U.S. patent application number 15/054760 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 | 20160177460 15/054760 |
Document ID | / |
Family ID | 52585960 |
Filed Date | 2016-06-23 |
United States Patent
Application |
20160177460 |
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; an oxygen generating electrode; a
raw material supplier; a moisture supplier; and a power controller.
The oxygen generating electrode has: an electron conductive metal
substrate; a protective layer that contains one or two or more
metals or metal oxides, the metal (metals) being selected from the
group consisting of Nb, Mo, Ta, and W, and that covers the metal
substrate; and a catalyst that contains 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, and that is held on the
surface of the protective layer.
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: |
52585960 |
Appl. No.: |
15/054760 |
Filed: |
February 26, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2014/004212 |
Aug 18, 2014 |
|
|
|
15054760 |
|
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Current U.S.
Class: |
204/230.2 |
Current CPC
Class: |
C25B 15/02 20130101;
C25B 1/10 20130101; C25B 3/04 20130101; C25B 11/0484 20130101; C25B
15/08 20130101; C25B 11/0447 20130101; C25B 11/04 20130101; Y02E
60/36 20130101; C25B 11/0405 20130101; Y02E 60/366 20130101; C25B
9/08 20130101 |
International
Class: |
C25B 15/08 20060101
C25B015/08; C25B 11/04 20060101 C25B011/04; C25B 15/02 20060101
C25B015/02; C25B 9/08 20060101 C25B009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2013 |
JP |
2013-179556 |
Claims
1. An electrochemical reduction device comprising: an electrolyte
membrane having proton conductivity; a reduction electrode that is
provided in contact with one major surface of the electrolyte
membrane and contains a reduction catalyst for producing a hydride
of an aromatic compound; 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, wherein the oxygen generating
electrode has: an electron conductive metal substrate; a protective
layer that contains one or two or more metals or metal oxides, the
metal (metals) being selected from the group consisting of Nb, Mo,
Ta, and W, and that covers the metal substrate; and a catalyst that
contains 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, and that is held on the surface of the protective
layer.
2. The electrochemical reduction device according to claim 1,
wherein the metal substrate is a molded body selected from the
group consisting of a metal fiber, a sintered body of a metal
porous body, a foamed molded body, and an expanded metal.
3. The electrochemical reduction device according to claim 1,
wherein the metal substrate contains titanium.
4. The electrochemical reduction device according to claim 1,
wherein the catalyst is iridium oxide.
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.
2013-179556, filed on Aug. 30, 2013 and International Patent
Application No. PCT/JP2014/004212, 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 improving the life of an electrolysis cell, in which
a hydride-producing reaction for an aromatic hydrocarbon compound
supplied in a liquid state is performed, 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
for improving the life of a device in which a hydride of an
aromatic compound is 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 that is provided in contact with one major surface of the
electrolyte membrane and contains a reduction catalyst for
producing a hydride of an aromatic compound; 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, in which the oxygen generating
electrode has: an electron conductive metal substrate; a protective
layer that contains one or two or more metals or metal oxides, the
metal (metals) being selected from the group consisting of Nb, Mo,
Ta, and W, and that covers the metal substrate; and a catalyst that
contains 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, and that is held on the surface of the protective
layer.
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 transitions of the potentials and
overpotentials of the oxygen generating electrodes in respective
electrode units of Example 1 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 that is provided in contact with one major surface of the
electrolyte membrane and contains a reduction catalyst for
producing a hydride of an aromatic compound; 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, in which the oxygen generating
electrode has: an electron conductive metal substrate; a protective
layer that contains one or two or more metals or metal oxides, the
metal (metals) being selected from the group consisting of Nb, Mo,
Ta, and W, and that covers the metal substrate; and a catalyst that
contains 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, and that is held on the surface of the protective
layer.
[0020] In the electrochemical reduction device of the
aforementioned embodiment, the metal substrate may be a molded body
selected from the group consisting of a metal fiber, a sintered
body of a porous metal, a foamed molded body, and an expanded
metal. The metal substrate may contain titanium. The catalyst may
be iridium oxide.
[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 (registered trademark) and FLEMION
(registered trademark). 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 VRef. Examples of the reference electrode 112 include a
standard hydrogen reduction electrode (reference electrode
potential VRef=0 V) and an Ag/AgCl electrode (reference electrode
potential VRef=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.VCA 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.VCA 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. The dense layer 142 is not essential, and may be omitted.
[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 (registered
trademark). 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 (registered
trademark) and FLEMION (registered trademark). 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 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.
[0046] 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 are held on the surface of a protective layer covering a
metal substrate having electron conductivity. 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. The metal
substrate plays a role as a substrate for holding the catalyst and
also functions as a current collecting member. The metal substrate
is a molded body 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. The protective layer is an electron conductive 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 protective
layer can be produced, for example, by vapor deposition, ion
plating, sputtering, chemical vapor deposition (CVD), electrolytic
deposition, and coating, etc.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] When toluene is used as the aromatic compound, reactions in
the electrode unit 100 are as follows.
3H.sub.2O.fwdarw.1.5O.sub.2+6H.sup.++6e.sup.-:E0=1.23 V
<Electrode Reaction at Oxygen Generating Electrode>
Toluene+6H.sup.++6e.sup.-.fwdarw.methylcyclohexane:E0=0.153 V (vs
RHE) <Electrode Reaction at Reduction Electrode>
[0053] 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.
[0054] Because organic substances (aromatic compounds and hydrides
thereof) are present at a high concentration on the reduction
electrode 120 side in the aforementioned electrochemical reduction
device 10, unlike in the conventional water electrolysis or brine
electrolysis, there is the possibility that cross-leaking of the
organic substances may occur on the oxygen generating electrode 130
side via the electrolyte membrane 110. Accordingly, substances that
adversely affect the oxygen generating electrodes 130, such as the
organic substances and compounds made by the organic substances
being oxidized by the oxygen generating electrode 130 having a
noble potential (e.g., oxygen-containing compounds such as an
aldehyde and carboxylic acid), may be present on the oxygen
generating electrode 130 side. In conventional water electrolysis
and brine electrolysis, there are problems in which: in the
electrode on the anode side, catalyst metals may aggregate, a
substrate may be corroded, or a catalyst metal layer and the
substrate may be peeled off from each other, by a small amount of
organic substances; however, in the electrochemical reduction
device 10, the concentration of the organic substances, to which
the oxygen generating electrode 130 is exposed, is much higher than
those in the existing water electrolysis and brine electrolysis
(orders are different from each other). In order to industrially
produce a hydride of an aromatic compound, it is required that the
oxygen generating electrode 130 maintains, for a predetermined
period of time, its electrode performance in a state in which large
amounts of aromatic compounds and oxygen-containing compounds
coexist together. Because the oxygen generating electrode 130 is
exposed to an acid, aromatic compounds, and oxygen, etc., all of
the metal substrate, the catalyst, and the interface between the
metal substrate and the catalyst are likely to be deteriorated.
[0055] In the electrochemical reduction device 10 under such a
situation, the aggregation of the catalysts 136, corrosion of the
metal substrate 132, and peeling of the catalyst 136 from the metal
substrate 132 can be suppressed under a condition in which large
amounts of aromatic compounds and oxygen-containing compounds
coexist together by holding the catalyst 136 in the metal substrate
132 covered with the protective layer 134 in the oxygen generating
electrode 130. As a result, the electrode performance of the
electrode unit can be maintained over a longer period of time. It
is particularly preferable that the protective layer 134 contains
at least one of Nb, Mo, Ta, W, and oxides of these metals, from the
viewpoint of both catalyst activity and durability.
Example
[0056] 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.
[0057] The configurations of electrode units of Example 1 and
Comparative Example 1 are shown in Table 1. The durability of each
of the electrode units was evaluated by using toluene as the
aromatic compound.
TABLE-US-00001 TABLE 1 EXAMPLE 1 COMPARATIVE EXAMPLE 1 DIFFUSION
MATERIAL CARBON PAPER CARBON PAPER LAYER MEAN FLOW PORE 42 42
DIAMETER dm (.mu.m) ELECTRON 100 100 CONDUCTIVITY (S/cm) THICKNESS
(.mu.m) 350 350 DENSE LAYER MATERIAL RATIO OF VULCAN TO PTFE = 1:1
RATIO OF VULCAN TO PTFE = 1:1 MEAN FLOW PORE 1 1 DIAMETER dm
(.mu.m) THICKNESS (.mu.m) 10 10 REDUCTION MATERIAL Pt (23 wt %), Ru
(27 wt %) Pt (23 wt %), Ru (27 wt %) ELECTRODE RATIO OF NAFION TO
CARBON RATIO OF NAFION TO CARBON CATALYST LAYER BLACK = 0.8:1 BLACK
= 0.8:1 THICKNESS (.mu.m) 300 300 ELECTROLYTE MATERIAL NAFION
NAFION MEMBRANE THICKNESS (.mu.m) 50 50 OXYGEN SUBSTRATE
SHEET-SHAPED TITANIUM FIBER SHEET-SHAPED TITANIUM FIBER GENERATING
(FIBER DIAMETER 25 .mu.m) (FIBER DIAMETER 25 .mu.m) ELECTRODE
PROTECTIVE LAYER TANTALUM OXIDE -- (THICKNESS 0.5 .mu.m) CATALYST
IrO.sub.2 (THICKNESS 1 .mu.m) IrO.sub.2 (THICKNESS 1 .mu.m)
[0058] FIG. 5 is a graph showing transitions of the potentials and
overpotentials of the oxygen generating electrodes in respective
electrode units of Example 1 and Comparative Example 1. A certain
concentration of toluene was supplied to each of the reduction
electrodes. It has been confirmed that, as shown in FIG. 5, the
electrode unit of Example 1, having a protective layer between a
metal substrate and a catalyst, can maintain an electrode
performance close to the initial performance over a longer period
of time, in comparison with the electrode unit of Comparative
Example 1, not having the protective layer. In Comparative Example
1, the ratio, at which the potential of the oxygen generating
electrode was increased after the hydride-producing reaction was
continued for 1000 hours, was 15.8%. On the other hand, the ratio
was suppressed to 1.2% in Example 1. Accordingly. it has been
confirmed that sufficient durability can be secured even if the
target durability of the electrode unit is set, for example, to be
5%/1000 h. In Comparative Example 1, the ratio, at which the
overpotential was increased after the hydride-producing reaction
was continued for 1000 hours, was 61.9%. On the other hand, the
ratio was suppressed to 4.5% in Example 1. Accordingly, it has been
confirmed that sufficient durability can be secured even if the
target durability of the electrode unit is set, for example, to be
10%/1000 h.
[0059] 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.
[0060] The embodiments described above will be summarized
below.
[Item 1]
[0061] An electrochemical reduction device comprising:
[0062] an electrolyte membrane having proton conductivity;
[0063] a reduction electrode that is provided in contact with one
major surface of the electrolyte membrane and contains a reduction
catalyst for producing a hydride of an aromatic compound;
[0064] an oxygen generating electrode provided in contact with the
other major surface of the electrolyte membrane;
[0065] a raw material supplier that supplies the aromatic compound
in a liquid state to the reduction electrode;
[0066] a moisture supplier that supplies water or humidified gas to
the oxygen generating electrode; and
[0067] 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, wherein
[0068] the oxygen generating electrode has: an electron conductive
metal substrate; a protective layer that contains one or two or
more metals or metal oxides, the metal (metals) being selected from
the group consisting of Nb, Mo, Ta, and W, and that covers the
metal substrate; and a catalyst that contains 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, and that is held on the
surface of the protective layer.
[Item 2]
[0069] The electrochemical reduction device according to Item 1,
wherein
[0070] the metal substrate is a molded body selected from the group
consisting of a metal fiber, a sintered body of a metal porous
body, a foamed molded body, and an expanded metal.
[Item 3]
[0071] The electrochemical reduction device according to Item 1 or
Item 2, wherein
[0072] the metal substrate contains titanium.
[Item 4]
[0073] The electrochemical reduction device according to any one of
Items 1 to 3, wherein
[0074] the catalyst is iridium oxide.
* * * * *