Utilization of TiO2 photocatalysts in green chemistry*

Environmental pollution and destruction on a global scale have drawn attention to the vital need for totally new environmentally friendly, clean chemical technologies and processes, the most important challenge facing chemical scientists in the field of green chemistry. Strong contenders as environmentally harmonious catalysts are photocatalysts that operate at room temperature and in a clean manner, while applications of such safe photocatalytic systems are urgently desired for the purification of polluted water, the decomposition of offensive atmospheric odors as well as toxins, the fixation of CO 2, and the decomposition of NOx and chlorofluorocarbons on a huge global scale. To address such enormous tasks, photocatalytic systems that are able to operate effectively and efficiently not only under UV light but also under the most environmentally ideal energy source, sunlight, must be established. To this end, we are moving in a positive direction with various practical applications already at hand, as is described. The present report involves 1) new approaches in the design and development of secondgeneration titanium oxide photocatalysts which can operate effectively under visible light and/or solar beam irradiation, 2) practical industrial applications of titanium oxide photocatalysts in Japan, and 3) recent advances in green chemistry in Japan. NEW APPROACHES IN THE DESIGN AND DEVELOPMENT OF SECOND-GENERATION TITANIUM OXIDE PHOTOCATALYSTS OPERATING UNDER VISIBLE LIGHT IRRADIATION As shown in Figs. 1a and 1b, when titanum oxides are irradiated with UV light that is greater than the bandgap energy of the catalyst (l < 380 nm), electrons and holes are produced in the conduction and valence bands, respectively. The electrons have a highly reactive reduction potential while the holes have a highly reactive oxidation potential, which together induce catalytic reactions on the catalyst surfaces—namely, photocatalytic reactions. Because of its similarity to the mechanism observed with the photosynthesis in plants, photocatalysis may also be referred to as artificial photosynthesis. As will be introduced in the Section “Practical Industrial Applications of Titanium Oxide Photocatalysts in Japan”, there are no limits to the possibilities and applications of titanium oxide photocatalysts and photocatalytic reactions as “environmentally harmonious catalysts”. However, as can be seen in Fig. 2a and unlike photosynthesis in green plants, the titanium oxide photocatalyst in itself does not allow the use of visible light and can make use of only 3–4% of solar beams that reach the earth. Therefore, to establish clean and safe photocatalytic reaction systems using the solar beam and/or visible light, it is vital to develop titanium oxide photocatalysts that can absorb visible light and operate with high efficiency under solar beam and/or visible light irradiation. We have applied the metal ion-implantation method to modify the electronic properties of titanium oxide photocatalysts by bombarding them with high-energy metal ions and have found that this advanced physical method is the most suitable and promising for the dramatic modification of the