Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-01-03
2001-07-24
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S330000, C502S339000, C502S344000, C502S347000, C502S350000, C501S095200, C252S514000
Reexamination Certificate
active
06265341
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a photocatalyst which can clean the environment by decomposing environmental contaminants, etc., and more particularly to a technical field of a photocatalyst which causes a conspicuous improvement in photocatalytic efficiency in quantum-mechanical terms by supporting nano-scale ultra-fine metal particles, which are capable of realizing a quantum size effect, on fine particles of a photocatalyst, and a highly functional base material which can clean the environment with high efficiency by holding the photocatalyst supporting ultra-fine metal particles on the surface of such a material.
BACKGROUND ART
A photocatalytic reaction of titanium dioxide was reported in
Nature
in 1972 and became known world-wide as the Honda-Fujishima effect. Since then, research has been conducted on the production of hydrogen and carbon dioxide by the decomposition of water or the decomposition of aqueous solutions of organic matter by means of titanium dioxide under irradiation with light; and today, a technique in which fine particles of titanium dioxide are held in the form of a thin film on tiles or window glass is in the process of being adapted for practical use in the decomposition of environmental contaminants, i.e., tobacco tar and organic matter such as bacteria, toxins produced by bacteria, etc.
Titanium dioxide is a powder-form metal oxide and is used after being dispersed in a solution in the case of decomposition of water or solutions. However, it is desirable that titanium dioxide adhere to window glass and bathroom tiles, or to the surfaces of construction materials, in the form of a uniform thin film even when titanium dioxide is used in particle form. Accordingly, methods such as sol-gel methods, spray pyrolysis methods using titanium acetate, etc., and dip coating methods, etc. have been developed. These techniques are described in “
Oyo Butsuri
(Applied Physics)”, Vol. 64, No. 8, p. 803 (1995), “
Kayak to Kogyo
(Chemistry and Industry)”, Vol. 48, No. 10, p. 1256 (1995), and “
Kayak to Kogyo
(Chemistry and Industry)”, Vol. 49, No. 6, p. 764 (1996). It has been shown that adhering oils and tobacco tar can be decomposed while being irradiated with ultraviolet light using a glass, etc. covered with such titanium dioxide. It is difficult to decompose inorganic matter such as dirt and dust, etc.; however, it has been reported that since such inorganic matter adheres with organic substances such as oils, etc. as a binder, the decomposition of organic matter also tends to prevent the adhesion of inorganic matter.
The principle of the action of fine particles of titanium dioxide on materials such as tiles, etc. is based on the photocatalytic characteristics of titanium dioxide as a semiconductor. If titanium dioxide is irradiated with light which exceeds the band gap energy, e.g., ultraviolet light, then the electrons in the valence electron band are excited and undergo a transition to the conduction band, so that positively charged holes are left in the valence electron band, thus producing electron-hole pairs. These electrons and holes move through the titanium dioxide and reach the surface; and the electrons are supplied to oxygen in the air so that O
2
−
(super-oxide anion) is produced and other substances are reduced. The holes not only cause direct oxidative decomposition of organic matter but also oxidize water molecules adhering to the surface so that strongly oxidizing hydroxyl radicals are formed, and other substances are oxidized by the oxidizing power of these hydroxyl radicals. The O
2
−
reportedly participates in this oxidation process; however, the detailed reaction circuit is still being studied. Thus, organic matter is decomposed into carbon dioxide and water by electron-hole pairs excited by light.
In this research, instances have been found in which the electrons and holes re-couple and disappear prior to the oxidation-reduction of external substances in cases where titanium oxide is used alone. Accordingly, it has been indicated that there are limits to the photocatalytic efficiency of titanium dioxide. The ordinary state of titanium dioxide is a powdered state; considering a single particle of titanium dioxide, there are countless lattice defects such as point defects and plane defects, etc. in the surface and interior portions of the particle. When electrons and holes excited in titanium dioxide by ultraviolet light encounter lattice defects in the process of movement, these electrons and holes are captured by the lattice defects and caused to re-couple. In some cases, furthermore, even if the electrons and holes are able to move to the surface, the electrons and holes re-couple when they approach each other. In order to ameliorate such problems, it is necessary to develop techniques for manufacturing titanium dioxide which is free of lattice defects, and techniques for separating electrons and holes at the surface. In regard to the former techniques, improvements have been made in crystal growth techniques; however, since these techniques have no direct connection with the present invention, a detailed description will be omitted here.
In regard to techniques for separating electrons and holes at the surface, a photocatalyst has been proposed in which an electrode which collects excited electrons is formed on the surface of titanium dioxide, so that holes are separated and collected on the surface of the titanium dioxide, while electrons are separated and collected on the surface of the metal electrode. If this approach is used, electrons can be efficiently collected on the surface of the metal electrode, and holes and electrons can be separated; accordingly, the probability of re-coupling would appear to be lowered. Photocatalysts of this type are referred to as “metal-supporting photocatalysts”, and are manufactured by forming metals conventionally used as catalysts, such as Pt (platinum) and Cu (copper), etc. on the surface of titanium dioxide. The idea here is that if the metals have a catalytic effect even when used alone, then a synergistic effect with the catalytic action of titanium dioxide should be exhibited.
Methods that have been developed for manufacturing such metal-supporting photocatalysts include: photo-deposition methods in which the semiconductor is suspended in an aqueous solution of a metal salt, after which a reducing agent is added and irradiation with light is performed; impregnation methods in which the semiconductor is immersed in an aqueous solution of a metal salt and dried, after which a reduction treatment is performed; chemical deposition methods in which the semiconductor is violently agitated in an aqueous solution of a metal salt and a reducing agent is added; and simultaneous precipitation methods in which an aqueous solution of a metal salt is added to the semiconductor raw material and simultaneous precipitation is performed, after which sintering is performed. Furthermore, other methods which have been developed include: kneading methods in which the semiconductor and a powdered metal are mixed in a mortar; shaking mixing methods in which the semiconductor and a powered metal are placed in a vessel and mixed by shaking using a shaker, etc.; and powdered metal addition methods in which the semiconductor and a powdered metal are separately added to reaction product solutions and are then suspended and mixed.
The present inventors successively investigated these methods but were unable to form micron-sized fine metal particles that have a particle diameter of 0.1 microns or greater on the surface of titanium dioxide. In other words, the inventors reached the conclusion that the formation of nano-scale ultra-fine metal particles capable of exhibiting a quantum size effect on the surface of a semiconductor is difficult as long as powdered metals or aqueous solutions of metal salts are used. Furthermore, in such conventional methods, the number of fine metal particles that can be supported on one titanium dioxide particle (i.e., the supported particle density) is limi
Harada Akio
Komatsu Teruo
Ueyama Ryosuke
Griffin Steven P.
Koda & Androlia
Komatsu Teruo
Nguyen Cam N.
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