 3 .mu.m to about 6 nm, and a surface
area in the range of about 500 m.sup.2/g to about 2000 m.sup.2/g.
2. A method of controlling the primary particle size of a mesoporous
silica, comprising: reacting NaOH with SiO.sub.2 in a water-based
reaction medium to form sodium silicate; reacting the sodium silicate,
a poly(ethyleneglycol)-block-poly(propyleneglycol)-block-poly(ethyleneglyco-
l), and an acetic acid in a water-based reaction medium to obtain
precipitates; and drying and calcining the precipitates to obtain
the mesoporous silica, wherein the Na:Si atomic ratio of the formed
sodium silicate is controlled by relatively controlling the amount
of each of NaOH and SiO.sub.2 when forming the sodium silicate and
the average particle size of the formed mesoporous silica is reduced
by increasing the Na:Si atomic ratio.
3. A method of preparing a mesoporous silica molecular sieve having
an average primary particle size in the range of about 250 nm to
about 700 nm, comprising: reacting sodium silicate with Na:Si atomic
ratio of about 1.5 to about 2.5 a poly(ethyleneglycol)-block-poly(propyleneglycol)-bloc-
k-poly(ethyleneglycol), and an acetic acid to obtain precipitates;
and drying and calcining the precipitates.
4. A method of preparing a mesoporous carbon molecular sieve, comprising:
filling a carbon precursor in pores of a template composed of a
mesoporous silica having an average primary particle size in the
range of about 250 nm to about 700 nm; thermally decomposing the
carbon precursor by heating the template having the carbon precursor
impregnated under a non-oxidizing atmosphere, to form a carbon structure
in the pores of the template; and removing the template using a
silica-soluble solution, thereby isolating the carbon structure.
5. A supported catalyst comprising: a mesoporous carbon molecular
sieve having an average primary particle size of less than about
500 nm, an average mesopore size in the range of about 3 nm to about
6 nm, and a surface area in the range of about 500 m.sup.2/g to
about 2000 m.sup.2/g; and catalytic metal particles dispersed in
and supported on the mesoporous carbon molecular sieve.
6. The supported catalyst of claim 5 wherein the mesoporous carbon
molecular sieve has an average primary particle size in the range
of about 250 nm to about 350 nm.
7. The supported catalyst of claim 5 wherein the mesoporous carbon
molecular sieve has a surface area in the range of about 1000 m.sup.2/g
to about 1500 m.sup.2/g.
8. A fuel cell comprising a cathode, an anode, and an electrolyte
membrane interposed between the cathode and the anode, wherein at
least one of the cathode and the anode contains the supported catalyst
comprising: a mesoporous carbon molecular sieve having an average
primary particle size of less than about 500 nm, an average mesopore
size in the range of about 3 nm to about 6 nm, and a surface area
in the range of about 500 m.sup.2/g to about 2000 m.sup.2/g; and
catalytic metal particles dispersed in and supported on the mesoporous
carbon molecular sieve.
9. The fuel cell of claim 8 wherein the mesoporous carbon molecular
sieve has an average primary particle size in the range of about
250 nm to about 350 nm.
10. The fuel cell of claim 8 wherein the mesoporous carbon molecular
sieve has a surface area in the range of about 1000 m.sup.2/g to
about 1500 m.sup.2/g.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to Korean Patent Application
No. 2003-83041 filed on Nov. 21 2003 in the Korean Intellectual
Property Office, the disclosure of which is incorporated herein
in its entirety by reference.
FIELD OF THE INVENTION
[0002] The present invention is related to a catalyst support,
and in particular, to a carbon-based catalyst support. In particular,
the present invention is related to a supported catalyst employing
the carbon-based catalyst support. Also, the present invention is
related to a fuel cell, and more particularly, to a fuel cell comprising
the supported catalyst employing the carbon-based catalyst support.
BACKGROUND
[0003] Fuel cells are inherently ultra-clean, highly reliable,
and have high power density and high energy-conversion efficiency.
Also, since fuel cells can operate at an ambient temperature and
can be fabricated in miniaturized form and hermetically sealed,
they can be extensively applied to power generating systems for
home and regional use, medical equipment, military equipment, space
equipment, and used as power sources for portable electrical/electronic
devices such as mobile telecommunications equipment.
[0004] The fuel cell produces electricity through the electrochemical
reaction of fuel, such as hydrogen, natural gas, and methanol, and
an oxidizing agent. In general, the fuel cell consists of two electrodes--an
anode and a cathode, which are sandwhiched around an electrode membrane.
The fuel is supplied to the anode where it is electrochemically
oxidized, an oxidizing agent, such as oxygen or air, is fed to the
cathode where it is electrochemically reduced, and the electrolyte
membrane acts as a path for transporting ions produced at the anode
to the cathode. Electrons generated at the anode by oxidation of
the fuel go through an external circuit, creating a flow of electricity.
The protons migrate through the electrolyte to the cathode, where
they reunite with the oxidization agent and the electrons to produce
water and heat.
[0005] A catalyst contained in the anode and the cathode to promote
the electrochemical reactions is very important in the fuel cell
having such structure. For example, in a polymer electrolyte membrane
fuel cell (PEMFC) both of the anode and the cathode generally contain
a carbon-supported platinum catalyst having platinum nanoparticles
dispersed in a microporous carbon support. Also, in a direct methanol
fuel cell the anode catalyst may be, for example, a PtRu alloy powder
or a carbon-supported PtRu catalyst having PtRu nanoparticles dispersed
in the microporous carbon support, and the cathode catalyst may
be, for example, a Pt particle powder or the carbon-supported platinum
catalyst having platinum nanoparticles dispersed in the microporous
carbon support.
[0006] A catalyst support for a fuel cell must exhibit porosity
to support and disperse catalytic metal particles and electroconductivity
to act as the path for the migration of electrons. In general, amorphous
microporous carbon powder known as activated carbon or carbon black
may be used as a catalyst support for the fuel cell.
[0007] An amorphous microporous carbon powder is generally prepared
by chemically and/or physically activating a raw material, such
as wood, peat, charcoal, coal, brown coal, coconut peel, and petroleum
coke, for example. Generally, the activated carbon has pores exhibiting
a diameter of less than about 1 nm and has a surface area of about
60 m.sup.2/g to about 1000 m.sup.2/g. In particular, Vulcan Black
and Kejten Black, which are commercial products most broadly used
as a catalyst support, have a surface area of about 230 m.sup.2/g
and about 800 m.sup.2/g, respectively, and have an average primary
particle size of less than about 100 nm. Amorphous microporous carbon
particles, however, have poor micropore interconnection. Specifically,
in a conventional DMFC, a supported catalyst using amorphous microporous
carbon particles as a support has lower reactivity than a catalyst
consisting of only metal particles. However, DMFCs employing metal
particle catalysts are not cost effective due to the high costs
associated with the metal particular catalysts. Thus, there is a
need to develop a carbon-based catalyst support that is capable
of improving the reactivity of the catalyst for fuel cells, such
as PEMFCs, PAFCs and DMFC.
[0008] For example, the mesoporous carbon molecular sieve, disclosed
in Korean Patent Laid-Open Publication No. 2001-0001127 is an example
of such a carbon-based catalyst support. This patent discloses a
method of preparing an ordered mesoporous carbon molecular sieve
using a mesoporous silica molecular sieve, which is prepared using
a surfactant as a template material. In this method based on nano-replication,
the ordered mesoporous silica molecular sieve, such as "MCM-48"
and "SBA-1", which has micropores connected three-dimensionally
by mesopores is used as a template to prepare an ordered mesoporous
carbon molecular sieve, such as "CMK-1" and "CMK-2",
which has micropores and mesopores with a uniform diameter and regularly
arranged.
[0009] The mesoporous carbon molecular sieve prepared as described
above may be used as a possible carbon-based catalyst support. However,
since the particle size of the mesoporous carbon molecular sieve
is larger than those of Vulcan Black and Kejten Black, there is
a need to improve the catalytic activity in the mesoporous carbon
molecular sieve.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to an improved mesoporous
carbon molecular sieve and a method of preparing the same. The mesoporous
carbon molecular sieve of the invention, may be used as a catalyst
support to improve the activity of a supported catalyst. Additionally,
the present invention is also directed to a supported catalyst employing
the mesoporous carbon molecular sieve as a support. The present
invention also provides a fuel cell employing the supported catalyst.
[0011] According to an aspect of the present invention, a mesoporous
carbon molecular sieve having an average primary particle size of
less than about 500 nm, an average mesopore size in the range of
about 3 nm to about 6 nm, and a surface area in the range of about
500 m.sup.2/g to about 2000 m.sup.2/g is provided.
[0012] According to another aspect of the present invention, a
method of preparing the mesoporous carbon molecular sieve is provided.
The molecular sieve may be prepared in the following manner. A mesoporous
silica molecular sieve having an average primary particle size in
the range of about 100 nm to about 700 nm is prepared, then, the
mesoporous carbon molecular sieve using the mesoporous silica molecular
sieve as a template is prepared.
[0013] An additional aspect of the present invention is directed
to a method of preparing a mesoporous silica molecular sieve having
an average primary particle size in the range of about 250 nm to
about 700 nm. The silica molecular sieve may be prepared in the
following manger. A reaction of Sodium silicate with Na:Si having
an atomic ratio of about 1.5 to about 2.5 a poly(ethyleneglycol)-block-poly(propyleneglycol)-bloc-
k-poly(ethyleneglycol), and acetic acid in a water-based reaction
medium may be employed to produce precipitates, and then, the precipitates
may be dried and calcined.
[0014] According to another aspect of the present invention, a
method of preparing the mesoporous carbon molecular sieve is provided.
The method may be carried out in the following manner. A carbon
precursor may be filled into pores of a template of the mesoporous
silica molecular sieve having an average primary particle size in
the range of about 250 nm to about 700 nm, then the carbon precursor
may be thermally degraded by heating the template having the carbon
precursor impregnated under a non-oxidizing atmosphere to form a
carbon structure in the pores of the template. Then, the template
may be removed using a silica-soluble solution in order to isolate
the carbon structure.
[0015] According to another aspect of the present invention, a
supported catalyst including the mesoporous carbon molecular sieve
having an average primary particle size of less than about 500 nm,
an average mesopore size in the range of about 3 nm to about 6 nm,
and a surface area in the range of about 500 m.sup.2/g to about
2000 m.sup.2/g; and catalytic metal particles dispersed in and supported
on the mesoporous carbon molecular sieve is provided.
[0016] According to another aspect of the present invention, a
fuel cell comprising a cathode, an anode, and an electrolyte membrane
interposed between the cathode and the anode, where at least one
of the cathode and the anode may contain the supported catalyst
according to the present invention is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 illustrates an XRD analysis result for a mesoporous
silica molecular sieve obtained in an Example of the present invention.
[0018] FIG. 2 is a particle size distribution chart of a mesoporous
carbon molecular sieve according to an Example of the present invention.
[0019] FIG. 3 is a SEM photograph of a mesoporous carbon molecular
sieve according to an Example of the present invention.
[0020] FIG. 4 illustrates an XRD analysis result for a supported
catalyst obtained in Example 7 and Example 8 of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention is directed to a mesoporous carbon
molecular sieve, which has the shape of a porous particle substantially
consisting of carbon. Unlike a conventional amorphous microporous
carbon powder having mainly micropores, the mesoporous carbon molecular
sieve of the present invention may have both mesopores and micropores
in an appropriate ratio. In the present invention, the term "micropores"
means pores having a diameter less than about 2 nm, the term "mesopores"
means pores having a diameter in the range of about 2 nm to about
10 nm, and a molecular sieve refers to porous particles having mesopores
and micropores with substantially uniform size.
[0022] In an embodiment of the present invention, pores of the
mesoporous carbon molecular sieve may or may not be regularly arranged.
Regardless of this, in the mesoporous carbon molecular sieve of
the present invention, micropores interconnect via mesopores or
mesopores interconnect via micropores. Accordingly, reactants can
be easily supplied, via mesopores, to micropores, and products created
in micropores can be easily discharged outside of supported catalyst
particles through mesopores. Such a mesoporous carbon molecular
sieve may be characterized by an average size (or diameter) of mesopores
and a surface area as well as an average primary particle size.
[0023] When the mesoporous carbon molecular sieve is used as a
support of a supported catalyst, as the primary particle size of
the mesoporous carbon molecular sieve decreases, electrochemical
activity of the supported catalyst increases compared with that
of a conventional Black supported platinum catalyst. The diffusion
of reactants and removal of products in the supported catalyst may
be smoothly performed by decreasing the primary particle size of
the mesoporous carbon molecular sieve having micropores and mesopores
regularly arranged, thus enabling all of the catalytic metal particles
present even in the micropores in the support to participate in
the electrochemical reaction.
[0024] There is an appropriate range for the average primary particle
size of the mesoporous carbon molecular sieve used as the support,
and the appropriate range is less than about 500 nm, and in particular,
in the range of about 250 nm to about 350 nm. Specifically, the
primary particle size may be about 300 nm.
[0025] The mesoporous carbon molecular sieve of the present invention
may be characterized as follows. The mesoporous carbon molecular
sieve of the present invention may have an average mesopore size
in the range of about 3 nm to about 6 nm and a surface area in the
range of about 500 m.sup.2/g to about 2000 m.sup.2/g. The average
mesopore size of the mesoporous carbon molecular sieve may be preferably
in the range of about 3.5 nm to about 5 nm. The surface area of
the mesoporous carbon molecular sieve may be in the range of about
800 m.sup.2/g to about 1500 m.sup.2/g. A method of preparing the
mesoporous carbon molecular sieve of the present invention may be
performed as follows. A mesoporous silica molecular sieve having
an average primary particle size in the range of about 250 nm to
about 700 nm may be prepared, and then, a mesoporous carbon molecular
sieve may be prepared by using the mesoporous silica molecular sieve
as a template. In a further embodiment, the mesoporous silica may
have an average primary particle size in the range of about 250
nm to about 500 nm, and more specifically, in the range of about
275 nm to about 325 nm. The method of the present invention is based
on "nano-replication". In other words, pores of the mesoporous
silica molecular sieve used as the template may be filled with carbon,
and then the silica molecular sieve may be removed using a silica-soluble
solution to isolate a carbon molecular sieve. Here, the average
primary particle size of the final carbon molecular sieve is proportional
to the average primary particle size of the mesoporous silica molecular
sieve. Thus, the method may include adjusting process of the particle
size of the mesoporous silica molecular sieve.
[0026] Additionally, the present invention may provide a method
of adjusting the average primary particle size of the mesoporous
silica molecular sieve. The method may include reacting NaOH with
SiO.sub.2 in a water-based reaction medium to form sodium silicate;
reacting the sodium silicate, a poly(ethyleneglycol)-block-poly(propyleneglycol)-block-poly(e-
thyleneglycol), and a pH controller in a water-based reaction medium
to obtain precipitates; and then, drying and calcining the precipitates
to obtain the mesoporous silica molecular sieve. The Na:Si atomic
ratio of the formed sodium silicate may be adjusted by relatively
adjusting the amounts of NaOH and SiO.sub.2 when forming the sodium
silicate, and the average particle size of the formed mesoporous
silica may be decreased by increasing the atomic ratio of Na:Si.
[0027] When forming sodium silicate, the water-based reaction medium
acts as a solvent for NaOH and as a dispersion medium for SiO.sub.2
particles. In order to form sodium silicate having the desired Na:Si
atomic ratio, NaOH and SiO.sub.2 may be added to the water-based
reaction medium in a relative amount corresponding to the desired
Na:Si atomic ratio. However, the absolute contents of NaOH and SiO.sub.2
are not particularly limited. When the reactants are too diluted
or concentrated, the reaction may not occur smoothly. Therefore,
the adding amount of NaOH may be typically in the range of about
10 parts to about 30 parts by weight based on 100 parts by weight
of the water-based reaction medium. The added amount of SiO.sub.2
may be in the range of about 10 parts to about 15 parts by weight
based on 100 parts by weight of the water-based reaction medium.
The reaction temperature and the reaction time in this step are
not particularly limited. When the reaction temperature is too low,
SiO.sub.2 may not dissolve well. When the reaction temperature is
too high, the composition of the whole solution may fluctuate. Therefore,
the reaction temperature may be in the range of about 60.degree.
C. to about 80.degree. C. The reaction time may be easily selected
to obtain sodium silicate at an appropriate yield according to other
reaction conditions and is typically in the range of about 0.5 hours
to about 2 hours. The resultant product may be a type of transparent
water-based solution containing sodium silicate.
[0028] Next, the sodium silicate, the poly(ethyleneglycol)-block-poly(prop-
yleneglycol)-block-poly(ethyleneglycol), and the pH controller may
be reacted in a water-based reaction medium to obtain precipitates.
In this step, the sodium silicate may be added to the water-based
reaction medium in a dried state or in the form of the transparent
water-based solution obtained in the previous step. The poly(ethyleneglycol)-block-poly(propyl-
eneglycol)-block-poly(ethyleneglycol) (P123 polymer (BASF Co.))
may be used as a template material to form micropores. As the pH
controller, a general acid may be used. A representative example
of the pH controller may be acetic acid.
[0029] When the amount of sodium silicate added to the water-based
reaction medium is too low, the mesoporous structure may not form.
When the amount of sodium silicate is too high, the amount of amorphous
SiO.sub.2 may increase. Therefore, the amount of sodium silicate
added to the water-based reaction medium may be in the range of
about 13 parts to about 20 parts by weight based on 100 parts by
weight of the water-based reaction medium.
[0030] When the amount of poly(ethyleneglycol)-block-poly(propyleneglycol)-
-block-poly(ethyleneglycol) added to the water-based reaction medium
is too little, mesopores may not form. When the amount of poly(ethyleneglycol)-block-poly(propyleneglycol)-block-poly(ethyleneglyco-
l) is too great, a mesoporous substance of non-desirable structures
may form or no structure may form. Therefore, the amount of poly(ethyleneglycol)-block-poly(propyleneglycol)-block-poly(ethyleneglyco-
l) added to the water-based reaction medium may be in the range
of about 1 part to about 5 parts by weight based on 100 parts by
weight of the water-based reaction medium.
[0031] Also, if the amount of acetic acid added to the water-based
reaction medium is too little, a large amount of NaOH may not be
neutralized, and the structure formation cannot be performed. When
the amount of the acetic acid added to the water-based reaction
medium is too great, the acidity increases, and the particle size
of silica substance cannot be controlled. Hence, the amount of acetic
acid added to the water-based reaction medium may be in the range
of about 2 parts to about 7 parts by weight based on 100 parts by
weight of the water-based reaction medium.
[0032] The order of adding the reactants to the water-based reaction
medium is not particularly limited. Typically, the transparent water-based
solution containing sodium silicate may be mixed with an aqueous
poly(ethyleneglycol)-block-poly(propyleneglycol)-block-poly(ethyl-
eneglycol) solution, and then the acetic acid may be added to the
mixture. In this step, the reaction temperature and the reaction
time are not particularly limited. However, when the reaction temperature
is too low, the poly(ethyleneglycol)-block-poly(propyleneglycol)-block-poly(ethyleneg-
lycol) may not dissolve well in the water-based reaction medium.
When the reaction temperature is too high, the composition of the
solution or the structure of the silica substance may fluctuate.
Therefore, the reaction temperature is typically in the range of
about 30.degree. C. to about 60.degree. C. The reaction time may
be easily selected to obtain precipitates at an appropriate yield,
according to other reaction conditions, and is typically in the
range of about 5 hours to about 48 hours. The resultant precipitates
are a type of white powder.
[0033] The resultant precipitates are separated and dried and then
subjected to calcination to obtain the mesoporous silica molecular
sieve having the desired average primary particle size. The separation
of precipitates may be performed in a conventional manner, such
as filtration/washing, and centrifuging. The drying temperature
and drying time of the precipitates are not particularly limited.
Typically, drying of the precipitates may be performed at room temperature
for a time period in the range of about 12 hours to about 36 hours.
Then, the dried precipitates may be calcined. The calcination may
be performed under an oxidizing atmosphere such as air. When the
calcination temperature is too low, micropores are not created since
the template material and other impurities remain. When the calcination
temperature is too high, the uniformity of the micropores may decrease.
Therefore, the calcination temperature may be in the range of about
450.degree. C. to about 700.degree. C. Also, when the calcination
time is too short, the template material and other impurities can
possibly remain. When the calcination time is too long, a large
amount of time may be spent to calcine. In view of this, the calcination
time may be in the range of about 5 hours to about 15 hours.
[0034] In order to adjust the average primary particle size of
the silica molecular sieve to the range of about 250 nm to about
700 nm, the Na:Si atomic ratio of sodium silicate may be adjusted
to about 1.5 to about 2.5. Accordingly, the method of preparing
the mesoporous silica molecular sieve having the average primary
particle size in the range of about 250 nm to about 700 nm includes
reacting sodium silicate with Na:Si atomic ratio of about 1.5 to
about 2.5 the poly(ethyleneglycol)-block-poly(prop- yleneglycol)-block-poly(ethyleneglycol),
and acetic acid to obtain precipitates; and drying and calcining
the precipitates.
[0035] As described above, the mesoporous carbon molecular sieve
of the present invention may be prepared using nano-replication.
In other words, the method of preparing the mesoporous carbon molecular
sieve may include filling a carbon precursor into pores of a template
composed of the mesoporous silica molecular sieve having an average
primary particle size in the range of about 250 nm to about 700
nm; thermally decomposing the carbon precursor by heating the template
having the carbon precursor impregnated under non-oxidizing atmosphere,
to form a carbon structure in the pores of the template; and removing
the template using a silica-soluble solution to isolate the carbon
structure.
[0036] The mesoporous silica molecular sieve having an average
primary particle size of about 250 nm to about 700 nm as described
above may be used as the template for the nano-replication. The
carbon precursor filled in the pores of the template may be any
material capable of being carbonized by thermal decomposition. Examples
of the carbon precursor include a polymer of a carbon-containing
compound capable of being polymerized. The polymerization includes
various types of polymerization, such as addition polymerization
and condensation polymerization, for example. Examples of the carbon-containing
compound capable of being polymerized include carbohydrates and
monomers, for example. Hereinafter, the carbon-containing compound
capable of being polymerized is referred to as a polymerizable carbon-containing
compound.
[0037] The carbohydrates are classified into monosaccharides, oligosaccharides,
and polysaccharides. In the present invention, monosaccharides,
oligosaccharides, and a mixture thereof may specifically be used.
Representative examples of the monosaccharides include glucose,
fructose, mannose, galactose, ribose, and xylose. These materials
may be used alone or in a combination of two or more. The oligosaccharides
are carbohydrates composed of two or more of monosaccharides joined
together by a glycoside link. Saccharides from disaccharides composed
of two monosaccharides to decasaccharides composed of ten monosaccharides
are collectively called oligosaccharides. The oligosaccharides may
include simple ones, which are composed of one type of monosaccharide,
and complicated ones, which may be composed of two or more types
of monosaccharides. Of the oligosaccharides, disaccharides are mainly
present in the natural world in an isolated state. Specific examples
of the disaccharides include sucrose contained in sugar canes, maltose
(malt sugar) which is a digested material of starch by amylase and
is a raw material of wheat gluten, lactose (milk sugar) contained
in the milk of mammals, and the like. Reducing groups of these saccharides
and hydroxy groups of saccharides or compounds except for the saccharides
may undergo dehydration condensation.
[0038] Representative examples of a monomer that can be used as
the polymerizable carbon-containing compound include furfuryl alcohol,
divinylbenzene, phenol-formaldehyde, resorcinol-formaldehyde and
the like.
[0039] The method of filling the carbon precursor in the pores
of the template may be carried out as follows. First, a mixture
containing the polymerizable carbon-containing compound and a liquid
carrier may be impregnated into the pores of the template. Then,
the polymerizable carbon-containing compound may be polymerized
in the template to form a polymer of the polymerizable carbon-containing
compound in the pores of the template. These processes may be repeated
one or two times or more.
[0040] The carrier is in a liquid state and acts as a solvent dissolving
the polymerizable carbon-containing compound and as a medium carrying
the polymerizable carbon-containing compound to the pores of the
template. The carrier may be selected from, for example, water,
an organic solvent, and a mixture thereof. In particular, the organic
solvent may be alcohol. More particularly, the alcohol may be ethanol.
Furfuryl alcohol may be used as the polymerizable carbon-containing
compound or as the carrier. Furfuryl alcohol used as the polymerizable
carbon-containing compound may also be the carrier.
[0041] The mixture may further comprise an acid. The acid may promote
the polymerization of the carbon precursor. The acid may be selected
from, for example, sulphuric acid, hydrochloric acid, nitric acid,
sulfonic acid, derivatives thereof, and a mixture of two or more
of the foregoing materials. Representative examples of the sulfonic
acid may include methylsulfonic acid and the like.
[0042] The concentrations of the respective constituents in the
mixture are not particularly limited provided that the purpose of
the present invention can be accomplished. For example, the concentrations
of the respective constituents in the mixture may be as follows.
[0043] When the concentration of the carrier is too low, impregnation
of the mixture into the template may not be easily performed. When
the concentration of the carrier is too high, the amount of carbon
filled in the template may be excessively decreased. Therefore,
the concentration of the carrier may be, for example, in the range
of about 300 parts to about 1000 parts by weight based on 100 parts
by weight of the polymerizable carbon-containing compound.
[0044] When the concentration of the acid is too low, the effect
of promoting the polymerization of the polymerizable carbon-containing
compound according to the addition of an acid may be trivial. When
the concentration of the acid is too high, the catalyzing effect
may be saturated. In view of this, the concentration of the acid
may be, for example, in the range of about 1 part to about 30 parts
by weight based on 100 parts by weight of the polymerizable carbon-containing
compound.
[0045] The polymerization of the polymerizable carbon-containing
compound in pores of the template may be performed by, for example,
heating, UV irradiation, and the like. When polymerising by heating,
a heating temperature too low may result in insufficient polymerization
and a heating temperature too high may decrease the uniformity of
the resulting carbon molecular sieve. In view of this, the heating
temperature of the template having the mixture impregnated therein
may be, for example, in the range of about 50.degree. C. to about
250.degree. C. Alternatively, the heating may be performed in two
steps of a first heating and a second heating. For example, the
first heating may be performed at a temperature of about 50.degree.
C. to about 150.degree. C. and the second heating may be performed
at a temperature of about 150.degree. C. to about 250.degree. C.
Through these heating processes, the carbon precursor can be polymerized
and the liquid carrier can be vaporized.
[0046] Thus, the carbon precursor, i.e., the polymer of the polymerizable
carbon-containing compound, filled in the template is converted
to the carbon structure via thermal decomposition. In other words,
the carbon precursor filled in pores of the template is carbonized
by thermal decomposition. The thermal decomposition may be performed,
for example, by heating the template having the carbon precursor
impregnated therein at a temperature in the range of about 400.degree.
C. to about 1400.degree. C. under a non-oxidizing atmosphere. The
non-oxidizing atmosphere may be selected from among a vacuum, a
nitrogen atmosphere, and inactive gas atmospheres. During this process,
the carbon precursor is carbonized and the carrier and/or acid is
removed via evaporation or decomposition.
[0047] After converting the carbon precursor filled in the template
to the carbon structure, the template which is a silica molecular
sieve may be removed by treating it with a solution capable of selectively
dissolving silica. Examples of the solution capable of selectively
dissolving only silica include an aqueous hydrofluoric acid solution,
an aqueous sodium hydroxide solution, and the like. It is known
that silica may be converted to a soluble silicate by alkaline fusion
or carbonate melting and reacted with HF to form erodible SiF.sub.4.
The template may be treated with the silica-soluble solution several
times depending on the type of silica molecular sieve used as the
template so as to remove the template. Also, ethanol may be further
added to the solution. Due to removal of the template, the carbon
molecular sieve can be separated from the pores of the template.
[0048] The supported catalyst of the present invention may comprise
the mesoporous carbon molecular sieve having mesopores with an average
size of about 250 nm and a maximum particle size of about 500 nm,
micropores with an average size in the range of about 3 nm to about
6 nm, a surface area in the range of about 500 m.sup.2/g to about
2000 m.sup.2/g; and catalytic metal particles dispersed in and supported
on the carbon molecular sieve.
[0049] In the supported catalyst of the present invention, the
mesoporous carbon molecular sieve of the present invention as described
above may be used as a support, and the catalytic metal particles
may be dispersed in pores of the support.
[0050] As described above, when the mesoporous carbon molecular
sieve may be used as the support of the supported catalyst, as the
primary particle size of the mesoporous carbon molecular sieve decreases,
electrochemical activity of the supported catalyst increases. By
reducing the primary particle size of the mesoporous carbon molecular
sieve having regularly arranged micropores and mesopores, the diffusion
of reactants and the removal of products in the supported catalyst
can occur smoothly, thus enabling all of the catalytic metal particles
even in the micropores of the support to participate in the electrochemical
reaction. However, when the primary particle size of the mesoporous
carbon molecular sieve is too small, particles of the supported
catalyst seriously agglomerate, thereby rather decreasing the coefficient
of catalyst utilization. There is an appropriate range for the average
primary particle size of the mesoporous carbon molecular sieve used
as the carrier, and the appropriate range is less than about 500
nm, and more specifically in the range of about 250 nm to about
350 nm, and more specifically about 300 nm. The supported catalyst
of the present invention has improved catalytic activity by using
the support having an average primary particle size within the above
range.
[0051] The catalytic metal that can be used for the supported catalyst
of the present invention is not particularly limited and examples
thereof include titanium (Ti), vanadium (V), chrome (Cr), manganese
(Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn),
aluminium (Al), molybdenum (Mo), selenium (Se), tin (Sn), platinum
(Pt), ruthenium (Ru), palladium (Pd), tungsten (W), iridium (Ir),
osmium (Os), rhodium (Rh), niobium (Nb), tantalum (Ta), lead (Pb),
and a mixture of two or more of the foregoing metals.
[0052] The catalytic metal may be appropriately selected depending
on a specific reaction to which the supported catalyst is to be
applied. Also, the catalytic metal may be a single metal or an alloy
of two or more metals.
[0053] Specifically, when the supported catalyst of the present
invention is used for a catalyst layer of a cathode or an anode
of a fuel cell such as PAFC, PEMFC, or the like, Pt may be generally
used as the catalytic metal. Further, when the supported catalyst
of the present invention is used for a catalyst layer of an anode
of DMFC, a Pt-Ru alloy may be generally used as the catalytic metal.
In this case, the atomic ratio of Pt-Ru may be typically about 0.5:1
to about 2:1. Further, when the supported catalyst of the present
invention is used for a catalyst layer of a cathode of DMFC, Pt
may be generally used as the catalytic metal.
[0054] When the average particle size of the catalytic metal particles
is too small, the catalyst may not catalyse the catalytic reaction.
When the average particle size of the catalytic metal particles
is too large, a reaction surface area is decreased, resulting in
reduced catalytic activity. In view of this, the average particle
size of the catalytic metal particles may be in range of about 1
nm to about 5 nm.
[0055] When the concentration of catalytic metal particles in the
supported catalyst is too low, the catalyst cannot be applied to
a fuel cell, and when the concentration of catalytic metal particles
in the supported catalyst is too high, the catalyst particle size
can increase. In view of this, the concentration of catalytic metal
particles in the supported catalyst may be in the range of about
50% to about 80% by weight based on the total weight of the supported
catalyst.
[0056] To prepare the supported catalyst of the present invention,
various known methods of preparing a supported catalyst can be used.
For example, the supported catalyst of the present invention can
be prepared by impregnating a solution of the catalytic metal precursor
is in the carrier and then by reducing the catalytic metal precursor.
[0057] The fuel cell of the present invention may comprise a cathode,
an anode, and an electrolyte membrane interposed between the cathode
and the anode, and at least one of the cathode and the anode contains
the supported catalyst of the present invention. Examples of the
fuel cell of the present invention include PAFC, PEMFC, or DMFC.
The construction of the fuel cells and the method of manufacturing
such fuel cells, are not particularly limited, and specific examples
thereof are known in various literatures in detail, and thus the
detailed description thereon will not be provided here.
EXAMPLES
Specific Examples 1-2
Preparation of a Solution of Sodium Silicate
[0058] To control the average primary particle size of the mesoporous
silica molecular sieve, sodium silicate solutions with different
Na:Si atomic ratios were prepared. Na:Si atomic ratios of sodium
silicate in the sodium silicate solutions prepared in Examples 1-2
were 1.5:1 and 2.5:1 respectively.
[0059] To prepare sodium silicate solutions of Examples 1-2 the
corresponding amount of NaOH was first dissolved in distilled water,
and then, Ludox HS-40 (Dupont, USA; the content of SiO.sub.2: 40%
by weight) was added thereto while stirring. The, contents of Sio.sub.2
added to each of the solutions were identically 10% by weight. Then,
the mixture was heated at a temperature of about 75.degree. C. for
about 30 minutes to obtain a clear solution. The conditions for
preparing the sodium silicate solutions of Examples 1-2 are summarized
in Table 1 below.
1 TABLE 1 Amount of Na:Si atomic Amount of Amount of Ludox ratio
of water (g) NaOH (g) HS-40 (g) generated used used used sodium
silicate Preparation 413.8 49.9 125 1.5 Example 1 Preparation 406.3
83.2 125 2.5 Example 2
Specific Examples 3-4
Preparation of a Mesoporous Silica Molecular Sieve
[0060] To prepare the mesoporous silica molecular sieves of Examples
3-4 P123 polymer was dissolved in distilled water and sodium silicate
solutions of Examples 1-2 respectively, were added to the resulting
solution while stirring. An acetic acid was added to the mixture
and heated at about 45.degree. C. for about 24 hours to produce
precipitates. The precipitates were filtered and washed with distilled
water and then dried in a vacuum oven at room 5 temperature for
about 24 hours. The dried precipitates were calcined at about 550.degree.
C. for about 10 hours. The conditions for preparing the mesoporous
silica molecular sieves of Examples 3-4 respectively, are summarized
in Table 2 below.
2 TABLE 2 Average primary Sodium silicate solution particle size
Amount Amount Na:Si Amount of resulting of water of P123 Solution
Amount atomic ratio of of acetic silica molecular (g) used (g) used
used (g) used sodium silicate acid (g) used sieve (nm) Specific
59.9 1.643 Preparation 10 1.5 1.498 700 Example 3 Example 1 Specific
59.9 1.643 Preparation 10 2.5 2.496 300 Example 3 Example 2
[0061] FIG. 1 is an XRD analysis result for the mesoporous silica
molecular sieves of Examples 3-4. Referring to FIG. 1 it is apparent
from a peak shown around 20 and the weaker peaks shown thereafter,
that the mesoporous silica molecular sieves of Examples 3-4 were
well-ordered. The particle size of each of the mesoporous silica
molecular sieves of Examples 3-4 was measured from its SEM photograph,
and the result is shown in Table 2.
Specific Examples 5-6
Preparation of a Mesoporous Carbon Molecular Sieve
[0062] The mesoporous carbon molecular sieve of Example 5 was prepared
using the mesoporous silica molecular sieve obtained in Example
3 as a template.
[0063] 0.94 g of sucrose was dissolved in 3.75 g of secondary distilled
water, and 0.11 g of 97% sulphuric acid was added to the aqueous
solution to prepare a polymerizable carbon-containing compound impregnating
solution. The impregnating solution was supported on the mesoporous
silica molecular sieve obtained in Example 3 using an impregnating
method. The impregnated silica molecular sieve was dried at about
100 .degree. C. for about 6 hours and heated at about 160.degree.
C. for 6 hours again. The silica molecular sieve that had undergone
the first impregnation and drying, was impregnated again in the
polymerizable carbon-containing compound impregnating solution,
and then the resultant was dried and heated in the same manner as
above. The amount of the impregnating solution used at the time
of the second impregnation was about 60% of the amount of the impregnating
solution used at the time of the first impregnation. The silica
molecular sieve which had undergone the second impregnation and
drying, was heated under a nitrogen atmosphere at about 200.degree.
C. for about 2 hours and then at about 900.degree. C. for about
2 hours to carbonize the polymer from sucrose in the silica molecular
sieve.
[0064] The silica molecular sieve having the carbonized sucrose
polymer was added to a solution of HF/H.sub.2O/EtOH mixture (10%
by weight of HF, 10% by weight of H.sub.20 and 80% by weight of
EtOH). The mixture was stirred at room temperature for about 2 hours,
and the silica molecular sieve was dissolved, allowing a carbon
structure formed in the silica molecular sieve template to be isolated.
The carbon materials (mesoporous carbon molecular sieve) was filtered
and washed three times with second distilled water. These dissolving-filtering-washing
processes were further performed two times to obtain a final carbon
structure, which was dried at about 100.degree. C. for about 12
hours.
[0065] Mesoporous carbon molecular sieve of Examples 2-6 was prepared
in the same manner as in Example 5 except that the mesoporous silica
molecular sieves of Example 4 were used as the template.
[0066] The physical properties of the mesoporous carbon molecular
sieves of Example 5 and Example 6 are summarized in Table 3 below.
3 TABLE 3 Example 5 Example 6 Size of template (nm) 300 700 used
Physical properties of prepared mesoporous carbon molecular sieve
Average primary 298 552 particle size (nm) Average size of 3.9 3.7
mesopore (nm) Surface area (m.sup.2/g) 1158 1510
[0067] FIG. 2 is a particle size distribution chart of the mesoporous
carbon molecular sieve of Example 2. Referring to FIG. 2 the average
primary particle size of the mesoporous carbon molecular sieve of
Example 5 is 298 nm and the maximum particle size of the mesoporous
carbon molecular sieve of Example 5 is about 500 nm. FIG. 3 is an
SEM photograph of the mesoporous carbon molecular sieve of Example
5. Referring to FIG. 3 the mesoporous carbon molecular sieve powder
according to Example 5 is composed of spherical particles having
a very uniform size.
Specific Examples 7-8
Preparation of a Supported Catalyst
[0068] The supported catalyst of Example 7 is a carbon supported
platinum catalyst using the mesoporous carbon molecular sieve obtained
in Example 5 as a support. The supported catalyst was prepared as
follows.
[0069] 0.5 g of the mesoporous carbon molecular sieve was mixed
with a solution of 0.9616 g of H.sub.2PtCl.sub.6 in 1.5 ml of acetone
and the mixture was dried in the air for about 4 hours. The mesoporous
carbon molecular sieve having an impregnated platinum precursor
was placed in a crucible and dried at about 60.degree. C. for more
than about 12 hours. After placing the crucible in an electric furnace,
nitrogen gas was supplied to the furnace for about 10 minutes, and
then the atmosphere of the electric furnace was switched to hydrogen
gas, thereby reducing the platinum precursor. This heating process
was performed once again, and the carbon supported platinum catalyst
with platinum loading amount of about 60% by weight was prepared.
[0070] The carbon supported platinum catalysts of Example 8 was
prepared in the same manner as in Example 7 except that the mesoporous
carbon molecular sieve of Example 6 was used as carriers.
Specific Examples 9-10
Manufacturing of a Fuel Cell
[0071] The fuel cell of Example 9 included a cathode containing
the supported catalyst of Example 7 an anode containing an PtRu
black catalyst, and a Nafion 117 electrolyte membrane. The loading
of platinum in the cathode was 3 mg/cm.sup.2 and the loading of
PtRu in the anode was 8 mg/cm.sup.2.
[0072] The fuel cell of Example 10 was manufactured in the same
manner as in Example 9 except that the carbon supported platinum
catalyst of Example 8 was used in the cathode.
[0073] The fuel cell of Example 11 was manufactured in the same
manner as in Example 10 except that the conventional carbon black
supported platinum catalyst was used in the cathode.
[0074] Evaluation of Performance of the Fuel Cell
[0075] The performance of each of the fuel cells of Example 9 and
Examples 10 and 11 was measured under the following conditions.
A 2M aqueous methanol solution was used as a fuel, and air was used
as an oxidizing agent. The working temperature of the fuel cells
was 40.degree. C. The evaluation results of the performance are
summarized in Table 4 below.
4 TABLE 4 Example 11 Example 9 Example 10 (Pt/black) Current density
42 81 60 (mA/cm.sup.2) @ 0.4 V, 40.degree. C. Particle size of Pt
2.8 3.1 8.0 catalyst (nm)
[0076] As is apparent from Table 4 the fuel cell of Example 10
has much higher current density generated at 0.4 V of potential
than the fuel cells of Examples 10 and 11. In other words, the mesoporous
carbon molecular sieve of the present invention can greatly improve
the electrochemical activity of the fuel cell.
[0077] However, comparing the fuel cells of Examples 10 and 11
it can be seen that the performance of a fuel cell is not necessarily
improved by using the mesoporous carbon molecular sieve. Since the
catalyst particle size of Example 8 is much less than that of Pt
Black catalyst, the fuel cell of Example 10 using the catalyst of
Example 8 must have higher performance than the fuel cell of Example
11 using the Pt Black catalyst. However, the performance of the
fuel cell of Example 11 is indeed higher than the fuel cell of Example
10. This is because the average primary particle size of the support
of 550 nm is too large to utilize all catalyst particles dispersed
in the support. Moreover, the catalyst of Example 8 prepared using
the mesoporous carbon molecular sieve of Example 5 with the particle
size distribution illustrated in FIG. 2 as a support has the primary
particle size of the support sufficient to utilize catalyst particles.
Thus, the fuel cell of Example 10 using the catalyst of Example
8 has the improved performance.
[0078] When the mesoporous carbon molecular sieve of the present
invention having an average primary particle size of 500 nm and
less is used as a carrier of a supported catalyst, the diffusion
of reactants and the removal of products in the supported catalyst
occurs the support can participate in the electrochemical reaction.
Accordingly, the supported catalyst of the present invention can
display improved catalytic activity by using the above support.
Also, the fuel cell using the supported catalyst of the present
invention can display improved electrochemical activity. |