U.S. patent application number 13/862116 was filed with the patent office on 2014-10-16 for subnanometer catalytic clusters for water splitting, method for splitting water using subnanometer catalyst clusters.
This patent application is currently assigned to UChicago Argonne, LLC. The applicant listed for this patent is UCHICAGO ARGONNE, LLC. Invention is credited to Larry A. Curtiss, Michael J. Pellin, Stefan Vajda.
Application Number | 20140305805 13/862116 |
Document ID | / |
Family ID | 51686044 |
Filed Date | 2014-10-16 |
United States Patent
Application |
20140305805 |
Kind Code |
A1 |
Vajda; Stefan ; et
al. |
October 16, 2014 |
SUBNANOMETER CATALYTIC CLUSTERS FOR WATER SPLITTING, METHOD FOR
SPLITTING WATER USING SUBNANOMETER CATALYST CLUSTERS
Abstract
The invention provides a catalytic electrode for converting
molecules, the electrode comprising a predetermined number of
single catalytic sites supported on a substrate. Also provided is a
method for oxidizing water comprising contacting the water with
size selected catalyst clusters. The invention also provides a
method for reducing an oxidized moiety, the method comprising
contacting the moiety with size selected catalyst clusters at a
predetermined voltage potential.
Inventors: |
Vajda; Stefan; (Lisle,
IL) ; Curtiss; Larry A.; (Downers Grove, IL) ;
Pellin; Michael J.; (Naperville, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UCHICAGO ARGONNE, LLC |
Chicago |
IL |
US |
|
|
Assignee: |
UChicago Argonne, LLC
Chicago
IL
|
Family ID: |
51686044 |
Appl. No.: |
13/862116 |
Filed: |
April 12, 2013 |
Current U.S.
Class: |
205/462 ;
204/290.01; 205/628 |
Current CPC
Class: |
Y02E 60/366 20130101;
C25B 1/04 20130101; C25B 3/04 20130101; Y02E 60/36 20130101; C25B
11/0426 20130101; C25B 11/0421 20130101; C25B 11/0405 20130101;
C25B 11/02 20130101; C25B 11/0452 20130101; C25B 11/0478 20130101;
C25B 11/0473 20130101 |
Class at
Publication: |
205/462 ;
204/290.01; 205/628 |
International
Class: |
C25B 11/02 20060101
C25B011/02; C25B 11/04 20060101 C25B011/04; C25B 3/04 20060101
C25B003/04; C25B 1/04 20060101 C25B001/04 |
Goverment Interests
CONTRACTUAL ORIGIN OF THE INVENTION
[0001] The United States Government has rights in this invention
pursuant to Contract No DE-AC02-06CH11357 between the United States
Government and UChicago Argonne, LLC representing Argonne National
Laboratory.
Claims
1. A catalytic electrode for converting molecules, the electrode
comprising a predetermined number of single catalytic sites
supported on a substrate.
2. The electrode as recited in claim 1 wherein the number of sites
is proportional to conversion rate of the molecules.
3. The electrode as recited in claim 1 wherein between 5 and 25
percent of the surface area of the substrate is in contact with the
catalytic sites.
4. The electrode as recited in claim 1 wherein the catalytic sites
each comprise between 3 and 50 atoms of catalytic moiety.
5. The electrode as recited in claim 4 wherein the catalytic moiety
is a metal selected from the group consisting of Pd, Pt, Co, Ir,
Pd, Pt, Al, Ag, Cu, Ru, Fe, and combinations thereof.
6. The electrode as recited in claim 1 wherein some of the
catalytic sites comprise a first moiety, and some of the catalytic
sites comprise a second moiety.
7. The electrode as recited in claim 1 wherein the substrate is an
electrically conductive material selected from the group consisting
of such as (ultra)nanocrystalline diamond, graphite, graphene,
carbon nanotubes and other; (ii) oxides such as alumina, iron
oxide, copper oxide, titania, magnesium oxide, zinc oxide,
zirconium oxide, hafnium oxide, tungsten oxide (iii) metals such as
copper, silver, aluminum, tantalum, tungsten and combinations
thereof.
8. The electrode as recited in claim 7 wherein the substrate
comprises substantially the entire support of the sites.
9. The electrode as recited in claim 7 wherein the electrically
conductive material overlays a foundation material.
10. A method for oxidizing water comprising contacting the water
with size selected catalyst clusters.
11. The method as recited in claim 10 wherein each of the catalysts
clusters comprise between 4 and 50 atoms of catalytic moiety.
12. The method as recited in claim 10 wherein the pH of the method
is greater than about 12.
13. The method as recited in claim 10 wherein the method utilizes
electrolysis and the voltages applied up to approximately 3V.
14. The method as recited in claim 10 wherein the method is
practiced at a temperature ranging between approximately 5.degree.
C. and 900.degree. C.
15. A method for reducing an oxidized moiety, the method comprising
contacting the moiety with size selected catalyst clusters at a
predetermined voltage potential.
16. The method as recited in claim 15 wherein each of the catalysts
clusters comprise between 5 and 50 atoms of catalytic moiety.
17. The method as recited in claim 15 wherein the voltages applied
in the method range from between approximately -2 V and +2.5 V with
respect to the standard hydrogen electrode.
Description
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to an electrode and a method for
facilitating electrochemical reactions, and more specifically this
invention relates to an electrode and a method for utilizing size
selected catalyst clusters to facilitate oxidizing and reducing
reactions.
[0004] 2. Background of the Invention
[0005] Efficient methods for splitting water are a key scientific
challenge for meeting the world's fuels needs over the next
century. Water oxidation (i.e., water splitting) is a key catalytic
step for electrical fuels generation. It consists of two reactions:
the hydrogen evolution reaction (HER) and the oxygen evolution
reaction (OER).
[0006] Oxygen evolution reactions (see Equations 1 and 2 infra)
require high over potentials (.eta.=0.3.about.0.5 V depending on
the catalytic material and support chosen). The overpotential,
.eta., is the potential required above the standard reaction
potential (1.23 V at pH=0). As such, the overpotential required
often results in undesirable effects such as higher use of energy,
degradation of catalyst, and reduction of support structures.
[0007] Even when the reaction occurs, turnover rates (the number of
water molecules converted per atom of catalyst metal) remain low.
This critical step occurs in acid via the reaction:
2H.sub.2O.fwdarw.4H++4e.sup.-+O.sub.2 Equation 1
and in base via the reaction:
4OH.sup.-.fwdarw.2H.sub.2O+4e.sup.-+O.sub.2 Equation 2
[0008] In a fuel cell, where the inverse acid OER reaction,
Equation 1, is employed to produce energy, this poor catalytic
performance results in the need for a high Pt catalyst loading on
the cathode. This adds significantly to the cost of electrodes.
[0009] In electrolyzers, reaction in base is often employed to
improve catalytic performance. However, additional effort is
necessary to identify more efficient, less expensive
electrocatalysts for the OER reaction.
[0010] The complexity of both the catalytic materials and the water
oxidation reaction makes understanding the catalytic site critical
to improving the process.
[0011] State of the art catalyst systems comprise films and
macroscale particles as catalytic surfaces. It was commonly
believed that these macro-sized morphologies were required to
withstand the harsh reaction conditions (extreme pH, temperatures,
pressures) associated with oxidation and reduction protocols.
However, these materials are ill-defined in their structure and
especially their composition. Typically, high metal/metal oxide
loadings are utilized. This results in only a very small fraction
of atoms on the surface actually participating in the reaction,
while the bulk of materials beneath the surface do not
participate.
[0012] Given the large size (more than 5 nanometers) of the
catalyst particles utilized in the state of the art, the
composition and size of the active site is unknown. Many of these
large size systems comprise platinum or other precious metals.
Therefore, a rational optimization of the electrocatalyst has
remained elusive, while also remaining very expensive.
[0013] State of the art catalysts also suffer from instability in
that they change in size, composition and/or morphology. This can
negatively impact their function and therefore the efficiency of
conversions. Lastly, catalytic systems suffer from degradation of
the support material, partially due to its own undesirable
reactivity. For example, oxide supports, and the most common
supports comprising amorphous carbon and other forms of carbon, may
have more defects (such as unfavorable reaction sites) than
graphite or diamond supports, but also may be relatively less
expensive.
[0014] In light of the foregoing, most state of the art
electrocatalysts are neither efficient nor specific enough to be
used on a large scale economically.
[0015] A need exists in the art for a catalyst system and a method
for catalysis that is applicable to many reactions, including water
electrolysis. The system and method should either completely
eliminate, or minimize the use of precious metals. The system and
method should also be self-healing so as to extend the longevity of
the system, and thereby enhance the efficiency and economics of
catalysis processes. And the system and method should incorporate
catalytic materials having reduced over potentials and increased
turnover rates.
SUMMARY OF INVENTION
[0016] An object of the invention is to provide a water splitting
system that overcomes many of the drawbacks of the prior art.
[0017] Another object of the present invention is to provide
electrocatalysts for water decomposition. A feature of the
invention is the use of clusters of catalysts, wherein each of the
clusters contain no more than about 50 atoms of catalytic moiety,
preferably between 3 and 50 atoms and more preferably between 4 and
30 atoms. An advantage of the invention is achieving the highest
utilization of catalyst, thereby rendering the electrocatalysts
very economical to use.
[0018] Still another object of the present invention is providing a
method for electro-oxidation or electro-reduction reactions. A
feature of the method is subjecting a fluid containing the moiety
to be converted water with catalyst comprising non-precious and
precious metal clusters supported on an electrically conductive
support wherein the clusters cover between 1 and 25 percent of the
surface of the support and each of the clusters comprise a
predetermined number of atoms. (An embodiment of the invention
comprises clusters covering between 1 and 5 percent of the surface
of one side of the support.) An advantage of the method is that it
provides a very efficient catalytic method wherein target moiety
conversions per catalyst cluster surface atom is between about 10
and 100 times higher than conversion rates seen in state of the art
films and catalytic nanoparticles larger than 1 nanometer. This
results in a cost that is relatively low compared to methods using
precious metals.
[0019] Yet another object of the present invention is providing a
self-healing catalyst system. A feature of the invention is the use
of some sized metal catalytic clusters (such as clusters consisting
of 4-50 metal atoms of catalytic material) to eliminate support
structure defects by covering them, (i.e. isolating them from
direct exposure to the environment) and therefore passivating them.
An advantage of the invention and the use of relatively higher atom
containing clusters on the same passivated support to enable
catalytic reactions. This enhances the activity of the catalyst
while extending the useful lifetime of the entire electrode.
[0020] Briefly, the invention provides a catalytic electrode for
converting molecules, the electrode comprising a predetermined
number of single catalytic sites supported on a substrate.
[0021] Also provided is a method for oxidizing water comprising
contacting the water with size selected catalyst clusters.
[0022] The invention also provides a method for reducing an
oxidized moiety, the method comprising contacting the moiety with
size selected catalyst clusters at a predetermined voltage
potential. In an embodiment of the invention, the voltages applied
in the method range from between approximately -2 V and +2.5 V with
respect to the standard hydrogen electrode. This voltage range
enables electrolysis related to oxygen evolution, hydrogen
evolution and CO.sub.2 reduction, and also high temperature
electrolysis protocols as also described herein.
BRIEF DESCRIPTION OF DRAWING
[0023] The invention together with the above and other objects and
advantages will be best understood from the following detailed
description of the preferred embodiment of the invention shown in
the accompanying drawings, wherein:
[0024] FIGS. 1A and 1B are graphs showing performance of electrodes
with different sized catalytic clusters, in accordance with
features of the present invention;
[0025] FIG. 2 is a table showing conversion rates, in accordance
with features of the present invention;
[0026] FIG. 3 is a schematic of CO.sub.2 reduction, in accordance
with features of the present invention;
[0027] FIG. 4 is a schematic of a cathode, in accordance with
features of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The foregoing summary, as well as the following detailed
description of certain embodiments of the present invention, will
be better understood when read in conjunction with the appended
drawings.
[0029] As used herein, an element or step recited in the singular
and preceded with the word "a" or "an" should be understood as not
excluding plural said elements or steps, unless such exclusion is
explicitly stated. Furthermore, references to "one embodiment" of
the present invention are not intended to be interpreted as
excluding the existence of additional embodiments that also
incorporate the recited features. Moreover, unless explicitly
stated to the contrary, embodiments "comprising" or "having" an
element or a plurality of elements having a particular property may
include additional such elements not having that property.
[0030] Water splitting is a key step to develop processes for
energy/fuel production that is independent of fossil fuels. In an
embodiment of the invention, the invented subnanometer catalyst
system offers energy efficient three dimensional electrocatalysts
with tailorable performance and at the highest possible level of
usage of catalytic metals (i.e., the most efficient use). Such
three dimensional systems comprise clusters containing more than
three atoms of reactive moiety. The system is operable in very
harsh environmental conditions (e.g., in alkaline conditions,
typically above 10 pH, and often higher than 13 pH). For example,
in an embodiment of the invention, electrochemical conditions of
pH13.1 at 1.8 V vs. a reference hydrogen electrode (RHE) were
accommodated.
[0031] Aside from facilitating oxidation in harsh environs, the
system also is applicable in reduction scenarios, for example in
the reduction of carbon dioxide via the schematic depicted in FIG.
3. In this scenario, water splitting is combined with CO.sub.2
conversion to produce fuels and or other valuable feedstocks (e.g.
C.sub.xH.sub.y and C.sub.xH.sub.yR.sub.z wherein R is an inorganic
or an organic (e.g., alky, allyl) moiety and x, y and z represent
molar ratios), as well as oxygenated products.
[0032] The invented system is applicable at a myriad of
temperatures ranging from 5 to 900.degree. C. For example, when
liquid water is oxidized, any temperature below 100.degree. C. is
suitable. However, if waste water, contaminated water, pressurized
water, or pressurized steam is being treated, (such as what may
occur in turbine applications), conversions occurring at above
100.degree. C., and as high as 900.degree. C. are envisioned.
However, typical "high" temperatures are between about
100-350.degree. C.
[0033] A pH range of from about 2 to 13, and pressures up to 15
atmospheres are accommodated by the invention. Preferred pressures
are up to about seven atmospheres.
[0034] The invention provides subnanometer (e.g., between
approximately 0.2 nm and 0.9 nm) electrocatalyst materials for
oxidation/reduction of fluids such as water, carbon dioxide,
organics (including contaminants in water) and combinations of
fluids. All of the catalytic clusters on a support can consist of a
single metal, or alternatively, several different metals. Suitable
metals include, but are not limited to Co, Ni, Pd, Pt, Ir, Ag, Al,
Ru, Fe, their alloys and combinations of these metals and alloys.
In an embodiment of the invention, some of the catalytic sites
comprise a first moiety, and some of the catalytic sites comprise a
second moiety, whereby the moieties are metals and or metal
oxides.
[0035] Cluster interaction with the support can be covalent,
noncovalent, ionic or combinations thereof. Detail of the
interaction and attachment of Co clusters to substrate are
disclosed by the inventors in Journal Physical. Chemistry, 2012,
116, pp 24027-24034, the entirety of which is incorporated herein
by reference.
[0036] The clusters are supported on electrically conducting
substrate, including but not limited to, various (i) carbon based
materials, such as (ultra) nanocrystalline diamond, graphite,
graphene, carbon nanotubes and other; (ii) oxides such as alumina,
iron oxide, copper oxide, titania, magnesium oxide, zinc oxide,
zirconium oxide, hafnium oxide, tungsten oxide (iii) metals such as
copper, silver, aluminum, tantalum, tungsten and combinations
thereof.
[0037] UNCD is particularly suitable for use in harsh experimental
conditions and demonstrates a very wide potential electrochemical
working window of about 3.5 V while displaying only minor
reactivity. The activity of the support is approximately one to two
orders of magnitude lower than that of the clusters (depending on
the support composition and cluster size/composition).
[0038] The instant system offers splitting systems that are lower
in cost and have higher efficiency than state of the art
electrolysis systems.
[0039] An embodiment of the invention provides a method for
oxidizing water in alkaline conditions using size-selected clusters
(between approximately 0.2 nm and 0.9 nm) of Co, Ni, Pd, Ir, Ag,
Al, their oxides and other metals and their oxides. An exemplary
catalyst system comprises three-dimensional clusters such as those
clusters containing more than 3 atoms. For example, 6-atom
clusters, and 17-atom clusters of palladium supported on an
electrically-conducting surface such as alumina and UNCD provide
good catalytic results, while for example smaller Pd clusters are
also active on TiO.sub.2 and/or ZnO supports.
[0040] This demonstrates that specific bonds in the
cluster/catalytic active site, such as bridging Pd--Pd sites are
active for the Oxygen Evolution Reaction (OER) depicted in Equation
1, supra, while in general, the size of the working catalyst may
vary with support and the reaction being pursued. The catalytic
activity of the clusters is based on the turnover rate per Pd
atom.
[0041] Notwithstanding the foregoing, the invented system and
method for catalysis is also applicable for use in acidic
conditions. Such conditions may be applicable in electrochemical
water splitting incorporating catalytic reduction scenarios. In
these instances, the invented cluster/support composites are
tolerant of pHs from between approximately 3 to 13.
Electrode Detail
[0042] FIG. 4 is a schematic diagram of exemplary cathodes used in
experiments by the inventors. An embodiment of the cathode
comprises a UNCD coated Si wafer with three soft-landed (4 mm
diameter) regions of clusters. A separate cathode was prepared for
each Pd cluster size (Pd4, Pd6, or Pd17).
[0043] Schematics of the cluster-covered area on the sample
immersed into the electrolyte showing three cluster spots of
.about.4 mm in diameter. The arrows below and above the central
immersion line indicate the incident X-ray beams which, in addition
to the determination of cluster area and position, were used to
characterize by GISAXS and GIXANES the potential differences in
particle size and oxidation state of the unimmersed (as deposited)
and in electrolyte immersed exposed clusters, respectively.
[0044] Cluster support can either be a homogeneous material, such
as free standing substrate, or a heterogeneous construction wherein
an electrically conductive material such as metal or UNCD overlays
a rigid foundation. The cathode structure depicted in FIG. 4
depicts a heterogeneously constructed cathode. Generally, the
support substrate is an electrically conductive material and/or a
1-100 nm thick coating overlaying the material, whereby the
material or the coating is selected from the group consisting of
various (i) carbon based materials, such as (ultra)nanocrystalline
diamond, graphite, graphene, carbon nanotubes and other; (ii)
oxides such as alumina, iron oxide, copper oxide, titania,
magnesium oxide, zinc oxide, zirconium oxide, hafnium oxide,
tungsten oxide (iii) metals such as copper, silver, aluminum,
tantalum, tungsten and combinations thereof.
[0045] In an embodiment of such a construction, silicon wafers
coated with a 0.3.mu. thick ultrananocrystalline diamond film
purchased from Advanced Diamond Technologies (UA25-100-0-p3) were
used as the support electrodes. All electrodes used in this study
were constructed from 15.times.18 mm size chips diced from the same
wafer. Dicing for example can be accomplished by scoring and
breaking the wafer or by using precision laser cutting. The latter
allows precision sample sizes.
[0046] UNCD is a preferred support material with a wide, flat
potential window showing excellent stability under the harsh
electrochemical oxidation conditions. The surface roughness of the
wafers was .about.7 nm RMS and the electrical conductivity of the
B-doped Silicon wafer/UNCD film combination of .about.1 ohm-cm was
sufficient to both allow controlled "soft-landing" of size-selected
Pd clusters and to allow electrochemical studies. The wafers were
used as purchased except for the dicing operation and for solvent
cleaning prior to size-selected cluster deposition.
Cluster Deposition
Detail
[0047] A myriad of methods for producing the clusters are utilized,
including those disclosed in the inventor's previous patents (U.S.
Pat. Nos. 8,143,189 and 8,148,293) both of which are incorporated
herein by reference in their entirety. Cluster-based catalysts were
prepared by depositing a narrow size-distribution of clusters
generated in a laser vaporization cluster source. A myriad of
metals are suitable as cluster constituents. Other cluster sources,
such as magnetron, cold discharge, electrospray, microplasma,
microwave or atomic layer deposition, chemical vapor deposition,
decomposition of organometallic and other precursors by photons,
small clusters produced in solution can be envisioned to produce
the clusters, For the purposes of illustration only, palladium
metal is discussed in this section, with the palladium clusters
produced in a laser vaporization source.
[0048] A molecular beam of palladium clusters was generated by
laser vaporization of a rotating palladium target using helium as a
buffer gas. This continuous cluster beam was guided through ion
optics and a quadrupole assembly. This assembly could be tuned to
select clusters of a single nuclearity. Then mass-selected,
positively charged clusters were directed onto the UNCD coated Si
wafer support. For the three cluster sizes chosen here Pd4, Pd6,
and Pd17,) the size distribution was 1, 2 and 3 atoms,
respectively, around the main size (i.e. 4+/-1, 6+/-2 and 17+/-3
atoms per cluster). The kinetic energy of the clusters was
controlled by electrical potentials placed on the wafer. This
"soft-landing" method allowed deposition without subsequent
agglomeration.
[0049] The amount of deposited palladium metal was determined by
real-time monitoring of the deposition flux on the support. The
total charge deposited could be determined with less than 5%
uncertainty of the targeted surface coverage. In order to avoid
possible aggregation of clusters on the surface, the surface
coverage was chosen to be around 0.1 atomic ML equivalent of
palladium metal for all samples, based on 1.53.times.10.sup.15
atoms/cm2 of a Pd (111) plane. Three spots were deposited on each
sample with size-selected clusters (see FIG. 4).
[0050] The position and diameter of the cluster-covered area was
later determined by GIXAS at 12-ID-C beamline of the Advanced
Photon Source (Argonne Laboratory, Argonne Ill.), with an estimated
uncertainty of 0.5 mm or better. The diameters of the cluster spots
(each cluster which in turn is comprised of approximately 10.sup.12
to 10.sup.14 clusters, depending cluster size, each cluster
physically isolated from each other) varied between 4 and 5 mm,
depending on cluster size and deposition conditions.
[0051] The inventors found that these clusters withstand the harsh
reaction conditions embodied in the oxidation and reduction
protocols discussed herein. These results were unexpected as the
electrocatalytic community's perception was that only larger, nm
sized (i.e. at least 1 nm), particles can survive such
conditions.
[0052] Surprisingly and unexpectedly, the inventors have discovered
that overpotential and turnover frequency can be predicted can be
determined based on the size of the cluster utilized. The number of
catalytic sites (number of deposited metal atoms as well as number
of clusters) is determined from the inventor's synthesis
method.
[0053] The per electron turn-over rate was calculated as follows:
i) converting current into charge/sec (Coulomb's law), ii) dividing
the charge/sec by the number of deposited atoms gives the number of
electrons turned over by a single catalytic moiety such as a Pd
atom. As producing an O.sub.2 molecule takes 4 electrons, dividing
the electron turn-over numbers gives the produced turn-over number
(TOR) for O.sub.2. Lastly, the per cluster turn over rates are
calculated by dividing the preceding O.sub.2 TOR with the number of
the atoms in the given sample (i.e., 4, 6 or 17).
[0054] Soft-landed Pd4, Pd6 and/or Pd17 clusters on an UNCD
Silicon-coated electrode shows stable electrochemical potentials
over several cycles. Soft landings are effected when a voltage
potential exists between free associated clusters and support of
between about 1 V and 17 V. Generally, suitable voltages are those
above which the clusters would otherwise decompose or dissolve into
solution. However, high voltages (about several tens or hundreds of
V) may result in heavy impingement of the cluster to the support
surface, leading to possible defects and embedment of the cluster
into the support system. While some support surfaces (e.g., those
comprised of graphite) are amenable to such embedment, others may
not be and defects or chinks in the surfaces often result. The
inventors have discovered that a soft landing with low atom number
clusters (e.g. 4 atoms) ameliorates any defects existing or
otherwise produced in the support surfaces. Synchrotron studies of
the electrodes show no evidence for evolution or dissolution of
either the electrode material or the clusters nor their aggregation
into larger, nanosized entities, as judged from high sensitivity
X-ray absorption and scattering experiments.
[0055] Subnanometer cobalt oxides with CoO-type oxidation states
are highly active and stable on various supports, those supports
including, but not limited to UNCD, graphite, graphene, carbon
nanotubes, iron oxide, titanium oxide, zinc oxide, zirconium oxide,
magnesium oxide, hafnium oxide and combinations thereof, including
supports made of nanosized particulates or nanoshapes made of these
materials and/or nanopatterned by physical or chemical methods.
[0056] The inventors have discovered that the overpotential and
turnover frequency are cluster-size dependant (i.e., are dependant
on the number of atoms comprising each cluster). This discovery was
made given the capability of the method disclosed herein to
formulate catalysts comprising single catalytic sites, and the
method to measure the number of such sites on the support
electrodes.
EXAMPLE 1
[0057] OER activity was measured for three Pd clusters (Pd4, Pd6
and Pd17). The activity was measured on an ultrananocrystalline
diamond film electrode at pH 13.6 (1 M NaOH). FIG. 1a shows linear
sweep voltammetry (LSV) curves of Pd4 (solid line), Pd6
(cross-hatched line) and Pd17 (dashed line) clusters after 500
second cleaning/preconditioning by chronoamperometry at about 1.7
V. A background curve (dotted line) for a UNCD coated electrode
without deposited Pd clusters is also shown. The blank UNCD used
was of the same dimensions and identical batch as the
cluster-covered samples.
[0058] The graphs in FIGS. 1A and 1B depict large increases in
anodic current between Pd6 or Pd17 clusters and the UNCD support.
In this example, about 10 percent of the total surface area of the
electrode was coated with Pd clusters, moreover within this area as
low as 0.1 atomic monolayer equivalent of Pd metal
(1.5.times.10.sup.14 atoms/cm.sup.2) was deposited. Generally,
between about 1 and 25 percent of the surface area of one side of a
support is suitable coverage, and preferably between about 5 and 20
percent coverage. Alternatively, more than one side of a support
can be covered with the same type clusters, with clusters
comprising different moieties and/or with clusters of different
sizes. This mix and matching provides higher efficacy as well as
multifunctionally to a cluster support substrates.
[0059] This example shows that activity of clusters increases with
the number of atoms comprising each of the clusters.
[0060] FIG. 1B shows anodic current between the clusters and the
support with background corrected (i.e., after subtraction of the
current of the blank UNCD support). This figure shows a drop of
current between Pd.sub.4 and the support, which indicates that a
greater fraction of UNCD surface defect (and undesirably active)
sites are blocked by the Pd.sub.4 clusters. This indicates that the
lower atom number clusters are efficient at healing support surface
defect, while larger atom clusters also possess added and desired
catalytic activity. This means that larger (greater than 4 atoms)
possess dual functionality of both healing support surface defects
and conferring high catalytic activity.
[0061] A feature of the system is using deposition current to
calculate the number of deposited clusters on the support
electrode. Atom number per cluster represents an upper bound on the
number of catalytic reaction sites. The background corrected
current, depicted in FIG. 1B, and the known concentration of
deposited clusters is used to calculate OER turnover rates. As
water oxidation is a four electron transfer process, the per
electron rate has been divided by a factor of four. The turnover
rates (depicted in FIG. 2) are compared to published studies of Pd
metal and Co oxide deposited on Au.
[0062] As shown in FIGS. 1 and 2, Pd.sub.4 clusters supported on
ultrananocrystalline diamond film electrodes exhibit a small
decrease in reactivity compared to UNCD electrodes by themselves.
The drop in activity seen with Pd4 deposition, compared to an
increase in activity seen with clusters having more than 4 atoms
indicates that activity depends on cluster size and numbers, with a
preferred number of atoms per cluster ranging from between 3 and 50
for clusters comprising only one catalytic moiety type and from
between 4 and 50 for clusters comprising more than one catalytic
moiety. The 3-50 atom cluster blocks/units described supra also
serve as assembly modules for constructs measured at the nanometer
(above 1 nm) or mesoscale level having specific, improved catalytic
propensities not seen with the instant (homogenous structure) large
particles described herein.
[0063] FIG. 2 further shows that individual clusters having more
than 4 atoms are more active than Pd metal film or webbing (column
3 of FIG. 2). All of the clusters showed stable performance during
repeated voltametric cycles, with currents within an estimated 5
percent uncertainty.
[0064] It is to be understood that the above description is
intended to be illustrative, and not restrictive. For example, the
above-described embodiments (and/or aspects thereof) may be used in
combination with each other. In addition, many modifications may be
made to adapt a particular situation or material to the teachings
of the invention without departing from its scope. While the
dimensions and types of materials described herein are intended to
define the parameters of the invention, they are by no means
limiting, but are instead exemplary embodiments. Many other
embodiments will be apparent to those of skill in the art upon
reviewing the above description. The scope of the invention should,
therefore, be determined with reference to the appended claims,
along with the full scope of equivalents to which such claims are
entitled. In the appended claims, the terms "including" and "in
which" are used as the plain-English equivalents of the terms
"comprising" and "wherein." Moreover, in the following claims, the
terms "first," "second," and "third," are used merely as labels,
and are not intended to impose numerical requirements on their
objects. Further, the limitations of the following claims are not
written in means-plus-function format and are not intended to be
interpreted based on 35 U.S.C. .sctn.112, sixth paragraph, unless
and until such claim limitations expressly use the phrase "means
for" followed by a statement of function void of further
structure.
[0065] As will be understood by one skilled in the art, for any and
all purposes, particularly in terms of providing a written
description, all ranges disclosed herein also encompass any and all
possible subranges and combinations of subranges thereof. Any
listed range can be easily recognized as sufficiently describing
and enabling the same range being broken down into at least equal
halves, thirds, quarters, fifths, tenths, etc. As a non-limiting
example, each range discussed herein can be readily broken down
into a lower third, middle third and upper third, etc. As will also
be understood by one skilled in the art all language such as "up
to," "at least," "greater than," "less than," "more than" and the
like include the number recited and refer to ranges which can be
subsequently broken down into subranges as discussed above. In the
same manner, all ratios disclosed herein also include all subratios
falling within the broader ratio.
[0066] One skilled in the art will also readily recognize that
where members are grouped together in a common manner, such as in a
Markush group, the present invention encompasses not only the
entire group listed as a whole, but each member of the group
individually and all possible subgroups of the main group.
Accordingly, for all purposes, the present invention encompasses
not only the main group, but also the main group absent one or more
of the group members. The present invention also envisages the
explicit exclusion of one or more of any of the group members in
the claimed invention.
* * * * *