U.S. patent application number 10/101175 was filed with the patent office on 2003-06-12 for method for incorporating metal nanoparticles in porous materials.
This patent application is currently assigned to National Inst. of Advanced Ind. Science and Tech.. Invention is credited to Tai, Yutaka, Tajiri, Koji, Tanemura, Sakae, Watanabe, Masao.
Application Number | 20030107024 10/101175 |
Document ID | / |
Family ID | 19183001 |
Filed Date | 2003-06-12 |
United States Patent
Application |
20030107024 |
Kind Code |
A1 |
Tai, Yutaka ; et
al. |
June 12, 2003 |
METHOD FOR INCORPORATING METAL NANOPARTICLES IN POROUS
MATERIALS
Abstract
The method of preparing the porous material incorporating
ultrafine metal particles comprises the following steps: (1)
preparing surface-protected ultrafine metal particles by reducing
metal ions in the presence of molecules such as dodecanethiol
molecules; (2) immersing a wet gel in a solution of the ultrafine
metal particles, thus forming an ultrafine metal particle/wet gel
composite in which the ultrafine metal particles are incorporated
in the wet gel; and (3) drying the ultrafine metal particle/wet gel
composite to form a porous body.
Inventors: |
Tai, Yutaka; (Aichi, JP)
; Tajiri, Koji; (Aichi, JP) ; Watanabe, Masao;
(Aichi, JP) ; Tanemura, Sakae; (Aichi,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
National Inst. of Advanced Ind.
Science and Tech.
Tokyo
JP
|
Family ID: |
19183001 |
Appl. No.: |
10/101175 |
Filed: |
March 20, 2002 |
Current U.S.
Class: |
252/500 ;
252/512 |
Current CPC
Class: |
Y10S 977/896 20130101;
Y10T 428/12014 20150115; Y10S 977/895 20130101; Y10S 977/773
20130101; B82Y 30/00 20130101; B01J 23/70 20130101; Y10T 428/12042
20150115; Y10T 428/2907 20150115; B01J 23/38 20130101; B01J 23/52
20130101; B01J 37/16 20130101; B01J 35/0013 20130101; Y10T 428/2924
20150115 |
Class at
Publication: |
252/500 ;
252/512 |
International
Class: |
H01B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 7, 2001 |
JP |
2001-374445 |
Claims
1. A method of preparing a porous material incorporating ultrafine
metal particles, comprising the steps of: preparing
surface-protected ultrafine metal particles by reducing metal ions
in the presence of thiol groups or other surface-protecting groups;
immersing a wet gel in a solution of the ultrafine metal particles,
thus forming an ultrafine metal particle/wet gel composite in which
the ultrafine metal particles are incorporated in the wet gel; and
drying the ultrafine metal particle/wet gel composite.
2. The method of preparing a porous material incorporating
ultrafine metal particles according to claim 1, wherein the metal
is one or more selected from the group consisting of gold, silver,
palladium and other noble metal, and iron, cobalt and other
transition metal.
3. The method of preparing a porous material incorporating
ultrafine metal particles according to claim 1, wherein the
ultrafine metal particles have a particle diameter of 1 to 20
nm.
4. The method of preparing a porous material incorporating
ultrafine metal particles according to claim 1, wherein the solvent
in which the ultrafine metal particles are dissolved is toluene,
hexane, and/or tetrahydrofuran.
5. The method of preparing a porous material incorporating
ultrafine metal particles according to claim 1, wherein the wet gel
used is a silica wet gel or an alumina wet gel.
6. The method of preparing a porous material incorporating
ultrafine metal particles according to claim 1, wherein the
ultrafine metal particle/wet gel composite is dried by natural
drying or supercritical drying.
7. A method of preparing a porous material incorporating ultrafine
metal particles, comprising the step of heating the porous material
prepared by the method defined in claim 1 to remove the
surface-protecting molecules.
8. A porous material supporting ultrafine metal particles,
comprising an ultrafine metal particle/aerogel composite, prepared
by the method defined in any of claims 1 through 7.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of preparing a
porous material incorporating ultrafine metal particles, and more
particularly to a method of preparing a porous material
incorporating ultrafine metal particles of diameter a few
nanometers that can be used, for example, as a catalyst or a
photonics material that uses a minute size effect, and to the
porous material incorporating ultrafine metal particles prepared
using the method.
[0003] 2. Background of the Invention
[0004] Moreover, it is expected that materials in which ultrafine
metal particles are dispersed through a transparent matrix should
be usable as photonics materials that make use of the highly
nonlinear third order optical properties characteristic of
ultrafine particles. The method generally used to prepare such
materials is to mix a metal salt into a solution of an alkoxide of
silicon or the like, and precipitate out ultrafine particles during
a matrix gelation process by heating and adding a reducing
agent.
[0005] Conventionally, the method most commonly used to prepare a
material in which ultrafine metal particles are incorporated in a
porous material is to first prepare a porous body having a
honeycomb shape or the like, then make this porous body come into
contact with a solution containing metal ions, and then carry out
heat treatment/reduction (reference: Koji Onishi, Shokubai--Sono
Himitsu o Saguru--(`Probing the Secrets of Catalysts`),
Dainippon-tosho, 1987). However, with this method, the porous body
must have sufficient strength so as not to be damaged by the
metal-introducing process, and hence the surface area of the porous
body per unit weight cannot be made very high, and thus the amount
of metal that can be introduced is limited. As a result, the
catalyst quantity per unit weight of the material cannot be made
very high. Moreover, because the fine metal particles are produced
through heat treatment/reduction, sintering occurs during the heat
treatment, resulting in relatively large fine particles of diameter
of the order of several microns.
[0006] Moving on, a method commonly used to disperse ultrafine
metal particles of diameter a few nm to a few tens of nm through a
support is to prepare the ultrafine metal particles in a gel
(reference: ed. Ueno, Mizukami and Sodezawa, Kinzoku-Arukokishido o
Mochiiru Shokubai Chosei (`Preparation of Catalysts using Metal
Alkoxides`), ICP, 1993). In this method, a salt or complex of the
metal that one wishes to incorporate is added to a solution of a
metal alkoxide that acts as a precursor of the support, and then
hydrolysis and gelation are carried out, followed by drying.
Ultrafine metal particles are then precipitated out into the
support by heating to a few hundred, or else instead of heating,
the ultrafine particles are produced at room temperature by adding
a reducing agent. By using this method, ultrafine metal particles
can be supported at any desired proportion in a fine support
network, but because the precipitation of the ultrafine metal
particles occurs at the heat treatment/reduction stage after the
gel has been dried, there is a problem that it is difficult to
control the size of the particles; moreover, in the case of heat
treatment, sintering of the particles occurs, and hence there is a
problem that relatively large ultrafine particles are produced.
[0007] Ultrafine particles of a metal such as gold of diameter a
few nm cannot exist as is in a solution, the air or the like, but
rather agglomeration occurs. However, by adsorbing protective
groups such as thiol groups onto the surfaces of the ultrafine
metal particles, such agglomeration can be inhibited. Specifically,
it is known that by reducing metal ions in chloroauric acid or the
like in the presence of an alkylthiol or the like, the metal can be
made to stably exist in the form of ultrafine particles of diameter
a few nm. Moreover, by controlling the preparation conditions, it
is possible to control the size of the ultrafine metal particles
produced.
[0008] However, art for dispersing such ultrafine metal particles
through a porous body in a form in which the surface-protecting
groups such as thiol groups have been removed so that usage as a
catalyst is possible, and art for dispersing such ultrafine metal
particles through a transparent inorganic substance so that
application to photonics elements is possible, has not yet been
developed.
[0009] With the foregoing in view, the present inventors carried
out assiduous studies with a goal of developing art for introducing
ultrafine metal particles of diameter down to a few nm that have
been pre-prepared with size control carried out into a porous
inorganic material in any desired proportion without changing the
size of the ultrafine particles. As a result, the present inventors
discovered that this goal can be attained by immersing a wet gel in
a solution of surface-protected ultrafine metal particles, drying
the resulting ultrafine metal particle/wet gel composite, and
heating the resulting dried body to remove the surface-protecting
molecules. As a result, the present inventors arrived at the
present invention.
SUMMARY OF THE INVENTION
[0010] A porous material incorporating ultrafine metal particles
and a method of preparing the same are provided. The method of
preparing the porous material incorporating ultrafine metal
particles comprises the following steps: (I) preparing
surface-protected ultrafine metal particles by reducing metal ions
in the presence of molecules such as dodecanethiol molecules; (2)
immersing a wet gel in a solution of the ultrafine metal particles,
thus forming an ultrafine metal particle/wet gel composite in which
the ultrafine metal particles are incorporated in the wet gel; and
(3) drying the ultrafine metal particle/wet gel composite to form a
porous body. Moreover, the surface-protecting molecules are
subsequently removed by heating the porous body.
[0011] It is thus an object of the present invention to manufacture
and provide an ultrafine metal particle/aerogel composite by
preparing ultrafine metal particles using a liquid phase method,
incorporating the ultrafine metal particles in a gel to produce an
ultrafine metal particle/wet gel composite, and then drying the
ultrafine metal particle/wet gel composite.
[0012] To solve the above problems, the present invention is
constituted from the following technical means.
[0013] (1) A method of preparing a porous material incorporating
ultrafine metal particles, comprising the steps of:
[0014] preparing surface-protected ultrafine metal particles by
reducing metal ions in the presence of thiol groups or other
surface-protecting groups;
[0015] immersing a wet gel in a solution of the ultrafine metal
particles, thus forming an ultrafine metal particle/wet gel
composite in which the ultrafine metal particles are incorporated
in the wet gel; and
[0016] drying the ultrafine metal particle/wet gel composite.
[0017] (2) The method of preparing a porous material incorporating
ultrafine metal particles described in (1) above, wherein the metal
is one or more selected from the group consisting of gold, silver,
palladium and other noble metal, and iron, cobalt and other
transition metal.
[0018] (3) The method of preparing a porous material incorporating
ultrafine metal particles described in (1) above, wherein the
ultrafine metal particles have a particle diameter of 1 to 20
nm.
[0019] (4) The method of preparing a porous material incorporating
ultrafine metal particles described in (1) above, wherein the
solvent in which the ultrafine metal particles are dissolved is
toluene, hexane, and/or tetrahydrofuran.
[0020] (5) The method of preparing a porous material incorporating
ultrafine metal particles described in (1) above, wherein the wet
gel used is a silica wet gel or an alumina wet gel.
[0021] (6) The method of preparing a porous material incorporating
ultrafine metal particles described in (1) above, wherein the
ultrafine metal particle/wet gel composite is dried by natural
drying or supercritical drying.
[0022] (7) A method of preparing a porous material incorporating
ultrafine metal particles, comprising the step of heating the
porous material prepared by the method described in (1) above to
remove the surface-protecting molecules.
[0023] (8) A porous material supporting ultrafine metal particles,
comprising an ultrafine metal particle/aerogel composite, prepared
by the method as described in any of (1) to (7) above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a photograph of a silica wet gel into which have
been adsorbed ultrafine gold particles from toluene;
[0025] FIG. 2 is a TEM photograph of a silica aerogel incorporating
ultrafine gold particles;
[0026] FIG. 3 shows the visible absorption spectrum of a silica
aerogel incorporating ultrafine gold particles; and
[0027] FIG. 4 shows the visible absorption spectra of a silica
aerogel incorporating ultrafine gold particles before and after
heat treatment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The present invention will now be described in more
detail.
[0029] As described above, one of the constituent elements of the
present invention comprises immersing a wet gel that will become
the porous support in a solvent solution containing ultrafine metal
particles that have been stabilized by adsorbing surface-protecting
groups such as thiol groups onto the surfaces thereof, before the
wet gel is immersed in the solvent solution, the liquid phase of
the wet gel is replaced with the solvent of the solvent solution.
As a result, because the ultrafine metal particles are stabilized,
the ultrafine metal particles are adsorbed onto the inner surfaces
of the wet gel spontaneously without undergoing any structural
changes such as agglomeration. Control can be carried out by
changing the concentration of the ultrafine metal particles
introduced, the concentration of the ultrafine metal particles in
the solution in which the wet gel is immersed, and the immersion
time. Moreover, another constituent element of the present
invention comprises carrying out natural drying or supercritical
drying using liquefied carbon dioxide or the like of the wet gel
into which the ultrafine metal particles have been adsorbed as
described above. As a result, a material can be obtained in which
the ultrafine metal particles are incorporated uniformly in a
incorporating porous body, with agglomeration not taking place and
hence the ultrafine metal particles having the same size as when
initially introduced from the solvent solution. Furthermore,
another constituent element of the present invention comprises
heating the incorporating porous body thus obtained. As a result,
the surface-protecting groups can be removed, thus obtaining a
material in which the ultrafine metal particles are dispersed
through the porous body in a form in which the surface-protecting
molecules have been removed, while hardly changing the size of the
metal cores of the ultrafine metal particles.
[0030] In the present invention, a noble metal such as gold, silver
or palladium, or a transition metal such as iron and cobalt, is
preferably used as the metal in the ultrafine metal particles.
Moreover, as the `surface-protecting groups such as thiol groups`,
a reagent suitable for stabilizing the ultrafine metal particles in
the solvent solution is used, for example a thiol such as
dodecanethiol or benzenethiol, or a phosphorus compound such as
triphenylphosphine. The ultrafine metal particles and the
stabilizing reagent are not limited to the examples given above,
but rather other combinations of ultrafine metal particles and
stabilizing reagent may be used so long as the metal is able to
exist stably in the solvent solution without agglomerating. In the
present invention, `surface-protecting groups such as thiol groups`
is thus defined to mean a reagent suitable for stabilizing the
ultrafine metal particles in the solvent solution as described
above. Moreover, ultrafine metal particles prepared using the
method described above generally have a particle diameter of about
1 to 20 nm, but the particle diameter is not particularly limited
to being in this range.
[0031] A silica wet gel, an alumina wet gel or the like can be used
as the wet gel that becomes the incorporating porous body. Such a
wet gel is generally prepared through hydrolysis of a metal
alkoxide, for example methyl silicate or ethyl silicate in the case
of a silica wet gel, and gelation. However, so long as a wet gel
having a strength sufficient to withstand the solvent replacement
described below and the like can be obtained, the material and
method of preparing the wet gel are not limited to those above.
Moreover, when preparing the wet gel, by diluting the metal
alkoxide with an alcohol such as ethanol before carrying out the
hydrolysis and gelation, wet gels of various metal oxide
concentrations can be obtained. Depending on this and the drying
method, porous bodies incorporating the ultrafine metal particles
of various densities can be obtained.
[0032] The solvent used when adsorbing the ultrafine metal
particles into the wet gel should be such that both the stabilized
ultrafine metal particles and the wet gel can exist stably;
examples include toluene, benzene, hexane, tetrahydrofuran and the
like and mixtures thereof, and also mixtures of the above with
insoluble solvents such as ethanol and acetonitrile.
[0033] The solvent in the liquid phase of the wet gel is replaced
with a series of solvents, with each solvent being miscible with
the last one, until the target solvent is reached. For example, in
the case that the target solvent is toluene and the wet gel has
been prepared through hydrolysis of a metal alkoxide and gelation,
then because the liquid phase of the wet gel is initially a mixture
of water and an alcohol such as ethanol, the liquid phase is first
replaced with the pure alcohol, and then the pure alcohol is
replaced with toluene. Each replacement of the liquid phase in the
wet gel is carried out by repeating 2 to 3 times an operation in
which the wet gel is immersed for 5 to 6 hours at room temperature
in the pure solvent to be replaced with so that the old solvent is
replaced with this new solvent. The immersion time and the number
of solvent replacements should of course be changed as appropriate
in accordance with the size of the wet gel.
[0034] The wet gel as described above is normally immersed in the
solution containing the stabilized ultrafine metal particles for
about 1 to 2 days at room temperature. With such an extent of
immersion, the ultrafine metal particles in the solution are
adsorbed onto the inner surfaces of the wet gel. Generally, the
solution containing the ultrafine metal particles is colored, and
the wet gel is colorless and translucent or almost transparent, and
hence the progress of the reaction can be observed from the wet gel
becoming colored and the solution becoming the original color of
the solvent, for example colorless and transparent. By changing the
concentration of the ultrafine metal particles in the solution, the
immersion time, and the ratio of the amount of the solution to the
amount of the wet gel, the concentration of the ultrafine metal
particles in the porous body finally obtained can be changed.
[0035] The wet gel into which the ultrafine metal particles have
been adsorbed is made into the porous body through natural drying
or supercritical drying. Natural drying is generally carried out by
leaving for a few days in the atmosphere at room temperature.
However, so long as a dried porous body can be obtained, the method
is not so limited; the conditions can be set variously in view of
the solvent type and the prevention of shrinkage during drying, for
example slight heating can be carried out or the drying can be
carried out under reduced pressure.
[0036] Moreover, in the case that supercritical drying is used, in
general a carbon dioxide medium is used, so that the temperature
necessary for the drying can be made low. The wet gel into which
the ultrafine metal particles have been adsorbed is put into an
autoclave, the autoclave is filled with the solvent in the liquid
phase of the wet gel, the liquid phase is replaced with liquefied
carbon dioxide under pressure, the carbon dioxide is made to be a
supercritical fluid under conditions above the critical conditions
for carbon dioxide, for example 50 and 10 MPa, and then the carbon
dioxide is removed while holding the temperature, thus obtaining
the porous body. According to this method, a very low density
porous body supporting ultrafine metal particles can be obtained.
Note that the supercritical medium should be such that the
temperature necessary for the drying is sufficiently low that
sintering of the ultrafine metal particles does not occur, but is
not limited to being carbon dioxide.
[0037] In the present invention, the surface-protecting molecules
are removed by heating the porous material incorporating the
ultrafine metal particles. The temperature at which the
surface-protecting molecules desorb can be estimated form
differential thermal analysis, thermogravimetric analysis or the
like. The desorption of the surface-protecting molecules from the
ultrafine metal particles in the porous material is achieved, for
example, by carrying out heat treatment in an electric furnace for
1 hour at a temperature about 10 above the desorption temperature.
Note, however, that so long as the heating apparatus, the
temperature and the heating time are sufficient for desorption of
the surface-protecting groups, the heating apparatus, the
temperature and the heating time are not limited to being as
above.
EXAMPLES
[0038] Specific examples of the present invention will now be
described. It should be noted, however, that the present invention
is not limited whatsoever by the following examples.
Example 1
[0039] (1) Preparation of Ultrafine Metal Particles
[0040] 30 ml of a 30 mmol chloroauric acid aqueous solution and 80
ml of a 50 mmol tetraoctylammonium bromide toluene solution were
mixed together, and agitation was carried out, thus extracting the
chloroaurate ions into the toluene phase. The toluene phase was
then separated off, 0.201 ml (0.842 mmol) of dodecanethiol was
added thereto, and agitation was carried out for 3 to 4 hours. 1.25
ml of a 0.4 mol sodium borohydride aqueous solution was then
instilled into the solution, and agitation was carried out for 3 to
4 hours, thus reducing the gold ions. The toluene phase was then
separated off, and then the solution was concentrated down to about
10 ml, before being mixed with 400 ml of ethanol. The resulting
mixed liquid was stored at -18, thus precipitating out ultrafine
metal particles. The ultrafine metal particles thus produced were
then purified twice by recrystallizing from the toluene-ethanol
mixed liquid. The metal cores of the particles produced had a mean
diameter of 2.6 nm. The half width of the particle diameter
distribution was about 2 nm.
[0041] (2) Preparation of Wet Gel
[0042] 51 g of tetramethyl silicate was mixed with 1078 of
methanol, and agitation was carried out. 36 g of ammonia water was
then added to the solution while continuing to agitate. The mole
ratio of the tetramethyl silicate to the methanol to the water at
this time was 1:10:6. After agitating for about 1 minute, the mixed
liquid was poured into a cylindrical mold (diameter 40 mm, depth 10
mm). The reaction liquid was left in the mold for about 1 hour, and
after it had been verified that the reaction liquid had solidified
into a jelly, the jelly was sealed with polyvinylidene chloride
film to prevent drying out. The jelly was then left for 1 day,
during which time gelation proceeded. The gel was then removed from
the mold, and was immersed in ethanol and left for at least 1 day.
To completely remove water and ammonia remaining in the gel, the
ethanol was then subsequently replaced 2 times.
[0043] (3) Adsorption of Ultrafine Metal Particles into the Wet
Gel
[0044] The solvent of the silica wet gel was replaced with 1:1
toluene-ethanol and then with toluene. To carry out the replacement
completely, the silica wet gel was immersed in the 1:1
toluene-ethanol twice and then in toluene three times, with each
immersion being carried out for at least 1 day.
[0045] The silica wet gel was then immersed in the toluene solution
of the ultrafine metal particles (concentration: 3 mg/50 ml). After
leaving for about 60 hours, the ultrafine gold particles in the
solution had been completely adsorbed into the silica wet gel, thus
forming the support. FIG. 1 shows a photograph of the wet gel
support.
Example 2
[0046] The wet gel support produced by the method of Example 1 was
subjected to supercritical drying using carbon dioxide, thus
obtaining an aerogel support.
[0047] Subsequently, the wet gel support obtained as described
above was put into an autoclave, and the autoclave was filled with
toluene. To replace the liquid phase part of the gel with liquefied
carbon dioxide gas (critical temperature 31.1, critical pressure
72.9 atmospheres), liquefied carbon dioxide gas was then injected
into the autoclave while pressurizing with a pressurizing pump.
Once the pressure had reached 90 atmospheres, the valve was
adjusted to maintain this pressure, and the state was then held for
2 hours at 20. To carry out the replacement completely, this
replacement operation was carried out 3 times. After the third
replacement had been completed, the valve was closed, thus
maintaining the pressure in the autoclave. The temperature in the
autoclave was then increased, thus increasing the pressure to 100
atmospheres. The valve was then adjusted, thus holding the
pressure. Once the temperature of the sample had exceeded 40, the
carbon dioxide gas in the autoclave was released such that the
pressure dropped at a rate of 1 atmosphere per minute.
[0048] The diameter of the metal cores of the ultrafine particles
in the aerogel support produced was about the same as before the
ultrafine particles were supported in the gel. A TEM photograph of
the aerogel support is shown in FIG. 2.
[0049] FIG. 3 shows the visible light absorption spectrum of an
aerogel support obtained using the method of the present invention.
It can be seen from the visible light absorption spectrum that the
silica aerogel, which is originally transparent in the visible
region, is colored by the ultrafine metal particles.
Example 3
[0050] When an aerogel support produced using the method of Example
2 above was subjected to thermal analysis, a weight loss
corresponding to desorption of the thiol molecules was observed at
about 290. Based on this data, such an aerogel support was heated
for 1 hour at 300 in a nitrogen atmosphere (flow rate 20 ml/min).
Visually inspecting the gel after the heating, there was no change
in the color compared with before the heating, and moreover the
visible light absorption spectra for before and after the heating
were almost the same. It is thus thought that the heating causes
hardly any change in the particle diameter. FIG. 4 shows the
visible light absorption spectra of the silica aerogel
incorporating the ultrafine metal particles before and after the
heating.
[0051] As described above, the present invention relates to a
porous material incorporating ultrafine metal particles and a
method of preparing the same. According to the present invention,
the following remarkable effects are produced: 1) Using
pre-prepared ultrafine metal particles of diameter 2 to 3 nm, a
porous body incorporating the ultrafine metal particles can be
prepared, with the particle size of the ultrafine metal particles
being maintained, and with it being possible to control the amount
of the ultrafine metal particles incorporated. 2) Moreover, the
ultrafine metal particles can be dispersed through the porous body
in a form in which the surface-protecting molecules have been
removed. 3) When using the porous body incorporating ultrafine
metal particles as a catalyst or the like, because the size of the
ultrafine metal particles in the porous body can be made small, the
surface area of the ultrafine metal particles can be made large,
and hence the catalytic efficiency can be improved. 4) Moreover,
with ultrafine metal particles of size a few nm there is expected
to be a quantum size effect, and hence it is anticipated that it
will be possible to apply the porous body incorporating ultrafine
metal particles to materials that use a quantum size effect such as
nonlinear optical materials.
* * * * *