U.S. patent application number 11/898544 was filed with the patent office on 2008-03-13 for electrode for electrolysis, electrolytic process using the electrode, and electrolytic apparatus using them.
This patent application is currently assigned to SANYO ELECTRIC CO., LTD.. Invention is credited to Mineo Ikematsu, Masahiro Iseki, Kazuhiro Kaneda, Kenta Kitsuka.
Application Number | 20080060947 11/898544 |
Document ID | / |
Family ID | 38805851 |
Filed Date | 2008-03-13 |
United States Patent
Application |
20080060947 |
Kind Code |
A1 |
Kitsuka; Kenta ; et
al. |
March 13, 2008 |
Electrode for electrolysis, electrolytic process using the
electrode, and electrolytic apparatus using them
Abstract
An object of the present invention is to provide an electrode
for electrolysis which is available by an easy fabrication process,
can produce ozone water at high efficiency and also can produce
hydrogen peroxide and OH radicals having a high oxidizing power by
the electrolysis of water at a low current density; an electrolytic
process using the electrode; and an electrolytic apparatus using
them. The electrode for electrolysis according to the present
invention has a substrate and a surface layer formed on the surface
thereof, the surface layer being made of anatase type titanium
oxide.
Inventors: |
Kitsuka; Kenta; (Gunma,
JP) ; Kaneda; Kazuhiro; (Saitama, JP) ;
Ikematsu; Mineo; (Ibaraki, JP) ; Iseki; Masahiro;
(Saitama, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
SANYO ELECTRIC CO., LTD.
|
Family ID: |
38805851 |
Appl. No.: |
11/898544 |
Filed: |
September 13, 2007 |
Current U.S.
Class: |
205/466 ;
204/242; 204/290.12; 205/464 |
Current CPC
Class: |
C25B 11/059 20210101;
C25B 11/093 20210101; Y02E 60/36 20130101; C02F 2209/23 20130101;
C25B 1/13 20130101; C02F 2001/46138 20130101; C02F 2201/46115
20130101; C02F 2201/46125 20130101; C25B 11/077 20210101; C02F
2201/4611 20130101; C02F 1/4618 20130101; C25B 1/30 20130101; C02F
2001/46185 20130101 |
Class at
Publication: |
205/466 ;
204/242; 204/290.12; 205/464 |
International
Class: |
C25B 1/04 20060101
C25B001/04; C25B 1/30 20060101 C25B001/30; C25B 11/04 20060101
C25B011/04; C25B 9/00 20060101 C25B009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 13, 2006 |
JP |
JP 2006-248251 |
Mar 7, 2007 |
JP |
JP 2007-57303 |
Claims
1. An electrode for electrolysis comprising a substrate and a
surface layer formed on the surface thereof, wherein the surface
layer is anatase type titanium oxide.
2. An electrode for electrolysis according to claim 1, wherein the
surface layer has a thickness not greater than 1 mm.
3. An electrode for electrolysis according to claim 1 or 2, wherein
the surface layer has a thickness from 200 nm to 600 nm.
4. An electrode for electrolysis according to any one of claims 1
to 3, wherein the substrate is conductive at least a contact
surface thereof with the surface layer.
5. An electrolytic process, which comprises electrolyzing water or
an electrolyte solution at a current density from 0.1 mA/cm.sup.2
to 2000 mA/cm.sup.2 while using an electrode for electrolysis as
claimed in any one of claims 1 to 4 as an anode.
6. An electrolytic process according to claim 5, wherein the
substance produced by the electrolytic process is at least one of
ozone, hydrogen peroxide and another active oxygen species.
7. An electrolytic apparatus, which comprises an electrode for
electrolysis as claimed in any one of claims 1 to 4 and
electrolyzes water or an electrolyte solution by using the
electrode in accordance with an electrolytic process as claimed in
claim 5 or 6.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an electrode for
electrolysis to be used in electrolysis for industrial or consumer
use, an electrolytic process using the electrode, and an
electrolytic apparatus using them.
[0002] Ozone is typically a very oxidative substance so that water
having ozone dissolved therein which is so-called ozone water is
expected to be utilized for various cleaning and sterilizing
treatments of water and sewerage systems, food, or the like and
cleaning treatment in a fabrication process of semiconductor
devices. As a process of producing ozone water, there is known a
process of dissolving, in water, ozone produced by ultraviolet
irradiation or electric discharge, or a process of producing ozone
in water by electrolysis of water.
[0003] In Japanese Patent Laid-Open No. 11-269686 (Patent Document
1), there is disclosed a process of producing oxygen, ozone and
hydrogen peroxide by electrolysis of water while using an electrode
substance having a conductive diamond structure as an anode. In
Japanese Patent No. 3298431 (Patent Document 2), there is also
disclosed a process of producing hydrogen peroxide and ozone by
electrolyzing water or the like in a plurality of cells.
[0004] The above-described process of Patent Document 1 however has
a problem that it uses diamond as an electrode substance, which
raises the cost of the apparatus itself.
[0005] The above-described process of Patent Document 2, on the
other hand, has also a problem that it is inferior in working
efficiency because ozone and hydrogen peroxide must be produced by
electrolysis in each of a plurality of cells so that an aqueous
solution having ozone and hydrogen peroxide dissolved therein
cannot be obtained by a single electrolytic operation.
SUMMARY OF THE INVENTION
[0006] The present invention has been made to overcome the
conventional technological problems. An object of the present
invention is to provide an electrode for electrolysis which can be
fabricated in an easy manner, can produce ozone at a high
efficiency by the electrolysis of water at a low current density,
and at the same time, can produce hydrogen peroxide and highly
oxidative OH radicals; an electrolytic process using the electrode;
and an electrolytic apparatus using the electrode and process.
[0007] In a first aspect of the present invention, there is
provided an electrode for electrolysis comprising a substrate and a
surface layer formed over the surface of the substrate, wherein the
surface layer is made of an anatase type titanium oxide.
[0008] In a second aspect of the present invention, there is
provided an electrode for electrolysis according to the
above-described aspect of the present invention, wherein the
surface layer has a thickness not greater than 1 mm.
[0009] In a third aspect of the present invention, there is
provided an electrode for electrolysis according to the
above-described aspects of the present invention, wherein the
surface layer has a thickness from 200 nm to 600 nm.
[0010] In a fourth aspect of the present invention, there is
provided an electrode for electrolysis according to the
above-described aspects of the present invention, wherein the
substrate is conductive at least a contact surface thereof with the
surface layer.
[0011] In a fifth aspect of the present invention, there is
provided an electrolytic process, which comprises electrolyzing
water or an electrolyte solution at a current density from 0.1
mA/cm.sup.2 to 2000 mA/cm.sup.2 while using any one of electrodes
as described in the above-described aspects of the present
invention as an anode.
[0012] In a sixth aspect of the present invention, there is
provided an electrolytic process, wherein a substance produced by
an electrolytic process as described in the above-described aspect
of the present invention is at least one of ozone, hydrogen
peroxide and another active oxygen species.
[0013] In a seventh aspect of the present invention, there is
provided an electrolytic apparatus which comprises an electrode for
electrolysis as described above in any one of the first to fourth
aspects of the present invention and electrolyzes water or an
electrolyte solution by an electrolytic process as described above
in the fifth or sixth aspect of the present invention.
[0014] The electrode for electrolysis according to the first aspect
of the present invention is equipped with a substrate and a surface
layer formed over the surface of the substrate. The surface layer
is a thin film so that it can perform electrolysis at a low current
density. It is made of an anatase titanium oxide so that it can
efficiently produce ozone. In addition, this titanium oxide
aggregates during the formation of an anatase type crystal
structure and allows the surface of the substrate to expose between
crystals. Thus, a plurality of pores appear in the surface layer.
Existence of these pores enables to realize the production of
hydrogen peroxide without suppressing the production efficiency of
ozone in an electrode reaction at an anode.
[0015] In the second aspect of the present invention, the thickness
of the surface layer is defined to 1 mm or less. Since the surface
layer can be formed as a thin film, electrons can transfer inside
of the electrode via an impurity level in the surface layer or by
Fowler-Nordheim tunneling. In the electrode reaction at an anode,
an empty level near the bottom of a conductor which is on an energy
level higher by about half of the band gap than a Fermi level can
receive electrons from an electrolyte, making it possible to cause
transfer of electrons on a higher energy level and thereby carry
out electrolysis at a low current density. Ozone can therefore be
produced efficiently.
[0016] According to the third aspect of the present invention, the
thickness of the surface layer according to the above-described
aspects of the present invention is defined to from 200 nm to 600
nm. Similar to the above-described aspects of the invention, the
surface layer can be formed as a thin film so that electrons can
transfer inside of the electrode via an impurity level in the
surface layer or by Fowler-Nordheim tunneling. In the electrode
reaction at an anode, an empty level near the bottom of a conductor
which is on an energy level higher by about half of the band gap
than a Fermi level can receive electrons from an electrolyte,
making it possible to cause transfer of electrons on a higher
energy level and thereby carry out electrolysis at a low current
density. Ozone can therefore be produced efficiently.
[0017] In addition, since the thickness of the surface layer is
defined to from 200 nm to 600 nm, ozone can be produced while
keeping a ratio of charge contributing to ozone production to total
charge applied to the electrode, that is, a current efficiency
within a high range; and improvement in the productivity of
electrodes and reduction in their production cost can be
realized.
[0018] This makes it possible to realize an electrode for
electrolysis having a still higher ozone production efficiency at a
low production cost.
[0019] According to the fourth aspect of the present invention, in
each of the above-described aspects of the present invention, the
substrate is conductive at least a contact surface thereof with the
surface layer so that it can be allowed to function as an electrode
as the electrode as described in the first to third aspects of the
present invention without depending on the conductivity of the
substrate itself.
[0020] According to the fifth aspect of the present invention, by
electrolyzing water or an electrolyte solution at a current density
from 0.1 mA/cm.sup.2 to 2000 mA/cm.sup.2 while using an electrode
for electrolysis according to the above-described aspects of the
present invention as an anode, at least one of ozone, hydrogen
peroxide and another active oxygen species can be produced
efficiently as in the sixth aspect of the present invention.
[0021] By the electrolysis of water or an electrolyte solution
according to such an electrolytic process, plural oxidizing
substances can be formed so that OH radicals having a relatively
short life can be formed easily at a site where they are required.
The oxidizing power of the OH radicals thus formed can be used
effectively.
[0022] The electrolytic apparatus according to the seventh aspect
of the present invention is equipped with an electrode for
electrolysis as described in any one of the above-described first
to fourth aspects of the present invention and electrolyzes water
or an electrolyte solution by an electrolytic process according to
the fifth or sixth aspect of the present invention. This
facilitates production of at least one of ozone, hydrogen peroxide
and another active oxygen species at a site where it is
required.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic cross-sectional view of an electrode
for electrolysis according to the present invention (Example
1);
[0024] FIG. 2 is a flow chart of a fabrication process of the
electrode for electrolysis according to the present invention
(Example 1);
[0025] FIG. 3 is a schematic cross-sectional view of an electrode
for electrolysis according to the present invention (Example
2);
[0026] FIG. 4 is a flow chart of a fabrication process of the
electrode for electrolysis according to the present invention
(Example 2);
[0027] FIG. 5 is an X-ray diffraction diagram of an electrode for
electrolysis according to the present invention;
[0028] FIG. 6 is a schematic view of an electrolytic apparatus
according to the present invention (Example 1, Example 2);
[0029] FIG. 7 shows ultraviolet absorption spectrum of an electrode
for electrolysis formed at a firing temperature of 550.degree. C.
as a function of current density (Example 1);
[0030] FIG. 8 shows ultraviolet absorption spectrum of an electrode
for electrolysis formed at a firing temperature of 600.degree. C.
as a function of current density (Example 1);
[0031] FIG. 9 shows ultraviolet absorption spectrum of an electrode
for electrolysis formed at a firing temperature of 650.degree. C.
as a function of current density (Example 1);
[0032] FIG. 10 shows ultraviolet absorption spectrum of an
electrode for electrolysis (Example 2);
[0033] FIG. 11 is a cross-sectional plan view of an electrode for
electrolysis (Example 3);
[0034] FIG. 12 is a schematic view of an electrolytic apparatus
according to the present invention (Example 3);
[0035] FIG. 13 is a graph showing a current efficiency for ozone
production as a function of a surface layer thickness of the
electrode for electrolysis (Example 3); and
[0036] FIG. 14 is a graph showing a current efficiency for ozone
production as a function of a surface layer thickness of the
electrode for electrolysis (Example 3).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0037] Referring to some drawings, Examples 1 and 2 will
hereinafter be described as preferred embodiments of the electrode
for electrolysis according to the present invention.
Example 1
[0038] FIG. 1 is a cross-sectional plan view of an electrode 1 for
electrolysis of Example 1 as one example of the electrode for
electrolysis according to the present invention. The electrode 1
for electrolysis has, as shown in FIG. 1, a substrate 2, an
intermediate layer 3 formed on the surface of the substrate 2, and
a surface layer 4 formed on the surface of the intermediate layer
3.
[0039] In the present invention, the substrate 2 is made of a
conductive material such as platinum (Pt), a valve metal such as
titanium (Ti), tantalum (Ta), zirconium (Zr) or niobium (Nb), an
alloy of two or more of these valve metals, or silicon. In
particular, silicon having a planarized surface is employed for the
substrate 2 to be used in the present Example.
[0040] The intermediate layer 3 is made of a metal resistant to
oxidation such as platinum, gold (Au), a metal oxide having
conductivity such as iridium oxide, palladium oxide or ruthenium
oxide, or an oxide superconductor, or a metal having conductivity
even after oxidation such as ruthenium (Ru), rhodium (Rh),
palladium (Pd), iridium (Ir) or silver (Ag) included in platinum
group elements. The metal oxide is not limited to an oxide that has
constituted the intermediate layer 3 in advance but a metal oxide
obtained by the oxidation as a result of hydrolysis may be
embraced. In the present Example, the intermediate layer 3 is made
of platinum.
[0041] When the substrate 2 is made of platinum, the surface of the
substrate 2 is, needless to say, made of platinum so that it is not
necessary to form the intermediate layer 3 particularly. The
formation of the substrate 2 by using platinum however causes a
rise in cost so that from the industrial viewpoint, it is preferred
to use an inexpensive material for the substrate 2 and form, on the
surface of the substrate 2, the intermediate layer 3 made of a
noble metal or the like. The constitution of the substrate 2 is not
limited to the above-described one insofar as the substrate 2 is
made of a substance having no conductivity such as a glass plate
and at least the contact surface of the substrate 2 with a surface
layer 4 which will be described later is covered with a material
having conductivity. This also makes it possible to suppress a rise
in the cost incurred for materials used for forming the substrate
2.
[0042] The surface layer 4 is formed in layers over the surface of
the substrate 2 with the intermediate layer 3. It is made of a
dielectric substance and covers the intermediate layer 3 therewith.
The surface layer 4 has a predetermined thickness. In the present
Example, it is greater than 0 but not greater than 1 mm, preferably
not greater than 2000 nm. The thickness of the surface layer 4 in
the present Example will be described specifically later, but the
thickness of it is more preferably less than 100 nm.
[0043] A fabrication process of the electrode for electrolysis
according to the present invention will next be described with
reference to the flow chart of FIG. 2. Silicon is employed as the
substrate 2. In this case, silicon into which an impurity such as
phosphorus (P) or boron (B) has been introduced to increase its
conductivity is preferred. Silicon having a very flat surface is
used. In the present Example, silicon is used as the material of
the substrate 2 but a conductive material as described above is
also usable.
[0044] First, in Step S1, the silicon substrate 2 is pre-treated
with 5% hydrofluoric acid to remove a natural oxide film formed on
the surface of the silicon substrate 2. The surface of the
substrate 2 is thereby planarized further. The pretreatment is not
always necessary. In step S2, the surface of the substrate 2 is
rinsed with pure water. In Step S3, the substrate 2 is introduced
into the chamber of a conventional sputtering apparatus, where film
formation is performed.
[0045] In Example 2, an intermediate layer 3 is formed on the
substrate 2 by sputtering. In the present Example, the intermediate
layer 3 is made of platinum so that film formation is carried out
using Pt (80 mm.phi.), which is an intermediate layer constituting
material, as a first target at room temperature for 20 minutes
under the conditions of an RF power of 100 W, an Ar gas pressure of
0.9 Pa, and a distance of 60 mm between the substrate 2 and target
(Step 3). As a result of this step, the intermediate layer 3 of
about 100 nm thick is formed on the surface of the substrate 2. In
the present Example, sputtering is employed for the formation of
the intermediate layer 3. The film formation method is not limited
thereto, but it may be CVD, vapor deposition, ion plating, plating
or the like.
[0046] A surface layer 4 is then formed on the surface of the
substrate 2 having the intermediate layer formed thereon. In the
present Example, the surface layer 4 is formed by spin coating so
that a solution of an organic titanium compound as a surface layer
constituting material is applied to the surface of the substrate 2
having the intermediate layer 3 formed thereon. In the present
Example, the surface layer 4 is made of titanium oxide so that an
organic titanium compound obtained by the coordination of a
functional group such as hydroxyl group, aldehyde group, alkyl
group, carboxyl group or alkoxyl group to titanium having a
coordination number of 4 is used. The titanium content (wt. %) in
this organic titanium compound solution is desirably from about
0.5% to 5%. In the present Example, an organic titanium compound
solution is used as the surface layer constituting material but the
material is not limited thereto. Titanium-containing compounds from
which substances other than titanium can be removed by firing, for
example, titanium chloride, titanium bromide and titanium iodide
may be used.
[0047] The surface layer constituting material is then added
dropwise to the surface of the substrate 2 having the intermediate
layer 3 formed thereon and a thin film is formed by spin coating.
This spin coating in the present Example is conducted for 5 seconds
at 1500 rpm and for 60 seconds at 3000 rpm, followed by drying at
room temperature and at 220.degree. C. each for 10 minutes (Step
5). By this step, the surface layer 4 made of the
titanium-containing surface-layer constituting material is formed
on the surface of the intermediate layer 3 of the substrate 2.
[0048] In step S6, the substrate 2 having the intermediate layer 3
and surface layer 4 formed thereover is fired (annealed) in a
muffle kiln at from 500.degree. C. to 800.degree. C., preferably
from 550.degree. C. to 650.degree. C. in an air atmosphere for 10
minutes, whereby the electrode 1 for electrolysis is obtained. By
this, the surface layer constituting material applied to the
surface of the intermediate layer 3 becomes titanium oxide
uniformly. Titanium oxide is formed by this firing and in this
state, the surface layer 4 has a thickness from about 20 nm to 100
nm. For the measurement of the thickness of the surface layer 4, a
fluorescent X-ray analyzer ("JSX-3220ZS Element Analyzer", trade
name; product of JEOL) is employed.
[0049] In the electrode 1 for electrolysis thus obtained, the
surface layer 4 is composed alone of titanium oxide. The
intermediate layer 3 forms platinum silicide with silicon of the
substrate 2. Silicon remains in the intermediate layer 3 and does
not penetrate into the surface layer 4. Similarly, platinum
constituting the intermediate layer 3 does not reach the inside of
the surface layer 4.
[0050] In the present Example, the surface layer 4 is composed only
of a dielectric substance so that a using amount of a noble metal
or an oxide thereof for the surface layer 4 can be decreased,
leading to a cost reduction.
Example 2
[0051] Referring to FIG. 3, a description will next be made of an
electrode 6 for electrolysis according to Example 2. FIG. 3 is a
cross-sectional plan view of the electrode 6 for electrolysis as
one example of the electrode for electrolysis according to the
present invention. As illustrated in FIG. 3, the electrode 6 for
electrolysis is composed of a substrate 7, an adhesion layer 8
formed on the surface of the substrate 7, an intermediate layer 9
formed on the surface of the adhesion layer 8, and a surface layer
10 formed on the surface of the intermediate layer 9.
[0052] The substrate 7 in the present invention is made of a
material similar to that constituting the substrate 2 used for the
electrode 1 for electrolysis in Example 1 so that the substrate 7
in Example 2 is made of silicon.
[0053] The adhesion layer 8 is formed on the surface of the
substrate 7 and serves to improve the adhesion between the
substrate 7 and the intermediate layer 9 formed of, for example,
platinum on the surface of the adhesion layer 8 and it is made of
titanium oxide or titanium nitride. In the present Example,
titanium oxide is employed.
[0054] The intermediate layer 9 is made of a material similar to
that constituting the intermediate layer 3 used in the electrode 1
for electrolysis according to Example 1. In Example 2, the
intermediate layer 9 is made of platinum.
[0055] In the present Example, when the substrate 7 is made of
platinum, the surface of it is, needless to say, made of platinum.
It is therefore not necessary to form the intermediate layer 9
particularly and is therefore not necessary to form the adhesion
layer 8. Use of platinum for the formation of the substrate 7,
however, leads to a substantial cost increase so that it is
preferred to use an inexpensive material for the formation of the
substrate 7 and then form the intermediate layer 9 composed of a
noble metal or the like on the surface of the substrate 7 via the
adhesion layer 8.
[0056] The surface layer 10 is formed in layers over the surface of
the substrate 7 with the intermediate layer 9. It is made of a
dielectric substance and covers the intermediate layer 9 therewith.
The surface layer 10 has a predetermined thickness. In the present
Example, it is greater than 0 but not greater than 1 mm, preferably
not greater than 2000 nm. The thickness of the surface layer 10 in
the present Example will be described specifically later, but the
thickness of it is more preferably less than 100 nm.
[0057] As the dielectric substance constituting the surface layer
10, anatase type titanium oxide is employed.
[0058] The fabrication process of the electrode for electrolysis
according to the present invention will next be described with
reference to the flow chart of FIG. 4. Silicon is used as the
substrate 7. In this case, silicon having a conductivity raised by
the introduction thereinto of phosphorus (P) or boron (B) as an
impurity is desired. Moreover, the silicon must have a very flat
surface. In the present Example, silicon is used as the substrate
7. The material is however not limited thereto but a conductive
material as described above and preferably having a planarized
surface may be used.
[0059] Similar to Example 1, the silicon substrate 7 is therefore
pre-treated with 5% hydrofluoric acid to remove a natural oxide
film formed on the surface of the silicon substrate 7. The surface
of the substrate 2 is thereby planarized further. The pretreatment
is not always necessary. The surface of the substrate 7 is then
rinsed with pure water. The substrate 7 is introduced into the
chamber of a conventional sputtering apparatus, in which film
formation is performed.
[0060] In the present Example, first, in Step S11, the adhesion
layer 8 is formed in order to improve, as described above, the
adhesion of the intermediate layer 9 to the surface of the
substrate 7. The adhesion layer 8 is formed on the substrate 7 by
reactive sputtering. The adhesion layer 8 is made of titanium oxide
so that film formation using Ti as a first target is performed at
room temperature for 10 minutes under the following conditions of
input power of 6.1 W/cm.sup.2, oxygen partial pressure
(Ar:O.sub.2=24:26) of 52% and film forming pressure of 0.6 Pa (Step
S3). By this step, the adhesion layer 8 composed of titanium oxide
and having a thickness of about 50 nm is formed on the surface of
the substrate 7. In the present Example, reactive sputtering is
employed for the formation of the adhesion layer 8. The film
formation method is not limited thereto, but it may be sputtering,
CVD, ion plating or plating or a combination thereof with thermal
oxidation.
[0061] In Step S12, the intermediate layer 9 is formed on the
surface of the substrate 7 having the adhesion layer 8 thereon. The
intermediate layer 9 is formed over the substrate 7 by sputtering
as in Example 1. In the present Example, the intermediate layer 9
is made of platinum so that film formation is carried out using Pt
(80 mm.phi.) as a first target at room temperature for about 1
minute and 10 seconds under the conditions of an input power of
4.63 W/cm.sup.2, an Ar gas pressure of 0.7 Pa. As a result of this
step, the intermediate layer 9 of about 200 nm thick is formed over
the substrate 7 having thereon the adhesion layer 8. In the present
Example, sputtering is employed for the formation of the
intermediate layer 9. The film formation method is not limited
thereto, but it may be CVD, vapor deposition, ion plating, plating
or the like.
[0062] The surface layer 10 is then formed over the substrate 7
having the intermediate layer 9 formed thereon. In the present
Example, the surface layer 10 is formed by spin coating so that a
solution of an organic titanium compound as a surface layer
constituting material is applied to the surface of the substrate 2
having the intermediate layer 9 formed thereon. In the present
Example, the surface layer 10 is made of titanium oxide so that an
organic titanium compound obtained by the coordination of a
functional group such as hydroxyl group, aldehyde group, alkyl
group, carboxyl group or alkoxyl group to titanium having a
coordination number of 4 is used. The titanium content (wt. %) in
this organic titanium compound solution is desirably from about
0.5% to 5%. In the present Example, an organic titanium compound
solution is used as the surface layer constituting material but the
material is not limited thereto. Titanium-containing compounds from
which substances other than titanium can be removed by firing, for
example, titanium chloride, titanium bromide and titanium iodide
may be used.
[0063] In Step S13, the surface layer constituting material is
added dropwise to the surface of the substrate 7 having the
intermediate layer 3 formed thereon and a thin film is formed by
spin coating. This spin coating in the present Example is conducted
at 1000 rpm for 10 seconds and at 3000 rpm for 30 seconds, followed
by drying at room temperature and at 200.degree. C. for each 10
minutes (Step S14). By this step, the surface layer 10 is formed of
the titanium-compound-containing surface-layer constituting
material on the surface of the intermediate layer 9 of the
substrate 7.
[0064] In step S15, the substrate 7 having the intermediate layer 9
and surface layer 10 thereover is fired (annealed) in a muffle kiln
at from 500.degree. C. to 800.degree. C., preferably from
550.degree. C. to 650.degree. C., at 650.degree. C. in the present
Example, in ambient atmosphere for 10 minutes, whereby the
electrode 6 for electrolysis is obtained. By this, the surface
layer constituting material applied to the surface of the
intermediate layer 9 becomes titanium oxide uniformly. In the
present Example, the above-described film forming operation was
repeated 14 times in total. The surface layer constituting material
becomes titanium oxide by the firing. The thickness of the surface
layer 10 in this state is from about 20 nm and 1000 nm. It is about
500 nm in the present Example. For the measurement of the thickness
of the surface layer 10, a fluorescent X-ray analyzer ("JSX-3220ZS
Element Analyzer", trade name; product of JEOL) is employed.
[0065] In the electrode 6 for electrolysis obtained in the
above-described manner, the surface layer 10 is composed only of
titanium oxide. Described specifically, the surface layer
constituting material is a titanium-containing compound, for
example, as described above, an organic titanium compound having,
in addition to titanium, a plurality of functional groups
coordinated therein, titanium chloride, titanium bromide or
titanium iodide but substances other than titanium such as
functional groups composed of an organic substance, chlorine,
bromine and iodine are removed when the material is fired.
Titanium, on the other hand, reacts with oxygen in the atmosphere
into titanium oxide.
[0066] Titanium oxide typically exists in three different
crystalline forms, that is, anatase, rutile and brookite.
Thermodynamically, the rutile type titanium oxide is a stable phase
in any temperature region, and the other types are metastable
phases. Brookite type titanium oxide is unstable compared with
another crystalline form. Owing to difficulty in synthesizing pure
crystals of it, industrial use of it is not common. Anatase type
titanium oxide is known to thermally transfer into rutile type one
at high temperatures, for example, 900.degree. C. or greater.
[0067] For the analysis of crystal structures of these anatase,
rutile and brookite type titanium oxides, X-ray diffraction (XRD)
is typically employed. This method enables analysis of the crystal
structure of titanium oxide constituting the surface layer 10.
[0068] FIG. 5 is an X-ray diffraction pattern of the surface layer
10 of the electrode 6 for electrolysis thus obtained. In this
pattern, titanium oxide that constitutes the surface layer 10 of
the electrode 6 for electrolysis obtained in the present Example
shows peaks specific to an anatase type crystal structure
(encircled are peaks specific to the anatase crystal structure and
the numeral appended to each peak is a plane index). This has
revealed that the titanium oxide film constituting the surface
layer 10 has an anatase type crystal structure.
[0069] In the present Example, the surface layer constituting
material containing a titanium compound is applied to the surface
of the substrate (the surface of the intermediate layer 9 in the
present Example) by spin coating, followed by firing at a
predetermined temperature, whereby the surface layer 10 made of
anatase type titanium oxide can be obtained. A method of forming
the surface layer 10 from the anatase type titanium oxide is
however not limited to this.
[0070] Another formation method of the surface layer 10 is thermal
CVD. In this thermal CVD, after successive formation of the
adhesion layer 8 and intermediate layer 8 on the surface of the
substrate 7 as in Example 1, an organic titanium compound which is
a surface layer constituting material is evaporated and introduced
into a reaction tube with an aid of a proper carrier gas. Chemical
reaction is then effected on the surface of the substrate 7 heated
to a high temperature, for example, from 500.degree. C. to
800.degree. C., preferably from 500.degree. C. to 600.degree.
C.
[0071] By this heat treatment, substances other than titanium of
the organic titanium compound, which is the surface layer
constituting material, for example, organic substances are removed
on the surface of the substrate 7 heated to high temperature and
only titanium reacts with oxygen in the atmosphere to form titanium
oxide on the surface of the substrate 7. Titanium oxide formed on
the surface of the substrate 7 (strictly, the surface of the
intermediate layer 9) constitutes a thin film (titanium oxide film)
having an anatase crystal structure.
[0072] The surface layer 10 made of anatase type titanium oxide can
also be formed, for example, by a dip method.
[0073] In any formation method of the surface layer 10, titanium
oxide having an anatase crystal structure is not formed uniformly
in the titanium oxide film formed on the surface of the
intermediate layer 9. During formation of crystals on the surface
of the high temperature substrate 7 by firing or thermal CVD,
titanium oxide aggregates into crystal grains and pores like
interstices are formed between crystal grains. These pores are
dotted in the surface layer 10 of the electrode 6 for electrolysis
and from the pores, a layer just under the surface layer 10, that
is, the intermediate layer 9 made of platinum is exposed.
[0074] In the present Example, the adhesion layer 8 made of
titanium oxide is formed on the surface of the substrate 7 made of
silicon so that it becomes possible to stop the formation of
platinum silicide due to direct diffusion of platinum constituting
the intermediate layer 9 into the substrate 7. In addition, by the
adhesion layer 8 made of titanium oxide, adhesion of platinum
constituting the intermediate layer 9 to the substrate 7 can be
improved. In addition, it becomes possible to constitute the
intermediate layer 9, which is to be formed on the surface of the
substrate 7, uniformly at an atomic level. This facilitates the
formation of the surface layer 10 on the surface of the
intermediate layer 9, that is, formation of titanium oxide having
an anatase crystal structure.
[0075] In the present Example, the surface layer 10 is composed
only of a dielectric substance so that it is possible to decrease
the using amount of a noble metal or novel metal oxide for the
surface layer 10 and thereby reducing the cost.
Example 3
[0076] Electrode 26 for electrolysis in Example 3 will next be
described with reference to FIG. 11. FIG. 11 is a cross-sectional
plan view of the electrode 26 for electrolysis as one example of
the electrode for electrolysis according to the present invention.
As illustrated in FIG. 11, the electrode 26 for electrolysis has a
substrate 27, an adhesion layer 28 formed on the surface of the
substrate 27, an intermediate layer 29 formed on the surface of the
adhesion layer 28, and a surface layer 30 formed on the surface of
the intermediate layer 29. This electrode 26 for electrolysis is
equipped, on the side of the substrate 27, with a titanium plate 31
as an electroconducting portion. The titanium plate 31 and the
intermediate layer 29 are made conductive via a silver paste 32
serving as a conductive material disposed on the end face of the
electrode 26. The silver paste 32 and titanium plate 31 are covered
with a sealing material 33 and do not contribute to
electrolysis.
[0077] In the present Example, the substrate 27 is made of a
material similar to that employed for the substrate 2 or 7 used for
the electrode 1 or 6 for electrolysis in the above-described
Examples so that the substrate 27 is also made of silicon in the
present Example.
[0078] The adhesion layer 28 is formed on the surface of the
substrate 27 and serves to improve the adhesion between the
substrate 27 and the intermediate layer 29 formed of, for example,
platinum on the surface of the adhesion layer 28. The adhesion
layer is made of titanium oxide, titanium nitride or the like. In
the present Example, titanium oxide is employed.
[0079] The intermediate layer 29 is made of a material similar to
that used for the intermediate layer 3 or 9 of the electrode 1 or 6
for electrolysis obtained in the above Example. In the present
Example, the intermediate layer 29 is comprised of platinum.
[0080] Also in the present Example, when the substrate 27 is made
of platinum, the surface of the substrate 27 is, needless to say,
made of platinum. It is therefore not necessary to form the
intermediate layer 29 particularly and needless to say, not
necessary to form the adhesion layer 28. The formation of the
substrate 27 using platinum causes a rise in cost so that from the
industrial viewpoint, it is preferred to employ an inexpensive
material for the substrate 27 and form the intermediate layer 29
made of a noble metal or the like on the surface of the substrate
27 via the adhesion layer 28.
[0081] The surface layer 30 is formed in layers over the surface of
the substrate 27 with the intermediate layer 29. It is made of a
dielectric substance and covers the intermediate layer 29
therewith. The surface layer 30 has a predetermined thickness. In
the present Example, it is greater than 0 but not greater than 1
mm, preferably from 200 nm to 600 nm.
[0082] As the dielectric substance constituting the surface layer
30, anatase type titanium oxide is used as in the above-described
Examples.
[0083] A fabrication process of the electrode 26 for electrolysis
in the present Example is substantially similar to that of the
electrode 6 for electrolysis in Example 2 (firing temperature:
650.degree. C.) so that description on it is omitted. With regards
to the surface layer 36 constituting the electrode 26 for
electrolysis, however, as a result of repetition of the
above-described film forming operation, the surface layer 30 has a
thickness of from about 200 nm to 600 nm. For the measurement of
the thickness of the surface layer 30, fluorescent X-ray analyzer
("JSX-3220ZS Element Analyzer", trade name; product of JEOL) is
employed.
[0084] In the electrode 26 for electrolysis available by the
above-described manner, the surface layer 30 is composed only of
titanium oxide. Described specifically, the surface layer
constituting material is a titanium-containing compound, for
example, as described above, an organic titanium compound having,
in addition to titanium, a plurality of functional groups
coordinated thereto, titanium chloride, titanium bromide or
titanium iodide but substances other than titanium such as
functional groups composed of an organic substance, chlorine,
bromine and iodine are removed when the material is fired.
Titanium, on the other hand, reacts with oxygen in the atmosphere
into titanium oxide.
[0085] Also in this case, titanium oxide constituting the surface
layer 30 has an anatase type crystal structure.
[0086] Also in this case, a surface layer constituting material
containing a titanium compound is applied to the surface of the
substrate (the surface of the intermediate layer 29 in the present
Example) by spin coating, followed by firing at a predetermined
temperature to obtain the surface layer 30 made of anatase type
titanium oxide. A method of forming the surface layer 30 from the
anatase type titanium oxide is however not limited to this.
[0087] As described above, the surface layer 30 may also be formed
by thermal CVD method, dip method, plating method or the like.
(Electrolytic Process Using Each Electrode for Electrolysis; and
Evaluation Thereof)
[0088] With reference to FIGS. 6 to 10, and FIGS. 12 to 14, a
description will next be made of the production of ozone and/or
hydrogen peroxide by electrolysis using the electrode 1, 6 or 27
for electrolysis fabricated in the above-described Examples.
[0089] First, tests on the electrodes 1 and 5 for electrolysis by
using an electrolytic apparatus 20 as illustrated in FIG. 6 will be
described. FIG. 6 is a schematic view of the electrolytic apparatus
20. The electrolytic apparatus 20 has a treatment tank 21, the
above-described electrode 1 or 6 for electrolysis as an anode, an
electrode 22 as a cathode, and a power supply 25 for applying a
direct current to these electrodes 1 (6) and 22. The apparatus is
equipped further with a cation exchange membrane (diaphragm:
"Nafion", trade name; product of Dupont) for partitioning the
treatment tank 21 into a region having the electrode 1 (6) and the
other region having the electrode 22. In this treatment tank 21,
simulated tap water 23 or 0.01 M HClO.sub.4 is retained as an
electrolyte solution. In the present Example, an electrolyte
solution such as simulated tap water or 0.01 M HClO.sub.4 is used
in the Test. Owing to the cation exchange membrane installed in the
apparatus, however, a substantially similar effect is available
even if pure water is used for the treatment.
[0090] The electrode 1 or 6 for electrolysis used in the test is
fabricated in accordance with the fabrication process employed in
the above Examples. As the electrode 1 for electrolysis used in the
electrolytic apparatus 20, three electrodes are fabricated in
total. They have surface layers 4 formed by firing at 550.degree.
C., 600.degree. C. and 650.degree. C., respectively. The surface
layer 10 of the electrode 6 for electrolysis is, on the other hand,
formed at a firing temperature of 650.degree. C. The electrodes 1
and 6 for electrolysis are evaluated by measuring ultraviolet
absorption of the electrolyte solution when the electrodes 1 and 6
for electrolysis are used as an anode.
[0091] On the other hand, platinum is used for the electrode 22 as
a cathode. An insoluble electrode obtained by firing platinum on
the surface of the titanium substrate 2, a platinum-iridium
electrode for electrolysis, or a carbon electrode may also be used
for the electrode as a cathode.
[0092] The electrolyte solution to be subjected to electrolytic
treatment using the electrode 1 for electrolysis is an aqueous
solution obtained by simulating tap water and this simulated tap
water 23 is composed of 5.75 ppm of Na.sup.+, 10.02 ppm of
Ca.sup.2+, 6.08 ppm of Mg.sup.2+, 0.98 ppm of K.sup.+, 17.75 ppm of
Cl.sup.-, 24.5 ppm of SO.sub.4.sup.2- and 16.5 ppm of
CO.sub.3.sup.2-. The electrolyte solution to be subjected to
electrolytic treatment using the electrode 6 for electrolysis is
0.01 M HClO.sub.4.
[0093] According to the above-described constitution, in the
electrolysis using the electrode 1 for electrolysis, 150 ml of
simulated tap water 23 is retained in the treatment tank 21, and
the electrode 1 for electrolysis and electrode 22 are immersed in
the simulated tap water. An area of each of the electrode 1 for
electrolysis and the electrode 22 in the present Example is set at
25 mm.times.15 mm. By the power supply 25, a constant current of 80
mA with a current density of about 20 mA/cm.sup.2, a constant
current of 160 mA with a current density of about 40 mA/cm.sup.2, a
constant current of 240 mA with a current density of about 60
mA/cm.sup.2 are each applied to the electrode 1 for electrolysis
and the electrode 22. On the other hand, in the electrolysis using
the electrode 6 for electrolysis, 0.01 M HClO4 is retained in the
treatment tank 21 and the electrode 6 for electrolysis and
electrode 22 are immersed in the solution. Electrolysis is
conducted under the conditions of a current density of 26.7
mA/cm.sup.2 and temperature of the solution of +15.degree. C.
[0094] It is to be noted that in the present Example, the
production amounts of ozone and hydrogen peroxide by the electrode
1 or 6 for electrolysis are determined and evaluated by measuring
ultraviolet absorption of the simulated tap water 23 or solution
five minutes after electrolysis under the above-described
conditions.
[0095] Next, referring to FIGS. 7 to 9, a description will be made
of substances produced using electrodes 1 for electrolysis obtained
at respective firing temperatures at each current density. FIG. 7
shows ultraviolet absorption of the electrode 1 for electrolysis
obtained at a firing temperature of 550.degree. C. at each current
density; FIG. 8 shows ultraviolet absorption of the electrode 1 for
electrolysis obtained at a firing temperature of 600.degree. C. at
each current density; and FIG. 9 shows ultraviolet absorption of
the electrode 1 for electrolysis obtained at a firing temperature
of 650.degree. C. at each current density. In any diagram, the
ordinate indicates absorbance and the abscissa indicates
wavelength. In these diagrams, A, B and C are test results at a
current density of about 20 mA/cm.sup.2, about 40 mA/cm.sup.2 and
about 60 mA/cm.sup.2, respectively, while D are test results of an
electrode for electrolysis indicated as control. The electrode for
electrolysis used in D is obtained by forming titanium on the
surface of the substrate or on the surface of the intermediate
layer by sputtering, and then thermally oxidizing it into the
surface layer 4. The electrode area of it is 15 mm.times.15 mm,
firing temperature is 500.degree. C. and current density is 10
MA/cm.sup.2.
[0096] FIG. 7 shows the test results of the electrode 1 for
electrolysis obtained by firing at 550.degree. C. According to
them, an absorption peak (from about 200 nm to 220 nm) of hydrogen
peroxide and an absorption peak (about 258 nm) of ozone are
observed at any current density and their absorbances are almost
equal or that of ozone is a little greater than that of hydrogen
peroxide. The results show that absorbances of hydrogen peroxide
and ozone increase with a rise in current density and that they are
large particularly at a current density of 60 mA/cm.sup.2
[0097] When the electrode 1 for electrolysis having a surface layer
4 formed by sputtering is used, on the other hand, an absorption
peak of ozone is observed but no absorption peak of hydrogen
peroxide is found. This suggests that it is impossible to produce
both ozone and hydrogen peroxide by the method of forming the
surface layer 4 by sputtering under the above-described conditions,
while it is possible to produce both ozone and hydrogen peroxide by
the method of forming the surface layer 4 by spin coating.
[0098] FIG. 8 shows test results of the electrode 1 for
electrolysis obtained by firing at 600.degree. C. According to
these results, an absorption peak (from about 200 nm to 220 nm) of
hydrogen peroxide is observed at any current density, but an
absorption peak (about 258 nm) of ozone is observed only at a
current density of 40 mA/cm.sup.2 and 60 mA/cm.sup.2 and no
absorption peak of ozone is found at a current density of 20
mA/cm.sup.2. In particular, an absorption peak of hydrogen peroxide
at a current density of about 20 mA/cm.sup.2 is higher than that at
current densities of about 40 and 60 mA/cm.sup.2. This means that
hydrogen peroxide is produced preferentially to ozone.
[0099] Owing to such preferential production, it becomes possible
to select a ratio of hydrogen peroxide to ozone produced by
electrolysis, particularly, production or non-production of ozone
by changing a current density. By carrying out electrolysis at a
current density changed depending on a desired substance, a
substance produced preferentially can be selected, leading to
improvement in versatility.
[0100] When an electrolyte solution is electrolyzed using the
electrode 1 for electrolysis obtained by firing at 600.degree. C.,
an absorption peak of ozone becomes higher and an absorption peak
of hydrogen peroxide becomes lower at a current density of 60
mA/cm.sup.2, compared with the absorption peaks of ozone produced
by the electrode 1 for electrolysis obtained by firing at
550.degree. C. as illustrated in FIG. 7. An absorption peak of
hydrogen peroxide at a current density of 40 mA/cm.sup.2, on the
other hand, becomes higher.
[0101] FIG. 9 shows test results of the electrode 1 for
electrolysis annealed at 650.degree. C. According to these results,
an absorption peak (from about 200 nm to 220 nm) of hydrogen
peroxide is observed at any current density, but an absorption peak
(about 258 nm) of ozone is observed only at current densities of
about 40 mA/cm.sup.2 and about 60 mA/cm.sup.2 and no absorption
peak of ozone is found at a current density of about 20
mA/cm.sup.2. In particular, an absorption peak of hydrogen peroxide
at a current density of about 20 mA/cm.sup.2 is higher than those
at current densities of about 40 and about 60 mA/cm.sup.2. This
means that hydrogen peroxide is produced preferentially to
ozone.
[0102] Owing to such preferential production, it becomes possible
to select a ratio of hydrogen peroxide to ozone produced by
electrolysis, particularly, production or non-production of ozone
by changing a current density. In particular, with an increase in
the current density to about 20 mA/cm.sup.2, the production amount
of hydrogen peroxide can be raised and at the same time, the
production amount of ozone can be reduced. In addition, by
adjusting the current density to about 20 mA/cm.sup.2, it is
possible to inhibit the production of ozone and produce only
hydrogen oxide. By carrying out electrolysis at a current density
changed depending on a desired substance, a substance produced
preferentially can be selected, leading to improvement in
versatility.
[0103] The absorption peak of ozone is observed at each of about 40
mA/cm.sup.2 and about 60 mA/dm.sup.2, but the absorption amount is
smaller compared with that in the case of electrolysis using the
electrode obtained by firing at 600.degree. C. Anyway, hydrogen
peroxide is produced preferentially to ozone at any current
density. It can be understood that this tendency is more pronounced
than in the case of FIG. 8 where the electrode 1 obtained by firing
at 600.degree. C. is used.
[0104] Accordingly, with an increase in the firing temperature of
the electrode 1 for electrolysis, hydrogen peroxide is produced
more preferentially. When the firing temperature is 600.degree. C.,
it is possible to produce ozone efficiently while producing
hydrogen peroxide.
[0105] When both ozone and hydrogen peroxide are produced, hydrogen
peroxide generated in the electrolyte solution is oxidized with
ozone having a higher oxidizing power into OH radicals (chemical
reaction formula A). The following is the chemical reaction formula
A: Chemical reaction formula A:
2O.sub.3+H.sub.2O.sub.2.fwdarw.2.OH+3O.sub.2
[0106] The OH radicals can exhibit a strong oxidizing power. By the
use of the electrode 1 for electrolysis to electrolyze an
electrolyte solution, a plurality of oxidizing substances can be
produced and OH radicals produced in accordance with these
reactions contribute to elimination of smells of mold and smoke,
which cannot be eliminated easily only by the power of ozone. In
this case, OH radicals have a relatively short life, but can be
produced easily at a site where it is required. The oxidizing power
of the OH radicals can therefore be utilized effectively.
[0107] Next, a substance produced by the electrolysis using the
electrode 6 for electrolysis will be described with reference to
FIG. 10. FIG. 10 shows the ultraviolet absorption of an electrolyte
solution electrolyzed using the electrode 6 for electrolysis. The
ordinate represents absorbance, while the abscissa represents
wavelength. According to this, an absorption peak (from about 200
nm to 220 nm) of hydrogen peroxide and an absorption peak (about
258 nm) of ozone are observed. An absorbance ratio of hydrogen
peroxide to ozone is about 2 but anyway, both hydrogen peroxide and
ozone are produced in the electrolyte solution. Test results shown
in FIG. 10 are different in production or non-production of ozone
from the test data, shown in FIG. 9, of the electrode 1 for
electrolysis, which has been fabricated by spin coating, at a
current density of about 20 mA/cm.sup.2. This difference is
presumed to occur because the current density of FIG. 10 is 26.7
mA/cm.sup.2, the electrode 6 for electrolysis has the adhesion
layer 8 between the intermediate layer 9 and substrate 7, and the
electrolyte solution employed for electrolysis is different.
[0108] Thus, both ozone and hydrogen peroxide can be produced in
the electrolyte solution by electrolyzing an electrolyte solution
by using the electrode 1 or 6 for electrolysis obtained in the
above-described Example as an anode. Both of them can be produced,
because, in the electrode 1 or 6 for electrolysis obtained in the
above-described Example, titanium oxide constituting the surface
layer 4 or 10 can be formed by spin coating as a relatively thin
film having a thickness not greater than 100 nm and in addition,
titanium oxide constituting the thin film has, as is apparent from
FIG. 5, an anatase type crystal structure.
[0109] Electrons are therefore presumed to transfer via an impurity
level in the surface layer 4 or 10, or by the Fowler-Nordheim
tunneling to the intermediate layer 3 comprised of a conductive
material.
[0110] When a metal electrode is used as an electrode for
electrolysis, an empty level just above the Fermi level typically
receives electrons from the electrolyte and an electrode reaction
at an anode occurs. When the electrode 1 or 6 for electrolysis
having the surface layer 4 or 10 according to the present invention
is used, on the other hand, owing to the surface layer made of a
dielectric substance, an empty level near the bottom of a conductor
which is on an energy level higher by about half of a band gap than
the Fermi level receives electrons from the electrolyte, whereby
the electrode reaction at an anode occurs.
[0111] Use of the electrode 1 or 6 for electrolysis according to
the present invention is therefore presumed to induce an increase
in the production efficiency because compared with the use of an
electrode for electrolysis made of platinum or the like, it causes
transfer of electrons at a higher energy level and triggers an
electrode reaction.
[0112] By applying an electric current to the electrode 1 for
electrolysis at a predetermined low current density, that is, from
0.1 mA/cm.sup.2 to 200 mA/cm.sup.2, preferably from 1 mA/cm.sup.2
to 1000 mA/cm.sup.2, the electrode thus obtained can therefore
produce ozone at a high efficiency and at the same time, produce
hydrogen peroxide.
[0113] In particular, the surface layer 10 of the electrode 6 for
electrolysis is, as described in Example 2, formed on the surface
(the surface of the intermediate layer 9 in the present Example) of
the substrate 7 by spin coating or thermal CVD of a
titanium-containing compound as a surface layer constituting
material so that the surface layer 10 is a thin film made of
anatase type titanium oxide. Aggregation of titanium oxide occurs
at the time of reaction on the surface of the substrate heated to
high temperature by firing or thermal CVD, whereby a plurality of
pores which expose the surface of the substrate 7 (strictly, the
surface of the intermediate layer 9 just below the surface layer
10, that is, the conductive intermediate layer 9 constituting the
contact surface between the substrate 2 and the surface layer 10)
between crystals are formed.
[0114] In the surface layer 10, a reaction for producing ozone as
described above proceeds on the surface of the titanium oxide thin
film and at the same time, a reaction for producing hydrogen
peroxide proceeds owing to the presence of pores from which the
intermediate layer 9 is exposed.
[0115] This makes it possible to produce ozone efficiently even by
the electrolysis at a low current density and in addition, to
produce hydrogen peroxide without suppressing the production
efficiency of ozone.
[0116] The surface layer 4 or 10 of the electrode 1 or 6 capable of
producing both ozone and hydrogen peroxide can be formed by spin
coating as described above. This enables fabrication of electrodes
for electrolysis at a relatively low cost, leading to cost
reduction of the corresponding equipment. The surface layer 4 or 10
is, when it is formed by thermal CVD as described above, able to
have good stability and therefore contributes to high production
efficiency.
[0117] Moreover, the surface constituting material to be used for
forming such surface layer 4 or 10 has a titanium content of from
0.5 to 5 wt. %. This facilitates formation of an anatase type
crystal structure and at the same time enables the formation of
pores large enough to realize the production of hydrogen peroxide
without suppressing the production efficiency of ozone when the
electrode 1 or 6 is used as an anode.
[0118] In Examples 1 and 2, as described above, the electrode 1 or
6 for electrolysis is fabricated by forming the intermediate layer
3 or 9 containing at least one of metals which are difficult to
oxidize, metal oxides having conductivity, or metals having
conductivity even if they are oxidized and then forming the surface
layer 4 or 10 as described above on the surface of the intermediate
layer 3 or 9. When the substrate 2 or 7 and the intermediate layer
3 or 9 are made of the same material, that is, a material
containing at least one of metals which are difficult to oxidize,
metal oxides having conductivity, or metals having conductivity
even if they are oxidized, an electrode capable of efficiently
producing ozone can be fabricated even without disposing the
intermediate layer 3 particularly. By forming, as in the present
invention, the intermediate layer 3 or 9 made of the
above-described material while covering the substrate 2 or 7
therewith, however, it is possible to fabricate the electrode 1 or
6 capable of efficiently producing ozone at a low production
cost.
[0119] Test on the electrode 26 for electrolysis, which has been
obtained in Example 3, by using an electrolytic apparatus 35 as
illustrated in FIG. 12 will next be described. FIG. 12 is a
schematic view of the electrolytic apparatus 35. The electrolytic
apparatus 35 has a treatment tank 36, the electrode 26 for
electrolysis as an anode, an electrode 22 as a cathode and a power
supply 37 for applying a direct current to these electrodes 26 and
22. The apparatus is equipped further with a cation exchange
membrane (diaphragm: "Nafion", trade name; product of Dupont) 24
for partitioning the treatment tank 36 into a region having the
electrode 26 and the other region having the electrode 22. It has
an agitator 38 in the region where the electrode 26 for
electrolysis is immersed as an anode.
[0120] In this treatment tank 36, simulated tap water 23 or 0.01 M
HClO.sub.4 is retained as an electrolyte solution. In the present
Example, an electrolyte solution such as simulated tap water or
0.01 M HClO.sub.4 is used in the test. Owing to the cation exchange
membrane installed in the apparatus, however, a substantially
similar effect is available even if pure water is treated. The
simulated tap water employed in this Test has a composition equal
to that used in the above-described Test.
[0121] The electrode 26 for electrolysis used in the test is that
obtained by the fabrication process as described in Example 3. For
the electrode 22 as a cathode, a material similar to that employed
in the above test, that is, platinum is used. An insoluble
electrode obtained by firing platinum on the surface of the
titanium substrate 2, a platinum-iridium electrode for
electrolysis, or a carbon electrode may also be used for the
electrode as a cathode.
[0122] A test on current efficiency for ozone production when
electrolysis is carried out using the electrode 26 for electrolysis
in the above-described constitution will next be described with
reference to FIGS. 13 and 14. In the Test as shown in FIG. 13, 150
ml of 0.01 M HClO.sub.4 is retained in both regions (respective
regions on the anode side and cathode side) in the treatment tank
36 and the electrode 26 for electrolysis and electrode 22 are
immersed therein respectively. The distance between these
electrodes is set at 10 mm and a constant current with a current
density of 26.7 mA/cm.sup.2 is applied to the electrode 26 for
electrolysis and electrode 22 from a power supply 37. The
temperature of the solution is set at +15.degree. C.
[0123] The electrode 26 for electrolysis in the present Example is
evaluated by measuring the production amount of ozone in the
solution after electrolysis for 5 minutes under the above-described
conditions by an indigo method (product of HACH) and then,
calculating a ratio of charge contributing to ozone production
relative to the total charge applied to the electrode, that is,
current efficiency based on the following chemical reaction formula
B: Chemical reaction formula B:
3H.sub.2O.fwdarw.O.sub.3+6H.sup.++6e.sup.-
[0124] In the Test in FIG. 13, seven electrodes 26 for electrolysis
are fabricated while adjusting the thickness of their surface layer
30 to 207.3 nm, 277.7 nm, 346.2 nm, 385.5 nm, 503.6 nm, 547.0 nm
and 724.7 nm, respectively. The amount of a current contributing to
ozone production in the electrolyte solution is measured and an
ozone production efficiency of these electrodes as a function of
the thickness of the surface layer 30 is compared based on a ratio
of charge contributing to ozone production relative to total charge
applied to the electrode, that is, current efficiency. For the
measurement of the thickness of the surface layer 30, a fluorescent
X-ray analyzer ("JSX-3220ZS Element Analyzer", trade name; product
of JEOL) is employed.
[0125] When stable perchloric acid (HClO.sub.4) is used as an
electrolyte solution, most of the current to be supplied to the
electrode usually contributes to the production of oxygen so that
ozone production hardly occurs. In Test as shown in FIG. 13, when
the thickness of the surface layer 30 of the electrode 26 for
electrolysis is 207.3 nm or 277.7 nm, a current efficiency for
ozone production becomes almost 0% and most of the current to be
supplied to electrolysis is consumed for the production of
oxygen.
[0126] When the thickness of the surface layer 30 of the electrode
26 for electrolysis is 346.2 nm, on the other hand, the current
efficiency for ozone production becomes about 1.7%. This suggests
that even in the electrolysis of stable perchloric acid as an
electrolyte solution, ozone is produced. The film thickness becomes
greater, for example, 385.5 nm, 503.6 nm, 547.0 nm or 724.7 nm,
ozone can be produced at a current efficiency as high as about
5.2%, about 6.9%, about 4.7% and about 5.4%, respectively. Also in
this case, most of the current which has remained after supply to
electrolysis is used for the production of oxygen. This will
equally apply hereinafter.
[0127] In Test shown in FIG. 14, on the other hand, 150 ml of
simulated tap water is retained in both regions (respective regions
on the anode side and cathode side) in the treatment tank 36 and
the electrode 26 for electrolysis and electrode 22 are immersed
therein respectively. The distance between these electrodes is set
at 10 mm and a constant current with a current density of 26.7
mA/cm.sup.2 or a constant current with a current density of 17.8
mA/cm.sup.2 is applied to the electrode 26 for electrolysis and
electrode 22 from a power supply 37. The temperature of the
solution is set at +15.degree. C.
[0128] In this test, the electrode 26 for electrolysis is evaluated
by measuring the production amount of ozone in the solution after
electrolysis for 5 minutes under the above-described conditions by
an indigo method (product of HACH) and calculating a ratio of
charge contributing to the production of ozone relative to the
total charge applied, that is, current efficiency based on the
following chemical reaction formula B: Chemical reaction formula B:
3H.sub.2O.fwdarw.O.sub.3+6H.sup.++6e.sup.-
[0129] In the Test in FIG. 14, ten electrodes 26 for electrolysis
are fabricated while adjusting the thickness of their surface layer
30 to 49.8 nm, 78.5 nm, 111 nm, 147 nm, 202 nm, 250 nm, 278 nm, 303
nm, 354 nm and 381 nm, respectively. The amount of a current
contributing to ozone production in the electrolyte solution is
measured and an ozone production efficiency of these electrodes as
a function of the thickness of the surface layer 30 is compared
based on a ratio of charge contributing to ozone production
relative to total charge applied, that is, current efficiency.
[0130] Under any current density conditions, a current efficiency
for ozone production is about 1.3% or less when the surface layer
30 of the electrode 26 for electrolysis has a thickness not greater
than 250 nm.
[0131] When the surface layer 30 of the electrode 26 for
electrolysis is adjusted to 278 nm thick, on the other hand, a
current efficiency for ozone production is about 2.5% (17.8
mA/cm.sup.2) or about 2.8% (26.7 mA/cm.sup.2), suggesting a marked
increase in the ozone production efficiency. When the thickness of
the surface layer 30 is increased further, a current efficiency for
ozone production rises with an increase in the film thickness.
[0132] Test results in FIGS. 13 and 14 show that a production
efficiency of ozone shows a marked increase when the thickness of
the surface layer 30 constituting the electrode 26 for electrolysis
exceeds a certain level. The thickness of the surface layer at
which the current efficiency for ozone production undergoes a
change differs depending on the kind of the electrolyte solution to
be electrolyzed. If the thickness of the surface layer 30 is 200 nm
or greater, it is possible to observe the thickness at which the
current efficiency for ozone production changes.
[0133] Adjustment of the thickness of the surface layer 30 to 200
nm or greater therefore makes it possible to produce ozone without
lowering "a ratio of charging contributing to ozone production to
the total charge" is (that is) "a current efficiency".
[0134] Based on the test results as shown in FIG. 13, when the
surface layer 30 of the electrode 26 for electrolysis has a certain
thickness, an increase in current efficiency for ozone production
cannot be observed. In view of the film forming operation of the
surface layer 30 of the electrode 26 for electrolysis, it is
therefore possible to obtain an electrode having a high ozone
production efficiency while improving the productivity of the
electrode 26 for electrolysis and reducing the production cost of
it by adjusting the thickness of the surface layer 30 to 600 nm or
less.
[0135] This makes it possible to fabricate an electrode for
electrolysis having a higher ozone production efficiency at a low
cost.
* * * * *