Molecular sieve abstract
A method for manufacturing a carbon molecular sieve with increased
microporosity; a method for manufacturing a carbon molecular sieve
with increased microporosity and improved structural regularity;
a carbon molecular sieve with increased microporosity; a carbon
molecular sieve with increased microporosity and improved structural
regularity; a catalyst for a fuel cell using the carbon molecular
sieve; and a fuel cell using the catalyst are provided.
Molecular sieve claims
What is claimed is:
1. A method for manufacturing a carbon molecular sieve, comprising:
(a) impregnating pores of a mesoporous silica molecular sieve, used
as a template, with a mixture of a silica oligomer, a condensable
or polymerizable carbon-containing compound, used as a carbon precursor,
and a liquid carrier; (b) polymerizing the carbon precursor to form
a carbon precursor polymer within the pores of the template; (c)
carbonizing the carbon precursor polymer using pyrolysis; and (d)
removing the template and the silica oligomer using a solution capable
of dissolving silica.
2. The method according to claim 1 wherein the template is selected
from the group consisting of MCM48 KIT-1 MSU-1 SBA-1 SBA-3
SBA-15 and SBA-16.
3. The method according to claim 1 wherein an average particle
size of the silica oligomer is 0.5 to 5 nm.
4. The method according to claim 1 wherein the carbon precursor
is a carbohydrate.
5. The method according to claim 4 wherein the carbohydrate is
selected from the group consisting of monosaccharide, oligosaccharide,
and a mixture thereof.
6. The method according to claim 1 wherein the carrier is selected
from the group consisting of water, an organic solvent, and a mixture
thereof.
7. The method according to claim 6 wherein the organic solvent
is alcohols.
8. The method according to claim 7 wherein the alcohol is ethanol.
9. The method according to claim 1 wherein the mixture further
comprises an acid.
10. The method according to claim 9 wherein the acid is selected
from the group consisting of sulfuric acid, hydrochloric acid, nitric
acid, sulfonic acid, and a mixture thereof.
11. The method according to claim 1 wherein step (b) comprises
heating the template at a temperature range of 50 to 250.
12. The method according to claim 1 wherein step (b) comprises:
(b-1) first heating the template at a temperature range of 50 to
150; and (b-2) second heating the template at a temperature range
of 150 to 250.
13. The method according to claim 1 wherein step (c) comprises
heating the template at 400 to 1400 under a non-oxidizing atmosphere.
14. The method according to claim 13 wherein the non-oxidizing
atmosphere is selected from the group consisting of a vacuum atmosphere,
a nitrogen atmosphere, and an inert gas atmosphere.
15. The method according to claim 1 wherein the silica dissolving
solution in step (d) is an aqueous fluoric acid solution or an aqueous
sodium hydroxide solution.
16. The method according to claim 1 further comprising once or
more repeating steps (a) and (b) before step (c).
17. A method for manufacturing a carbon molecular sieve, the method
comprising: (a) impregnating micropores of an ordered mesoporous
silica molecular sieve, used as a template, having the mesopores
and the micropores that are responsible for the connections between
the mesopores, with a first mixture of a condensable or polymerizable
carbon-containing compound, used as a carbon precursor, and a liquid
carrier; (b) polymerising the carbon precursor within the micropores
of the template to form a carbon precursor polymer within the micropores
of the template; (c) impregnating the mesopores of the template
with a second mixture of a silica oligomer, a condensable or polymerizable
carbon-containing compound, used as a carbon precursor, and a liquid
carrier; (d) polymerising the carbon precursor within the mesopores
of the template to form a carbon precursor polymer within the mesopores
of the template; (e) carbonizing the carbon precursor polymers within
the template using pyrolysis; and (f) removing the template and
the silica oligomer using a solution capable of dissolving silica.
18. The method according to claim 17 wherein the template is selected
from the group consisting of SBA-15 and SBA-16.
19. The method according to claim 17 wherein an average particle
size of the silica oligomer in the second mixture is 0.5 to 5 nm.
20. The method according to claim 17 wherein the carbon precursors
in the first and the second mixtures are a carbohydrate.
21. The method according to claim 20 wherein the carbohydrate
is selected from the group consisting of monosaccharide, oligosaccharide,
and a mixture thereof.
22. The method according to claim 17 wherein the liquid carriers
in the first and the second mixtures are each selected from the
group consisting of water, an organic solvent, and a mixture thereof.
23. The method according to claim 22 wherein the organic solvent
is alcohols.
24. The method according to claim 23 wherein the alcohol is ethanol.
25. The method according to claim 17 wherein the first and the
second mixtures further comprise an acid.
26. The method according to claim 25 wherein the acid is selected
from the group consisting of sulfuric acid, hydrochloric acid, nitric
acid, sulfonic acid, and a mixture thereof.
27. The method according to claim 17 wherein step (b) comprises
heating the template at a temperature range of 50 to 250.
28. The method according to claim 17 wherein step (b) comprises:
(b-1) first heating the template at a temperature range of 50 to
150; and (b-2) second heating the template at a temperature range
of 150 to 250.
29. The method according to claim 17 wherein step (d) comprises
heating the template at a temperature range of 50 to 250.
30. The method according to claim 17 wherein step (d) comprises:
(d-1) first heating the template at a temperature range of 50 to
150; and (d-2) second heating the template at a temperature range
of 150 to 250.
31. The method according to claim 17 wherein step (e) comprises
heating the template at 400 to 1400 under a non-oxidizing atmosphere.
32. The method according to claim 31 wherein the non-oxidizing
atmosphere is selected from the group consisting of a vacuum atmosphere,
a nitrogen atmosphere, and an inert gas atmosphere.
33. The method according to claim 17 wherein the silica dissolving
solution in step (f) is an aqueous fluoric acid solution or an aqueous
sodium hydroxide solution.
34. The method according to claim 17 further comprising once or
more repeating steps (c) and (d) before step (e).
Molecular sieve description
BACKGROUND OF THE INVENTION
This application claims the priority of Korean Patent Application
No: 2003-9097 filed on Feb. 13 2003 in the Korean Intellectual
Property Office, the disclosure of which is incorporated herein
in its entirety by reference.
1. Field of the Invention
The present invention relates to a molecular sieve and more particularly,
to a carbon molecular sieve. The present invention also relates
to a fuel cell and more particularly, to a catalyst and a catalyst
support for a fuel cell.
2. Description of the Related Art
Originally, the term "molecular sieve" was the trade
name of a synthetic zeolite commercially manufactured by Linde Ltd.
(USA). A molecular sieve has excellent adsorption capacity due to
its many fine pores with uniform diameter. As derived from its literal
meaning, a molecular sieve is used to selectively sieve molecules.
Presently, the term "molecular sieve" is an generic name
in the field of science technology and refers to a porous material
in which uniform-sized pores are arranged in a three dimensional
manner.
A molecular sieve selectively adsorbs a certain size of molecules
due to its uniform pore size. Therefore, a molecular sieve can be
widely used as a catalyst, a catalyst support, or an adsorbent.
A carbon molecular sieve is mainly made of a carbon material. A
carbon molecular sieve has many advantages, such as excellent thermal
stability, hydrothermal stability, chemical resistance, and lipophilicity,
when compared to a metal oxide based molecular sieve such as zeolite.
A carbon molecular sieve can also be used for various purposes,
such as a catalyst support, an adsorbent, a sensor, and an electrode
material.
As one example of a method for synthesis of a carbon molecular
sieve, there is disclosed a pyrolysis process of a natural vegetable
material, such as coconut, or a synthetic polymer. According to
this method, however, pore size and porosity can only be limitedly
increased by pyrolysis temperature adjustment and post-treatment
with oxygen.
Another example of a method for synthesis of a carbon molecular
sieve is disclosed in Korean Patent Application Laid-Open Publication
Nos. 2001-1127 and 2002-84372. According to these methods, a mesoporous
silica molecular sieve is used as a template. A carbohydrate is
subjected to adsorption into the template, polymerization and pyrolysis.
The template is then removed to thereby produce a carbon molecular
sieve with a structural regularity of uniform-sized pores. However,
a disadvantage exists in that a silica molecular sieve used as a
template must be newly designed to adjust the surface area of the
carbon molecular sieve and the volume ratio of its micropores and
mesopores. In addition, it is difficult to control the volume ratio
between micropores and mesopores.
Fuel cells are clean energy sources capable of reducing dependence
on fossil energy, with a high output density and high energy conversion
efficiency. In addition, fuel cells can be operated at room temperature
and can be miniaturized and packed. Therefore, fuel cells can be
widely used in the fields of zero emission vehicles, domestic power
systems, mobile communication equipment, medical instruments, military
equipment aerospace equipment, and portable electronic devices.
A polymer electrolyte membrane fuel cell (PEMFC) and a direct methanol
fuel cell (DMFC) are electric power generating systems that allow
electrochemical reaction of hydrogen or methanol, water, and oxygen
to produce direct current electricity. These fuel cells comprise
an anode and a cathode which receive a liquid/gaseous reactant and
a proton conducting membrane interposed between the two electrodes.
At the anode, an anode catalyst dissociates hydrogen or methanol
to generate protons. The generated protons are transported through
the proton conducting membrane to the cathode. At the cathode, the
protons react with oxygen by the cathode catalyst. Therefore, in
such structured fuel cells, the role of a catalyst is very important.
Currently, in a PEMFC, platinum (Pt) particles supported on a carbon
support are used as both anode and cathode catalysts. In DMFCs,
platinum-ruthenium (Pt--Ru) black is used as an anode catalyst and
Pt particles by themselves or Pt particles supported on a carbon
support are used as a cathode catalyst. Because metal black by themselves
provide excellent catalytic activity, a supported metal catalyst
system is rarely used in a DMFC. However, because a large portion
of costs incurred in a DMFC is caused by a catalyst, in considering
cost effectiveness, the amount of a used catalyst needs to be decreased.
Therefore, many efforts have been made to research a carbon support
capable of providing improved catalyst activity and dispersion over
a currently used carbon support with no structural regularity, in
order to reduce the catalyst amount used in an anode and a cathode.
SUMMARY OF THE INVENTION
The present invention provides a method for manufacturing a carbon
molecular sieve with increased microporosity.
The present invention also provides a method for manufacturing
a carbon molecular sieve with increased microporosity and improved
structural regularity.
The present invention also provides a carbon molecular sieve with
increased microporosity.
The present invention also provides a carbon molecular sieve with
increased microporosity and improved structural regularity.
The present invention also provides a catalyst for a fuel cell
containing the aforementioned carbon molecular sieve.
The present invention also provides a fuel cell employing the aforementioned
catalyst.
According to an aspect of the present Invention, there is provided
a method for manufacturing a carbon molecular sieve, comprising
the steps of (a) impregnating pores of a mesoporous silica molecular
sieve, used as a template, with a mixture of a silica oligomer,
a condensable or polymerizable carbon-containing compound, used
as a carbon precursor, and a liquid carrier; (b) polymerizing the
carbon precursor to form a carbon precursor polymer within the pores
of the template; (c) carbonizing the carbon precursor polymer using
pyrolysis; and (d) removing the template and the silica oligomer
using a solution capable of dissolving silica selectively.
According to another aspect of the present invention, there is
provided a method for manufacturing a carbon molecular sieve, comprising
the steps of (a) impregnating micropores of an ordered mesoporous
silica molecular sieve, used as a template, having the mesopores
and the micropores that are responsible for the connections between
the mesopores, with a first mixture comprising a condensable or
polymerizable carbon-containing compound, used as a carbon precursor,
and a liquid carrier; (b) polymerizing the carbon precursor within
the micropores of the template to form a carbon precursor polymer
within the micropores of the template; (c) impregnating the mesopores
of the template with a second mixture comprising a silica oligomer,
a condensable or polymerizable carbon-containing compound, used
as a carbon precursor, and a liquid carrier; (d) polymerizing the
carbon precursor within the mesopores of the template to form a
carbon precursor polymer within the mesopores of the template; (e)
carbonizing the carbon precursor polymers within the template using
pyrolysis; and (f) removing the template and the silica oligomer
using a solution capable of dissolving silica selectvely.
According to another aspect of the present invention, there is
provided a carbon molecular sieve having mesopores and micropores.
The total volume of pores, with a size of 80 nm or less, is 1.0
cm.sup.3 /g or more and microporosity is 35% or more.
According to another aspect of the present invention, there is
provided a catalyst for a fuel cell comprising a porous catalyst
support and catalytic metals dispersed on the catalyst support,
wherein the catalyst support is the aforementioned carbon molecular
sieve.
According to yet another aspect of the present invention, there
is provided a fuel cell using the aforementioned catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and advantages of the present invention
will become more apparent by describing in detail exemplary embodiments
thereof with reference to the attached drawings in which:
FIG. 1A is an X-ray diffraction (XRD) pattern of a carbon molecular
sieve according to an embodiment of the present invention;
FIG. 1B is a nitrogen adsorption-desorption curve of a carbon molecular
sieve according to an embodiment of the present invention;
FIG. 2A is an XRD pattern of a carbon molecular sieve according
to another embodiment of the present invention; and
FIG. 2B is a nitrogen adsorption-desorption curve of a carbon molecular
sieve according to another embodiment of the present invention;
DETAILED DESCRIPTION OF THE INVENTION
A method for manufacturing a carbon molecular sieve according to
a first aspect of the present invention comprises the steps of (a)
impregnating pores of a mesoporous silica molecular sieve, used
as a template, with a mixture of a silica oligomer, a condensable
or polymerizable carbon-containing compound, used as a carbon precursor,
and a liquid carrier; (b) polymerizing the carbon precursor to form
a carbon precursor polymer within -the pores of the template; (c)
carbonizing the carbon precursor polymer using pyrolysis; and (d)
removing the template and the silica oligomer using a solution capable
of dissolving silica selectively.
In this method, when the mixture containing the silica oligomer
and the carbon precursor undergoes polymerization and pyrolysis,
the mesopores of the template are filled with solid carbon that
includes the silica oligomer. In step (d), the template and the
silica oligomer are removed altogether. Removing the silica oligomer
creates micropores. In addition to the micropores created by removing
the silica oligomer, many other micropores are present in the solid.
The microporosity and micropore size of a carbon molecular sieve
can be adjusted by controlling the particle size and content of
the silica oligomer.
As used herein, the term "mesopore" refers to a pore
with a size of about 1 to about 80 nm and the term "micropore"
refers to a pore with a size of about 1 nm or less.
Hereinafter, the method for manufacturing a carbon molecular sieve
according to a first aspect of the present invention will be described
in more detail.
Examples of the mesoporous silica molecular sieve used as a template
include a cubic MCM-48 (la3d), another cubic SBA-1 (Pn3m), SBA-16
(Im3m), KIT-1 or MSU-1 having a three-dimensional disordered pore
network, and SBA-15 of which one-dimensional mesopores are interconnected
through micropores.
MCM-48 may be prepared using a method introduced by Kim et al.
[Chem. Commun., 1998 259]. SBA-1 may be prepared using a method
introduced by Kim and Ryoo [Chem. Mater., 1999 11 487]. SBA-15
and SBA-16 may be prepared using a method introduced by Kim and
Stucky [Chem. Commun., 20001159].
The silica oligomer may be obtained by hydrolysis of tetraalkylorthosilicate.
More particulary, the silica oligomer may be prepared by reacting
tetraethylorthosilicate (TEOS) with an aqueous solution of hydrochloric
acid. There are no particular limitations to an average particle
size of the silica oligomer. The silica oligomer can have different
average particle sizes depending on the degree of polymerization.
After the removal of the silica oligomer, micropores with the same
size as the average particle size of the silica oligomer are formed
on the carbon molecular sieve. For example, a silica oligomer with
an average particle size of about 0.5 to about 5 nm may be used.
In this case, a carbon molecular sieve having the increased porosity
of pores with a size of about 0.5 to 5 nm can be obtained. In order
to increase the porosity of micropores with a size of about 1 nm
or less on a carbon molecular sieve, it is preferable to limit the
average particle size of the silica oligomer to about 1 nm or less.
The carbon precursor is a condensable or polymerizable carbon-containing
compound. Examples of the carbon precursor include a carbohydrate
and a monomer.
A carbohydrate is classified into monosaccharide, oligosaccharide,
and polysaccharide. Preferably, monosaccharide, oligosaccharide,
or a mixture thereof is used. Representative examples of monosaccharide
include glucose, fructose, mannose, galactose, ribose, and xylose.
They may be used alone or in combination. Oligosaccharide contains
two or more monosaccharide units joined by a glycoside bond. The
term, "oligosaccharide" is a general name that can describe
disaccharide through decasaccharide. Oligosaccharide may be classified
into a simple oligosaccharide containing the same monosaccharides
and a complex oligosaccharide containing two or more different monosaccharides.
Oligosacchaddes which exist in the free state in nature are mainly
disaccharides. Exemplary disaccharides are sucrose, which is contained
in sugar canes; maltose, which is a product obtained by amylase
hydrolysis of starch and is a raw material of wheat-gluten; and
lactose, which is found in mammal's milk.
A reducing group and a hydroxyl group on a carbohydrate or a hydroxyl
group on a compound except for a carbohydrate may undergo a dehydrocondensation
process.
A representative example of the monomer used as the carbon precursor
is furfuryl alcohol.
The liquid carrier acts as a dispersing medium for the silica oligomer,
a solvent for dissolving the carbon precursor, and a transport medium
for transporting the carbon precursor and the silica oligomer into
the pores of the template. The liquid carrier may be selected from
water, an organic solvent, and a mixture thereof. Preferably, the
organic solvent is alcohols. More preferably, the alcohol is ethanol.
Furfuryl alcohol may be used as both the carbon precursor and the
liquid carrier. Furfuryl alcohol used as the carbon precursor, may
act as the liquid carrier by itself.
The mixture may further comprise an acid. The acid catalyzes the
condensation or polymerization of the carbon precursor. The acid
may be selected from sulfuric acid, hydrochloric acid, nitric acid,
sulfonic acid, a derivative thereof, and a mixture thereof. Representative
sulfonic acid is methylsulfonic acid.
There are no particular limitations to the content of each component
in the mixture, as long as the objects of the present invention
are accomplished. However, it is preferable that the content of
each component in the mixture is as follows.
If the content of the silica oligomer is too low, microporosity
is slightly increased. On the other hand, if it is too high, the
structure of the resultant carbon molecular sieve may be destroyed.
Therefore, it is preferable that the content of the silica oligomer
is within a range of about 1 to about 40 parts by weight based on
100 parts by weight of the carbon precursor.
If the content of the liquid carrier is too low, it is difficult
to impregnate the template with the mixture. On the other hand,
if it is too high, the filling density of carbonaceous components
in the template may be significantly reduced. For this reason, disadvantageously,
steps (a) and (b) must be overly repeated. Therefore, it is preferable
that the content of the carrier is within a range of about 300 to
about 1000 parts by weight based on 100 parts by weight of the
carbon precursor.
If the content of the acid is too low, the facilitation effect
of the condensation or polymerization of the carbon precursor by
the acid addition may be slight. On the other hand, because the
facilitation effect is not continuously increased in proportion
to the added amount of the acid, excess addition of the acid is
also not preferable. Therefore, it is preferable that the content
of the acid is within a range of about 1 to about 30 parts by weight
based on 100 parts by weight of the carbon precursor.
In step (a), the pores of the template, i.e., the pores of the
mesoporous silica molecular sieve are filled with the mixture of
the silica oligomer, the condensable or polymerizable carbon-containing
compound used as a carbon precursor, and the liquid carrier. Various
impregnation processes may be used. For example, the mixture and
the template can be simply mixed to initiate a contact with each
other. Consequently, through such contact, the template is impregnated
with the mixture.
In step (b), the carbon precursor, filled in the template, undergoes
polymerization. After the polymerization, the carbon precursor polymer
is formed within the pores of the template. The polymerization of
the carbon precursor may be accomplished by heating. If the heating
temperature is too low, the polymerization is slightly carried out.
While, if it is too high, the uniformity of the obtained carbon
molecular sieve may be decreased. Therefore, it is preferable that
the heating temperature for the polymerization is within a range
of about 50 to about 250.degree. C. The heating may be divided into
two steps, i.e., first heating and second heating. For example,
the first heating may be carried out at a temperature range of about
50 to about 150.degree. C. and the second heating at a temperature
range of about 150 to about 250.degree. C. During the heating, the
carbon precursor undergoes polymerization. At the same time, the
liquid carrier may be dried.
In step (c), the carbon precursor polymer within the pores of the
template is carbonized by pyrolysis. For this, the template containing
the carbon precursor polymer may be subjected to heating at about
400 to about 1400.degree. C. under a non-oxidizing atmosphere.
The non-oxidizing atmosphere may be selected from a vacuum atmosphere,
a nitrogen atmosphere, and an inert gas atmosphere. During the carbonizing
process, the liquid carrier and/or acid are removed by evaporation
or decomposition.
In step (d), the template and the silica oligomer are removed using
a solution capable of dissolving silica. Examples of the solution
include an aqueous fluoric acid solution and an aqueous sodium hydroxide
solution. The silica is converted to soluble silicate by alkaline
melting or carbonate fusion, or the silica reacts with fluoric acid
(HF) to form very erosive SiF.sub.4. According to the type of the
silica molecular sieve used as the template, the template and the
silica oligomer may be treated several times with the silica dissolving
solution, or may be treated with a silica dissolving solution which
further comprises ethanol. When the template is removed, a carbon
molecular sieve is obtained. Furthermore, the silica oligomer contained
in the carbon molecular sieve is also removed, thus the microporosity
of the carbon molecular sieve is increased.
Before step (c), steps (a) and (b) may be repeated once or more.
Thus, a larger amount of the carbon precursor polymers may be formed
in the pores of the template.
Hereinafter, the method for manufacturing a carbon molecular sieve
according to a second aspect of the present invention will be described.
The method for manufacturing a carbon molecular sieve according
to the second aspect of the present invention comprises the steps
of (a) impregnating micropores of an ordered mesoporous silica molecular
sieve, used as a template, having the mesopores and the micropores
that provide the connections between the mesopores, with a first
mixture of a condensable or polymerizable carbon-containing compound,
used as a carbon precursor, and a liquid carrier, (b) polymerizing
the carbon precursor within the micropores of the template to form
a carbon precursor polymer within the micropores of the template;
(c) impregnating the mesopores of the template with a second mixture
of a silica oligomer, a condensable or polymerizable carbon-containing
compound, used as a carbon precursor, and a liquid carrier; (d)
polymerizing the carbon precursor within the mesopores of the template
to form a carbon precursor polymer within the mesopores of the template;
(e) carbonizing the carbon precursor polymers within the template
using pyrolysis; and (f) removing the template and the silica oligomer
using a solution capable of dissolving silica selectively.
This method is characterized in that an ordered mesoporous silica
molecular sieve having mesopores and micropores in which the micropores
provide connections between the mesopores, is used as a template
and the micropores and mesopores of the template are filled with
different mixtures. That is, the silica oligomer is contained in
the mesopores but not in the micropores of the template.
A carbon molecular sieve manufactured according to the second aspect
of the present invention comprises carbon meso-rods and carbon micro-rods.
The carbon micro-rods provide connections between the carbon meso-rods.
Each carbon meso-rod has a diameter corresponding to the size of
each mesopore of the template and each carbon micro-rod has a diameter
corresponding to the size of each micropore of the template. Due
to the absence of the silica oligomer, the microporous carbon rods
have strength sufficient to maintain the structural regularity of
the carbon molecular sieve. On the other hand, the carbon meso-rods
have increased microporosity due to the spaces created by the removal
of the silica oligomer.
According to the second aspect of the present invention, the ordered
mesoporous silica molecular sieve having mesopores and micropores
in which the micropores provide connections between the mesopores
is used as a template. Illustrative examples are SBA-15 and SBA-16.
An average particle size of the silica oligomer may be about 0.5
to about 5 nm. Accordingly, an obtained carbon molecular sieve contains
mesoporous carbon rods with the increased ratio of pores with a
size of about 0.5 to about 5 nm. In order to increase micropores
with a size of about 1 nm or less in a carbon molecular sieve, it
is preferable that the average particle size of the silica oligomer
is about 1 nm or less.
The carbon precursors and carriers, as used in the first and the
second mixtures, may be the same as mentioned above.
The first and the second mixtures may further comprise an acid.
The acid may be selected from sulfuric acid, hydrochloric acid,
nitric acid, sulfonic acid, a derivative thereof, and a mixture
thereof.
There are no particular limitations to the content of each component
in the first mixture provided that each component can accomplish
the objects of the present invention. However, it is preferable
that the content of each component in the first mixture is as follows.
If the content of the liquid carrier in the first mixture is too
low, it is difficult to impregnate the template with the first mixture.
On the other hand, if it is too high, the filling density of carbonaceous
components in the template may be significantly reduced, and thus
disadvantageously, impregnating and polymerizing must be repeated
several times. Therefore, it is preferable that the content of the
carrier in the first mixture is within a range of about 300 to about
1000 parts by weight based on 100 parts by weight of the carbon
precursor in the first mixture.
If the content of the acid in the first mixture is too low, the
facilitation effect of the condensation or polymerization of the
carbon precursor by the acid addition may be slight. On the other
hand, because the facilitation effect is not continuously increased
in proportion to the added amount of the acid, excess addition of
the acid is also not preferable. Therefore, it is preferable that
the content of the acid in the first mixture is within a range of
about 1 to about 30 parts by weight based on 100 parts by weight
of the carbon precursor in the first mixture.
There are no particular limitations to the content of each component
in the second mixture as long as the objects of the present invention
are accomplished. However, it is preferable that the content of
each component in the second mixture is as follows.
If the content of the silica oligomer in the second mixture is
too low, microporosity is barely increased. On the other hand, if
it is too high, the structure of the obtained carbon molecular sieve
may be destroyed. Therefore, it is preferable that the content of
the silica oligomer in the second mixture is within a range of about
1 to about 40 parts by weight based on 100 parts by weight of the
carbon precursor in the second mixture.
If the content of the liquid carrier in the second mixture is too
low, it is difficult to impregnate the template with the second
mixture. On the other hand, if it is too high, the filling density
of carbonaceous components in the template may be significantly
reduced, thus disadvantageously, impregnating and polymerizing must
be repeated several times. Therefore, it is preferable that the
content of the carrier in the second mixture is within a range of
about 300 to about 1000 parts by weight based on 100 parts by weight
of the carbon precursor in the second mixture.
If the content of the acid in the second mixture is too low, the
facilitation effect of the condensation or polymerization of the
carbon precursor by the acid addition may be slight. On the other
hand, because the facilitation effect is not continuously increased
in proportion to the added amount of the acid, excess addition of
the acid is also not preferable. Therefore, it is preferable that
the content of the acid in the second mixture is within a range
of about 1 to about 30 parts by weight based on 100 parts by weight
of the carbon precursor in the second mixture.
In step (a), the micropores of the template are filled with the
first mixture. Various impregnation processes may be used. For example,
the first mixture and the template can be simply mixed to initiate
a contact with each other. Through a capillary phenomenon caused
by such contact, the template is impregnated with the first mixture.
In order to selectively fill the micropores of the template with
the first mixture, it is preferable to allow the volume of the first
mixture to approximate to that of the micropores of the template.
For example, the content of the carbon precursor in the first mixture
may be about 10 to about 50 parts by weight based on 100 parts by
weight of the carbon precursor in the second mixture.
In step (b), the first mixture-containing template may be heated
at a temperature range of about 50 to about 250.degree. C. The heating
may comprise (b-1) first heating the first mixture-containing template
at a temperature range of 50 to 150.degree. C. and (b-2) second
heating the previously heated template at a temperature range of
150 to 250.degree. C.
In step (d), the second mixture-containing template may be heated
at a temperature range of about 50 to about 250.degree. C. The heating
may comprise (d-1) first heating the second mixture-containing template
at a temperature range of 50 to 150.degree. C. and (d-2) second
heating the previously heated template at a temperature range of
150 to 250.degree. C.
In step (e), the template containing the carbon precursor polymer
may be subjected to heating at about 400 to about 1400.degree.
C. under a non-oxidizing atmosphere. The non-oxidizing atmosphere
may be selected from a vacuum atmosphere, a nitrogen atmosphere,
and an inert gas atmosphere.
The silica dissolving solution in step (f) may be an aqueous fluoric
acid or an aqueous sodium hydroxide solution.
Before step (e), steps (c) and (d) may be repeated once or more.
Other detailed descriptions of the second aspect of the present
invention are as described in the first aspect of the present invention.
As used herein, the term "microporosity" refers to the
volume percentage of micropores with a size of 1 nm or less to the
total volume of pores with a size of 80 nm or less. In carbon molecular
sieves which are manufactured according to the embodiments of the
present invention and comprise mesopores and micropores, the total
volume of pores with a size of 80 nm or less is about 1.0 cm.sup.3
/g or more, and the microporosity is about 35% or more.
The method according to the second aspect of the present invention
provides an ordered carbon molecular sieve containing carbon meso-rods
and carbon micro-rods. The carbon micrrods provide connections between
the carbon meso-rods. The carbon meso-rods have a diameter corresponding
to the size of the mesopores of the template. The carbon micro-rods
have a diameter corresponding to the size of the micropores of the
template. The carbon meso-rods form the internal structure of the
carbon molecular sieve while in a state of being supported by the
carbon micro-rods. The spaces between the carbon meso-rods provide
substantially uniform sized mesopores. The carbon meso-rods have
increased microporosity due to the removal of the silica oligomer.
The carbon molecular sieve of the present invention may be used
for various purposes, including a catalyst carrier, an adsorbent,
a sensor, and an electrode material.
A carbon molecular sieve of the present invention may be used in
a catalyst for a fuel cell. Generally, a catalyst for a fuel cell
may comprise a porous catalyst support and catalytic metals positioned
on pores of the catalyst carrier. According to the present invention,
a catalyst for a fuel cell uses the aforementioned carbon molecular
sieve as the catalyst support. The catalytic metals may be titanium
(Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt
(Co), nickel (Ni), copper (Cu), zinc (Zn), aluminum (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), or a mixture thereof. The catalyst
for a fuel cell of the present invention may be prepared, for example,
using a method comprising impregnating a porous catalyst support
with a catalytic metal precursor solution and reducing the catalytic
metal precursor. Such a method is disclosed in various documents
and thus the detailed description thereof is omitted.
The catalyst of the present invention may be used in, for example,
a phosphoric acid fuel cell, a polymer electrolyte membrane fuel
cell, and a direct methanol fuel cell. There are no particular limitations
to the structures and the preparation methods of these fuel cells.
Illustrative examples are disclosed in various documents and thus
the detailed descriptions thereof are omitted.
Hereinafter, the present invention will be described in more detail
with reference to examples but is not limited thereto.
EXAMPLE 1
First Aspect of the Present Invention
0.94 g of sucrose, 3.75 g of distilled water, and 0.11 g of a 97%
by weight aqueous solution of sulfuric acid were mixed to prepare
a solution A. 0.63 g of TEOS, 0.63 g of ethanol, and 0.21 g of a
10.sup.-3 M aqueous solution of hydrochloric acid were mixed and
stirred at 40.degree. C. for one hour to prepare a solution B containing
a silica oligomer.
1.0 g of calcined SBA-15A was impregnated with a mixture of the
solution A and the solution B.
The SBA-15 so impregnated was dried at 100.degree. C. for 6 hours
and again heated at 160.degree. C. for 6 hours. The heated SBA-15
was once again subjected to the above processes except that the
amount of each of the solution A and the solution B was reduced
to 60%.
The obtained SBA-15 was heated at 900.degree. C. under a nitrogen
atmosphere for two hours.
Then, the SBA-15 was added into a mixed HF-H.sub.2 O-EtOH solution
(10% by weight of HF, 10% by weight of H.sub.2 O, 80% by weight
of EtOH) and stirred at room temperature for two hours. The resultant
was filtered and washed with deionized secondary distilled water
five times. The stirring, filtering, and washing were repeated twice.
The carbon molecular sieve that was finally obtained, was dried
at 100.degree. C. for 12 hours.
An X-ray diffraction (XRD) pattern and a nitrogen adsorption-desorption
curve of the carbon molecular sieve of Example 1 are shown in FIG.
1A and FIG. 1B, respectively. In addition, the porosity of the carbon
molecular sieve of Example 1 was measured and the result is presented
in Table 1.
TABLE 1 Total pore Mesopore Micropore Surface volume volume volume
area (<80 nm) (1-80 nm) (<1 nm) Micro- Sample (m.sup.3 /g)
(cm.sup.3 /g) (cm.sup.3 /g) (cm.sup.3 /g) porosity CMK-3 1480 1.44
0.99 0.45 0.31 Example 1 1950 1.67 1.05 0.62 0.37 Increased 32%
16% 6% 38% 20% effect
The result of porosity of CMK-3 a carbon molecular sieve of Korean
Patent Application Laid-Open Publication No. 2002-84372 is also
presented in Table 1. CMK-3 is a carbon molecular sieve using sucrose
as a carbon precursor and SBA-15 as a template. A silica oligomer
was not used in CMK-3 manufacturing as disclosed in the cited reference.
A surface area, a total volume of pores with a size of 80 nm or
less, a volume of mesopores with a size of 1 to 80 nm , a volume
of micropores with a size of 1 nm or less, and a microporosity,
i.e., the ratio of the micropore volume to the total pore volume,
of each carbon molecular sieve are listed in Table 1.
In the carbon molecular sieve of Example 1 a surface area was
increased by about 32%, a total pore volume by about 16%, a mesopore
volume by about 6%, a micropore volume by about 38%, and microporsosity
by about 20%, when compared to CMK-3.
It can be seen from the results that the use of a silica oligomer
can increase the porosity or microporosity of the carbon molecular
sieve, and the microporosity of the carbon molecular sieve can be
adjusted by controlling the amount or size of a silica oligomer.
EXAMPLE 2
Second Embodiment of the Present Invention
1.0 g of calcined SBA-15 was impregnated with a first mixture of
0.125 g of sucrose, 0.014 g of a 97% by weight aqueous solution
of sulfuric acid, and 0.5 g of distilled water.
The impregnated SBA-15 was dried at 100.degree. C. for 6 hours
and then again heated at 160.degree. C. for 6 hours.
1.25 g of sucrose, 0.14 g of a 97% by weight aqueous solution of
sulfuric acid, and 5.0 g of distilled water were mixed to prepare
a solution A. 0.75 g of TEOS, 0.25 g of a 10.sup.-3 M aqueous solution
of HCl, and 0.75 g of ethanol were mixed and stirred at 40.degree.
C. for one hour to prepare a solution B containing a silica oligomer.
The solution A and the solution B were mixed to prepare a second
mixture. The heated SBA-15 was impregnated with the second mixture.
Then, the SBA-15 was dried at 100.degree. C. for 6 hours and again
heated at 160.degree. C. for 6 hours. The heated SBA-15 was again
impregnated with the second mixture and again underwent drying and
heating.
The resultant SBA-15 was heated at 900.degree. C. under a nitrogen
atmosphere for two hours.
Thus treated SBA-15 was added into a mixed HF-H.sub.2 O-EtOH solution
(10% by weight of HF, 10% by weight of H.sub.2 O, 80% by weight
of EtOH) and stirred at room temperature for two hours. The resultant
was filtered and washed with secondary distilled water three to
five times. The stirring, filtering, and washing were repeated twice.
The finally obtained carbon molecular sieve was dried at 100.degree.
C. for 12 hours.
An XRD pattern and a nitrogen adsorption-esorption curve of the
carbon molecular sieve of Example 2 are shown in FIG. 2A and FIG.
2B, respectively. In addition, the porosity of the carbon molecular
sieve of Example 2 was measured and the result is presented in Table
2.
TABLE 2 Total pore Mesopore Micropore Surface volume volume volume
area (<80 nm) (1-80 nm) (<1 nm) Micro- Sample (m.sup.3 /g)
(cm.sup.3 /g) (cm.sup.3 /g) (cm.sup.3 /g) porosity CMK-3 1480 1.44
0.99 0.45 0.31 Example 2 1513 1.56 1.00 0.56 0.36 Increased 2% 8%
1% 24% 20% effect
As shown in Table 2 in the carbon molecular sieve of Example 2
a surface area was increased by about 2%, a total pore volume by
about 8%, a mesopore volume by about 1%, a micropore volume by about
24%, and microporsosity by about 20%, when compared to CMK-3. These
results again demonstrate that use of a silica oligomer can increase
the porosity or microporosity of the carbon molecular sieve.
Referring to FIG. 2A, a peak was observed at about 2 degrees. This
indicates that carbon meso-rods for the carbon molecular sieve of
Example 2 are regularly arranged.
Accordingly, the carbon molecular sieve, manufactured according
to the second aspect of the present invention, has increased microporosity
and maintains the strength of carbon micro-rods sufficient to provide
connection between carbon meso-rods.
EXAMPLE 3
Catalyst for Fuel Cell
1 g of the carbon molecular sieve of Example 2 was put into a plastic
envelope. Then, a solution of 1.099 g of H.sub.2 PtCl.sub.6 in 1
ml of acetone was added to the plastic envelope and mixed for 5
to 10 minutes. The resultant mixture was dried in the air for four
hours. The dried carbon carrier was transferred to a crucible and
again dried at 60.degree. C. The crucible was placed in an electric
furnace and a nitrogen gas was fed into the furnace for 10 minutes.
The nitrogen gas was then replaced with a hydrogen gas and then
catalyst reduction was carried out at 200.degree. C. After the hydrogen
gas was again replaced with a nitrogen gas, the temperature of the
furnace was raised to 250.degree. C. for 10 minutes, maintained
at that temperature for two hours, and cooled to room temperature.
A slurry containing the Pt/C catalyst thus prepared, a commercially
available ionomer, and isopropyl alcohol (IPA) was coated on a carbon
paper using a spray method so that the loading of Pt reached 3 mg/cm.sup.3
and then heated to prepare a cathode. As for an anode, a commercially
available Pt-Ru alloy catalyst was used and an amount of a supported
catalyst was 8 mg/cm.sup.3. Nafion 115 (Dupont) was used as an electrolyte
membrane. A 2 M solution of aqueous methanol and air were fed into
the unit cell so prepared and the performance of the cell was measured.
Comparative Example 1
Use of Carbon Black
A unit cell was prepared in the same manner as in Example 3 except
that for preparation of a cathode, a Pt/C catalyst (E-TEK) containing
activated carbon powder as a catalyst carrier, instead of a carbon
molecular sieve, was used and the loading of platinum supported
catalyst was 3 mg/cm.sup.3. The performance of the unit cell was
measured in the same manner as in Example 3.
A current density of each fuel cell of Example 3 and Comparative
example 1 was measured at a cell voltage of 0.3 V. According to
the results, a fuel cell of Example 3 exhibited a current density
of 112 mA/cm.sup.3 whereas a fuel cell of Comparative example 1
exhibited a current density of 85 mA/cm.sup.3. It can be seen from
the results that a carbon molecular sieve of the present invention
has improved catalytic activity, when compared to a conventional
activated carbon powder.
As apparent from the above description, a carbon molecular sieve
manufactured according to the first aspect of the present invention
has increased surface area and/or microporosity.
A carbon molecular sieve manufactured according to the second aspect
of the present invention has increased surface area and/or microporosity,
while simultaneously maintaining the structural regularity.
The carbon molecular sieve of the present invention can be more
effectively used as a catalyst carrier that supports catalytic metals
because of the increased surface area and/or microporosity.
A catalyst using the carbon molecular sieve of the present invention
as a catalyst carrier and a fuel cell using the catalyst exhibit
enhanced catalytic activity and efficiency, when compared to using
a conventional activated carbon powder as a catalyst carrier.
While the present invention has been particularly shown and described
with reference to exemplary embodiments thereof, it will be understood
by those of ordinary skill in the art that various changes in form
and details may be made therein without departing from the spirit
and scope of the present invention as defined by the following claims. |