Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-05-24
2003-12-09
Nguyen, Cam N. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S350000, C502S208000
Reexamination Certificate
active
06660686
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photocatalyst and to a process for producing the same.
2. Related Background Art
Methods employing photocatalysts are among the existing techniques for achieving effective utilization of solar energy. Photocatalysts are substances that function as catalysts activated by light irradiation, typical known ones including homogeneous catalysts which employ metal complexes and non-homogeneous catalysts which employ semiconductor catalysts.
The mechanism by which a photocatalyst exhibits a catalytic effect upon light irradiation is theorized to be as follows, for semiconductor catalysts, Specifically, a semiconductor has a band structure in which conduction bands and valence bands are partitioned by forbidden bands of appropriate width, and irradiation of light of energy exceeding the band gap causes excitation of the valence band electrons to the conduction bands. As a result, electron holes are produced in the valence bands and electrons in the conduction bands, and these electron holes and electrons induce an oxidation or reduction reaction by a mechanism similar to electrolysis.
The band gap size and the potential of the conduction band and valence band are important factors contributing to the catalytic activity. In addition to these, other contributing factors include the life and ease of movement of the generated electrons and electron holes, as well as the charge separation, and the overvoltage and reactivity site of the oxidation-reduction reaction.
As photocatalysts with such functions there are known semiconductor catalysts such as TiO
2
, ZnO, Ta
2
O
5
, CdS, GaP, SiC, K
4
Nb
6
O
17
, K
2
La
2
Ti
3
O
10
, Na
2
Ti
6
O
13
, BaTi
4
O
9
and K
3
Ta
3
Si
2
O
13
, and it has been confirmed that hydrogen and (oxygen are produced when these catalysts are powdered, suspended in water and irradiated with light.
With certain types of photocatalysts, it has been confirmed that formic acid, formaldehyde, methanol, methane and the like are produced upon light irradiation while carbon dioxide is passed through their aqueous suspensions. It has also been reported that catalysts with high dispersion of the semiconductor catalyst TiO
2
exhibit activity in pores of the insulator zeolite during the conversion reaction of carbon dioxide to hydrocarbons.
In addition to those mentioned above, titanosilicate zeolites (TS-1, TS-2) are also known as problem which hampers their practical implementation.
It is an object of the present invention, which has been accomplished in light of such technical problems, to provide a photocatalyst which can induce efficient photocatalytic reaction even when used in a small amount and with a small light irradiation area, and which can thereby decompose water and the like at an adequate reaction rate. It is another object of the invention to provide a process for producing the photocatalyst.
As a result of much diligent research aimed at achieving the aforementioned object, the present inventors have discovered that a photocatalytic function is exhibited by porous materials composed of compounds with a basic framework having titanium atoms and phosphorus atoms bonded by way of oxygen atoms and porous materials composed of compounds with a basic framework having zirconium atoms and phosphorus atoms bonded by way of oxygen atoms. The present invention has been completed upon the further discovery that such photocatalysts can be used for efficient photocatalytic reaction even in small amounts and with a small light irradiation area, thus allowing decomposition of water and the like at a sufficiently high reaction rate.
In other words, the photocatalyst of the invention is a photocatalyst comprising a porous material, materials exhibiting catalytic activity in photocatalytic reactions. Such zeolites are porous materials with pares (micropores) of generally 0.3-1.3 nm in size, and it is believed that their catalytic activity is exhibited by their moderate solid acidity. In recent years it has become possible to) synthesize titanosilicate mesoporous materials with larger pores mesopores) than zeolite, measuring 1.5-50 nm in size, and such substances have also been reported to exhibit activity in photocatalytic reactions.
Because these photocatalysts allow utilization of solar energy for sundry chemical reactions, they are being studied for effective use in numerous fields, such as production of hydrogen and oxygen by decomposition of water, synthesis of hydrocarbons such as methane and methanol from carbon dioxide and water, and purification of harmful substances such as NO
x
and dioxin.
SUMMARY OF THE INVENTION
Nevertheless, these photocatalysts of the prior art have slow reaction rates in photocatalytic reactions for decomposition of water or fixing of carbon dioxide, and since in order to adequately accelerate the reactions it has been necessary to use large amounts of catalyst and accomplish light irradiation over a wide area, this has constituted a wherein the porous material comprises a compound with a basic framework having metal atoms bonded to phosphorus atoms by way of oxygen atoms, the metal atoms being selected from the group consisting of titanium atoms and zirconium atoms.
The present invention includes the first to third preferred embodiments of the photocatalyst as described below.
First Embodiment
The first embodiment of the present invention is a photocatalyst comprising a porous material, wherein the porous material comprises a compound with a basic framework having titanium atoms bonded to phosphorus atoms by way of oxygen atoms and the porous material has a median pore diameter of from 0.2 nm to less than 1.5 nm.
It is believed that in the photocatalyst according to the first embodiment, which has a basic framework with titanium atoms and phosphorus atoms bonded by way of oxygen atoms, the electron holes and electrons produced by light irradiation efficiently contribute to the catalytic reaction so that the efficiency of the catalytic reaction is enhanced even with a small light irradiation area. Also, since the photocatalyst of the first embodiment has pores of the size specified above (micropores), it is possible to greatly increase the contact area for contact of water, for example, thereby increasing the number of reaction sites for the photocatalytic reaction and allowing efficient decomposition of water even with a small photocatalyst amount.
The basic framework of the photocatalyst of the first embodiment preferably has the composition represented by the following general formula (1).
Ti
x
P
y
O
z
(1)
wherein x is a number of from 0.5 to 1.5, y is a number of from 0.5 to 1.5 and z is a number of from 3 to 6.
A photocatalyst of the first embodiment having the composition represented by general formula (1) above exhibits improved efficiency of light absorption and efficiency of the light irradiation-induced charge separated electron holes and electrons participating in the photocatalytic reaction.
According to the first embodiment, the porous material preferably has a pore volume of from 0.05 to 0.5 ml/g. If the porous material has a pore volume within this range, the contact area will tend to increase for contact with water, for example, thereby increasing the number of reaction sites for the photocatalytic reaction and tending to increase the efficiency of the photocatalytic reaction.
Second Embodiment
The second embodiment of the present invention is a photocatalyst comprising a porous material wherein the porous material comprises a compound with a basic framework having titanium atoms bonded to phosphorus atoms by way of oxygen atoms and the porous material has the median pore diameter of from 1.5 nm to 30 nm, further wherein the value of the total volume of pores with a diameter in the range of ±40% of the median pore diameter divided by the total pore volume is from 0.4 to 1.
Since the photocatalyst according to the second embodiment is composed of a compound with a basic framework having titanium atoms and phosphorus atoms bon
Bhaumik Asim
Inagaki Shinji
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