U.S. patent application number 13/659354 was filed with the patent office on 2013-02-28 for porous metal dendrites as gas diffusion electrodes for high efficiency aqueous reduction of co2 to hydrocarbons.
This patent application is currently assigned to The Trustees of Columbia University in the City of New York. The applicant listed for this patent is The Trustees of Columbia University in the City of. Invention is credited to Ed Chen.
Application Number | 20130048506 13/659354 |
Document ID | / |
Family ID | 45004447 |
Filed Date | 2013-02-28 |
United States Patent
Application |
20130048506 |
Kind Code |
A1 |
Chen; Ed |
February 28, 2013 |
Porous Metal Dendrites as Gas Diffusion Electrodes for High
Efficiency Aqueous Reduction of CO2 to Hydrocarbons
Abstract
An electrolytic cell system to convert carbon dioxide to a
hydrocarbon that includes a first electrode including a substrate
having a metal porous dendritic structure applied thereon; a second
electrode, and an electrical input adapted for coupling to a source
of electricity, for applying a voltage across the first electrode
and the second electrode.
Inventors: |
Chen; Ed; (New York,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Trustees of Columbia University in the City of; |
New York |
NY |
US |
|
|
Assignee: |
The Trustees of Columbia University
in the City of New York
New York
NY
|
Family ID: |
45004447 |
Appl. No.: |
13/659354 |
Filed: |
October 24, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2011/038601 |
May 31, 2011 |
|
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13659354 |
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61349744 |
May 28, 2010 |
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Current U.S.
Class: |
205/317 ;
204/233; 204/242; 204/275.1; 204/277; 204/284; 205/462 |
Current CPC
Class: |
C25B 11/035 20130101;
Y02P 30/40 20151101; C10G 2/50 20130101; C10G 2400/20 20130101;
C25B 3/04 20130101 |
Class at
Publication: |
205/317 ;
204/242; 204/233; 204/275.1; 204/277; 204/284; 205/462 |
International
Class: |
C25B 15/08 20060101
C25B015/08; C25B 3/00 20060101 C25B003/00; C25D 9/02 20060101
C25D009/02; C25B 11/03 20060101 C25B011/03 |
Claims
1. An electrolytic cell system to convert carbon dioxide to a
hydrocarbon comprising: (a) a first electrode including a substrate
having a metal porous dendritic structure applied thereon; (b) a
second electrode; and (c) an electrical input adapted for coupling
to a source of electricity, for applying a voltage across the first
electrode and the second electrode.
2. The electrolytic cell system of claim 1, wherein the metal is
selected from platinum, gold, silver, zinc, cobalt, nickel, tin,
palladium and copper.
3. The electrolytic cell system of claim 2, wherein the metal is
copper.
4. The electrolytic cell system of claim 1, wherein the substrate
is selected from copper, copper foil, glassy carbon and
titanium.
5. The electrolytic cell system of claim 1, wherein at least one of
the first electrode and the second electrode is at least partially
saturated with carbon dioxide.
6. The electrolytic cell system of claim 1, further comprising an
electrolyte source capable of being introduced into a region in
between the first electrode and the second electrode of the
electrolytic cell system.
7. The electrolytic cell system of claim 6, wherein the electrolyte
is selected from a bicarbonate salt, sodium chloride, carbonic
acid, hydrogen, potassium and methanol.
8. The electrolytic cell system of claim 6, further comprising a
membrane to dissolve carbon dioxide in the electrolyte.
9. The electrolytic cell system of claim 5, further comprising a
conduit to pass carbon dioxide directly to the surface of the first
electrode.
10. The electrolytic cell system of claim 1, further comprising a
source of a metal porphryrin salt capable of being introduced into
a region in between the first electrode and the second electrode of
the electrolytic cell system.
11. The electrolytic cell system of claim 10, wherein the metal
porphyrin salt is a metal chlorophyllin salt.
12. The electrolytic cell system of claim 11, wherein the metal
chlorphyllin salt is copper chlorophyllin.
13. An electrode for an electrolytic cell system comprising a
substrate with a metal porous dendritic structure applied
thereon.
14. The electrode of claim 13, wherein the metal is selected from
platinum, gold, silver, zinc, cobalt, nickel, tin, palladium and
copper.
15. The electrode of claim 14, wherein the metal is copper.
16. A method of converting carbon dioxide to a hydrocarbon
comprising: providing an electrolytic cell that includes (a) a
first electrode including a substrate having a metal porous
dendritic structure applied thereon; (b) a second electrode, and
(c) an electrical input adapted for coupling to a source of
electricity, for applying a voltage across the first electrode and
the second electrode; introducing a source of carbon dioxide to the
electrolytic cell; and applying the voltage across the first
electrode and the second electrode.
17. The method of claim 16, wherein the metal is selected from
platinum, gold, silver, zinc, cobalt, nickel, tin, palladium and
copper.
18. The method of claim 16, wherein the metal is copper.
19. The method of claim 18, wherein the copper dendritic structure
is prepared by a process that includes adding copper chlorophyllin
to the electrolytic cell and electrodepositing the copper
chlorophyllin on the first electrode.
20. The method of claim 16, wherein the carbon dioxide is obtained
from an air stream, a combustion exhaust stream, or a pre-existing
carbon dioxide source.
21. The method of claim 16, wherein the hydrocarbon is
ethylene.
22. A method for preparing an electrode for use in an electrolytic
cell comprising: (a) providing an electrolytic cell; (b) applying a
solution of a metal porphyrin salt to the electrolytic cell; and
(c) applying electricity to plate the metal porphyrin salt on the
substrate.
23. The method of claim 22, wherein the metal porphyrin salt is a
metal chlorphyllin salt.
24. The method of claim 23, wherein the metal chlorophyllin salt is
copper chlorophyllin.
25. The method of claim 22, wherein the metal porphyrin salt is
pulse plated or reverse pulse plated on the substrate.
26. The method of claim 22, wherein the metal porphyrin salt is
applied to the substrate using high current density to create
hydrogen bubble templates on the surface of the substrate.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International Patent
Application Serial No. PCT/US2011/038601 filed May 31, 2011, which
claims priority to U.S. Provisional Application Ser. No.
61/349,744, filed May 28, 2010, the contents of both of which are
hereby incorporated by reference in their entireties herein.
BACKGROUND
[0002] Existing carbon infrastructure costs make a transition from
a fossil fuel economy particularly difficult. Thus, intermediate
solutions which abate CO.sub.2 emissions while also producing
valuable products would be particularly useful. The use of
electrolytic cells in the reduction of CO.sub.2 to methane and
other hydrocarbons, electrolytically, at room temperatures, with a
saturated solution of carbon dioxide and an electrolyte, can be a
highly economic means of producing natural gas from carbon
dioxide.
SUMMARY
[0003] One aspect of the presently disclosed subject matter
provides an electrolytic cell system to convert carbon dioxide to a
hydrocarbon (e.g., ethylene) that includes a first electrode
including a substrate having a metal porous dendritic structure
applied thereon; a second electrode, and an electrical input
adapted for coupling to a source of electricity, for applying a
voltage across the first electrode and the second electrode.
[0004] In one embodiment, the metal porous dentritic structure is a
metal selected from platinum, gold, silver, zinc, cobalt, nickel,
tin, palladium and copper. In one embodiment, the metal porous
dendritic structure is a copper dendritic structure. In one
embodiment, the substrate is selected from copper, copper foil,
glassy carbon and titanium. The first electrode and/or the second
electrode can be at least partially saturated with carbon
dioxide.
[0005] The presently disclosed electrolytic cell system can further
include an electrolyte source capable of being introduced into a
region in between the first electrode and the second electrode of
the electrolytic cell system. In one embodiment, the electrolyte is
selected from a bicarbonate salt (e.g., potassium hydrogen
carbonate), sodium chloride, carbonic acid, hydrogen, potassium and
methanol.
[0006] In one embodiment, the electrolytic cell system can include
a membrane to dissolve carbon dioxide in the electrolyte. In an
alternative embodiment, the electrolytic cell system includes a
conduit to pass carbon dioxide directly to the surface of the first
electrode.
[0007] The presently disclosed electrolytic cell system can further
include a source of a metal porphryrin salt capable of being
introduced into a region in between the first electrode and the
second electrode of the electrolytic cell system. For example, the
metal porphyrin salt can be a metal chlorophyllin salt, such as
copper chlorophyllin.
[0008] Another aspect of the presently disclosed subject matter
provides an electrode for an electrolytic cell system comprising a
substrate with a metal porous dendritic structure applied thereon.
The metal can be selected from platinum, gold, silver, zinc,
cobalt, nickel, tin, palladium and copper.
[0009] Another aspect of the presently disclosed subject matter
provides a method of converting carbon dioxide to a hydrocarbon
(e.g., ethylene) that includes providing an electrolytic cell that
includes a first electrode including a substrate having a metal
porous dendritic structure applied thereon; a second electrode, and
an electrical input adapted for coupling to a source of
electricity, for applying a voltage across the first electrode and
the second electrode; introducing a source of carbon dioxide to the
electrolytic cell; and applying the voltage across the first
electrode and the second electrode.
[0010] The metal of the pourous dendritic structure can be selected
from platinum, gold, silver, zinc, cobalt, nickel, tin, palladium
and copper. The copper dendritic structure can be prepared, for
example, by a process that includes adding copper chlorophyllin to
the electrolytic cell and electrodepositing the copper
chlorophyllin on the first electrode.
[0011] In one embodiment, the carbon dioxide is obtained from an
air stream, a combustion exhaust stream, or a pre-existing carbon
dioxide source.
[0012] Another aspect of the presently disclosed subject matter
provides a method for preparing an electrode for use in an
electrolytic cell that includes providing an electrolytic cell;
applying a solution of a metal porphyrin salt to the electrolytic
cell; and applying electricity to plate the metal porphyrin salt on
the substrate. The metal porphyrin salt can be a metal
chlorophyllin salt (e.g., copper chlorophyllin).
[0013] In one embodiment, the metal porphyrin salt is pulse plated
or reverse pulse plated on the substrate. According to an
alternative embodiment, the metal porphyrin salt is applied to the
substrate using high current density to create hydrogen bubble
templates on the surface of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 depicts a cyclic voltametry (CV) of copper and
platinum electrodes immersed in a 0.1 M sodium bicarbonate solution
saturated with carbon dioxide. The A) red line graph is a CV of a
piece of copper foil 0.2 grams in mass, while B) the blue line is a
CV of porous copper dendritic electrode with a mass of 5 mg.
Current densities for the porous copper is much higher, despite the
differences in mass. As reproduced in black and white, the "red"
line (A) are the two lines that have the highest i/A at about -1.5
volts and the "blue lines" (B) are the two lines that have the
lowest i/A at about -1.5 volts.
[0015] FIG. 2 is a photograph of copper deposits grown at 150
mA/cm.sup.2 with PVA.
[0016] FIG. 3 is a photograph of copper deposits grown at 150
mA/cm.sup.2 with no additive.
[0017] FIG. 4 is a photograph of copper grown with a PEG additive
on glassy carbon.
[0018] FIG. 5 is a cyclic voltametry (CV) a high acid copper
solution 10 g/L Cu, and 32 g/L of sulfuric acid (left diagram) and
CV of high acid copper solution 10 g/L Cu and 32 g/L sulfuric acid
with 1% chlorophyllin additive.
[0019] FIG. 6 is a photograph of chlorophllyn residue on porous
structure during reverse pulse application, magnified 400 times.
The chlorophllyn membrane is attracted to the anode, and is
selectively pulled off of the growing fractal front while remaining
in the recessed regions, containing loss of surface area while
increasing growth of surface area.
[0020] FIG. 7 is a photograph of copper particles formed with
chlorophyllin additive and 10 alternating pulses of -0.32
A/cm.sup.2 and 0.1 A/cm.sup.2. Copper structures can be resolved
down to 50 nm, and take on a non-spherical form which display
higher surface areas.
[0021] FIG. 8 is a photograph of copper particles formed with
chlorophyllin additive and 10 alternating pulses of -0.32
A/cm.sup.2 and 0.1 A/cm.sup.2. Copper structures can be resolved
down to 50 nm, and take on a non-spherical form which provide
higher surface areas.
[0022] FIG. 9 is a photograph of a dendritic fractal cluster
produced under a pulsating regime of 500 pulses with a current
density of 0.69 A/cm.sup.2.
[0023] FIG. 10 is a photograph of a dendritic structure after a
pulsating regime of 1000 pulses with a current density of 0.69
A/cm.sup.2.
[0024] FIG. 11 is a photograph showing the beginnings of the
dendritic copper foam beginning to form.
[0025] FIG. 12 is a photograph of copper PDS for visual
characterization magnified 30 times.
[0026] FIG. 13 is a photograph of copper PDS magnified 300 times
and 5000 times.
[0027] FIG. 14 is a photograph of a dendrite structure magnified
20,000 times.
DETAILED DESCRIPTION
[0028] The presently disclosed subject matter provides a method of
converting CO.sub.2 to methanol, methane and other hydrocarbons in
an electrolytic cell. In one embodiment, the method includes
introducing an electrolyte saturated with CO.sub.2 to an
electrolytic cell that includes a substrate with a metal plated
thereon, and applying electricity to the electrolytic cell to
electrochemically reduce the CO.sub.2. The metal can be selected
from, for example, Pt, Au, Ag, Zn, Co, Pb, Ni, Pd and Cu. In one
embodiment the substrate is plated with a metal porous dendritic
structure, such as a copper porous dendritic structure. Substrates
can include, but are not limited to, glassy carbon and titanium.
Electrolytes can include, but are not limited to, sodium chloride,
sodium carbonate, sodium bicarbonate and potassium hydrogen
carbonate.
[0029] The presently disclosed subject matter also provides an
electrolytic cell system that includes an electrolyte saturated
with carbon dioxide, a cathode that includes a substrate with a
metal plating, and a source of electricity capable of being applied
to the electrolytic cell. The metal can be selected from, for
example, Pt, Au, Ag, Zn, Co, Ni and Cu. In one embodiment the
substrate is plated with a metal porous dendritic structure, such
as a a copper porous dendritic structure. Substrates can include,
but are not limited to, glassy carbon and titanium.
[0030] In one embodiment, a metal porous dendritic structure is
obtained using a metal porphyrin salt. As used herein, "porphyrin"
refers to a cyclic structure composed of four pyrrole rings
together with four nitrogen atoms and two replaceable hydrogens for
which various metal atoms can readily be substituted. Porphyrins
may be substituted or unsubstituted. An example of a porphyrin is
chlorophyllin. Porphyrins, many of which are naturally-occurring,
can be obtained from commercial sources. Alternatively, porphyrins
can be synthesized. See, e.g., P. Rothemund (1936): "A New
Porphyrin Synthesis. The Synthesis of Porphin," J. Am. Chem. Soc.
58 (4): 625-627; P. Rothemund (1935): "Formation of Porphyrins from
Pyrrole and Aldehydes". J. Am. Chem. Soc. 57 (10): 2010-2011, each
of which hereby incorporated by reference.
[0031] In one embodiment of the presently disclosed subject matter,
an electrode is prepared by pulse and reverse pulse plating a
substrate with a copper porous dendritic structure using a copper
chlorophyllin salt as one of the copper sources. This electrode can
be used in the methods and systems described herein.
[0032] Metal Porous Dendritic Structures (PDS) (e.g., Copper Porous
Dendritic Structures) can be a high performance material in the
catalysis of carbon dioxide as well as air capture and electrolytic
reduction of CO.sub.2 due to the high surface areas as well as the
absorptive catalytic capacity of Copper PDS. For example, copper
PDS can solve one of the major difficulties in the electrolytic
reduction of CO.sub.2, as presented in the literature--constructing
a electrode which maximizes adsorption of gaseous CO.sub.2 in the
reduction reaction with H.sub.2 on the cathode surface. This can
allow a commercially feasible process linking electrolytic
reduction with air capture, and, in certain embodiments, create a
standard temperature and pressure (STP) Fischer Tropsch (FT)
device.
[0033] Electrodes can be created using a plating mechanism which
has been described. See, e.g., Nikolic N D, K I Popov, Lj. J.
Pavlovic, M G Pavlovic. "The Effect of Hydrogen Codeposition on the
Morphology of Copper Electrodeposits. I. The Concept of Effective
Overpotential:" Journal of Electroanalytical Chemistry, 558 (2006)
88-98, which is hereby incorporated by reference.
[0034] According to one non-limiting embodiment, a bath of copper
sulfate and sulfuric acid solution (10 g/L Cu, 32 g/L
H.sub.2SO.sub.4) can be prepared. An Autolab 4800 Potentiostat can
be used with a glassy carbon and copper PDS cathode and a platinum
wire anode. Copper Chlorophyllin salt
(C.sub.34H.sub.31CuN.sub.4O.sub.6.3Na Sigma Commercial Grade) can
be added to the solution at 1% by weight. Because chlorophyllin is
characterized by anodic attraction, the reverse pulse regime
creates regions of chlorophyllin membranes covering the dendritic
structures, creating additional diffusion-limited growth of
dendrites of a smaller scale. Pulse and reverse pulse
electrodeposition can be used to form microporous, copper PDS (SEM
photos included). A current density of pulsating regimes of -0.015
and 0.01 can be used, which translates into a current density of
-0.32 A/m.sup.2 and 0.21 A/m.sup.2 of 15 ms and 5 ms respectively.
This regime can be repeated numerous times (e.g., 10,000 times),
which creates a small pore on the glassy carbon. A microporous
correl structure results.
[0035] The presently disclosed subject matter provides electrodes
grown in this manner, as well as electroless plating of other nobel
metals such as, but not limited to, Pt, Au, Ag, as well as other
metals such as Zn, Co, Ni to the copper template to
electrochemically reduce CO.sub.2 to hydrocarbons (e.g., ethylene)
using electricity in an electrolytic cell which can use sodium
bicarbonate or potassium bicarbonate as the electrolyte, or
methanol. CO.sub.2 can be dissolved into electrolyte using a
membrane, such as a liquicell membrane. In certain embodiments,
potentials can vary from, for example, -0.5 V to -3 V vs. SHE.
[0036] Emobidments of the presently disclosed subject matter
provides rapid electrochemical reduction of CO.sub.2 to
hydrocarbons at current efficiencies of more than, for example, 100
times more than copper foil per gram. Unique products can also be
produced on the electrode including C.sub.2 to C.sub.6
hydrocarbons, formate, ethylene, propane, and methanol. In one
embodiment, ethylene is the primary hydrocarbon produced by the
electrolytic cell system.
[0037] Using the pulse reverse pulse technique along with copper
chlorophyllin additive, BET surface areas were measured between 20
to 41 m.sup.2/gram. Use of these electrodes can profitably produce
valuable hydrocarbons from carbon dioxide, producing near carbon
neutral fuels, while also taking advantage of future and existing
carbon credits for offsetting emissions.
[0038] The presently disclosed subject matter provides for the
electrolytic reduction of carbon dioxide. Further embodiments
provide a process linking electrolytic reduction with air capture,
creating a standard temperature and pressure (STP) Fischer Tropsch
(FT) device. The mechanics of dendrite formation and review of the
theoretical literature on fractal catalyst simulations is also
provided.
[0039] Porous dendritic metal foams can be used in electrocatalytic
applications, particularly the conversion of CO.sub.2 directly to
useful hydrocarbons, such as ethylene. Furthermore, because these
catalysts are both produced and applied in an electrochemical
environment, any lost catalyst area can be rapidly regenerated in
situ. These possible applications extend to porous copper,
platinum, and gold structures on reactions such as the
electrocatalytic reduction of CO.sub.2 to C.sub.2-C.sub.6
hydrocarbons, methanol, CO, hydrogen, formate, and other organic
compounds, with hydrocarbons being produced at large molar
percentages and current densities. The high surface area, coupled
with the microporous structure creates outsized absorptivity, while
the continuous structure of the foam allows for high electrical
conductivity. Finally, continuous absorption of product species
leads to further conversion of methane to higher hydrocarbons. BET
surface characterization and SEM scans show that dendritic
structures have higher surface areas than bulk materials as well as
non-dendritic powders. Furthermore, the electrocatalytic and
catalytic activities are tested using cyclic voltammetry and
calculations indicate that copper PDS have nearly a full order of
magnitiude higher BET surface area than dendritic powder, and more
than 100 times the electrochemical activity on reduction of carbon
dioxide to methane and other hydrocarbons than commercial copper
foil.
EXAMPLES
[0040] The present application is further described by means of the
examples, presented below. The use of such examples is illustrative
only and in no way limits the scope and meaning of the invention or
of any exemplified term. Likewise, the invention is not limited to
any particular preferred embodiments described herein. Indeed, many
modifications and variations of the presently disclosed subject
matter will be apparent to those skilled in the art upon reading
this specification. The invention is therefore to be limited only
by the terms of the appended claims along with the full scope of
equivalents to which the claims are entitled.
[0041] The experiment is conducted in three portions. The first
experiment involved growing porous fractals which maintained their
stability and cohesion to the surface of the substrate. The second
phase of the experiments were conducted to determine the surface
area of the dendritic pores, compared to spherical copper powder,
and dendritic copper powder. Finally, the third phase of the
experiment involved testing the efficiency of the copper PDS
electrode for electrocatalytic effects on the reduction of CO.sub.2
to higher hydrocarbons.
[0042] One purpose of the growth phase of the experiments are to
grow fractal surfaces which can be tested for catalytic activity.
Initially, only titanium and glassy carbon produced dendritic
structures on their surfaces in this particular example. This is
due to the low nucleation densities achieved on the surface of
these two substrates. Low nucleation densities result in high
current densities, which also have correspondingly high electric
potentials. Ultimately glassy carbon is used as substrate for
experiments because of the low nucleation densities achieved due to
the low conductivity of the glassy carbon, as well as the
repeatibility of the surface of glassy carbon. Low nucleation
densities on the surface of titanium are due to inconstitent
oxidation patterns. Finally, glassy carbon is a substrate of choice
in the literature when studying copper crystal growth.
[0043] A bath of copper sulfate and sulfuric acid solution (10 g/L
Cu, 32 g/L H.sub.2SO.sub.4) is prepared. An Autolab 4800
Potentiostat is used with a glassy carbon with copper PDS cathode
and a platinum wire anode. Copper chlorophyllin salt
(C.sub.34H.sub.31CuN.sub.4O.sub.6.3Na Sigma Commercial Grade) is
added to the solution at 1% by weight. Because chlorophyllin is
characterized by anodic attraction, as observed through visual
inspection, the reverse pulse regime creates regions of
chlorophyllin membranes covering the dendritic structures, creating
additional diffusion-limited growth of dendrites of a smaller scale
by limiting the exposure of cathodic surface area and concentrating
a high current density on the tips of new dendrites while
preventing structures previously grown from smoothing out with more
copper particles. The surface of dendrites after an anode phase of
a pulse is shown below to demonstrate the chlorophyllin anodic
attraction. Other additives used in experiments were PVA, PEG, and
PVP. Results of nucleation for each can be displayed.
[0044] Pulse and reverse pulse electrodeposition are used to form
microporous, copper PDS. A current of pulsating regimes of -0.015 A
and 0.01 A are used, which translated into a current density of
-0.32 A/m.sup.2. and 0.21 A/m.sup.2 of 15 ms and 5 ms respectively.
This regime is repeated 10,000 times, which creates a small pore on
the glassy carbon substrate. A microporous carrel structure
results. The conceptual advantages of pulse and reverse pulse
plating for standard electroplating applications is discussed in a
review by Chandrasekar and Pushpavanam (2007). It creates
dissolution, and the potential of new nucleations. Other metals
such as zinc and iron, which are known to produce dendrites, can
also be used as templates for copper and other metal electrodes
through electroless plating.
[0045] To measure the surface area of the copper PDS, as well as to
provide a comparison with other copper powders and structures, BET
surface area measurements were conducted. The theory of BET surface
area measurements can be found in Brunauer, S., P. H. Emmett and E.
Teller, J. Am. Chem. Soc., 1938, 60, 309. doi:10.1021/ja01269a023,
which is hereby incorporated by reference. The substrate is removed
carefully from the electrolyte. If too many pulses are used, pores
can lose their structural integrity. Too few pulses, and the pores
can be too readily oxidized upon contact with air. Pores are rinsed
with deionized water to remove residues of sulfuric acid, then
acetone is used to remove deionized water and prevent redissolution
of copper PDS. The pore is degassed for a period of six hours at
100.degree. C. in a Quantchrome Nova 3000 Surface Area Analyzer
under a nitrogen atmosphere to prevent oxidation. When degassed at
significantly higher temperatures, the PDS can lose its structure,
or can be oxidized into a hard brown crust. When degassed at lower
temperatures, residues can react with copper and can turn the
powder into a blue residue.
[0046] Additional dendritic powder, which are dendritic copper
grown at high curent densities without a pulsing regime, are
collected and analyzed as free copper which did not remain on
substrate. Furthermore, commercially available spherical copper
powder is also analyzed. After sample is degassed, pores are then
measured for BET surface area by scraping dendritic pores from
glassy carbon substrate into a sealed vacuum tube which is
evacuated to set pressures, and partial vapor pressures measured
with a transducer at each pressure point.
[0047] The resulting foam was collected in free form from a tube
chamber within the electrolytic cell. This setup is necessary to
maintain the high current densities necessary to produce the foam,
while also allowing for the flow of copper ions into the cell. Once
the tube is filled with copper, the resulting product is washed
with deionized water and placed in an argon atmosphere to prevent
oxidation of copper powder. A copper correl structure is also grown
on a glassy carbon substrate. The BET surface area of the intact
coral structure is measured. Cyclic voltametry is performed on the
copper electrode on the oxidation of CO.sub.2 to methanol to
compare the activity of the fractal catalyst with the activity of a
flat geometry deposit.
[0048] In the third section of the experiment, copper PDS grown on
glassy carbon substrate is tested as a electrocatalyst for the
reduction of carbon dioxide to higher hydrocarbons. A sealed
electrolytic cell is constructed isolating the anode from the
cathode so that samples of gas produced could be collected.
[0049] A 0.1 M Na.sub.2CO.sub.3 is prepared with deionized water,
and saturated with carbon dioxide by bubbling gas through solution
for one hour. A piece of commercially available, thin copper foil
(Alfa Aesar Cu foil Puratronic, 99.9999% (metal basis), 0.25 mm
thick) is used as an electrode in the reduction process. A MetroOhm
Autolab 4800 potentiostat is used, and a platinum wire
counterelectrode is used as well. Gas phase products are analyzed
using gas chromotography. A volume of 100 ml is extracted from the
cell after running the cell for 10 minutes to purge all air from
the system. The production of hydrogen and CO is not detected by
the GC. Its weight is determined to be 0.2 grams, which is
approximately 40 times the weight of the copper dendritic
electrode, which had a weight of 0.00503 grams. However, it's
apparent surface area is the same when projected to a two
dimensional plane.
[0050] This solution was then used as electrolyte for tests. In the
literature, regardless of the electrolyte used, whether it was
KHCO.sub.3, Na.sub.2CO.sub.3, or simply salt, the resulting
products did not change (Shibata et al 2008). A copper foil sheet
was prepared to have roughly the same surface area as the substrate
of glassy carbon. A cyclic voltametry was conducted to determine
the difference in activities between the copper foil electrode and
the copper PDS electrode. Gas was collected from the sealed
cathodic chamber and removed with a syringe. The resulting gases
were analyzed using an Agilent Gas Chromatography to confirm the
production of methane and ethylene. Furthermore, visual inspect
found that there was an oil slick on the surface of the water,
though this substance was not analyzed.
Results:
[0051] This results demonstrate that reactions are occurring at the
surface of copper fractal catalysts which do not occur on the
copper foil catalyst. Furthermore, the results demonstrate that
fractal catalysts have a high surface area which also have a high
electrical conductance. Finally, it shows that fractal catalysts
are two orders of magnitude more efficient per gram than copper
foil.
[0052] A copper gas diffusion electrode is fabricated which
addresses one of the major needs for improvement--making room
temperature and pressure, aqueous electrochemical reduction of
carbon dioxide to higher hydrocarbons feasible; this electrode is
at least two orders of magnitude more active per gram than an
equivalent copper foil. Furthermore, a CV (Cyclic Voltametry)
demonstrated that additional reactions occur in the copper PDS
electrode as compared to a copper foil electrode, giving some
support to the hypothesis that geometrical effect can play a
significant role in selectivity of products. A method was found to
grow surfaces which are significantly more complex, as measured by
BET surface area, than those produced in the literature using
techniques which have not yet been reported, namely the addition of
chlorophyllin. The interface of the copper surface also can be used
as a template for other catalysts, providing the potential for
creating unique electrocatalytic alloys.
[0053] Copper PDS electrodes demonstrated electrochemical reduction
of CO.sub.2 to hydrocarbons with a peak occurring at a slightly
lower potential. Because this process occurs due to adsorption on
electrode surfaces, it is possible the gaseous diffusion electrodes
would produce higher yields than a simple foil electrode. Copper
PDS has very significant surface area and a very low volumetric
density. In addition, copper PDS displays many irregularities on
its surface, a condition that has been found to be conducive to
catalytic reactions, perhaps due to local concentrations in
electric field potentials at boundary discontinuities. It is
interesting to note that when these structures were placed into the
saturated solution of sodium bicarbonate, bubbles nucleated at a
far higher rate on the structures, than elsewhere in the solution
or other electrodes.
[0054] When the CV is run comparing the copper PDS electrode and a
copper foil electrode, an interesting effect is detected. It is
immediately apparent that the copper structured into a fractal
produced rates of reduction higher than the electrode with a mass
almost 40 times greater than the copper dendrite. This is
unexpected. The peaks occur at nearly the same places for both the
copper foil and the copper foam, though the copper foam displays a
slightly lower peak voltage, making the process energetically more
efficient. The PDS peak is relatively longer, which can imply there
are two competitive processes occurring: the production of methane
and perhaps other higher hydrocarbons from hydrogen and carbon
dioxide. As the potentials increase, the higher activity of the
copper PDS, as compared to the copper foil is apparent. The copper
PDS is almost 4 times more efficient, and almost 160 times more
efficient per gram.
[0055] Finally, the CV showed another interesting effect for copper
PDS gaseous diffusion electrodes. On the negative sweep of the CV,
the oxidation peaks for the copper gas diffusion electrode differed
from the peaks of the foil electrode. The gas diffusion electrodes
showed two peaks, while the foil electrode only showed a single
peak. The dual peaks implies that two reverse reactions are
occurring, each of a slightly different reaction energy, as shown
in FIG. 1.
[0056] It is interesting that significant production of side
reactions occur, and are likely due to the porous structure of the
electrode, since the gas reactions only occur when gas is captured
on the porous electrode and adsorbed onto the surface of the
catalytic metal. Highly dispersed metal particles are not a better
geometry when compared with a porous structure, given the
requirements of gaseous adhesion for electrolytic conversion of
carbon dioxide to occur. Instead, the unique, fractal geometry of
the internal surface of the structure creates a reaction surface,
which also traps individual reactant and product gas molecules and
confines them within what is essentially a knudsen diffusion
regime, though the scalar accuracy of this non-binding proposal can
be verified with simulations. The production of ethylene and
methane were confirmed with gas chromatography with dendritic
copper grown at high current densities.
Results of Experimental Growth of Dendritic Copper:
[0057] The result of the first phase of experiments, which is to
grow catalytic fractal surfaces, resulted in SEM scans that showed
PVP (FIG. 2) prevents formation of dendritic nucleations, while no
additives, i.e., no PVP, resulted in irregular deposits (FIG. 3).
However, PEG created conditions which allowed for dendritic
structures to form on the substrate, though dendrites were much
larger than those grown with chlorophyllin (FIG. 4).
[0058] Copper chlorophyllin is used because of the chelated copper
at the center of the molecule, as well as its characteristic anodic
attraction during pulsing cycles. SEM photographs are taken of the
structures after a short number of pulses, as well as longer
pulses. From these photographs, it can be seen that the addition of
chlorophyllin resulted in the further branching of the nucleating
copper particles. Photographs are given from spherical nucleation
of copper obtained without additives, to the nucleation of copper
with more structure on smaller scales with chlorophyllin as an
additive. Nucleation without additives can be controlled to 100 nm.
However, using a chlorophyllin additive, structures can be resolved
to 50 nm and particles take on a popcorn-like structure. A CV of
plating solution with chlorophyllin and without chlorophyllin shows
that chlorophyllin increases the resistivity of the solution
significantly. Because of the small concentrations of chlorophyllin
added to achieve this effect, it is likely that the resistance
effect occurs at the electrode surface rather than due to lowering
the conductivity of the electrolyte itself. In FIG. 5 below, the
figure on the left shows a high acid copper solution without the
addition of chlorophyllin. The second graph on the right of FIG. 5
shows the same high acid copper solution with chlorophyllin. In
FIG. 5, the top line refers to copper plating and the bottom line
refers to copper dissolution. A close examination of the CV shows
that the two initial peaks in the forward sweep and reverse sweep
occur at the same potentials. However, in the forward sweep of the
electrolyte with chlorophyllin, the peak oxidation of copper into
ionic form is suppressed, and the final peak occurs at a potential
that is 0.3 volts higher than in the solution without the additive.
This shows that some another process is occurring at the anode
during reduction of copper. This likely occurs due to adhered
chlorophyllin desorbing off the surface of the electrodes as
potential increased. Furthermore, the peak plating current is also
lower in the solution with chlorophyllin, giving support to the
idea that chlorophyllin increases the resistivity of the system by
preventing the flow of ions.
[0059] In an embodiment represented by this example, copper
chlorophyllin is found to have a considerable effect upon the
structure of the copper crystals. Chlorophyllin undergoes anodic
attraction during alternating pulses, creating a situation in which
the chlorophyllin coats the developing dendritic fractal structure,
creating regions of even higher thermodynamic instability allowing
additional growth of dendrites on the already complex surface. FIG.
6 shows an anodic pulse, and the resulting chlorophyllin film
coating the copper electrode.
[0060] The chlorophyllin can produce this effect because it is
selectively pulled from the fractal structure in a way that exposes
surfaces to rough, protruding points which promote additional
dendritic growth. Based on the electrochemistry of chlorophyllin,
and its effect on the electrodeposition of copper, proper surface
areas can be determined on which reactions can occur. Copper
nanostructures are resolved down to length scales as low as 10
nanometers, nearly a full order of magnitude smaller than those
previously reported in the literature. FIG. 7 and FIG. 8 show a
highly structured dendritic copper particle resolved to 500 nm.
These particles have much higher complexities than other particles
reported in the literature, and most likely form due to the
interaction of the process with copper chlorophyllin. FIG. 9 and
FIG. 10 show the result of dendritic agglomeration after 500 pulses
and 1000 pulses.
[0061] The result of high current densities running through the
relatively low number of nucleating particles results in higher
formation of hydrogen bubbles, since the potentiostat is set, in
this embodiment, to deliver a constant current rather than a
constant potential. Thus the total current is distributed over a
much smaller surface area. While dendrites still formed at current
densities insufficient to produce hydrogen, hydrogen bubbles serve
as a template for the formation or micropores as well as the
protrution of dendrites into the micropores. Because the interface
of gas and liquid in bubbles form thin channels of liquid, the
growth of copper becomes diffusion limited, creating dendritic
structures of varying crystal structures. Furthermore, this
dendritic pore forms a single, conductive crystal, with the
potential to vibrate and transmit phonons through its structure.
FIG. 11 shows the incipient formation of copper PDS after 2000
pulses. FIG. 12 shows the final copper PDS grown on glass carbon.
The diameter of the pore is about 2 mm and it protrudes about 1 mm
off the surface of the glass carbon. FIG. 13 shows closeups of the
fully formed copper PDS, which take the form of buds, leaves,
stalks and stems. The space between pore openings are filled with
dendritic copper, structured down to only a few nanometers, as
shown in FIG. 14. The outside surface of PDS can be further
controlled based on, for example, a program of finishing
pulses.
Surface Area Results
[0062] Multiple BET surface areas are taken. While not being bound
by any particular theory, it is believed that due to the high
reactivity of the copper dendritic powder with air, measurements
varied from 19.2 meters square per gram to 41.2 meters squared per
gram. Weighing errors are also likely due to the small amount of
dendritic powder available to be weighed. However, the median
measurement is 29.45, and this corresponds well to 20 meters square
per gram previously reported as the BET measurement of a copper tin
foam alloy grown under similar conditions, and due to the hydrogen
bubble templating. See Shin, H, M. Liu, Copper-Tin electrodes for
lithium batteries," Adv. Functional Materials 15, No 4, April
(2008). The increased surface area can be due to the dendritic
structures protruding from the pores--structures which the Cu--Sn
alloy does not possess.
[0063] A <150 micron copper powder (Alrich 99.999% pure <150
micron powder) is tested, a surface area of 1.28 is obtained.
However with the fractal powder a BET surface area of 4.26 is
obtained despite the visibly larger particle sizes. When the
structure is held intact on a piece of glassy carbon, the BET
surface area is almost 1 order of magnitude higher, with readings
ranging between 19.5 m.sup.2/gram and 41.2 m.sup.2/gram. The high
range in the results could be due to errors in degassing the
samples. Because of the low stability of the samples used in this
Example, degassing at temperatures above 150.degree. C. caused the
sample to fall apart, constrict, or otherwise disappear. Thus,
degassing is conducted at a relatively low temperature and not all
gas can have been equally driven away from the samples. However,
this error would tend to bias measurements downwards rather than
upwards since sample absorptiveness and sample mass will be
underestimated.
TABLE-US-00001 TABLE 1 Sample Mass v. BET Measurement Sample Mass
BET Measurement 0.0055 19.5 0.0022 26.3 0.0032 31.99 0.0052 41.2
0.003 22.4 Average 28.278
[0064] Interestingly, a comparison of BET surface measurements
gives insight into the different surface areas of powders. Using 2
grams of copper powder, <150 microns in diameter, this powder
has a surface area of only 1.14 meters squared per gram. Even this
is a high reading, as there exist reports of copper powders with
surface areas as low as 0.5 meters squared per gram. Even dendritic
powder grown under high current densities showed a surface area of
4.15 meters squared per gram. Thus, the dendritic structures
themselves, as well as the arrangement of the dendritic metals into
a foam, both contribute to the increased absorptivity of the
dendritic foam. This lends further support to the unique and
positive geometries of the dendritic microporous foam, along with
the combination of dendritic structures and micropores, both
contributing to the gas absorption capabilities of the foam.
[0065] The average pore size of the foam in this example is 10 to
50 microns, which is consistent with those reported in the
literature. In certain embodiments that employ higher system
pressures, pore sizes can be reduced through the reduction in
bubble size of template hydrogen gas. While not being bound by any
particular theory, it is believed that the tips of dendrites could
be resolved to 50 nanometers, and display a highly textured surface
which is also self-similar across multiple scales.
[0066] The high surface area, as well as the electrical
conductivity of this material, are noteworthy. The use of these
structures in the reduction of CO.sub.2 to ethylene, methane,
and/or other hydrocarbons electrolytically, at room temperatures,
with nothing more than a saturated solution of carbon dioxide and
sodium bicarbonate, can be a highly economic means of producing
natural gas from carbon dioxide.
Applications for CO.sub.2 to Liquid Fuels:
[0067] Direct electrochemical reduction of CO.sub.2 allows for a
simpler process and also, by avoiding high temperature and pressure
reactors, also provides the process production rates to take
advantage of baseload surplus electricity (Gatrell 2008). Because
of the high absorptivity of these structures, the absorptive
resins, in one particular embodiment, are to be used in an
electrolytic cell, optionally functionalized onto the copper, to
produce a direct means electrolytic reduction of CO.sub.2 to
ethylene, methane and/or other hydrocarbons on the surface of the
resin support.
[0068] Copper and platinum display catalytic activity on various
toxic and undesirable substances, pollutants, residues, and
greenhouse gases. Copper is a particularly good catalyst because of
its relative low cost, as well as its proven applications in the
breakdown and detoxification of organic compounds. High surface
area copper can provide rapid decomposition and neutralization of
toxins such as hydrazine, trichloroethylene, nitrobenzene, and
phenols, as well as the potential for applications in other fields,
such as the electrolytic reduction of carbon dioxide to methane,
methanol, and other hydrocarbons, and rapid, high current energy
generation in fuel cells. Solely for pupose of convenience, this
section will discuss the electrolytic reduction of CO.sub.2 on
copper electrodes.
[0069] The electrochemical reduction of CO.sub.2 on a Cu electrode
has gained attention for the removal and conversion of CO.sub.2 to
more useful products, the electrocatalytic activity of Cu
electrodes, and the electrode activity as a function of different
electrolyte concentrations, temperatures and pressures. See Lee,
Jaeyoung, Yongsug Tak: "Electrocatalytic activity of Cu electrode
in electroreduction of CO.sub.2; Electrochimica Acta 46 2001
3015-3022; Cabrera, Carlos R., Hector De. Jesus Cardona, and
Cynthia del Moral: "Voltammetric study of CO.sub.2 reduction at Cu
electrodes under different KHCO.sub.3 concentrations, temperatures,
and CO.sub.2 Pressures." Journal of Electroanalytical Chemistry 513
(2001) 45-51.
[0070] The creation of porous gas electrodes, which facilitate the
conversion of saturated CO.sub.2 to unsaturated CO.sub.2, is one
major need in improving the efficiency and commercial feasibility
of electrolytic reduction of carbon dioxide to ethylene, methane,
methanol as well as formic acid and other hydrocarbons. It is
hypothesized by Gattrell et al (2006) that the reduction of
CO.sub.2 to CH.sub.4 occurs not from dissolved CO.sub.2 but from
gas phase CO.sub.2, due to adsorbed CO.sub.2 on the surface of the
electrode, as well as the adsorption of CH.sub.4 and higher
hydrocarbons on the surface of the gas electrode. The first
reaction is CO.sub.2+e-.fwdarw.CO.sub.2ads.sup.-. For many other
catalysts which have high CO adsorption, the production of CO is
favored. CO+ both physisorbs and chemisorbs onto copper and is
enhanced by surface defects and can form temporary carbonate
structures with the copper.
[0071] The reactions involved for the electrolytic reduction of
CO.sub.2 to higher chain hydrocarbons are given below; all
reactions are given vs. Standard Calomel Electrode. (Collin and
Sauvage 1989):
CO.sub.2+2H.sup.++2e.sup.-.fwdarw.CO+2H.sub.2O E.sup.o'=-0.52
V.
CO.sub.2+2H.sup.++2e.sup.-.fwdarw.HCOOH E.sup.o'=-0.61 V.
CO.sub.2+8H.sup.++8e.sup.-.fwdarw.HCHO+H.sub.2O E.sup.o'=-0.48
V.
CO.sub.2+8H.sup.++8e.sup.-.fwdarw.CH.sub.3OH+H.sub.2O
E.sup.o'=-0.38 V.
CO.sub.2+8H.sup.++8e.sup.-.fwdarw.CH.sub.4+2H.sub.2O E.sup.o'=-0.24
V.
[0072] A rough calculation of the cost of methane can be
calculated. A high current efficiency of 60% hydrocarbons, with the
balance being hydrogen, formate, and CO can be achieved using a
simple copper foil. Under complete conversion of CO.sub.2 to
methane, one would obtain 44 grams CO.sub.2 per 16 grams of
methane. The price of 1 mmBTU of natural gas is $4.304
(www.nymex.com). There are 97 cubic feet in 100000 BTU of natural
gas and thus there are 970 cubic feet in 1 mmBTU of natural gas.
This converts to 27467.341 liters. Using the ideal gas law, PV=nRT,
the number of moles of methane can be determined. Thus, there are
1225.49 moles in 1 mmBTU of natural gas. Which will require the
same amount of moles of carbon dioxide to produce, Assuming the
cost of carbon dioxide is $100/ton and 1 ton is 1000 kg, 1 ton of
CO.sub.2 would produce 1000000 g/53922 g=18.5453 mmBTUs of natural
gas. Thus, based on the raw material cost of carbon dioxide, the
market value of the product would be 18.5453*4.304=$79.82. With a
carbon offset price of $35 per ton of CO.sub.2 it is conceivable
that natural gas can be produced from CO.sub.2 profitably if the
cost of baseload electricity would be negligible in this process.
This also assumes a 60% efficiency with other valuable products
which produce, neglected in the analysis, 1 mol of CO.sub.2 would
require 8 mols of electrons or 13.33 coulombs of energy. Assuming 1
cent per kilowatt hour, it would cost 13.33 cents per kilomol of
CO.sub.2 produced, since each mole of CO.sub.2 requires 13.33
coulombs of electricity operating at 60% efficiency. The minimum
current that can be used in the reaction would be on the order of
0.5*10.sup.-2 assuming a resistance of 100 ohms. Based on this
number, the minimum power requirements for the reaction to proceed
is 100*(0.5*10.sup.-2).sup.2 or 0.0025 watts or 0.0000025
kilowatts. To reduce 1 ton of CO.sub.2 to methane would require
13.33*1225.5*18.55 Columbs. At an amperage of 0.5*10.sup.-2
C/second as the lower bound, 1 ton of CO.sub.2 would require
(13.33*1225.5*18.55)/0.005=60606244.7 seconds or 16326 hours. Thus
16326*0.0000025=0.040815 kilowatt hours. Of course one would not
expect the reaction to proceed at such low amperages, and one could
turn up the reaction rates very significantly and still only incur
relatively reasonable electricity costs. The primary costs are
CO.sub.2 feed stock, and water.
[0073] Interestingly, a mixture of Fischer Tropsch products of up
to C.sub.6 hydrocarbons are produced from CO.sub.2 at room
temperature. Carbon chain products are observed for copper
electrodes which had been polished under an acid solution. See
Shibata, Hirokazu Jacob A. Moulijn and Guido Mul: Enabling
Electrocatalytic Fischer-Tropsch Synthesis from carbon dioxide Over
Copper-Based Electrodes. Gattrell et al (2007) propose that five
cells connected in series would be able to convert 97% of Carbon
Dioxide fed into the system producing a final product of hythane.
Other studies have been conducted on the effect of copper crystal
structure on selectivities (Hori et al 2003) and the effect of
alloying other metals to copper (Mho et al 2000).
[0074] Other methods of electrocatalytic conversion of CO.sub.2
involve alternative metals such as TiO.sub.2, Pt, as well as using
methanol as an electrolyte instead of water (Centi et al 2007).
Higher H.sup.+ concentration increased the yield of hydrocarbons in
the reduction of CO.sub.2. The results at different temperatures
and KHCO.sub.3 concentrations support the idea of the presence of
CO as an adsorbed intermediate and the existence of a region of
lower pH near the electrode surface, respectively. Different
pressures also change the current efficiency and products at the
copper electrode. Voltammetric study of CO.sub.2 reduction at Cu
electrodes under different KHCO.sub.3 concentrations, temperatures
and CO.sub.2 pressures (DeJesus-Cardona et al 2002).
[0075] Hori et al (1993) tested the selectivity of various metals
for CO production from CO.sub.2 and found that
Au>Ag>Cu>Zn>>Cd>Sn>In>Pb>Ti>Hg,
though copper is still the best producer of hydrocarbons
electrolytically. Mediation with metal porphyrins is also studied
and found to be an effective means of electrolytic reduction of
CO.sub.2. See, Ogura, Kotaro, Ichiro Yoshida: Electrocatalytic
Reduction of CO.sub.2 to Methanol part 9: Mediation with Metal
Porphyrins." Journal of Molecular Catalysis, 47 (1998) 51-57,
hereby incorporated by reference. Copper chlorophyllin, which will
reduce carbon dioxide in air at the same rate as a leaf, is of
particular note. Losada et al (1995) used polymer films of
polypyrrole cobalt(II) to reduced CO.sub.2 electrolytically.
[0076] While poisoning of catalyst has been reported to occur as a
major problem of deposition (Yano et al 2001), Hori et al (2005)
determined that the deactivation of copper electrodes are, in
reality, due to impurities in the prepared solution which could be
eliminated through pretreatment. Bockris discusses the importance
of preelectrolysis of electrolyte solutions when studying electrode
processes in detail (Bockris 1993, Bokris 1970). Catalyst poisoning
is not detected on the surface of the catalyst by the CV. The CV
shows the current flow as a function of potential. Current is
directly proportional to the reduction of CO.sub.2 to CH.sub.4.
[0077] One of the major difficulties with using electrochemical
cells for industrial conversion of CO.sub.2 to hydrocarbons is the
large geometries necessary for foil electrodes to produce
industrial quantities. These structures are especially useful for
electrocatalytic application which require high interfacial surface
areas or high absorptions of gas reactant species. The
extraordinarily high activity potential of the presently disclosed
copper chlorophyllin catalyst is due to the high surface area of
the dendritic structures as well as the micropores and nanopores in
the scaling, self-similar structure, which allows for rapid
absorption of reactant species. These structures absorb a notable
amount of gas, as determined by a BET test.
[0078] Aqueous electroreduction of CO.sub.2 to ethylene, methane
and other hydrocarbons could be a significant strategy for
upgrading the value of CO.sub.2 to enhance the economic feasibility
of air capture and other CCS (carbon capture and storage)
technologies. This is particularly true with ionic resin exchange
membranes which capture CO.sub.2, as the technology requires the
immersion of the CO.sub.2 saturated membranes into water to
facilitate the desorption of CO.sub.2. During the desorption
process, the resulting solution can be saturated with CO.sub.2 and
fed into an electrolytic cell for the conversion of the gas into
hydrocarbons. This could be facilitated with copper dendritic gas
diffusion electrodes, which would allow for a high efficiency
conversion with the minimal use of copper, a catalyst that is
already cheap and plentiful.
[0079] Furthermore, Fisher-Tropsch (FT) synthesis can also be
conducted from the higher hydrocarbons produced from the initial
copper electrodes. Fisher Trospch synthesis can also be conducted
electrolytically at room temperature. The limiting factor again is
the solubility of the gas in the electrolyte, as well as the
ability of electrodes to adhere gaseous reactant species.
[0080] Through the use of double templating, copper dendrites can
be converted to metals thermodynamically preferred, such as gold,
silver, and platinum. Furthermore, by producing zinc dendrites,
electrolytically, a similar process can be used to produce iron and
cobalt dendritic electrodes which produce similar gaseous effects.
Table 2 below gives the reduction potentials of important
electrolytic reduction reaction which can be utilized with double
templating to produce gaseous electrodes with high
efficiencies.
TABLE-US-00002 TABLE 2 Standard Potentials. Source: Handbook of
Chemistry and Physics, 86.sup.th Edition. Half-reaction E.degree.
(V) Zn2+ + 2 Zn(Hg) -0.7628 Zn2+ + 2 Zn(s) -0.7618 Cr3+ + 3 Cr(s)
-0.74 Fe2+ + 2 Fe(s) -0.44 C2OO2(g) + 2 + + 2 HOOCCOOH(aq) -0.43
Cr3+ + e- Cr2+ -0.42 Cd2+ + 2 Cd(s) -0.40 Cu2O(s) + H2O + 2 2 u(s)
+ 2OH- -0.360 Co.sup.2+ + 2 ==> Co(s) -0.28 Ni.sup.2+ + 2 ==>
Ni(s) -0.25 Pb.sup.2+ + 2 ==> Pb(s) -0.13 2CO.sub.2(g) +
2H.sup.+ + 2 ==>HCOOH(aq) -0.11 2HCOOH(aq) + 2 .sup.+ + 2
==>HCHO(aq) + H.sub.2O -0.03 2 .sup.+ + 2 ==>H.sub.2(g)
0.0000 C(s) + 4 .sup.+ + 4 ==>CH.sub.4(g) +0.13 2HCHO(aq) + 2
.sup.+ + 2 ==>CH.sub.3OH(aq) +0.13 Re.sup.3+ + 3 ==>Re(s)
+0.300 Cu.sup.2+ + 2 ==> Cu(s) +0.340 Cu.sup.+ + e.sup.- ==>
Cu(s) +0.520 2CO(g) + 2 .sup.+ + 2 ==>C(s) + H.sub.2O +0.52
I.sup.3- + 2 ==> 3 +0.53 I.sub.2(s) + 2 ==> 2 +0.54
PtCl.sub.4.sup.2- + 2 ==>Pt(s) + 4 l.sup.- +0.758 Ag.sup.+ +
e.sup.- ==> Ag(s) +0.7996 Pd.sup.2+ + 2 ==> Pd(s) +0.915 Au
[AuCl.sub.4].sup.- + 3 ==> Au(s) + 4 l.sup.- +0.93 Au
[AuBr2].sup.- + e.sup.- ==>Au(s) + 2 .sup.- +0.96 Au
[AuCl2].sup.- + e.sup.- ==>Au(s) + 2 l.sup.- +1.15 Au.sup.3+ + 3
==>Au(s) +1.52 Au.sup.+ + e.sup.- ==> Au(s) +1.83
[0081] Any metal with a more positive electromotive potential can
undergo electroless plating, in which metal ions which have a
higher EMF will spontaneously exchange ions with the metal of a
lower EMF. Thus, from the porous copper structure, platinum,
silver, palladium and gold can be plated electrolessly to form
dendritic pores of a similar structure. Furthermore, if zinc leaves
are grown instead of copper leaves, a larger array of potential
porous dendritic electrodes could be produced from a wide variety
of metals, since zinc has a relatively low EMF. Thus, an
electroless process could be used to replace zinc with chromium,
iron, nickel or cobalt, all of which can play significant roles in
Fischer Tropsch synthesis.
[0082] A further application of the presently disclosed subject
matter is the use of carbon nanotubes as electrodes for the further
refining of hydrocarbons into FT synthetic fuels. Since the
experiments performed are conducted on glassy carbon, a relatively
low surface area substrate with a low conductivity and activity
(Rozwadowskp 1979), improvements in current efficiencies for
reduction of carbon dioxide can be obtained if glassy carbon
substrates are replaced with a carbon nanotube substrate as a
heterogeneous catalyst support due to the increased absorptive,
conductance, and electrochemical activity of nanotubes (Planeix
1994). Many uses of nanostructured electrodes have already been
found for electrolytic applications (Wang 2004) for such
applications as sensors (Pietrobon et al 2009, Welch et al 2006),
fuel cells (Lien et at 2005), and fuel conversion (Tong 2007) and
reforming of methane (Pawelec 2006). Direct plating of metal
catalyst particles has found some success, though chemical means
have been the dominant method of electrode preparation (Yao et al
2004, Yang et al 2009). Nanotubes are already a promising route for
high pressure and temperature FT synthesis (Prinsloo et al 2002,
Serp et al 2003), including the direct impregnation of high
activity catalysts such as cobalt (Choi et al 2002) onto carbon
nanotube structures, which has been shown to increase yields of
lighter hydrocarbons and lower the peak temperatures of the
reaction (Tavasoli et al 2008, Lu 2007) as well as selectivities of
specific hydrocarbons (Lordi et al 2001). A combination of the
capacity of nanotubes to adsorb and store hydrogen (Mishra et al
2008), as well as its demonstrated high electrochemical activity
when decorated with noble and near noble metals (Sun et al 2005,
Tang et al 2004, Li et al 2004, Tsai et al 2007, Georgakilas et al
2007) along with the ability of porous copper dendrite to adsorb
carbon dioxide and hydrocarbons, make the combination of the two
particularly interesting for reduction of CO.sub.2 as well as FT
synthesis. No studies on CNT electrolytic reduction of CO.sub.2
have been reported in the literature.
Means of Producing Shape Controlled Nanoparticles:
[0083] This section will review electrochemical mechanisms of
producing fractals and dendrites as well as other shape-controlled
nanoparticles. It will first discuss electrodeposition and some of
the dendritic structures produced with this process. It will then
discuss sonochemistry and sonoelectrochemistry as another means of
producing fractal nanostructures. Finally, the mechanisms of
fractal formation for copper is briefly discussed.
[0084] Electrodeposition:
[0085] Electrodeposition has found application for creating
nanostructures with unique properties. Electrodeposition provides a
high degree of control and repeatability for production of
nanoparticles, including shape control as well as size control,
depending upon the applied currents and potentials, as well as
nucleation characteristics of electrode materials. See Liu, H. F.
Favier, K Ng, M P Zach, and R M Penner: "Size Selective
Electrodeposition of Meso-scale Metal Particles: a general method."
Electrochimica Acta 47 (2001) 671-677; Radisic, Aleksandar Philippe
M. Vereecken, James B. Hannon, Peter C. Searson, and Frances M.
Ross: "Quantifying Electrochemical Nucleation and Growth of
Nanoscale Clusters Using Real-Time Kinetic Data, Nanoletters (2006)
Vol, 6 No 2. 238-242.
[0086] Furthermore, this technique is well understood, and is both
economical and fast. Finally, the product of electrodeposition can
be harvested directly from electrodes rather than slowly separated
out of a mixture, which is often the case in the production of
nanoparticles through chemical means. Electrodeposition has been
used to produce nanowires directly on carbon nanotubes.
Electrodeposition goes a long way towards solving the problem most
nanoparticles face: the lack of stability that other methods such
as chemical reduction as well as the method of microwave
irradiation which are more difficult to structure into a stable,
repeatable configurations. Particles can be deposited directly onto
a supporting structure such as nanotubes. Catalytic metals relevant
to FT synthesis can be deposited unto carbon nanotubes and other
carbon substrates such as glassy carbon as supports include
platinum and platinum-ruthenium, gold and silver. See, e.g., Auer
E, Freund A, Pietsch J, Tacke T: Carbons as Supports for Industrial
Precious Metal Catalysts. Appl Catal A. 1998; 173: 259-71.
[0087] Sonoelectrochemistry:
[0088] Sonoelectrochemistry has also been used to produce fractal
and dendritic nanostructures. In order to understand this method,
Sonochemistry must first be discussed, and involves using an
ultrasonic horn to agitate liquid systems. Sonochemical effects
occur because of acoustic cavitation which form as the peaks and
troughs of an ultrasonic wave pass rapidly through the liquid
medium creating regions of rarification and attenuation, See
Adewuyi, Yusuf G: "Sonochemistry: Environmental Science and
Engineering Applications." Ind Eng. Chem. Res. 2001 40(22),
4681-4715 DOT 10.1021/ie0100961; Mason, Timothy J., "Large Scale
Sonochemical Processing: Aspiration and Actuality." Ultrasonics
Sonochemistry 7 (2000) 145-149. This causes formation of bubbles,
which are then caused to implode by the moving pressure wave of
sound. This results in two regions of enhanced chemical activity:
in the gas within the bubbles which reach temperatures of up to
5200 K, and along the boundary between the water and the gas phase,
which can reach temperatures of 1900K. Hydrodynamic models of
cavitation also estimate pressures of reach betwen 1000-10000 bars.
See Suslick, Kennth S. Taeghwan Hyeon, and Mingming Fang:
"Nanostructured Materials Generated by High-Intensity Ultrasound:
Sonochemical Synthesis and Catalytic Studies." Chem. Material. 1996
8, 2172-2179. (Suslick et al 1986).
[0089] Sonochemistry has been used to produce iron oxide
nanoparticles when they are ligands of organic particles. These
iron nanoparticles of 20 nm clusters of 2-3 nm smaller
subcomponents displayed kinetics of up to 10 times higher than the
bulk form. Furthermore, this method is also reported to improve
iron selectivity when loaded onto a silica substrate through
sonification. Sonification also produces OH radicals which explains
many of its effects in the environmental engineering practices,
such as phytocatalysis of pollutants. However, the also gives it
potential for functionalizing the surface of carbon nanotubes as
well as the surfaces of electrodes and catalyst supports for
catalyzing reactions such as methanation. See Tong, Hao, Hu Lin Li,
Xiao Gang Zhang: Ultrasonic Synthesis of Highly Dispersed Pt
Nanoparticles Supported on MWCNTs and Their Electrocatalytic
Activity Towards Methanol Oxidation. Carbon 45 (2007)
2424-2432.
[0090] Sonoelectrochemistry couples the power ultrasound to
electrochemistry. Kinetics and cavitation are the two main avenues
through which sonoelectrochemistry produce its unique results on
the nanoscale. Microjets are generated at the electrode surface by
the cavitation events with speeds of up to 100 msec. The setup
should include an ultrasonic immersion horn probe in which the horn
tip can be placed inside the electrochemical cell, producing a
sonoelectrochemical cell. The other components would be a graphite
counter electrode, Ar inlet degassing unit, Pyrex reservoir to
maintain thermal conditions, a Titanium tipped sonic horn, an SCE
reference electrode, and Pt 102 resistance thermocouple. A thorough
review of the setup can be found in Compton Richard G, John C.
Eklund, Frank Marken, Thomas O Rebbitt, Richard P. Akkermans and
David N. Waller. "Dual Activation: Coupling Ultrasound to
Electrochemistry--An Overview." Electrochimica Acta Vol 42 No 18 pp
2919-2927, which is hereby incorporated by reference.
[0091] According to the literature, sonication enhances current
densities of the electrochemical system. Even uncontrolled
electrode geometries placed in an ultrasonic bath, enhancing
currents to 10 times high than unsonicated electrodes. On glassy
carbon electrode surfaces, significant pitting is observed.
However, activation of the carbon is also observed, and is
theorized to arise from OH-- radicals produced in the cavitation
bubble, which then react with the surface of the glassy carbon
electrode. This activation is not likely due to increased BET
surface area. OH-- fictionalization of glassy electrodes led to
higher rates of electrodeposition of PbO.sub.2. Other aspects which
can be controlled are the frequency and intensity, pulsing
intervals and lengths, gases in dissolved in solution, pressure and
temperature, concentration of solute, and geometry and location of
sonic sources. The enhanced reactions spurred by
sonoelectrochemical practices are due to a thinning of the
diffusion layer between the electrolyte and electrode.
Sonoelectrification of CNTs with SbSI has been used to prepare
nanorods with the CNT matrix and Co/Fe alloys are also produced
with this method. Nowak et al (2009) in their study use the high
pressures and temperatures formed by the cavitation bubbles to form
nanorods within the CNTs.
[0092] Additives such as PVA have been used in the sonochemical
process to prevent the agglomeration of particles as they are
deposited. Hass et al (2008) used a sonoelectrochemical method to
synthesize copper dendrite nanostructures. See Haas, Iris,
Sangaraju Shanmugam, and Aharon Gedanken, "Synthesis of Copper
Dendrite Nanostructures by a Sonoelectrochemical Method." Chem.
Eur. J. 2008, 14, 4696-4703. Because sonochemistry relies on
ultrasonic pulses that produce small bubbles which collapse very
quickly (Compton 1997), this can explain why dendritic structures
form. Lead Oxide nanostructures are created using ultrasonic pulses
on a glassy carbon electrode (Garcia et al 1998). It is likely that
these dendritic structures form as a powder, which are later linked
together on the carbon matrix by the intereaction between the
polymer chains which hold the particles together and prevent them
from agglomerating, as well as the interaction between the PVA and
the carbon matrix. PVA functions by forming a polymer matrix which
creates this effect, while the --OH group allows for electric
interaction between particles, which would be prevented from
occurring by the surfactant PVP. They concluded that neither the
electrode, nor the pulseform or pretreatment made any difference in
the dendritic structures formed, and instead these formed only
after on the carbon-copper matrix used in TEM studies. Haas
reported that the BET surface area of the dendritic structure is
less than 2 m.sup.2/gram.
[0093] Haas does not explain the mechanism of dendrite and fractal
formation beyond suggesting the electronic interaction. However,
given the results of the formation of copper dendrite foam, it
seems likely that dendritic powder formation occurs due to the same
mechanism, whereby bubble formation create diffusion limited
conditions which promote the formation of dendritic structures.
Their method is interesting in that they have a 300 ms pulse of
electricity followed by a 250 ultrasonic pulse. The electric pulse
causes the copper to be reduced into a polymer matrix formed on the
PVA, which is then ablated off the electrode by the ultrasonic
pulse. Then searching for the deposition of dendritic fractal
structures which had dimensions between 1.74 and 1.76, and had
details of up to 50 nm in resolution, these dendrites are dependent
upon the interaction between the colloid solution and the interface
on which it is prepared to be scanned rather than from any inherent
activity from the sonoelectrochemical cell. The major contribution
of the sonoelectrochemical cell is to create nanoparticles from
reduction of copper, and then the stabilization of these
nanoparticles by the PVA. Intriguingly, the dendritic structures
only formed on a copper carbon grid, which is used as preparation
material for TEM study. Perhaps, by creating a electrical matrix on
the surface of carbon nanotubes, it can be possible to load
nanotubes with dendrites. The use of surfactants has also been
reported to create tin nanorods in conjunction with a sonochemical
method (Qiu et al 2005). Dendritic crystal growth occurs in
electrochemical conditions far from thermodynamic equilibrium.
Dendrites tend to grow under mass transport limited conditions. At
conditions far from thermodynamic equilibrium, surface energy is no
longer the dominant factor in crystal formation (Choi, Kyoung-shin
2008).
Dendrite Formation:
[0094] Dendrites are also the most efficient way to distribute
surface area in a three dimensional structure while maintaining a
coherent, single structure. Other dispersion methods optimize the
total catalytic surface area, without maintaining a coherent shape
that also preserves the charge transport properties of the metal.
While interest in the formation of non-noble nanoparticles and
structures have been growing due to the relatively high stability
of copper nanoparticles, the presently disclosed subject matter
relates to uses of porous copper dendritic structures. One advance
in copper dendritic structures has come where the porous dendritic
structures grown under high current densities can be used as a
template to electrolessly exchange copper ions with platinum ions,
creating a dendritic structure that is fully platinum. These
structures have been shown to increase the current density of the
electrocatalytic reduction of O.sub.2 over 2.5 times.
[0095] This section will discuss crystal growth in copper and the
conditions necessary for dendrite formation to occur. High ionic
concentrations will not necessarily affect the crystal growth rate,
as the current applied determines the amount of a substance
deposited. However, the concentration will affect whether
deposition occurs in a mass transport limited regime. The
literature suggests that branching growth even at low
overpotentials result from an uneven distribution of potential
across the surface of the crystal structure. The reduction of
Cu.sup.2+ to Cu.sup.+ depends on the concentration of Cu.sup.2+ as
the Nernst equation shows:
E.sub.red=E.sup.o-0.05916.times.log([Cu.sup.+]/[Cu.sup.2+] at
T=298.15 K. 11)
[0096] This mechanism can be used to produce dendrites at low
overpotentials in low ion concentrations. Different crystal growth
regimes can be established depending on the over potential. The
growth rate of crystals depends upon the overpotential applied to
the electrochemical system. The overpotential, is defined as
N=|E.sub.appl-E.sub.red|. The higher the overpotential, the further
the system is from equilibrium.
[0097] Mass transport is the most important factor in dendritic
crystal growth. Mass transport-limited growth occurs when the rate
of crystal growth is greater than the availability of ions in the
immediate mass transport boundary layer. Imperfections in the
crystal faces create a nonlinear effect in these conditions, as the
apexes of the imperfections grow at a higher rate than the receded
faces, further increasing the differences between the apexes and
valleys of the crystal faces. At high overpotentials in relatively
low concentrations of metal ions, a diffusion boundary layer forms
around the electrode, which leads the deposition into a mass
transport limited regime. High overpotentials also increase the
number of crystal branches as well as the total surface area per
volume.
[0098] Some factors will prevent the system from reaching a
diffusion limited regime. At high concentrations of metal ions, the
transport regime cannot become diffusion limited. Higher
temperatures also tend to increase the size of the boundary layer
and mitigate the depletion zone, thus leading the system to remain
outside the diffusion limited regime for higher overpotentials.
This is true in the growth of zinc crystals as discussed in the
literature. At higher temperatures, the rate of mass transport and
the rate of diffusion across the boundary layer increase. Finally,
any other factors which tend to contribute to mass transfer, such
as stirring rates or short plating pulses would also mitigate the
growth of dendrites. Furthermore, capping protruding edges would
also tend to reduce the formation of dendrites, by directing
crystal growth toward non-dendritic protrusions on the
electrode.
[0099] The effect of organic additives on dendrite growth has been
studied. One means to study the interaction of additives and
crystal growth is to introduce additives into the plating solution
after initial growth has already occurred. This allows for the
study of crystal faces which might have otherwise been dissolved by
the additive. Additives change crystal structure primarily by
changing the kinetics and thermodynamics of crystal growth. PVP has
been used to prevent the growth of dendrites (Haas 2006) by capping
the protruding nucleations. PVP is attracted to the cathode, and is
a non polar capping agent, preventing electric interaction between
ions and nucleated metals. PEG and PVA on the other hand, do not
necessarily promote the growth of dendrites, but change the
morphology of the deposits. Finally, chlorophillin is an
interesting substance because it both contains a copper core, while
also displaying anodic attraction. No studies have been conducted
on chlorophyllin to date, as known to the inventors.
[0100] Choi, Kyoung-Shin (2008) discusses shape control through
electrodeposition and the use of additives. Different crystal
planes have different chemical and physical properties.
Furthermore, control of branch growth also changes the distribution
of crystals and the connectivity which can play a critical role in
the optimization of surface structure. Surfactants such as sodium
dodecyl sulfate absorbs to the {111} crystal plane, which slows the
growth of branch structures, as the {111} plane is the furthest
protruding plane. On the other hand Cl- interact with the {100}
direction, resulting in retardation of growth along this axis. The
degree of hinderance depends upon the concentration of additives.
Initially it is thought that pH is the dominant influence in shape
evolution, though later studies showed that the Cl- ion is the
determining factor (Choi, Kyoung-Shin (2008)).
[0101] There have only been a few studies published about the
control of copper morphology on the nano and micro scales.
Dendrites tend to grow under mass transport limited conditions, far
from thermodynamic equilibrium where surface energy is no longer
the dominant factor in crystal formation (Choi 2008). Imperfections
in the crystal faces creates a nonlinear effect in these
conditions, as the apexes of the imperfections grow at a higher
rate than the receded faces, further increasing the differences
between the apexes and valleys of the crystal faces. Furthermore,
as reported by Nikolic et al (2009), as well as Shin et al (2003),
when mass transfer limited deposition occurs in the hydrogen
evolution regime, evolving hydrogen bubbles form a template around
which copper dendrites can form. Manipulating the pause-to-pulse
ratio gives greater control over the size of micropores, while
variation on voltages can give some control over the morphology of
the dendrites (Nikolic 2007). The resulting foam maintains its
structural integrity, unlike other dendritic deposits. When grown
without additives, Shin reported copper is structured down to
hundreds of nanometers. Nikolic et al (2006) reports that the size
of the copper grains decrease with increasing over potentials of
550 mV to 1000 mV. A similar copper-tin foam structure is
characterized using BET surface measurements to have 20
m.sup.2/gram (Shin and Liu 2005).
[0102] Dendrites form a tree-like structure with a backbone as well
as leaves. The physical connection between the crystals of the
leaves as well as the backbone crystal allow nanocrystals to act as
a single crystal, conducting phonons and electrons as a single
structure (Choi 2008). The continuous structure of metallic
dendritic structures can provide the first clue as to novel
catalyst actions as will be further sketched out in this thesis.
Porous dendritic structures occur because of mass transport limited
branching growth. The hydrogen bubbles evolved during
electrodeposition of copper at high potentials results in the
formation of diffusion limited regions near the cathode. These
diffusion limited regions produce branching structures while the
bubbles create a template for the development of porous dendritic
structures.
Theory and Models of Fractal Geometry
[0103] Recently, Copper PDS have been synthesized from copper, as
well as other metals such as tin, to form metallic foam with high
surface area and high adsorptive characteristics. Many experts in
catalysis dismiss the notion of dendritic surfaces as being
economically viable for applications due to the assumed short
lifetimes of their surfaces. While fractal distributions of
catalytic metals have been proposed, only a few multi-scale
structures which display self similarity have been synthesized.
Furthermore, these structures are usually too delicate to find
practical use. However, copper PDS have a higher stability than
other fractal distributed catalysts grown at the submicron scale,
as these dendritic structures are structured on both a microscale
and macroscale and form a continuous structure, rather than a
powder. Anecdotal observations of the structure recorded an ability
to resist oxidation while maintaining cohesion within an aqueous
environment. These structures are completely metallic and display
surface areas which are orders of magnitudes higher than metals in
their bulk form. Their shape follow fractal geometries, which
display self-similarity across multiple scales, and surfaces which
grow with the complexity of the surface roughness. PDS have the
potential to capture the theoretical effects of fractal surfaces.
Studies have demonstrated that catalytic metals arranged in a
fractal geometry show higher kinetic rates at lower
temperatures.
Review of Fractals and Catalysts.
[0104] Because of the complex interfaces displayed by fractal
surfaces, theorists have posited that these surfaces could possess
unique applications for catalysis, as well as unique mass and heat
transfer properties. Rates of reactions are affected by diffusion
effects as well as surface area effects. Geometric effects have
also been discussed, and become significant at smaller scales.
Catalyst surfaces have been found to have a random fractal, or
multi-fractal geometry. Furthermore, catalyst surfaces in
simulations have been found to have a significant effect on the
rates of reactions of the catalysts, especially those limited by
knudsen diffusion, where diffusion occurs along a long pore and
collissions occur frequently (Sheintuch 2001). However, these
theoretical studies only quantified existing catalyst supports and
their conclusions mainly pertained to issues of mass transfer,
rather than to the kinetic effect different fractal geometries of
metal catalysts themselves might have on catalytic reactions.
[0105] Computer simulations have been conducted in exploring the
potential effects of fractal surfaces. Authors have posited that
heterogeneous fractal surfaces and fractal pore structures can
produce novel effects such as enhanced mass transfer and
selectivity. Pfeifer and Avnir (1983) state in their article a
power law relating the size of an object R and fractal dimension D
to its chemical interaction property.
A.about.s.sup.(2-D)/2 for monolayer coverage with s=cross sectional
area of particles. 1)
A.about.R.sup.D-3 for adsorbates, where R is the radius of
particles 2)
dV/dp.about.p.sup.2-D for pores where dV is the infinitesimal pore
volume with radius>dp. 3)
[0106] The dendritic porous copper foam would have a combination of
contributions from equation 1) and equation 3). These power laws
state: that the more complicated the surface, the higher the
surface area available for adsorption; and, that the larger the
radius and the larger the fractal dimension, the higher the
chemical interaction property. Furthermore in Avnir (1991) the
catalyst activity is described with another relatively intuitive
equation:
A.about.R.sup.D where A is the chemical activity of the particle
4).
[0107] Other studies have made correlations between the activity of
a catalyst and its fractal dimension, as a higher fractional
dimension implies a more complex surface with more surface area.
The justification for this is that porous dendritic structures are
controlled fractal surfaces, rather than random fractal surfaces.
Meankin's (1986) simulation of catalyst selectivity in random
fractals finds small effects due to the unique geometries of random
fractals. However, controlled fractal surfaces, can have very
specific effects on different types of chemical reactions catalyzed
by the base metals beyond those found by Meakin. For example, the
porous dendritic structures described in this paper, might have a
geometry which deflects gas particles into paths which maximize the
number of impacts with the catalyst surface.
Fractal Cage
[0108] The mechanisms simulated are based on the inner recesses of
the fractals to have a higher ability to absorb a particle of a
specific size, and thus create new products. However, the
fundamental mechanism of action would be similar: that though the
distribution of catalysis events is equal on all surfaces, the
distribution of diffusion absorption events varies greatly as some
surfaces are harbored from certain objects (Meakin 1986), perhaps
because of their geometry.
[0109] Under certain geometries with involutions, the
concentrations of different species of chemicals depending on their
molecular mass, would be limited by the depth of scale. It is
possible a specific geometry will catalyze a specific reaction to
completion very quickly so that production rates are fast which the
dominant products produced at one scale, the products reactions
occurring on another. There is a multi component recursive product
chain of products produced in one of the involutions, as on
reactant becomes the reactants on the next. The consumptions of
small products, creates a gradient for new monomers to diffuse into
the regions, and the random kinetic activity between inflowing
monomers will displace new monomers. Certain regions of the fractal
surface become inaccessible to diffusion gradients (Meakin 1986).
There regions can act as a harbor for creation of a specific type
of chemical species.
[0110] One can assume that metals are delocalized electron shells
which have the capacity to absorb kinetic energy from surrounding
molecules, while also imparting electronic energy to reacting
species. If one were to assume that metals, which are high
conductors of heat, do not possess kinetic energy when in solid
state, then each molecule that strikes the surface of the
delocalized shell of an electron will impart some fraction of its
energy, 1/f, to metal surface plus a constant, c, amount of energy
which is the attractive surface energy of the metal. The reacting
species will subsequently slow down. Species which have a low
enough kinetic energy below the surface binding energy of the metal
will stick to the surface of the metal. When two demobilized
reactant molecules come in contact on the surface of the metal, the
vibrational energy their reaction creates can be high enough for
them to leave the surface of the metal.
[0111] If this is true, then a fractal geometry would be
significantly better than another standard Euclidian geometry.
Surface area is not the primary determinant of catalyst activity,
in itself. Rather, surface area is only important in increasing the
number of collisions with reactant molecules. However, an optimized
2 dimensional coating of catalyst particles will still be of a
lower efficiency than a porous fractal geometry because fractal
geometries maximize the collisions per molecule by directing the
trajectory of molecules after the collision towards another metal
surface in the vicinity, whose angle directs particles towards
another internal wall of the porous dendritic cage. Reactant
molecules are trapped within the interfacial spaces and slow down
dramatically faster because of multiple collisions. Because of the
potential for multiple collisions, even molecules which are moving
with initial kinetic energy which exceeds the surface binding
energy of the metal, can be demobilized after multiple collisions
with the surface of the metal.
[0112] If we assume a flat surface, as is the case with most
catalyst loading geometries, then we should assume a fast moving
molecule will likely have at most a single collision. Especially if
the loading of catalyst particles are only a small percentage of
the total surface area on a supporting structure. Since reactant
molecules can only have a single collision, their kinetic energy
cannot exceed c. However, for multiple collisions, a molecule can
have kinetic energy E*(1-f).sup.i where i is the total number of
collisions for a given molecule and E is the initial kinetic energy
of the molecule upon entering the cage. It will also gain energy
from collisions with other molecules and collisions with infrared
photons emitted by the surface of the metal. However, if the
catalyst surface is a polycrystalline with higher heat and phonon
conductivity than the surrounding region, and it is connected to an
effectively infinite heat sink, the infrared radiation given off by
the metallic catalyst surface would likely not exceed ambient
temperatures, despite what is an effective hot spot trap within the
fractal pore, since the metal can be said to absorb the kinetic
energy throughout its delocalized electron shell, and only has
average excitation equal to the average kinetic energy it
absorbs.
[0113] The pore can become a hotspot because when first exposed to
the ambient environment with a given temperature T, which has a
corresponding Kinetic energy KE, carried primarily by the movement
of molecules. As these molecules come into contact with the opening
of the hole, it has a probability p for every unit of time t of
getting trapped by the cage. Furthermore, there is a probability q
that a particle will escape from the trap where q depends on the
number of particles trapped by the cage t with q<p. At some
point, p and q will equilibrate and the average number of particles
per unit volume will be greater within the fractal trap than
outside the fractal trap. Furthermore, particles can constantly
lose kinetic energy based on each collisions with the surface of
the metal, as the metal carries away the energy from particles with
higher kinetic energy. Thus, if the average number of collisions
for a molecule within some involution of the fractal surface is S,
and assuming a uniform distribution of energies. The proportion of
molecules which could be captured by a surface with binding energy
B would be those molecules which have an energy E.ltoreq.B. If we
assume a normal Gaussian distribution for the kinetic energy of
particles within the system, then the proportion of particles which
have an energy less than B for a flat surface would be the
cumulative density function of a normal Gaussian distribution where
x equal in this case to the normalized number (B-E)/E and the
probability of binding would be
.PHI. ( x ) = 1 2 .pi. .intg. - .infin. ( B - E ) / E - t 2 / 2 t 5
) ##EQU00001##
[0114] On the other hand, assuming there is a slight loss of
kinetic energy by particles in a collision where f is the average
proportion of energy lost for each particle after a collision with
a metallic surface, while also including energies gained from
random collisions with infrared radiation and collisions with other
molecules. Assuming that one closes off the opening of the pore
system to prevent any new particles from entering, then the new
average kinetic energy of the pocket would be
E*(1-f).sup.i 6)
[0115] Since the average kinetic energy of the system is shifted to
a lower energy if one assumes a closed set of particles and a heat
sink attached to the metal phonon conductor. Regarding the heat
sink, the fractal structure, since it is a continuous structure,
will have an increase of kinetic energy transferred through it as
phonons, though it does not violate the second law, since these
phonons are attached to a glassy carbon surface, which is catalyzed
at relatively low temperatures of approximately 200 degrees
Celsius.
[0116] The probability of a particle exceeding the binding energy
of the metal is:
.PHI. ( x ) = 1 2 .pi. .intg. - .infin. ( B - E * ( 1 - f ) i ) / (
E * ( 1 - f ) i ) - t 2 / 2 t 7 ) ##EQU00002##
[0117] Finally, the difference between a flat catalyst surface, and
a surface which is arranged in a fractal trap would be:
.PHI. ( x ) = 1 2 .pi. .intg. ( B - E ) / E ( B - E * ( 1 - f ) i )
/ ( E * ( 1 - f ) i ) - t 2 / 2 t 8 ) ##EQU00003##
[0118] One simulation (Phillips et al 2003) conducted with Fluent
CFD using Gambit mesh generator, utilized the Cantor set generator,
a 76% reduction in the active surface area, the calculated drop in
mass transfer to the active surfaces is only reduced by 2.25% when
the reactions are diffusion limited. With each iteration, the total
length is shortened by 1/3. However, even after infinite
iterations, where the effective length is 0, under this study, the
simulations show that the total rate of mass transfer falls
asymptotically to a fixed value.
[0119] The present invention is not to be limited in scope by the
specific embodiments described herein. Indeed, various
modifications of the invention in addition to those described
herein will become apparent to those skilled in the art from the
foregoing description and the accompanying figures.
[0120] Patents, patent applications, publications, product
descriptions, and protocols are cited throughout this application,
the disclosures of which are incorporated herein by reference in
their entireties for all purpose.
* * * * *