U.S. patent application number 11/585578 was filed with the patent office on 2007-06-21 for aerogel and metallic compositions.
This patent application is currently assigned to Aerogel Composite, LLC. Invention is credited to Can Erkey, Hiroaki Hara.
Application Number | 20070142222 11/585578 |
Document ID | / |
Family ID | 27407575 |
Filed Date | 2007-06-21 |
United States Patent
Application |
20070142222 |
Kind Code |
A1 |
Erkey; Can ; et al. |
June 21, 2007 |
Aerogel and metallic compositions
Abstract
Metallic aerogel compositions comprising an aerogel, e.g., RF or
carbon aerogel, having metallic particles dispersed on its surface
are disclosed. The aerogel compositions can have a uniform
distribution of small metallic particles, e.g., 1 nanometer average
particle diameter. Also disclosed are processes for making the
aerogel compositions comprising contacting an aerogel with a
supercritical fluid containing a metallic compound. The aerogel
compositions are useful, for example in the manufacture of fuel
cell electrodes.
Inventors: |
Erkey; Can; (South Windsor,
CT) ; Hara; Hiroaki; (West Hartford, CT) |
Correspondence
Address: |
MCCORMICK, PAULDING & HUBER LLP
CITY PLACE II
185 ASYLUM STREET
HARTFORD
CT
06103
US
|
Assignee: |
Aerogel Composite, LLC
Bloomfield
CT
University of Connecticut
Farmington
CT
|
Family ID: |
27407575 |
Appl. No.: |
11/585578 |
Filed: |
October 24, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10327300 |
Dec 20, 2002 |
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11585578 |
Oct 24, 2006 |
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60343700 |
Dec 27, 2001 |
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60390174 |
Jun 19, 2002 |
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60412755 |
Sep 23, 2002 |
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Current U.S.
Class: |
502/237 |
Current CPC
Class: |
H01M 4/9075 20130101;
C04B 30/00 20130101; B01J 35/1047 20130101; H01M 4/926 20130101;
C04B 20/0056 20130101; B01J 37/084 20130101; H01M 4/923 20130101;
B01J 37/03 20130101; B01J 37/0203 20130101; C04B 2111/00853
20130101; C04B 38/0022 20130101; B01J 35/1061 20130101; B01J 21/18
20130101; Y02E 60/50 20130101; B01J 35/1042 20130101; C04B
2111/00413 20130101; C04B 14/34 20130101; B01J 35/10 20130101; H01M
4/925 20130101; H01M 4/9083 20130101; C04B 41/009 20130101; B01J
35/1023 20130101; B01J 23/40 20130101; C04B 41/5001 20130101; H01M
4/9008 20130101; C04B 38/0022 20130101; C04B 14/34 20130101; C04B
35/52 20130101; C04B 38/0045 20130101; C04B 38/009 20130101; C04B
41/5001 20130101; C04B 41/0072 20130101; C04B 41/4554 20130101;
C04B 41/4582 20130101; C04B 41/51 20130101; C04B 2103/007 20130101;
C04B 20/0056 20130101; C04B 14/386 20130101; C04B 14/34 20130101;
C04B 20/008 20130101; C04B 30/00 20130101; C04B 14/028 20130101;
C04B 14/34 20130101; C04B 30/00 20130101; C04B 14/064 20130101;
C04B 14/34 20130101; C04B 30/00 20130101; C04B 14/302 20130101;
C04B 14/34 20130101; C04B 41/009 20130101; C04B 30/00 20130101 |
Class at
Publication: |
502/237 |
International
Class: |
B01J 21/08 20060101
B01J021/08 |
Claims
1-32. (canceled)
33. A method for producing metal particles or mixed metal particles
dispersed on a particulate substrate comprising: a. exposing an
organometallic and the particulate substrate to a supercritical or
near supercritical fluid under conditions to form a mixture of the
fluid and the organometallic; b. allowing the mixture to remain in
contact with the substrate for a time sufficient to deposit
dispersed organometallic onto the substrate; c. venting the
mixture; d. thereby adsorbing the organometallic onto the
substrate; and then e. reducing the dispersed organometallic to
dispersed metal particles with a reducing agent.
34. The method of claim 1, wherein the substrate comprises a
carbonaceous material.
35. The method of claim 34, wherein the metal particles are
nanoparticles.
36. The method of claim 1, wherein the organometalllic comprises
1,5-cyclooctadiene dimethyl platinum [Pt(COD)Me.sub.2].
37. The method of claim 33, wherein the metal particles are
nanoparticles.
38. The method of claim 37, whrein the nanoparticles are about 1 nm
to about 4 nm in average diameter.
39. The method of claim 33, wherein the metal particles are noble
metal particles.
40. The method of claim 33, wherein the metal particles comprise
platinum, iridium, ruthenium, rhodium, palladium, chromium, gold,
silver, nickel, cobalt, or a mixture thereof, or an alloy
thereof.
41. The method of claim 33, wherein the metal particles comprise
platinum.
42. The method of claim 33, wherein the metal particles comprise
silver.
43. The method of claim 33, wherein the metal particles comprise
ruthenium.
44. The method of claim 33, wherein the metal particles are mixed
metal particles.
45. The method of claim 33, wherein the fluid comprises carbon
dioxide, ethane, or propane.
46. The method of claim 33, wherein the reducing is by addition of
a reducing agent.
47. The method of claim 46, wherein the reducing agent comprises
hydrogen.
48. The method of claim 33, wherein the organometallic is adsorbed
while in the mixture.
49. The method of claim 33, wherein the organometallic is adsorbed
when the mixture is vented.
50. The method of claim 33, wherein in step (a), at least some of
the organometallic dissolves in the fluid.
51. The method of claim 33, wherein in step (a), all or
substantially all of the organometallic dissolves in the fluid.
52. The method of claim 33, wherein the method produces a supported
particulate catalyst suitable for use in a fuel cell.
53. A method for producing particulate substrate-supported
dispersed metallic particles comprising: a. mixing an
organometallic in a supercritical or near supercritical fluid to
form a mixture; b. exposing a particulate substrate to the mixture
of a) under supercritical or near supercritical conditions for a
period of time sufficient to deposit dispersed organometallic on
the substrate; c. venting the mixture; d. thereby adsorbing the
organometallic onto the substrate; and then e. reducing the
organometallic to dispersed metal particles with a reducing
agent.
54. A method for producing particulate substrate-supported
dispersed metallic particles comprising; a. adding a particulate
substrate and an organometallic to a reactor; b. adding a
supercritical fluid to the reactor to form a mixture with the
organometallic; c. allowing the organometallic to remain in contact
with the substrate for a time sufficient to deposit dispersed
organometallic onto the substrate; d. venting the reactor; e.
thereby adsorbing the organometallic onto the substrate; and then
f. adding a gaseous reducing agent to the reactor; and g.
contacting the reducing agent and organometallic until the
organometallic is reduced to dispersed metal particles.
55. The method of claim 33, wherein the particulate substrate is a
porous particulate substrate.
56. A method for producing metal particles or mixed metal particles
dispersed on a surface of a substrate comprising; exposing an
organometallic and the surface to a supercritical or near
supercritical fluid in a reactor under conditions to form a mixture
of the fluid and the organometallic; allowing the mixture to remain
in contact with the surface for a time sufficient to deposit
dispersed organometallic onto the surface; depressurizing the
reactor; and then reducing the dispersed organometallic to
dispersed metal or mixed metal particles.
57. The method of claim 57, wherein the substrate is a porous
substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation application of Ser. No.
10/327,300 filed Dec. 20, 2002, which claims priority from the
following patent applications: U.S. Ser. No. 60/343,700, filed on
Dec. 27, 2001, U.S. Ser. No. 60/390,174, filed on Jun. 19, 2002,
and U.S. Ser. No. 60/412,755, filed on Sep. 23, 2002.
FIELD OF THE INVENTION
[0002] The present invention generally relates to aerogel
compositions and more specifically to aerogels having metallic
particles dispersed therein, e,g, carbon aerogels loaded with
platinum, and their preparation.
BACKGROUND OF THE INVENTION
[0003] Aerogels, are porous materials that are produced by
polycondensation reactions known in the art as the "sol-gel
process". A common feature among aerogels is their small
inter-connected pores. The aerogel chemical composition,
microstructure and physical properties can be controlled at the
nanometer scale due to sol-gel processing. There are three major
types of aerogels- inorganic, organic and carbon aerogels.
Inorganic aerogels can be obtained by supercritical drying of
highly cross-linked and transparent hydrogels synthesized by
polycondensation of metal alkoxides. Silica aerogels are the most
well known inorganic aerogels. Organic aerogels can be synthesized
by supercritical drying of the gels obtained by the sol-gel
polycondensation reaction of monomers such as, for example,
resorcinol with formaldehyde, in aqueous solutions. Carbon aerogels
can be obtained by pyrolizing the organic aerogels at elevated
temperatures.
[0004] Aerogels, e.g., carbon aerogels (also referred to in the art
as carbon foams) have been produced by various methods for a
variety of applications. These prior processes are exemplified by
U.S. Pat. No. 4,806,290 issued Feb. 21, 1989; U.S. Pat. No.
4,873,218 issued Oct. 10, 1989; U.S. Pat. No. 4,997,804 issued Mar.
5, 1991; U.S. Pat. No. 5,086,085 issued Feb. 4, 1992; and U.S. Pat.
No. 5,252,620 issued Oct. 12, 1993. Typically, efforts have been
directed to the development of carbon aerogels for use as
electrodes and include all forms of carbon aerogels, monolithic,
granular or microspheres. Such electrodes find use, for example, in
energy storage devices, e.g., capacitors and batteries, as well as
for fuel cells, e.g., proton exchange membrane ("PEM") fuel cells
and electrocapacitive deionization devices, etc. These efforts are
exemplified by U.S. Pat. No. 5,260,855 issued Nov. 9, 1993; U.S.
Pat. No. 5,529,971 issued Jun. 25, 1996; U.S. Pat. No. 5,420,168
issued May 20, 1995; U.S. Pat. No. 5,508,341 issued Apr. 16, 1996;
and U.S. Pat. No. 6,010,798, issued Jan. 4, 2000.
[0005] Additives can be incorporated into aerogels to make aerogel
compositions (also referred to herein as "aerogel composite"). The
role of the additives is to enhance the properties of pure aerogels
or to impart additional desirable properties depending on the
application. In general, aerogel composites are typically prepared
using two different methods. The first one involves adding the
additive to the sol prior to polymerization and the second method
involves contacting the produced aerogel with a liquid or gaseous
stream containing the additive.
[0006] Ye et al., Can. J. Chem. 75:1666-1673 (1997) disclose the
preparation of polyacrylonitrile/platinum aerogel composites by
dipping carbonized polyacrylonitrile ("PAN") aerogels in
hexachloroplatinic (H.sub.2PtCl.sub.6) solution. The precursor
(H.sub.2PtCl.sub.6) was added prior to the gelation stage. It is
disclosed that incorporating the platinum precursor before the
gelation stage resulted in a more homogeneous distribution of
platinum.
[0007] Pajonk et al., Preparation of Catalysts VII, 1998, 167
(1997), disclose a method to make carbon aerogels and load platinum
onto the aerogels, whereby resorcinol-formaldehyde ("RF") aerogels
were obtained by polymerization in acetone instead of water and
perchloric acid was used as the catalyst. After curing and
supercritical extraction of acetone, the samples were pyrolyzed.
Subsequently, the samples were impregnated with H.sub.2PtCl.sub.6
in acetone. Then, acetone was supercritically extracted and the
sample was calcined and reduced with hydrogen. The dispersion of
platinum was reported to be 23% and the platinum content was
reported to be 0.44 wt %.
[0008] U.S. Pat. No. 5,851,947, issued Dec. 22, 1998, discloses a
method for incorporating noble metals into inorganic aerogels. The
metal precursors were added to the sol. After gelation, the ethanol
was removed by supercritical drying.
[0009] Miller et al., J. Electrochem Soc., 144 (No. 12) (1997);
Lanngmuir 15:799-806 (1999) disclose the deposition of ruthenium
nanoparticles on carbon aerogels. Carbon aerogels were prepared and
impregnated with ruthenium 2,4 pentanedionate by chemical vapor
impregnation.
[0010] Maldonado-Hodar et al., Carbon 37, 1199-1205 (1999),
disclose a series of carbon aerogels containing Pt, Pd and Ag.
Pt(NH.sub.3)4Cl.sub.2, PdCl.sub.2 and Ag(CH.sub.3COO) were used as
the polymerization catalyst in the initial solution for preparation
of RF aerogels. After curing, water was exchanged with acetone and
acetone was extracted by supercritical carbon dioxide.
Subsequently, the aerogels were pyrolyzed in flowing nitrogen.
[0011] U.S. Pat No. 5,789,027, issued Aug. 4, 1998, discloses
methods for depositing a film of material on the surface of a
substrate by i) dissolving a precursor of the material into a
supercritical or near-supercritical solvent to form a supercritical
or near-supercritical solution; ii) exposing the substrate to the
solution, under conditions at which the precursor is stable in the
solution; and iii) mixing a reaction reagent into the solution
under conditions that initiate a chemical reaction involving the
precursor, thereby depositing thematerial onto the solid substrate,
while maintaining supercritical or near-supercritical conditions.
The patent also discloses similar methods for depositing material
particles into porous solids, and films of materials on substrates
or porous solids having material particles deposited in them.
[0012] Processes such as described above often have inadequate
control over the manner in which the metallic particles are
incorporated, thereby providing aerogel compositions having
inconsistent metal particle sizes and broad particle size
distributions. This has been one of the factors which have
inhibited the commercialization of aerogels, particularly for use
in PEM fuel cells which currently require large amount of platinum
to obtain an acceptable level of performance. Decreasing the amount
of platinum used in fuel cells would be beneficial for fuel cell
based power generation systems to compete with internal combustion
engines.
[0013] Accordingly, aerogel compositions comprising aerogels having
metallic particles, e.g., platinum, dispersed within, and processes
for making such aerogels, are desired. Desirably, such aerogel
compositions would contain metal particles having a small particle
size, e.g., 4 nanometers or less, with a narrow particle size
distribution.
SUMMARY OF THE INVENTION
[0014] In accordance with the present invention, aerogel
compositions comprising an aerogel having metallic particles
dispersed on the surface of the aerogel, i.e., in the pores,
wherein the metallic particles have an average particle size of
about 4 nanometers or less, based on the number of metallic
particles. Typically, the average particle size of the metallic
particles is about 3 nanometers or less, preferably from 1 to 2
nanometers and more preferably about 1 nanometer.
[0015] Quite surprisingly in accordance with the present invention,
it has been found that the aerogel compositions can have a very
narrow particle size distribution of the metallic particles.
Typically, when the metallic particles have an average particle
size of about 4 nanometers or less, less than about 20% of the
metallic particles have a particle size of about 5 nanometers or
greater, based on the number of metallic particles. Typically, when
the metallic particles have an average particle size of about 3
nanometers, less than about 20% of the metallic particles have a
particle size of about 4 nanometers or greater and less than about
20% of the metallic particles have a particle size of about 2
nanometers or less. Typically, when the metallic particles have an
average particle size of about 2 nanometers, less than about 20% of
the metallic particles have a particle size of about 3 nanometers
or greater and less than about 20% of the metallic particles have a
particle size of about 1 nanometer or less. Preferably, less than
about 20% of the metallic particles have a particle size of about 3
nanometers or greater and less than about 20% of the particles have
a particle size of less than about 1 nanometer. More preferably,
when the metallic particles have an average particle size of about
1 nanometer, less than about 20% of the metallic particles have a
particle size of about 2 nanometers or greater and less than about
20% of the metallic particles have a particle size of less than
about 1 nanometer, based on the number of metallic particles.
[0016] By virtue of the present invention, it is now possible to
utilize less metal, e.g., platinum, in the aerogel compositions
than used in coventional aerogel compositions, and still provide
equivalent or enhanced performance in catalytic applications, e.g.,
PEM fuel cells.
[0017] Further in accordance with the present invention, there are
provided processes for making metallic aerogel compositions,
comprising contacting an aerogel with a supercritical fluid
comprising a metallic compound, e.g., an organometallic compound.
Quite advantageously, the metallic compound can be incorporated
into the aerogel at various times during the manufacturing. For
example, the metallic compound can be incorporated after the
supercritical extraction of the liquid polymerization medium, e.g.,
water, from the cured aerogel (also referred to as the "cured
hydrogel"). Alternatively, the metallic compound can be
incorporated into the aerogel after pyrolysis. Additionally, the
polymerization can be conducted in the presence of the metallic
compound.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The particular aerogel used in the compositions of the
present invention is not critical. For example, the aerogels can be
organic, e.g., RF aerogels, or inorganic, e.g., silica aerogels.
Further, organic aerogels can be pyrolized to form carbon aerogels.
As used herein, the term "aerogel" includes all aerogel forms,
i.e., inorganic aerogels, organic aerogels, carbon aerogels and
xerogels (gels formed when hydrogels are air dried instead of
supercritically dried).
[0019] Likewise, the particular precursors used to make the
aeorgels are not critical. Typical precursors used to make silica
aerogels, for example, include tetramethyl orthosilicate (TMOS,
Si(OCH.sub.3).sub.4), and tetraethyl orthosilicate (TEOS,
Si(OCH.sub.2CH.sub.3).sub.4) Other precursors can be selected by
those skilled in art to make other inorganic aerogels containing
oxides such as silica, alumina, titania, vanadia, niobia, zirconia,
tantala, or mixtures thereof. Examples of precursors, i.e.,
monomers, used to make organic aerogels include include resorcinol,
phenol, catechol, chloroglucinol, and other polyhydroxybenzene
compounds that react in the appropriate ratio with formaldehyde or
furfural, e.g., resorcinol-furfural, resorcinol-formaldehyde,
phenol-resorcinol-formaldehyde, catechol- formaldehyde, and
chloroglucinol-formaldehye. Further details concerning the
selection of suitable precursors to make the desired aerogels are
known to those skilled in the art. Such materials are commercially
available.
[0020] The particular method for producing the aerogels is not
critical to the present invention. Organic aerogels are typically
produced as follows. The process in general requires first that the
reactants, i.e., monomers, are mixed with a catalyst and may
include the addition of metals. A gel formed by polymerization is
then dried in a solvent exchange and extraction step. The resulting
organic aerogel is then pyrolyzed in an inert atmosphere to form a
carbon aerogel. Specifically, the process to prepare the gels
proceeds through a sol-gel polymerization of certain
multifunctional organic monomers in a solvent, typically water,
leading to the formation of highly cross-linked, transparent gels
("hydrogel sol"). For example, in a preferred aspect of the
invention, one mole of resorcinol (1,3-dihydroxybenzene) condenses
in the presence of a basic catalyst with two moles of formaldehyde.
Mildly basic catalysts such as sodium carbonate are preferred. In
this polymerization, resorcinol is a trifunctional monomer capable
of adding formaldehyde in the 2-, 4-, and/or 6-ring positions. The
substituted resorcinol rings condense with each other to form
nanometer-sized clusters in solution. Eventually, the clusters
crosslink through their surface groups (e.g., --CH.sub.2 OH) to
form the hydrogel sol. Further details of the reaction are known in
the art, e.g. see U.S. Pat. Nos. 4,997,804 and 4,873,218. Other
patents which describe the preparation of aerogels include U.S.
Pat. Nos. 6,432,886, 6,364,953, 6,307,116, 5,908,896, 5,879,744,
5,851,947 and 5,306,555.
[0021] The size of the clusters can be regulated by the
concentration of catalyst in the resorcinol-formaldehyde (RF)
mixture. More specifically, the mole ratio of resorcinol (R) to
catalyst (C), R/C, controls the surface area and electrochemical
properties of the resulting gel. Preferably, in accordance with the
present invention, the R/C ratio is from about 50 to 300. Other
commonly referenced ratios include resorcinol (R) to formaldehyde
(F), RF and resorcinol (R) to water (W), R/W. Typically, the R/F
and R/W molar ratios are in the range of about 0.01 to 10.
[0022] Then, the hydrogel sol is typically cured for a time and
temperature sufficient to stabilize the aerogel structure and form
a cured hydrogel. Typical curing times range from 2 hours to 5
days, or more. Typical curing temperatures range from 25 C to 150
C. Pressures greater than 1 atmosphere ("atm") can be used if
desired to decrease the curing time. After curing, RF aerogels are
typically dark red or black in color, and substantially
transparent. The next step in organic aerogel preparation is to dry
the hydrogel sol. If the polymerization solvent is removed from
these gels by simple evaporation, large capillary forces are
exerted on the pores, forming a collapsed structure, i.e., xerogel.
In order to preserve the gel skeleton and minimize shrinkage, it is
preferable to perform the drying step under supercritical
conditions (described hereinafter). Other drying steps may also be
conducted, if desired, usually before the supercritical extraction
step. For example, it is common to conduct a solvent exchange step
where the cured hydrogel is contacted with an exchange solvent,
e.g., acetone, to form a dried aerogel, prior to subjecting the
dried aerogel to supercritical extraction, because water is
immiscible with liquid carbon dioxide, a common supercritical
fluid. Also, as an alternative, or in addition, to the exchange
step, surfactants may be used to remove water from the cured
hydrogel. The highly porous material obtained from this removal
operation is the organic aerogel. By appropriate adjustment of
drying conditions, a hybrid structure having characteristics of
both a xerogel and an aerogel may be produced. For example, such a
hybrid may be produced as a result of a partial evaporation of the
gel solvent under conditions promoting xerogel formation followed
by evaporation of the remaining solvent under conditions promoting
aerogel formation. The resulting hybrid structure would then be
dried under supercritical conditions and pyrolyzed. Preparation of
other xerogel-aerogel hybrids may be produced by first evaporating
under conditions promoting aerogel formation and completing the
evaporation under xerogel-promoting conditions.
[0023] As noted above, one means for removing water from the
hydrogel to form an organic aerogel is by extraction of the gel
under supercritical conditions. As used herein, a "supercritical
fluid" (also referred to in the art as "supercritical solution" or
"supercritical solvent") is one in which the temperature and
pressure of the fluid are greater than the respective critical
temperature and pressure of the fluid. A supercritical condition
for a particular fluid refers to a condition in which the
temperature and pressure are both respectively greater than the
critical temperature and critical pressure of the particular
fluid.
[0024] A "near-supercritical fluid" is one in which the reduced
temperature (actual temperature measured in Kelvin divided by the
critical temperature of the solution (or solvent) measured in
Kelvin) and reduced pressure (actual pressure divided by critical
pressure of the fluid) of the fluid are both greater than 0.8 but
the fluid is not a supercritical fluid. A near-supercritical
condition for a particular fluid refers to a condition in which the
reduced temperature and reduced pressure are both respectively
greater 0.8 but the condition is not supercritical. Under ambient
conditions, the fluid can be a gas or liquid. The term fluid is
also meant to include a mixture of two or more different individual
fluid. As used herein, the term "supercritical fluid" and
"supercritical conditions" are intended to include near
supercritical fluids and near supercritical conditions
respectively.
[0025] The temperature and pressure of the extraction process
depend on the choice of supercritical fluid. Generally, the
temperature is less than 250 C and often less than 100C, while the
pressure is typically between 50 to 500 atm.
[0026] Solvents that can be used as supercritical fluids are well
known in the art and are sometimes referred to as dense gases
(Sonntag et al., Introduction to Thermodynamics, Classical and
Statistical, 2nd ed., John Wiley & Sons, 1982, p. 40). Suitable
solvents for use as a supercritical fluid include, for example,
carbon dioxide, ethane, propane, butane, pentane, dimethyl ether,
ethanol, water and mixtures thereof. Carbon dioxide is a preferred
supercritical fluid for use in accordance with the present
invention. For example, at 333K and 150 atm, the density of
CO.sub.2 is 0.60 g/cm.sup.3; therefore, with respect to CO.sub.2,
the reduced temperature is 1.09, the reduced pressure is 2.06, and
the reduced density is 1.28. Carbon dioxide is a particularly good
choice of supercritical fluid. Its critical temperature (31.1 C.)
is close to ambient temperature and thus allows the use of moderate
process temperatures (<80 C.). The time required for
supercritical drying depends on the thickness of the gel. Further
details concerning the selection of suitable supercritical fluids
and extraction conditions are known to those skilled in the art,
see e.g., McHugh et al, Supercritical Fluid Extraction: Principles
and Practice; Butterworths: Boston, 1986).
[0027] In cases where the cured hydrogels are of sufficiently high
density, such as greater than about 40 wt % solids, the pore
network may have sufficient inherent strength to withstand the
drying process without resort to supercritical drying conditions.
Thus, carbon dioxide may be bled from the vessel under
nonsupercritical conditions. Nonsupercritical drying is
particularly attractive because of reduced processing time. To
maximize crosslinking and further increase the density of the gels,
a cure cycle may be desired.
[0028] Following the solvent exchange/extraction step and any cure
cycle, the organic aerogel is typically pyrolyzed at elevated
temperatures of about 400 C to 2000 C, typically in a conventional
inert atmosphere of nitrogen, argon, neon or helium to form a
pyrolized aerogel, e.g., carbon aerogel. The pyrolysis temperatures
can alter the surface area and structure of the pyrolized aerogel.
In particular, higher surface areas are achieved at lower
temperatures. The resulting aerogels, independent of the procedure
by which they are pyrolyzed, are black and not transparet due to
the visible absorption properties of the carbon matrix.
[0029] The aerogels of the present invention typically have a
surface area of from about 100 to 2000 meters squares per gram
("m2/g"), a pore volume of from about 0.5 to 10 cubic centimeters
per gram ("cm.sup.3/g"), and a density of from about 0.01 to 2.0
grams per cubic centimeter ("g/cm.sup.3"). Such properties can be
readily determined by those skilled in the art. For example,
surface area and pore volume can be determined by the BET method,
S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc. 60, 309
(1938) and density can be determined by using a pycnometer, the
details of which are known to those skilled in the art.
[0030] The amount of the aerogel in the metallic aerogel
compositions of the present invention is typically from about 20 to
99.9 wt %, more typically from about 40 to 99 wt % and often from
about 50 to 90 wt %, based on the total weight of the composition,
i.e., total solids (metallic particle plus aerogel exclusive of any
liquids). The amount of the metallic particle in the metallic
aerogel compositions of the present invention is typically from
about 0.1 to 80 wt %, more typically from about 1 to 60 wt % and
often from about 10 to 50 wt %, based on the total weight of the
composition.
[0031] The particular metallic particles used in the compositions
of the present invention are not critical. The metallic particles
may be in the form of free metal, i.e., zero valence, or ionic,
e.g., in the form of a metallic compound. Examples of suitable
metals include typical metals used in catalysis, although the
invention is not limited to particular metals. Typical metals
include iron, cobalt, magnesium, nickel, titanium, chromium,
copper, platinum, gold, silver, rhodium, ruthenium, palladium,
iridium, and the like. Preferred metals include platinum, rhodium,
palladium, iridium, silver, gold and mixtures thereof.
[0032] Preferably, the metallic compound (precursor) is provided in
the form of an organometallic compound. Typically, the
organometallic compounds comprise a transition metal bound to one
or more organic ligands. Some examples of useful organometallic
compounds contain the following classes of ligands:
beta-diketonates (e.g., Cu(hfac).sub.2 or Pd(hfac).sub.2, where
hfac is an abbreviation for 1,1,1,5,5,5-hexafluoroacetylacetonate),
alkyls (e.g., Zn(ethyl).sub.2 or
dimethyl(cyclooctadiene)platinum(II) (CODPtMe.sub.2)), allyls (e.g.
bis(allyl)zinc or W(.pi..sup.4-allyl).sub.4), dienes (e.g.,
CODPtMe.sub.2), or metallocenes (e.g.,
Ti(.pi..sup.5-C.sub.5H.sub.5).sub.2 or
Ni(.pi..sup.5-C.sub.5H.sub.5).sub.2)--Preferred organometallic
compounds include dimethyl(cyclooctadiene)platinum(II), tetraamine
platinum (II) chloride, platinum(II)hexafluoroacetylacetone,
(trimethyl)methylcyclopentadienylplatinum(IV),
bis(cyclopentadienyl)ruthenium,
bis(ethylcyclopentadienyl)ruthenium(II),
bis(pentamethylcyclodienyl)ruthenium,
(methylcyclopentadienyl)(1,5-cyclooctadiene)iridium(I), and
mixtures thereof. For a list of additional potential organometallic
compounds, see for example, M. J. Hampden-Smith and T. T. Kodas,
Chem. Vap. Deposition, 1:8 (1995). Further details concerning the
selection of suitable Organometallic compounds to make the desired
aerogel compositions are known to those skilled in the art. Such
materials are commercially available.
[0033] In accordance with the present invention, the metallic
particles have an average particle size of about 4 nanometers or
less, based on the number of metallic particles. Typically, the
average particle size of the metallic particles is about 3
nanometers or less, preferably from 1 to 2 nanometers and more
preferably about 1 nanometer. As used herein, the term "average
particle size" means the average diameter (also referred to in the
art as "effective diameter"). A preferred technique for measuring
the average particle size is to measure the diameter of a
representative number of particles from an electron micrograph,
e.g., from a transmission electron microscope ("TEM") and calculate
an average. Another method is hydrogen or CO chemisorption where
the total metal surface area is measured. This information can then
be used to calculate an average metal diameter. Further details
concerning techniques for measuring the average particle size of
the metallic particles are known to those skilled in the art.
[0034] Furthermore, in accordance with the present invention it is
not necessary to include a reaction reagent to promote the
deposition of the metallic compound onto the surface of the
aerogel, such as required in chemical vapor deposition or chemical
fluid deposition processes, e.g., H.sub.2, H.sub.2S, O.sub.2 or
N.sub.2O--Preferably in accordance with the present invention, the
metallic compound is deposited in the substantial absence a
reaction reagent (of the metallic compound). Preferably, the
supercritical fluid containing the metallic compound comprises less
than 5 wt %, more preferably less than 1 wt % and most preferably
less than 0.1 wt % of a reaction reagent, based on the total weight
of the supercritical fluid, reaction reagent and metallic compound.
Preferably, in accordance with the present invention there is no
chemical change to the metallic compound during the supercritical
deposition of the metallic compound onto the aerogel surface. As
described hereinafter, when a chemical change is desired, e.g.,
reduction with hydrogen, it is not conducted until after the
metallic compound is deposited onto the aerogel.
[0035] Quite surprisingly in accordance with the present invention,
it has been found that the aerogel compositions can have a very
narrow particle size distribution of the metallic particles.
Typically, when the metallic particles have an average particle
size of about 4 nanometers or less, less than about 20% of the
metallic particles have a particle size of about 5 nanometers or
greater, based on the number of metallic particles. Typically, when
the metallic particles have an average particle size of about 3
nanometers, less than about 20% of the metallic particles have a
particle size of about 4 nanometers or greater and less than about
20% of the metallic particles have a particle size of about 2
nanometers or less. Typically, when the metallic particles have an
average particle size of about 2 nanometers, less than about 20% of
the metallic particles have a particle size of about 3 nanometers
or greater and less than about 20% of the metallic particles have a
particle size of about 1 nanometer or less. Preferably, less than
about 20% of the metallic particles have a particle size of about 3
nanometers or greater and less than about 20% of the particles have
a particle size of less than about 1 nanometer. More preferably,
when the metallic particles have an average particle size of about
1 nanometer, less than about 20% of the metallic particles have a
particle size of about 2 nanometers or greater and less than about
20% of the metallic particles have a particle size of less than
about 1 nanometer, based on the number of metallic particles. The
particle size distribution can readily be determined by generating
a histogram of the particle sizes from the TEM micrographs
described above.
[0036] In accordance with a preferred aspect of the invention,
there is provided a process for making a metallic aerogel
composition, comprising contacting an aerogel with a supercritical
fluid comprising a metallic compound. The concentration of the
metallic compound should be sufficient to provide the desired
amount of the metallic particle dispersed within the aerogel.
[0037] In another aspect of the invention, the metallic compound
can be added along with the reactants, e.g., monomers, in the
preparation of the hydrogel sol. This can be conducted in addition
to, or instead of the, the contacting of the aerogel with a
supercritical fluid comprising a metallic compound. In still yet
another aspect of the invention, the metallic particle can be
impregnated into the surface of the aerogel by techniques known to
those skilled in the art.
[0038] The metallic aerogel compositions can be used with the metal
present in an ionic state or in the free metal state. If the free
metal state is desired, the metal in ionic form can be reduced by
any method known to those skilled in the art, e.g., by conducting a
second pyrolysis step, e.g., at a temperature of from about
500.degree. to 2000.degree. C., or by contacting the metallic
aerogel composition with a reduction gas such as, for example,
hydrogen, to form a reduced metal aerogel. Preferably, the metallic
particles have a surface area of at least about 50 m.sup.2/g,
preferably at least about 100 M.sup.2/g, more preferably at least
about 200 m.sup.2/g, and most preferably at least about 300
M.sup.2/g.
[0039] In one preferred aspect of the invention, there is provided
a process for making a metallic aerogel composition, comprising:
[0040] polymerizing at least two monomers in a liquid medium to
form a polymerization product comprising a hydrogel sol and the
liquid medium; [0041] curing the hydrogel sol to form a cured
hydrogel; [0042] removing at least a portion of the liquid medium
from the cured hydrogel to form an organic aerogel; [0043]
pyrolizing the organic aerogel to form a pyrolized aerogel; and
[0044] contacting the pyrolized aerogel with a supercritical fluid
comprising a metallic compound to form a metallic aerogel.
[0045] In another preferred aspect of the invention, there is
provided a process for making a metallic aerogel composition,
comprising: [0046] polymerizing at least two monomers in a liquid
medium to form a polymerization product comprising a hydrogel sol
and the liquid medium; [0047] curing the hydrogel sol to form a
cured hydrogel; [0048] optionally removing at least a portion of
the liquid medium from the cured hydrogel to form a dried aerogel;
[0049] contacting the dried aerogel or the cured aerogel with a
first supercritical fluid to form an organic aerogel; [0050]
pyrolizing the organic aerogel to form a pyrolized aerogel; and
[0051] optionally contacting the pyrolized aerogel with a second
supercritical fluid to form a metallic aerogel; characterized in
that at least one of said first supercritical fluid or said second
supercritical fluid comprises a metallic compound.
[0052] Thus, in accordance with this aspect of the present
invention, either the first supercritical fluid or the second
supercritical fluid, or both, comprises the metallic compound.
[0053] The processes of the present invention may comprise further
steps as desired. For example, a preferred aspect of the invention
further comprises the step of contacting the metallic aerogel with
a polymer electrolite, e.g, Nafion.TM. Solutions, a polymer
electrolyte available from E.I. duPont de Nemours and Company,
Wilmington, Del. as a 5% solution of perfluorosulfonic acid/PTFE
copolymer in the (H+) form. Preferably, the contacting is conducted
after incorporation of the metallic particles into the aerogel.
[0054] The particular form of the compositions of the present
invention is not critical. Typical forms include particles,
extrudate, pellets, films, coatings, fibers and the like. Likewise,
the compositions of the present invention can have a variety of end
uses such as, for example, for use in fuel cell electrodes, as
catalysts for chemical reactions, e.g., hydrogenation or
dehydrogenation, oxidation, isomerization, reforming,
hydrocracking, polymerization, etc. Use of the compositions of the
present invention as fuel cell electrodes, e.g., PEM electrodes, is
especially preferred.
[0055] Certain preferred aspects of the present invention are
exemplified as follows.
[0056] One exemplification of the present invention for producing
platinum loaded carbon aerogels includes placing a platinum
precursor (for example, Pt(NH.sub.3).sub.4(Cl.sub.2) into a
solution together with resorcinol, formaldehyde and sodium
carbonate. Resorcinol and formaldehyde polymerize and thereafter
cured. Water is then extracted from the matrix using acetone.
Subsequent supercritical carbon dioxide extraction of acetone
results in a platinum complex uniformly distributed in an organic
resorcinol-formaldehyde (RF) aerogel matrix. The matrix is
subsequently subjected to pyrolysis under a nitrogen atmosphere
resulting in a carbon aerogel loaded with platinum metal.
[0057] The curing time of the RF aerogels can be decreased by
carrying out the curing in the temperature range of from about
50-200.degree. C., preferably from about 100-200.degree. C., and
more preferably from about 110-140.degree. C., by conducting the
process, for example, in glass lined steel vessels under pressure
greater than 1 atm, e.g., 1 to 5 atm. Moreover, surfactants may be
used to remove the water directly from RF aerogels without the need
for acetone exchange.
[0058] Another exemplification of the present invention for
producing platinum loaded carbon aerogels includes contacting an RF
aerogel sample with a super critical CO.sub.2 solution containing a
platinum precursor (for example, CODpt(CH.sub.3).sub.2) dissolved
therein. The platinum precursor is adsorbed onto the aerogel.
Hydrogen is then used to reduce the platinum precursor to platinum
metal. The RF aerogel is then depressurized and subjected to
pyrolysis resulting in a carbon aerogel loaded with platinum
metal.
[0059] In this exemplification, instead of using hydrogen to reduce
the platinum precursor to a platinum metal, the RF aerogel may be
depressurized and then subjected to pyrolysis under a nitrogen
atmosphere. The platinum metal results due to the thermal reduction
of the platinum precursor.
[0060] Another exemplification of the present invention includes
impregnating a carbon aerogel with a platinum precursor (for
example, CODptMe.sub.2). The platinum precursor is converted to a
finely dispersed platinum metal within the aerogel matrix by
pyrolizing under a nitrogen atmosphere. The atmosphere can also be
a mixture of hydrogen with any inert gas.
[0061] Another exemplification of the present invention includes a
silica aerogel impreganted with an organometallic complex
containing platinum (Pt) using supercritical carbon dioxide
(scCO.sub.2) as solvent medium.
[0062] Preparation conditions such as reactant (monomer)
concentrations, curing times and temperatures, impregnation
conditions, pyrolysis temperatures and pressures can all be changed
appropriately to control the properties of the resulting materials.
For example, it was realized that platinum loaded RF aerogels
prepared using the method according to the second exemplification
turned black while degassing at 200.degree. C. This indicated that
platinum was acting as a catalyst in the pyrolysis. Therefore, in
accordance with the present invention, substantially lower
pyrolysis temperatures may be employed. This is important in the
incorporation of the aerogels into other materials such as
membranes which often do not survive the usual high pyrolysis
temperatures.
[0063] The invention is hereafter described with respect to the
examples which are not intended to limit the scope of the claims
which follow.
EXAMPLE 1
Preperation of RF Aerogels
[0064] The RF aerogels were synthesized by the reaction of
resorcinol with formaldehyde. For each run, 2 grams ("g") of
resorcinol was dissolved in 2.38 g of water in a test tube.
Subsequently, 0.019 g of sodium carbonate and 2.95 g of
formaldehyde solution were added.
[0065] The tube was then sealed by a rubber stopper and the
contents mixed by shaking. The tube was kept at room temperature
for one(1) day, at 50.degree. C. for one (1) day and at 90.degree.
C. for three (3) days. At the end of the first day, the solution in
the tube gelled and had a yellow-orange color. The gel
progressively became darker during the curing period and was dark
red-black in the end. At the end of the 90.degree. C. period, the
monolith was taken out of the test tube and immersed in
approximately 200 milliliters ("ml") acetone. There it was kept for
a period of two (2) days. Subsequently, acetone was extracted
supercritically.
[0066] Supercritical carbon dioxide extraction was conducted using
a high-pressure vessel (internal volume of 54 cm.sup.3), custom
manufactured from 316 stainless steel and equipped with two
sapphire windows (diameter=1.25'', thickness=0.5''), sealed on both
sides with poly(ether ether ketone) ("PEEK") seals.
[0067] In a typical experiment, the vessel was filled with acetone
and the monolith was placed in the vessel. The vessel was charged
very slowly with CO.sub.2 from a syringe pump (ISCO, Lincoln,
Nebr., Model 260D). The vessel was connected to a back pressure
regulator which kept the system pressure at 200 bars. The acetone
was displaced by liquid CO.sub.2 as evident by the transformation
from a two phase to a single phase system. The vessel was then
heated to the desired temperature, 50.degree. C., by a
recirculating heater/cooler (Fisher) via a machined internal
coil.
[0068] Extraction was continued for a period of around four (4)
hours until no acetone could be detected in the effluent stream.
This process usually took approximately 400 g of CO.sub.2. The
temperature was controlled during each experiment with a variation
of 0.5.degree. C. The pressure was measured using a pressure
transducer (Omega Engineering, Stamford, Conn., Model PX01K1-5KGV).
At the end of the extraction period, the vessel was slowly
depressurized at 50.degree. C. Depressurization took approximately
three (3) hours.
[0069] Once the depressurization was complete, the vessel was
opened and the monolith removed as RF aerogel. It weighed 2.9 g
(theoretical yield=3.09 g) indicating that almost all of the
resorcinol and formaldehyde polmerized and very little weight loss
occurred during processing. The shape and volume of the monolith
was conserved during the extraction and depressurization
process.
[0070] The properties of the RF aerogel were investigated by BET
nitrogen adsorption. Part of the aerogel was crushed into a powder
and analyzed. Part of it was broken down into large pieces and one
of the pieces was analyzed. The BET surface area (SA) and pore size
distribution of RF and carbon aerogels (CA) were obtained from at
least ninety (90) point nitrogen adsorption/desorption analysis
with a Micromeritics ASAP 2010 instrument. All samples were
outgassed at 100.degree. C. (for RF) and 200.degree. C. (for CA)
prior to any adsorption-desorption measurements. BET SA was
calculated from the linear part of the BET equation over the
relative pressure range of 0.03 to 0.30. Pore size distributions
were derived from the incorporated software of the instrument using
the Barrett, Joyner and Halenda ("BJH") method, the details of
which are known to those skilled in the art.
[0071] Powder Form: TABLE-US-00001 BET Surface Area: 888 m.sup.2/g
Pore Volume: 1.24 cm.sup.3/g Average Pore Diameter: 5.5 nanometers
("nm") (from BJH desorption)
[0072] Large Piece Form: TABLE-US-00002 BET Surface Area: 873
m.sup.2/g Pore Volume: 1.49 cm.sup.3/g Average Pore Diameter: 6.3
nm (from BJH desorption)
[0073] The data indicate that the pore structure stayed
substantially intact when crushed. There seems to be a light
decrease in the pore volume and average pore diameter upon
crushing. The shapes of the hysteresis loops for both crushed and
uncrushed samples are Type E.
EXAMPLE 2
Acetone--Water Exchange
[0074] To investigate the effect of immersion time in acetone, 3
monoliths were prepared using the procedure and reactant
compositions given above in Example 1. One sample was left for one
(1) day, one sample was left for two (2) days and one sample was
left for five days in an acetone bath followed by a one hour
sonication in fresh acetone. Subsequently, acetone was extracted
from the samples using supercritial carbon dioxide as described
above in Example 1.
[0075] After depressurization, the weights of the pellets ranged
from 2.84 to 2.90 g indicating that immersion time in acetone had
no effect on the amount of water displaced. It is belived that
sonication of the samples in acetone accelerated the acetone-water
exchange process dramatically.
EXAMPLE 3
Preparation of Carbon Aerogels
[0076] RF aerogels made in accordance with Example 1 were converted
to carbon aerogels by pyrolysis in an inert nitrogen atmosphere. RF
aerogel in monolithic form was placed in a quartz tube. The quartz
tube was paced in a tubular oven. One end of the tube was connected
to a nitrogen cylinder.
[0077] The flow rate of the nitrogen was controlled using a needle
valve placed after the regulator. The other end of the tube was
connected to a soap bubble meter to measure the flow rate of
nitrogen. The flow rate of nitrogen was adjusted to 100
cm.sup.3/min and the oven was heated to 1000.degree. C. under
flowing nitrogen. The heating rate was approximately 5.degree.
C./min. The temperature inside the oven reached 1020.degree. C. in
approximately six (6) hours.
[0078] The oven was kept at this temperature for another five (5)
hours. Subsequently, the oven was turned off and cooled overnight
with nitrogen flowing. The material removed from the tube was black
and its size appeared to have shrunk compared to the original
material.
[0079] Large Piece Form: TABLE-US-00003 BET Surface Area: 741
m.sup.2/g Pore Volume: 0.77 cm.sup.3/g Average Pore Diameter: 3.4
nm (from BJH desorption)
[0080] The analysis indicates that loss of surface area was not
significant during pyrolysis. However, the pore volume decreased
and the average pore diameter also decreased in accordance with the
volume contraction observed.
EXAMPLES 4-11
Effect of Reactant Concentrations on the Properties of Aerogels
[0081] To investigate the effects of concentrations of the
reactants and catalyst on the properties of the aerogels, a variety
of solutions were prepared and processed as explained above. The
characteristics of the aerogels are shown in Table 1.
TABLE-US-00004 TABLE 1 Effect of Reactant Composition on Properties
of Aerogels Average Pore Diameter Pore Example BET SA nm Volume No.
R/C R/W m.sup.2/g BET BJH cm.sup.3/g 4-C.sub.12RF 99.1 0.08 888.6
5.6 5.5 1.24 5-C.sub.12CA 99.1 0.08 741.2 4.1 4.3 0.78 6-C.sub.13RF
50.0 0.08 864.0 5.3 4.5 1.1 7-C.sub.13CA 50.0 0.08 682.4 3.1 3.2
0.52 8-C.sub.11RF 91.2 0.04 765.4 11.0 9.9 2.10 9-C.sub.11CA 91.2
0.04 723.4 8.6 7.9 1.55 10-C.sub.49RF 200.0 0.02 636.7 [[16.5]]
[[17.1]] [[2.63]] 19.3 20.7 3.03 11-C.sub.49CA 200.0 0.02 629.2
[[19.3]] [[20.7]] [[3.03]] 16.5 17.1 2.63
EXAMPLE 12
Impregnation of Platinum on Aerogel
[0082] RF aerogels made in accordance with Example 1 were
impregnated with an organometallic complex containing platinum
which was subsequently reduced to platinum metal and carbon aerogel
by pyrolysis.
[0083] 0.8 g RF aerogels (2 pieces) was placed in the vessel
described above in Example 1 together with finely ground 0.150 g
dimethyl(cyclooctadiene)platinum (II) (CODPtMe.sub.2)--The vessel
was heated to 80.degree. C. and charged with carbon dioxide to 2250
psig. The system was depressurized after 6 hours and weighed 0.85 g
indicating that 50 milligrams ("mg") of the Pt complex had adsorbed
on the RF aerogel. The RF aerogel pieces were pyrolyzed as
described in Example 3 resulting in 0.42 g of platinum loaded
carbon aerogel. The estimated platinum content was 7 wt %.
[0084] The sample was also characterized for its platinum content.
The sample was first digested with nitric and hydrochloric based on
Method EPA 3010. Subsequently, the samples were put in a hot block
for about four (4) hours and then the solution was analyzed by
Inductively Coupled Mass Spectrometry (Perkin Elmer, Norwalk,
Conn., Model OPTIMA 3300 XL, with AS 91 Autosampler, based on EPA
Method 6010B. The sample had a platinum content of 5.6 wt %, which
is close to the value calculated gravimetrically.
EXAMPLE 13
Metallic Compound Added to Sol
[0085] 2 g of resorcinol was dissolved in 1.5 g of water in a test
tube. To this solution, 0.019 g of sodium carbonate was added and
the tube was shaken until clear solution was obtained. In a
separate vial, 0.05 g of tetraamine platinum (II) chloride,
Pt(NH.sub.3).sub.4Cl.sub.2 was dissolved in 1.18 g water and the
solution was added to the tube. The tube was sealed by a rubber
stopper.
[0086] The tube was kept at room temperature for one (1) day, at
50.degree. C. for one (1) day and at 90.degree. C. for three (3)
days. At the end of the first day, the solution in the tube gelled
and had a brownish color. After three days at 90.degree. C., the
tube was taken out of the oven and cooled. The monolith had an
orange color and there was no weight loss. The monolith was
immersed in an acetone bath for two days. After sonication in fresh
acetone for one hour, acetone was extracted by scCO.sub.2 as
described above. The platinum loaded RF aerogel was pyrolyzed as
described above in Example 3 giving platinum loaded carbon aerogel.
The results of the BET analysis and hydrogen chemisorption studies
are given below:
[0087] Powder Form: TABLE-US-00005 BET Surface Area: 545 m.sup.2/g
Pore Volume: 1.34 cm.sup.3/g Average Pore Diameter: 20.5 nm
Hydrogen Chemisorption: [0088] Metal Dispersion: 31% based on the
original amount of platinum placed into the sol [0089] Metallic
Surface Area: 0.79 M.sup.2/g sample
EXAMPLE 14
Metallic Compound Added to Sol
[0090] 2 g of resorcinol were dissolved in 1.5 g of water in a test
tube. To this solution, 0.01 9 g of sodium carbonate was added and
the tube was shaken until a clear solution was obtained. In a
separate via, 0.05 g of potassium tetrachloroplatinate was
dissolved in 1.18 g of water and the solution was added to the
tube. The tube was sealed by a rubber stopper. The tube was kept at
room temperature for one (1) day, at 50.degree. C. for one (1) day
and at 90.degree. C. for three (3) days. At the end of the first
day, the solution in the tube gelled and had a brownish color.
After three days at 90.degree. C., the tube was taken out of the
oven and cooled. The monolith had an orange color and there was no
weight loss. The monolith was immersed in an acetone bath for two
days. After sonication in fresh acetone for one hour, acetone was
extracted by supercritical CO.sub.2 as described above and the
sample pyrolyzed.
EXAMPLE 15
Platinum Deposition on Aerogel with Carbon Dioxide
[0091] A carbon aerogel prepared as described above in Example 3
was broken down into small pieces and 1.229 g of small pices 4
carbon aerogel, 250 mg of finely ground
dimethyl(cyclooctadiene)platinum (II)(CODPtMe.sub.2) and a magnetic
stir bar were put into a 50 ml high pressure vessel equipped with a
rupture disk assembly, 2 sapphire windows and a pressure
transducer. The vessel was placed on a magnetic stirrer and was
heated to 80.degree. C. and charged with carbon dioxide to 4000
psig. The vessel was kept at these conditions for a period of 24
hours. After 24 hours, the vessel was depressurized and
CODPtMe.sub.2 loaded carbon aerogel was removed out of the vessel.
It weighed 1.449 g indicating that 220 mg of the precursor was
adsorbed into the carbon aerogel.
EXAMPLE 16
Reduction of Platinum
[0092] 593.2 mg of the CODPtMe.sub.2 loaded carbon aerogel of
Example 15 was placed in a quartz tube and heated to 350.degree. C.
under flowing nitrogen in a furnace such as used for pyrolysis in
Example 3. It was kept at 350.degree. C. for a period of 6 hours.
During this period, CODPtMe.sub.2 was converted to platinum metal.
Subsequently, the furnace was turned off and the tube was allowed
to cool under flowing nitrogen. The pieces of platinum loaded
carbon aerogels were removed from the tube and crushed into
particles. Analysis of the particles by TEM indicated the presence
of uniformly distributed small Pt crystallites with an average size
of 1 nm within the carbon aerogel matrix.
[0093] Additional samples were prepared in accordance with the
procedures set forth in Examples 15 and 16, the results of which
are set forth in Table 2, below. TABLE-US-00006 TABLE 2 Properties
of Platinum Loaded Carbon Aerogel Particles Pt Loading Sample R/C
R/W R/F wt(%) Pt Size (nm) 16a-C.sub.69B 100 0.08 0.5 13 NA
16b-C.sub.30 100 0.08 0.5 18 2 16c-C.sub.45 100 0.08 0.5 20 2
16d-C.sub.48 100 0.08 0.5 30 3 16e-C.sub.12 100 0.08 0.5 12 1
16f-C.sub.49 200 0.02 0.5 43 NA
[0094] The platinum particle size was determined using a
Transmission Electron Microscope (TEM) (Model 2010, FAS JEOL).
EXAMPLE 17
Reduction of Platinum
[0095] 614.1 mg of the CODPtMe.sub.2 loaded carbon aerogel of
Example 15 was placed in a quartz tube and heated to 500.degree. C.
under flowing nitrogen in a furnace such as used for pyrolysis in
Example 3. It was kept at 500.degree. C. for a period of 6 hours.
During this period, CODPtMe.sub.2 was converted to platinum metal.
Subsequently, the furnace was turned off and the tube was allowed
to cool under flowing nitrogen. The pieces of platinum loaded
carbon aerogels were removed from the tube and crushed into
particles. Analysis of the particles by TEM indicated the presence
of uniformly distributed small Pt crystallites with an average size
of 1.2 nm within the carbon aerogel matrix.
EXAMPLE 18
Platinum Loading on Silica Aerogel
[0096] Random sizes of silica aerogels were purhcased from
Marketech International, Inc., and the organic precursor,
dimethyl(cyclooctadiene)platinum (II)(CODPtMe.sub.2), from STREM
Chemicals and was used without any purification. The silica aerogel
had a whitish tint and was transparent.
[0097] A certain amount of this monolithic aerogel (842.5 mg) was
placed in a high-pressure vessel (internal volume of 54 cc), custom
manufactured from 316 stainless steel and equipped with two
sapphire windows (diameter=1.25'', thickness=0.5''), sealed on both
sides with PEEK seals, together with 374.6 mg of finely ground of
the Pt precursor.
[0098] The vessel was heated to 80.degree. C. and charged with
carbon dioxide to 4000 psig. In approximately 2.5 hours, all of the
CODPtMe.sub.2 dissolved and was adsorbed into the aerogel. The
vessel was kept at these conditions for 24 hours to ensure that
adsorption equilibrium was achieved. The system was depressurized
slowly for 3 hours at 60.degree. C. and then cooled to room
temperature. The sample was taken out of the vessel and weighed.
The final weight was 1059.3 mg which corresponds to a 216.8 mg
loading of the precursor (approximately 20 wt %). The aerogel
composite was still intact and its color turned black but was still
vaguely transparent indicating some conversion of the precursor to
platinum metal under these conditions.
[0099] A chunk of this (804.4 mg) aerogel composite was placed in a
quartz tube and then heated to 300.degree. C. using a Thermolyne
Tube Furnace (Model F21125) with flowing nitrogen gas at 100 ml/min
for 6 hours. The aerogel was cooled to room temperature and
weighed. The final weight was 736.0 mg which corresponds to 13% Pt
loading, indicating the conversion of the precursor to the platinum
metal was complete. The color of the composite was pitch black and
color was uniform throughout. It was nontransparent but intact.
[0100] Those skilled in the art will recognize that the invention
has been described with reference to specific aspects and that
other aspects are intended to be within the scope of the claims
which follow. For example, additional materials may be incorporated
into the aerogel during its preparation, or incorporated into the
aerogel after its formation, to achieve desired properties, e.g.,
electrical conductivity. In addition, unless otherwise defined, all
technical and scientific terms used herein have the same meaning as
commonly understood by one of ordinary skill in the art to which
this invention belongs. Although methods and materials similar or
equivalent to those described herein can be used in the practice or
testing of the present invention, the preferred methods and
materials are described above. All publications, patent
applications, patents, and other references mentioned herein are
incorporated by reference in their entirety. In case of conflict,
the present specification, including definitions, will control.
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