U.S. patent application number 15/613613 was filed with the patent office on 2018-06-28 for process to prepare metal nanoparticles or metal oxide nanoparticles.
This patent application is currently assigned to UNIVERSITEIT LEIDEN. The applicant listed for this patent is UNIVERSITEIT LEIDEN. Invention is credited to Nuria GARCIA-ARAEZ, Marcus KOPER, Paramaconi RODRIGUEZ, Alexei YANSON.
Application Number | 20180179653 15/613613 |
Document ID | / |
Family ID | 44629405 |
Filed Date | 2018-06-28 |
United States Patent
Application |
20180179653 |
Kind Code |
A1 |
YANSON; Alexei ; et
al. |
June 28, 2018 |
PROCESS TO PREPARE METAL NANOPARTICLES OR METAL OXIDE
NANOPARTICLES
Abstract
The invention is directed to a process to prepare metal
nanoparticles or metal oxide nanoparticles by applying a cathodic
potential as an alternating current (ac) voltage to a solid
starting metal object which solid metal object is in contact with a
liquid electrolyte comprising a stabilising cation. The invention
is also directed to the use of the nanoparticles as a catalyst.
Inventors: |
YANSON; Alexei; (Leiden,
NL) ; KOPER; Marcus; (Leiderdorp, NL) ;
RODRIGUEZ; Paramaconi; (Birmingham, GB) ;
GARCIA-ARAEZ; Nuria; (Southampton, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UNIVERSITEIT LEIDEN |
Leiden |
|
NL |
|
|
Assignee: |
UNIVERSITEIT LEIDEN
Leiden
NL
|
Family ID: |
44629405 |
Appl. No.: |
15/613613 |
Filed: |
June 5, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13810804 |
Mar 28, 2013 |
9695521 |
|
|
PCT/EP2011/062101 |
Jul 14, 2011 |
|
|
|
15613613 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 35/0013 20130101;
C09D 11/52 20130101; B01J 23/468 20130101; B23K 35/262 20130101;
C25C 5/02 20130101; B01J 37/348 20130101; B01J 23/462 20130101;
B22F 2998/00 20130101; C25D 1/006 20130101; B82Y 40/00 20130101;
C02F 1/725 20130101; B22F 2998/00 20130101; B01J 23/89 20130101;
B23K 35/025 20130101; C25C 5/00 20130101; B01J 35/006 20130101;
B01J 23/464 20130101; B82Y 30/00 20130101; B22F 1/0018 20130101;
B01J 23/40 20130101; B01D 53/94 20130101; Y02W 10/37 20150501; B01J
35/00 20130101 |
International
Class: |
C25D 1/00 20060101
C25D001/00; C25C 5/02 20060101 C25C005/02; C25C 5/00 20060101
C25C005/00; B01J 23/40 20060101 B01J023/40; B01J 23/46 20060101
B01J023/46; C09D 11/52 20140101 C09D011/52; C02F 1/72 20060101
C02F001/72; B82Y 40/00 20110101 B82Y040/00; B82Y 30/00 20110101
B82Y030/00; B23K 35/26 20060101 B23K035/26; B23K 35/02 20060101
B23K035/02; B01J 37/34 20060101 B01J037/34; B01J 35/00 20060101
B01J035/00; B01J 23/89 20060101 B01J023/89 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 19, 2010 |
NL |
2005112 |
Claims
1. Process to prepare metal nanoparticles or metal oxide
nanoparticles by applying a cathodic potential as an alternating
current (ac) voltage to a solid starting metal object which solid
metal object is in contact with a liquid electrolyte comprising a
stabilising cation.
2. Process according to claim 1, wherein the liquid electrolyte
comprises water.
3. Process according to any one of claims 1-2, wherein the metal or
metals are chosen from the groups of the Periodic Table of Elements
according to IUPAC starting at 3 to and including group 15.
4. Process according to claim 3, wherein the metal is chosen from
the group consisting of Y, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb,
Mo, Ru, Ag, Ta, W, Re, Os, Ir, Pt, Au, Al, Si, Ga, Ge, As, In, Sn,
Sb, Te, Tl, Pb and Bi.
5. Process according to any one of claims 1-4, wherein the solid
starting metal is an alloy of two or more metals.
6. Process according to claim 5, wherein the alloy is chosen from
the group consisting of PtNi, PtIr, PtRh, PtRu, PtCo, PtMo, PtAu,
PtAg, PtRuMo, PtFe, AuCu, PtCu, PtOs, PtSn, PtBi, CuNi, CoNi, AgCu,
AgAu, NiSn and SnAg, SnAgCu.
7. Process according to claim 6, wherein the alloy is a SnAg or
SnAgCu alloy.
8. Process according to any one of claims 1-7, wherein the
stabilising cation is an alkali, alkaline earth, ammonium or an
alkyl ammonium cation.
9. Process according to any one of claims 1-8, wherein two
electrodes are in contact with the liquid electrolyte and wherein
one electrode is composed of the solid starting material and
wherein only to said electrode a cathodic potential is applied.
10. Process according to any one of claims 1-9, wherein the
nanoparticles are separated from the liquid electrolyte.
11. Process according to claim 10, wherein separation is performed
by means of centrifugal force to obtain a phase rich in
nanoparticles and a phase of electrolyte, re-using the electrolyte
in the process according to claims 1-9, diluting the phase rich in
nanoparticles with water and redispersion by sonication.
12. Use of the nanoparticles as obtained by the process according
to any one of claims 1-11 as a catalyst.
13. Use according to claim 12 as a catalyst in fuel-cell
reactions.
14. Use according to claim 13 as a catalyst in the oxidation of
hydrogen, ethanol, formic acid, ammonia, borohydride and other
organic compounds and the reduction of oxygen, nitrates and
nitrites.
15. Use according to claim 12 as a catalyst for neutralization of
exhaust gases.
16. Use according to claim 14 as a catalyst for waste water
treatment.
17. Use of the nanoparticles as obtained by the process according
to any one of claims 1-11 as an electrochemical sensor.
18. Use of the nanoparticles as obtained by the process according
to any one of claims 1-11 in photovoltaics.
19. Use of the nanoparticles as obtained by the process according
to any one of claims 1-11 as part of a conductive nano ink.
20. Use of silver nanoparticles as obtained by the process
according to any one of claims 1-11 as part of an anti-microbial
device or composition.
21. Lead-free soldering paste comprising nanoparticles as obtained
by the process according to any one of claims 7-11.
Description
CROSS REFERENCES TO RELATED APPLICATION
[0001] This application is a Continuation Application of U.S. Ser.
No. 13/810,804 which has a 371(c) date of Mar. 28, 2013, which is a
35 U.S.C. .sctn. 371 National Phase Entry Application of
International Application No. PCT/EP2011/062101 filed Jul. 14,
2011, which designates the U.S., and which claims benefit under 35
U.S.C. .sctn. 119(b) of Netherlands Application 2005112 filed Jul.
19, 2010, the contents of which are herein incorporated by
reference in their entirety.
BACKGROUND OF THE INVENTION
[0002] The invention is directed to a process to prepare metal
nanoparticles or metal oxide nanoparticles.
[0003] US-A-2010/0072434 describes a process to prepare gold
nanoparticles by first preparing a polymer surfactant in an alcohol
solvent and heated to its boiling point. To the boiling mixture a
metal seed, i.e. chloroplatanic acid is added and subsequently
HAuCl4 salt is added after which gold nano particles form around
the seed.
[0004] WO-A-2007/055663 describes a process to prepare a metal nano
particle wherein a metal precursor compound is contacted with a
reducing agent and a capping agent to generate a reaction mixture.
Exemplified metal precursor is HAuCl.sub.4, exemplified reducing
agent is NaBH.sub.4 and the exemplified capping agent is amino
acid. To this mixture sonication is applied to generate a plurality
of metal colloidal particles and depositing the metal particles on
a support, i.e. TiO.sub.2, to prepare a catalyst.
[0005] US-A-2009/0325795 describes a process to prepare platinum
nanoparticles wherein first a chemical compound like potassium
tetrachloroplatinate is prepared. To this compound potassium iodide
(KI) is added and subsequently the mixture is reduced to form the
platinum nanoparticles.
[0006] At the heart of all known methods developed to make
nanoparticles lies the reduction reaction of metal cations. The art
is to limit the number of reduced cations per nanoparticle and keep
its size as constant as possible, and that is invariably achieved
by adding extra chemical stabilizers most methods of nanoparticles
synthesis involve micelles or colloids. These contaminate the final
product and adversely affect its performance, e.g. in catalysis or
biological applications. In addition, the prior art processes have
the problem that they involve multiple chemical synthesis steps.
Furthermore these processes require additional chemicals to act as
for example reducing agent, capping agent, polymer and/or a
surfactant.
[0007] In a paper titled The Phenomenon of the Formation of
Metallic Dust from Cathodes' as read by Prof. Dr. Fritz Haber at
the Second Meeting of the American Electrochemical Society on Sep.
17, 1902 an experiment was described wherein black clouds form
around a lead cathode when the direct current density is increased.
The same is observed for tin, bismuth, thallium, arsenic, antimony
and mercury. The same paper mentions that in an acid solution a
platinum wire cathode becomes black and spongy. Starting with the
same platinum cathode in an alkaline solution only a slightly
roughened surface was observed.
[0008] US-A-2009/0218234 describes a process to prepare titania
nanowires by applying an electrical potential between an anode and
a cathode as part of an electrolytic cell. The surfaces of both
anode and cathode comprise a titanium surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a SEM image of formation of nanoparticles having a
size of <50 nm which were coated on the surface of the platinum
wire (bar=1 mm scale).
[0010] FIG. 2 is a high-magnification SEM image of FIG. 1 of
formation of nanoparticles having a size of <50 nm which were
coated on the surface of the platinum wire (bar=500 nm scale.
[0011] FIG. 3 is a transmission electron microscope (TEM) image
showing that platinum nanoparticles formed having a size ranging
from 4 to 30 nm when a platinum wire was used.
[0012] FIG. 4 shows an Energy Dispersive X-ray (EDX) spectrum of
the platinum nanoparticles showing that lattice spacing, as well as
Energy Dispersive X-ray (EDX) spectrum measured correspond exactly
to platinum.
[0013] FIG. 5 showed transmission electron microscope (TEM) image
showing gold nanoparticles formed having a size ranging from 4 to
30 nm when a gold wire was used.
[0014] FIG. 6 shows an Energy Dispersive X-ray (EDX) spectrum of
the gold nanoparticles showing that lattice spacing, as well as
Energy Dispersive X-ray (EDX) spectrum measured correspond exactly
to gold.
[0015] FIGS. 7A-7C show results of the platinum-nanoparticles
modified using gold electrodes (solid lines) versus commercially
obtained nanoparticles (dashed line). FIG. 7A shows blank
voltammogram recorded in 0.5 M H2SO4 at a scan rate of 50 mV/s
(solid line for platinum nanoparticles; dashed line for commercial
nanoparticles). FIG. 7B shows the results of electrooxidation of CO
measured after adsorption of CO at E=0.1 V vs. RHE (RHE=Reversible
Hydrogen Electrode) for 1 minute and consequent purging with of the
CO dissolved in solution for 30 minutes. The CO electrooxidation
was recorded with a scan rate of 20 mV/s (solid line for platinum
nanoparticles; dashed line for commercial nanoparticles). FIG. 7C
shows results of catalysis for methanol oxidation which was
measured in a 0.5 M H2SO4+0.5 M CH3OH solution at a scan rate of 50
mV/s (solid line for platinum nanoparticles; dashed line for
commercial nanoparticles).
[0016] FIG. 8 shows the negative dc offset results in the formation
of the nanoparticles via a reduction (cathodic) process at the
alloy wire electrode. In the case of the alloys PtRh, PtRu, PtIr,
AuCo, AuCu and FeCo, the applied ac potential varied between -10
and 0 V vs. the glassy carbon counter electrode.
[0017] FIG. 9 shows particle size of the nanoparticles as assessed
by transmission electron microscopy (TEM), including energy
dispersive x-ray analysis (EDX), and x-ray diffraction (XRD) for
Rh, Rh80Pt70, Rh30Rt70, Rh10, Rt90 and Pt.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The object of the present invention is to provide a more
simple process to prepare nanoparticles or metal oxide
nanoparticles.
[0019] This object is achieved by the following process. Process to
prepare metal nanoparticles or metal oxide nanoparticles by
applying a cathodic potential as an alternating current (ac)
voltage to a solid starting metal object which solid metal object
is in contact with a liquid electrolyte comprising a stabilising
cation.
[0020] Applicants found that metal nanoparticles and metal oxide
nanoparticles can be obtained in less process steps than the prior
art methods while requiring less additional chemical compounds. For
example, the liquid electrolyte comprising the stabilising cation
can be reused in the process according to the invention. Because
less additional chemicals are used, more pure nanoparticles are
obtained. Without wishing to be bound to the following theory
applicants believe that at the strongly negative electrode
potentials employed during the process, highly non-equilibrium
(clusters of) negative metal anions are formed, which serve as
precursors for the formation of the nanostructures and
nanoparticles.
[0021] The 1902 paper as described above did not suggest that
nanoparticles are formed when applying the conditions of the
process according to the present invention.
[0022] For the present invention the term nanoparticles will have
the meaning of any particle having a smallest dimension of less
than 1000 nm, preferably less than 200 nm, and more preferably less
than 100 nm. The smallest dimension will be the smallest diameter
of the particle. The nanoparticles as obtained by the process are
obtained in a liquid as a solid/liquid suspension. The process
includes processes which make particles of which more than 50 wt %
are nanoparticles, more preferably of which more than 80 wt % are
nanoparticles and even more specific wherein more than 95 wt % of
the particles are nanoparticles as defined above.
[0023] The liquid electrolyte may be any liquid which has the
ability to oxidise the above described anionic metal to its
metallic state. Another feature of the electrolyte is that it has
so-called mobile charges, i.e. it must have the ability to transfer
a current from a cathode to an anode. Such mobile charges are
suitably cations and anions. An example of an electrolyte is a
molten salt of NaOH or an aqueous solution of NaCl. Preferably the
electrolyte comprises water.
[0024] The electrolyte will comprise a stabilising cation. Suitable
stabilising cations are those which do not or only very slightly
reduce on the surface of the cathode under the conditions of the
process according to the present invention. Such reduction would
result in the formation of a layer of this compound on the surface
of the metal object resulting in effectively terminating the
formation of the metal nanoparticles. Applicants found that the
stabilising cation is preferably an alkali or an alkaline earth
cation. Examples of suitable cations of this type are Na.sup.+,
Li.sup.+, K.sup.+, Cs.sup.+, Mg.sup.2+, Ca.sup.2+ and Ba.sup.2+.
Applicants further found that ammonium cation or a n-alkylammonium
cation can be used as the stabilising cation (with n ranging
between 1 and 4) and wherein the alkyl groups can be any alkyl
group having 1 to 10 carbon atoms, more preferably wherein the
alkyl group is a methyl, ethyl, n-propyl, iso-propyl, n-butyl or
tert-butyl group. A suitable n-alkylammonium cation is
tetra-tert-butylammonium. Applicants found that the choice for the
accompanying anion is not critical. Examples of suitable anions are
Cl.sup.-, SO.sub.4.sup.2-, HSO.sub.4.sup.-, ClO.sub.4.sup.-,
F.sup.-, NO.sub.3.sup.-, PO.sub.4.sup.3-, HPO.sub.4.sup.2-,
H.sub.2PO.sub.4.sup.-, BO.sub.3.sup.3-, HBO.sub.3.sup.2-,
H.sub.2BO.sub.3.sup.-, and OH.sup.-.
[0025] Preferred liquid electrolyte comprises a stabilising cation,
water and an anion. It is found that the pH of the liquid
electrolyte at the start of the process is not critical. Increasing
the concentration of the stabilising cation in the electrolyte has
resulted in a faster formation of the nanoparticles. A preferred
electrolyte is an aqueous solution comprising an alkali or alkaline
cation as the stabilising cation in a concentration between 0.1
mol/l and saturation. It must be understood that with concentration
is meant the average concentration in the bulk of the solution and
not any local higher or lower concentration.
[0026] The metal or metals are preferably chosen from the groups of
the Periodic Table of Elements starting at 3 to and including group
15 and more preferably Groups 3 to and including group 13, wherein
the groups are numbered according to the system adopted by IUPAC.
Suitable metals of these groups are Y, Ti, V, Mn, Fe, Co, Ni, Cu,
Zn, Zr, Nb, Mo, Ru, Rh, Ag, Ta, W, Re, Os, Ir, Pt, Au, Al, Si, Ga,
Ge, As, In, Sn, Sb, Te, Tl, Pb and/or Bi and more suitably the
metals as illustrated in the examples. Applicants found that when
starting from Ru and applying an alternating current nanoparticles
of ruthenium oxide are formed. The invention is also directed to
such a process. Ruthenium oxide nanoparticles are advantageously
used as part of a capacitor or as part of a catalyst in a catalytic
gas phase oxidation of a hydrogen chloride.
[0027] The starting metal object may be an alloy of two or more
metals or may consists substantially of one metal. By substantially
pure metal is meant that the composition comprises more than 98 wt
% of the metal or especially even more than 99.5 wt % of the metal.
Applicants found that when starting from an alloy object
nano-particles are obtained which also are alloys of two or more
metals. Examples of possible alloys are PtNi, PtIr, PtRh, PtRu,
PtCo, PtMo, PtAu, PtAg, PtRuMo, PtFe, AuCu, PtCu, PtOs, PtSn, PtBi,
CuNi, CoNi, AgCu, AgAu and NiSn. This list may be expanded with
SnAg, SnAgCu, SnCuNi, PtPd, SnBi, SnZn, SnZn--Bi, SnCoCu, SnCu,
SnIn and SnSb and more suitably the alloy combinations as
illustrated in the examples. It has been found possible to prepare
nanoparticles starting from an alloy. Applicants thus provide a
method to rapidly synthesize metal alloy nanoparticles with
pre-defined composition, structure and catalytic properties. Most
prior art methods of chemical synthesis of alloy and core shell
nanoparticles involve (co-)reduction of metal salts within micelles
or colloids. Other route is the so-called carbonyl synthesis route.
While much work has been done to perfect these methods for making
nanoparticles with predefined properties, they present inherent
inconveniences. First is the inevitable presence of the organic
components employed during the synthesis on the surface of the
nanoparticles, which is also applicable to pure metal
nanoparticles. These surfactants or capping materials contaminate
the final product and adversely affect its performance, for example
in catalysis or in biological applications. Gas-phase synthesis
like laser vaporization of solid targets, magnetron sputtering, or
ion sputtering allow making clean nanoparticles including alloys
and mixtures, but the technique suffers from low yields and the
relatively wide size distribution and particle agglomeration. The
impregnation and incipient wetness method of supported nanoparticle
synthesis often results in relatively large particles with a
non-uniform composition due to support irregularities, and
segregation of metals due to the differences in their
reducibility.
[0028] Nano-particles comprising of a mixture of different
materials may also be obtained by performing the process according
to the invention wherein a metal different from the metal of the
solid starting metal object is present as a cation in the solution.
In this manner it is possible to make bimetallic nanoparticles of
both metals, metal-on-oxide nanoparticles (when one of the two
metals, i.e. Ti, makes TiO.sub.2 nanoparticles), or mixed oxides
nanoparticles.
[0029] Applicants further found that when the potential is applied
as an alternating current (ac) voltage a suspension of
nanoparticles in the liquid electrolyte is obtained. In a situation
of alternating current the movement of the electric charge
periodically reverses direction. It is believed that the
alternating current enables the nanoparticles as formed when the
metal object is in the cathodic state to leave the metal object
when said object is in its anodic state. Preferred frequencies
range from 1 to 10000 Hz. The preferred voltage amplitude is
between 1 V and 30 V. Applicants found that the process proceeds
well in this range. Higher voltages may be applied but will not
substantially improve the speed of the reaction. The form of the
alternating current may be any form, for example square, sinusoidal
or triangular.
[0030] The required voltage is more cathodic than the so-called
potentials of zero charge. In order to proceed with an acceptable
speed of formation of nanoparticles, the applied voltage is more
cathodic than the onset of the hydrogen evolution reaction. The
optimum voltage will for example depend on the metal, the liquid
electrolyte and the stabilising cation and the desired size of the
nanoparticle. A suitable voltage is between 0.1 V and 30 V more
cathodic than the onset of the hydrogen evolution reaction.
Applicants found that the process proceeds well in this range.
Higher voltages may be applied but will not substantially improve
the speed of the reaction. Illustrative conditions will be provided
in the examples.
[0031] The application of the alternating current (ac) voltage can
be made between two metal objects with the same composition, in
contact with the liquid electrolyte. This will result in that
nanoparticles will be formed at both objects and eventually totally
dissolve into the liquid electrolyte. Alternatively, the ac voltage
can be applied between two electrodes of different composition.
Both electrodes are in contact with the liquid electrolyte and at
least one of the electrodes is the solid starting metal object. If
both electrodes are active for the formation of nanoparticles, a
mixture of two types of nanoparticles can be obtained.
Alternatively, one of the electrodes may be of an inert material,
like graphite, and therefore, nanoparticles will only be formed on
the other electrode. Preferably to only one of the electrodes a
cathodic potential is applied, alternating between the chosen
potential and a value below the voltage at which the onset of the
hydrogen evolution reaction starts. This is also referred to as
applying a so-called DC off-set, resulting in that only the
electrode to which the cathodic potential is applied remains
negative during the entire cycle. This is advantageous because at
the cathodic electrode hydrogen will be selectively produced and at
the other, anodic, electrode, oxygen will be produced. The hydrogen
as produced and optionally in combination with the oxygen as
produced can be used as fuel, suitably as fuel to generate
electricity in a fuel cell for performing the process according to
the present invention. This further improves the efficiency of the
present process. The cathodic electrode will be composed of the
desired nano-particle metal or alloy. The anodic electrode may be
composed of platinum or graphite, which materials will be inert
under the anodic conditions.
[0032] The nanoparticles as present in the liquid electrolyte are
suitably separated from the liquid electrolyte to obtain an end or
an intermediate product. Separation can be achieved by many
different methods, like precipitation or filtration. Preferably
said separation is performed by means of centrifugal force to
obtain a phase rich in nanoparticles and a phase of electrolyte.
Possible separation processes which use a centrifugal force are a
hydrocyclone or a centrifuge. Preferably a centrifuge is used.
Suitably the electrolyte is re-used in the process according to the
invention as described above. The phase rich in nanoparticles is
subsequently diluted with water, preferably deionised water or
ultrapure water. In order to redisperse the nanoparticles in the
added water it is preferred to apply sonication. Sonification may
be performed at conditions known to the skilled person. In order to
effectively remove any remaining liquid electrolyte, the steps of
centrifugation, dilution with water and sonication can be repeated
several times, preferably between 3 and 10 times.
[0033] The nanoparticles as obtained may be dissolved to form a
suspension in any suitable liquid, suitably water. Surfactants or
polymers may be added to achieve a more stable suspension. Examples
of suitable surfactants or polymers are Cetyl Trimethyl Ammonium
Bromide (CTAB), Tetradecyl Trimethyl Ammonium Bromide (TTAB),
surfactants of the Brij.RTM. type as obtainable from Sigma-Aldrich
and Polyvinylpyrrolidone (PVP). The suspended nanoparticles may be
used as an intermediate product suited to transport the
nanoparticles from the manufacturer to its end user or as a means
to affix the nanoparticle to said support.
[0034] For some catalytic end uses of the nanoparticles it is
preferred to affix the nanoparticles to the surface of a suitable
support. Generally, any support capable of supporting and providing
adequate dispersion for the nanoparticles can be used. Preferably,
the support is stable in the local environment where the catalyst
is to be used. The support has a surface area and/or porosity
sufficient to provide dispersion of the nanoparticles. However, a
support with increased porosity provides more intimate contact
between reactants and catalytic material. Examples of suitable
solid supports are silica gels, derivatized plastic films, glass
beads, cotton, plastic beads, alumina gels, polymer resins, a
zeolite, a molecular sieve, a carbon, an inorganic oxide, an
inorganic hydroxide, a mixed inorganic hydroxides or mixed
inorganic oxides. Specific examples of these supports are further
described in WO-A-2007/055663, which publication is hereby
incorporated by reference.
[0035] The nanoparticles are preferably fixed to the support by
mixing the support with the suspension followed by washing with a
solvent to remove the excess of the surfactant. The catalyst may
optionally be calcined. Alternatively the nanoparticles can be
loaded on a support by first preparing a reverse microemulsion of
the nanoparticle as described in WO-A-2008/101602. This publication
also describes an alternative support suited for the nanoparticles
prepared according to the present invention. The alternative
support are fabrics from activated carbon fibers, acrylonitril
fibers, glass fibers, ceramic fibers, metal fibers or fleece
composite oxides of activated carbon fibers.
[0036] The catalyst comprising the nanoparticles as obtained by the
present process may be used in various reactions, such as
hydrogenation, hydrotreating, hydrocracking, hydroisomerisation,
hydrofinishing, reforming, Fischer-Tropsch reactions and methanol
to olefin reactions. Suitably the metal or metal alloy comprises a
Group VIII metal. The nanoparticles are also suited as catalyst in
a fuel cell to catalyse the oxidation of methanol or alternatively
ethanol. Methanol and ethanol are particularly attractive as a fuel
due to their compatibility with existing infrastructure and its
high energy density. The main problem of methanol oxidation on
platinum is the formation of poison species (CO) as an intermediate
that is difficult to oxidize. Therefore it is desirable to produce
platinum nanoparticles with high catalytic activity towards the
oxidation of both methanol or ethanol and CO. It has been found
that the platinum nanoparticles and platinum alloy particles,
especially nanoparticles of PtRh and PtRu, as prepared by the
process according to the present invention have improved catalytic
activity for the oxidation reaction of methanol and the reduction
of nitrate. It has been also shown that these nanoparticles have
improved catalytic activity for the oxidation reaction of CO.
[0037] The nanoparticles are also suited as catalyst in a fuel cell
to catalyse the oxidation of hydrogen, ethanol, formic acid,
ammonia, borohydride and other organic compounds and the reduction
of oxygen. Suitably the metal or metal alloy is platinum, copper,
gold, rhodium, nickel or a platinum alloy. The nanoparticles are
also suited as catalyst for neutralization of exhaust gases from,
for example, an automobile engine or industry. Suitably the metal
or metal alloy is gold or a platinum alloy. The nanoparticles are
also suited as catalyst for waste water treatment for, for example,
reduction of nitrates and nitrites. Suitably the metal or metal
alloy is gold, platinum, rhodium or a platinum alloy. The
nanoparticles can also be used as electrodes in electrochemical
sensors in, for example, HPLC. The nanoparticles can also be used
in photovoltaics or photocatalysis. Suitably nanoparticles of
titanium oxide are used for this application. The nanoparticles may
also find use as part of a conductive nano ink. Suitably gold,
silver or copper nanoparticles are used for this application.
[0038] Nanoparticles comprising silver as prepared according to the
present invention may advantageously find use as part of an
anti-microbial device or composition, for example as part of wound
dressing, clothing, filters, cloth, ointment or paint.
[0039] The invention also relates to nano lead-free soldering paste
used for electronic element welding and surface packaging.
Soldering is an indispensable technology used for the
interconnection and packaging of electronic products. For the last
decades, Sn--Pb solder alloys have been the preferred
interconnection materials used in such applications. This
preference can be attributed to their numerous advantages, such as
low cost, low melting temperature, good workability and ductility,
excellent mechanical properties, and good soldering and wetting
behaviour on several substrate materials such as Cu, Ag, Pd, Au,
and respective alloys. Increased environmental and health concerns
regarding the toxicity of Pb have, however, lead many countries to
legislate the ban of Pb from many electronic applications. It is
therefore of utmost importance for the electronics community to
find appropriate substitutes to replace Sn--Pb solders. The nano
lead-free soldering paste has the advantage over non-nano soldering
paste in that the melting point of the nano lead-free soldering
paste is lower. This is obviously advantageous for soldering
applications. Known methods to prepare nanoparticles for such a
soldering paste are described in Journal of Electronic Materials,
Vol. 38, No. 2, 2009 pages 351-355. In this publication it is
mentioned that the Sn-2.62Ag-0.34Cu nanoparticles are prepared by a
so-called consumable-electrode direct current arc technique
resulting in a particle size ranging between 10 and 100 nm.
Applicants now found that with the process according to the present
invention nanoparticles are prepared having the smaller size and
having much narrower size distribution. Thus the majority of the
particles have a smaller size resulting in an even more improved
lowering of the melting point.
[0040] The nano lead-free soldering paste consists of nano
lead-free solder powder and soldering flux, and according to the
mass percent, the nano lead-free soldering paste contains the
following components: 85 to 94 weight percent of nano particles and
6 to 15 weight percent of soldering flux, wherein, the nano
particles are preferably nanoparticles prepared by the process
according to the present invention wherein the starting solid metal
object is made of an alloy comprising Sn and at least one other
metal. A preferred other metal is Ag. Possible alloys are Sn--Ag,
Sn--Ag--Cu, Sn--Bi, Sn--Zn, Sn--Zn--Bi, Sn--Co--Cu, Sn--Cu, Sn--In,
or Sn--Sb and wherein the nanoparticles thus obtained are composed
of the starting alloy. A preferred alloy is SnAg, wherein the
content of Ag in the alloy is preferably between 2 and 5 wt %. An
example is 96.5Sn-3.5Ag (weight ratio). Another suitable alloy is
SnAgCu alloy wherein the content of Ag may vary from 2 to 5 wt %
and the content of Cu may vary from 0.1 to 1.5 wt %. An example is
Sn-3 Ag-0.5 Cu (wherein the values are weight percentage).
[0041] The invention will be illustrated with the following
non-limiting examples
Example 1
[0042] A 100 .mu.m diameter platinum wire having a purity of 99.99
wt % was submerged by 1 mm in an aqueous solution containing 10 M
NaOH. Said solution was prepared with ultrapure water (Millipore
MilliQ gradient A10 system, 18.2 M.OMEGA. cm, 3 ppb total organic
carbon) and NaOH (99.9% from Sigma-Aldrich). For 1000 seconds a
cathodic potential of -10 V dc (direct current) was maintained.
Vitreous carbon is used as anode to rule out the possibility of
formation of interfering species by anode dissolution. The platinum
wire was rinsed with ultrapure water and observed using SEM
(scanning electron microscope). The SEM images as shown in FIGS. 1
and 2 showed the formation of nanoparticles having a size of <50
nm which were coated on the surface of the platinum wire. It was
observed that even the vigorous H2 gas evolution during the
cathodic treatment was not able to dislodge the particles from the
surface.
Example 1a and 1b
[0043] Example 1 was repeated except that the metal was Ir or Re.
The cathodic potential was -30 V (dc) and the electrolyte was an
aqueous solution of 1 M NaClO.sub.4. The solution of NaClO.sub.4
was prepared from NaClO.sub.4 99.9% from Sigma-Aldrich. The
experiment was continued for 30 minutes. Inspection of the
cathodes, after that the electrolyte was removed by washing with
deionised water, showed that the surface of the Ir and Re cathode
was coated with nanoparticles of Ir and Re respectively.
Example 2
[0044] Example 1 was repeated except that an alternating current at
100 Hz and 20 V peak to peak (p-p) square wave ac was applied in 10
M NaOH. After around 100 seconds the platinum wire part as
submerged in the liquid electrolyte was totally `dissolved`. The
aqueous solution had turned black, and consists of a suspension of
nanoparticles. The electrolyte in the nanoparticles solution was
subsequently removed by successive steps of centrifugation at 3000
rpm using a Hettich EBA 20 Centrifuge), dilution with ultrapure
water and sonication in an ultrasonic bath at 40 kHz using a
Branson ultrasonic cleaner Model 2510. The cleaned suspension of
nanoparticles was deposited on a grid for further investigation
using a transmission electron microscope (TEM). It was found that
platinum nanoparticles had formed having a size ranging from 4 to
30 nm. Lattice spacing as well as Energy Dispersive X-ray (EDX)
spectrum measured correspond exactly to platinum. FIG. 3 shows the
TEM image. FIG. 4 shows the EDX spectrum.
Comparative Experiment A
[0045] Example 1 and 2 was repeated, except that instead of an
aqueous solution containing NaOH, an aqueous acid solution of
H.sub.2SO.sub.4, HClO.sub.4 or HCl was used, with concentration
between 1 M and 10 M. Under the remaining conditions of Example 1
and 2 no formation of metal nanoparticles was observed.
Example 3
[0046] Example 2 was repeated using a gold wire instead of the
platinum wire. In this way, a solution with gold nanoparticles is
obtained, which was cleaned as described in example 2. After
further investigation of said solution using a transmission
electron microscope (TEM) it was found that gold nanoparticles had
formed having a size ranging from 4 to 30 nm. Lattice spacing as
well as EDX spectrum measured correspond exactly to gold. FIG. 5
shows the TEM image. FIG. 6 shows the EDX spectrum.
Examples 4
[0047] Example 2 was repeated wherein the following aqueous
electrolyte compositions were used: 10 M NaOH, 1 M NaOH, 1 M
tetra-tert-butylammonium hydroxide, 1 M LiOH, 1 M CsOH, 1 M KOH, 1
M NH.sub.4Cl, 1 M NaCl, 1 M NaClO.sub.4, 0.5 M K.sub.3PO.sub.4, 0.5
M BaCl.sub.2, 1 M NH.sub.4F, 1 M Na.sub.2SO.sub.4, 1 M NaNO.sub.3,
1 M H.sub.3BO.sub.3+1 M NaH.sub.2BO.sub.3 and 1 M NaF. In all
examples the formation of platinum nanoparticles in the solution
was observed.
Example 5
[0048] Example 2 was repeated wherein the following aqueous
electrolyte composition was used: 10 M NaOH. The voltage was varied
according to the following list: 28 V p-p, 24 V p-p, 16 V p-p, 12 V
p-p, 8 V p-p, 4 V p-p. For all voltages the formation of platinum
nanoparticles in solution was observed.
Examples 6
[0049] Example 2 was repeated wherein the following electrolyte
composition was used: 10 M NaOH and wherein the metal was varied:
Ni, Rh, Ag, Nb, Al, Co, Mo, Y, V, and Ru. The formation of
nanoparticles in the solution was observed for all metals. In case
of ruthenium a ruthenium oxide nanoparticle was isolated from the
solution.
Examples 7
[0050] Example 2 was repeated wherein the following electrolyte
composition was used: 1 M NaOH and wherein the metal was Cu, V, In,
Rh and Mn. The formation of nanoparticles of said metals in the
solution was observed.
Example 8
[0051] Example 6 was repeated except that the metal was Au and the
voltage was 30 V p-p. The formation of gold nanoparticles in the
solution was observed.
Example 9
[0052] Example 2 was repeated except that the electrolyte was
molten NaOH. The formation of platinum nanoparticles was
observed.
Example 10
[0053] Example 9 was repeated at 10 V p-p alternating current. The
formation of platinum nanoparticles was observed.
Examples 11
[0054] Example 7 was repeated wherein the following metal alloys
were used as cathode: Pt.sub.90Rh.sub.10, Pt.sub.80Rh.sub.20,
Pt.sub.70Rh.sub.30, Pt.sub.80Ir.sub.20, Pt.sub.50Ni.sub.50,
Pt.sub.95.2Ru.sub.4.8, wherein the subindexes in the alloys refer
to the weight percentage. For all alloys the formation of
nanoparticles was observed. The applicants have found that these
nanoparticles exhibit an electrochemical behaviour in sulfuric acid
solutions that clearly indicates that the nanoparticles are not a
mixture of Pt nanoparticles and the other metal nanoparticles.
Consequently, the chemical composition of the synthesized
nanoparticles must be that of an alloy.
Example 12
[0055] Example 11 was repeated for all alloys using a 10 M NaOH
electrolyte solution. For all alloys the formation of alloy
nanoparticles was observed as in Example 11.
Examples 13
[0056] Example 2 was repeated at different frequencies of the
alternating current: 10 Hz, 50 Hz, 100 Hz, 500 Hz, 1000 Hz and
10000 Hz. At all frequencies the formation of platinum
nanoparticles was observed.
Example 14
[0057] The solution of platinum nanoparticles as obtained in
Example 2 was subjected to a centrifuge at 3000 rpm to separate the
majority of the electrolyte from the nanoparticles. The remaining
concentrated nanoparticles solution was diluted with ultrapure
water and redispersed by sonication in an ultrasonic bath at 40
kHz. The steps of centrifugation, dilution with water and
sonication were repeated six times. At the end, the nanoparticle
solution had pH=7 and the concentration was of 1 mgram of Pt in 1
ml of solution. 3 .mu.l of said solution was deposited on a flat
gold electrode having a diameter of 3 mm. The water content of the
nanoparticle solution was dried under a flux of argon. The thus
deposited platinum nanoparticles remain attached to the gold
surface during the following electrochemical experiments. The
platinum-nanoparticles-modified gold electrode was transferred to
an electrochemical cell. The blank voltammogram was recorded in 0.5
M H.sub.2SO.sub.4 at a scan rate of 50 mV/s (FIG. 7A, solid curve).
Then, the electrooxidation of CO was measured after adsorption of
CO at E=0.1 V vs. RHE (RHE=Reversible Hydrogen Electrode) for 1
minute and consequent purging with of the CO dissolved in solution
for 30 minutes. The CO electrooxidation was recorded with a scan
rate of 20 mV/s. The results are shown in FIG. 7B, solid curve.
Then, the catalysis for methanol oxidation was measured in a 0.5 M
H.sub.2SO.sub.4+0.5 M CH.sub.3OH solution at a scan rate of 50 mV/s
(FIG. 7C, solid curve). Current was normalized per gram of
platinum.
Comparative Experiment B
[0058] Example 5 is repeated using commercially obtained
nanoparticles from the Tanaka Kikinzoku Inc company. The
as-received nanoparticles were dispersed on Vulcan.RTM. carbon with
50% wt loading, and had an average size of 5 nm diameter. A
solution of of 1 mgram of Pt in 1 ml of solution was obtained by
adding ultrapure water. Dispersion of the nanoparticles in solution
was achieved by sonication for one hour. Afterwards, 3 .mu.l of
said solution was deposited on a glassy carbon electrode having a
diameter of 3 mm as in Example 14. The water content of the
nanoparticle solution was dried under a flux of argon. The thus
deposited platinum nanoparticles remain attached to the glassy
carbon surface during the following electrochemical experiments.
The platinum-nanoparticles-modified glassy carbon electrode was
transferred to an electrochemical cell. Further cleaning of the
nanoparticles is achieved by cycling the electrode between 0.05 V
and 1.1 V vs. RHE for 80 cycles in 0.5 M H.sub.2SO.sub.4 at 50
mV/s. Afterwards, the electrode was transferred to a new
electrochemical cell, and the blank voltammogram was recorded in
0.5 M H.sub.2SO.sub.4 at a scan rate of 50 mV/s (FIG. 7A, dashed
curve). Then, CO and methanol electrooxidation were recorded as
described in example 5. The results are shown in FIGS. 7B and 7C,
dashed curves.
[0059] FIG. 7B clearly shows that CO oxidation takes place at ca
0.1 V lower overpotential in Example 14 as compared to the results
of Experiment B. Further the charge under the CO stripping peak is
about two times lower for Example 14. The first observation shows
that it is energetically more favourable to oxidize CO with the
catalyst comprising the platinum nanoparticles as obtained by the
present invention. The second observation indicates that our
nanoparticles are .about.1.5 times larger on average, which leads
to a lower effective surface area per gram of platinum.
[0060] FIG. 7C shows that the current towards methanol oxidation is
markedly higher in Example 14 (solid line) than in the comparative
experiment B (dashed line). Although the nanoparticles of Example
14 are .about.1.5 times larger on average than the ones used in
Experiment B (which leads to a lower effective surface area per
gram of platinum) it is nevertheless observed that the intrinsic
catalytic activity of the nanoparticles of Example 14
overcompensates for this effect in such a way that the maximum
methanol oxidation current is double that of Experiment B using the
commercial nanoparticles. Thus it is concluded that the platinum
nanoparticles as prepared according to this invention are
catalytically more active than the existing platinum nanoparticles
of Experiment B.
Example 14
[0061] The cathodic corrosion method described here is a simple,
clean and fast way of synthesis of nanoalloys with high catalytic
performance. On the example of a series of PtRh alloys we show that
this one-step method can convert a bulk alloy electrode into an
aqueous suspension of nanoparticles, retaining the composition and
the crystal lattice structure of the starting alloy. Compared to
pure metals, these alloy nanocatalysts are more active towards the
CO and methanol oxidation and the nitrate reduction reactions.
Nanoparticles made of PtRu, PtIr, PtNi, AuCo, AuCu and FeCo bulk
alloys demonstrate the universality of this synthesis method.
[0062] An alloy wire having a diameter of 0.12 mm was immersed for
2.5 mm in an electrolyte. In the case of the alloys PtRh, PtRu,
PtIr, AuCo, AuCu and FeCo, the electrolyte was 1 M NaOH. In the
case of alloys PtNi and PtCo, the electrolyte was a saturated
solution of H.sub.2SO.sub.4. An ac potential versus a high surface
area glassy carbon counter electrode was applied, until all
submerged metal is converted into a black suspension of metal
nanoparticles. A negative dc offset ensures that the formation of
the nanoparticles proceeds via a reduction (cathodic) process at
the alloy wire electrode. In the case of the alloys PtRh, PtRu,
PtIr, AuCo, AuCu and FeCo, the applied ac potential varied between
-10 and 0 V vs. the glassy carbon counter electrode (see FIG. 8).
In the case of the alloy PtNi, the applied ac potential varied
between -4 and +1 V vs. the glassy carbon counter electrode. In the
case of the alloy PtCo, the applied ac potential varied between -5
and +2 V vs. the glassy carbon counter electrode. A black
suspension coming off the electrode was observed throughout the
experiment. Once the wire is atomized, the suspension of
nanoparticles is centrifuged and washed with MilliQ water.
[0063] After cathodic atomization the nanoparticles were
characterized by transmission electron microscopy (TEM), including
energy dispersive x-ray analysis (EDX), and x-ray diffraction
(XRD). As is expected for clean, unprotected nanoparticles, they
show a certain degree of agglomeration, while careful visual size
analysis gives a rather compact size distribution (see FIG. 9). EDX
composition analysis confirms within a 5% error that the
composition of the nanoparticles remains the same as the starting
wire material.
Example 15
[0064] The electrochemical properties of the PtRh nanoparticles
prepared in Example 14 were measured by recording a current-voltage
characteristic of an electrochemical interface under controlled
conditions (a so-called blank voltammogram). The nanoparticles were
supported on a gold electrode in sulfuric acid. It is well known
that the voltammetric response of gold in this media does not show
any significant signal in the potential region under study. Prior
to each experiment, the blank voltammogram of the bare gold was
checked in order to avoid contamination from previous samples. Two
important potential regions were identified. The features between
0.05 V and 0.4 V are related to the adsorption/desorption of a
monolayer of hydrogen and anions, and between 0.4 and 0.9 V to the
adsorption/desorption of OH species on Rh.
[0065] It was found that as the content of Rh is increased in the
alloy, in the lower potential region the two peaks of hydrogen
shift toward more negative potentials, becoming one single peak
characteristic of Rh. In the higher potential region the charge
related to the adsorption/desorption of OH increases with the
concentration of Rh in the sample.
[0066] The electrochemical oxidation of methanol on bimetallic
catalysts is an important and widely studied reaction due to
applications in low temperature fuel cells, with a number of
studies on PtRh alloys. As for the reduction of nitrate, Rh is
generally considered to be most active monometallic
electrocatalyst. Remarkably little attention has been given to PtRh
alloys for this reaction.
[0067] Two important behaviours were found:
1) The maximum negative current corresponding to the reduction of
nitrate increases proportionally to the content of Rh, but
surprisingly the Rh.sub.80Pt.sub.20 nanoalloy exhibits an even
higher current density than pure Rh nanoparticles, also when
compared in terms of current density per electrochemically active
surface area. The maximum current density observed in the sample
Rh.sub.80Pt.sub.20 is 560 .mu.Acm.sup.-2 while the maximum current
density observed in the pure Rh sample for the same concentration
for nitrate is 400 .mu.Acm.sup.-2. Previous results for Rh
nanoparticles and Rh massive electrode report maximum currents of
500 .mu.Acm.sup.-2 (in a 10 times more concentrated solution) and
300 .mu.Acm.sup.-2. 2). The Rh.sub.80Pt.sub.20 sample also exhibits
a smaller difference between the reduction current in the negative
and the positive scans, suggesting that the reactivity of
intermediate species (nitrite, NO) leads to less hysteresis
compared to the other samples. While Rh is generally considered the
best monometallic catalyst for nitrate reduction, the observation
that our Rh.sub.80Pt.sub.20 nanoparticles are superior to pure Rh
is new and significant. The high activity of Rh is usually ascribed
to its ability to strongly bind (oxy-)anions.
[0068] In the voltametric response of the supported nanoparticles
towards the oxidation of methanol it was found that the samples of
Rh and Rh.sub.80Pt.sub.20 show no (or negligible) activity towards
the oxidation of methanol in agreement with previous results on
bulk electrodes. This is due to the inhibition of methanol
adsorption by the strong adsorption of poisoning CO on Rh. The
samples with higher platinum content, however, show significant
oxidation currents in the potential region between 0.4 and 0.9 V.
The voltammetric profiles of the three high content platinum
samples show very similar behaviour, the most important
characteristic being the hysteresis between the positive and the
negative scan. The lower oxidation currents during the positive
scan are due to the initial CO poisoning of the surface. At higher
potentials this CO is oxidized off the surface, and during the
negative scan the oxidation of methanol is not limited by the
presence or formation of the poison and for that reason the
currents are higher. The alloys Pt.sub.90Rh.sub.10 and
Pt.sub.70Rh.sub.30 present a hysteresis of about 60 mV between the
positive and the negative scan, which is 40 mV smaller than the
hysteresis of the Pt sample. This is probably due to the higher
reactivity of these alloys toward the oxidation of
surface-poisoning CO. It is therefore clear that, compared to pure
metals; PtRh alloys show better catalytic activity for the
oxidation of methanol and CO, which are reactions of special
interest for fuel cells and, in the gas phase, for automotive
exhaust catalysis.
Example 16
[0069] To demonstrate the universality of our cathodic atomization
method, we have prepared nanoparticles from Pt.sub.80Ir.sub.20,
Pt.sub.95Ru.sub.5 and Pt.sub.50Ni.sub.50 alloy wires, and also
AuCo, AuCu and FeCo alloys in a method as described in Example 14.
In all cases TEM analysis shows nanoparticles with a mean size
below 10 nm, EDX confirms the bimetallic composition, and the shift
of x-ray diffraction lines indicates proper alloying.
Electrochemical studies of CO and methanol oxidation on PtRu
nanoalloys, and the oxygen reduction reaction on PtNi nanoalloys,
as prepared by
* * * * *