U.S. patent application number 15/054610 was filed with the patent office on 2016-06-23 for electrochemical reduction device.
This patent application is currently assigned to JX NIPPON OIL & ENERGY CORPORATION. The applicant listed for this patent is JX NIPPON OIL & ENERGY CORPORATION. Invention is credited to Yoshihiro Kobori, Kota Miyoshi, Kojiro Nakagawa, Shinji Oshima, Yasushi Sato.
Application Number | 20160177459 15/054610 |
Document ID | / |
Family ID | 52585959 |
Filed Date | 2016-06-23 |
United States Patent
Application |
20160177459 |
Kind Code |
A1 |
Sato; Yasushi ; et
al. |
June 23, 2016 |
ELECTROCHEMICAL REDUCTION DEVICE
Abstract
An electrochemical reduction device includes: an electrolyte
membrane; a reduction electrode including a reduction electrode
catalyst layer, a diffusion layer, and a dense layer provided
between the diffusion layer and the reduction electrode catalyst
layer; an oxygen generating electrode; a raw material supplier that
supplies the aromatic compound in a liquid state to the reduction
electrode; a moisture supplier that supplies water or humidified
gas to the oxygen generating electrode; and a power controller that
externally applies an electric field such that the reduction
electrode has an electronegative potential and the oxygen
generating electrode has an electropositive potential.
Inventors: |
Sato; Yasushi; (Tokyo,
JP) ; Miyoshi; Kota; (Tokyo, JP) ; Nakagawa;
Kojiro; (Tokyo, JP) ; Kobori; Yoshihiro;
(Tokyo, JP) ; Oshima; Shinji; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JX NIPPON OIL & ENERGY CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JX NIPPON OIL & ENERGY
CORPORATION
Tokyo
JP
|
Family ID: |
52585959 |
Appl. No.: |
15/054610 |
Filed: |
February 26, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2014/004211 |
Aug 18, 2014 |
|
|
|
15054610 |
|
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Current U.S.
Class: |
204/230.2 |
Current CPC
Class: |
C25B 9/10 20130101; C25B
11/0447 20130101; C25B 11/0484 20130101; C25B 9/08 20130101; C25B
3/04 20130101; Y02E 60/36 20130101; Y02E 60/366 20130101; C25B
11/035 20130101; C25B 11/04 20130101; C25B 15/08 20130101; C25B
11/0405 20130101; C25B 15/02 20130101 |
International
Class: |
C25B 15/08 20060101
C25B015/08; C25B 15/02 20060101 C25B015/02; C25B 11/04 20060101
C25B011/04; C25B 9/08 20060101 C25B009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2013 |
JP |
2013-179555 |
Claims
1. An electrochemical reduction device comprising: an electrolyte
membrane having proton conductivity; a reduction electrode
including a reduction electrode catalyst layer that is provided in
contact with one major surface of the electrolyte membrane and has
an electron conductive material and a metal containing one or both
of Pt and Pd supported by the electron conductive material, a
diffusion layer that is provided near to the other major surface of
the reduction electrode catalyst layer, the other major surface
being opposite to the electrolyte membrane, and that causes a
liquid aromatic compound and a hydride of the aromatic compound to
pass through, and a dense layer provided between the diffusion
layer and the reduction electrode catalyst layer; an oxygen
generating electrode provided in contact with the other major
surface of the electrolyte membrane; a raw material supplier that
supplies the aromatic compound in a liquid state to the reduction
electrode; a moisture supplier that supplies water or humidified
gas to the oxygen generating electrode; and a power controller that
externally applies an electric field such that the reduction
electrode has an electronegative potential and the oxygen
generating electrode has an electropositive potential.
2. The electrochemical reduction device according to claim 1,
wherein a mean flow pore diameter of the dense layer is 20 .mu.m or
less.
3. The electrochemical reduction device according to claim 1,
wherein a thickness of the dense layer (t) is 1
.mu.m.ltoreq.t.ltoreq.50 .mu.m.
4. The electrochemical reduction device according to claim 1,
wherein the dense layer contains a mixture of an electron
conductive powder and a water repellent.
5. The electrochemical reduction device according to claim 4,
wherein amass ratio of the electron conductive powder to the water
repellent in the mixture is 4 or more.
6. The electrochemical reduction device according to claim 1,
wherein the diffusion layer has a fiber-like shape or a shape in
which many particles are solidified.
7. The electrochemical reduction device according to claim 1,
wherein the diffusion layer contains a carbon fiber.
8. The electrochemical reduction device according to claim 1,
wherein the diffusion layer contains a material having an electron
conductivity of 1.0.times.10.sup.-2 S/cm or more.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from the prior Japanese Patent Application No.
2013-179555, filed on Aug. 30, 2013 and International Patent
Application No. PCT/JP2014/004211, filed on Aug. 18, 2014, the
entire content of each of which is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a technique for
electrochemically hydrogenating an aromatic compound.
[0004] 2. Description of the Related Art
[0005] It is known that a cyclic organic compound such as
cyclohexane or decalin can be efficiently obtained by hydrogenating
at least one benzene ring of a corresponding aromatic hydrocarbon
compound (benzene or naphthalene) with the use of hydrogen gas.
This reaction requires reaction conditions of high temperature and
high pressure, and hence it is not suitable for small and medium
scale manufacturing of cyclic organic compounds. It is also known
that, on the other hand, an electrochemical reaction using an
electrolysis cell proceeds without gaseous hydrogen being required
and under relatively mild reaction conditions (from normal
temperature to approximately 200.degree. C. and normal pressure)
because water can be used as a hydrogen source.
[0006] [Patent Document 1] JP 2003-045449
[0007] [Patent Document 2] JP 2005-126288
[0008] [Patent Document 3] JP 2005-239479
[0009] As an example in which a benzene ring of an aromatic
hydrocarbon compound such as toluene is electrochemically
hydrogenated, a method is reported, in which toluene that has been
vaporized into a gaseous state is sent to the reduction electrode
side to obtain methylcyclohexane, a benzene-ring hydrogenated
substance (hydride), without going through a state of hydrogen gas
in a configuration similar to that of water electrolysis (see
Patent Document 1). In this method, however, the amount of
substance that can be transformed per electrode area or time
(current density) is not large, and it has been difficult to
industrially produce a hydride of an aromatic hydrocarbon
compound.
[0010] In order to address the problem, the present inventors have
studied a method in which a liquid aromatic hydrocarbon compound is
directly introduced into the reduction electrode side of an
electrolysis cell. This method has the advantage that an
electrolytic hydride-producing reaction can be caused to proceed at
a higher current density in comparison with a method in which a
vaporized aromatic hydrocarbon compound is introduced. However, a
technique, for causing a hydride-producing reaction for an aromatic
hydrocarbon compound supplied in a liquid state to proceed more
efficiently, has not been studied sufficiently, and there remains a
challenge to be addressed.
SUMMARY OF THE INVENTION
[0011] The present invention has been made in view of such a
challenge, and a purpose of the invention is to provide a technique
in which a hydride of an aromatic compound can be produced at a
high efficiency by an electrochemical reaction.
[0012] An embodiment of the present invention is an electrochemical
reduction device. The electrochemical reduction device comprises:
an electrolyte membrane having proton conductivity; a reduction
electrode including a reduction electrode catalyst layer that is
provided in contact with one major surface of the electrolyte
membrane and has an electron conductive material and a metal
containing one or both of Pt and Pd supported by the electron
conductive material, a diffusion layer that is provided near to the
other major surface of the reduction electrode catalyst layer, the
other major surface being opposite to the electrolyte membrane, and
that causes a liquid aromatic compound and a hydride of the
aromatic compound to pass through, and a dense layer provided
between the diffusion layer and the reduction electrode catalyst
layer; an oxygen generating electrode provided in contact with the
other major surface of the electrolyte membrane; a raw material
supplier that supplies the aromatic compound in a liquid state to
the reduction electrode; a moisture supplier that supplies water or
humidified gas to the oxygen generating electrode; and a power
controller that externally applies an electric field such that the
reduction electrode has an electronegative potential and the oxygen
generating electrode has an electropositive potential.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Embodiments will now be described, by way of example only,
with reference to the accompanying drawings, which are meant to be
exemplary, not limiting, and wherein like elements are numbered
alike in several figures, in which:
[0014] FIG. 1 is a schematic view illustrating a schematic
configuration of an electrochemical reduction device according to
an embodiment;
[0015] FIG. 2 is an exploded perspective view illustrating a
schematic configuration of an electrode unit included in the
electrochemical reduction device according to an embodiment;
[0016] FIG. 3 is a side view illustrating a schematic configuration
of an electrode unit included in the electrochemical reduction
device according to an embodiment;
[0017] FIG. 4 is a schematic view illustrating an enlarged example
of a sectional structure of an oxygen generating electrode; and
[0018] FIG. 5 is a graph showing I-V curves obtained by respective
electrode units of Examples from 1 to 5 and Comparative Example
1.
DETAILED DESCRIPTION OF THE INVENTION
[0019] An embodiment of the present invention is an electrochemical
reduction device. The electrochemical reduction device comprises:
an electrolyte membrane having proton conductivity; a reduction
electrode including a reduction electrode catalyst layer that is
provided in contact with one major surface of the electrolyte
membrane and has an electron conductive material and a metal
containing one or both of Pt and Pd supported by the electron
conductive material, a diffusion layer that is provided near to the
other major surface of the reduction electrode catalyst layer, the
other major surface being opposite to the electrolyte membrane, and
that causes a liquid aromatic compound and a hydride of the
aromatic compound to pass through, and a dense layer provided
between the diffusion layer and the reduction electrode catalyst
layer; an oxygen generating electrode provided in contact with the
other major surface of the electrolyte membrane; a raw material
supplier that supplies the aromatic compound in a liquid state to
the reduction electrode; a moisture supplier that supplies water or
humidified gas to the oxygen generating electrode; and a power
controller that externally applies an electric field such that the
reduction electrode has an electronegative potential and the oxygen
generating electrode has an electropositive potential.
[0020] In the electrochemical reduction device of the
aforementioned embodiment, the diffusion layer may have a
fiber-like shape or a shape in which many particles are solidified.
The diffusion layer may contain a carbon fiber. Additionally, the
diffusion layer may contain a material having electron conductivity
of 1.0.times.10.sup.-2 S/cm or more. The dense layer may contain a
mixture of an electron conductive powder and a water repellent.
[0021] Combinations of the aforementioned respective elements will
also be within the scope of the present invention sought to be
patented by the present patent application.
[0022] Hereinafter, embodiments of the present invention will now
be described with reference to the drawings. In the figures, like
numerals represent like constituting elements and the description
thereof will be appropriately omitted.
Embodiment
[0023] FIG. 1 is a schematic view illustrating a schematic
configuration of an electrochemical reduction device 10 according
to an embodiment. FIG. 2 is an exploded perspective view
illustrating a schematic configuration of an electrode unit
included in the electrochemical reduction device 10 according to an
embodiment. FIG. 3 is a side view illustrating a schematic
configuration of an electrode unit included in the electrochemical
reduction device 10 according to an embodiment.
[0024] As illustrated in FIG. 1, the electrochemical reduction
device 10 includes an electrode unit 100, a power controller 20, an
organic material storage tank 30, a water storage tank 40, a
gas-water separation unit 50, and a controller 60.
[0025] The power controller 20 is, for example, a DC/DC converter
for converting the output voltage of a power source into a
predetermined voltage. The positive electrode output terminal of
the power controller 20 is connected to an oxygen generating
electrode (positive electrode) 130 of the electrode unit 100. The
negative electrode output terminal of the power controller 20 is
connected to a reduction electrode (negative electrode) 120 of the
electrode unit 100. Thereby, a predetermined voltage is applied
between the oxygen generating electrode 130 and the reduction
electrode 120 of the electrode unit 100. A reference pole may be
provided in the power controller 20 in order to detect the
potentials of the positive and negative electrodes. A reference
pole input terminal is connected to a reference electrode 112
provided on the later-described electrolyte membrane 110. The
outputs of the positive electrode output terminal and the negative
electrode output terminal of the power controller 20 are controlled
by the controller 60 such that the potentials of the oxygen
generating electrode 130 and the reduction electrode 120 become
desired ones based on the potential of the reference electrode 112.
A power source is not particularly limited, and normal system power
may be used, or electrical power derived from natural energy such
as sunlight or wind power can also be used preferably.
[0026] The organic substance storage tank 30 stores an aromatic
compound. The aromatic compound used in the present embodiment is
an aromatic hydrocarbon compound or a nitrogen-containing
heterocyclic aromatic compound, which contains at least one
aromatic ring. Examples of the aromatic compound include, for
example, benzene, naphthalene, anthracene, diphenylethane,
pyridine, pyrimidine, pyrazine, quinoline, isoquinoline,
N-alkylpyrrole, N-alkylindole, N-alkyldibenzopyrrole, and the like.
From one to four hydrogen atoms of an aromatic ring of anyone of
the aforementioned aromatic hydrocarbon compounds or
nitrogen-containing heterocyclic aromatic compounds may be
substituted by alkyl groups. However, the "alkyl" of any one of the
aromatic compounds is a C.sub.1-6 straight or branched alkyl group.
For example, alkylbenzenes include toluene, ethyl benzene, and the
like, dialkylbenzenes include xylene, diethylbenzene, and the like,
and trialkylbenzenes include mesitylene and the like.
Alkylnaphthalenes include methylnaphthalene. An aromatic ring of
any one of the aromatic hydrocarbon compounds or
nitrogen-containing heterocyclic aromatic compounds may have from 1
to 3 substituents. In the following description, the aromatic
hydrocarbon compounds and the nitrogen-containing heterocyclic
aromatic compounds may be collectively referred to as "aromatic
compounds". It is preferable that the aromatic compound is a liquid
at room temperature. When a mixture of two or more of the
aforementioned aromatic compounds is used, the mixture should be a
liquid. Consequently, the aromatic compound can be supplied to the
electrode unit 100 in a liquid state without being heated,
pressurized, or the like, and hence the configuration of the
electrochemical reduction device 10 can be simplified. The
concentration of the aromatic hydrocarbon compound in a liquid
state is 0.1% or more, preferably 0.3% or more, and more preferably
0.5% or more.
[0027] The aromatic compound stored in the organic material storage
tank 30 is supplied to the reduction electrode 120 of the electrode
unit 100 by a first liquid supplier 32. As the first liquid
supplier 32, for example, various types of pumps such as a gear
pump or a cylinder pump, or a gravity flow device or the like can
be used. As the aromatic compound, an N-substitution product of the
aforementioned aromatic compound may be used. A circulation pathway
is provided between the organic material storage tank 30 and the
reduction electrode 120 of the electrode unit 100. The aromatic
compound, a hydride of which has been produced by the electrode
unit 100, and an unreacted aromatic compound are stored in the
organic material storage tank 30 after passing through the
circulation pathway. No gas is generated by a major reaction that
proceeds at the reduction electrode 120 of the electrode unit 100,
but if gas such as hydrogen is by-produced, a gas-liquid separation
device may be provided in the middle of the circulation
pathway.
[0028] The water storage tank 40 stores, for example, ion exchange
water, pure water, an aqueous solution in which an acid such as
sulfuric acid is added to ion exchange water or pure water, or the
like (hereinafter, they are simply referred to as "water"). The
water stored in the water storage tank 40 is supplied to the oxygen
generating electrode 130 of the electrode unit 100 by a second
liquid supplier 42. As the second liquid supplier 42, for example,
various types of pumps such as a gear pump or a cylinder pump, or a
gravity flow device or the like can be used, similarly to the first
liquid supplier 32. A circulation pathway is provided between the
water storage tank 40 and the oxygen generating electrode 130 of
the electrode unit 100. The water that has not been reacted in the
electrode unit 100 is stored in the water storage tank 40 after
passing through the circulation pathway. The gas-water separation
unit 50 is provided in the middle of a pathway through which
unreacted water is sent back to the water storage tank 40 from the
electrode unit 100. With the gas-water separation unit 50, gas such
as oxygen generated by the electrolysis of water in the electrode
unit 100 is separated from water and discharged to the outside of
the system.
[0029] As illustrated in FIGS. 2 and 3, the electrode unit 100
includes the electrolyte membrane 110, the reduction electrode 120
having a reduction electrode catalyst layer 122, a diffusion layer
140, and a dense layer 142, the oxygen generating electrode 130, a
separator 150, and a current collector 160. In FIG. 1, the
electrode unit 100 is illustrated in a simplified way in which the
separator 150 and the current collector 160 are omitted.
[0030] The electrolyte membrane 110 is formed of a material
(ionomer) having proton conductivity, and inhibits substances from
getting mixed or being diffused between the reduction electrode 120
and the oxygen generating electrode 130 while selectively
conducting protons. The thickness of the electrolyte membrane 110
is preferably from 5 to 300 .mu.m, more preferably from 10 to 150
.mu.m, and most preferably from 20 to 100 .mu.m. If the thickness
of the electrolyte membrane 110 is less than 5 .mu.m, the barrier
property of the electrolyte membrane 110 is deteriorated, so that
cross-leaking is likely to occur. On the other hand, if the
thickness thereof is more than 300 .mu.m, ion transfer resistance
becomes too large, which is not preferred.
[0031] The area resistance of the electrolyte membrane 110, that
is, the ion transfer resistance per geometric area thereof is
preferably 2000 m.OMEGA.cm.sup.2 or less, more preferably 1000
m.OMEGA.cm.sup.2 or less, and most preferably 500 m.OMEGA.cm.sup.2
or less. If the area resistance of the electrolyte membrane 110 is
more than 2000 m.OMEGA.cm.sup.2, proton conductivity may become
insufficient. Examples of the material having proton conductivity
(i.e., a cation-exchanging ionomer) include perfluorosulfonic acid
polymers such as NAFION.TM. and FLEMION.TM.. The ion exchange
capacity (IEC) of the cation exchange ionomer is preferably from
0.7 to 2 meq/g, and more preferably from 1 to 1.2 meq/g. If the ion
exchange capacity thereof is less than 0.7 meq/g, ion conductivity
may become insufficient. On the other hand, if the ion exchange
capacity of the cation exchange ionomer is more than 2 meq/g, the
solubility of the ionomer in water is increased, and hence the
strength of the electrolyte membrane 110 may become
insufficient.
[0032] On the electrolyte membrane 110, the reference electrode 112
may be provided in an area spaced apart from the reduction
electrode 120 and the oxygen generating electrode 130 so as to
contact the electrolyte membrane 110, as illustrated in FIG. 1.
That is, the reference electrode 112 is electrically isolated from
the reduction electrode 120 and the oxygen generating electrode
130. The reference electrode 112 is held at a reference electrode
potential V.sub.Ref. Examples of the reference electrode 112
include a standard hydrogen reduction electrode (reference
electrode potential V.sub.Ref=0 V) and an Ag/AgCl electrode
(reference electrode potential V.sub.Ref=0.199 V), but the
reference electrode 112 is not limited thereto. When the reference
electrode 112 is installed, it is preferable to install it on the
surface of the electrolyte membrane 110 on the reduction electrode
120 side.
[0033] The current I flowing between the reduction electrode 120
and the oxygen generating electrode 130 is detected by a current
detection unit 113 illustrated in FIG. 1. The value of the current
I detected by the current detection unit 113 may be inputted to the
controller 60 to be used for the control of the power controller 20
by the controller 60.
[0034] A potential difference .DELTA.V.sub.CA between the reference
electrode 112 and the reduction electrode 120 is detected by a
voltage detection unit 114 illustrated in FIG. 1. The value of the
potential difference .DELTA.V.sub.CA detected by the voltage
detection unit 114 may be inputted to the controller 60 to be used
for the control of the voltage controller 20 by the controller
60.
[0035] The reduction electrode 120 is provided in contact with one
major surface of the electrolyte membrane 110. The reduction
electrode 120 is a laminated body in which the reduction electrode
catalyst layer 122, the dense layer 142, and the diffusion layer
140 are laminated in this order from the electrolyte membrane 110
side.
[0036] The reduction electrode catalyst layer 122 is provided in
contact with one major surface of the electrolyte membrane 110. The
reduction electrode catalyst layer 122 includes a reduction
catalyst for producing a hydride of an aromatic compound. The
reduction catalyst used in the reduction electrode catalyst layer
122 contains at least one of Pt and Pd. Additionally, the reduction
catalyst may be composed of a metal composition that contains a
first catalyst metal (noble metal) made of at least one of Pt and
Pd, and one or two or more second catalyst metals selected from the
group consisting of Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Sn, W, Re,
Pb, and Bi. In this case, the form of the metal composition is an
alloy of the first catalyst metal and the second catalyst metal, or
an intermetallic compound made of the first catalyst metal and the
second catalyst metal. The ratio of the first catalyst metal to the
total mass of the first catalyst metal and the second catalyst
metals is preferably from 10 to 95 wt %, more preferably from 20 to
90 wt %, and most preferably from 25 to 80 wt %. If the ratio of
the first catalyst metal is less than 10 wt %, durability may be
deteriorated in terms of resistance to dissolving, and the like. On
the other hand, if the ratio of the first catalyst metal is more
than 95 wt %, the properties of the reduction catalyst become
closer to those of a noble metal alone, and hence the electrode
activity may become insufficient. In the following description, the
first catalyst metal and the second catalyst metals may be
collectively referred to as a "catalyst metal."
[0037] The aforementioned catalyst metal may be supported by an
electron conductive material (support). The electron conductivity
of the electron conductive material is preferably
1.0.times.10.sup.-2 S/cm or more, more preferably
3.0.times.10.sup.-2 S/cm or more, and most preferably
1.0.times.10.sup.-1 S/cm or more. If the electron conductivity of
the electron conductive material is less than 1.0.times.10.sup.-2
S/cm, sufficient electron conductivity may not be imparted.
Examples of the electron conductive material include electron
conductive materials containing any one of porous carbon, a porous
metal, and a porous metal oxide as a major component. Examples of
the porous carbon include carbon black such as KETJEN BLACK
(registered trademark), acetylene black, and VULCAN.TM.. The BET
specific surface area of the porous carbon, measured by a nitrogen
adsorption method, is preferably 100 m.sup.2/g or more, more
preferably 150 m.sup.2/g or more, and most preferably 200 m.sup.2/g
or more. If the BET specific surface area of the porous carbon is
less than 100 m.sup.2/g, it is difficult to uniformly support the
catalyst metal. Accordingly, the rate of utilizing the surface of
the catalyst metal is decreased, possibly causing the catalyst
performance to be deteriorated. Examples of the porous metal
include, for example, Pt black, Pd black, a Pt metal deposited in a
fractal shape, and the like. Examples of the porous metal oxide
include oxides of Ti, Zr, Nb, Mo, Hf, Ta and W. Besides these,
examples of the porous electron conductive material for supporting
the catalyst metal include nitrides, carbides, oxynitrides,
carbonitrides, partially-oxidized carbonitrides of metals such as
Ti, Zr, Nb, Mo, Hf, Ta, and W (hereinafter, they are collectively
referred to as porous metal carbonitrides and the like). The BET
specific surface areas of the porous metal, the porous metal oxide,
the porous metal carbonitrides and the like, measured by a nitrogen
adsorption method, are preferably 1 m.sup.2/g or more, more
preferably 3 m.sup.2/g or more, and most preferably 10 m.sup.2/g or
more. If the respective BET specific surface areas of the porous
metal, the porous metal oxide, the porous metal carbonitrides and
the like are less than 1 m.sup.2/g, it is difficult to uniformly
support the catalyst metal. Accordingly, the rate of utilizing the
surface of the catalyst metal is decreased, possibly causing the
catalyst performance to be deteriorated.
[0038] To the reduction electrode catalyst layer 122, a material
having electron conductivity, such as the aforementioned electron
conductive oxide or carbon black, may be added separately from the
electron conductive compound for supporting the catalyst metal.
Thereby, the number of electron-conducting paths among reduction
catalyst particles can be increased, and hence the resistance per
geometric area of a reduction catalyst layer can be lowered in some
cases.
[0039] The reduction electrode catalyst layer 122 may contain, as
an additive, a fluorine-based resin such as polytetrafluoroethylene
(PTFE).
[0040] The reduction electrode catalyst layer 122 may contain an
ionomer having proton conductivity. The reduction electrode
catalyst layer 122 preferably contains, at a predetermined mass
ratio, an ion conductive material (ionomer) having a structure that
is identical or similar to that of the aforementioned electrolyte
membrane 110. Thereby, the ion conductivity of the reduction
electrode catalyst layer 122 can be improved. When the catalyst
support is porous, the ion conductivity of the reduction electrode
catalyst layer 122 is particularly and greatly improved with the
reduction electrode catalyst layer 122 containing an ionomer having
proton conductivity. Examples of the ionomer having proton
conductivity (i.e., a cation-exchanging ionomer) include
perfluorosulfonic acid polymers such as NAFION.TM. and FLEMION.TM..
The ion exchange capacity (IEC) of the cation exchange ionomer is
preferably from 0.7 to 3 meq/g, more preferably from 1 to 2.5
meq/g, and most preferably from 1.2 to 2 meq/g. When the catalyst
metal is supported by porous carbon (carbon support), the mass
ratio I/C of the cation-exchanging ionomer (I) to the carbon
support (C) is preferably from 0.1 to 2, more preferably from 0.2
to 1.5, and most preferably from 0.3 to 1.1. If the mass ratio I/C
is less than 0.1, it may be difficult to obtain sufficient ion
conductivity. On the other hand, if the mass ratio I/C is more than
2, the thickness of an ionomer coating over the catalyst metal is
increased, and hence the contact of the aromatic compound, a
reactant, with a catalyst-active site may be inhibited or the
electrode activity may be decreased due to a decrease in the
electron conductivity.
[0041] It is also preferable that the reduction catalyst is
partially coated with the ionomer included in the reduction
electrode catalyst layer 122. Thereby, three elements (aromatic
compound, proton, electron), which are necessary for an
electrochemical reaction in the reduction electrode catalyst layer
122, can be efficiently supplied to a reaction site.
[0042] The dense layer 142 is provided, of two major surfaces of
the reduction electrode catalyst layer 122, in contact with one
major surface thereof that is opposite to the other major surface
that the electrolyte membrane 110 contacts. The dense layer 142 has
a function in which a liquid aromatic compound and a hydride of an
aromatic compound, which have a relatively low surface tension, are
caused to pass through it, while liquid water having a relatively
high surface tension is suppressed from passing through it.
[0043] The diffusion layer 140 is provided, of two major surfaces
of the dense layer 142, in contact with one major surface thereof
that is opposite to the other major surface that the reduction
electrode catalyst layer 122 contacts. The diffusion layer 140 is
formed of a porous material or fiber that causes the liquid
aromatic compound and hydride of an aromatic compound, which have
been supplied from the later-described separator 150, to permeate
and that has good electron conductivity. Additionally, it is
preferable that the material that forms the diffusion layer 140 has
a high affinity with the aforementioned aromatic compounds. As the
material that forms the diffusion layer 140, for example, carbon
paper, and woven cloth or non-woven cloth of carbon, etc., can be
used. When carbon paper is used as the material that forms the
diffusion layer 140, the thickness of the diffusion layer 140 is
preferably from 50 to 1000 .mu.m, and more preferably from 100 to
500 .mu.m. Additionally, it is preferable that the electron
conductivity of the material that forms the diffusion layer 140 is
10.sup.-2 S/cm or more.
[0044] Besides these, a material can be used for the diffusion
layer 140, in which fine particles of a corrosion-resistant alloy
such as Cr--Mo are solidified. In this case, it is preferable to
perform, on the surface of the metal, a treatment such as a water
repellent treatment, lipophilic treatment, or the like.
[0045] The dense layer 142 is formed, for example, by coating a
kneaded product in a paste state, which is obtained by kneading an
electron conductive powder and a water repellent, to the diffusion
layer 140 so as to fill the void thereof and then by drying it.
Conductive carbon such as VULCAN.TM. can be used as the electron
conductive powder. A fluorine resin such as a tetrafluoroethylene
resin (PTFE) can be used as the water repellent. The ratio of the
electron conductive powder to the water repellent can be
appropriately determined to be within a range in which desired
electron conductivity and water repellency are obtained, but when
VULCAN.TM. and PTEE are used as the electron conductive powder and
the water repellent, respectively, the ratio is, for example, from
4:1 to 1:1 (VULCAN:PTEE).
[0046] The mean flow pore diameter (dm) of the dense layer 142 is
preferably from 100 nm to 20 .mu.m, and more preferably from 500 nm
to 5 .mu.m. Additionally, the thickness of the dense layer 142 is
preferably from 1 to 50 .mu.m, and more preferably from 2 to 20
.mu.m. When the dense layer 142 is formed to be recessed from the
surface of the diffusion layer 140, the average thickness of the
dense layer 142 itself, including the portion recessed into the
diffusion layer 140, is defined to be the thickness.
[0047] By providing the dense layer 142 on one major surface of the
reduction electrode catalyst layer 122, the one major surface being
opposite to the other major surface thereof that the electrolyte
membrane 110 contacts, water can be easily retained in a proton
conductive portion such as the ionomer (electrolyte) in the
reduction electrode catalyst layer 122 and the cathode side of the
electrolyte membrane 110. Thereby, the proton reduction reaction at
the reduction electrode 120 side can be facilitated more easily in
an environment in which a large amount of organic substances such
as aromatic compounds are present.
[0048] The separator 150 is laminated on one major surface of the
diffusion layer 140, the one major surface being opposite to the
electrolyte membrane 110. The separator 150 is formed of a carbon
resin, or a corrosion-resistant alloy such as Cr--Ni--Fe,
Cr--Ni--Mo--Fe, Cr--Mo--Nb--Ni, or Cr--Mo--Fe--W--Ni. One or more
groove-like flow channels 152 are provided in the surface of the
separator 150 on the diffusion layer 140 side. The liquid aromatic
compound supplied from the organic substance storage tank 30
circulates through the flow channel 152, and the liquid aromatic
compound permeates the liquid diffusion layer 140 from the flow
channel 152. The form of the flow channel 152 is not particularly
limited, but for example, a straight flow channel or a serpentine
flow channel can be employed. When a metal material is used for the
separator 150, the separator 150 may have a structure formed by
sintering sphere-like or pellet-like metal fine powders.
[0049] The oxygen generating electrode 130 is provided in contact
with the other major surface of the electrolyte membrane 110. The
oxygen generating electrode 130 contains, as a catalyst, one or two
or more metals or metal oxides, the metal (metals) being selected
from the group consisting of Ru, Rh, Pd, Ir, and Pt. These
catalysts may be dispersedly supported or coated by or on a metal
substrate having electron conductivity such as: a metal fiber
(fiber diameter: e.g., from 10 to 30 .mu.m) of a metal or an alloy
whose major components are metals, the metal or metals being
selected from the group consisting of Ti, Cr, Mn, Fe, Co, Ni, Cu,
Zn, Nb, Mo, Ta, and W; a mesh (diameter: e.g., from 500 to 1000
.mu.m); a sintered body of a porous metal; a foamed molded body
(form); an expanded metal; or the like. When these catalysts are
layered, the thickness of the catalyst layer is preferably from 0.1
to 10 .mu.m, and more preferably from 0.2 to 5 .mu.m. In
particular, when IrO.sub.2 is used as a catalyst, manufacturing
costs can be reduced by coating a metal substrate with a thin film
because IrO.sub.2 is expensive. The surface of the metal substrate
may be covered with an electron conductive protective layer made of
one or two or more metals or metal oxides, the metal (metals) being
selected from the group consisting of Nb, Mo, Ta, and W. The
thickness of the protective layer is preferably from 0.05 to 2
.mu.m, and more preferably from 0.1 to 1 .mu.m. The metal substrate
plays a role as a substrate for holding the catalyst, and also
functions as a current collecting member.
[0050] FIG. 4 is a schematic view illustrating an enlarged example
of a sectional structure of the oxygen generating electrode 130. In
the oxygen generating electrode 130 of this example, a catalyst 136
is held on the surface of a protective layer 134 covering a metal
substrate 132. The metal substrate 132 is an expanded metal formed
of a metal material such as Ti. The protective layer 134 is formed
of one or two or more metals or metal oxides, the metal (metals)
being selected from the group consisting of Nb, Mo, Ta, and W. The
catalyst 136 is formed of one or two or more metals or metal
oxides, the metal (metals) being selected from the group consisting
of Ru, Rh, Pd, Ir, and Pt. Thus, by covering the metal substrate
132 with the protective layer 134, the durability of the metal
substrate 132 can be improved.
[0051] As illustrated in FIGS. 2 and 3, the current collector 160
electrically connected to the aforementioned oxygen generating
electrode 130 is provided in the electrode unit 100. The current
collector 160 has a substrate portion 162 and a connection portion
164, both formed of a metal having good electron conductivity, such
as copper or aluminum.
[0052] The substrate portion 162 is a planar metal member installed
so as to be spaced apart from the oxygen generating electrode 130
by a predetermined distance. The space between the substrate
portion 162 and the oxygen generating electrode 130 is used for the
circulation of the water supplied to the oxygen generating
electrode 130.
[0053] The connection portion 164 of the present embodiment is a
strip-shaped metal member whose short side is curved. A plurality
of the connection portions 164 are provided in combination so as to
be spaced apart from each other by predetermined intervals in a
state in which one long side of each connection portion 164 is
fixed to the substrate portion 162. The other long side thereof is
electrically connected to the oxygen generating electrode 130. Each
connection portion 164 and the oxygen generating electrode 130 may
be fixed by welding, etc. Because the connection portion 164 has a
shape in which the short side is curved, it can play a role of a
spring. Accordingly, the contact between the connection portion 164
and the oxygen generating electrode 130 can be made more reliable
by pressing the substrate portion 162 toward the oxygen generating
electrode 130. The connection portion 164 is strip-shaped in the
present embodiment, but the shape of the connection portion 164 is
not limited thereto. The connection portion 164 may be formed, for
example, by a spring-like wire, etc.
[0054] In the present embodiment, liquid water is supplied to the
oxygen generating electrode 130, but humidified gas (for example,
air) may be used instead of liquid water. In this case, the
dew-point temperature of the humidified gas is preferably from room
temperature to 100.degree. C., and more preferably from 50 to
100.degree. C.
[0055] When toluene is used as the aromatic compound, reactions in
the electrode unit 100 are as follows.
<Electrode Reaction at Oxygen Generating Electrode>
[0056] 3H.sub.2O.fwdarw.1.5O.sub.2+6H.sup.++6e.sup.-:E0=1.23 V
<Electrode Reaction at Reduction Electrode>
[0057] Toluene+6H.sup.++6e.fwdarw.methylcyclohexane:E0=0.153 V (vs
RHE)
[0058] In other words, the electrode reaction at the oxygen
generating electrode and that at the reduction electrode proceed in
parallel, and the protons generated by the electrolysis of water
occurring due to the electrode reaction at the oxygen generating
electrode 130 is supplied to the reduction electrode 120 via the
electrolyte membrane 110. The proton supplied to the reduction
electrode 120 is used for producing a hydride of an aromatic
compound in the electrode reaction at the reduction electrode
120.
[0059] According to the electrochemical reduction device 10
described above, an aromatic compound is uniformly and quickly
supplied to a reaction site of the reduction electrode catalyst
layer 122 and a hydride of the aromatic compound is promptly
removed from the reaction site, by providing the diffusion layer
140. Thereby, the hydride-producing reaction for an aromatic
compound can be promoted. In particular, even if the concentration
of an aromatic compound is decreased, a decrease in the
hydride-producing reaction for the aromatic compound can be
suppressed. Additionally, a certain amount of moisture can be
retained in the reduction electrode catalyst layer 122 by providing
the dense layer 142 between the diffusion layer 140 and the
reduction electrode catalyst layer 122, and hence the electrolyte
membrane 110 and the reduction electrode catalyst layer 122 are
suppressed from being dried and the hydride-producing reaction for
an aromatic compound can be further promoted.
Examples
[0060] Hereinafter, examples of the present invention will be
described, which do not intend to limit the scope of the invention,
but are presented as preferred illustrative examples of the
invention.
[0061] The configurations of respective electrode units of Examples
from 1 to 5 and Comparative Example 1 are shown in Table 1. The
respective electrode units were evaluated by using toluene as an
aromatic compound. Differences among the respective electrode units
of Examples from 1 to 5 and Comparative Example 1 are as follows:
That is, the electrode units of Examples from 1 to 5 have a dense
layer, while that of Comparative Example 1 does not have a dense
layer. The mean flow pore diameters of the dense layers of Examples
from 1 to 5 were set to be 3.2 .mu.m, 1.1 .mu.m, 0.76 .mu.m, 1
.mu.m, and 23 .mu.m, respectively. The electron conductive powders
of the dense layers of Examples from 1 to 5 were VULCAN.TM., and
the water repellents thereof were PTFE. However, the ratio of the
VULCAN to the PTEE was set to be 1:1 in Examples 1, 2, 4, and 5,
and that was set to be 4:1 in Example 3. The thicknesses of the
dense layers of Examples from 1 to 5 were set to be 8.6 .mu.m, 8.5
.mu.m, 8.4 .mu.m, 0.76 .mu.m, and 9.6 .mu.m, respectively.
TABLE-US-00001 TABLE 1 EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 DIFFUSION
MATERIAL CARBON PAPER CARBON PAPER CARBON PAPER LAYER MEAN FLOW
PORE DIAMETER dm (.mu.m) 42 42 42 ELECTRON CONDUCTIVITY (S/cm) 100
100 100 THICKNESS (.mu.m) 350 350 350 DENSE LAYER MATERIAL RATIO OF
VULCAN RATIO OF VULCAN RATIO OF VULCAN TO PTFE = 1:1 TO PTFE = 1:1
TO PTFE = 4:1 MEAN FLOW PORE DIAMETER dm (.mu.m) 3.2 1.1 0.76
THICKNESS (.mu.m) 8.6 8.5 8.4 REDUCTION MATERIAL Pt (23 wt %), Ru
(27 wt %) Pt (23 wt %), Ru (27 wt %) Pt (23 wt %), Ru (27 wt %)
ELECTRODE RATIO OF NAFION TO RATIO OF NAFION TO RATIO OF NAFION TO
CARBON BLACK = CARBON BLACK = CARBON BLACK = 0.8:1 0.8:1 0.8:1
THICKNESS (.mu.m) 300 300 300 ELECTROLYTE MATERIAL NAFION NAFION
NAFION MEMBRANE THICKNESS (.mu.m) 50 50 50 OXYGEN SUBSTRATE
TITANIUM MESH TITANIUM MESH TITANIUM MESH GENERATING PROTECTIVE
LAYER TANTALUM OXIDE TANTALUM OXIDE TANTALUM OXIDE ELECTRODE
CATALYST IrO.sub.2 IrO.sub.2 IrO.sub.2 THICKNESS (.mu.m) 300 300
300 COMPARATIVE EXAMPLE 1 EXAMPLE 4 EXAMPLE 5 DIFFUSION MATERIAL
CARBON PAPER CARBON PAPER CARBON PAPER LAYER MEAN FLOW PORE
DIAMETER dm (.mu.m) 42 42 42 ELECTRON CONDUCTIVITY (S/cm) 100 100
100 THICKNESS (.mu.m) 350 350 350 DENSE LAYER MATERIAL -- RATIO OF
VULCAN RATIO OF VULCAN TO PTFE = 1:1 TO PTFE = 1:1 MEAN FLOW PORE
DIAMETER dm (.mu.m) -- 1 23 THICKNESS (.mu.m) -- 0.76 9.6 REDUCTION
MATERIAL Pt (23 wt %), Ru (27 wt %) Pt (23 wt %), Ru (27 wt %) Pt
(23 wt %), Ru (27 wt %) ELECTRODE RATIO OF NAFION TO RATIO OF
NAFION TO RATIO OF NAFION TO CATALYST CARBON BLACK = 0.8:1 CARBON
BLACK = 0.8:1 CARBON BLACK = 0.8:1 LAYER THICKNESS (.mu.m) 300 300
300 ELECTROLYTE MATERIAL NAFION NAFION NAFION MEMBRANE THICKNESS
(.mu.m) 50 50 50 OXYGEN SUBSTRATE TITANIUM MESH TITANIUM MESH
TITANIUM MESH GENERATING PROTECTIVE LAYER TANTALUM OXIDE TANTALUM
OXIDE TANTALUM OXIDE ELECTRODE CATALYST IrO.sub.2 IrO.sub.2
IrO.sub.2 THICKNESS (.mu.m) 300 300 300
[0062] In each electrode unit, the density of a current flowing
between a reduction electrode and an oxygen generating electrode,
occurring when the potential difference between the two electrodes
was changed, was measured. FIG. 5 is a graph showing I-V curves
obtained by the respective electrode units of Examples from 1 to 5
and Comparative Example 1. As shown in FIG. 5, it has been
confirmed that, when the potential differences between the
reduction electrode and the reference electrode are made the same
as each other, the absolute values of the current densities are
further increased in Examples from 1 to 5, in comparison with that
in Comparative Example 1. It has been confirmed that the absolute
values of the current densities are remarkably increased
particularly in Examples from 1 to 3. Additionally, the
concentration of the toluene supplied to the reduction electrode
was detected under a condition in which the absolute value of the
current density was set to be 50 mA/cm.sup.2. A converted substance
amount was determined from a change in the toluene concentration by
using gas chromatography. Additionally, an electricity amount was
determined from the time integration of current time transition in
each of the electrochemical measurement devices. Then, with the
converted substance amount being divided by the electricity amount,
Faraday efficiency was calculated. The obtained results with
respect to the Faraday efficiencies are shown in Table 2. The
absolute value of the current density at a boundary point (point
indicated by mark "X" in FIG. 5) was measured, the boundary point
being located between: a region where the hydrogenation of toluene
is dominant, in other words, the Faraday efficiency is almost 100%
and the hydrogenation of toluene selectively proceeds; and a region
where the Faraday efficiency becomes less than 100% and the
hydrogenation of toluene and hydrogen generation competitively
proceed. The obtained results with respect to the boundary points
are shown in Table 2.
TABLE-US-00002 TABLE 2 COMPARATIVE EXAMPLE 1 EXAMPLE 2 EXAMPLE 3
EXAMPLE 1 EXAMPLE 4 EXAMPLE 5 FARADAY EFFICIENCY (%) 96 97 99 73 78
79 BOUNDARY POINT: ABSOLUTE VALUE 72 83 94 32 38 41 OF CURRENT
DENSITY (mA/cm.sup.2)
[0063] As shown in Table 2, it has been confirmed that, when the
absolute value of the current density is set to be 50 mA/cm.sup.2,
the Faraday efficiencies in Examples from 1 to 5, in each of which
the density layer is included, are further increased in comparison
with that in Comparative Example 1.
[0064] It has been confirmed that the Faraday efficiencies in
Examples from 1 to 3, in each of which the thickness of the density
layer is 1 .mu.m or more, are further increased in comparison with
that in Example 4 in which the thickness thereof is less than 1
.mu.m. Further, it has been confirmed that the Faraday efficiencies
in Examples from 1 to 3, in each of which the mean flow pore
diameter of the density layer is 20 .mu.m or less, is still further
increased in comparison with that in Example 5 in which the mean
flow pore diameter is more than 20 .mu.m. Particularly in Examples
from 1 to 3, a larger current was able to be caused to flow in a
state in which the Faraday efficiencies was almost 100%. That is, a
larger amount of toluene was able to be hydrogenated per unit time.
Among them, in Example 3 in which the ratio of the electron
conductive powder to the water repellent was 4 or more, the Faraday
efficiency was further improved in comparison with Examples 1 and 2
in each of which the ratio thereof was less than 4.
[0065] The present invention is not limited to the aforementioned
embodiments, and various modifications, such as a design change,
can be added thereto based on knowledge of those skilled in the
art, and any embodiment to which such modifications are added can
also be included in the scope of the present invention.
[0066] For example, a separator, including a diffusion layer for
uniformly diffusing water to the oxygen generating electrode 130
and a flow channel for circulating water to be supplied to the
oxygen generating electrode 130, may be installed on the side of
the oxygen generating electrode 130 similarly to the side of the
reduction electrode 120, instead of the current collector 160.
[0067] The embodiments described above will be summarized
below.
[Item 1]
[0068] An electrochemical reduction device comprising:
[0069] an electrolyte membrane having proton conductivity;
[0070] a reduction electrode including a reduction electrode
catalyst layer that is provided in contact with one major surface
of the electrolyte membrane and has an electron conductive material
and a metal containing one or both of Pt and Pd supported by the
electron conductive material, a diffusion layer that is provided
near to the other major surface of the reduction electrode catalyst
layer, the other major surface being opposite to the electrolyte
membrane, and that causes a liquid aromatic compound and a hydride
of the aromatic compound to pass through, and a dense layer
provided between the diffusion layer and the reduction electrode
catalyst layer;
[0071] an oxygen generating electrode provided in contact with the
other major surface of the electrolyte membrane;
[0072] a raw material supplier that supplies the aromatic compound
in a liquid state to the reduction electrode;
[0073] a moisture supplier that supplies water or humidified gas to
the oxygen generating electrode; and
[0074] a power controller that externally applies an electric field
such that the reduction electrode has an electronegative potential
and the oxygen generating electrode has an electropositive
potential.
[Item 2]
[0075] The electrochemical reduction device according to Item 1,
wherein
[0076] a mean flow pore diameter of the dense layer is 20 .mu.m or
less.
[Item 3]
[0077] The electrochemical reduction device according to Item 1 or
Item 2, wherein
[0078] a thickness of the dense layer (t) is 1
.mu.m.ltoreq.t.ltoreq.50 .mu.m.
[Item 4]
[0079] The electrochemical reduction device according to any one of
Items 1 to 3, wherein
[0080] the dense layer contains a mixture of an electron conductive
powder and a water repellent.
[Item 5]
[0081] The electrochemical reduction device according to Item 4,
wherein
[0082] a mass ratio of the electron conductive powder to the water
repellent in the mixture is 4 or more.
[Item 6]
[0083] The electrochemical reduction device according to any one of
Items 1 to 5, wherein
[0084] the diffusion layer has a fiber-like shape or a shape in
which many particles are solidified.
[Item 7]
[0085] The electrochemical reduction device according to any one of
Items 1 to 6, wherein
[0086] the diffusion layer contains a carbon fiber.
[Item 8]
[0087] The electrochemical reduction device according to any one of
Items 1 to 7, wherein
[0088] the diffusion layer contains a material having an electron
conductivity of 1.0.times.10.sup.-2 S/cm or more.
* * * * *