U.S. patent application number 13/805865 was filed with the patent office on 2013-05-23 for carbon dioxide immobilization unit.
This patent application is currently assigned to SONY CORPORATION. The applicant listed for this patent is Hiroki Mita, Hideki Sakai, Yuichi Tokita. Invention is credited to Hiroki Mita, Hideki Sakai, Yuichi Tokita.
Application Number | 20130126336 13/805865 |
Document ID | / |
Family ID | 45469377 |
Filed Date | 2013-05-23 |
United States Patent
Application |
20130126336 |
Kind Code |
A1 |
Sakai; Hideki ; et
al. |
May 23, 2013 |
CARBON DIOXIDE IMMOBILIZATION UNIT
Abstract
There is provided a carbon dioxide immobilization unit capable
of easily immobilizing carbon dioxide in the form of an organic
acid or a carbohydrate under a normal environment. An anode and a
cathode both having a surface where an oxidoreductase is present
are disposed to face each other with a proton conductor in between.
Then, when electric power is externally supplied to the carbon
dioxide immobilization unit, in the anode, water is decomposed to
produce protons, and in the cathode, an organic acid or a
carbohydrate is produced from the protons produced in the anode and
carbon dioxide. At this time, while a carbon dioxide supply section
supplies a high concentration of carbon dioxide to the cathode,
oxygen produced in the anode and the organic acid or hydrocarbon
produced in the cathode are removed from a reaction system through
an oxygen removal section and a product recovery section,
respectively.
Inventors: |
Sakai; Hideki; (Kanagawa,
JP) ; Mita; Hiroki; (Kanagawa, JP) ; Tokita;
Yuichi; (Kanagawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sakai; Hideki
Mita; Hiroki
Tokita; Yuichi |
Kanagawa
Kanagawa
Kanagawa |
|
JP
JP
JP |
|
|
Assignee: |
SONY CORPORATION
Tokyo
JP
|
Family ID: |
45469377 |
Appl. No.: |
13/805865 |
Filed: |
July 8, 2011 |
PCT Filed: |
July 8, 2011 |
PCT NO: |
PCT/JP2011/065681 |
371 Date: |
December 20, 2012 |
Current U.S.
Class: |
204/225 ;
204/242; 204/277 |
Current CPC
Class: |
C25B 3/04 20130101; C25B
1/02 20130101; C25B 9/12 20130101; C25B 11/00 20130101 |
Class at
Publication: |
204/225 ;
204/242; 204/277 |
International
Class: |
C25B 3/04 20060101
C25B003/04; C25B 9/12 20060101 C25B009/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 16, 2010 |
JP |
2010-162130 |
Claims
1. A carbon dioxide immobilization unit comprising at least: a
first electrode decomposing water to produce protons; a second
electrode producing an organic acid or a carbohydrate from the
protons produced in the first electrode and carbon dioxide; and a
proton conductor transferring the protons produced in the first
electrode to the second electrode, wherein an oxidoreductase is
present on a surface of the first electrode or a surface of the
second electrode, or both.
2. The carbon dioxide immobilization unit according to claim 1,
further comprising a carbon dioxide supply section supplying carbon
dioxide to the second electrode.
3. The carbon dioxide immobilization unit according to claim 2,
wherein the carbon dioxide supply section supplies a gas containing
carbon dioxide in concentration of 0.028 to 100 vol % both
inclusive.
4. The carbon dioxide immobilization unit according to claim 3,
comprising: an oxygen removal section removing oxygen produced in
the first electrode; and a product recovery section extracting the
organic acid or the carbohydrate produced in the second
electrode.
5. The carbon dioxide immobilization unit according to claim 1,
wherein the first electrode is a dipping type electrode which is
directly in contact with a liquid phase or is in contact with the
liquid phase with a separator in between, and the second electrode
is a semi-dipping type electrode which is directly in contact with
the liquid phase or is in contact with the liquid phase with a
separator in between, as well as is in contact with a vapor phase
with a gas-liquid separator film in between.
6. The carbon dioxide immobilization unit according to claim 1,
wherein the first electrode or the second electrode, or both are
formed of a conductive porous material.
Description
TECHNICAL FIELD
[0001] The present invention relates to a carbon dioxide
immobilization unit using an oxidoreductase. More specifically, the
invention relates to a carbon dioxide immobilization unit producing
an organic acid or a carbohydrate from carbon dioxide.
BACKGROUND ART
[0002] Biofuel cells using an oxidoreductase as a reaction catalyst
have been attracting an attention as next-generation fuel cells
with high capacity and high safety, since the biofuel cells
effectively extract electrons from glucose, ethanol, and the like
which are not usable in fuel cells using a typical industrial
catalyst.
[0003] FIG. 5 is a diagram schematically illustrating an electric
power generation principle of a biofuel cell using an enzyme. For
example, in the case of a biofuel cell using glucose as a fuel as
illustrated in FIG. 5, in an anode 101, glucose is decomposed by an
enzyme immobilized on a surface thereof to extract electrons
(e.sup.-) and to produce protons (H.sup.+). On the other hand, in a
cathode 102, water (H.sub.2O) is produced from the protons
(H.sup.+) transported from the anode 101 through a proton conductor
103, the electrons (e.sup.-) transmitted through an external
circuit, and oxygen (O.sub.2) in, for example, air.
[0004] FIG. 6 is a diagram schematically illustrating an electric
power generation principle of a methanol type biofuel cell.
Moreover, as illustrated in FIG. 6, a biofuel cell using methanol
as a fuel to generate electric power has been proposed in related
art (for example, refer to PTL 1). In this biofuel cell, alcohol
dehydrogenase (ADH), formaldehyde genase (FalDH), and formate
dehydrogenase (FateDH) are immobilized on the surface of the anode
101.
[0005] Then, in the anode 101, methanol (CH.sub.3OH) is decomposed
by these enzymes to extract electrons (e.sup.-) and to produce
protons (H.sup.+), and then to produce carbon dioxide (CO.sub.2).
On the other hand, in the cathode 102, water (H.sub.2O) is produced
from the protons (H.sup.+) transported from the anode 101 through
the proton conductor 103, the electrons (e.sup.-) transmitted
through the external circuit, and oxygen (O.sub.2) in, for example,
air.
[0006] On the other hand, in related art, there are proposed
methods of storing and producing hydrogen with use of a unit
including a formic acid decomposition section and a formic acid
production section (refer to PTLs 2 and 3). In this unit for formic
acid production and decomposition, the formic acid production
section produces formic acid through allowing hydrogen and carbon
dioxide to react with each other by a catalyst for formic acid
production, and then stores hydrogen in the form of formic
acid.
[0007] Moreover, in the formic acid decomposition section, the
formic acid produced in the formic acid production section is
decomposed into hydrogen and carbon dioxide by a catalyst for
formic acid decomposition. Hydrogen produced by this decomposition
reaction is used for an arbitrary purpose such as a fuel cell. On
the other hand, carbon dioxide as a by-product is transmitted to
the formic acid production section to be used for formic acid
production.
CITATION LIST
Patent Literature
[0008] [PTL 1] Japanese Unexamined Patent Application Publication
No. 2004-71559 [0009] [PTL 2] Japanese Unexamined Patent
Application Publication No. 2009-78200 [0010] [PTL 3] Japanese
Unexamined Patent Application Publication No. 2010-83730
SUMMARY OF INVENTION
[0011] However, in the techniques described in the above PTLs 2 and
3, hydrogen used as a reducing agent to produce formic acid from
carbon dioxide is not stably present under a normal environment;
therefore, there is an issue that extra energy is necessary to
obtain hydrogen as a raw material. Thus, a user-friendly technique
of immobilizing carbon dioxide in the form of formic acid or a
carbon compound such as carbohydrate has not yet established.
[0012] Therefore, it is a main object of the present invention to
provide a carbon dioxide immobilization unit capable of easily
immobilizing carbon dioxide in the form of an organic acid or a
carbohydrate under a normal environment.
[0013] A carbon dioxide immobilization unit according to the
invention includes at least: a first electrode decomposing water to
produce protons; a second electrode producing an organic acid or a
carbohydrate from the protons produced in the first electrode and
carbon dioxide; and a proton conductor transferring the protons
produced in the first electrode to the second electrode, in which
an oxidoreductase is present on a surface of the first electrode or
a surface of the second electrode, or both.
[0014] Here and in the following description, a surface of an
electrode includes an outer surface of the electrode and an inner
surface of a gap in an inside of the electrode.
[0015] The carbon dioxide immobilization unit may further include a
carbon dioxide supply section supplying carbon dioxide to the
second electrode. In this case, the carbon dioxide supply section
may supply a gas containing carbon dioxide in concentration of
0.028 to 100 vol % both inclusive.
[0016] Moreover, the carbon dioxide immobilization unit may include
an oxygen removal section removing oxygen produced in the first
electrode; and a product recovery section extracting the organic
acid or the carbohydrate produced in the second electrode.
[0017] Further, the first electrode may be a dipping type electrode
which is directly in contact with a liquid phase or is in contact
with the liquid phase with a separator in between, and the second
electrode may be a semi-dipping type electrode which is directly in
contact with the liquid phase or is in contact with the liquid
phase with a separator in between, as well as is in contact with a
vapor phase with a gas-liquid separator film in between.
[0018] Furthermore, the first electrode or the second electrode, or
both may be formed of, for example, a conductive porous
material.
[0019] According to the present invention, as the oxidoreductase is
used, carbon dioxide is allowed to be easily immobilized in the
form of an organic acid or a carbohydrate through only inputting
electric power to the carbon dioxide immobilization unit without
using hydrogen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a diagram schematically illustrating a principle
of a carbon dioxide immobilization unit according to an embodiment
of the invention.
[0021] FIG. 2 is a diagram schematically illustrating an electrode
configuration of an anode 1 illustrated in FIG. 1 which is of a
dipping type.
[0022] FIG. 3 is a diagram schematically illustrating an electrode
configuration of a cathode 2 illustrated in FIG. 1 which is of a
semi-dipping type.
[0023] FIG. 4 is a diagram schematically illustrating a principle
of a carbon dioxide immobilization unit according to a modification
example of the above-described embodiment of the invention.
[0024] FIG. 5 is a diagram schematically illustrating an electric
power generation principle of a biofuel cell using an enzyme.
[0025] FIG. 6 is a diagram schematically illustrating an electric
power generation principle of a methanol type biofuel cell.
DESCRIPTION OF EMBODIMENTS
[0026] An embodiment of the present invention will be described in
detail below referring to the accompanying drawings.
[0027] It is to be noted that the present invention is not limited
to the following embodiment. Moreover, description will be given in
the following order.
[0028] 1. Embodiment
[0029] (An example of a carbon dioxide immobilization unit
producing formic acid from carbon dioxide)
[0030] 2. Modification Example
[0031] (An example of a carbon dioxide immobilization unit
producing methanol from carbon dioxide)
1. First Embodiment
Entire Configuration of Cell
[0032] FIG. 1 is a diagram schematically illustrating a carbon
dioxide immobilization unit according to an embodiment of the
invention. Moreover, FIG. 2 is a diagram schematically illustrating
an electrode configuration of an anode 1 (a first electrode) which
is of a dipping type, and FIG. 3 is a diagram schematically
illustrating an electrode configuration of a cathode 2 (a second
electrode) which is of a semi-dipping type. As illustrated in FIG.
1, the carbon dioxide immobilization unit according to the
embodiment includes the anode 1 and the cathode 2 which are
disposed to face each other with a proton conductor 3 in
between.
[0033] In the carbon dioxide immobilization unit, an oxidoreductase
is present on a surface of the anode 1 or a surface of the cathode
2, or both, and an organic acid such as formic acid or a
carbohydrate such as glucose is produced from carbon dioxide
(CO.sub.2) by reaction opposite to reaction in a biofuel cell in
related art. Here and in the following description, a surface of an
electrode includes an outer surface of the electrode and an inner
surface of a gap in an inside of the electrode.
[0034] [Anode 1]
[0035] In the anode 1, water (H.sub.2O) is oxidatively decomposed
to produce oxygen (O.sub.2) and to extract protons (H.sup.+) and
electrons (e.sup.-). Therefore, the anode 1 adopts a dipping type
electrode configuration in which the anode 1 is directly in contact
with a liquid phase such as an electrolytic solution 13 including a
buffer substance or is in contact with the liquid phase with a
separator 14 made of nonwoven or the like in between as illustrated
in FIG. 2. It is to be noted that, in the electrode configuration
illustrated in FIG. 2, the electrolytic solution 13 serves as the
proton conductor 3.
[0036] An electrode configuring the anode 1 is not specifically
limited; however, for example, an electrode including, on a surface
of an electrode 11 made of a conductive porous material, an enzyme
immobilization layer 12 where an oxidoreductase or the like is
immobilized may be used. As the conductive porous material used in
this case, a known material may be used, and in particular, a
carbon-based material such as porous carbon, carbon pellets, carbon
felt, carbon paper, or a laminate of carbon fiber or carbon
microparticles is suitable.
[0037] Moreover, examples of the oxidoreductase immobilized on the
surface of the anode 1 include bilirubin oxidase (BOD), laccases,
and ascorbate oxidase. Moreover, an electron mediator may be
immobilized, together with the above-described enzyme, on the
surface of the anode 1 to decompose water by reaction of the enzyme
and the electron mediator. In this case, as the electron mediator,
a compound having a quinone skeleton is preferably used, and in
particular, a compound having a naphthoquinone skeleton is
suitable. More specifically, 2-amino-1,4-naphthoquinone (ANQ),
2-amino-3-methyl-1,4-naphthoquinone (AMNQ),
2-methyl-1,4-naphthoquinone (VK3),
2-amino-3-carboxy-1,4-naphthoquinone (ACNQ) or the like may be
used.
[0038] It is to be noted that, as the compound having the quinone
skeleton, in addition to the compound having the naphthoquinone
skeleton, for example, anthraquinone or a derivative thereof may be
used. Moreover, if necessary, one kind or two or more kinds of
other compounds functioning as electron mediators may be
immobilized together with the compound having the quinone skeleton.
Further, the anode 1 is not limited to an electrode having a
surface on which an oxidoreductase is immobilized, and, for
example, an electrode to which a microorganism including an
oxidoreductase and functioning as a reaction catalyst is attached
may be used, as long as the oxidoreductase is present on a surface
of the electrode.
[0039] [Cathode 2]
[0040] On the other hand, in the cathode 2, an organic acid such as
formic acid or a carbohydrate such as glucose is produced from
carbon dioxide (CO.sub.2), and the protons (H.sup.+) and the
electrons (e) produced in the anode 1. Therefore, the cathode 2
adopts an air-exposure type electrode configuration in which an
electrode is directly in contact with a vapor phase to allow carbon
dioxide to be sufficiently supplied thereto, or a semi-dipping type
electrode configuration, as illustrated in FIG. 3, in which an
electrode is in contact with the vapor phase with a gas-liquid
separation film 25 in between. In the case where the cathode 2 is
the semi-dipping type electrode, the cathode 2 is also directly in
contact with the liquid phase such as the electrolytic solution 13
including the buffer substance, or is also in contact with the
separator 24 made of nonwoven in between, as illustrated in FIG.
3.
[0041] Moreover, as the cathode 2, for example, an electrode
including an enzyme immobilization layer 22 on a surface of an
electrode 21 made of a conductive porous material may be used. As
the conductive porous material forming the cathode 2, a known
material may be also used, and in particular, a carbon-based
material such as porous carbon, carbon pellets, carbon felt, carbon
paper, or a laminate of carbon fiber or carbon microparticles is
suitable.
[0042] On the other hand, the enzyme immobilized on the surface of
the cathode 2 is allowed to be appropriately selected depending on
a product, and, for example, when formic acid is produced, formate
dehydrogenase (FDH) may be used. Moreover, when glucose is
produced, glucose dehydrogenase (GDH) may be used.
[0043] In addition, electron transfer enzymes, ATP synthases,
enzymes relating to saccharometabolism, for example, known enzymes
such as hexokinase, glucose phosphate isomerase,
phosphofructokinase, fructose bisphosphate aldolase,
triosephosphate isomerase, glyceraldehydephosphate dehydrogenase,
phosphoglyceromutase, phosphopyruvate hydratase, pyruvate kinase,
L-lactate dehydrogenase, D-lactate dehydrogenase, pyruvate
dehydrogenase, citrate synthase, aconitase, isocitrate
dehydrogenase, 2-oxoglutarate dehydrogenase, succinyl-CoA
synthetase, succinate dehydrogenase, fumarase, and malonate
dehydrogenase may be usable.
[0044] Moreover, coenzyme oxidase or an electron mediator is
preferably immobilized, together with the enzyme such as FDH, on
the surface of the cathode 2. Examples of a coenzyme used in this
case include NADH and NADPH, and diaphorase reducing an oxidant
thereof (such as NAD.sup.+ or NADP.sup.+). Moreover, examples of
the electron mediator immobilized together with these enzymes
include potassium hexacyanoferrate, potassium ferricyanide, and
potassium octacyanotungstate.
[0045] It is to be noted that the cathode 2 is also not limited to
an electrode having a surface on which an oxidoreductase is
immobilized, and, for example, an electrode to which a
microorganism including an oxidoreductase and functioning as a
reaction catalyst is attached may be used, as long as the
oxidoreductase is present on a surface of the electrode. Moreover,
as illustrated in FIG. 3, in the electrode 21 configuring the
cathode 2, a vapor-liquid coexistence layer 23 in which a vapor
phase and a liquid phase coexist may be further formed on an outer
side of the enzyme immobilization layer 22 where the enzyme or the
like is immobilized.
[0046] [Proton Conductor 3]
[0047] The proton conductor 3 may be a material not having
electronic conductivity and capable of transporting protons
(H.sup.+), and an electrolytic solution including a buffer
substance is typically used. In this case, for example, when a
separator (for example, cellophane, nonwoven, or the like)
impregnated with an electrolytic solution is sandwiched between
electrodes, a short circuit is preventable while protons are
conducted. Moreover, as the proton conductor 3, a separator having
proton conductivity such as an ion-exchange resin film having a
fluorine-containing carbon sulfonic acid group may be used.
[0048] On the other hand, it is preferable that the carbon dioxide
immobilization unit according to the embodiment include a carbon
dioxide supply section 5 supplying carbon dioxide or a gas
containing carbon dioxide to the cathode 2. Moreover, it is more
preferable that the carbon dioxide immobilization unit further
include an oxygen removal section 4 removing oxygen produced in the
anode 1, and a product recovery section 6 extracting the organic
acid or the carbohydrate produced in the cathode 2.
[0049] [Oxygen Removal Section 4]
[0050] To accelerate the above-described anode reaction, it is
preferable to remove oxygen (O.sub.2) existing around the anode 1.
A method of doing so, that is, the configuration of the oxygen
removal section 4 is not specifically limited; however, for
example, the oxygen removal section 4 may have a configuration in
which a solution around the anode 1 is allowed to flow, and
deoxidized water is supplied to emit a solution including oxygen.
Thus, the oxygen concentration in the solution around the anode 1
is allowed to be reduced; therefore, a decline in anode reaction is
preventable.
[0051] [Carbon Dioxide Supply Section 5]
[0052] To accelerate cathode reaction, it is preferable to supply a
sufficient amount of carbon dioxide (CO.sub.2) to the cathode 2.
Therefore, in the carbon dioxide immobilization unit according to
the embodiment, carbon dioxide or a gas containing carbon dioxide
is supplied from the carbon dioxide supply section 5 to the cathode
2 or its surrounding. The gas supplied from the carbon dioxide
supply section 5 may be a gas containing carbon dioxide in
concentration equivalent to or higher than the concentration of
carbon dioxide in air, and, for example, in the case where the
concentration of carbon dioxide in air is 0.028 vol %, a gas
containing carbon dioxide in concentration of 0.028 to 100 vol %
both inclusive may be supplied to the cathode 2. Thus, the
concentration of carbon dioxide around the cathode 2 is allowed to
be maintained at a high level, and reaction efficiency is allowed
to be enhanced.
[0053] As the gas supplied from the carbon dioxide supply section 5
to the cathode 2, for example, exhaust from a thermal power station
or a vehicle may be used, and dry ice, exhalation, and the like may
be also used. A gas containing a high concentration of carbon
dioxide is easily obtainable through using such a gas, and the
organic acid or the carbohydrate is efficiently obtainable.
[0054] [Product Recovery Section 6]
[0055] To accelerate the above-described cathode reaction, it is
preferable to remove a product (the organic acid or the
carbohydrate) existing around the cathode 2. A method of doing so,
that is, the configuration of the product recovery section 6 is not
specifically limited; however, for example, a method of recovering
a product contained in a solution around the cathode 2 through
allowing the solution to flow and converting the product into a
salt to precipitate the salt, or a method of recovering the product
through allowing an absorbent such as activated carbon to absorb
the product is applicable.
[0056] Thus, the concentration of the product in the solution
around the cathode 2 is allowed to be reduced, thereby preventing a
decline in cathode reaction. It is to be noted that, in the case
where such a product recovery section 6 is included, it is
preferable that the cathode 2 be a semi-dipping type electrode.
Therefore, as the solution around the cathode 2 flows, the product
is allowed to be immediately removed from the cathode 2 and
recovered.
[0057] [Operation]
[0058] Next, the operation of the carbon dioxide immobilization
unit according to the embodiment will be described below. As
illustrated in FIG. 1, when input electric power is externally
supplied to the carbon dioxide immobilization unit according to the
embodiment, the following reaction proceeds.
[0059] More specifically, in the anode 1, water (H.sub.2O) is
oxidized by the oxidoreductase in the enzyme immobilization layer
12 disposed on the surface to extract protons (H.sup.+) and
electrons (e.sup.-). For example, the oxygen removal section 4
allows oxygen (O.sub.2) produced by this reaction to exit from the
carbon dioxide immobilization unit. On the other hand, the protons
(H.sup.+) are transferred to the cathode 2 through the proton
conductor 3, and the electrons (e.sup.-) are transmitted to the
cathode 2 through an external circuit.
[0060] Moreover, in the cathode 2, an organic acid or a
carbohydrate is produced from the protons (H.sup.+) and the
electrons (e.sup.-) produced in the anode 1, and carbon dioxide
(CO.sub.2) which is supplied from, for example, the carbon dioxide
supply section 5 and is present in a vapor phase or a liquid phase
in contact with the cathode 2. It is to be noted that, for example,
the product recovery section 6 allows the organic acid or the
carbohydrate produced by this reaction to exit from the carbon
dioxide immobilization unit.
[0061] Thus, in the carbon dioxide immobilization unit according to
the embodiment, as the oxidoreductase is used, the organic acid or
the carbohydrate is allowed to be easily produced through only
inputting electric power to the carbon dioxide immobilization unit
without using hydrogen. Moreover, in a field where carbon dioxide
is emitted, the carbon dioxide immobilization unit is allowed to
immobilize carbon dioxide more efficiently, and a useful carbon
compound is obtainable.
[0062] For example, to immobilize carbon dioxide, a method of
trapping carbon dioxide deep in the ground, or the like is
considered in related art; however, an enormous amount of energy is
necessary in the method or the like, and downsizing is difficult.
On the other hand, the carbon dioxide immobilization unit according
to the embodiment is allowed to immobilize carbon dioxide as a
useful compound while using energy (electric power), and has a
small and simple configuration; therefore, the carbon dioxide
immobilization unit is applicable to a wide range of fields.
2. Modification Example of First Embodiment
Entire Configuration of Cell
[0063] In the above-described embodiment, the carbon dioxide
immobilization unit producing formic acid from carbon dioxide is
described; however, the present invention is not limited thereto,
and the carbon dioxide immobilization unit is allowed to produce a
carbohydrate such as methanol or glucose in addition to the organic
acid such as formic acid.
[0064] FIG. 4 is a diagram schematically illustrating a principle
of a carbon dioxide immobilization unit according to a modification
example of the above-described embodiment. It is to be noted that,
in FIG. 4, like components are denoted by like numerals as of the
carbon dioxide immobilization unit according to the first
embodiment illustrated in FIG. 1 and will not be further described.
As illustrated in FIG. 4, the carbon dioxide immobilization unit
according to the modification example also includes an anode 31 and
a cathode 32 which are disposed to face each other with the proton
conductor 3 in between.
[0065] [Cathode 32]
[0066] In the carbon dioxide immobilization unit according to the
modification example, three kinds of NAD.sup.+-dependent
dehydrogenases as dehydrogenase groups are immobilized in the
cathode 32, and methanol (CH.sub.3OH) is produced from CO.sub.2
through a plurality of steps. More specifically, formic acid is
produced from carbon dioxide by formate dehydrogenase (FateDH).
Then, the formic acid is converted into formaldehyde by
formaldehyde genase (FalDH), and then methanol is produced by
alcohol dehydrogenase (ADH).
[0067] [Operation]
[0068] When input electric power is supplied to the carbon dioxide
immobilization unit according to the modification example, the
following reaction proceeds. More specifically, in the anode 31,
water (H.sub.2O) is oxidized by the oxidoreductase existing on the
enzyme immobilization layer 12 disposed on the surface to extract
protons (H.sup.+) and electrons (e.sup.-). For example, the oxygen
removal section 4 allows oxygen (O.sub.2) produced by this reaction
to exit from the carbon dioxide immobilization unit. On the other
hand, the protons (H.sup.+) are transferred to the cathode 32
through the proton conductor 3, and the electrons (e.sup.-) are
transmitted to the cathode 2 through an external circuit.
[0069] Moreover, in the cathode 32, formic acid, formaldehyde, and
methanol are produced from the protons (H.sup.+) and the electrons
(e.sub.-) produced in the anode 31, and carbon dioxide (CO.sub.2)
which is supplied from, for example, the carbon dioxide supply
section 5 and is present in a vapor phase or a liquid phase in
contact with the cathode 32. Then, if necessary, product recovery
sections 36a to 36c allow formic acid, formaldehyde, and methanol
produced in such a manner, respectively, to exit from the carbon
dioxide immobilization unit.
[0070] In the carbon dioxide immobilization unit according to the
modification example, as carbon dioxide is allowed to be reduced by
multiple-step reaction, production of methanol which is
commercially more beneficial than formic acid and has not been
proposed in carbon dioxide immobilization units in related art is
achievable. Moreover, in the carbon dioxide immobilization unit
according to the modification example, three kinds of intermediate
products including formic acid are allowed to be recovered, if
necessary. It is to be noted that configurations and effects of the
carbon dioxide immobilization unit according to the modification
example other than those described above are similar to those in
the above-described embodiment.
* * * * *