SURFACTANTREMOVAL FROM PALLADIUM NANOPARTICLES
BACKGROUND
Palladium and palladium alloy nanoparticles can be used as catalysts,
particularly in fuel cells used to produce electrical energy. For example, in a hydrogen fuel
cell, a palladium catalyst can be used to oxidize hydrogen gas into protons and electrons at
the anode of the fuel cell. At the cathode of the fuel cell, the palladium catalyst triggers the
oxygen reduction reaction (ORR), leading to formation of water.
Fuel cell performance depends in part on the available surface area of the
palladium nanoparticles. Fuel cell performance generally increases when the surface area of
the palladium nanoparticles is increased. In addition to size, the shape of the palladium
nanoparticles can also be selected in order to further increase the oxygen reduction reaction
(ORR) activity. Surfactants are commonly used during nanoparticle formation to control
the particle size and shape. The surfactants bind to the nanoparticles as they are shaped and
sized.
Once the nanoparticles have been formed, the surfactants used for shaping
and sizing the particles need to be removed. Some surfactants can be removed by washing
and low temperature heat treatment. Other surfactants, however, require long washing
times (as long as weeks in special solvents) or high temperature treatment at temperatures
above 300 °C. For some catalyst nanoparticles, high temperature treatment presents
problems. For example, at 300 °C, cubic palladium nanoparticles may lose their shape and
increase in particle size. As a result, using high temperature treatment to remove surfactants
from the nanoparticles removes benefits the surfactants were intended to provide.
SUMMARY
A method for removing a surfactant from a palladium nanoparticle includes
exposing the palladium nanoparticle to hydrogen and removing the surfactant from the
palladium nanoparticle.
A method includes synthesizing the palladium nanoparticle using a
surfactant. The surfactant influences a geometric property of the palladium nanoparticle
and bonds to the palladium nanoparticle. The method also includes exposing the palladium
nanoparticle to hydrogen to remove the surfactant from the palladium nanoparticle.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a simplified schematic of a method for removing surfactant from a
palladium nanoparticle.
Fig. 2 is a simplified schematic of a method for exposing a bond between a
surfactant and a palladium nanoparticle to hydrogen using an electric potential.
Fig. 3 is a simplified schematic of a method for exposing a bond between a
surfactant and a palladium nanoparticle to hydrogen using hydrogen gas.
Fig. 4 is a simplified schematic of a method for removing surfactant from a
palladium nanoparticle.
Fig. 5 is a simplified schematic of a method for preparing a palladium
nanoparticle.
DETAILED DESCRIPTION
The present invention provides a simple and efficient method for removing
surfactants from palladium nanoparticles. Electrochemical and chemical processes use
hydrogen to weaken the adsorption of surfactant on a palladium nanoparticle. The method
provides a simple and efficient way to remove surfactants from palladium nanoparticles
without using high temperatures.
Surfactants are often used to modify the size and shape of palladium
nanoparticles that serve as fuel cell catalysts. Surfactant micelles present during the
formation of palladium nanoparticles affect the geometry of the nanoparticles. Particular
surfactants and surfactant concentrations can be used to form palladium nanoparticles
having the specific sizes and shapes needed to meet fuel cell performance requirements.
Suitable surfactants for. sizing and shaping palladium nanoparticles include
polyvinylpyrrolidone (PVP), and chlorine- and bromine-based salts. The surfactants bind to
the palladium nanoparticles as they shape the nanoparticles. These surfactants must be
removed from the palladium nanoparticles before they are used as catalysts in order for the
nanoparticles to be fully accessible by reactants.
As noted above, surfactants are typically removed by washing the
nanoparticles (with or without low temperature heat treatment) or high temperature
treatment. Each of these surfactant removal methods has drawbacks. Some surfactants can
be removed only after extremely long washing times. Long periods of time spent washing
the nanoparticles increases the time and costs required for production of the final
nanoparticle catalyst. High temperature treatments typically require temperatures above
300 °C and can have deleterious effects on palladium nanoparticles. As noted above, at 300
°C, palladium nanoparticles may lose their shape and increase in size. High temperature
treatment to remove surfactants from the nanoparticles can eliminate the size and shape
modifications the surfactants were used to provide.
Instead of using a lengthy washing method or a harmful high temperature
method, the present invention uses molecular hydrogen to weaken bonds between a
nanoparticle and the surfactant used to shape and/or size the nanoparticle. Fig. 1 illustrates
a simplified schematic of a method for removing surfactant from a nanoparticle. Method 10
includes exposing the nanoparticle to hydrogen (step 12) and removing the surfactant from
the nanoparticle (step 14). The hydrogen penetrates into the palladium particles and
expands the palladium-palladium lattice distance. This expansion of the palladiumpalladium
lattice distance weakens adsorption of surfactants and facilitates removal of the
surfactants from the nanoparticle. As discussed below, hydrogen exposure step 1 can be
performed by electrochemical or chemical methods.
Fig. 2 illustrates a simplified schematic of electrochemical method 16 for
removing a surfactant from a nanoparticle. In step 18, an electric potential is applied to the
nanoparticle. Suitable electric potentials are at or below the potential where hydrogen
adsorption/absorption and hydrogen evolution occurs. In exemplary embodiments, the
electric potential measures between about -0.2V and about 0.35V against a reversible
hydrogen electrode. In even more exemplary embodiments, the electric potential measures
between about -0.2V and about 0.1V against a reversible hydrogen electrode. In one
particular embodiment, the electric potential measures about -0.05V against a reversible
hydrogen electrode.
In step 20, the electric potential is maintained for a time sufficient to allow
hydrogen to penetrate into the nanoparticle. The nanoparticle acts as an electrode and
hydrogen forms as a result of a multistep reaction. First, adsorbed hydrogen atoms form at
the surface of the nanoparticle
H30 + + e H ads + H20 (1)
where H ads is an adsorbed hydrogen atom at the nanoparticle surface. The adsorbed
hydrogen atoms combine to form molecular hydrogen
H ads + Hads ® H2 (2)
or a further electrochemical reaction produces molecular hydrogen
H ads + H30 + + e ® H2 + H20 (3)
depending on the electrode potential. The formed hydrogen penetrates into the palladium
nanoparticle forming palladium hydrides and expanding the palladium nanoparticle lattice.
Palladium hydride is metallic palladium that contains a substantial quantity of hydrogen
within its crystal lattice. At room temperature and atmospheric pressure, palladium can
absorb up to 900 times its own volume of hydrogen. The adsorption of surfactants becomes
weaker and the surfactants easily desorb from the palladium surface due to the lattice
expansion. In some cases, molecular hydrogen is not necessary. Atomic hydrogen formed
in reaction (1) can also be absorbed by palladium and cause lattice expansion.
In exemplary embodiments, the electric potential is maintained for less than
about five minutes. In one particular embodiment, the electric potential is maintained for no
more than about one minute. Steps 18 and 20 are typically performed at room temperature
(between about 15 °C and about 30 °C). Electrolytes are used during steps 18 and 20.
Suitable electrolytes include dilute aqueous acids such as 0.1 M perchloric acid (HC10 ).
Fig. 3 illustrates a simplified schematic of chemical method 22 for removing
surfactants on a palladium nanoparticle. Instead of forming hydrogen on the surface of the
palladium nanoparticle, molecular hydrogen is delivered to the palladium nanoparticle
without involving an electrochemical reaction. In step 24, a sized and shaped palladium
nanoparticle Having surfactant is placed in a vessel. In step 26, hydrogen gas is added to the
vessel so that molecular hydrogen is absorbed by the palladium nanoparticle resulting in
lattice expansion of the palladium nanoparticle. The absorbed hydrogen weakens the bonds
between the palladium nanoparticle and surfactant, allowing the surfactant to be easily
removed.
Once a sufficient amount of hydrogen has been absorbed by the palladium
nanoparticle, the bonds between the surfactant and the palladium nanoparticle will have
weakened enough so that the surfactant merely desorbs from the nanoparticle (i.e. the
surfactant "falls off the nanoparticle). The hydrogen present at the surface of the
palladium nanoparticle or within the palladium crystal lattice also requires no further
treatment. Any hydrogen present will leave on its own prior to or during use as a fuel cell
catalyst.
Fig. 4 illustrates a simplified schematic of a method for removing surfactant
from a nanoparticle having post-processing steps. Method 28 includes the steps of method
10 (exposing the nanoparticle to hydrogen and removing the surfactant from the
nanoparticle) as well as washing step 30 and filtering step 32. The palladium nanoparticle
is washed with water. Washing step 30 provides for the removal of impurities as well as
any solvent used during steps 18 and 20. Palladium nanoparticles treated according to the
methods above can also be filtered in step 32 to further purify the nanoparticles.
Fig. 5 illustrates a simplified schematic of a method for preparing a
palladium nanoparticle. Method 34 includes combining a palladium nanoparticle with a
surfactant (step 36). In step 36, the surfactant modifies a geometric property (e.g., size,
shape, etc.) of the nanoparticle. The surfactant bonds to the nanoparticle as a result of the
geometric modification of step 36. Once the geometric property of the nanoparticle has
been modified, the nanoparticle is exposed to hydrogen in step 38 in order to remove the
surfactant from the nanoparticle. Step 38 is performed according to method 16 or method
22 described above. Following step 38, the surfactant is no longer bonded to the
nanoparticle and the nanoparticle is ready for use.
To summarize, hydrogen is used to weaken a bond between a palladium
nanoparticle and the surfactant used to shape and size the nanoparticle. Hydrogen can be
formed at the surface of the palladium nanoparticle using an electrochemical method so that
it is absorbed by the nanoparticle. Hydrogen can also be added to the nanoparticle
environment so that it is absorbed by the nanoparticle. The exposure of the nanoparticle to
hydrogen expands the nanoparticle' s lattice structure and weakens the bond between the
surfactant and the nanoparticle, allowing the surfactant to be easily removed. The method
described herein allows for simple, quick and efficient surfactant removal without using
deleterious high temperatures or requiring long processing times.
Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize that changes may be made
in form and detail without departing from the spirit and scope of the invention.

CLAIMS:
1. A method for removing a surfactant from a palladium nanoparticle, the
method comprising:
exposing the palladium nanoparticle to hydrogen; and
removing the surfactant from the palladium nanoparticle.
2. The method of claim 1, wherein the palladium nanoparticle comprises a
chemical substance selected from the group consisting of palladium, palladium alloys and
combinations thereof.
3. The method of claim 1, wherein exposing the palladium nanoparticle to
hydrogen comprises:
applying an electric potential to the palladium nanoparticle, wherein the
electric potential applied to the palladium nanoparticle is at or below
a potential required for hydrogen adsorption/absorption and hydrogen
evolution; and
maintaining the electric potential for a time sufficient for hydrogen to
penetrate into the palladium nanoparticle.
4. The method of claim 3, wherein the electric potential applied to the
palladium nanoparticle measures from about -0.2V to about 0.1 V.
5. The method of claim 4, wherein the electric potential applied to the
palladium nanoparticle measures about -0.05V.
6. The method of claim 3, wherein the electric potential is applied to the
palladium nanoparticle for less than about five minutes.
7. The method of claim 6, wherein the electric potential is applied to the
palladium nanoparticle for no more than about one minute.
8. The method of claim 3, wherein the electric potential is applied to the
palladium nanoparticle while the palladium nanoparticle is at a temperature between about
15 °C and about 30 °C.
9. The method of claim 3, wherein the electric potential is applied to the
palladium nanoparticle in the presence of a dilute acid.
10. The method of claim 3, further comprising:
washing the palladium nanoparticle with water after applying the electric
potential to the palladium nanoparticle.
1 . The method of claim 3, further comprising:
filtering the palladium nanoparticle after applying the electric potential to the
palladium nanoparticle.
12. The method of claim 2, wherein exposing the palladium nanoparticle to
hydrogen comprises:
placing the palladium nanoparticle in a vessel; and
adding hydrogen gas to the vessel so that hydrogen is absorbed by the
palladium nanoparticle.
13. A method comprising:
synthesizing a palladium nanoparticle using a surfactant, wherein the
surfactant influences a geometric property of the palladium
nanoparticle and bonds to the palladium nanoparticle; and
exposing the palladium nanoparticle to hydrogen to remove the surfactant
from the palladium nanoparticle.
14. The method of claim 13, wherein the palladium nanoparticle comprises a
chemical substance selected from the group consisting of palladium, palladium alloys and
combinations thereof.
15. The method of claim 13, wherein exposing the palladium nanoparticle to
hydrogen comprises:
applying an electric potential to the palladium nanoparticle wherein the
electric potential applied to the palladium nanoparticle is at or below
a potential required for hydrogen adsorption/absorption and hydrogen
evolution; and
maintaining the electric potential for a time sufficient for hydrogen to
penetrate into the palladium nanoparticle to weaken a bond between
the surfactant and the palladium nanoparticle.
16. The method of claim 15, wherein the electric potential applied to the
palladium nanoparticle measures from about -0.2V to about 0.1V.
17. The method of claim 16, wherein the electric potential applied to the
palladium nanoparticle measures about -0.05V.
18. The method of claim 15, wherein the electric potential is applied to the
palladium nanoparticle for less than about five minutes.
19. The method of claim 8, wherein the electric potential is applied to the
palladium nanoparticle for no more than about one minute.
20. The method of claim 14, wherein exposing the bond between the surfactant
and the palladium nanoparticle to molecular hydrogen comprises:
placing the palladium nanoparticle in a vessel; and
adding hydrogen gas to the vessel so that hydrogen is absorbed by the
palladium nanoparticle.