U.S. patent application number 12/371196 was filed with the patent office on 2009-09-24 for titanium oxide photocatalyst, process for producing the same and application.
This patent application is currently assigned to SUMITOMO METAL INDUSTRIES, LTD.. Invention is credited to Haruhiko Kajimura, Yasuhiro Masaki, Sadanobu Nagaoka, Katsuhiro Nishihara, Kouji Oda, Tadashi Ogasawara, Katsumi Okada, Shiji Shimosaki, Michiyasu Takahashi, Munetoshi Watanabe, Tadashi Yao.
Application Number | 20090239738 12/371196 |
Document ID | / |
Family ID | 28457577 |
Filed Date | 2009-09-24 |
United States Patent
Application |
20090239738 |
Kind Code |
A1 |
Okada; Katsumi ; et
al. |
September 24, 2009 |
Titanium Oxide Photocatalyst, Process For Producing The Same And
Application
Abstract
Titanium oxide-based photocatalysts which contain a metal halide
in titanium oxide and which are prepared from titanium oxide and/or
its precursor, which may optionally be heat treated, by contact
with a reactive gas containing a metal halide of the formula
MX.sub.n or MOX.sub.n (wherein M=a metal, X=a halogen, and n=an
integer) with heating stably develop a high photocatalytic activity
with visible light irradiation. The photocatalysts may subsequently
be stabilized by contact with water or by heat treatment, and/or
promoted by contact with a heteropoly acid and/or an isopoly acid
so as to include a metal complex in the titanium oxide.
Photocatalysts prepared in this manner exhibit novel ESR features.
The present invention also provides methods for preparing these
photocatalysts, a photocatalyst dispersion and a photocatalytic
coating fluid containing such a photocatalyst, and photocatalytic
functional products and methods for their manufacture using the
photocatalyst.
Inventors: |
Okada; Katsumi; (Nara-shi,
JP) ; Nishihara; Katsuhiro; (Amagasaki-shi, JP)
; Masaki; Yasuhiro; (Osaka-shi, JP) ; Kajimura;
Haruhiko; (Kobe-shi, JP) ; Takahashi; Michiyasu;
(Nishinomiya-shi, JP) ; Yao; Tadashi;
(Amagasaki-Shi, JP) ; Ogasawara; Tadashi;
(Nishinomiya-shi, JP) ; Watanabe; Munetoshi;
(Osaka, JP) ; Shimosaki; Shiji; (Amagasaki-shi,
JP) ; Oda; Kouji; (Kobe-shi, JP) ; Nagaoka;
Sadanobu; (Kobe-shi, JP) |
Correspondence
Address: |
THE WEBB LAW FIRM, P.C.
700 KOPPERS BUILDING, 436 SEVENTH AVENUE
PITTSBURGH
PA
15219
US
|
Assignee: |
SUMITOMO METAL INDUSTRIES,
LTD.
Osaka
JP
OSAKA TITANIUM TECHNOLOGIES CO., LTD.
Hyogo
JP
|
Family ID: |
28457577 |
Appl. No.: |
12/371196 |
Filed: |
February 13, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10508871 |
May 25, 2005 |
7521133 |
|
|
12371196 |
|
|
|
|
Current U.S.
Class: |
502/227 |
Current CPC
Class: |
B01D 2255/802 20130101;
B01J 21/063 20130101; B01J 37/0238 20130101; B01J 37/22 20130101;
B01J 27/132 20130101; B01D 53/8662 20130101; B01D 53/8612 20130101;
B01J 23/18 20130101; B01J 27/135 20130101; B01D 53/8609 20130101;
B01J 35/004 20130101; B01D 53/8634 20130101; B01D 53/8628 20130101;
B01J 35/002 20130101 |
Class at
Publication: |
502/227 |
International
Class: |
B01J 27/135 20060101
B01J027/135 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2002 |
JP |
2002-83222 |
Nov 5, 2002 |
JP |
2002-321101 |
Jan 21, 2003 |
JP |
2003-12492 |
Claims
1-13. (canceled)
14. A method for the preparation of a titanium oxide-based
photocatalyst characterized in that titanium oxide and/or its
precursor is brought into contact with a reactive medium containing
a metal halide of the formula MX.sub.n or MOX.sub.n (wherein M=a
metal, X=a halogen, and n=an integer).
15. The method for the preparation of a titanium oxide-based
photocatalyst as set forth in claim 14 which further includes
subjecting the titanium oxide and/or its precursor to contact with
water and/or heat treatment after the contact with the reactive
medium.
16. The method for the preparation of a titanium oxide-based
photocatalyst as set forth in claim 14 which further includes
performing contact with a heteropoly acid and/or an isopoly acid
after the contact with the reactive gas, or after subsequent
contact with water and/or heat treatment.
17-24. (canceled)
25. A method for the manufacture of a photocatalytic functional
product characterized by applying a coating fluid to a surface of a
substrate, wherein said coating fluid is characterized by
comprising a titanium oxide-based photocatalyst in a solvent, and
wherein said titanium oxide-based photocatalyst is characterized by
comprising a titanium oxide and an additional metal compound; and
developing a photocatalytic activity by irradiation with visible
light the additional metal compound comprising at least one metal
halide.
26. A method for the preparation of a photocatalytic coating fluid
characterized by mixing a binder with a photocatalyst dispersion,
wherein said photocatalyst dispersion characterized by comprising a
titanium oxide-based photocatalyst dispersed in a solvent, wherein
said photocatalyst has an average particle diameter of at most 500
nm, wherein said titanium oxide-based photocatalyst is
characterized by comprising a titanium oxide and an additional
metal compound in titanium oxide; and developing a photocatalytic
activity by irradiation with visible light, and wherein the
additional metal compound comprising at least one metal halide.
27. A method for the manufacture of a photocatalytic functional
product characterized by depositing titanium oxide and/or its
precursor on a surface of a substrate followed by bringing the
surface into contact with a reactive medium containing a metal
halide of the formula MX.sub.n or MOX.sub.n (wherein M=a metal, X=a
halogen, and n=an integer).
28. The method as set forth in claim 27 which further includes
subjecting the surface of the substrate to contact with water
and/or heat treatment after the contact with the reactive
medium.
29. A The method as set forth in claim 27 which further includes
bringing the surface of the substrate into contact with a
heteropoly acid and/or an isopoly acid after the contact with the
reactive gas, or after subsequent contact with water and/or heat
treatment.
30. The method as set forth in claim 15 wherein the heat treatment
is carried out in a temperature range of 373-873 K.
31. The method as set forth in claim 14 wherein the metal halide is
at least one compound selected from TiCl.sub.4, VOCl.sub.3,
SnCl.sub.4, SbCl.sub.5, SiCl.sub.4, WCl.sub.6, and BiCl.sub.3.
32. The method as set forth in claim 31 wherein the metal halide is
TiCl.sub.4.
33. The method as set forth in claim 14 which further includes
subjecting the titanium oxide and/or its precursor or the substrate
to heat treatment in a temperature range of 323-823 K or to wet
processing prior to the contact with the reactive gas.
34. The method as set forth in claim 14 wherein the contact with
the reactive gas is performed in a temperature range of 323-873
K.
35. The method as set forth in claim 14 wherein the reactive medium
is gaseous and its base gas is a nonoxidizing gas.
36. The method as set forth in claim 35 wherein the base gas
comprises hydrogen and/or nitrogen.
37. The method as set forth in claim 14 wherein the titanium oxide
and/or its precursor is prepared via a step of neutralizing at
least one compound selected from titanium tetrachloride, titanium
sulfate, and titanyl sulfate with a nitrogen-containing base.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/508,871 filed May 25, 2005. The entire contents of the
above-referenced application are incorporated herein by
reference.
TECHNICAL FIELD
[0002] This invention relates to a titanium oxide-based
photocatalyst which can exhibit a photocatalytic activity not only
when irradiated with ultraviolet light but also when irradiated
with visible light and to a method of preparing the same. The
invention also relates to a photocatalytic functional product and a
coating fluid containing such a photocatalyst and to a method of
manufacturing the photocatalytic functional product.
BACKGROUND ART
[0003] In recent years, the photocatalytic activity of titanium
oxide has been applied to various environmental depollution
techniques including deodorization, prevention of microbial growth,
and prevention of fouling. Anatase titanium oxide, which is
commonly used as a photocatalyst, has a band gap of about 3.2 eV,
and its photocatalytic reaction proceeds by the action of
ultraviolet light having a wavelength of about 380 nm or shorter.
Therefore, in order to develop its photocatalytic activity, it is
necessary for the titanium oxide to be irradiated with ultraviolet
light, which results in problems such as a limitation in places
where the photocatalyst can be located and in the fields to which
it can be applied.
[0004] If it becomes possible to use visible light, which plentiful
in sunbeams and room lamps, as an energy source to activate a
photocatalyst, the reactivity of the photocatalyst will be
strengthened, and it will become possible to utilize a
photocatalyst in various locations. For this reason, attempts have
been made to develop photocatalytic materials capable of being
activated by irradiation with visible light.
[0005] For example, JP-A 09-262484 (1997) discloses anatase
TiO.sub.2 having a visible light activity (i.e., capability of
being activated by visible light) which is characterized by
containing vanadium or chromium incorporated by means of ion
implantation. A titanium oxide-based photocatalyst which is made
responsive to visible light by incorporating nitrogen in the
titanium oxide crystals is reported in JP-A 2001-205103, and one
which is made responsive by introducing stable lattice defects into
the titanium oxide crystals is reported in WO 00/10706.
[0006] The fact that a visible light photocatalytic activity occurs
with titanium oxide having oxygen defects (lattice defects in the
positions of oxygen) is described in Nippon Kagaku Kaishi 8, pp.
1084-1091 (1986) and JP-A 10-146530 (1998). Oxygen defects can be
identified and quantified most effectively by measurement of an ESR
(electron spin resonance) spectrum.
[0007] A photocatalyst made of titanium oxide which has oxygen
defects and is activated by visible light irradiation is reported
in Japanese Patent No. 3,252,136. That photocatalyst is
characterized by an ESR spectrum in which a signal having a g-value
range of 2.003-4 is observed when the spectrum is measured in the
dark in a vacuum at 77 K, and the intensity of this signal is
increased when the spectrum is measured under irradiation with
visible light in a vacuum at 77K.
[0008] The titanium oxide photocatalyst reported in JP-A
2001-190953 has an ESR spectrum with at least three peaks in a
g-value range of 1.903-2.030 (i.e., the range in which the g value
is 1.903-2.030), and the highest peak among these peaks exists in a
g-value range of 1.990-2.020.
[0009] The presence of foreign metal ions or defects in titanium
oxide is effective at making the titanium oxide responsive to
visible light, but on the other hand, it worsens the
reproducibility of development of photocatalytic activity and may
decrease the photocatalytic activity itself, since deactivation of
a photocatalytic response which is accompanied by isolation of
electrons and holes may originate from such ions and defects.
Another problem of the conventional photocatalysts capable of
responding to visible light including those which contain nitrogen
is that their preparation requires a device for ion implantation or
sputtering, and such a device is not suitable for use in mass
production.
[0010] It is an object of the present invention to provide a
photocatalyst which can stably exhibit a high visible light
photocatalytic activity, a method for its preparation which is
suitable for mass production, a photocatalytic functional product
using the photocatalyst, a coating fluid which can be used to
manufacture the photocatalytic functional product, and a method of
manufacturing the photocatalytic functional product.
DISCLOSURE OF THE INVENTION
[0011] The present invention is based on the following findings (1)
to (4):
[0012] (1) incorporation of a metal compound such as a metal halide
into titanium oxide results in the development of photocatalytic
activity by the action of visible light;
[0013] (2) when titanium oxide having a metal compound incorporated
therein is treated by contact with water and/or heat treatment to
convert the metal compound into a metal hydroxide or oxide, it
still retains its visible light photocatalytic activity at a
sufficient level for practical purposes while it is stabilized;
[0014] (3) when titanium oxide having a metal compound incorporated
therein is treated by contact with a heteropoly acid and/or an
isopoly acid, either directly or after the treatment with water
and/or heat treatment, so as to incorporate a polynuclear metal
complex therein, its photocatalytic activity is further increased;
and
[0015] (4) these materials show novel ESR spectra, so a material
which exhibits a high visible light photocatalytic activity can be
identified by its ESR spectrum.
[0016] The present invention provides a titanium oxide-based
photocatalyst identified by any of the following (a) to (d) and
preferably having the property described in the following (e).
[0017] (a) It comprises titanium oxide which contains a different
metal compound and preferably a metal halide, a metal complex, a
metal hydroxide, or a metal oxide, the photocatalyst exhibiting its
photocatalytic activity by irradiation with visible light.
[0018] (b) It comprises titanium oxide which contains a metal
halide and preferably a metal chloride, the photocatalyst having a
thermal desorption spectrum in which a peak of a molecular ion or a
fragment ion of a halogen-containing substance appears at a
temperature of 623 K or higher.
[0019] (c) It exhibits its photocatalytic activity by irradiation
with visible light and has an ESR spectrum measured at a
temperature of at least 5 K in which a peak is observed in a
g-value range of 1.950-2.030 (i.e., the range in which the g value
is 1.950-2.030) when the spectrum is measured with visible light
irradiation and that peak is not substantially observed when the
spectrum is measured in the dark.
[0020] (d) It exhibits its photocatalytic activity by irradiation
with visible light and has an ESR spectrum measured at room
temperature in a substantially oxygen-free atmosphere in which the
highest peak observed in a g-value range of 1.950-2.030 when
irradiated with visible light has a peak intensity with a half life
of at least 3 minutes after the irradiation with visible light is
stopped.
[0021] (e) It has an ESR spectrum measured at a temperature lower
than 77K in which a peak appears in a g-value range of
1.986-1.994.
[0022] In the present invention, the term Avisible light.apprxeq.
indicates light having a wavelength of 400 nm or longer.
[0023] The identity of the metal compound or metal halide in the
titanium oxide-based photocatalyst described in (a) or (b) above is
not clearly known at present, but it contributes to the
photocatalytic function through some interaction of a central metal
ion or a coordinated (metal) ion included in the metal halide,
hydroxide, oxide, or complex with titanium oxide.
[0024] Particularly when the titanium oxide is of the visible light
absorbing type such as one containing a lower valence titanium ion
(Ti.sup.3+, Ti.sup.2+), an oxygen-defective titanium oxide which
contains oxygen defects, or a mixture of these, or when its
response to visible light is conferred by incorporation of an
element or ion other than Ti and oxygen, such as a chromium ion, an
iron ion, a vanadium ion, nitrogen, sulfur, or chlorine, the
above-described interaction is increased, and the photocatalyst has
an increased activity. Therefore, such titanium oxide is
preferred.
[0025] In the case where the metal compound is a metal complex, it
is preferably a polynuclear metal complex such as a heteropoly acid
or an isopoly acid. In this case, it is very advantageous that the
titanium oxide is of an oxygen-defective type due to the activity
of the photocatalyst which can be further increased.
[0026] In the case of a metal complex with a halogen ligand, there
is the possibility of the halogen ligand interacting with titanium
oxide. When the metal compound is a metal halide, it is
particularly preferable that at least part of the metal halide be
chemically bonded to the surface of the titanium oxide.
[0027] The present invention also provides methods of preparing the
above-described titanium oxide-based photocatalyst.
[0028] A first method comprises bringing titanium oxide and/or its
precursor into contact with a reactive medium containing a metal
halide of the formula MX.sub.n or MOX.sub.n (wherein M=a metal, X=a
halogen, and n=an integer). This method can be used to readily
prepare a titanium oxide-based photocatalyst which is defined by
any of (a)-(c) and (e) above with good reproducibility.
[0029] A second method comprises subjecting the product obtained by
contact with the reactive medium in the first method to
post-treatment by contact with water and/or by heat treatment. This
post-treatment can be used to prepare a titanium oxide-based
photocatalyst defined by (d) above.
[0030] A third method comprises further subjecting the product
obtained by contact with the reactive medium by the first method or
obtained by post-treatment comprising contact with water and/or
heat treatment by the second method to contact with a heteropoly
acid and/or an isopoly acid. This method can be used to prepare a
titanium oxide-based photocatalyst containing a metal complex.
[0031] In order to obtain a titanium oxide-based photocatalyst
having a further increased activity in any of the above-described
methods, it is preferred to subject the titanium oxide and/or its
precursor to heat treatment at 323-823 K or wet processing as
pretreatment before contact with the reactive medium.
[0032] A titanium oxide-based photocatalyst according to the
present invention can be used in various forms such as a powder,
film (including thin film), fiber, and the like. In particular, a
substrate having the titanium oxide-based photocatalyst deposited
or fixed on the surface thereof (in the form of a film, for
example) can be used as a photocatalytic functional product having
a photocatalytic activity.
[0033] A preferable photocatalytic functional product has a film
comprising the above-described titanium oxide-based photocatalyst
and a binder component on the surface of a substrate, the film
containing 5-95 mass % of the titanium oxide-based
photocatalyst.
[0034] The present invention further provides a dispersion and a
coating fluid containing the photocatalyst which can be used to
manufacture the photocatalytic functional product.
[0035] A photocatalytic functional product according to the present
invention can be manufactured by a method comprising applying the
coating fluid to the surface of a substrate.
[0036] In another method, a photocatalytic functional product can
be manufactured by depositing titanium oxide and/or its precursor
on the surface of a substrate followed by bringing the surface into
contact with a reactive medium containing a metal halide of the
formula MX.sub.n or MOX.sub.n (wherein M, X, and n are as indicated
above).
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 shows a thermal desorption spectrum (TDS) of a
titanium oxide-based photocatalyst according to the present
invention which was prepared in Example 2(a).
[0038] FIG. 2 shows a thermal desorption spectrum (TDS) of another
titanium oxide-based photocatalyst according to the present
invention which was prepared in Example 2(b).
[0039] FIG. 3 shows an ESR spectrum measured at 5 K in air of a
titanium oxide-based photocatalyst according to the present
invention which was obtained in Example 1.
[0040] FIG. 4 shows an ESR spectrum measured at 293 K and 5 K in
air of a titanium oxide-based photocatalyst according to the
present invention which was obtained in Example 5.
[0041] FIG. 5 shows an ESR spectrum measured at 293 K and 5 K in
air of a known titanium oxide-based photocatalyst having visible
light activity (used in Comparative Example 2).
[0042] FIG. 6 is a graph showing the decrease, after turnoff of the
light, in the intensity of the highest peak observed in a g-value
range of 1.950-2.030 in ESR spectra of the titanium oxide-based
photocatalysts of Example 5 and Comparative Example 2 which were
measured in a deoxidized atmosphere.
[0043] FIG. 7 is a graph showing the relationship between the
content of a titanium oxide-based photocatalyst in a photocatalytic
film and the visible light photocatalytic activity (rate of
CO.sub.2 formation) in an example.
[0044] FIG. 8 is a graph showing the relationship between the
thickness of a photocatalytic film and the visible light
photocatalytic activity (rate of CO.sub.2 formation) in an
example.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0045] A titanium oxide-based photocatalyst according to the
present invention develops its photocatalytic activity by the
action of visible light. As described earlier, the term Avisible
light.apprxeq. means radiation having a wavelength of 400 nm or
longer.
[0046] A conventional titanium oxide-based photocatalyst does not
develop its photocatalytic activity with light having a wavelength
of 390 nm or longer, so it will not exhibit any substantial
photocatalytic activity when irradiated with visible light. In
contrast, a titanium oxide-based photocatalyst according to the
present invention exhibits its photocatalytic activity when
irradiated with visible light. Of course, it also functions as a
photocatalyst when irradiated with ultraviolet light. In addition,
it exhibits its photocatalytic activity when irradiated with near
ultraviolet light having a narrow wavelength range of 390-400 nm.
This is another different property which is not found in a
conventional titanium oxide-based photocatalyst. These unique
photocatalytic properties are observed commonly with all
photocatalysts according to various embodiments of the present
invention which will be described below.
[0047] In one aspect, a titanium oxide-based photocatalyst capable
of developing its photocatalytic activity by irradiation with
visible light according to the present invention contains a
different metal compound (namely, a metal compound other than
titanium oxide) in titanium oxide.
[0048] The Adifferent metal compound.apprxeq. may be present either
inside or outside the titanium oxide particles, or both inside and
outside the particles. The metal compound includes a metal halide,
a metal hydroxide, a metal oxide, a metal complex, and the like.
Other metal compounds may be used provided that their presence
causes the titanium oxide to exhibit visible light photocatalytic
activity. In the case of a metal halide or a metal hydroxide, at
least part of the compound is preferably bonded to the surface of
titanium oxide by a chemical reaction. For example, a metal halide
can be bonded to the surface of titanium oxide by reacting with the
hydroxyl groups existing on the surface of titanium oxide to cause
a dehydrohalogenation reaction. Alternatively, the metal halide may
be partly bonded to the surface of titanium oxide in this manner
and partly adsorbed by that surface.
[0049] The metal halide which is present in a titanium oxide-based
photocatalyst according to the present invention may be in the form
of a metal halide such as titanium tetrachloride itself, which is
generally represented by MX.sub.n (wherein M=a metal, X=a halogen,
and n=an integer). However, the metal halide is usually chemically
bonded to the surface of titanium oxide and thus converted into the
form of --O-M-X.sub.n (wherein O is oxygen from the surface of
titanium oxide), which is formed by partial dehydrohalogenation of
the metal halide, or into the form --O-M-(L).sub.mX.sub.n= (wherein
L is a ligand such as oxo (O), hydroxyl, ammine (ammonia), an
alkylamine, an alkylphosphine, phenylphosphine, an alkyl, acetate,
carbonyl, or a nitrogen-containing heterocyclic group such as
pyridyl and piperidyl, m is an integer, and n= is zero or an
integer), which is formed when the residual halogen of
--O-M-X.sub.n further undergoes hydrolysis or another reaction. The
metal halide may be converted into a metal hydroxide in which all
of the halogens X of the halide are converted into hydroxyl groups
by hydrolysis, or it may be converted into a metal oxide in which
the metal hydroxide further undergoes dehydration (condensation) to
form a metal oxide network.
[0050] The halogen may be a single element selected from fluorine,
chlorine, bromine, and iodine, or it may be a mixture of two or
more of these elements. Chlorine is particularly preferred since it
can confer the highest photocatalytic activity.
[0051] The metal compound may be a halogen-free compound such as a
metal complex. The metal complex which exists in titanium oxide may
be either in the form of a metal complex itself which is generally
represented by ML.sub.n (wherein M=a metal, L=a ligand, n=an
integer) or in a form such as --O-M(L).sub.n derived from the
complex when it is chemically bonded to the surface of titanium
oxide.
[0052] Examples of the ligand L are oxo (O, oxygen), hydroxyl,
ammine (ammonia), an alkylamine, an alkylphosphine,
phenylphosphine, an alkyl, acetate, carbonyl, and a
nitrogen-containing heterocyclic group such as pyridyl and
piperidyl. When a plurality of ligands are present, they may be the
same or different from each other.
[0053] The metal compound may also be a compound such as a
polynuclear metal complex which contains a plurality of metals
which may be the same or different from each other. It is also
possible to include two or more different metal compounds in
titanium oxide. Among polynuclear metal complexes, a heteropoly
acid (anion) and an isopoly acid (anion) which are oxo acid-type
polynuclear metal complexes are preferred because of their ability
to confer an increased visible light photocatalytic activity.
Examples of a heteropoly acid include PW.sub.12O.sub.40.sup.3-,
SiW.sub.12O.sub.40.sup.3-, and PMo.sub.12O.sub.40.sup.3-. Examples
of an isopoly acid include Mo.sub.7O.sub.24.sup.6- and
W.sub.7O.sub.24.sup.6-. The counter cations of these anions may be
protons or other cations such as Cs.sup.+ or NH.sub.4.sup.+. They
may be either chemically bonded to the surface of titanium oxide or
merely adsorbed by that surface.
[0054] Regardless of whether the metal compound is a metal halide,
a metal hydroxide, a metal oxide, or a metal complex, examples of
the metal M include Ti, Si, Zr, Al, B, C, P, Mg, Sc, Cr, Co, Zn,
Hf, Fe, Sb, W, V, Sn, Bi, Mn, Mo, Cs, Ge, As, and Ce. Of these, P,
Si, W, Mo, Cs, Ge, As, and Ce are metals which are preferred when
the metal compound is a metal complex of the heteropoly acid or
isopoly acid type. In particular, a high photocatalytic activity is
obtained when the metal M is at least one metal selected from Ti,
Si, V, Sn, Sb, W, Nb, Bi, P, Mo, Cs, Ge, As, and Ce, and thus such
metals are preferred.
[0055] The content of the metal compound in titanium oxide is
preferably at least 0.1 mass % and at most 300 mass % as metal
based on the mass of titanium oxide. A content of less than 0.1
mass % may be insufficient to confer to titanium oxide the desired
photocatalytic activity developed with visible light. If the
content of the metal compound is excessive, the properties of the
metal compound appear strongly, leading to a decrease in
photocatalytic activity. When the metal compound is a metal halide
introduced into titanium oxide by the below-mentioned contact
treatment or a metal hydroxide or oxide formed by decomposition
(hydrolysis or thermal decomposition) of the halide, it is
difficult for the metal compound to be included in titanium oxide
in an amount larger than 10 mass %. Therefore, in such a case, the
upper limit of the content of the metal compound is generally on
the order of 10 mass %, although a higher content is acceptable if
it is possible. When the metal compound is a heteropoly acid or an
isopoly acid, it can be introduced into titanium oxide in a much
higher amount. However, a content thereof in excess of 300 mass %
causes a significant decrease in photocatalytic activity.
[0056] The above-described titanium oxide-based photocatalyst which
contains a metal halide is characterized by a thermal desorption
spectrum (also called TDS or TPD spectrum) measured by
temperature-programmed desorption gas spectrometry in which a peak
of a molecular ion or a fragment ion of a halogen-containing
substance appears in a temperature range above 573 K.
[0057] Measurement of a thermal desorption spectrum is carried out
by measuring the spectrum of gases which are desorbed from a sample
by use of a mass spectrograph while the temperature of the sample
is elevated at a constant rate by infrared heating or other heating
method after the sample has been thoroughly dried and then
exhausted to about 10.sup.-6-10.sup.-9 Torr. The measurement of the
temperature is preferably performed on the surface of the sample
for accurate identification. A molecular ion or fragment ion
normally has a plurality of peaks in a thermal desorption
spectrum.
[0058] Even if titanium oxide contains a halogen, when all the
peaks of a molecular ion or a fragment ion of a halogen-containing
substance observed in a thermal desorption spectrum appear in a
temperature range lower than 573 K, the interaction of the halogen
or halogen-containing substance with titanium oxide is weak,
resulting in an extremely weak photocatalytic activity, so such a
material will not substantially function as a photocatalyst with
visible light. Titanium oxide having at least one peak in a
temperature range above 573 K in a thermal desorption spectrum can
be expected to exhibit a high photocatalytic activity with visible
light. The peak is preferably in a temperature range above 623 K.
In such a case, the photocatalytic activity with visible light is
further increased.
[0059] With respect to the case where the halogen is chlorine,
non-limiting examples of a metal halide having a peak of a
molecular ion or a fragment ion above 573 K in a thermal desorption
spectrum include titanium chlorides, titanium oxychloride, and a
polymer of these. All the metal halides exemplified above have a
peak at a temperature above 623 K.
[0060] In another aspect of the present invention, a titanium
oxide-based photocatalyst which exhibits visible light
photocatalytic activity according to the present invention has an
ESR spectrum with novel features.
[0061] Specifically, in a first feature, at least one peak is
observed in an ESR spectrum measured with irradiation by visible
light at a temperature of at least 5 K in a g-value range of
1.950-2.030, and this peak is not substantially observed in an ESR
spectrum measured in the dark. The expression Anot substantially
observed.apprxeq. indicates that the peak is lower than the limit
of detection of the ESR measuring apparatus. Some photocatalysts
according to the present invention show the first feature of an ESR
spectrum throughout a temperature range of from 5 K to room
temperature. Other photocatalysts show the first feature only at a
low temperature of 5K or in the vicinity thereof and no longer show
it at room temperature.
[0062] Irradiation with visible light for measurement of ESR
spectra can be performed using a halogen lamp or a xenon lamp and
is preferably continued for at least 1 minute. If the irradiated
light includes a considerable amount of ultraviolet rays, the peak
intensities of an ESR spectrum are decreased, so it is preferable
to irradiate only visible light consisting substantially of rays
having a wavelength of 400 nm or longer by filtering the light, if
necessary. For the first feature, the atmosphere for ESR
measurement is not limited. Thus, whether the measurement
atmosphere is air, an inert gas, or a vacuum, the ESR spectrum
shows the first feature.
[0063] Measurement of an ESR spectrum in the dark is conducted in
light-shielding conditions after a titanium oxide-based
photocatalyst to be measured is stored in the dark or in
light-shielding conditions for at least 3 days at the same
temperature as that of the ESR measurement whether the
photocatalyst has a history of irradiation with light or not.
Subsequent to this measurement in the dark, if the photocatalyst is
irradiated with visible light and its ESR spectrum is measured, the
resulting spectrum has an ESR peak in a g-value range of
1.950-2.030, which peak does not appear in the ESR spectrum
measured in the dark.
[0064] Some titanium oxide-based photocatalysts having an ESR
spectrum with the above-described first feature show a second
feature in an ESR spectrum measured at room temperature in a
substantially oxygen-free atmosphere in which the highest peak
observed in a g-value range of 1.950-2.030 when irradiated with
visible light has a peak intensity with a half life of at least 3
minutes after the irradiation with visible light is stopped. A
photocatalyst having visible light activity can be identified only
by the presence of the second feature in its ESR spectrum. The half
life of the intensity of this peak after extinction is preferably
at least 4 minutes for further increasing the photocatalytic
activity. Since the intensity of a peak in an ESR spectrum is
generally indicative of the concentration of an active species, the
half life of the intensity of a peak generally corresponds to the
period elapsed until the concentration of an active species
decreases to a half its initial concentration.
[0065] The term Asubstantially oxygen-free.apprxeq. indicates that
the atmosphere has an oxygen partial pressure of at most 0.1 Torr.
The atmosphere may be either a vacuum or a pure inert gas. The
second feature can be seen when an ESR spectrum is measured at room
temperature in such an atmosphere.
[0066] Peaks in an ESR spectrum with a g-value in the range of
1.950-2.030 constitute a triplet signal (having three peaks), and
this signal is considered to be attributable to an active species
in which oxygen defects participate, as discussed in the Japanese
article Catalyst 9 (the 21st proceeding), pp. 45-48 (1963). The
active species plays the major role in visible light photocatalytic
reaction.
[0067] The active species can be present both inside titanium oxide
particles and on the surface thereof, and in either case, it is
attenuated (extinguished) and decayed by recombination or by a
reaction with a substance such as oxygen. The above-described first
feature in an ESR spectrum indicates that the decay of the active
species is very fast in the presence of oxygen. From this, it is
presumed that a photocatalyst according to the present invention
has a structure in which the active species readily reacts with
oxygen, for example, it is mostly located on the surface of
titanium oxide. Therefore, when the photocatalyst is made to
function in normal conditions, namely, in an oxygen-containing
atmosphere (typically in air), it exhibits a very high
photocatalytic activity by irradiation with visible light due to a
prompt reaction with oxygen of the active species formed by
irradiation with visible light.
[0068] On the other hand, the second feature in an ESR spectrum
that the intensity of the highest peak of the triplet signal has a
half life of at least 3 minutes when the spectrum is measured at
room temperature in a deoxidized atmosphere indicates that the
decay of the above-described active species, which plays the major
role in a visible light photocatalytic reaction, is very slow in a
deoxidized atmosphere. Since any reaction with oxygen cannot occur
under such conditions, it is thought that the active species which
gives the highest peak intensity and which exists inside the
particles or on the surface thereof has a structure which is
essentially difficult to recombine.
[0069] As a result, with a photocatalyst having the second feature
in addition to the first feature, the active species formed by
irradiation with visible light preferentially reacts with oxygen in
an oxygen-containing atmosphere with little deactivation which can
occur inside the titanium oxide particles or the surface thereof,
and the photocatalytic reaction proceeds sequentially in an
efficient manner so that the visible light photocatalytic activity
continues.
[0070] It is preferable that a titanium oxide-based photocatalyst
having the above-described first and second features further have a
third feature, which is an ESR spectrum measured at a temperature
lower than 77K in which a peak appears in the very narrow g value
range of 1.986-1.994. As a result, it has a further increased
photocatalytic activity. The third feature is observed whether the
atmosphere of ESR spectrum measurement is air, a vacuum, or an
inert gas. The temperature of measurement is lower than 77 K,
preferably lower than 30 K, and more preferably 5 K. The lower the
temperature, the higher the peak.
[0071] The structure of titanium oxide in a titanium oxide-based
photocatalyst according to the present invention may be either
amorphous or crystalline, or it may be partially crystalline. When
it includes a crystalline structure, the crystal form may be any of
anatase, rutile, and brookite, or it may be a mixture of these. In
order to achieve the highest visible light photocatalytic activity,
the major crystal form of titanium oxide is desirably anatase.
[0072] The titanium oxide is preferably of a type which absorbs
visible light, such as titanium oxide containing lower valence
titanium ions, oxygen-defective titanium oxide, or a mixture of
these. Oxygen-defective titanium oxide is particularly preferred.
Whether the titanium oxide constituting a photocatalyst has visible
light absorptivity can be determined by whether the photocatalyst
assumes a color. When the titanium oxide is of the visible
light-absorbing type, most of the energy of sunbeams and artificial
light sources (such as fluorescent tubes and halogen lamps) can be
used to decompose organic substances (such as pollutants and
harmful substances), and thus the reaction efficiency is increased
compared to a conventional photocatalyst activated by ultraviolet
light. In particular, in the case of using a heteropoly acid or an
isopoly acid as a metal compound included in the photocatalyst, if
the titanium oxide is of the oxygen-defective type, a titanium
oxide-based photocatalyst having a very high visible light
photocatalytic activity can be achieved.
[0073] When titanium oxide including lower valence titanium ions,
oxygen-defective titanium oxide, or a mixture of these is generally
represented by the formula TiO.sub.(2-Y), the value of Y is
preferably 0.5>Y>0. If Y is greater than 0.5, it becomes
almost impossible to achieve a photocatalytic activity with
titanium oxide containing a metal compound.
[0074] It is also possible to apply the present invention to
titanium oxide to which visible light responding ability is
imparted by another technique (such as incorporation of Cr, Fe, or
V ions), thereby increasing the visible light responding ability of
the titanium oxide.
[0075] A titanium oxide-based photocatalyst according to the
present invention can be prepared by a method comprising bringing
titanium oxide or its precursor into contact with a reactive medium
containing a metal halide of the formula MX.sub.n or MOX.sub.n
(wherein M=a metal, X=a halogen, and n=an integer and more
specifically n=the valence number of the metal in the case of
MX.sub.n and n=[the valence number minus 2] in the case of
MOX.sub.n), and optionally followed by contact with water and/or
heat treatment.
[0076] The raw material which is used in this method is titanium
oxide, a titanium oxide precursor, or a mixture of both. The
Atitanium oxide precursor.apprxeq. means a compound which is
converted into titanium oxide in its major structure upon heating.
Specific examples of a titanium oxide precursor include titanium
hydroxide, hydrated titanium oxide, titanium oxychloride, and a
partial hydrolysate of a titanium alkoxide.
[0077] When the raw material is titanium oxide, its structure may
be either amorphous or crystalline, or it may be a mixture of
these. In the case of a raw material containing crystalline
titanium oxide, the crystal form may be anatase, rutile, or a
mixture of these. For the purpose of obtaining titanium oxide
having a high photocatalytic activity, it is desirable to use
anatase titanium oxide as a raw material, and a more desirable raw
material is anatase titanium oxide containing some amorphous
moieties which still remain therein. As mentioned before, the raw
material may be titanium oxide having visible light responding
ability provided by another technique.
[0078] The titanium oxide or its precursor which is used as a raw
material may be either a commercially available product or a
prepared product formed by a reaction, but the use of a prepared
product is preferred. The prepared product can be obtained by, for
example, hydrolyzing an aqueous solution or a solution in an
organic solvent of a titanium compound such as a titanium alkoxide,
titanyl sulfate, titanium sulfate, or titanium tetrachloride in the
presence of water, an acid, or a base followed by calcinating the
hydrolysate, if necessary, or by calcinating a titanium compound in
an oxygen-containing atmosphere.
[0079] The raw material which is used in the present invention is
preferably titanium oxide and/or its precursor prepared by a
process comprising hydrolyzing at least one titanium compound
selected from titanyl sulfate, titanium sulfate, and titanium
tetrachloride in the presence of a nitrogen-containing base (i.e.,
ammonia or an amine). This makes it possible to obtain a more
active titanium oxide-based photocatalyst. The titanium compound
used as a raw material can be subjected to hydrolysis either as is
or in aqueous solution, but the use of an aqueous solution is
desirable from the standpoint of handling.
[0080] Titanium oxide and/or its precursor is brought into contact
with a reactive medium which contains a metal halide. The reactive
medium may be either gaseous or liquid, but in order to distribute
a metal halide uniformly on the titanium oxide, the use of a
gaseous medium is desirable. The following description will be made
mainly with respect to the case in which a gaseous reactive medium
(hereinafter referred to as a reactive gas) is used.
[0081] The contact of titanium oxide and/or its precursor with a
reactive gas may be performed at room temperature, but preferably
it is done at an elevated temperature. A preferable temperature for
this contact is at least 323 K and at most 873 K. If the
temperature is outside this range, it may be, made impossible to
impart adequate visible light responding ability to titanium oxide.
A more preferred temperature range is at least 473 K and at most
673 K.
[0082] The duration of contact with the reactive gas varies
depending on the temperature. It is at least 1 minute and at most
several hours. For commercial operation, the duration is desirably
within 120 minutes. For example, in the case of the temperature of
contact being 573 K, a photocatalyst having adequate performance
can be obtained if the duration of contact is from 5 to 40
minutes.
[0083] As described previously, the metal halide which is present
in the reactive gas is a compound of the formula MX.sub.n or
MOX.sub.n. The metal M in the formula MX.sub.n may be any metal
capable of forming a liquid or gaseous halide in the
above-described temperature range for contact such as Ti, Si, Zr,
Al, B, C, P, Mg, Sc, Cr, Co, Zn, Hf, Fe, Sb, W, V, Sn, Bi, and Mn.
An example of a compound of the formula MOX.sub.n is VOCl.sub.3.
The halogen X may be any of F, Cl, Br, and I, but as described
earlier, it is preferably Cl from the standpoint of photocatalytic
activity.
[0084] Preferable metal halides are TiCl.sub.4, VOCl.sub.3,
SnCl.sub.4, SbCl.sub.5, SiCl.sub.4, WCl.sub.6, and BiCl.sub.3. The
use of a reactive gas which contains one or more of these halides
allows the preparation of a titanium oxide-based photocatalyst
having a high visible light activity.
[0085] In particular, TiCl.sub.4 is the most preferred in view of
availability, ease of handling, and ability to prepare a
photocatalyst having a high activity with visible light. TiCl.sub.4
is liquid at room temperature and has a relatively high vapor
pressure, so it is readily vaporized to prepare a reactive gas.
[0086] The metal halide, when allowed to contact with titanium
oxide at an elevated temperature, is bound by the surface of the
titanium oxide, and at the same time it can reduce the titanium
oxide matrix to introduce oxygen defects therein, thus converting
the titanium oxide into one capable of absorbing visible light.
This effect is particularly significant when a base gas (dilution
gas) of the reactive gas contains hydrogen and/or nitrogen, and a
photocatalyst having an increased visible light activity can be
obtained.
[0087] The reactive gas is generally a mixed gas of a metal halide
and a base gas (e.g., a carrier gas used for vaporization of the
metal halide by bubbling). The base gas is preferably one or more
gases selected from non-oxidizing gases such as hydrogen, nitrogen,
argon, carbon monoxide, and ammonia. However, it may contain
oxidizing gases such as oxygen and/or steam insofar as their total
amount is at most 3 vol %. In this case, attention should be paid
to side reactions between a non-oxidizing gas and an oxidizing
gas.
[0088] It is preferable that at least part of the base gas be
constituted by hydrogen and/or nitrogen since, as described above,
a photocatalyst having an increased photocatalytic activity can be
obtained if the base gas contains at least one of hydrogen and
nitrogen.
[0089] The content of the metal halide in the reactive gas is
limited by the temperature of the gas and the vapor pressure of the
compound. In general, it is preferably at most 10 vol %. Even if it
is made higher than 10 vol %, there is no significant difference in
the properties of the resulting titanium oxide-based photocatalyst,
and the yield is decreased, which is disadvantageous from the
viewpoint of economy.
[0090] As described previously, the reactive medium may be liquid.
Even in such a case, it is possible to obtain a titanium
oxide-based photocatalyst having visible light photocatalytic
activity. The liquid reactive medium is generally a solution of a
metal halide dissolved in a solvent. Examples of a suitable solvent
are hydrocarbons such as hexane, heptane, and benzene;
tetrahydrofuran, dioxane, and DMF, although it is not limited to
these. Titanium oxide and/or its precursor is added to a solution
of a metal oxide and heated, if necessary, resulting in the
formation of a photocatalyst according to the present invention
which contains a metal halide. After it is recovered from the
liquid reactive medium, the titanium oxide and/or its precursor is
dried with heating, if necessary, to allow the solvent to
evaporate. Then, it may be further subjected to the undermentioned
post-treatment and particularly heat treatment. When the raw
material is a titanium oxide precursor, it is converted to titanium
oxide by such heat treatment.
[0091] Before the raw material which is titanium oxide and/or its
precursor is brought into contact with a reactive medium containing
a metal halide and preferably a reactive gas as described above,
the raw material may be subjected to heat treatment as
pretreatment. Pretreatment by heat of titanium oxide or its
precursor which is used as a raw material causes removal of water
from the surface of the raw material, crystallization of the
material, or modification of surface functional groups, which
favorably affects the reaction of the raw material with the metal
halide during contact with the reactive medium. As a result, a
photocatalyst having an increased activity can be obtained.
[0092] The temperature for such heat treatment performed as
pretreatment is preferably at least 323 K and at most 823 K and
more preferably at least 373 K and at most 673 K. The atmosphere
for this heat treatment is not limited, and the heat treatment may
be performed in a vacuum, in air, in an atmosphere of an inert gas
such as nitrogen or argon, or in ammonia. The duration of this heat
treatment varies depending on the temperature, but usually it is
preferably between about 30 minutes and about 120 minutes. When the
raw material is a titanium oxide precursor, it may be converted
into titanium oxide during pretreatment by performing the heat
treatment for pretreatment in air.
[0093] The pretreatment of the raw material may be effected by wet
processing rather than heat treatment. The wet processing may be
performed not only at atmospheric pressure but also at an elevated
pressure and temperature in an autoclave (i.e., by hydrothermal
treatment). The wet processing is preferably performed using an
aqueous solution containing a nitrogen-containing substance such as
ammonia, an ammonium salt, an amine, or hydrazine, thereby making
it possible to obtain a highly active photocatalyst. The
temperature for wet processing can be from room temperature to
about 473 K and preferably is in the range of from 373 K to 473 K.
Usually, the duration of processing is preferably 120 minutes or
longer.
[0094] The raw material which is titanium oxide and/or its
precursor and which may optionally have been pretreated in the
above-described manner is brought into contact with a reactive
medium containing a metal halide, thereby resulting in the
preparation of a titanium oxide-based photocatalyst according to
the present invention in which a different compound is included in
titanium oxide. The photocatalyst exhibits a very high visible
light photocatalytic activity. The photocatalyst has an ESR
spectrum which shows the above-described first feature when
measured at 5 K and preferably also shows the above-described third
feature. In addition, the photocatalyst preferably has the
above-described thermal desorption spectrum.
[0095] The titanium oxide-based photocatalyst prepared by the
above-described method may be subjected to post treatment by
contact with water and/or by heat treatment. Contacting titanium
oxide and/or its precursor with a reactive medium containing a
metal halide causes a chemical reaction to occur between functional
groups (such as hydroxyl groups) existing on the surface of the
titanium oxide or its precursor and the metal halide. Thus, the
resulting photocatalyst includes a halogen-containing reaction
product attached to the surface thereof and in some cases further
includes hydrogen chloride formed as a by-product or unreacted
metal halide adsorbed by the surface.
[0096] The post treatment causes decomposition or removal of the
halogen-containing substances attached to or adsorbed by the
surface of titanium oxide and conversion of the metal compound or
compounds included in the titanium oxide to another metal compound
or compounds such as a hydroxide and/or oxide, thus stabilizing the
photocatalyst. However, the visible light photocatalytic activity
of the photocatalyst is decreased by the post treatment.
Nevertheless, a titanium oxide-based photocatalyst prepared by the
above-described method retains its visible light photocatalytic
activity at a level high enough for practical use even after being
subjected to such post treatment. In addition, the post treatment
makes the photocatalyst chemically stable and easy to handle, so
from a practical standpoint, it is preferable to subject the
photocatalyst to post treatment.
[0097] When the metal halide included in the reactive medium is a
titanium halide, the metal compound included in the resulting
photocatalyst is converted into titanium hydroxide or oxide during
post treatment, and the photocatalyst no longer contains a halogen.
It is difficult to distinguish the resulting halogen-free
post-treated product from a conventional titanium oxide by chemical
composition. However, the oxygen defects introduced by a reaction
during contact with the reactive medium still remain in the
post-treated product and make the product show an ESR spectrum
which is different from that of a conventional titanium oxide,
thereby making it possible to identify a photocatalyst according to
the present invention by an ESR spectrum. Thus, whereas a
conventional titanium oxide gives an ESR spectrum which does not
show any of the above-described first to third features, an ESR
spectrum of a post-treated product according to the present
invention shows the first and the second features and preferably
also the third feature. In addition, it shows the first feature in
a wide temperature range of from 5 K to room temperature.
[0098] Post treatment can be performed by contact with water and/or
by heat treatment. Although either treatment alone has an adequate
effect, both treatments may be performed sequentially. In this
case, the order of treatment is not limited.
[0099] Post treatment of the photocatalyst by contact with water
results in hydrolysis of halogen-containing substances which are
chemically bonded to the surface of the photocatalyst as well as
removal of halogen-containing adsorbates by washing off from that
surface. The contact with water can be performed by placing the
photocatalyst into water and allowing the resulting mixture to
stand or stirring it. Another method such as spraying the
photocatalyst with water may be employed. The water which is used
may be just water, or it may contain a suitable ingredient such as
a base (e.g., a nitrogen-containing base such as ammonia and an
amine) in water. After treatment, the recovered photocatalyst is
preferably dried.
[0100] When a coating fluid, which is described later, is prepared,
if water is used as a dispersing medium, the photocatalyst is
brought into contact with water, so it is necessarily subjected to
post treatment by contact with water. However, since that treatment
is accompanied by release of halogen-containing substances to the
coating fluid, the post treatment is preferably performed
separately before preparation of the coating fluid.
[0101] Post treatment by heat treatment causes the
halogen-containing substances bonded to or adsorbed by the surface
of the photocatalyst to be removed by thermal decomposition or
evaporation. Such heat treatment may also cause crystallization of
titanium oxide to proceed, thereby leading to an increased activity
of the photocatalyst. The temperature for heat treatment is
preferably in the range of 373-873 K, since if it is outside this
range, the above results of heat treatment may not be achieved
significantly. A more preferable temperature range is between 473 K
and 673 K. The atmosphere for heat treatment may be air, an inert
gas, or a vacuum. The duration of heat treatment is not limited,
but from a practical standpoint, it is usually in the range of
60-180 minutes.
[0102] A photocatalyst according to the present invention which
contains a metal complex can be prepared, after a metal halide is
incorporated into titanium oxide in the above-described manner and
optionally the resulting photocatalyst is subjected to post
treatment by contact with water and/or heat treatment, by
subjecting the product to a chemical reaction to introduce a
desired metal complex into titanium oxide. However, there are cases
in which a photocatalyst containing a metal complex is formed
merely by contact of titanium oxide with a metal halide with or
without subsequent post treatment.
[0103] In particular, a photocatalyst according to the present
invention which contains a metal complex like a heteropoly acid or
an isopoly acid can be prepared by bringing titanium oxide and/or
its precursor into contact with a metal halide as described above
and subsequently with a heteropoly acid or an isopoly acid. The
contact with a heteropoly acid or an isopoly acid may be performed
after the above-described post treatment, i.e., treatment with
water or heat treatment to remove any halides which are present on
the surface of the photocatalyst prepared by contact with a metal
halide.
[0104] The contact may be performed using a solution of a
heteropoly acid or an isopoly acid by immersion, precipitation, or
kneading. The solvent used for the solution is preferably a polar
solvent such as water, acetone, an alcohol, THF, or pyridine. The
contact treatment with a heteropoly or isopoly acid is preferably
carried out with heating. Alternatively or additionally, heat
treatment may be performed in a temperature range of 373-773 K
after the solvent used for the contact treatment is removed by
distillation. Such heat treatment is preferable since it allows the
metal complex to be firmly fixed to the titanium oxide, thereby
increasing the activity of the photocatalyst.
[0105] A photocatalyst according to the present invention in which
a heteropoly acid and/or an isopoly acid is included as a metal
complex has an ESR spectrum having the above-described first
feature at a temperature of 5 K and preferably the third
feature.
[0106] A photocatalyst according to the present invention may be
supported on a carrier or adsorbent having no photocatalytic
activities such as silica, alumina, activated carbon, and zeolite.
It may be doped with a promoter like a noble metal such as
platinum, ruthenium, or palladium with the object of improving the
reaction efficiency or the like.
[0107] The photocatalyst may be in the form of particles, fibers, a
coating film (including a thin film), and the like. The form may be
selected depending on the purpose of application. Particles
encompass from fine particles on the order of one nanometer to
granulated particles on the order of several millimeters, and the
size and shape of the particles are not limited. In the case of a
coating film, it is commonly fixed on the surface of a substrate,
and its thickness is not limited. When a photocatalyst is shaped
into any desired form such as a coating film or fiber, the shaped
form normally comprises a binder in addition to particles of a
titanium oxide-based photocatalyst. The presence of a binder makes
it possible to increase the thickness of a coating film or diameter
of a fiber and improve the strength and workability of the coating
film or fiber. In place of or in addition to a binder, a shaping
additive may be used.
[0108] A titanium oxide-based photocatalyst according to the
present invention can be used as a photocatalytic functional
product by depositing it on the surface of a substrate for
fixation. The form of fixation may be selected depending on the
surface shape of the substrate and the purpose of application and
is not limited to a specific form, but it is typically a coating
film.
[0109] The material of the substrate is not limited, and it may be
any of a metallic material such as a carbon steel, plated steel,
chromated steel, enameled steel, stainless steel, aluminum,
titanium, and various alloys; an inorganic material such as a
ceramic, glass, porcelain, and quartz; and an organic material such
as a plastic, resin, and activated carbon. A composite material in
which two or more different materials are combined such as a coated
steel sheet may also be employed. However, some substrates having
an organic material on their surface or made entirely of an organic
material may be deteriorated or degraded by the oxidizing ability
of the photocatalyst. In such cases, the surface of the substrate
on which a photocatalyst is fixed should previously be coated with
a material which is not degraded by the photocatalyst.
[0110] The shape of the substrate is also not limited, and it may
be any shape such as a sheet or plate, a panel, fibers (including
woven and nonwoven fabric), a mesh, or a pipe. The substrate may be
a fabricated object having a complicated shape desired for a final
product, or it may be even an object which is already installed or
in service. The surface of the subject may be either porous or
non-porous.
[0111] The above-described photocatalytic functional product may be
manufactured by either of the following two methods.
[0112] In a first method, a coating fluid comprising particles of a
titanium oxide-based photocatalyst according to the present
invention dispersed in a solvent is prepared and applied to a
substrate. The coating fluid may consist essentially of a
photocatalyst and a solvent, but preferably it further comprises a
binder.
[0113] The coating fluid may be prepared by merely adequately
mixing the titanium oxide-based photocatalyst with a solvent or a
solvent and a binder. However, a titanium oxide-based photocatalyst
prepared by the above-described method typically has a small
average diameter of primary particles in the range of several
nanometers to several hundreds nanometers, so it is in the form of
an agglomerated powder of such primary particles, and the average
diameter of the agglomerated powder is as large as several tens of
micrometers, for example. Therefore, if the coating fluid is
prepared by mere mixing of the components, most particles of the
photocatalyst remain in an agglomerated state.
[0114] In a preferred embodiment of the present invention,
particles of a titanium oxide-based photocatalyst are subjected to
dispersion treatment in a solvent to form a dispersion of the
photocatalyst in the solvent, from which the coating fluid is
prepared. In this manner, it is made possible to form a thinner and
more uniform photocatalytic coating which has significantly
improved film properties and photocatalytic activity.
[0115] The photocatalyst in the dispersion preferably has an
average particle diameter of 500 nm or smaller. If the average
particle diameter of the photocatalyst is larger than 500 nm,
powdering and peeling of a coating film tends to occur readily. In
addition, the storage stability of the dispersion becomes worse,
since precipitation of the particles tend to occur readily. The
average particle diameter of the photocatalyst is more preferably
300 nm or smaller and most preferably 200 nm or smaller. Ideally,
all the particles of the photocatalyst are s in the form of primary
particles.
[0116] The solvent in which the photocatalyst particles are
dispersed (namely, a dispersion medium) includes water such as
distilled water, deionized water, and ultrapure water; alcohols
such as methanol, ethanol, and 2-propanol; ketones such as methyl
ethyl ketone; aromatic hydrocarbons such as benzene, toluene, and
xylene; and the like. These solvents may optionally be used in
combination.
[0117] The dispersion treatment is preferably carried out by mixing
the photocatalyst with a solvent in such an amount that the solids
content of the resulting dispersion is in the range of from a few
mass % to 30 mass %. If the solids content is outside this range,
the dispersibility of the photocatalyst may decrease. A dispersing
agent or a deflocculant may be added, if necessary. Examples of a
dispersing agent are of the carbonyl or sulfone type, and examples
of a deflocculant are hydrochloric acid and sulfuric acid. A base
or an acid may be added for pH adjustment.
[0118] The dispersion treatment is preferably performed such that
the titanium oxide-based photocatalyst is sufficiently
deagglomerated while its bulk structure and surface structure are
maintained as much as possible. Although it is possible to use a
paint shaker, which is commonly used in the preparation of a
coating fluid, for the dispersion treatment, it is preferred that
the dispersion treatment be performed by use of a means which is
capable of more powerful dispersion such as a media mill, a rotary
knife for shearing dispersion, thin film rotation, and ultrasonic
dispersion. Two or more dispersing means may be utilized in
combination.
[0119] If the resulting dispersion contains coarse particles, they
are preferably removed by filtration or centrifugal separation.
This is because peeling or powdering of a coating film tends to
originate from coarse particles. A solvent may be added to the
dispersion obtained by the dispersion treatment to adjust its
solids content.
[0120] The dispersion can be directly used as a coating fluid and
applied to a substrate. A photocatalyst in the form of fine
particles having an average diameter of 500 nm or smaller can form
a film even in the absence of a binder, leading to the formation of
a coating film which consists essentially of particles of the
photocatalyst. However, since such film is low in strength and
adhesion, a binder solution may be applied to the film for
impregnating openings between the photocatalyst particles with the
binder solution.
[0121] A preferable coating fluid further comprises a binder in
addition to a photocatalyst and a solvent. The solvent may be the
same as described above, but it is selected such that the binder is
dissolved or emulsified therein. By mixing a dispersion of a
titanium oxide-based photocatalyst prepared as above with a binder,
it is possible to prepare a coating fluid which has improved
dispersibility of the photocatalyst and good storage stability and
which is capable of forming a coating film having a high
photocatalytic activity.
[0122] The amount of the binder is adjusted such that the content
of the titanium oxide-based photocatalyst in the resulting coating
film is in the range of 5-95 mass %. A coating film containing less
than 5 mass % of the photocatalyst exhibits little photocatalytic
activity. If the content of the photocatalyst is more than 95 mass
%, the amount of the binder is so small that the film forming
properties of the coating fluid are deteriorated leading to the
formation of a film which tends to peel off readily. The content of
the photocatalyst is preferably from 30 to 90 mass % and more
preferably at least 50 mass % in order to obtain an adequate
photocatalytic activity.
[0123] As the binder component, a sol of a metal oxide such as
silica, alumina, titania, magnesia, and zirconia (which becomes a
gel in a dry film); an organic silane compound; and an organic
resin such as a silicone resin, a fluororesin, a urethane resin,
and an acrylic resin can be used. If the oxidizing ability of the
photocatalyst is high enough to decompose some binder components,
it is desirable to use a binder which is difficult to decompose
such as a metal oxide sol or a silicone resin. When a good
formability or high strength is required for a photocatalytic
functional product to be manufactured, the required properties can
be guaranteed by adding an appropriate amount of an organic resin
such as a fluororesin, an acrylic resin, or a urethane resin to the
above-described binder component which is difficult to
decompose.
[0124] A preferred binder component is a silicon compound such as
silica (e.g., a silica sol), a hydrolysate/condensate of an organic
silane compound, or a silicone resin. Silica may be a silica sol
(silica colloid) formed by hydrolysis and condensation of a
silicate (e.g., ethyl silicate). As an organic silane compound, one
having film forming properties such as an alkoxysilane or a silane
coupling agent may be used.
[0125] The coating fluid may contain additional components other
than those described above. Examples of such additional components
include a titanium oxide-based photocatalyst having no visible
light activity (e.g., a conventional titanium oxide-based
photocatalyst), and a carrier when the photocatalyst is in the form
of particles supported on the carrier. A minor additive such as a
coloring agent (preferably an inorganic pigment) may be included in
the coating film.
[0126] The application of the coating fluid can be carried out by a
method selected from various well-known methods depending on the
properties of the coating fluid and the shape of a substrate to be
coated. After application, the coating film is dried (and
optionally cured) with heating, if necessary. The temperature for
drying (curing) can be determined according to the composition of
the coating fluid (type of the solvent and binder), the highest
temperature to which the substrate is resistant, and the like.
[0127] A photocatalyst-containing coating film formed on a
substrate preferably has a thickness of at least 0.5 .mu.m. A
coating film thinner than 0.5 .mu.m gives a very low photocatalytic
activity by irradiation with visible light due to a very low amount
of the photocatalyst contained therein. The film thickness may be
selected depending on the catalytic performance required for a
product and the manufacturing costs acceptable for the products. In
view of the stability of the catalytic performance and the
catalytic activity, the film thickness is preferably at least 3
.mu.m and more preferably at least 5 .mu.m. There is no upper limit
on the film thickness, but in view of the costs and saturation of
the effects, the film thickness is preferably at most 30 .mu.m and
more preferably at most 25 .mu.m.
[0128] In a second method of manufacturing a photocatalytic
functional product, titanium oxide and/or its precursor is
previously deposited on the surface of a substrate, which is then
brought into contact with a reactive medium containing a metal
halide of the formula MX.sub.n or MOX.sub.n (wherein M, X, and n
are as indicated above) while the substrate is subjected to heat
treatment.
[0129] In the second method, the deposition of titanium oxide
and/or its precursor on the surface of a substrate can be performed
by preparing a coating fluid containing titanium oxide and/or its
precursor followed by application of the fluid and drying in the
same manner as just mentioned. Since the titanium oxide and/or its
precursor must be brought into contact with a reactive medium in
the next step, it is preferable that a binder other than titania
not be used in the coating fluid or, if it is present, that its
amount be as small as possible.
[0130] A preferable method for deposition comprises preparing a
solution-type coating fluid containing a partially hydrolyzed
titanium compound (e.g., a partial hydrolysate of a titanium
alkoxide) or a titania sol dissolved in a solvent and applying it
to the surface of a substrate so as to form a coating film
substantially consisting solely of a titanium oxide precursor on
the substrate surface after drying.
[0131] Another preferable method is to use a coating fluid prepared
from particles of titanium oxide (or its insoluble precursor) and a
titania sol as a binder, resulting in the formation of a coating
film consisting substantially of titanium oxide and its precursor
on the surface of a substrate.
[0132] The method of depositing titanium oxide and/or its precursor
on the surface of a substrate is not limited to the above-described
coating methods, and well-known vapor phase film forming methods
such as CVD and PVD may be utilized, although they entail
complicated operations and increased costs.
[0133] After titanium oxide and/or its precursor is deposited on
the surface of a substrate to form a film, the surface of the
substrate (more accurately, the film of titanium oxide and/or its
precursor formed thereon) is brought into contact with a reactive
medium containing a metal halide. The contact with the reactive
medium may be performed in the same manner as described earlier
with respect to the preparation of a photocatalyst. As described
there, the reactive medium may be either liquid or gaseous, and
preferably it is gaseous, namely, a reactive gas. The contact with
the reactive gas is preferably performed with heating, as described
before, as long as the substrate can resist such heating.
[0134] The substrate having titanium oxide and/or its precursor
deposited thereon may be subjected to heat treatment as
pretreatment prior to contact with the reactive medium. In
addition, the substrate after it has contacted the reactive medium
may be subjected to post treatment by contact with water and/or by
heat treatment in order to stabilize the photocatalyst formed on
the surface of the substrate. Such post treatment is preferable
from a practical standpoint.
[0135] When it is desired to include a metal complex in the
photocatalyst deposited on the substrate, a desired ligand may be
introduced by a chemical reaction as described for the preparation
of a photocatalyst. When the metal complex is a heteropoly acid
and/or an isopoly acid, after a substrate having a film of titanium
oxide and/or its precursor formed thereon is treated by contact
with a reactive medium and optionally to post treatment by contact
with water and/or heat treatment, the substrate is then treated by
contact with a heteropoly acid and/or an isopoly acid and then by
heat treatment, if necessary.
[0136] As in the first method, a binder-free dispersion of titanium
oxide fine particles can be applied to a substrate to deposit
titanium oxide thereon, the deposited titanium oxide is then
brought into contact with a reactive medium, and thereafter it may
be impregnated with a binder, if necessary.
[0137] A titanium oxide-based photocatalyst and a photocatalytic
functional product according to the present invention which have
been described above develop their photocatalytic activities by
irradiation with not only ultraviolet light but also with only
visible light having a wavelength of 400 nm or longer and thus
exhibit excellent effects on decomposition, removal or safening of
various harmful or deposited substances.
[0138] In actual use, the photocatalyst may be used in an
environment in which it can contact a substance or substances to be
decomposed and it can be irradiated with visible light. The light
source may be any which includes or emits visible light having a
wavelength of 400 nm or longer. For example, sunbeams, fluorescent
tubes, halogen lamps, black lights, xenon lamps, and mercury vapor
lamps can be used.
[0139] Harmful substances include gases present in air such as
NO.sub.x, SO.sub.x, flons (chlorofluorocarbons), ammonia, and
hydrogen sulfide; organic compounds such as aldehydes, amines,
mercaptans, alcohols, BTX, and phenols; organic halogen compounds
such as trihalomethanes, trichloroethylene, and flons; various
agricultural chemicals such as herbicides, fungicides, and
insecticides; various biological oxygen-demanding substances such
as proteins and amino acids; surfactants; inorganic compounds such
as cyanogen compounds and sulfur compounds; various heavy metal
ions; and underwater microorganisms including bacteria,
actinomyces, fungi, and algae. Deposited substances indicate
substances directly deposited on the surface of a photocatalyst or
a photocatalytic functional product, and they include
microorganisms such as E. coli, staphylococci, Pseudomonas
aeruginosa, and molds; as well as fats and oils, tars from
tobaccos, finger prints, streaky marks of rain, and dirt.
[0140] The following examples are given to illustrate the present
invention. In the examples, the content of a metal halide in a
photocatalyst is that in mass % as metal. All parts in the examples
are by mass unless otherwise indicated.
Example 1
Preparation of a Titanium Oxide-Based Photocatalyst
[0141] 0.5 grams of a commercially available anatase titanium oxide
powder having an ultraviolet photocatalytic activity (ST-01
manufactured by Ishihara Sangyo, partially amorphous) were put into
a quartz tube (with a diameter of 33 mm), and the tube was purged
by argon and then heated to 573 K. Subsequently, a reactive gas
containing 1.4 vol % of titanium tetrachloride (TiCl.sub.4) in
hydrogen gas was passed through the quartz tube to cause the
titanium oxide to contact the gas, and this contact treatment was
continued for 20 minutes at 573 K. The flow rate of the reactive
gas was 2.0 L/min. Then, after the atmosphere in the tube was
replaced by argon, the tube was slowly cooled to room temperature,
and the resulting titanium oxide-based photocatalyst according to
the present invention was recovered from the tube.
[0142] The photocatalyst contained a titanium halide which was
bonded to the surface of the titanium oxide by a
dehydrochlorination reaction of titanium tetrachloride with the
hydroxyl groups present on the surface of the titanium oxide. The
amount of Ti metal introduced into the photocatalyst by the contact
treatment was calculated based on the height gain measured after
the treatment and the halogen content of the photocatalyst.
[0143] The ESR spectra and photocatalytic activity (in an
acetaldehyde decomposition test) of the photocatalyst were measured
by the following methods. The absorption spectrum of the material
was also measured, and it showed that the material was absorptive
in the visible wavelength region of 400 nm or longer.
[0144] Measurement of ESR Spectrum
[0145] A sample sealed in a Teflon.TM. tube for ESR measurement in
air was irradiated with visible light from a 200 W halogen lamp for
at least 10 minutes at room temperature, and then the ESR spectrum
of the sample was measured with continued irradiation. Thereafter,
the sample in the tube was placed in the dark and the measurement
of an ESR spectrum was performed without irradiation with visible
light.
[0146] The measurement of an ESR spectrum was performed under the
following conditions: [0147] Measuring equipment: X-band (9 GHz
band) electron spin resonance spectrometer manufactured by Japan
Electron Optics Laboratory (JES-RE2X) [0148] Detection limit of ESR
signals: 1E 10/mT [=1E 10 spins (unpaired electrons) contributing
to the same ESR signal in the sample]
[0149] Magnetic scanning range: 330 V 25 mT
[0150] Width of magnetic modulation: 0.1 mT
[0151] Magnetic modulation frequency: 100 KHz
[0152] Scanning time: 2 minutes
[0153] Number of scans: 1
[0154] Amplification factor: 500
[0155] Delay time: 0.1 seconds
[0156] Microwave output: 0.1 mW.
[0157] The g value was calculated as follows:
[0158] Based on the third signal (g=2.0303) and the fourth signal
(g=1.981) of a Mn.sup.2+/MgO marker (attached to the equipment),
the g value of an ESR signal of the sample was calculated by the
following equation:
g=2.0303!(2.0303!1.981)HL3/(L3+L4)
[0159] wherein
[0160] L3: difference in magnetic field between the third peak of
the marker and the ESR signal of the sample; and
[0161] L4: difference in magnetic field between the fourth peak of
the marker and the ESR signal of the sample.
[0162] With respect to the intensity of an ESR peak, since the
detected ESR signals are differential signals, the difference
between the maximum and minimum values of a differential signal was
taken as the intensity of the ESR signal.
[0163] ESR spectra measured at 5 K are shown in FIG. 3. As can be
seen from this figure, an ESR spectrum measured at 5 K with visible
light irradiation had peaks in a g-value range of 1.950-2.030, and
these peaks were not observed in an ESR spectrum measured in the
dark (the above-described first feature). An additional peak in a
g-value range of 1.986-1.994, which was different from the triplet
signal attributable to oxygen defects and having g value ranges of
2.002-2.010, 2.020-2.028, and 1.982-1.998, appeared in the ESR
spectrum (the above-described third feature).
[0164] Measurement of Photocatalytic Activity (Acetaldehyde
Decomposition Test)
[0165] A sample (0.3 grams) was placed on a dish measuring 40 mm
square, and the dish was put into a quartz reaction cell. The cell
was connected to a closed circulating line system (having a total
internal volume of about 3.0 liter), and a gas prepared by diluting
acetaldehyde with a nitrogen gas containing 20 vol % of oxygen (to
an acetaldehyde concentration of about 240 ppm) was introduced into
the system. While the gas was circulated in the system, the
reaction cell was irradiated with light from a 250 W high-pressure
mercury-vapor lamp through an ultraviolet cutoff filter (Toshiba
L42). Due to the performance of the filter, the irradiated light
included a slight amount of near ultraviolet light having a
wavelength in the region of 390 nm to 400 nm, but the most part (at
least 99.9% in energy intensity) was visible light having a
wavelength of 400 nm or longer. The reaction was monitored by
periodically measuring the concentration of carbon dioxide, which
was formed by decomposition of acetaldehyde, by an automated gas
chromatograph connected to the circulating line. The photocatalytic
activity was evaluated in terms of the rate of carbon dioxide
formation. The result is shown in Table 1.
Comparative Example 1
[0166] The same commercially available titanium oxide powder as was
used in Example 1 as a raw material was used as a photocatalyst as
is with no treatment and was subjected to an acetaldehyde
decomposition test with visible light irradiation in the same
manner as described in Example 1. The result is shown in Table 1.
The titanium oxide, which was active with ultraviolet light, had an
absorption spectrum having no absorption peaks in the visible
wavelength region and thus did not absorb visible light.
Example 2
[0167] (a) Titanium oxide was prepared according to the method
described in Handbook of Catalytic Experiments (published by
Kodansha Scientific, Tokyo). Namely, 200 cc of an ice-cooled
aqueous solution of TiCl.sub.4 (Ti content of 16.5%) was
neutralized by adding an aqueous ammonia solution (28%) with
stirring until an alkaline pH was obtained. The resulting
precipitate was collected by filtration, then thoroughly washed
with water, and then vacuum dried at 353 K to prepare a titanium
oxide powder.
[0168] The crystal form of the crystalline portion of the titanium
oxide powder thus obtained was anatase. The titanium oxide powder
was treated by contact with a reactive gas containing 1.4 vol % of
TiCl.sub.4 in hydrogen in the same manner as described in Example 1
to prepare a titanium oxide-based photocatalyst according to the
present invention.
[0169] (b) The titanium oxide-based photocatalyst obtained in (a)
above was further subjected to post treatment by heat treatment at
573 K in an argon atmosphere for 20 minutes. This heat treatment in
an argon atmosphere resulted in removal of a considerable part of
the metal halide which had been introduced into the titanium oxide
during the previous heat treatment with the reactive gas.
[0170] The titanium oxide-based photocatalysts obtained in (a) and
(b) above were subjected to determination of the amount of Ti of
the metal halide (metal compound) present therein and to an
acetaldehyde decomposition test by the above-described methods. The
results are also included in Table 1.
[0171] Both the products of (a) and (b) showed the above-described
first and third features in their ESR spectra. In addition, the
product of (b) showed the above-described second feature.
TABLE-US-00001 TABLE 1 Metal content.sup.1) Rate of CO.sub.2
formation Number (mass %) (.mu.mol/h) Example 1 1.2 11.3
Comparative Example 1 0 <0.1 Example 2(a) 1.3 39.0 Example 2(b)
0.156 0.7 .sup.1)Amount of Ti present as a titanium chloride
[0172] As can be seen from a comparison between Examples 1-2 and
Comparative Example 1, it was confirmed that a titanium oxide-based
photocatalyst which had been treated so as to include a metal
halide (titanium tetrachloride) according to the present invention
exhibited satisfactory photocatalytic activity by irradiation with
visible light. In contrast, with a commercially available titanium
oxide which had not been subjected to such treatment (Comparative
Example 1), no substantial photocatalytic activity was observed by
irradiation with visible light.
[0173] By comparison between Examples 2(a) and 2(b), it was found
that the photocatalytic activity increased as the content of metal
halides increased. The content of metal halides was not
significantly different between the photocatalysts of Example 1 and
Example 2(a), but the photocatalytic activity in Example 2(a) was
significantly higher. This is thought to be because the conditions
of a titanium oxide such as crystallinity and particle diameter
have great influences on the structure and location in which the
metal halides are present to render the titanium oxide to be
capable of developing its photocatalytic activity with visible
light.
[0174] The thermal desorption spectrum (TDS) of the titanium
oxide-based photocatalysts obtained in Examples 2(a) ane 2(b) were
measured in the following manner. The results are shown in FIGS. 1
and 2, respectively.
[0175] Measurement of Thermal Desorption Spectrum
[0176] The measurement of a thermal desorption spectrum was
conducted using a high resolution thermal desorption spectrometer
(EMD-WA1000S manufactured by Electronic Science Co., Tokyo). A
sample was placed in a platinum cell and thoroughly evacuated to
10.sup.-6 Torr. Subsequently, while the temperature of the sample
was raised at a rate of 1 K/sec, any gases desorbed from the sample
were analyzed by a mass spectrograph.
[0177] As can be seen from FIGS. 1 and 2, in either the
photocatalyst of Example 2(a) (FIG. 1) in which 1.3 mass % of metal
halides were introduced in the titanium oxide by contact with a
reactive gas and the photocatalyst of Example 2(b) (FIG. 2) in
which the photocatalyst of Example 2(a) was further heat-treated in
an argon atmosphere thereby reducing the content of metal halides
to 0.156 mass %, molecular ions of halogen-containing substances or
their fragment ions such as TiCl.sub.2 (M/Z=118), TiC.sub.3
(M/Z=154), TiCl.sub.4 (M/Z=189), and HCl (M/Z=36) were detected.
Among these, the peaks of the first three ions of titanium
chlorides appeared in a temperature range above 623 K. Thus, either
of these photocatalysts had a peak of a molecular ion or fragment
ion of a halogen-containing substance at a temperature above 623 K
in its thermal desorption spectrum.
Example 3
[0178] Titanium oxide-based photocatalysts according to the present
invention were prepared in the same manner as described in Example
1 except that the reactive gas containing 1.4 vol % of TiCl.sub.4
in hydrogen gas was replaced by a reactive gas containing about 1
vol % of VOCl.sub.3, SnCl.sub.4, SbCl.sub.5, SiCl.sub.4, WCl.sub.6,
BiCl.sub.6, FeCl.sub.3, ZnCl.sub.4 or TiCl.sub.4 in argon gas. The
results of an acetaldehyde decomposition test which was performed
on these photocatalysts by the above-described method with visible
light irradiation are shown in Table 2.
TABLE-US-00002 TABLE 2 Rate of CO.sub.2 formation Run No. Metal
halide (.mu.mol/h) 1 VOCl.sub.3 0.4 2 SnCl.sub.4 1.1 3 SbCl.sub.5
0.3 4 SiCl.sub.4 0.7 5 WCl.sub.6 2.4 6 BiCl.sub.3 7.7 7 FeCl.sub.3
0.1 8 ZrCl.sub.4 0.1 9 TiCl.sub.4 11.3
[0179] It is apparent that all the metal halides were able to
provide a titanium oxide-based photocatalyst having a visible light
photocatalytic activity. Among these halides, TiCl.sub.4,
BiCl.sub.6, and WCl.sub.6 were able to impart a higher
photocatalytic activity.
Example 4
[0180] Titanium oxide-based photocatalysts according to the present
invention were prepared in the same manner as described in Example
1 except that the conditions (temperature and duration) for contact
with the reactive gas containing 1.4 vol % of TiCl.sub.4 in
hydrogen gas were varied as shown in Table 3. An acetaldehyde
decomposition test was performed on these photocatalysts by the
above-described method with visible light irradiation. The test
results are shown in Table 3 along with the conditions for the heat
treatment.
TABLE-US-00003 TABLE 3 Conditions of contact Rate of CO.sub.2
formation Run No. (temp. H duration) (.mu.mol/h) 1 323 K H 20
minutes 0.1 2 373 K H 20 minutes 0.4 3 373 K H 120 minutes 0.5 4
423 K H 20 minutes 0.6 5 473 K H 20 minutes 3.5 6 523 K H 20
minutes 8.8 7 573 K H 20 minutes 11.3 8 623 K H 20 minutes 4.8 9
673 K H 20 minutes 2.5 10 773 K H 5 minutes 4.2 11 773 K H 20
minutes 0.1 12 873 K H 20 minutes 0.1
[0181] A titanium oxide-based photocatalyst having a visible light
photocatalytic activity could be obtained by performing the
treatment with a reactive gas at a temperature of 323 K or higher.
The temperature for the treatment at which a high photocatalytic
activity was obtained was in the range of 373-773 K, and in
particular, better results are achieved in the range of 473-773 K.
A photocatalyst having a high activity could be obtained by a
longer duration of treatment when the temperature was lower or by a
shorter duration of a treatment when the temperature was
higher.
Example 5
[0182] A titanium oxide powder used as a raw material was prepared
by neutralizing an aqueous solution of TiCl.sub.4 with an aqueous
ammonia solution in the same manner as described in Example 2
except that the concentration of the aqueous TiCl.sub.4 solution
was half that in Example 2, i.e., 8.25% as Ti.
[0183] 1.0 gram of the titanium oxide powder was treated by contact
with a reactive gas containing 1.4 vol % of TiCl.sub.4 in hydrogen
in the same manner as described in Example 1. Subsequently, the
resulting treated product was subjected to post treatment by
contact with water three times wherein each time 1.0 gram of the
product was stirred in 10 grams of water and then collected by
filtration. Thereafter, the treated product was dried for at least
24 hours at 323 K to prepare a titanium oxide-based photocatalyst
according to the present invention.
[0184] The results of X-ray diffractometric analysis showed that
the titanium oxide-based photocatalyst which had an orange color
had crystal forms which were predominantly anatase, but the
coexistence of brookite in an extremely small amount was also
observed.
[0185] Measurements of ESR spectra in air (at 5 K and 293 K) and
the visible light photocatalytic activity (by an acetaldehyde
decomposition test) were performed on the photocatalyst according
to the present invention in the same manner as described in Example
1. The results are shown in FIG. 4 and Table 4. The photocatalyst
had visible light absorptivity.
[0186] In this example, an ESR spectrum in a deoxidized atmosphere
was also measured in the following manner. A sample was put into a
quartz tube for ESR measurement and vacuum sealed (about 10.sup.-3
Torr) using a rotary vacuum pump. At a predetermined temperature in
the range of from 5 K to 293 K (room temperature) set by a
cryostat, the sample was irradiated with visible light for at least
10 minutes before its ESR spectrum was measured while continuing
the irradiation. When the half life of a peak intensity was
subsequently determined, after the irradiation with visible light
was stopped, measurements of ESR spectra were performed at
predetermined intervals to follow the decrease of the peak
intensity. The results are shown in FIG. 6.
Comparative Example 2
[0187] A known titanium oxide-based photocatalyst in powder form
having visible light activity was used to perform measurements of
ESR spectra in air and in a deoxidized atmosphere and an
acetaldehyde decomposition test with visible light irradiation in
the same manner as described in Example 5. The results are shown in
FIGS. 5 and 6 and Table 4. The photocatalyst had a color and had
visible light absorptivity.
TABLE-US-00004 TABLE 4 Comparative Items measured Example 5 Example
2 ESR (293 K, 5K): Irradiated with Yes Yes presence of a visible
light peak in a g-value range of 1.950-2.030 In the dark No Yes
Half life in a vacuum of the highest 4.8 min. 1.7 min. peak in the
above g value range ESR (5 K): presence of a peak in a Yes No
g-value range of 1.986-1.994 Rate of CO.sub.2 formation
(.mu.mole/h) 3.5 2.5
[0188] As shown in Table 4, the titanium oxide-based photocatalyst
according to the present invention exhibited a higher activity than
a known photocatalyst which was active with visible light even
after stabilized by post treatment which was performed by contact
with water. Thus, it was found that the photocatalyst still
maintained its visible light photocatalytic activity at a level
sufficient for practical use after the photocatalytic activity was
decreased by the post treatment.
[0189] As shown in FIG. 4, in the ESR spectra of the photocatalyst
according to the present invention, a triplet signal attributed to
unpaired electrons captured by oxygen defects was observed in a
g-value range of 1.950-2.030 (in which the peak appearing in the
lowest magnetic field overlapped a peak of the manganese marker).
This indicates that the titanium oxide was of the visible light
absorbing type having oxygen defects.
[0190] In addition, at each temperature of 293 K (room temperature)
and 5 K, the peaks in a g-value range of 1.950-2.030 which were
observed by measurement with irradiation by visible light were not
observed by measurement in the dark (the above-described first
feature). In contrast, in Comparative Example 2, these peaks
observed with visible light irradiation were also observed in the
dark, and extremely large peaks were observed by measurement at 5 K
in the dark (FIG. 5).
[0191] From the result of measurement at 5 K shown in FIG. 4, with
the photocatalyst according to the present invention, a new peak,
which was not observed by measurement at room temperature, appeared
in a g-value range of 1.986-1.994 (around 1.99) (the
above-described third feature). In contrast, with a photocatalyst
of Comparative Example 2 shown in FIG. 5, such peak was not
observed in the results measured at 5 K.
[0192] From the result of variation in the intensity of the highest
peak (g=2.005) measured in a deoxidized atmosphere, i.e., variation
in the concentration of the catalytically active species, after
extinction shown in FIG. 6, the decrease occurred slowly with the
photocatalyst according to the present invention, and the half life
of the peak intensity was 4.8 minutes (the above-described second
feature). In contrast, with the comparative example, the decrease
occurred rapidly, and the half life was 1.7 minutes.
[0193] From these results of ESR spectra, it is suggested that
there was a big difference in the structure as an oxide between the
photocatalyst according to the present invention and that in the
comparative example. A titanium oxide-based photocatalyst having
improved visible light photocatalytic activity can be obtained when
its ESR spectra have at least one of the features defined in the
present invention.
Example 6
[0194] A titanium oxide-based photocatalyst according to the
present invention was prepared following the method described in
Example 5 except that the same commercially available titanium
oxide powder as used in Example 1 (ST-01) was used as a raw
material in place of the prepared powder of titanium oxide used in
Example 5.
[0195] The resulting photocatalyst was subjected to an acetaldehyde
decomposition test in the same manner as described in Example 1. As
a result, the rate of CO.sub.2 formation was 1.2 .mu.mol/h.
[0196] As indicated in Comparative Example 1, the raw material
powder of titanium oxide had no photocatalytic activity since its
rate of CO.sub.2 formation was less than 0.1 .mu.mol/h. From such a
commercially available titanium oxide, a titanium oxide-based
photocatalyst of the present invention which has visible light
photocatalytic activity and is easy to handle can be prepared by a
method disclosed herein.
Example 7
[0197] Titanium oxide-based photocatalysts according to the present
invention were prepared in the same manner as described in Example
5 except that the conditions for preparation were varied as shown
in Table 5.
[0198] Example 5 corresponds to Run No. 7 of Table 5. The heat
treatment for pretreatment and any post treatment were carried out
in air unless otherwise indicated. In Runs Nos. 14 and 15, post
treatment was conducted by heat treatment under the conditions
shown in the table, in place of contact with water. In Run No. 16,
contact with a reactive gas was performed using a nitrogen gas
containing titanium tetrachloride (1.4 vol %). In Run No. 29,
pretreatment was conducted in an ammonia (2 vol %)-argon
atmosphere. In Run No. 30, pretreatment was performed by wet
hydrothermal treatment in a Teflon.TM.-lined autoclave using water
containing ammonium chloride (1 M) and hydrochloric acid (1 M)
(under the conditions shown in the table).
[0199] Using the resulting photocatalysts, an acetaldehyde
decomposition test was performed by the above-described method with
visible light irradiation. The test results are shown in Table 5
along with the conditions for preparation.
Comparative Example 3
[0200] The titanium oxide powder prepared in Example 5 as a raw
material was calcined for 1 hour at 573 K in air to obtain a
photocatalyst. This sample was also subjected to an acetaldehyde
decomposition test by the above-described method with visible light
irradiation. The test result is shown in Table 5 (as Run No. 31)
along with the conditions for preparation.
TABLE-US-00005 TABLE 5 Rate of Pretreatment Contact with Post
CO.sub.2 Run (Temp. reactive gas treatment formation No. Duration)
(Temp. Duration) (K Minutes) (.mu.mol/h) 1 (None) 323 K 20 min.
(Contact with 1.2 water) 2 (None) 373 K 20 min. (Contact with 1.7
water) 3 (None) 423 K 20 min. (Contact with 1.8 water) 4 (None) 423
K 120 min. (Contact with 2.8 water) 5 (None) 423 K 20 min. (Contact
with 1.8 water) 6 (None) 523 K 20 min. (Contact with 2.2 water) 7
(None) 573 K 20 min. (Contact with 3.5 water) 8 (None) 598 K 20
min. (Contact with 3.9 water) 9 (None) 623 K 20 min. (Contact with
3.1 water) 10 (None) 673 K 20 min. (Contact with 2.4 water) 11
(None) 673 K 6 min. (Contact with 3.0 water) 12 (None) 723 K 20
min. (Contact with 0.8 water) 13 (None) 773 K 20 min. (Contact with
0.3 water) 14 (None) 373 K 20 min. (Contact with 3 water) 15 (None)
573 K 20 min. 573 60 3.2 16 (None) 573 K 20 min. 573 60 2.6
(N.sub.2) 17 373 K 60 min. 573 K 20 min. (Contact with 3.5 water)
18 473 K 60 min. 573 K 20 min. (Contact with 3.8 water) 19 573 K 60
min. 573 K 20 min. (Contact with 3.3 water) 20 623 K 60 min. 573 K
20 min. (Contact with 2.7 water) 21 673 K 60 min. 573 K 20 min.
(Contact with 2.3 water) 22 773 K 60 min. 573 K 20 min. (Contact
with 2.2 water) 23 473 K 60 min. 573 K 10 min. (Contact with 3.5
water) 24 473 K 60 min. 573 K 20 min. (Contact with 3.8 water) 25
473 K 60 min. 573 K 30 min. (Contact with 3.5 water) 26 473 K 60
min. 573 K 40 min. (Contact with 3.9 water) 27 473 K 60 min. 573 K
60 min. (Contact with 1.8 water) 28 473 K 60 min. 573 K 120 min.
(Contact with 1.2 water) 29 473 K 60 min. 573 K 20 min. (Contact
with 4.1 (NH.sub.3--Ar) water) 30 473 K 3 60 min, 573 K 20 min.
(Contact with 4.3 (NH.sub.4Cl--HCl) water) 31.sup.1 (None) 573 K 60
min. (None) 1 (calcination in air) 32.sup.2 (None) 573 K 20 min.
(None) 39.0 .sup.1Run No. 31 is a comparative example in which the
same raw material was merely calcined in air. .sup.2Run No. 32 is
the same as Example 2(a).
[0201] Titanium oxide-based photocatalysts according to the present
invention which exhibited visible light photocatalytic activity
could be obtained by subjecting titanium oxide to contact with a
reactive gas containing a metal halide (titanium tetrachloride in
this example) followed by post treatment. The temperature range for
that contact in which a photocatalyst having an increased
photocatalytic activity was obtained was 373-676 K. In particular,
better results were achieved in the range of 523-673 K.
[0202] Even when a satisfactory activity was not obtained because
of a low temperature at which the contact of a raw material with a
reactive gas was conducted, the activity could be increased by
subsequently performing heat treatment as post treatment. With
respect to the duration of contact with a reactive gas, it was
found that a longer duration was desirable for contact at a lower
temperature while a shorter duration was desirable for contact at a
higher temperature. In the case of contact at 573 K, a highly
active photocatalyst was obtained when the duration was
approximately between 10 and 40 minutes. A photocatalyst having
visible light activity could also be prepared by replacing
hydrogen, which was used as the base gas for a reactive gas, by
nitrogen.
[0203] With respect to the heat treatment prior to contact with the
reactive gas (as pretreatment), it was found that a photocatalyst
having an increased activity was obtained if the heat treatment was
performed in the temperature range of 373-573 K. In addition, it
was confirmed that pretreatment performed in an ammonia-containing
atmosphere which is a reducing atmosphere or by a wet process in a
solution was also effective.
[0204] In contrast, the photocatalyst of Run No. 31 (Comparative
Example 3) which was prepared by mere calcination in air without
treatment by contact with a reactive gas containing titanium
tetrachloride had a significantly decreased visible light
photocatalytic activity compared to the photocatalyst of Run No. 32
(Example 2(a)).
Example 8
[0205] In exactly the same manner as in Example 5, a prepared raw
material of titanium oxide was treated by contact with a
TiCl.sub.4-containing gas and then post treatment by contact with
water to give a titanium oxide-based photocatalyst having visible
light photocatalytic activity after drying.
[0206] The resulting titanium oxide powder was then impregnated
with an aqueous solution prepared by dissolving
H.sub.3PW.sub.12O.sub.40 or H.sub.3SiW.sub.12O.sub.40 in distilled
water (to about 50% concentration). After 4 hours, the solution was
removed by filtration, and the recovered titanium oxide powder was
dried overnight at 50.degree. C. Subsequently, the powder was
heat-treated for 2 hours at 773 K for fixation of the impregnated
compound to prepare two titanium oxide-based photocatalysts
containing a heteropoly acid on the surface.
[0207] The content of the metal complex in these photocatalysts
expressed as metal content relative to the weight of titanium oxide
was 77 mass % for the catalyst containing H.sub.3PW.sub.12O.sub.40
and 63 mass % for the catalyst containing
H.sub.3SiW.sub.12O.sub.40.
[0208] These photocatalysts were subjected to an acetaldehyde
decomposition test by the above-described method with visible light
irradiation. The photocatalytic activity expressed in terms of the
rate of CO.sub.2 formation was 7.7 .mu.mol/h for the
H.sub.3PW.sub.12O.sub.40-containing catalyst and 7.0 mmol/h for the
H.sub.3SiW.sub.12O.sub.40-containing catalyst. Compared to the
catalyst as subjected to the post treatment, it was found that the
photocatalytic activity was significantly increased by introducing
a metal complex into the photocatalyst. Thus, the introduction of a
metal complex makes it possible to significantly increase the
photocatalytic activity while maintaining the stability achieved by
the post treatment.
Example 9
[0209] This example illustrates the manufacture of photocatalytic
functional products according to the present invention.
[0210] (a) A titanium oxide film having a thickness of about 100 nm
was formed on a glass cloth by CVD using TiCl.sub.4 as a
film-forming material.
[0211] The glass cloth having a titanium oxide film on the surface
was treated by contact with a reactive gas containing 1.4 vol % of
TiCl.sub.4 in hydrogen gas to thus cause the titanium oxide film to
contain a titanium chloride therein. The treatment was carried out
in the same manner as described in Example 1 except that the
above-described glass cloth was used in place of the titanium oxide
powder. In this manner, a photocatalytic functional product having
a film of a titanium oxide-based photocatalyst according to the
present invention on the surface of a glass cloth was obtained.
[0212] A sample of the photocatalytic functional product was
subjected to an acetaldehyde decomposition test with visible light
irradiation in the same manner as described in Example 1. The size
of the sample was 16 cm.sup.2. The rate of CO.sub.2 formation
achieved in the test was 0.35 .mu.mol/h.
[0213] (b) A photocatalytic coating fluid was prepared by adding 3
parts of the titanium oxide-based photocatalyst powder according to
the present invention prepared in Example 1, 1 part of the titanium
oxide powder used in Example 1 as a raw material (ST-01) which was
a conventional photocatalyst, and 2 parts (as SiO.sub.2) of a
silica colloid (Snowtex 0 manufactured by Nissan Chemical
Industries, 20 mass % SiO.sub.2) as a binder to 36 parts of a mixed
solvent of alcohol and water and thoroughly mixing them using a
paint shaker (manufactured by Red Devil).
[0214] The coating fluid was applied by a bar coater onto the
surface of a precoated steel sheet, the surface having a previously
formed primer coating (thickness: about 1 .mu.m) based on a
silicone resin. The applied coating was dried by heating for 3
minutes at 423 K to prepare a photocatalytic functional product
having a coating film containing a titanium oxide-based
photocatalyst according to the present invention on the surface of
the precoated steel sheet as a substrate. The thickness of the
coating film (photocatalytic layer) was about 1 .mu.m. A sample of
the photocatalytic functional product was subjected to an
acetaldehyde decomposition test with visible light irradiation as
in (a) above. The rate of CO.sub.2 formation was 0.25
.mu.mol/h.
Comparative Example 4
[0215] (a) The glass cloth prepared in Example 9(a) which had a
titanium oxide film on the surface was used as a test material as
is (without treatment by contact with a reactive gas) and subjected
to an acetaldehyde decomposition test with visible light
irradiation in the same manner as in Example 9. The rate of
CO.sub.2 formation was 0.0 .mu.mol/h.
[0216] (b) A coating fluid was prepared in the same manner as
described in Example 9(b) except that the titanium oxide-based
photocatalyst powder according to the present invention was
replaced by 3 parts of the conventional photocatalytic titanium
oxide powder (ST-01) (the total amount of this powder which was
used being 4 parts). The coating fluid was used to form a coating
film on the surface of a precoated steel sheet in the same manner
as in Example 9(b). The resulting comparative photocatalytic
functional product was subjected to an acetaldehyde decomposition
test with visible light irradiation. The rate of CO.sub.2 formation
was 0.0 .mu.mol/h.
Example 10
[0217] This example further illustrates the manufacture of
photocatalytic functional products according to the present
invention.
[0218] (a) A titanium oxide colloid sol was prepared by adding 3
parts of hydrazine to 1 part of an aqueous TiCl.sub.4 (16.5 mass %)
solution. After the sol was subjected to deionization using a
dialysis tube, an appropriate amount of ethanol was added, and the
sol was applied onto a quartz substrate and dried at 373 K to form
a titanium oxide coating film on the quartz substrate.
[0219] The titanium oxide film was treated by contact with a
reactive gas which contained 1.4 vol % of TiCl.sub.4 in hydrogen.
The treatment was carried out in the same manner as described in
Example 1 except that the above-described quartz substrate having a
titanium oxide film formed thereon was used in place of the
titanium oxide powder. Subsequently, the quartz substrate was
heat-treated for 1 hour at 673 K in air for post treatment to
obtain a photocatalytic functional product according to the present
invention having a film of a titanium oxide-based photocatalyst.
The film had a thickness of about 0.6 .mu.m. The photocatalytic
functional product was subjected to an acetaldehyde decomposition
test with visible light irradiation in the same manner as in
Example 9. The rate of CO.sub.2 formation was 1.1 .mu.mol/h.
[0220] (b) A photocatalytic coating fluid was prepared by adding 3
parts of the titanium oxide-based photocatalyst powder according to
the present invention prepared in Example 5, 1.5 parts of a
conventional photocatalytic titanium oxide powder (ST-01), and 1.5
parts (as SiO.sub.2) of a silica colloid (which was the same as
used in Example 9(b)) as a binder to 20 parts of a mixed solvent of
alcohol and water and thoroughly mixing them using a paint
shaker.
[0221] The coating fluid was applied and dried in the same manner
as described in Example 9(b) to prepare a photocatalytic functional
product having a coating film containing about 50 mass % of a
titanium oxide-based photocatalyst according to the present
invention on the surface of a precoated steel sheet as a substrate.
The thickness of the coating film was about 0.8 .mu.m. When the
photocatalytic functional product was subjected to an acetaldehyde
decomposition test with visible light irradiation by the
above-described method, the rate of CO.sub.2 formation was 0.45
.mu.mol/h.
Comparative Example 5
[0222] (a) The quartz substrate having a titanium oxide film
prepared in Example 10(a) was directly heat treated for 1 hour at
673 K in air, without previous treatment by contact with a reactive
gas, to prepare a test material. The test material was subjected to
an acetaldehyde decomposition test with visible light irradiation
in the same manner as in Example 9. The rate of CO.sub.2 formation
was 0.19 .mu.mol/h.
[0223] (b) A coating fluid was prepared in the same manner as
described in Example 10(b) except that the titanium oxide-based
photocatalyst powder according to the present invention was
replaced by 3 parts of the conventional photocatalytic titanium
oxide powder (ST-01) (the total amount of this powder which was
used being 4.5 parts). The coating fluid was used to form a coating
film containing a photocatalyst consisting of titanium oxide on the
surface of a precoated steel sheet in the same manner as in Example
10(b). The resulting comparative photocatalytic functional product
was subjected to an acetaldehyde decomposition test with visible
light irradiation in the same manner as in Example 9. The rate of
CO.sub.2 formation was 0.0 mmol/h.
Example 11
[0224] A photocatalyst dispersion with solids content of 10 mass %
was prepared by mixing 30 parts of a titanium oxide-based
photocatalyst according to the present invention prepared in
Example 5 with 270 parts of distilled water and subjecting the
mixture to dispersion with zirconia beads (having a diameter of 0.4
mm) for 60 minutes using a media mill.
[0225] The average particle diameter of the resulting dispersion,
which was measured by a particle size distribution analyzer LA-700
manufacture by Horiba, was about 310 nm. The average particle
diameter of the photocatalyst before the dispersion treatment was
about 34 .mu.m.
[0226] A coating fluid was prepared by adding to 100 parts of the
photocatalyst dispersion 50 parts of a silica colloid (Snowtex O
manufactured by Nissan Chemical Industries, 20 mass % SiO.sub.2), 4
parts of methyltriethoxysilane, and 150 parts of methanol and then
shaking the mixture well for 60 minutes using a paint shaker.
[0227] The coating fluid was applied and dried in the same manner
as described in Example 9(b) except that drying was performed for
10 minutes, and thus a photocatalytic functional product having a
coating film containing a titanium oxide-based photocatalyst on the
surface of a precoated steel sheet as a substrate was obtained. The
coating film had a thickness of about 0.8 .mu.m and contained about
42 mass % of the photocatalyst.
[0228] The photocatalytic functional product was subjected to an
acetaldehyde decomposition test with visible light irradiation in
the same manner as in Example 9. The rate of CO.sub.2 formation was
0.72 .mu.mol/h. As can be seen from comparison with Example 10(b),
the photocatalytic activity was improved by preparing a coating
fluid after the photocatalytic particles had been subjected to
dispersion.
Example 12
Preparation of Coating Fluids
[0229] After from 0.05 parts to 1 part of the titanium oxide-based
photocatalyst powder prepared in Example 5 were subjected to
dispersion, the resulting dispersion was added to a mixed solvent
of ethanol and water (10 parts of ethanol and 30 parts of water)
along with 1 part (as SiO.sub.2) of a silica colloid (Snowtex OXS
manufactured by Nissan Chemical Industries, 10 mass % SiO.sub.2,
particle diameter: 4-6 mm) used as a binder and thoroughly mixed
using a paint shaker to prepare coating fluids.
[0230] In addition, in order to form a coating film which did not
contain the silica binder, a coating fluid was prepared by adding
only 1 part of the same titanium oxide-based photocatalyst as above
to the mixed solvent followed by mixing in the same manner.
[0231] Formation of Photocatalytic Coating Films
[0232] These coating fluids were used for coating by dipping
therein a 100 mm-square quartz plate having a thickness of 0.5 mm
at room temperature, and the coating was dried for 10 minutes at
150.degree. C. A photocatalytic functional product was obtained by
repeating the dipping and drying as required until a photocatalytic
coating film having a thickness of 1.0.A-inverted.0.1 .mu.m was
formed on the quartz plate.
[0233] Measurement of Photocatalytic Activity (Acetaldehyde
Decomposition Test)
[0234] The measurement of photocatalytic activity was performed in
the same manner as described in Example 1 except that a 40
mm-square test piece which was cut from the quartz plate having a
photocatalytic coating film (photocatalytic functional product) to
be tested was put in the quartz reaction cell and that the total
internal volume of the closed circulating line system was 1.0
liter. The results are shown in Table 6 and FIG. 7.
TABLE-US-00006 TABLE 6 Film thickness = 1.0 .A-inverted. 0.1 .mu.m
Parts by mass of Content of photocatalyst Rate of CO.sub.2 Run
photocatalyst in in coating film formation No. coating fluid (mass
%) (.mu.mol/h) 1 0.05 5 0.12 2 0.5 34 1.58 3 1 50 2.42 4 100 100
3.22 (Note) Photocatalyst = titanium oxide-based photocatalyst
having visible light activity
[0235] A photocatalytic coating film in which the content of a
titanium oxide-based photocatalyst was at least 5 mass % exhibited
photocatalytic activity by irradiation with visible light. Compared
to the photocatalytic activity of the coating film with 100 mass %
of the photocatalyst (100 mass % film), the photocatalytic activity
of a film containing 30 mass % of the photocatalyst reached about
50% the activity of the 100 mass % film, and that of a film
containing about 50 mass % of the photocatalyst reached about 75%
the activity of the 100 mass % film. The increment in visible light
photocatalytic activity achieved by an increased content of the
photocatalyst approached almost zero when the content was about 90
mass %. Therefore, a preferable content of the photocatalyst in the
coating film was 30-90 mass % and more preferably 50-90 mass %.
[0236] The coating film with 100 mass % of the titanium oxide-based
photocatalyst had the highest visible light photocatalytic
activity, but due to the absence of a binder, it had a very weak
bonding of the photocatalytic powder and was readily peeled off
from the quartz plate by rubbing with a finger, for example.
Example 13
[0237] The formation of a photocatalytic coating film having a
thickness of about 1 .mu.M and the measurement of visible light
photocatalytic activity were carried out in the same manner as
described in Example 12 using 1 part of the titanium oxide-based
photocatalyst powder prepared in Example 8 which contained a metal
complex and 1 part of colloidal silica (the same as used in Example
1) as a binder component.
[0238] The content of the titanium oxide-based photocatalyst in the
photocatalytic film was 50 mass %. Compared to the photocatalytic
film of Example 12 having the same content, the rate of CO.sub.2
formation achieved by measurement of visible light photocatalytic
activity increased by a factor of about 2.
Example 14
[0239] The procedure of Example 12 was repeated with the amount of
the titanium oxide-based photocatalyst in a coating fluid being
fixed at 1 part while the number of repetitions of dipping of the
quartz plate in the coating fluid and subsequent drying was varied
to prepare photocatalytic functional products having a
photocatalytic coating film formed on the quartz plate in which the
thickness of the film varied in the range of 0.5-25.5 .mu.m. The
content of the titanium oxide-based photocatalyst having visible
light activity in the coating film was constant and 50 mass % in
all the runs. The results of measurement of visible light
photocatalytic activity performed on these photocatalytic
functional products in the same manner as described in Example 12
are shown in Table 7 and FIG. 8.
TABLE-US-00007 TABLE 7 Parts by mass of Film Rate of CO.sub.2 Run
photocatalyst in % photocatalyst in thickness formation No. coating
fluid coating film (.mu.m) (.mu.mol/h) 1 1 50 0.5 1 2 1 50 1 2.4 3
1 50 2.5 4.5 4 1 50 6.6 8.4 5 1 50 19.6 12 6 1 50 25.5 12.5 (Note)
Photocatalyst = titanium oxide-based photocatalyst having visible
light activity
[0240] A photocatalytic coating film having a thickness of 0.5
.mu.m or greater exhibited photocatalytic activity by irradiation
with visible light. Compared to the photocatalytic activity of the
coating film having a thickness of 25.5 .mu.M, the photocatalytic
activity of a film having a thickness of at least 3 .mu.m reached
at least 50% and that of a film having a thickness of at least 5
.mu.m reached at least 60%, thereby providing a satisfactorily high
visible light photocatalytic activity.
INDUSTRIAL APPLICABILITY
[0241] A titanium oxide-based photocatalyst and a photocatalytic
functional product according to the present invention can be
mass-produced efficiently and exhibit a high photocatalytic
activity by irradiation not only with ultraviolet light but also
with visible light. Compared to a conventional titanium oxide
photocatalyst which is active with ultraviolet light, more
effective utilization of light is made possible, so they are
expected to have extremely potent photocatalytic activities and
strong effects on environmental depollution. In addition, they make
it possible to utilize a photocatalytic effect in those locations
in which the quantity of ultraviolet light is so small that a
conventional titanium oxide photocatalyst could not exhibit a
satisfactory effect, such as locations oriented to the north or in
the shade and the interior of a house in which substantially only
the light of fluorescent lamps is usable.
* * * * *