This type of photocatalysts are finding expanded applications because energy used in catalytic reactions is light energy, such as sunlight, that is inexhaustible. For example, titanium dioxide (TiO2), a kind of photocatalyst, particularly titanium oxide in an anatase crystal form, produces excited electrons and positive holes upon exposure to light energy (ultraviolet light), and the excited electrons and positive holes produce active oxygen species, such as O2−, O−, and •OH (wherein the dot • represents an unpaired electron and means that the species attended with the dot • is a free radical), in the presence of oxygen and water on the surface of the catalyst. Applications utilizing free radical properties of the active oxygen species have been proposed such as air purification applications where nitrogen oxides (NOx) in the air are oxidized with the active oxygen species and consequently converted to a harmless reaction product (nitric acid), and degradation of bacteria through the oxidation of organic matter, that is, the so called antimicrobial applications.
In the course of the oxidation of the nitrogen oxides with the active oxygen species, nitrogen dioxide (NO2) is produced as an intermediate that is further oxidized and finally converted to nitric acid. As a result of the production of nitric acid, the nitrogen oxides in the air are reduced, and the air is purified. For this reason, the presence of the active oxygen species and the nitrogen oxide or nitrogen dioxide is indispensable for enhancing the percentage reduction of nitrogen oxides. Since, however, nitrogen dioxide is a relatively chemically stable compound (gas), the produced nitrogen dioxide leaves the reaction system. This lowers the efficiency of the oxidation with the active oxygen species, resulting in lowered percentage reduction of the nitrogen oxides. Use of porous adsorbents, such as activated carbon, is considered effective for preventing nitrogen dioxide from leaving the reaction system. As is apparent from the following description, this method is not always effective.
Specifically, when nitrogen dioxide, which has left, is adsorbed on the above adsorbent, the nitrogen dioxide often remains within pores of the adsorbent without being released. For this reason, in some cases, the adsorbed nitrogen dioxide is placed outside the system of oxidation with the active oxygen species and does not undergo the oxidation reaction and hence cannot be converted to nitric acid as a final product. Thus, the nitrogen oxides are not finally converted to nitric acid. This inhibits the reduction of nitrogen oxides. In this case, it should be noted that nitrogen dioxide adsorbed onto the adsorbent in a region where nitrogen dioxide can be present together with the active oxygen species and is in the reaction system, that is, in a region close to the photocatalyst, is oxidized to produce nitric acid. Since, however, the region close to the photocatalyst occupies only a small proportion of the whole material adsorption region (including pores) in the absorbent, it can be said that the proportion of nitrogen dioxide, which could not be oxidized to nitric acid, is high. That is, the adsorbent merely adsorbs and holds nitrogen dioxide, and the percentage reduction of nitrogen oxides by conversion to nitric acid does not appear to be satisfactory.
The present invention has been made with a view to solving the above problems, and an object of the present invention is to further improve the efficiency of a catalytic reaction, in which a photocatalyst participates, or to improve the percentage reduction of the reactant applied to the catalytic reaction through the conversion of the reactants to the final product. Another object of the present invention is to supplement the function of a photocatalyst.