U.S. patent application number 15/516495 was filed with the patent office on 2018-03-01 for materials and methods for the electrochemical reduction of carbon dioxide.
The applicant listed for this patent is OHIO STATE INNOVATION FOUNDATION. Invention is credited to Joshua BILLY, Anne CO, Eric COLEMAN, Kendahl WALZ.
Application Number | 20180057950 15/516495 |
Document ID | / |
Family ID | 55631534 |
Filed Date | 2018-03-01 |
United States Patent
Application |
20180057950 |
Kind Code |
A1 |
CO; Anne ; et al. |
March 1, 2018 |
MATERIALS AND METHODS FOR THE ELECTROCHEMICAL REDUCTION OF CARBON
DIOXIDE
Abstract
Disclosed are methods for electrochemically reducing carbon
dioxide to provide a product. The methods can comprise contacting
the carbon dioxide with an electroreduction catalyst in an
electrochemical cell, and applying a potential to the
electrochemical ceil to form the product. The electroreduction
catalyst can comprise a nanoporous Cu catalyst, a nanoporous Cu-M
catalyst, or a combination thereof, where M is a metal chosen from
Pt, Ir, Pd, Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd, Tl, and
Ti. The product can comprise a C.sub.2-C.sub.3 alkane, a
C.sub.2-C.sub.3 alkene, a C.sub.2-C.sub.3 alcohol, a
C.sub.2-C.sub.3 carboxylic acid, a C.sub.2-C.sub.3 aldehyde, or a
combination thereof.
Inventors: |
CO; Anne; (Columbus, OH)
; BILLY; Joshua; (Newark, DE) ; COLEMAN; Eric;
(Hillsboro, OR) ; WALZ; Kendahl; (Columbus,
OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
OHIO STATE INNOVATION FOUNDATION |
Columbus |
OH |
US |
|
|
Family ID: |
55631534 |
Appl. No.: |
15/516495 |
Filed: |
October 1, 2015 |
PCT Filed: |
October 1, 2015 |
PCT NO: |
PCT/US15/53532 |
371 Date: |
April 3, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62058121 |
Oct 1, 2014 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06Q 10/10 20130101;
Y02A 90/10 20180101; G16H 40/20 20180101; B01D 2257/504 20130101;
C25B 11/04 20130101; C25B 11/0473 20130101; G16H 10/60 20180101;
G16H 50/30 20180101; C25B 3/04 20130101; G06F 19/328 20130101 |
International
Class: |
C25B 3/04 20060101
C25B003/04; C25B 11/04 20060101 C25B011/04 |
Claims
1. A method for electrochemically reducing carbon dioxide to
provide a product, the method comprising contacting the carbon
dioxide with an electroreduction catalyst in an electrochemical
cell, and applying a potential to the electrochemical cell to form
the product, wherein the electroreduction catalyst comprises a
nanoporous Cu catalyst, a nanoporous Cu-M catalyst, or a
combination thereof, where M is a metal chosen from Pt, Ir, Pd, Ag,
Au, Rh, Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd, Tl, and Ti; and wherein
the product comprises a C.sub.2-C.sub.3 alkane, a C.sub.2-C.sub.3
alkene, a C.sub.2-C.sub.3 alcohol, a C.sub.2-C.sub.3 carboxylic
acid, a C.sub.2-C.sub.3 aldehyde, or a combination thereof.
2. The method of claim 1, wherein the electroreduction catalyst
comprises nanoparticles having an average particle size of from 10
nm to 500 nm, as determined by scanning electron microscopy
(SEM).
3. The method of claim 2, wherein the nanoparticles have a BET
surface area of from 10 m.sup.2/g to 40 m.sup.2/g.
4. The method of claim 1, wherein the electroreduction catalyst
comprises a nanoporous Cu-M catalyst where M is a metal chosen from
Pt, Ir, Pd, Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd, Tl, and
Ti.
5. The method of claim 4, wherein the electroreduction catalyst
comprises a nanoporous Cu--Ru catalyst.
6. The method of claim 1, wherein the product comprises ethanol,
propanol, or a combination thereof.
7. The method of claim 6, wherein the wherein the product comprises
propanol.
8. The method of claim 7, wherein the propanol is formed at a
Faradaic efficiency of from 0.5% to 15%.
9. The method of claim 1, wherein the method is selective for the
formation of C.sub.2-C.sub.3 alcohols over methanol, such that the
C.sub.2-C.sub.3 alcohols are formed with at least 10 times greater
Faradaic efficiency than methanol.
10. The method of claim 1, wherein the product comprises ethane,
ethylene, or a combination thereof.
11. The method of claim 1, wherein the method is selective for the
formation of C.sub.2-C.sub.3 alkanes over methane, such that the
C.sub.2-C.sub.3 alkanes are formed with at least 10 times greater
Faradaic efficiency than methane.
12. The method of claim 1, wherein the electrochemical cell is a
divided electrochemical cell comprising a working electrode
comprising the electroreduction catalyst in a first cell
compartment, a counter electrode in a second cell compartment, and
a solid electrolyte membrane interposed between the working
electrode and the counter electrode, both the first cell
compartment and the second cell compartment further comprising an
aqueous solution of an electrolyte; wherein contacting the carbon
dioxide with the electroreduction catalyst comprises introducing
the carbon dioxide into the first cell compartment of the divided
electrochemical cell; and wherein applying a potential to the
electrochemical cell comprises applying a negative voltage and a
positive voltage to the working electrode and the counter
electrode, respectively, to reduce the carbon dioxide to form the
product.
13. The method of claim 12, wherein the electrolyte comprises an
alkali metal bicarbonate.
14. The method of claim 13, wherein the alkali metal bicarbonate is
potassium bicarbonate.
15. The method of claim 1, wherein the applied potential is from
-0.15 V to -1.8 V vs. a reversible hydrogen electrode.
16. An electrochemical cell for electrochemically reducing carbon
dioxide to provide a product, the electrochemical cell comprising a
working electrode comprising an electroreduction catalyst in a
first cell compartment, wherein the electroreduction catalyst
comprises a nanoporous Cu catalyst, a nanoporous Cu-M catalyst, or
a combination thereof, where M is a metal chosen from Pt, Ir, Pd,
Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd, Tl, and Ti; a
counter electrode in a second cell compartment; and a solid
electrolyte membrane interposed between the working electrode and
the counter electrode; both the first cell compartment and the
second cell compartment further comprising an aqueous solution of
an electrolyte.
17. The cell of claim 16, wherein the electroreduction catalyst
comprises a nanoporous Cu-M catalyst where M is a metal chosen from
Pt, Ir, Pd, Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd, Tl, and
Ti.
18. The cell of claim 17, wherein the electroreduction catalyst
comprises a nanoporous Cu--Ru catalyst
19. A method for preparing an organic compound defined by Formula
II from a carboxylic acid defined by Formula I and a carboxylic
acid defined by Formula I' according to the equation below
##STR00002## where R and R' interdependently represent hydrogen, a
substituted or unsubstituted alkyl group, a substituted or
unsubstituted alkenyl group, a substituted or unsubstituted alkynyl
group, a substituted or unsubstituted aryl group, a substituted or
unsubstituted heteroaryl group, a substituted or unsubstituted
alkylaryl group, or a substituted or unsubstituted alkylheteroaryl
group, the method comprising contacting the carboxylic acid defined
by Formula I and the carboxylic acid defined by Formula I' with an
electroreduction catalyst in an electrochemical cell, and applying
a potential to the electrochemical cell to form the organic
compound defined by Formula II, wherein the electroreduction
catalyst comprises a nanoporous Cu catalyst, a nanoporous Cu-M
catalyst, or a combination thereof, where M is a metal chosen from
Pt, Ir, Pd, Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd, Tl, and
Ti.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of U.S. Provisional
Application No. 62/058,121, filed. Oct. 1, 2014, which is hereby
incorporated herein by reference in its entirety.
BACKGROUND
[0002] The combustion of fossil fuels in activities such as the
electricity generation, transportation, and manufacturing produces
billions of tons of carbon dioxide annually. Research since the
1970s indicates increasing concentrations of carbon dioxide in the
atmosphere may be responsible for altering the Earth's climate,
changing the pH of the ocean and other potentially damaging
effects. Countries around the world, including the United States,
are seeking ways to mitigate emissions of carbon dioxide.
[0003] Converting carbon dioxide into economically valuable
materials (e.g., fuels and/or industrial chemicals) offers an
attractive strategy for mitigating carbon dioxide emissions.
Laboratories around the world have attempted for many years to use
electrochemistry and/or photochemistry to convert carbon dioxide to
economically valuable products. However, existing methods for the
conversion of carbon dioxide suffer from many limitations,
including the stability of systems used in the process, the
efficiency of systems, the selectivity of the systems or processes
for a desired chemical, the cost of materials used in
systems/processes, the ability to control the processes
effectively, and the rate at which carbon dioxide is converted. No
commercially available solutions tier converting carbon dioxide to
economically valuable fuels or industrial chemicals currently
exist.
SUMMARY
[0004] Disclosed are methods for electrochemically reducing carbon
dioxide to provide one or more products (e.g., fuels and/or
industrial chemicals). Methods for electrochemically reducing
carbon dioxide to provide a product can comprise contacting the
carbon dioxide with an electroreduction catalyst in an
electrochemical cell, and applying a potential to the
electrochemical cell to form the product. The applied potential can
be from -0.10 V to -1.8 V (e.g., from -0.15 V to -1.8 V, or from
-0.25 V to -1.6 V) vs. a reversible hydrogen electrode.
[0005] The electroreduction catalyst can comprise a nanoporous Cu
catalyst; a nanoporous Cu-M catalyst, where M is a metal chosen
from Pt, Ir, Pd, Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd,
Ti, and Ti; or a combination thereof in some embodiments, the
electroreduction catalyst be a nanoporous Cu-M catalyst, where M is
a metal chosen from Pt, Ir, Pd, Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe, Re,
Ga, In, Cd, Tl, and Ti. In some embodiments, the electroreduction
catalyst be a nanoporous Cu--Ru catalyst.
[0006] In some embodiments, the electroreduction catalyst can
comprise nanoparticles having an average particle size of from 10
nm to 500 nm e.g., from 10 nm to 250 nm, from 10 nm to 150 nm, from
20 nm to 100 nm, from 20 nm to 80 nm, or from 80 nm to 100 nm), as
determined by scanning electron microscopy (SEM). The nanoparticles
can have a BET surface area of from 5 m.sup.2/g to 40 m.sup.2/g
(e.g., from 10 m.sup.2/g to 40 m.sup.2/g, from 10 m.sup.2/g to 20
m.sup.2/g, or from 20 m.sup.2/g to 40 m.sup.2/g).
[0007] The electrochemical reduction of carbon dioxide can produce
one or more products. The electrochemical reduction of carbon
dioxide can be selective towards the formation of C.sub.2 and/or
C.sub.3 species. For example, the one or more products can comprise
a C.sub.2-C.sub.3 alkane, a C.sub.2-C.sub.3 alkene, a.
C.sub.2-C.sub.3 alcohol, a C.sub.2-C.sub.3 carboxylic acid, a
C.sub.2-C.sub.3 aldehyde, or a combination thereof. In some
embodiments, the one or more products can comprise a
C.sub.2-C.sub.3 alkane aid/or a C.sub.2-C.sub.3 alkene (e.g.,
ethane, ethylene, or a combination thereof). In certain embodiment,
the method is selective for the formation of C.sub.2-C.sub.3
alkanes over methane, such that the C.sub.2-C.sub.3 alkanes are
formed with at least 10 times greater Faradaic efficiency than
methane. In some embodiments, the one or more products can comprise
a C.sub.2-C.sub.3 alcohol (e.g., ethanol, propanol, or a
combination thereof). In certain embodiments, the one or more
products can comprise propanol. In some cases, the propanol can be
formed at a Faradaic efficiency of from 0.5% to 15%. In certain
embodiment, the method is selective for the formation of
C.sub.2-C.sub.3 alcohols over methanol, such that the
C.sub.2-C.sub.3 alcohols are formed with at least 10 times greater
Faradaic efficiency than methanol.
[0008] The electrochemical cell can be a divided electrochemical
cell that comprises a working electrode comprising the
electroreduction catalyst in a first cell compartment, a counter
electrode in a second cell compartment, and a solid electrolyte
membrane interposed between the working electrode and the counter
electrode. Both the first cell compartment and the second cell
compartment can further comprise an aqueous solution of an
electrolyte. For example, the first cell compartment can further
comprise an aqueous solution of an electrolyte in electrochemical
contact with the working electrode disposed in the first cell
compartment, and the second cell compartment can further comprise
an aqueous solution of an electrolyte in electrochemical contact
with the counter electrode disposed in the second cell compartment.
In these embodiments, contacting the carbon dioxide with the
electroreduction catalyst can comprise introducing the carbon
dioxide into the first cell compartment of the divided
electrochemical cell (e.g., bubbling the carbon dioxide
into/through the aqueous solution of the electrolyte). Applying a
potential to the electrochemical cell can comprise applying a
negative voltage and a positive voltage to the working electrode
and the counter electrode, respectively, to reduce the carbon
dioxide to form the product. The electrolyte can comprise an alkali
metal bicarbonate (e.g., potassium bicarbonate or sodium
bicarbonate).
[0009] Also provided are systems (e.g., electrochemical cells) and
electroreduction catalysts that can be used in conjunction with the
methods described herein. For example, provided herein are
electrochemical cells that comprise a working electrode comprising
an electroreduction catalyst described herein in a first cell
compartment; a counter electrode in a second cell compartment; and
a solid electrolyte membrane interposed between the working
electrode and the counter electrode. Both the first cell
compartment and the second cell compartment can further comprise an
aqueous solution of an electrolyte. For example, the first cell
compartment can further comprise an aqueous solution of an
electrolyte in electrochemical contact with the working electrode
disposed in the first cell compartment, and the second cell
compartment can further comprise an aqueous solution of an
electrolyte in electrochemical contact with the counter electrode
disposed in the second cell compartment.
DESCRIPTION OF DRAWINGS
[0010] FIG. 1 is an exploded view of an electrochemical cell that
can be used in conjunction with the electrochemical reduction of
carbon dioxide described herein.
[0011] FIG. 2 is a process flow diagram schematically illustrating
the methods fir the electrochemical reduction of carbon dioxide
described herein.
[0012] FIG. 3 shows scanning electron microscopy (SEM) images of
nanoporous, open cell copper foams prepared using four different
etching regimes: panel A (top left) 30 at % CuAl alloy etched in 6
M NaOH, at 80.degree. C. and 10 mA/g; panel B (top right) 30 at %
CuAl alloy etched in 6 M NaOH, at 80.degree. C. and 10 mA/g; panel
C (bottom right) 30 at % CuAl alloy etched in 6 M NaOH, at
80.degree. C. and 100 mA/g; and panel D (bottom left) 17 at % CuAl
alloy etched in 6 M NaOH, at 80.degree. C. and 0 mA/g.
[0013] FIG. 4 is a plot illustrating the effect of the etching
temperature (.degree. C.) and etching current (mA/g) on the pore
size (nm) and ligament size (nm) of nanoporous, open cell copper
foams.
[0014] FIG. 5 shows an SEM image of a nanoporous CuRu catalyst.
[0015] FIG. 6 shows an overlay plot of x-ray photoelectron
spectroscopy (XPS) spectra of a post-electrolysis CuRu electrode at
varying depths. XPS spectra of the surface indicate a lack of
ruthenium present on the surface. Repeated Ar.sup.+ etching reveals
the presence of ruthenium metal within a few atomic layers below
the surface.
[0016] FIG. 7 is a plot of the Faradaic efficiencies for gaseous
CO.sub.2 reduction products (top) and liquid CO.sub.2 reduction
products (bottom) as a function of applied potential (versus the
reversible hydrogen electrode) obtained using a nanoporous Cu
catalyst.
[0017] FIG. 8 is a plot of the Faradaic efficiencies for gaseous
CO.sub.2 reduction products (top) and liquid CO.sub.2 reduction
products (bottom) as a function of applied potential versus the
reversible hydrogen electrode) obtained using a nanoporous Cu--Ru
catalyst.
DETAILED DESCRIPTION
[0018] Provided herein are methods for electrochemically reducing
carbon dioxide to provide one or more products (e.g., fuels and/or
industrial chemicals). Methods for electrochemically reducing
carbon dioxide to provide a product can comprise contacting the
carbon dioxide with an electroreduction catalyst in an
electrochemical cell, and applying a potential to the
electrochemical cell to form the product.
[0019] The applied potential can be -0.10 V or less (e.g., -0.15 V
or less, -0.20 V or less, -0.25 V or less, -0.30 V or less, -0.35 V
or less, -0.40 V or less, -0.45 V or less, -0.50 V or less, -0.55 V
or less, -0.60 V or less, -0.65 V or less, -0.70 V or less, -0.75 V
or less, -0.80 V or less, -0.85 V or less, -0.90 V or less, -0.95 V
or less, -1.0 V or less, -1.05 V or less, -1.10 V or less, -1.15 V
or less, -1.20 V or less, -1.25 V or less, -1.30 V or less, -1.35 V
or less, -1.40 V or less, -1.45 V or less, -1.50 V or less, -1.55 V
or less, -1.60 V or less, -1.65 V or less, -1.70 V or less, or
-1.75 V or less) vs. a reversible hydrogen electrode. The applied
potential can be at least -1.8 V (e.g., at least -1.75 V, at least
-1.70 V, at least -1.65 V, at least -1.60 V, at least -1.55 V, at
least -1.50 V, at least -1.45 V, at least -1.40 V, at least -1.35
V, at least -1.30 V, at least -1.25 V, at least -1.20 V, at least
-1.15 V, at least -1.10 V, at least -1.05 V, at least -1.0 V, at
least -0.95 V, at least -0.90 V, at least -0.85 V, at least -0.80
V, at least -0.75 V, at least -0.70 V, at least -0.65 V, at least
-0.60 V, at least -0.55 V, at least -0.50 V, at least -0.45 V, at
least -0.40 V, at least -0.35 V, at least -0.30 V, at least -0.25
V, at least -0.20 V, or at least -0.15 V) vs. a reversible hydrogen
electrode.
[0020] The applied potential can range from any of the minimum
values described above to any of the maximum values described
above. For example, the applied potential can be from -0.10 V to
-1.8 V (e.g., from -0.15 V to -1.8 V, from -0.25 V to -1.6 V, from
-0.35 V to -1.0-V, or from -1.0 V to -1.6 V) vs. a reversible
hydrogen electrode.
[0021] The electrochemical reduction of carbon dioxide can produce
one or more products. The electrochemical reduction of carbon
dioxide can be selective towards the formation of C.sub.2 and/or
C.sub.3 species. Without wishing to be bound by theory, the
electroreduction catalysts described herein possess disordered
surfaces that include, for example, a large number of grain
boundaries that are believed to facilitate carbon-carbon bond
formation during the electrochemical reduction of CO.sub.2. As a
consequence, the catalysts and methods described herein can favor
the formation of C.sub.2 and/or C.sub.3 species.
[0022] For example, in some cases, the one or more products can
comprise a C.sub.2-C.sub.3 alkane (e.g., ethane, propane, or a
combination thereof), a C.sub.2-C.sub.3 alkene (e.g., ethylene,
propylene, or a combination thereof), a C.sub.2-C.sub.3 alcohol
(e.g., ethanol, propanol, or a combination thereof), a
C.sub.2-C.sub.3 carboxylic acid (e.g., acetic acid, propionic acid,
or a combination thereof), a C.sub.2-C.sub.3 aldehyde (e.g.,
acetaldehyde, propanal, or a combination thereof), or a combination
thereof. The electrochemical reduction of carbon dioxide can also
produce other species, such as carbon monoxide, formic acid, or a
combination thereof.
[0023] In some embodiments, the one or more products can comprise a
C.sub.2-C.sub.3 alkane and/or a. C.sub.2-C.sub.3 alkene (e.g.,
ethane, ethylene, or a combination thereof). In some examples, the
one or more products can comprise ethane, and the ethane can be
formed at a Faradaic efficiency of from 0.5% to 15%. In some
examples, the one or more products can comprise ethylene, and the
ethylene can be formed at a Faradaic efficiency of from 0.5% to
15%. In certain embodiments, the method can be selective for the
formation of C.sub.2-C.sub.3 alkanes over methane, such that the
C.sub.2-C.sub.3 alkanes are formed with at least 10 times greater
(e.g., at least 15 times greater, at least 20 times greater, at
least 25 times greater, at least 50 tittles greater, or at least
100 times greater) Faradaic efficiency than methane. In certain
examples, methane can be formed at a Faradaic efficiency of less
than 0.5% (e.g., less than 0.1%, or less than 0.05%).
[0024] In some embodiments, the one or more products can comprise a
C.sub.2-C.sub.3 alcohol (e.g., ethanol, propanol, or a combination
thereof). In certain embodiments, the one or more products can
comprise propanol. In some cases, the propanol can be formed at a
Faradaic efficiency of from 0.5% to 15%. In certain embodiment, the
method is selective for the formation of C.sub.2-C.sub.3 alcohols
over methanol, such that the C.sub.2-C.sub.3 alcohols are formed
with at least 10 times greater Faradaic efficiency than methanol.
In certain examples, methanol can be formed at a Faradaic
efficiency of less than 0.5% (e.g., less than 0.1%, or less than
0.05%).
[0025] In some embodiments, the methods for electrochemically
reducing carbon dioxide can produce formic acid at relatively high
Faradaic efficiencies and low overpotentials. For example, in some
examples, formic acid can be formed at a Faradaic efficiency of at
least at least 10% (e.g., at least 15%, at least 20%, or at least
25%) at an overpotential of from -0.65 V to -0.95 V (e.g., at an
overpotential of -0.65 V, -0.70 V, -0.75 V, -0.80 V, -0.85 V, -0.90
V, or -0.95 V) vs. a reversible hydrogen electrode. In some
examples, formic acid can be formed at a Faradaic efficiency of at
least at least 5% (e.g. at least 10%, at least 15% or at least 20%)
at an overpotential of from -0.25 V to -0.55 V (e.g., at an
overpotential of -0.25 V, -0.30 V, -0.35 V, -0.40 V, -0.45 V, -0.50
V, or -0.55 V) vs. a reversible hydrogen electrode.
[0026] As discussed above, methods for electrochemically reducing
carbon dioxide to provide a product can comprise contacting the
carbon dioxide with an electroreduction catalyst. The
electroreduction catalyst can comprise a nanoporous Cu catalyst; a
nanoporous Cu-M catalyst, where M is a metal chosen from Pt, Ir,
Pd, Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd, Tl, and Ti; or
a combination thereof.
[0027] The nanoporous Cu catalyst can be a nanoporous, open-cell
copper foam. Nanoporous, open-cell copper foams are known in the
art, and can be prepared from alloys of copper and a second, less
noble metal (e.g., aluminum, zinc, magnesium, tin, etc.). The
second, less noble metal can be selectively removed, for example by
etching the alloy (a process also referred to as selective leaching
or dealloying), to provide a porous copper material. This process
can involve contacting an alloy of copper and a second, less noble
metal with an etchant for a period of time effective to selectively
leach the second, less noble metal from the copper and form a
porous copper support. An appropriate etchant can be selected in
view of the identity of the second, less noble metal. For example,
in some embodiments, the nanoporous, open-cell copper foam can be
prepared by etching CuAl alloy (e.g., by contacting the CuAl alloy
with a suitable etchant, for example a base such as aqueous sodium
hydroxide, for a period of time effective to selectively leach the
aluminum from the copper) to form a nanoporous, open-cell copper
foam.
[0028] The relative amounts of copper and the second, less noble
metal in the alloy used to form the nanoporous, open-cell copper
foam can be varied in order to influence the properties of the
resulting nanoporous, open-cell copper foam (and thus the resulting
catalytic properties of the material). In some embodiments, the
alloy of copper and a second, less noble metal (e.g., aluminum) can
comprise at least 10 atomic percent (at %) copper (e.g., at least
15 at % copper, at least 20 at % copper, at least 25 at % copper,
at least 30 at % copper, at least 35 at % copper, at least 40 at %
copper, or at least 45 at % copper). In some embodiments, the alloy
of copper and a second, less noble metal (e.g., aluminum) can
comprise 50 at % or less copper (e.g., 45 at % or less copper, 40
at % or less copper, 35 at % or less copper, 30 at % or less
copper, 25 at % or less copper, 20 at % or less copper, or 15 at %
or less copper). In some embodiments, the alloy of copper and a
second, less noble metal (e.g., aluminum) can comprise at least 50
at % of the second, less noble metal (e.g., at least 55 at % of the
second, less noble metal, at least 60 at % of the second, less
noble metal, at least 65 at % of the second, less noble metal, at
least 70 at % of the second, less noble metal, at least 75 at % of
the second, less noble metal, at least 80 at % of the second, less
noble metal, or at least 85 at % of the second, less noble metal).
In some embodiments, the alloy of copper and a second, less noble
metal (e.g., aluminum) can comprise 90 at % or less of the second,
less noble metal (e.g., 85 at % or less of the second, less noble
metal, 80 at % or less of the second, less noble metal, 75 at % or
less of the second, less noble metal, 70 at % or less of the
second, less noble metal, 65 at % or less of the second, less noble
metal, 60 at % or less of the second, less noble metal, or 55 at %
or less of the second, less noble metal).
[0029] The relative amounts of copper and the second, less noble
metal (e.g., aluminum) in the alloy (e.g., CuAl) used to form the
nanoporous, open-cell copper foam can range from any of the minimum
values described above to any of the maximum values described
above. For example, the alloy of copper and a second, less noble
metal (e.g., aluminum) can comprise from 10 to 50 at % copper and
from 50 to 90 at % of the second, less noble metal (e.g., Al). In
certain embodiments, the alloy used to form the nanoporous,
open-cell copper foam can be a CuAl alloy that comprises from 10 to
50 at % copper and from 50 to 90 at % aluminum (e.g., from 10 to 30
at % copper and from 70 to 90 at % aluminum).
[0030] The nanoporous Cu-M catalyst can be a nanoporous, open-cell
Cu-M alloy foam. Nanoporous, open-cell Cu-M alloy foams can be
prepared by galvanically depositing a metal M (e.g., Pt, Ir, Pd,
Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd, Tl, and Ti) on a
nanoporous, open-cell copper foam to form the nanoporous, open-cell
Cu-M alloy foam. Methods for producing the nanoporous Cu-M catalyst
can comprise galvanically depositing a catalytically effective
amount of a desired metal (M) on a nanoporous, open-cell copper
foam (e.g., at a temperature greater than 5.degree. C.) to form a
Cu-Nil precursor catalyst; and conditioning the Cu-M precursor
catalyst to form the nanoporous Cu-M catalyst.
[0031] Galvanic deposition can involve contacting the nanoporous,
open-cell copper foam with a solution (e.g., an aqueous solution)
comprising an M-containing species (e.g., a Pt, Ir, Pd, Ag, Au, Rh,
Ru, Zn, Sn, Ni, Fe, Re, Ga, In, Cd, Ti, or Ti-containing species).
The M-containing species can comprise a suitable metal complex that
can participate in a spontaneous galvanic-reaction with the copper
in the nanoporous, open-cell copper foam. By way of example, in the
case of the galvanic deposition of Pt, the Pt-containing species
can comprise a platinum metal complex that can participate in a
spontaneous galvanic-reaction with the copper in the porous copper
support, such as PtCl.sub.4.sup.2-, PtCl.sub.6.sup.2-, or
combinations thereof.
[0032] In some embodiments, the nanoporous, open-cell copper foam
can be disposed on a surface (e.g., the surface of an electrode) in
contact with the solution comprising the M-containing species
during galvanic deposition. In certain embodiments, the surface
comprising the nanoporous, open-cell copper foam can be rotated
during galvanic deposition. The surface can be rotated at a rate
effective to induce a laminar flow of the solution comprising the
M-containing species towards and across the surface on which the
nanoporous, open-cell copper foam is disposed during galvanic
deposition. This can drive uniform deposition of the metal on the
nanoporous, open-cell copper foam. In certain embodiments, the
surface can be rotated at a rate of from 250 rpm to 2000 rpm (e.g.,
from 250 rpm to 1500 rpm, or from 250 rpm to 750 rpm).
[0033] The galvanic deposition can be performed at varying
temperatures to provide nanoporous Cu-M catalysts having the
desired properties for a particular catalytic application. In some
embodiments, the galvanic deposition can be performed at a
temperature greater than 5.degree. C. (e.g., at least 10.degree.
C., at least 15.degree. C., at least 20.degree. C., at least
25.degree. C., at least 30.degree. C., at least 35.degree. C., at
least 40.degree. C., at least 0.45.degree. C., at least 50.degree.
C., at least 55.degree. C. at least 60.degree. C., at least
65.degree. C., at least 70.degree. C., at least 75.degree. C., at
least 80.degree. C., at least 85.degree. C., at least 90.degree.
C., at least 95.degree. C., at least 100.degree. C., at least
110.degree. C. at least 120.degree. C., at least 130.degree. C., at
least 140.degree. C., at least 150.degree. C., at least 160.degree.
C., at least 170.degree. C., at least 180.degree. C., or at least
190.degree. C.). In some embodiments, the galvanic deposition can
be performed at a temperature of 200.degree. C. or less (e.g.,
190.degree. C. or less, 1.80.degree. C. or less, 170.degree. C. or
less, 160.degree. C. or less, 150.degree. C. or less, 140.degree.
C. or less, 130.degree. C. or less, 120.degree. C. or less,
110.degree. C. or less, 100.degree. C. or less, 95.degree. C. or
less, 90.degree. C. or less, 85.degree. C. or less, 80.degree. C.
or less, 75.degree. C. or less, 70.degree. C. or less, 65.degree.
C. or less, 60.degree. C. or less, 55.degree. C. or less,
50.degree. C. or less, 45.degree. C. or less, 40.degree. C. or
less, 35.degree. C. or less, 30.degree. C. or less, 25.degree. C.
or less, 20.degree. C. or less, 15.degree. C. or less, or
10.degree. C. or less).
[0034] The galvanic deposition can be performed at a temperature
ranging from any of the minimum temperature values described above
to any of the maximum temperatures described above. For example, in
some embodiments, the metal is galvanically deposited at a
temperature of from 5.degree. C. to 200.degree. C. (e.g., from
5.degree. C. to 170.degree. C., from 5.degree. C. to 150.degree.
C., from 5.degree. C. to 120.degree. C., from 5.degree. C. to
90.degree. C., from 5.degree. C. to 90.degree. C., from 25.degree.
C. to 90.degree. C., from 5.degree. C. to 60.degree. C., or from
25.degree. C. to 60.degree. C.).
[0035] The galvanic deposition can be performed for varying periods
of time, so as to provide nanoporous Cu-M catalysts having a molar
ratio of Cu:M desired for use in a particular catalytic
application. For example, the nanoporous, open-cell copper foam can
be maintained is maintained in contact with the solution comprising
the M-containing species for a period of tune effective to form a
nanoporous Cu-M catalyst having desired a molar ratio of Cu:M.
[0036] The molar ratio of Cu:M in the nanoporous Cu-M catalyst can
be determined by Inductively Coupled. Plasma-Mass Spectroscopy
(ICP-MS). In some embodiments, molar ratio of Cu:M in the
nanoporous Cu-M catalyst can be at least 1:2 (e.g., at least 1:1,
at least 1.25:1, at least 1.5:1, at least 1.6:1, at least 1.7:1, at
least 1.8:1, at least 1.9:1, at least 2:1, at least 2.1:1, at least
2.2:1, at least 2.3:1, at least 2.4:1, at least 2.5:1, at least
5:1, at least 10:1, at least 25:1, at least 50:1, at least 100:1,
at least 150:1, at least 200:1, or at least 250:1). In some
embodiments, molar ratio of Cu:M in the nanoporous Cu-M catalyst
can be 500:1 or less (e.g., 250:1 or less, 200:1 or less, 150:1 or
less, 100:1 or less, 50:1 or less, 25:1 or less, 10:1 or less, 5:1
or less, 2.5:1 or less, 2.4:1 or less, 2.3:1 or less, 2.2:1 or
less, 2.1:1 or less, 2:1 or less, 1.9:1 or less, 1.8:1 or less,
1.7:1 or less, 1.6:1 or less, 1.5:1 or less, 1.25:1 or less, or 1:1
or less).
[0037] The molar ratio of Cu:M in the nanoporous Cu-M catalyst can
range from any of the minimum ratios described above to any of the
maximum ratios described above. For example, the molar ratio of
Cu:M in the nanoporous Cu-M catalyst, as determined by ICP-MS, can
range from 1:2 to 500:1 (e.g., from 1:2 to 250:1; from to 500:1;
from 1:1 to 250:1; from 5:1 to 500:1; from 10:1 to 500:1; from
0.5:1 to 2.5:1, from 1:1 to 2.5:1, or from 1.5:1 to 2.2:1).
[0038] Following galvanic deposition, the Cu-M precursor catalyst
can be conditioned to form the nanoporous Ca-M catalyst.
Conditioning can involve electrochemical dealloying of the Cu-M
precursor catalyst to form the nanoporous Cu-M catalyst. For
example, the Cu-M precursor catalyst can be conditioned by repeated
electrochemical cycling (e.g., 50 cycles) of the Cu-M precursor
catalyst between 0.5 V and 1.2 V at 25.degree. C. in
N.sub.2-saturated 0.1 M HClO.sub.4 to dealloy/stabilize the
catalyst.
[0039] In some embodiments, the electroreduction catalyst (e.g.,
the nanoporous Cu catalyst; the nanoporous Cu-M catalyst, where M
is a metal chosen from Pt, Ir, Pd, Ag, Au, Rh, Ru, Zn, Sn, Ni, Fe,
Re, Ga, In, Cd, Tl, and Ti; or a combination thereof) can be
processed to reduce the particle size of the electroreduction
catalyst prior to use in conjunction with the methods described
herein. For example, in some embodiments, the electroreduction
catalyst can be formed into nanoparticles prior to use in
conjunction with the methods described herein.
[0040] The electroreduction catalyst can be formed into
nanoparticles prior to use in conjunction with the methods
described herein using any suitable method known in the art. The
nanoparticles formed by the process can be spherical or
non-spherical in shape. In certain embodiments, the nanoparticles
can be discrete, spherical nanoparticles. In some embodiments, the
population of nanoparticles formed by this process is monodisperse.
The nanoparticles can optionally comprise nanopores. In some
embodiments, the nanopores can interconnect, so as to form a
network of nanopores spanning the nanoparticles.
[0041] "Monodisperse" and "homogeneous size distribution," as used
herein, and generally describe a population of particles where all
of the particles are the same or nearly the same size. As used
herein, a monodisperse distribution refers to particle
distributions in which 80% of the distribution (e.g., 85% of the
distribution, 90% of the distribution, or 95% of the distribution)
lies within 25% of the median particle size (e.g., within 20% of
the median particle size, within 15% of the median particle size,
within 10% of h median particle size, or within 5% of the median
particle size).
[0042] "Mean particle size" or "average particle size", are used
interchangeably herein, and generally refer to the statistical mean
particle size of the particles in a population of nanoparticles.
The diameter of an essentially spherical particle can refer to the
physical diameter of the spherical particle. The diameter of a
non-spherical nanoparticle can refer to the largest linear distance
between two points on the surface of the nanoparticle. Mean
particle size can be measured using methods known in the art, such
as evaluation by scanning electron microscopy.
[0043] In some embodiments, the electroreduction catalyst can
comprise nanoparticles having an average particle size, as measured
by scanning electron microscopy (SEM), of at least 10 nm (e.g., at
least 15 nm, at least 20 nm, at least 25 nm, at least 30 nm, at
least 35 nm, at least 40 nm, at least 45 nm, at least 50 nm, at
least 55 nm, at least 60 nm, at least 65 nm, at least 70 nm, at
least 75 inn, at least 80 nm, at least 85 inn, at least 90 inn, at
least 95 inn, at least 100 nm, at least 150 nm, at least 200 nm, at
least 250 nm, at least 300 nm, at least 350 nm, at least 400 nm, or
at least 450 nm). In some embodiments, the electroreduction
catalyst can comprise nanoparticles having an average particle
size, as measured by SEM, of 500 nm or less (e.g., 450 nm or less,
400 nm or less, 350 nm or less, 300 nm or less, 250 nm or less, 200
nm or less, 150 nm or less, 100 nm or less, 95 urn or less, 90 nm
or less, 85 nm or less, 80 nm or less, 75 nm or less, 70 ruin or
less, 65 nm or less, 60 nm or less, 55 nm or less, 50 nm or less,
45 nm or less, 40 nm or less, 35 nm or less, 30 nm or less, 25 urn
or less, 20 nm or less, or 15 urn or less).
[0044] The electroreduction catalyst can comprise nanoparticles
having an average particle size, as measured by SEM, ranging from
any of the minimum values described above to any of the maximum
values described above. For example, the electroreduction catalyst
can comprise nanoparticles having an average particle size, as
measured by SEM, of from 10 nm to 500 nm (e.g., from 10 run to 250
nm, from 10 nm to 150 nm, from 20 nm to 100 run, from 20 nm to 80
nm, or from 80 nm to 100 nm, from 10 nm to 80 nm, from 25 nm to 80
nm, or from 50 nm to 80 nm).
[0045] In some embodiments, the electroreduction catalyst can have
a specific surface area of at least 5 m.sup.2/g, as measured using
the Brunauer-Emmett-Teller (BET) method (e.g., at least 10
m.sup.2/g, at least 15 m.sup.2/g, at least 20 m.sup.2/g, at least
25 m.sup.2/g, at least 30 m.sup.2/g, or at least 35 m.sup.2/g). In
some embodiments, the electroreduction catalyst can have a specific
surface area of 40 m.sup.2/g or less, as measured using the BET
method (e.g., 35 m.sup.2/g or less, 30 m.sup.2/g or less, 25
m.sup.2/g or less, 20 m.sup.2/g or less, 15 m.sup.2/g or less, or
10 m.sup.2/g or less).
[0046] The electroreduction catalyst can have a specific surface
area ranging from any of the minimum values described above to any
of the maximum values described above. For example, the
electroreduction catalyst can have a specific surface area of from
5 m.sup.2/g to 40 m.sup.2/g, as measured using the BET method
(e.g., from 10 m.sup.2/g to 40 m.sup.2/g, from 10 m.sup.2/g to 25
m.sup.2/g, from 10 m.sup.2/g to 20 m.sup.2/g, from 20 m.sup.2/g to
40 m.sup.2/g, or from 10 m.sup.2/g to 15 m.sup.2/g).
[0047] The electroreduction catalysts can be disposed on a
conductive substrate (e.g., the surface of an electrode, such as a
copper electrode (e.g., copper foil) or carbon electrode) to form
an electrode for use in conjunction with the methods described
herein.
[0048] For some applications, including many catalytic
applications, it may be of interest to deposit the electroreduction
catalysts described herein on a support, such as a carbonaceous
support. Accordingly, also provided are compositions comprising a
electroreduction catalyst described herein deposited on a support,
such as a carbonaceous support. The carbonaceous support may
comprise any type of carbon that suitably supports the
electroreduction catalyst to provide a catalyst having suitable
activity. The carbonaceous support can comprise an amorphous
carbon, a crystalline or graphitic carbon, or a vitreous or glassy
carbon. Also, the carbonaceous support can be in any suitable form
(e.g., in the form of a powder, fiber, or flake), and can have any
suitable crystallographic orientation, crystallite size, interlayer
spacing, density, particle size, or particle shape. The
carbonaceous support can comprise a carbon selected from Ketjen
Black, carbon black, lamp black, acetylene black, mesocarbon,
graphite, pyrolytic graphite, single-wall carbon nanotubes,
multi-wall carbon nanotubes, Vulcan carbon, and carbon fiber. In
some embodiments, the carbonaceous support can have an average
particle size of from 0.01 .mu.m to 10 .mu.m. The supported
electroreduction catalysts can also be disposed on a conductive
substrate to provide an electrode.
[0049] The electrochemical cell can be, for example, a divided
electrochemical cell. An exploded view of an example
electrochemical cell that can be used in conjunction with the
electrochemical reduction of carbon dioxide described herein is
illustrated in FIG. 1. As shown in FIG. 1, the electrochemical cell
(100) can comprise a working electrode (102) comprising the
electroreduction catalyst in electrochemical contact with a first
cell compartment (104) and a counter electrode (106) in
electrochemical contact with a second cell compartment (108). A
solid electrolyte membrane (110) (e.g., a cation exchange membrane
such as a. Nafion.RTM. membrane) can be interposed between the
working electrode (102) and the counter electrode (106).
[0050] Both the first cell compartment and the second cell
compartment can further comprise an aqueous solution of an
electrolyte. For example, the first cell compartment can further
comprise an aqueous solution of an electrolyte in electrochemical
contact with the working electrode disposed in the first cell
compartment, and the second cell compartment can further comprise
an aqueous solution of an electrolyte in electrochemical contact
with the counter electrode disposed in the second cell compartment.
Any suitable electrolyte can be used. For example, the electrolyte
can be selected to as to be compatible with carbon dioxide present
in the system (e.g., so as not to precipitate upon introduction of
carbon dioxide into the electrochemical cell). For example, the
electrolyte can comprise potassium bicarbonate, sodium hydrogen
carbonate, potassium chloride, potassium sulfate, or potassium
phosphate. In certain embodiments, the electrolyte can comprise an
alkali metal bicarbonate (e.g., potassium bicarbonate or sodium
bicarbonate).
[0051] The electrochemical cell can further comprise a gas inlet
(112) that can be used to introduce carbon dioxide into the first
cell compartment of the divided electrochemical cell. The cell can
further include other features to facilitate operation, including
an electrolyte inlet (114) and an electrolyte outlet (116) fluidly
connected to the second cell compartment that can be used to
introduce electrolyte into the electrochemical cell, and a product
outlet (118) that can be used to remove products from the first
cell compartment. If desired, the electrochemical cell can further
include a reference electrode configured to measure the potential
when the electrochemical cell is used for electrocatalysis.
[0052] In these embodiments when electrochemical cells of this
design are utilized, contacting the carbon dioxide with the
electroreduction catalyst can comprise introducing the carbon
dioxide into the first cell compartment of the divided
electrochemical cell (e.g., bubbling the carbon dioxide
into/through the aqueous solution of the electrolyte). Applying a
potential to the electrochemical cell can comprise applying a
negative voltage and a positive voltage to the working electrode
and the counter electrode, respectively, to reduce the carbon
dioxide to form the product. The reaction can be performed at room
temperature (i.e., 23.degree. C.) and standard pressure of CO.sub.2
(1 bar). Methods can further include obtaining one or more products
of the reduction reaction from the first cell compartment and/or
(in the case of multiple products) separating one or more products
to obtain a desired product from the reduction reaction.
[0053] The electroreduction catalysts described herein can also be
used in other catalytic applications. For example, the
electroreduction catalysts can also be used as an electrocatalyst
for the electrolytic coupling of carboxylic acids (i.e. the Kolbe
reaction). For example, provided herein are methods for preparing
an organic compound defined by Formula II from a carboxylic acid
defined by Formula I and a carboxylic acid defined by Formula I'
according to the equation below
##STR00001##
where R and R' interdependently represent hydrogen, a substituted
or unsubstituted alkyl group (e.g., a substituted or unsubstituted
C.sub.1-C.sub.12 alkyl group), a substituted or unsubstituted
alkenyl group (e.g., a substituted or unsubstituted
C.sub.2-C.sub.12 alkenyl group), a substituted or unsubstituted
alkynyl group (e.g., a substituted or unsubstituted
C.sub.2-C.sub.12 alkynyl group), a substituted or unsubstituted
aryl group (e.g., a substituted or unsubstituted phenyl group), a
substituted or unsubstituted heteroaryl group (e.g., a substituted
or unsubstituted C.sub.3-C.sub.10 heteroaryl group), a substituted
or unsubstituted alkylaryl group, or a substituted or unsubstituted
alkylheteroaryl group. The method can comprise contacting the
carboxylic acid defined by Formula I and the carboxylic acid
defined by Formula I' with an electroreduction catalyst described
herein in an electrochemical cell, and applying a potential to the
electrochemical cell to form the organic compound defined by
Formula II.
[0054] The examples below are intended to further illustrate
certain aspects of the methods, systems, and compositions described
herein, and are not intended to limit the scope of the claims.
Examples
[0055] Materials and Methods
[0056] Sodium hydroxide pellets (Certified ACS, 97%), methanol
(Certified ACS, 99.8%), phosphoric acid (85% in water), and
potassium bicarbonate (USP/FCC grade) were purchased from Fisher
Scientific. Copper foil (99.98%, 0.5 mm thick) and Nafion.RTM.
membrane (NRE-212) were purchased from Sigma Aldrich. Nation.RTM.
solution (5% w/w) was purchased from Alfa Aesar. Ruthenium
trichloride hydrate (>99%) was purchased from Pressure Chemical
Co. Hydrogen, carbon dioxide, and helium (all 99.995%) were
purchased from Praxair. Copper/aluminum rods were made in-house,
using a composition of 83 at % aluminum. The potassium bicarbonate
electrolyte solution was pre-electrolyzed and filtered before use,
using deionized water for dilution (MilliQ), Advantage A10).
[0057] Preparation of Nanoporous Copper Foams
[0058] A CuAl rod of known percent composition was mechanically cut
into smaller coins. The CuAl coins were placed into 6 M NaOH, which
was heated to 80.degree. C. and stirred for 24 hours to etch out
the Al, forming nanoporous Cu foams.
[0059] If desired, the porosity of the nanoporous Cu foams can be
further tuned with temperature (e.g., by varying the heating
temperature from 0.degree. C. to 100.degree. C.), strength of the
NaOH etchant (e.g., by varying the concentration of the NaOH
etchant from 0.1 to 6 M), etching current (e.g., by varying the
etching current from 0 to 300 mA/g), or a combination thereof. FIG.
3 includes SEM images of nanoporous Cu foams prepared at various
conditions. By varying process conditions, nanoporous Cu foams
possessing tunable pores and correspondingly a range of materials
with tunable surface area can be obtained. BET surface area
analysis shows that the surface area of the nanoporous Cu foams can
be tuned between 5 m.sup.2/g and 40 m.sup.2/g.
[0060] The nanoporous copper foams were then soaked in DI water for
12 hours and rinsed repeatedly. The foams were then dried in an
oven at 60.degree. C. for 1 hour. The foams were then reduced under
H.sub.2 atmosphere in a tube furnace (Lindberg Blue M, Thermo
Scientific) at 0.400.degree. C. for 2 hours. The nanoporous copper
foams were then stored in a vacuum sealed container until use.
[0061] Preparation of Nanoporous Copper Coating Solution
[0062] A nanoporous copper foam was crushed. 0.015 g of the crushed
nanoporous copper foam was then mixed with 10 mL of methanol in a
glass cylinder. The mixture was degassed for 1 minute; the mixture
was then sonicated for 10 minutes (FS30D, Fisher Scientific). 40
.mu.L of Nation.RTM. solution was added, and the mixture was
sonicated for an additional 10 minutes.
[0063] Preparation of Nanoporous Copper Electrodes
[0064] Copper foils were cut to an appropriate size and
electropolished in 85% phosphoric acid at 1.4 V vs a platinum wire
counter electrode for 10 minutes. The foils were then rinsed in DI
water and dried under N.sub.2. The foil was then placed into a
custom built aluminum box and the nanoporous copper coating
solution was cast onto the foil. The solution was then placed in an
oven at 60.degree. C. fir approximately 1 hour, or until dry. The
nanoporous copper foil was then reduced under H.sub.2 on a hot
plate in the following order: 200.degree. C. (20
min.).fwdarw.100.degree. C. (10 min.).fwdarw.RT (5 min.). A water
layer was applied until the foil was placed into an electrochemical
flow cell.
[0065] Preparation of Nanoporous Copper/Ruthenium Electrodes
[0066] Copper foils were cut to an appropriate size and
electropolished in 85% phosphoric acid at 1.4 V vs a platinum wire
counter electrode for 10 minutes. The foils were then rinsed in DI
water and dried under N.sub.2. The foil was then placed into a
custom built aluminum box and the nanoporous copper coating
solution was cast onto the foil. The solution was then placed in an
oven at 60.degree. C. fir approximately 1 hour, or until dry.
Approximately 0.100 g of ruthenium chloride monohydrate was mixed
with 300 Mt of DI water. The mixture was heated to 100.degree. C.
and stirred, then purged using N.sub.2, gas for 30 minutes. The
nanoporous copper coated foil was submerged into the ruthenium
solution for 1 hour and galvanic displacement of ruthenium for
copper occurred. The nanoporous copper/ruthenium foil was then
reduced under 1-12 on a hot plate in the following order:
200.degree. C. (20 min.).fwdarw.100.degree. C. (10 min.).fwdarw.RT
(5 min.). A water layer was applied until the foil was placed into
an electrochemical flow cell.
[0067] CO.sub.2 Electroreduction
[0068] Electroreduction experiments were performed at ambient
temperature and pressure in a custom two-compartment
electrochemical cell (FIG. 1). The cell was sealed using Viton.RTM.
gaskets. The counter electrode was a dimensionally stabilized
anode. The electrolyte was 0.1 M KHCO.sub.3 saturated with
CO.sub.2. The pH of the saturated solution was 6.8. The working and
counter compartments were separated using a Nation.RTM. cation
exchange membrane. An RHE reference electrode was used for all
experiments. The RHE was created by flowing H.sub.2 over a piece of
black Pt gauze in the 0.1 M KHCO.sub.3 electrolyte. The pH of the
solution at the RHE was 9.2. A volume of 26 mL of electrolyte in
the working compartment was held static throughout the experiment
and continuously saturated with CO.sub.2 at a flow rate of
approximately 10 mL/min. Headspace gas was vented directly to the
gas chromatograph (7890A, Agilent Technologies) through a sampling
loop. The combined CO.sub.2 and product gas flow rate was measured
at the end of the sampling loop by a soap bubble flow meter (Model
520, Fisher Scientific). Electrolyte in the counter compartment was
continuously replenished to maintain ionic conductivity of the
solution
[0069] Product Analysis
[0070] Potentiostatic experiments were performed for 65 minutes per
data point (Model 253A, Princeton Applied Research). Injections
were made into the gas chromatograph every 13 minutes. The gas
chromatograph had two columns to which the product gases were split
to: a Haysep Q column equipped with a mass spectrometer, and a
Molesieve 5A column equipped with a thermal conductivity detector.
The mass spectrometer was used to quantify C.sub.2H.sub.4 and
C.sub.2H.sub.6 concentration and the thermal conductivity detector
was used to quantify H.sub.2 and CO concentration.
[0071] The liquid electrolyte in the working compartment of the
electrolysis cell was collected at the end of the experiment and
analyzed using an AVIII 400 MHz NMR spectrometer. NMR samples were
prepared by mixing 0.8 mL of the collected electrolyte with 0.1 mL
D.sub.2O and 0.1 mL of 100 ppm acetonitrile as an internal
standard. A water suppression method was used to measure the
.sup.1H spectrum. This allowed for identification and
quantification of formate, acetone, ethanol, and propanol.
[0072] Ex situ experiments were performed to observe the
nanostructured material and determine the amount of ruthenium
present after galvanic displacement. Samples used in ex situ
experiments were reduced under H.sub.2 and heat in the manner
described above
[0073] Results and Discussion
[0074] Characterization of Catalysts and Electrodes
[0075] FIG. 5 is an SEM image of nanoporous CuRu catalyst for use
in CO.sub.2 reduction. The SEM shows relatively uniform
distribution of CuRu nanoparticles in the range of 80-100 nm.
[0076] X-ray photoelectron spectroscopy (XPS) was used to
characterize the nanoporous copper/ruthenium electrode. XPS spectra
were obtained on a Kratos XPS using monochromatic Al K.alpha.
radiation. Spectra were obtained from both the surface and several
atomic layers below the surface of newly made and used electrodes.
See FIG. 6.
[0077] The copper 2p peaks clearly show the presence of a copper
oxide layer, which should be expected, as these samples were
handled briefly in air before being transported to XPS for
analysis. The Cu LMM peaks show a strong signal at 917.5 eV (KE),
correlating strongly with Cu (II). It is possible that both Cu (I)
and Cu (II) are present. Etching the surface using Ar.sup.+
ion-bombardment resulted in a complete removal of the oxide
layer.
[0078] Ruthenium 3d peaks are found at a similar binding energy as
adventitious carbon. Newly made CuRu samples are shown to have
RuO.sub.2 present on the surface while ruthenium metal is found
just below the surface, after an Ar.sup.+ etch. CuRu that has been
used for CO.sub.2 reduction shows metallic Ru several atomic layers
below the surface while Ru is not detected on the top most surface
layer. The data suggests that Ru migrated from the surface to the
hulk of the sample and was thereafter evenly dispersed throughout
the material.
[0079] Electrocatalytic CO.sub.2 Activity and Selectivity
[0080] Electroreduction of carbon dioxide on the nanoporous Cu and
nanoporous CuRu catalyst resulted in the product distribution shown
in FIGS. 7 and 8 for the gas and liquid products respectively. The
electrochemical activity of both nanoporous Cu and nanoporous CuRu
catalysts is typically around 10 mA/cm.sup.2. The onset of CO.sub.2
reduction was not observed until an applied potential of -0.35 V.
At this potential, carbon monoxide and formate were the only
CO.sub.2 reduction products from both nanoporous Cu and nanoporous
CuRu catalyst. A lower onset potential implies lower power
requirement for the electrosynthesis process to occur. Generally,
more hydrocarbon products are produced on the nanoporous CuRu
catalyst compared to nanoporous Cu. On nanoporous CuRu, CO reached
a maximum efficiency of 13% at -0.65 V while the formation of
formate reached 30% at -0.95 V on nanoporous Cu. Ethane is
consistently produced on the nanoporous CuRu catalysts with an
onset at -0.55 V. Significant amounts of ethylene is produced
>15%, at -0.65 V, which is almost 100 mV less overpotential
required than typically observed on copper foil surfaces. Ethane is
another compound that is not typically formed on Cu catalyst. The
conversion of CO.sub.2 directly to ethane has only been detected on
Fe, Co, and Ni catalysts at high pressures of 50 to 60 atmospheres
and large overpotential of -1.65 V. A significant amount of ethane
(.about.5%) was detected at early as -0.55 V, an over 1100 mV
advantage over existing catalysts. On nanoporous CuRu, no methane
or methanol was observed at any of the applied potentials. Ethanol
and propanol were detected in the liquid phase with propanol
reaching a maximum efficiency of 6.5%. The onset potential of -0.85
V, one of the lowest onset potential reported for propanol
formation.
[0081] The devices, systems, and methods of the appended claims are
not limited in scope by the specific devices, systems, and methods
described herein, which are intended as illustrations of a few
aspects of the claims. Any devices, systems, and methods that are
functionally equivalent are intended to fall within the scope of
the claims. Various modifications of the devices, systems, and
methods in addition to those shown and described herein are
intended to fall within the scope of the appended claims. Further,
while only certain representative devices, systems, and method
steps disclosed herein are specifically described, other
combinations of the devices, systems, and method steps also are
intended to fall within the scope of the appended claims, even if
not specifically recited. Thus, a combination of steps, elements,
components, or constituents may be explicitly mentioned herein or
less, however, other combinations of steps, elements, components,
and constituents are included, even though not explicitly
stated.
[0082] The term "comprising" and variations thereof as used herein
is used synonymously with the term "including" and variations
thereof and are open, non-limiting terms. Although the terms
"comprising" and "including" have been used herein to describe
various embodiments, the terms "consisting essentially of" and
"consisting of" can be used in place of "comprising" and
"including" to provide for more specific embodiments of the
invention and are also disclosed. Other than where noted, all
numbers expressing geometries, dimensions, and so forth used in the
specification and claims are to be understood at the very least,
and not as an attempt to limit the application of the doctrine of
equivalents to the scope of the claims, to be construed in light of
the number of significant digits and ordinary rounding
approaches.
[0083] Unless defined otherwise, all technical and scientific terms
used herein have the same meanings as commonly understood by one of
skill in the art to which the disclosed invention belongs.
Publications cited herein and the materials for which they are
cited are specifically incorporated by reference.
* * * * *