U.S. patent application number 11/103676 was filed with the patent office on 2006-05-18 for multicomponent nanoparticles formed using a dispersing agent.
This patent application is currently assigned to Headwaters Nanokinetix, Inc.. Invention is credited to Sukesh Parasher, Michael Rueter, Bing Zhou.
Application Number | 20060105910 11/103676 |
Document ID | / |
Family ID | 36387150 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060105910 |
Kind Code |
A1 |
Zhou; Bing ; et al. |
May 18, 2006 |
Multicomponent nanoparticles formed using a dispersing agent
Abstract
Nanoparticles include a plurality of two or more dissimilar
components selected from the group of noble metals, base transition
metals, alkali earth metals, and rare earth metals and/or different
groups of the periodic table of elements. The two or more
dissimilar components are dispersed using a dispersing agent such
that the nanoparticles have a substantially uniform distribution of
the two or more dissimilar components. The dispersing agents can be
poly functional small organic molecules, polymers, or oligomers, or
salts of these. The molecules of the dispersing agent bind to the
particle atoms to overcome same-component attractions, thereby
allowing dissimilar components to form heterogeneous nanoparticles.
Dissimilar components such as iron and platinum can be complexed
using the dispersing agent to form substantially uniform
heterogeneous nanoparticles. The nanoparticles can be used alone or
applied to a support. At least a portion of the dispersing agent
can be removed by reduction and/or oxidation.
Inventors: |
Zhou; Bing; (Cranbury,
NJ) ; Parasher; Sukesh; (Lawrenceville, NJ) ;
Rueter; Michael; (Plymouth Meeting, PA) |
Correspondence
Address: |
WORKMAN NYDEGGER;(F/K/A WORKMAN NYDEGGER & SEELEY)
60 EAST SOUTH TEMPLE
1000 EAGLE GATE TOWER
SALT LAKE CITY
UT
84111
US
|
Assignee: |
Headwaters Nanokinetix,
Inc.
Lawrenceville
NJ
|
Family ID: |
36387150 |
Appl. No.: |
11/103676 |
Filed: |
April 12, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10990616 |
Nov 17, 2004 |
|
|
|
11103676 |
Apr 12, 2005 |
|
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Current U.S.
Class: |
502/338 |
Current CPC
Class: |
B01J 37/0211 20130101;
B01J 23/8906 20130101; B01J 37/0219 20130101; B01J 35/0013
20130101; B01J 21/18 20130101; Y10S 977/81 20130101 |
Class at
Publication: |
502/338 |
International
Class: |
B01J 23/745 20060101
B01J023/745 |
Claims
1. A multicomponent nanoparticle material, comprising: a plurality
of nanoparticles having a size less than about 100 nm formed from
at least two dissimilar nanoparticle components selected from
different members of the group consisting of noble metals, base
transition metals, alkali earth metals, and rare earth metals,
wherein at least about 50% of the nanoparticles include two or more
dissimilar nanoparticle components.
2. A multicomponent nanoparticle material according to claim 1,
wherein at least about 75% of the nanoparticles include two or more
dissimilar nanoparticle components.
3. A multicomponent nanoparticle material according to claim 1,
wherein at least about 85% of the nanoparticles include two or more
dissimilar nanoparticle components.
4. A multicomponent nanoparticle material according to claim 1,
wherein at least about 95% of the nanoparticles include two or more
dissimilar nanoparticle components.
5. A multicomponent nanoparticle material according to claim 1,
wherein at least about 99% of the nanoparticles include two or more
dissimilar nanoparticle components.
6. A multicomponent nanoparticle material according to claim 1,
wherein the plurality of nanoparticles have a size within a Range
of NR, excluding outliers, that is about 0.2 to about 5 times the
value of NR.sub.avg.
7. A multicomponent nanoparticle material according to claim 1,
wherein the plurality of nanoparticles have a size within a Range
of NR, excluding outliers, that is about 0.33 to about 3 times the
value of NR.sub.avg.
8. A multicomponent nanoparticle material according to claim 1,
wherein the plurality of nanoparticles have a size within a Range
of NR, excluding outliers, that is about 0.5 to about 2 times the
value of NR.sub.avg.
9. A multicomponent nanoparticle material as in claim 1, wherein
the at least two dissimilar components are alloyed.
10. A multicomponent nanoparticle material as in claim 1, wherein
the at least two dissimilar components are selected from the group
of component combinations comprising noble metal-base transition
metal, base transition metal-base transition metal, metal
oxide-noble metal, metal oxide-metal oxide.
11. A multicomponent nanoparticle material as in claim 1, wherein
at least one of the nanoparticle components comprises a base
transition metal.
12. A multicomponent nanoparticle material as in claim 11, wherein
the base transition metal comprises iron.
13. A multicomponent nanoparticle material as in claim 1, further
comprising a support material to which the nanoparticles are
attached.
14. A multicomponent nanoparticle material as in claim 1, wherein a
substantial portion of the nanoparticles are less than about 20 nm
in diameter.
15. A multicomponent nanoparticle material as in claim 1, wherein a
substantial portion of the nanoparticles are less than about 6 nm
in diameter.
16. A multicomponent nanoparticle material as in claim 1, wherein
the nanoparticles are catalytically active.
17. A multicomponent nanoparticle material, comprising: a plurality
of nanoparticles having a size less than about 100 nm formed from
at least two dissimilar metal nanoparticle components selected from
different groups of the periodic table of elements, wherein at
least about 50% of the nanoparticles include two or more dissimilar
metal nanoparticle components.
18. A multicomponent nanoparticle material according to claim 17,
wherein at least about 75% of the nanoparticles include two or more
dissimilar metal nanoparticle components.
19. A multicomponent nanoparticle material according to claim 17,
wherein at least about 95% of the nanoparticles include two or more
dissimilar metal nanoparticle components.
20. A multicomponent nanoparticle material according to claim 17,
wherein the plurality of nanoparticles have a size within a Range
of NR, excluding outliers, that is about 0.2 to about 5 times the
value of NR.sub.avg.
21. A method of preparing a multicomponent nanoparticle material,
comprising: (a) preparing a first solution of a first plurality of
nanoparticle atoms selected from the group consisting of noble
metals, base transition metals, alkali earth metals, rare earth
metals, and nonmetals; (b) preparing a second solution of a second
plurality of nanoparticle atoms, the second plurality of
nanoparticle atoms being a different member of the group consisting
of noble metals, base transition metals, alkali earth metals, rare
earth metals, and nonmetals than the first plurality of
nanoparticle atoms or (ii) a component select; (c) mixing together
the first solution, second solution, and a dispersing agent
selected from the group consisting of polyfunctional small organic
molecules, polymers, oligomers, and combinations thereof in order
to form a component complex; (d) causing or allowing the component
complex to form nanoparticles having a size less than about 100 nm
and bound to the dispersing agent; and (e) removing the dispersing
agent from the component complex by at least one of reduction or
oxidation in order to yield the multicomponent nanoparticles.
22. A method as in claim 21, wherein (c) yields one or more of a
suspension, solution or colloid.
23. A method as in claim 21, wherein the molar ratio of dispersing
agent functional groups to nanoparticle atoms is in a range of
about 0.01:1 to about 40:1.
24. A method as in claim 21, wherein the dispersing agent is
selected from the group consisting of glycolic acid, oxalic acid,
malic acid, citric acids, pectins, amino acids, celluloses,
polyacrylates, polyvinylbenzoates, polyvinyl sulfate, polyvinyl
sulfonates including sulfonated styrene, polybisphenol carbonates,
polybenzimidizoles, polypyridine, sulfonated polyethylene
terephthalate, polyvinyl alcohol, polyethylene glycol,
polypropylene glycol, and combinations thereof.
25. A method as in claim 21, wherein (c) further comprises
contacting the nanoparticles with a support material.
26. A multicomponent nanoparticle material manufactured according
to the method of claim 21.
Description
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 10/990,616, filed Nov. 17, 2004, the
disclosures of which is incorporated herein in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. The Field of the Invention
[0003] The invention is in the field of nanoparticles and/or
catalysts that incorporate such nanoparticles. More particularly,
the present invention relates to multi-component nanoparticles made
using a dispersing agent that helps bring together and distribute
different (e.g., dissimilar) components within the
nanoparticles.
[0004] 2. The Relevant Technology
[0005] Nanoparticles are becoming increasingly more important in
many industrial processes and products. Nanoparticles find use in a
variety of applications, including catalysis and nanomaterials.
Catalytic applications include uses for both supported and
unsupported nanoparticles of various components, including precious
metals, base metals, and oxides. Nanomaterial applications include
uses for light blocking, pigmentation, UV absorption, antimicrobial
activity, chemical mechanical polishing, and others.
[0006] While useful nanoparticles may include only a single
component (element or compound), it may be the case that
advantageous properties can be achieved if the nanoparticles were
to contain two or more distinct components to form a multicomponent
nanoparticle. In general, combinations of two or more metals can
have a variety of beneficial effects. In the case of catalysts, the
use of different elements can modify the catalytic activity to
improve an important performance parameter such as activity or
selectivity, or they may make the catalyst particle or crystal more
resistant to some deleterious effect, such as chemical poisoning or
mechanical attrition. In the case of nanomaterials, the inclusion
of two or more components would be expected to add additional
functionality to the particles, such as combining light blocking
function with UV absorption or anti-microbial activity.
Alternatively, additional components might be expected to stabilize
or strengthen the nanoparticles.
[0007] While there is a strong motivation for producing
multicomponent nanoparticles, it is difficult, if not impossible,
to manufacture particles that contain two or more dissimilar
components. This problem is particularly true of small
nanoparticles. Recently, academia and industry have made
significant advancements toward making very small particles. In
some cases, the sizes of the particles are near or below 1
nanometer.
[0008] While nanometer sized particles are very advantageous for
producing desired properties such as increased catalytic activity
and unique material properties, the very smallness of such
particles makes it difficult, if not impossible, to create
multicomponent nanoparticles that include dissimilar components or
elements within the same nanoparticle. One reason for this
difficulty is that similar or like elements or compounds have a
greater affinity for each other than to dissimilar materials. This
same-component attraction means each component has a propensity to
combine and form particles with itself rather than forming a
mixture with other, dissimilar components. As a result,
multicomponent nanoparticle mixtures are largely heterogeneous,
composed of two or more distinct particle compositions, each
relatively rich in one component and largely depleted or devoid of
the other dissimilar components.
[0009] In general, the composition of particles, including the
distribution of different components among and between the
particles, is driven by thermodynamics. The chance of finding
multiple components in any given particle depends to a large extent
on the size of the particles being formed. Where the particles are
relatively large, the probability is higher that two dissimilar
components can be compounded within a single particle and/or form
an alloy. As the size of the particles decreases, however, the
likelihood of finding multiple components within a single particle
decreases dramatically. At the nanometer scale, it is virtually
impossible to consistently and predictably compound two or more
dissimilar elements within a single nanoparticle using known
procedures. Small nanoparticles tend to be all of one component or
another.
[0010] Part of the problem with forming multicomponent nano-sized
particles is that conventional methods used to form nano-sized
particles are performed at relatively low temperatures since high
temperatures can causes nanoparticles to undesirably sinter or
agglomerate together to form larger particles. Unfortunately, at
such low temperatures, the thermodynamics of nanoparticle formation
favors formation of single-component particles, as described above.
On the other hand, raising the temperature sufficiently to overcome
thermodynamic barriers to multicomponent formation causes
agglomeration of smaller to larger particles. Consequently,
conventional particle formation methods are not able to form
nano-sized particles in which a substantial portion of the
nanoparticles contain two or more components in each particle.
[0011] Another factor that significantly affects the uniformity of
multicomponent particles is the dissimilarity of the components.
For example, two noble metals such as palladium and platinum are
typically more easily combined together within particles because
their electronic and chemical properties are similar. In contrast,
a noble metal such as platinum and a base metal such as iron have
different electronic and chemical properties and are thus much more
difficult, if not impossible, to compound together in a single
nanoparticle using conventional manufacturing methods. In many
cases, compounding dissimilar components does not produce a viable
nanoparticle system because of the lack of uniformity in the
distribution of the components throughout the nanoparticles. This
is particularly true in the case of catalyst particles that require
both catalyst components to be in close proximity and/or to be
alloyed together to generate the desired catalytic activity.
[0012] R. W. J. Scott et al., JACS Communications, 125 (2003) 3708,
state: " . . . at present there are no methods for preparing nearly
monodisperse, bimetallic nanoparticles that are catalytically
active . . . ." X. Zhang and K. Y. Chan, Chem. Mater., 15 (2003)
451, teach: "A number of techniques have been used for producing
nanoparticles, including vapor phase techniques, sol-gel methods,
sputtering, and coprecipitation. The synthesis of mixed metal
nanoparticles is attracting a lot of recent interest for their
catalytic properties . . . . The synthesis of mixed metal
nanoparticles is a complex problem because of the composition
control in addition to size and size distribution control.
Platinum-ruthenium bimetallic catalysts have been prepared by
co-impregnation methods but without good control of particle size,
particle size distribution, and chemical composition." R. W. J.
Scott et al., JACS Communications 127 (2005), 1380, disclose: "Most
other methods for preparing supported bimetallic nanoparticles in
the <5 nm size range lead to phase segregation of the two metals
and thus poor control over the composition of individual
particles." K. Hiroshima et al, Fuel Cells, 2 (2002) 31, teach:
"The preparation of a highly dispersed alloy catalyst typically
requires heat treatment, which is necessary to form an alloy but
promotes particle aggregation. As a result, alloy catalysts usually
have lower surface areas."
[0013] Therefore, what are needed are multicomponent nanoparticles
that include different components that are more evenly dispersed
among the particles. Furthermore, what is needed are compositions
and processes that can be used to bring together and compound
different (e.g., dissimilar) components together in individual
nanoparticles without destroying the nanometer size of the
particles.
BRIEF SUMMARY OF THE INVENTION
[0014] The present invention relates to nanoparticle compositions
that overcome the limitations of the prior art by providing "nano"
sized particles that are composed of two or more components in a
desired distribution. During manufacture, a dispersing agent binds
the two or more components and maintains them in close proximity
during nanoparticle formation in order to control the arrangement
and/or distribution of the components in the nanoparticle
material.
[0015] In an exemplary embodiment, the multicomponent compositions
of the present invention include a plurality of nanoparticles
having a size less than about 100 nm. According to one embodiment,
the plurality of nanoparticles includes at least two dissimilar
nanoparticle components selected from different ones of the
following groups: noble metals, base transition metals, alkali
metals, alkaline earth metals, rare earth metals, and nonmetals. In
an alternative embodiment, the multicomponent composition is made
from two dissimilar nanoparticle components selected from two or
more different groups of the periodic table of elements. The
components that form the nanoparticles can be elements or compounds
such as elemental metals or metal oxides.
[0016] Preferably, at least about 50% of the nanoparticles include
two or more dissimilar components. More preferably, at least about
75% of the nanoparticles include two or more dissimilar components,
even more preferably at least about 85% of the nanoparticles
include two or more dissimilar components, and most preferably at
least about 95% of the nanoparticles include two or more dissimilar
components. It is within the scope of the invention for at least
about 99% (or essentially all) of the nanoparticles to include two
or more dissimilar components.
[0017] The present invention also includes a method to produce the
uniform multicomponent nanoparticles. In general, the process
includes preparing first and second solutions of dissimilar
components and mixing them together with a dispersing agent to form
a component complex. The molecules of the dispersing agent bind to
at least a portion of the molecules of the first and second
components to sufficiently overcome the same-component attractions
such that the components can be arranged randomly or according to
the molecular arrangement of the dispersing agent within the
suspension. In some cases the component complex forms a suspension
of nanoparticles. In other cases, the component complex is a
precursor to the formation of nanoparticles (e.g., which may be
formed by attaching the component complex to a support and/or
removing at least a portion of the dispersing agent from the
component complex).
[0018] In one embodiment, a suspension of nanoparticles can be used
as an active catalyst while remaining in suspension form. In
another embodiment, the nanoparticles can be attached to or formed
on a solid support by suitable impregnation or attachment methods.
The nanoparticles can also be separated from some or all of the
liquid to form a concentrate of nanoparticles or a dry powder. As
needed, the suspension can be chemically modified to stabilize the
nanoparticles (e.g., prevent agglomeration), adjust pH, or
otherwise adjust composition to suit an end use application. In one
embodiment, the nanoparticles can be isolated by removing the
dispersing agent from the nanoparticles, such as under reducing
conditions (e.g., by reducing under H.sub.2 gas or using strong
reducing catalysts such as lithium aluminum hydride, sodium
hydride, sodium borohydride, sodium bisulfite, sodium thiosulfate,
hydroquinone, methanol, aldehydes, and the like, or by oxidation
such as by using molecular oxygen, hydrogen peroxide, organic
peroxides, and the like).
[0019] In an exemplary embodiment, the nanoparticles of the present
invention are also of a substantially uniform size such that the
particle size distribution (or deviation) is extremely narrow. The
substantially uniform particle size distribution produces a
nanoparticle material with more consistent properties and activity
throughout the material.
[0020] The nanoparticles and methods of the present invention
provide many advantages for making novel nanomaterials such as
catalysts and/or for improving the activity and performance of
existing nanomaterials. Novel nanomaterials are possible because
dissimilar components, which typically do not form uniform
particles, can be combined using one or more dispersing agents such
that most or all of the particles have the two or more components
in each particle. Because each nanoparticle contains a mixture or
alloy of the two or more components, each nanoparticle has the
intended or desired characteristic needed to produce the properties
of the multicomponent material.
[0021] Unlike the nanoparticles of the prior art, the dissimilar
components in the nanoparticles of the present invention are evenly
dispersed among the nanoparticles. The dispersing agent overcomes
the tendency for like components to agglomerate and form
homogeneous particles but instead helps form multicomponent
particles. In many cases, the functionality of the material depends
on forming heterogeneous (i.e. multicomponent) particles rather
than forming a heterogeneous mixture of homogeneous (i.e., single
component) particles, as is typically seen in the prior art. The
proper dispersing and mixing of the two or more components
according to the present invention imparts beneficial
characteristics, such as those described above.
[0022] Another advantage of the present invention is that the
dispersing agents are readily available and relatively inexpensive.
Still another advantage of the inventive process is that it is
highly flexible in that it works well with a variety of components
and thus can be used to improve many new and existing catalysts and
nanomaterials. Furthermore, existing and novel catalysts can be
stabilized thereby providing opportunities to use the nanoparticles
in new processes or improve the resistance of the nanoparticles to
degradation.
[0023] These and other advantages and features of the present
invention will become more fully apparent from the following
description and appended claims as set forth hereinafter.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
I. Introduction and Definitions
[0024] The present invention is directed to nanoparticles and
nanoparticle materials made from two or more different components.
The multicomponent nanoparticles are formed using a dispersing
agent. In an exemplary embodiment, the dispersing agent binds to
the components and determines in part the molecular arrangement of
the components. The dispersing agent is able to ensure that two or
more different components are distributed between and among
nanoparticles in a desired distribution. Nanoparticles according to
the invention can be used as catalysts with improved and/or novel
catalytic activity and/or to form nanomaterials having superior
properties.
[0025] For purposes of this invention, the term "nanoparticles" or
"nano-sized particles," means particles with a diameter of less
than about 100 nanometers (nm).
[0026] The term "component complex" refers to a solution, colloid,
or suspension in which a bond or coordination complex is formed
between a dispersing agent and one or more different types of
particle atoms. The "bond" between the control agent and particle
atoms can be ionic, covalent, electrostatic, or it can involve
other bonding forces such as coordination with nonbonding
electrons, van der Waals forces, and the
[0027] The term "minority component" means the component in a
multicomponent nanoparticle with the lesser concentration within
the particle. In the case where two or more components have
essentially the same concentration within the particle, evidenced
by the fact that the determination of a minority is statistically
impractical, then either component is considered to be the minority
component.
[0028] For purposes of disclosure and the appended claims, the term
"Number Ratio" or "NR" is equal to N.sub.A/N.sub.B where N.sub.A is
the number (or moles) of atoms of a more numerous component A in a
given nanoparticle or set of nanoparticles, and N.sub.B is the
number (or moles) of atoms of a less numerous component B in the
nanoparticle or set of nanoparticles. For a particular nanoparticle
i, NR can be expressed as the specific value (NR.sub.i). The
average NR for all of the nanoparticles in a given set of
nanoparticles is expressed as the average value (NR.sub.avg).
[0029] In most cases, the individual NR values corresponding to the
various particles within a given sample or set of nanoparticles do
not equal a single discrete value but fall within a range of NR
values (i.e., the "Range of NR"). The Range of NR for a given
sample of set of nanoparticles having at least two different
nanoparticle components within each particle has an upper value
NR.sub.max and a lower value NR.sub.min.
II. Multicomponent Nanopaticle Compositions
[0030] A. Nanoparticle Forming Component Complexes
[0031] As discussed above, two or more dissimilar atoms, molecules
or components are joined together into multicomponent nanoparticles
by means of a dispersing agent. The dissimilar components and the
dispersing agent form one or more types of component complexes from
which the multicomponent nanoparticles are formed. Thus, component
complexes include one or more different types of component atoms
complexed with one or more different types of dispersing agents.
When so complexed, the component atoms are arranged in such a
manner that the components either (i) form dispersed nanoparticles
in solution or (ii) that upon or after contact with a support, the
component complex forms dispersed nanoparticles. In either case,
the dispersing agent can form a component complex to produce
nanoparticles that are dispersed, stable, uniform, and/or desirably
sized. In the case where the component complex has not yet resulted
in the formation of nanoparticles, it may be proper to refer to
this complex as a nanoparticle-forming intermediate complex.
[0032] 1. Particle Component Atoms or Molecules
[0033] Any two or more elements or groups of elements that can form
catalysts or nanomaterials can be used to form component complexes
according to the present invention. As the primary component,
metals or metal oxides are preferred. Exemplary metals can include
base transition metals, rare earth metals, noble metals, and rare
earth metals. Nanoparticles may also comprise non-metal atoms,
alkali metals and alkaline earth metals. A catalyst compound
comprising two or more different types of atoms is referred to as a
molecule. Where catalytic activity is desired, elements or groups
of elements can be selected that exhibit primary catalytic
activity, as well as promoters and modifiers.
[0034] Examples of base transition metals include, but are not
limited to, chromium, manganese, iron, cobalt, nickel, copper,
zirconium, tin, zinc, tungsten, titanium, molybdenum, vanadium, and
the like. These can be used in various combinations with each
other, and/or in combinations with other different and/or
dissimilar metals such as noble metals, alkali metals, alkaline
earth metals, rare earth metals, or non-metals.
[0035] Molecules such as ceramics and metal oxides can also be used
in the nanoparticles of the present invention. Examples include
iron oxide, vanadium oxide, aluminum oxide, silica, titania,
yttria, zinc oxide, zirconia, cerium oxide, and the like.
[0036] Examples of noble metals, also referred to as precious
metals, include platinum, palladium, iridium, gold, osmium,
ruthenium, rhodium, rhenium, and the like. Noble metals can be used
in combination with other different and/or dissimilar elements,
such as base transition metals, alkali metals, alkaline earth
metals, rare earth metals, or non-metals.
[0037] Examples of rare earth metals include, but are not limited
to, lanthanum and cerium. These can be used alone, in various
combinations with each other, and/or in combinations with other
different and/or dissimilar elements, such as base transition
metals, noble metals, alkali metals, alkaline earth metals, or
non-metals.
[0038] Examples of non-metals include, but are not limited to,
phosphorus, oxygen, sulfur, antimony, arsenic, and halides, such as
chlorine, bromine and fluorine. At least some of the foregoing are
typically included as functionalizing agents for one or more
metals, such as those listed above.
[0039] When added to an appropriate solvent or carrier to form a
suspension, as described below, component atoms can be added in
elemental form; however, the component atoms are typically in ionic
form so as to more readily dissolve or disperse within the solvent
or carrier. For example, metal components can be added in the form
of salts or other compounds. Components that are compounds
themselves, such as oxides, can be added to a liquid medium in the
appropriate compound form, or may be in a different chemical form
that is converted to the appropriate chemical form during
nanoparticle formation. In the case of a metallic component, the
atoms may be in the form of a metal halide, nitrate or other
appropriate salt that is readily soluble in the solvent or carrier,
e.g., metal phosphates, sulfates, tungstates, acetates, citrates,
or glycolates.
[0040] 2. Dissimilar Components
[0041] In an exemplary embodiment, the nanoparticles of the present
invention include two or more dissimilar components. Two components
are dissimilar where the unique electronic configuration of each
component creates same-component attractions that, absent a
dispersing agent according to the present invention, significantly
affect or dominate the thermodynamics of particle formation and/or
arrangement. For example, iron is dissimilar from platinum. When
forming nanoparticles of platinum and iron using conventional
methods, most, if not all, of the platinum atoms form homogeneous
particles with other platinum atoms, and most, if not all, of the
iron atoms form homogeneous particles with other iron atoms. Absent
the use of a dispersing agent according to the present invention,
the dissimilarity of iron and platinum atoms creates same-component
attractions that predominate over other thermodynamic forces during
particle formation or arrangement. The result is generally a
heterogeneous mixture of largely homogeneous nanoparticles. In
contrast, the use of one or more dispersing agents as disclosed
herein overcomes such thermodynamic barriers and causes dissimilar
components to be compounded together so as to yield multicomponent
nanoparticles that include two or more dissimilar components in
each of a substantial portion, if not essentially all, of the
nanoparticles.
[0042] According to one embodiment, the dissimilar components
comprise one or more components selected from each of at least two
groups comprising (i) noble metals, (ii) base transition metals,
(iii) alkali metals, (iv) alkaline earth metals, (v) rare earth
metals, and (vi) non metals. That is, the dissimilar components
according to this embodiment comprise at least one component (a)
selected from one of groups (i)-(vi) and at least one other
component (b) selected from at least one other of groups
(i)-(vi).
[0043] In an alternative embodiment, dissimilar components are
selected from different groups of the periodic table of elements
(i.e., different columns of the periodic table). The dissimilar
components according to this embodiment comprise at least one
component (a') selected from one column of the periodic table and
at least one other component (b') selected from at least one other
column of the periodic table. Components selected from different
groups of the periodic table are often dissimilar because of the
difference in the number of valence electrons. As a non-limiting
example of components formed from different groups of the periodic
table, uniform nanoparticles may be composed of a mixture of
titania and zinc oxide.
[0044] It is within the scope of the invention for the dissimilar
components to comprise different base transition metals. Although
sometimes categorized together for simplicity, different base
transition metals often exhibit dissimilar properties. These
dissimilarities often create same-component attractions, which make
different base transition metals difficult to combine or alloy in a
dispersed manner. Likewise metal oxides can be difficult to
combine. Those skilled in the art are familiar with atoms and
molecules that are difficult or impossible to combine or alloy due
to dissimilarities in the two components.
[0045] 3. Dispersing Agents
[0046] One or more types of dispersing agents are selected to
promote the formation of multicomponent nanoparticles that have a
desired composition or distribution. Dispersing agents within the
scope of the invention include a variety of organic molecules,
polymers, and oligomers. The dispersing agent comprises individual
molecules that mediate in the formation of the multicomponent
nanoparticles.
[0047] In general, useful dispersing agents include organic
compounds that can form a complex with the component atoms or
molecules used to make nanoparticles in the presence of an
appropriate solvent or carrier, and optionally promoters and/or
support materials. The dispersing agent is able to interact and
complex with particle component atoms or molecules that are
dissolved or dispersed within an appropriate solvent or carrier
through various mechanisms, including ionic bonding, covalent
bonding, van der Waals interaction, hydrogen bonding, or
coordination bonding involving non-bonding electron pairs.
[0048] To provide the interaction between the dispersing agent and
the particle component atoms or molecules, the dispersing agent
includes one or more appropriate functional groups. In one
embodiment, the functional groups comprise a carbon atom bonded to
at least one electron-rich atom that is more electronegative than
the carbon atom and that is able to donate one or more electrons so
as to form a bond or attraction with a particle component atom.
Preferred dispersing agents include functional groups which have
either a negative charge, one or more lone pairs of electrons, or a
positive charge that can be used to complex or bond to a particle
component atom. These functional groups allow the dispersing agent
to have a strong binding interaction with dissolved particle
component atoms or molecules, which, in the case of metals, are
preferably in the form of positively charged ions in solution.
[0049] The dispersing agent may be a natural or synthetic compound.
In the case where the nanoparticle component atoms are metals and
the dispersing agent is an organic compound, the complex so formed
is an organometallic complex.
[0050] In one embodiment, the functional groups of the dispersing
agent comprise carboxyl groups, either alone or in combination with
other types of functional groups. In other embodiments, the
functional groups may include one or more of a hydroxyl, a
carboxyl, a carbonyl, an amine, a thiol, an ester, an amide, a
nitrile, a nitrogen with a free lone pair of electrons, a ketone,
an aldehyde, a sulfonic acid, an acyl halide, a sulfonyl halide,
and combinations of these. Examples of suitable dispersing agents
include glycolic acid, oxalic acid, malic acid, maleic acid, citric
acid, pectins, amino acids, celluloses, combinations of these, and
salts of any of these.
[0051] Suitable polymers and oligomers within the scope of the
invention include, but are not limited to, polyacrylates,
polyvinylbenzoates, polyvinyl sulfate, polyvinyl sulfonates
including sulfonated styrene, polybisphenol carbonates,
polybenzimidizoles, polypyridine, sulfonated polyethylene
terephthalate. Other suitable polymers include polyvinyl alcohol,
polyethylene glycol, polypropylene glycol, and the like. The
dispersing agent can also be an inorganic compound (e.g.,
silicon-based) or a salt of any of the foregoing.
[0052] It may be advantageous to provide an amount of dispersing
agent so as to provide an excess of functional groups relative to
the number of particle component atoms or molecules. Including an
excess of functional groups helps ensure that all or substantially
all of the particle component atoms or molecules are complexed by
the dispersing agent, which is particularly beneficial in the case
where at least one of the nanoparticle components is expensive,
such as in the case of noble metals. Providing an excess of
dispersing agent can also help ensure the availability of
functional groups for bonding the nanoparticle complex to a support
where a supported nanoparticle is desired. It is also believed that
employing an excess of functional groups helps yield nanoparticles
that are more evenly dispersed in the particle system. Excess
dispersing agent molecules are believed to intervene and maintain
spacing between dispersing agent molecules. The excess dispersing
agent molecules can increase spacing and dispersion in a suspension
as well as aid in spacing nanoparticles upon deposition to a
support surface.
[0053] In addition to the foregoing, it may also be useful to
express the molar ratio of dispersing agent to the particle
component atoms in a nanoparticle suspension. In one embodiment,
the molar ratio of dispersing agent molecules to particle component
atoms is in the range of about 0.01:1 to about 40:1. Preferably,
the molar ratio of dispersing agent molecules to particle component
atoms is in a range of about 0.1:1 to about 35:1, most preferably
in a range of about 0.5:1 to about 30:1.
[0054] In some cases, a more useful measurement is the molar ratio
between dispersing agent functional groups and particle component
atoms. For example, in the case of a divalent metal ion two molar
equivalents of a monovalent functional group would be necessary to
provide the theoretical stoichiometric ratio. It may be desirable
to provide an excess of dispersing agent functional groups to (1)
ensure that all or substantially all of the particle component
atoms are complexed, (2) bond the nanoparticles to a support, a and
(3) help keep the nanoparticles segregated so that they do not
clump or agglomerate together. In general, it will be preferable to
include a molar ratio of dispersing agent functional groups to
particle component atoms in a range of about 0.5:1 to about 40:1,
more preferably in a range of about 1:1 to about 35:1, and most
preferably in a range of about 3:1 to about 30:1.
[0055] As discussed below, the nanoparticles can be supported on a
support surface. It is believed that when a support material is
added to a suspension of nanoparticles, the dispersing agent acts
to uniformly disperse the complexed component atoms and/or
suspended nanoparticle complexes onto the support material.
[0056] In addition to the foregoing, the dispersing agent can be
selected in order to act as an anchor between the nanoparticles and
a support material or substrate. Preferably, the support substrate
has a plurality of hydroxyl or other functional groups on the
surface thereof which are able to chemically bond to one or more
functional groups of the dispersing agent, such as by way of a
condensation reaction. One or more additional functional groups of
the dispersing agent are also bonded to one or more atoms within
the nanoparticle, thereby anchoring the nanoparticle to the
substrate.
[0057] While the dispersing agent has the ability to inhibit
particle agglomeration in the absence of being anchored to a
support, chemically bonding the nanoparticle to the substrate
surface through the dispersing agent is an additional and
particularly effective mechanism for preventing particle
agglomeration since the nanoparticles thereby become fixed in
space.
[0058] B. Solvents and Carriers
[0059] A solvent or carrier may be used as a vehicle for the
particle component atoms (typically in the form of an ionic salt)
and/or the dispersing agent. The solvent may be an organic solvent,
water or a combination thereof. Organic solvents that can be used
include alcohols, ethers, glycols, ketones, aldehydes, nitrites,
and the like. Preferred solvents are liquids with sufficient
polarity to dissolve the metal salts. They include water, methanol,
ethanol, normal and isopropanol, acetonitrile, acetone,
tetrahydrofuran, ethylene glycol, dimethylformamide,
dimethylsulfoxide, methylene chloride, and mixtures thereof.
[0060] Other chemical modifiers may also be included in the liquid
mixture. For example, acids or bases may be added to adjust the pH
of the mixture. Surfactants may be added to adjust the surface
tension of the mixture, or to stabilize the nanoparticles.
[0061] The solvent for the nanoparticle components may be a neat
solvent, but it is preferable to include an acid to yield an acidic
solution, as acids aid in the dissolution of the nanoparticle
components. The solvent solution may be acidified with any suitable
acid, including organic and inorganic acids. Preferred acids
include mineral acids such as sulfuric, phosphoric, hydrochloric,
nitric, and the like, or combinations thereof. While it is possible
to use an acid in a wide range of concentrations, it is generally
only necessary to use relatively dilute solutions to accomplish the
desired solubility enhancement. Moreover, concentrated acid
solutions may present added hazard and expense. Thus, dilute acid
solutions are currently preferred.
[0062] C. Supports and Support Materials
[0063] As discussed above, it is within the scope of the invention
for the nanoparticles to be isolated on a support surface. The
support material may be organic or inorganic. According to one
embodiment, the supported nanoparticles may function as a catalyst.
In the case of a supported catalyst, the support material can be
chemically inert in the chemical reaction environment, or the
support material may itself serve a catalytic function
complementary to the function of the supported nanocatalyst
particles.
[0064] Any solid support material known to those skilled in the art
as useful nanoparticle supports can be used as supports for the
dispersed nanoparticles of the present invention. The support may
be selected from a variety of physical forms. Exemplary supports
may be porous or non-porous. They may be 3-dimensional structures,
such as a powder, granule, tablet, extrudate, or the like. Supports
may be in the form of 2-dimensional structures, such as a film,
membrane, coatings, or the like. It is even conceivable for the
support to be a 1-dimensional structure, such as ultra thin fibers
or filaments.
[0065] A variety of materials, alone or in combination, can
comprise the support. One exemplary class of support materials
preferred for some applications includes porous inorganic
materials. These include, but are not limited to, alumina, silica,
silica gel, titania, kieselguhr, diatomaceous earth, bentonite,
clay, zirconia, magnesia, as well as the oxides of various other
metals, alone or in combination. They also include the class of
porous solids collectively known as zeolites, natural or synthetic,
which have ordered porous structures.
[0066] Another useful class of exemplary supports includes
carbon-based materials, such as carbon black, activated carbon,
graphite, fluoridated carbon, and the like. Other useful classes of
support materials include organic solids (e.g., polymers), metals
and metal alloys.
[0067] In the case where the nanoparticles are attached to a
support, the nanoparticles can be deposited in a wide range of
loadings on the support material. The loading can range from 0.01%
to 90% by weight of the total weight of the supported
nanoparticles. The preferred loading will depend on the
application. In the case where porous solids are used as the
support material, it is preferred that the surface area of the
support be at least 20 m.sup.2/g, and more preferably more than 50
m.sup.2/g.
[0068] D. Distribution of Components Within the Nanoparticles
[0069] At least a portion of the nanoparticles within a preparation
of nanoparticles manufactured according to the invention will
include two or more (e.g., both) of the nanoparticle components. In
a preferred embodiment, at least about 50% of the nanoparticles
include two or more of the nanoparticle components. More
preferably, at least about 75% of the nanoparticles within the
preparation include two or more of the nanoparticle components,
even more preferably at least about 85% of the nanoparticles
include two or more of the nanoparticle components, and most
preferably at least about 95% of the nanoparticles within the
preparation include two or more of the nanoparticle components. It
is within the scope of the invention for at least about 99% (i.e.,
essentially all) of the nanoparticles within a preparation
according to the invention to include two or more of the
nanoparticle components.
[0070] Because a substantial proportion of the nanoparticles
prepared according to the invention include two or more
nanoparticle components, the benefits derived from having the
components in a single particle are more uniformly distributed
throughout the nanoparticles compared to heterogeneous mixtures of
homogeneous particles. Consequently, the overall nanoparticle
material or catalyst has an increased display of these beneficial
properties.
[0071] According to another aspect of the invention, the degree of
dispersion of the two or more components within nanoparticles
prepared according to the invention can be measured by the Number
Ratio (NR) or Range of NR for a given set of nanoparticles having
two or more components. As mentioned above, the Number
Ratio=N.sub.A/N.sub.B, where N.sub.A is the number (or moles) of
atoms of a more numerous component A within a nanoparticle or set
of nanoparticles according to the invention, and N.sub.B is the
number (or moles) of atoms of a less numerous component B within
the nanoparticle or set of nanoparticles. The value of NR can be
expressed as an average value (NR.sub.avg) for all of the
nanoparticles in a given set or as the specific value (NR.sub.i)
for a particular nanoparticle i.
[0072] In an ideal case, the value NR.sub.i for each nanoparticle i
in a given set of inventive nanoparticles equals NR.sub.avg. In
this case, each particle i has an equal distribution of components
A and B. The present invention also contemplates controlling the
dispersion of components in bi- or multi-component nanoparticles
such that the Range of NR values for all of the nanoparticles in a
particular sample is within a desired range. As mentioned above,
the Range of NR has an upper value NR.sub.max and a lower value
NRmin. As NR.sub.max and NR.sub.min deviate less from NR.sub.avg,
the Range of NR becomes narrower, which indicates that the
nanoparticles are more uniform.
[0073] In a preferred embodiment, the value of NR.sub.max does not
exceed about 5 times the value of NR.sub.avg, more preferably does
not exceed about 3 times the value of NR.sub.avg, and most
preferably does not exceed about 2 times the value of
NR.sub.avg.
[0074] Conversely, the value of NR.sub.min is preferably at least
about 0.2 times the value of NR.sub.avg, more preferably at least
about 0.33 times the value of NR.sub.avg, and most preferably at
least about 0.5 times the value of NR.sub.avg.
[0075] Given the foregoing, the Range of NR is therefore preferably
about 0.2 to about 5 times the value of NR.sub.avg, more preferably
about 0.33 to about 3 times the value of NR.sub.avg, and most
preferably about 0.5 to about 2 times the value of NR.sub.avg. It
will be appreciated that the foregoing ranges do not count
"outliers" (i.e., particles that do not form correctly and that
excessively deviate from NR.sub.avg as to be outside the Range of
NR). Whereas the NR of the "outliers" may in some cases count
toward the NR.sub.avg, they do not fall within the "Range of NR" by
definition.
[0076] In a preferred embodiment, at least about 50% of the
individual nanoparticles in a given preparation will have an
NR.sub.i within the Range of NR. More preferably, at least about
75% of the individual nanoparticles within the preparation will
have an NR.sub.i within the Range of NR, even more preferably at
least about 85% of the individual nanoparticles within the
preparation will have an NR.sub.i within the Range of NR, and most
preferably at least about 95% of the individual nanoparticles
within the preparation will have an NR.sub.i within the Range of
NR. It is within the scope of the invention for at least about 99%
of the individual nanoparticles within a preparation according to
the invention to have an NR.sub.i within the Range of NR.
[0077] In contrast to the relatively narrow Range of NR for
nanoparticles made according to the present invention,
nanoparticles in the art, to the extent they can be made as all,
have very wide Ranges of NR.sub.i, in some cases ranging from zero
to infinity, indicating that some particles have essentially none
of one component, and other particles have essentially none of the
other component.
[0078] The following two simple numerical examples provide
non-limiting examples of nanoparticles of the present invention
having desired Ranges of NR. Consider a case where component B
comprises 1% of a bimetallic nanoparticle mixture, and component A
comprises the balance in a given set of nanoparticles. In this,
case the NR.sub.avg for the set of nanoparticles is approximately
100. The preferred Range of NR for the set nanoparticles is thus 20
to 500, which translates to a range of 0.2% to 5% of component B in
the individual nanoparticles that contain both components. The more
preferred range for NR is 33 to 300, translating to a composition
range of 0.33% to 3% of component B in the individual nanoparticles
that contain both components. The most preferred range for NR.sub.i
is 50 to 200, or a composition range of 0.5% to 2% component B in
the individual nanoparticles that contain both components.
[0079] In a second simple numerical example, consider a case where
component A and component B are each present in equal quantities of
50% of the total, such that the overall NR.sub.avg is 1. In this
case, the preferred range of NR.sub.i is 0.2 to 5, corresponding to
a composition range of 16% to 83% of component B in the individual
nanoparticles that contain both components. The more preferred
range of NR.sub.i is 0.33 to 3, corresponding to a composition
range of 25% to 75% component B in the individual nanoparticles
that contain both components. Finally, the most preferred range of
NR.sub.i is 0.5 to 2 or a composition range of 33% to 67% component
B in the individual nanoparticles that contain both components.
[0080] As discussed above, the dispersing agents according to the
present invention are used to provide the desired dispersion and
uniformity that is characteristic of the nanoparticles of the
present invention. Using the dispersing agents according to the
present invention, the above-mentioned uniformity as defined by the
Range of NR can be obtained.
[0081] In one embodiment, the dispersing agent remains as a
constituent of the nanoparticles. The inventors of the present
invention have found by infrared spectroscopy that characteristic
features attributable to the dispersing agent can be present in the
final nanoparticle product, indicating that the dispersing agent
persists beyond the nanoparticle production steps. In one
embodiment, the dispersing agent is believed to be a stabilizing
component in the final catalyst or nanoparticle material. For
example, the dispersing agent can provide a desirable anchoring
effect of the particle to a support which prevents migration and
agglomeration of nanoparticles, even under relatively severe
operating conditions. However, even where the dispersing agent is
not used as an anchor to a support material (e.g., in the absence
of a support material or where the dispersing agent does not bond
to the support material), the dispersing agent can have a
stabilizing effect.
[0082] While it is possible that the multicomponent nanoparticles
may contain a true multicomponent compound, alloy, or crystal
structure in which the components are in an ordered arrangement,
this is not required. In one embodiment, each nanoparticle can be
composed of a mixture of components regardless of how they are
combined or arranged. The components can be present as relatively
isolated atoms, as small atomic clusters, or decorated. They can
also be present as amorphous particles. The components can be
present as crystallites including alloys. Component crystals can
have relatively random crystal face exposures; or they can have a
controlled or selective exposure of particular crystal faces.
[0083] The statistical distribution or uniformity made possible by
the dispersing agent of the present invention allows for
nanocatalysts and nanomaterials with new and/or improved materials
and/or catalytic properties. Maximizing multicomponent catalyst and
nanomaterial properties may depend on the proximity of the two
components. The substantially uniform distribution of components
between and among nanoparticles provides a greater possibility for
different components to come into proximity with one another to
provide a desired functionality or property.
[0084] The dispersing agent also makes it possible to select very
precise ratios of components by controlling the average percent
composition. Because the individual multicomponent nanoparticles
have a percent composition that varies very little from the average
composition, the percent composition of the individual
nanoparticles can be more precisely controlled by adjusting the
starting materials to control the average percent composition.
III. Methods of Making Multicomponent Nanoparticles
[0085] General processes for manufacturing multicomponent
nanoparticles according to the invention can be broadly summarized
as follows. Two or more types of particle atoms and one or more
types of dispersing agents are selected. The particle atoms and the
dispersing agent are reacted or combined together to form a
plurality of component complexes (collectively referred to as the
"component complex"). The component complex is generally formed by
first dissolving the particle atoms and dispersing agent(s) in an
appropriate solvent or carrier and then allowing the dispersing
agent to recombine the dissolved component atoms as the component
complex so as to form a solution or suspension. In one embodiment,
multicomponent nanoparticles form in the suspension. Alternatively,
nanoparticles may form upon or after the component complex is
disposed on a support surface. If desired, at least a portion of
the dispersing agent can be removed to expose the multicomponent
nanoparticles. The dispersing agent may form a chemical bond with
the support material in order to thereby anchor the nanoparticles
to the support.
[0086] A more specific example for making multicomponent
nanoparticles according to the invention includes providing two or
more types of particle component atoms in solution (e.g., in the
form of an ionic salt), providing a dispersing agent in solution
(e.g., in the form of a carboxylic acid salt), and reacting the
particle component atoms with the dispersing agent to form a
component complex (i.e., a solution, suspension or colloid of
component atoms complexed with the dispersing agent). The particle
component atoms can be provided in any form so as to be soluble or
dispersible in the solvent or carrier that is used to form the
component complex. In the case where the particle component atoms
comprise one or more metals, salts of these metals can be formed
that are readily soluble in the solvent or carrier. In the case
where the component atoms include noble metals, it may be
advantageous to use noble metal chlorides and nitrates, since
chlorides and nitrate of noble metals are more readily soluble than
other salts. Chlorides and nitrates of other metal particle atoms,
such as base transition metals and rare earth metals may likewise
be used, since chlorides and nitrates are typically more soluble
than other types of salts.
[0087] The component atoms can be added to the solvent or carrier
singly or in combination to provide final nanoparticles that
comprise mixtures of various types of particle atoms. For example,
a bimetallic iron/platinum catalyst can be formed by first forming
a precursor solution into which is dissolved an iron salt, such as
iron chloride, and a platinum salt, such as chloroplatinic acid. In
general, the composition of the final nanoparticles will be
determined by the types of particle component atoms used to form
the component complex. Therefore, control of the amounts of
component atoms added to the solution, colloid or suspension
provides a convenient method for controlling the relative
concentrations of the different types of component atoms in the
final multicomponent nanoparticles.
[0088] The dispersing agent is added to the solvent or carrier in a
manner so as to facilitate association with the particle component
atoms in order to form the component complex. Some dispersing
agents may themselves be soluble in the solvent or carrier. In the
case of dispersing agents that include carboxylic acid groups, it
may be advantageous to form a metal salt of the acids (e.g., an
alkali or alkaline earth metal salt). For example, polyacrylic acid
can be provided as a sodium polyacrylate salt, which is both
readily soluble in aqueous solvent systems and able to react with
catalyst metal salts to form a metal-polyacrylate complex, which
may be soluble or which may form a colloidal suspension within the
solvent or carrier.
[0089] In general, component complexes according to the invention
comprise the particle atoms and dispersing agent, exclusive of the
surrounding solvent or carrier. Therefore, it is within the scope
of the invention to create a component complex in solution, or as a
colloid or suspension, and then remove the solvent or carrier so as
to yield a dried component complex. The dried component complex can
be used in this form, or it can be reconstituted as a solution,
colloid or suspension by adding an appropriate solvent.
[0090] In the case where the nanoparticles of the invention are to
be formed on a solid support material, the component complex
solution is physically contacted with the solid support. Contacting
the component complex with the solid support is typically
accomplished by means of an appropriate solvent within the
component complex solution, colloid or suspension in order to apply
or impregnate the component complex onto the support surface.
[0091] Depending on the physical form of the support material, the
process of contacting or applying the component complex to the
support may be accomplished by a variety of methods. For example,
the support may be submerged or dipped into a solution, colloid, or
suspension comprising a solvent or carrier and the component
complex. Alternatively, the solution, colloid, or suspension may be
sprayed, poured, painted, or otherwise applied to the support
material. Thereafter, the solvent or carrier is removed, optionally
in connection with a reaction step that causes the dispersing agent
to become chemically bonded or adhered to the support.
[0092] If desired, at least a portion of the nanoparticles can be
exposed by removing a at least a portion of the dispersing agent,
such as by reduction (e.g., hydrogenation) or oxidation. Hydrogen
is one preferred reducing agent. Instead of, or in addition to,
using hydrogen as the reducing agent, a variety of other reducing
agents may be used, including lithium aluminum hydride, sodium
hydride, sodium borohydride, sodium bisulfite, sodium thiosulfate,
hydroquinone, methanol, aldehydes, and the like. The reduction
process may be conducted at a temperature between 20.degree. C. and
500.degree. C., and preferably between 100.degree. C. and
400.degree. C.
[0093] In some cases, such as where it is desired for a portion of
the dispersing agent to remain as an anchoring agent, oxidation may
only be suitable when the particle atoms do not include noble
metals, since noble metals might catalyze the oxidation of the
entire dispersing agent, leaving none for anchoring. In such cases,
oxidation may be more suitable, for example, in the case where the
particle atoms comprise transition metals and the support is
non-combustible (e.g., silica or alumina rather than carbon black,
graphite or polymer membranes). According to an exemplary
embodiment, oxidation may be carried out using oxygen, hydrogen
peroxide, organic peroxides, and the like.
[0094] In one embodiment, the process of removing the dispersing
agent to expose the particle atoms is carefully controlled to
ensure that enough of the dispersing agent remains so as to
reliably maintain a dispersed catalyst. Removing the dispersing
agent to the extent that little or none of it remains to disperse
or anchor the nanoparticles has been found to reduce the stability
of the nanoparticles, particularly when the catalyst is subjected
to harsh reaction conditions during use. Nevertheless, it is within
the scope of the invention to remove all or substantially all of
the dispersing agent in order to yield free multicomponent
nanoparticles that are neither anchored to a support or otherwise
complexed with a dispersing agent to any degree.
[0095] Supported nanoparticles can be optionally heat-treated to
further activate the nanoparticles. It has been found that, in some
cases, subjecting the nanoparticles to a heat treatment process
before initially using the nanoparticles causes the nanoparticles
to be more active initially. The step of heat treating the
nanoparticles may be referred to as "calcining" because it may act
to volatilize certain components within the nanoparticles. The heat
treatment process may be carried in inert, oxidizing or reducing
atmospheres.
[0096] In some cases it may be desirable to maintain at least some
of the nanoparticle components in a non-zero oxidation state during
the heat treatment process in order to increase the bond strength
between the dispersing agent and the nanoparticles. Increasing the
bond between the dispersing agent and the nanoparticles is believed
to increase the dispersion of the nanoparticles and/or the
distribution of components within the particles by reducing the
tendency of nanoparticles to migrate and/or agglomerate together
when exposed to higher temperatures. This is particularly true in
the case of supported multicomponent nanoparticles.
[0097] Where the nanoparticles are subjected to a heat treatment
process, the process is preferably carried out at a temperature in
a range of about 50.degree. C. to about 300.degree. C., more
preferably in a range of about 100.degree. C. to about 250.degree.
C., and most preferably in a range of about 125.degree. C. to about
200.degree. C. The duration of the heat treatment process is
preferably in a range of about 30 minutes to about 12 hours, more
preferably in a range of about 1 hour to about 5 hours.
[0098] An important feature of the heat treating step according to
the present invention is that it does not degrade the nanoparticles
or reduce catalytic activity. The dispersing agent provides the
stability needed to subject the nanoparticles to higher
temperatures without destroying or partially destroying the
nanoparticles. Further stability may be possible where the particle
component atoms are bonded to the dispersing agent and then
maintained in a non zero-oxidation state, which enhances the bond
between the component atoms and the active complexing groups of the
dispersing agent.
[0099] The following exemplary procedures where used to prepare
iron-platinum multicomponent nanoparticles according to the
invention. By showing that iron and platinum can be compounded
together to form heterogeneous multicomponent nanoparticles, the
examples demonstrate that two very dissimilar materials having very
strong same-component attractions can, in fact, be compounded
together using a dispersing agent. From this it may be expected
that any two or more dissimilar materials can be compounded
together using the compositions and methods described herein.
EXAMPLE 1
Nanoparticle Suspension
[0100] An Iron (III) solution was prepared by dissolving 2.32 g of
FeCl.sub.3 in 4 ml HCl and 996 ml de-ionized water to produce a
0.08 wt % solution of Fe (III). A Pt solution was prepared by
dissolving 0.2614 g H.sub.2PtCl.sub.6 (from Strem Chemicals) in
1000 ml de-ionized water to make 0.01 wt % solution of Pt. To make
a 6.75 wt % solution of polyacrylate, 15 g of a 45 wt % poly
acrylate solution (Aldrich with MW ca. 1,200) was diluted to 100
grams with de-ionized water.
[0101] To prepare 2.4 grams of a 10% Fe and 0.2% Pt supported
nanoparticles, 300 ml of the 0.08 wt % Fe solution was mixed with
48 ml of the 0.010 wt % Pt solution and 40 ml of the 6.75 wt %
polyacrylate solution. The ratio of Fe:polyacrylate was 1:1. The
solution was then diluted to 4000 ml with de-ionized water. This
solution was purged by 100 ml/min N.sub.2 for 1 hour. Then the
N.sub.2 was replaced with 130 ml/min H.sub.2 for 16 minutes. The
flask was then held overnight. The Fe--Pt solution resulted in the
formation of a suspension of nanoparticles.
EXAMPLE 2
[0102] Supported nanoparticles were prepared by first preparing a
solution of Fe--Pt particles according to Example 1. 24 g of Black
Pearls 700 were impregnated by 4000 ml of the Fe--Pt solution or
suspension prepared according to Example 1. The slurry was heated
by an IR lamp under rotation until all the liquid was evaporated.
The obtained samples were kept in an oven at 100.degree. C. The
sample was packed in a reduction unit between two layers of
glass-wool. The sample was then treated by the following procedure:
purged by 100 ml/min N.sub.2 for 15 minute and then with 100 ml/min
H.sub.2 at the following temperatures and for the following amount
of time: 25.degree. C. (0.5 h), then 90.degree. C. (2 h), then
90.degree. C. (2 h), then 300.degree. C. (17 h). The sample was
then cooled to room temperature in 100 ml/min H.sub.2. It was then
purged by 100 ml/min of N.sub.2 for one hour.
EXAMPLE 3
[0103] 8.13 g FeCl.sub.3 was mixed with 16.5 g 70 wt % glycolic
acid and diluted with water to 100 g. After overnight agitation,
the FeCl.sub.3 was totally dissolved. To this solution 2.8 g 0.01
wt % Pt solution from Example 1 was added. This solution was used
to impregnate 140 g CaCO.sub.3. After the same drying and
activation procedure as for Example 1, an alloy sample with 2% Fe
and 0.02% Pt was formed.
[0104] The multicomponent nanoparticle materials produced in
examples 1, 2, and 3 had nanoparticles in which essentially all the
nanoparticles included both iron and platinum, which would be
virtually thermodynamically impossible using heat compounding
techniques.
EXAMPLE 4
[0105] Any of Examples 1-3 is modified in order to compound
together two or more dissimilar components in which at least one of
the components is selected from one of the following groups and at
least one other of the components is selected from another of the
following groups: noble metals, base transition metals, alkali
metals, alkaline earth metals, rare earth metals, and
nonmetals.
[0106] The dispersing agent may be one or more of any of the
dispersing agents described herein. A substantial portion of the
nanoparticles manufactured thereby include two or more dissimilar
components in each of the nanoparticles.
EXAMPLE 5
[0107] Any of Examples 1-3 is modified in order to compound
together two or more dissimilar components in which at least one of
the components is selected from one group of the periodic table of
elements and at least one other of the components is selected from
another group of the periodic table of elements.
[0108] The dispersing agent may be one or more of any of the
dispersing agents described herein. A substantial portion of the
nanoparticles manufactured thereby include two or more dissimilar
components in each of the nanoparticles.
[0109] The present invention may be embodied in other specific
forms without departing from its spirit or essential
characteristics. The described embodiments are to be considered in
all respects only as illustrative and not restrictive. The scope of
the invention is, therefore, indicated by the appended claims
rather than by the foregoing description. All changes which come
within the meaning and range of equivalency of the claims are to be
embraced within their scope.
* * * * *