Design and Development of Second-generation TiO2 Photocatalyst Operating under Solar Beam Irradiation for the Decomposition of NOx

本课题设计开发了一种既能吸收可见光，又能在紫外光照射下高效工作，又能在可见光或太阳光照射下高效工作，将NO分解为N_2和0_2的TiO_2光催化剂。到目前为止，已知TiO_2光催化剂用于大气中氮氧化物（NOx）分解反应的光催化反应仅在TiO2光催化剂的波长小于380nm的紫外光照射下进行，允许使用3%的太阳光束。为了更有效地利用太阳光束，我们研究了采用先进的金属离子注入技术对TiO_2光催化剂进行改性。这种金属离子注入和随后催化剂在723 K左右的氧中煅烧相结合，使TiO_2的吸收带向可见光区大量移动，其程度取决于注入的Cr离子的量。在波长大于450nm的可见光照射下，在275 K的温度下，注入Cr的TiO_2催化剂也能成功地将NO分解成N_2、0_2和N_2O。此外，在带隙紫外光照射下，反应速率与未注入TiO_2催化剂完全相同。在常温下的室外日光照射下，注入Cr离子的TiO_2催化剂的光催化活性比未注入Cr离子的TiO_2催化剂高数倍。可以说，目前的研究为许多创新的可能性开辟了道路，显著地解决了紧迫的环境问题，也可以被认为是太阳能利用的重要突破。

In this research project, the design and development of TiO_2 photocatalysts which can absorb visible light and operate efficiently and effectively not only under ultraviolet light but also upon irradiation with visible light or solar beam for the decomposition of NO into N_2 and 0_2 were carried out. Until now, photocatalytic reactions involving TiO_2 photocatalysts for the decomposition reaction of nitrogen oxides (NOx) in the atmosphere are known to proceed only under irradiation of TiO2 photocatalysts with ultraviolet light having shorter wavelength than 380nm, allowing the use of 3% of solar beam. In order to use solar beam more efficiently, we have elucidated the modification of TiO_2 photocatalyst by applying the advanced metal ion-implantation with various transition metal ions such as Cr highly accelerated by high voltage. A combination of such metal ion-implantation and the subsequent calcination of the catalyst in oxygen at around 723 K enables a large shift in the absorption bands of TiO_2 toward visible-light regions, its extent depending on the amount of Cr ions implanted. Such Cr ion-implanted TiO_2 catalysts successfully decompose NO into N_2, 0_2 and N_2O even at 275 K under irradiation with visible light longer than 450nm. Furthermore, the reaction proceeded at exactly the same rate to that of the unimplanted TiO_2 catalysts under band-gap UV light irradiation. Under outdoor solar light irradiation at ordinary temperatures, the Cr ion-implanted TiO_2 catalyst exhibited photocatalytic reactivity several times higher than the original unimplanted TiO_2 photocatalyst. It can be said that present research has opened the way to many innovative possibilities, significantly to address urgent environmental problems, and can also be considered an important breakthrough in the utilization of solar energy.