Photocatalyst

Details Photocatalyst Photocatalyst

2002-07-26

2004-06-01

Bell, Mark L. (Department: 1755)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Metal, metal oxide or metal hydroxide

C502S350000, C502S326000, C502S200000, C502S216000, C502S222000, C502S223000, C502S242000, C423S239100, C423S610000

Reexamination Certificate

active

06743749

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a photocatalyst that can produce effects when exposed to visible light.
BACKGROUND ART
Conventionally, various materials such as, for example, TiO
2
(titanium dioxide), CdS (cadmium sulfide), WO
3
(tungsten trioxide), and ZnO (zinc oxide) are known as materials for producing photocatalytic effects. These photocatalytic materials are semiconductors which absorb light to produce electrons and holes, and cause various chemical reactions and disinfection effects. Currently, the only material put in practice is TiO
2
, because TiO
2
is superior in consideration of toxicity and stability with respect to exposure to water, acids, and bases.
However, because of the band gap value of TiO
2
(Eg=3.2 eV in an anatase crystal), the operational light of the TiO
2
photocatalyst is limited to ultraviolet light having a wavelength of less than 380 nm. In order to allow satisfactory operation under sunlight, indoors, or in a vehicle, and to improvement catalytic activity when light of weak intensity is irradiated, there is strong demand for development of a material which can realize catalytic activity when irradiated by visible light having a wavelength longer than or equal to 380 nm.
For example, Japanese Patent Laid-Open Publication No. Hei 9-262482 discloses modification of material through ion implantation of a metal element such as, for example, Cr (chromium), and V (vanadium) to an anatase TiO
2
which has a high catalytic activity, in order to shift the light absorption edge of TiO
2
towards a longer wavelength and to thereby enable operation of a TiO
2
catalyst under visible light. Although doping of Cr, V, or the like has been reported since the early 1970's, none of the early reports disclose that operation by visible light is enabled. In Japanese Patent Laid-Open Publication No. Hei 9-262482, operation by visible light are enabled by using a special doping method, ion implantation, for Cr, V, or the like.
In the above conventional art, operability under visible light of a TiO
2
photocatalyst is enabled through ion implantation of a metal element to TiO
2
. However, ion implantation of a metal element is likely to require large and expensive apparatus. To this end, there is a demand for synthesizing the TiO
2
photocatalyst through other methods, such as, for example, synthesis in solution or sputtering. However, photocatalyst created through these methods cannot operate by visible light. It is considered that this is because aggregation of the dopant, Cr, occurs or because oxides such as Cr
2
O
3
are formed during the crystallization processes. As described, in the conventional art, there has been a problem that, in order to enable operation of TiO
2
by visible light using a metal element, ion implantation of the metal element is required.
DISCLOSURE OF INVENTION
One object of the present invention is to realize a TiO
2
photocatalyst capable of operating in the visible light in addition to the ultraviolet range by using a novel material and without using costly production methods such as ion implantation.
According to a first aspect of the present invention, there is provided a photocatalyst comprising, as an inner material, a titanium compound (Ti—O—N or Ti—O—S) in which a nitrogen atom (N) or a sulfur atom (S) substitutes for a portion of the oxygen site of crystals of titanium oxide (for example, TiO
2
), is doped at an interstitial site of the crystal lattices of titanium oxide, or is placed at the grain boundary of polycrystalline assembly of titanium oxide crystals, and wherein a charge separation material is partially supported on the surface of the titanium compound.
Ti—O—N or Ti—O—S are titanium compounds obtained by introducing nitrogen or sulfur to titanium oxide crystals and have an active photocatalytic function not only when exposed to light in the ultraviolet range, but also under light in the visible range. Therefore, the photocatalytic function similar to that in TiO
2
can be obtained with visible light as the operational light.
Moreover, a charge separation material can be partially supported on the surface of Ti—O—N or Ti—O—S. As the charge separation material, for example, at least one of Pt, Pd, Ni, RuO
x
(for example, RuO
2
), NiO
x
(for example, NiO), SnO
x
(for example, SnO
2
), Al
x
O
y
(for example, Al
2
O
3
), ZnO
x
(for example, ZnO), and SiO
x
(for example, SiO
2
) may be selected. Such a charge separation material acts as a promoter and facilitates separation of charges produced as a result of irradiation of light. That is, a metal element such as Pt, Pd, and Ni selectively captures electrons and an oxide such as RuO
x
(for example, RuO
2
), NiO
x
(for example, NiO), SnO
x
(for example, SnO
2
), Al
x
O
y
(for example, Al
2
O
3
), ZnO
x
(for example, ZnO), and SiO
x
(for example, SiO
2
) selectively captures holes. Therefore, by partially supporting these materials on the surface of the photocatalytic material, the probability of recombination of electrons and holes produced by the photocatalytic reaction is reduced and, thus, reduction in activity caused by the recombination of electrons and holes can be prevented.
It is preferable that the ratio, X %, of number of atoms of N in Ti—O—N be 0<X<13. A similar ratio is preferable for the S in Ti—O—S. It is also preferable that, when the metal element or oxide is assumed to be uniformly supported, the amount of the metal element or oxide on the surface which acts as a promoter corresponds to a thickness of 0.1 angstrom (Å) to 10 Å. In the case of SiO
x
, it is preferable that the corresponding amount be 10 Å to 500 Å. In reality, these promoters on the surface forms an island-like structure and may not be present entirely over the Ti—O—N or Ti—O—S surface.
According to another aspect of the present invention, it is preferable that the Ti—O—N or Ti—O—S is used as an inner material, a titanium oxide layer is formed on the surface of the inner material, and a charge separation material is partially supported on the surface of the titanium oxide layer.
By employing such a structure, it is possible for the inner Ti—O—N or Ti—O—S to absorb light in the range from ultraviolet to visible while allowing catalytic reaction by the titanium oxide on the surface and the charge separation material partially supported thereon. Titanium oxide is inexpensive and stable, and an effective catalytic reaction can be realized while preventing recombination of electrons and holes by Pt, Pd, Ni, RuO
x
(for example, RuO
2
), NiO
x
(for example, NiO), SnO
x
(for example, SnO
2
), Al
x
O
y
(for example, Al
2
O
3
), ZnO
x
(for example, ZnO), and SiO
x
(for example, SiO
2
).
According to another aspect of the present invention, there is provided a photocatalytic material comprising an oxide crystal of a metal element M1, the oxide crystal having a photocatalytic function, in which a nitrogen atom or a sulfur atom substitutes for a portion of the oxygen sites of the oxide crystals, is doped at an interstitial site of the crystal lattices of oxide, or is placed at the grain boundary of the polycrystalline body of oxide crystals, and wherein at least one metal element M2 of vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc(Zn), ruthenium (Ru), rhodium (Rh), rhenium (Re), osmium (Os), palladium (Pd), platinum (Pt), iridium (Ir), niobium (Nb), and molybdenum (Mo) substitutes for a portion of the M1 sites of the oxide crystal, is doped at an interstitial site of the crystal lattices of the oxide, or is placed at the grain boundary of the polycrystalline body of the oxide crystals.
Here, it is preferable that the compositional ratio of nitrogen exceeds 0 and is less than 13 in the ratio percent of number of atoms and that the compositional ratio of various metal elements exceeds 0 and is less than 5 in the ratio percent of number of atoms. The compositional ratio of sulfur is similar to that of nitrogen.
In such a photocatalytic material, the absorption edges in the light ab

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