Oxide photocatalysts

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

SEM and EDS analysis of the platinum-doped tungsten oxide photocatalyst after sintering showed the presence of platinum particles on the surface of the tungsten oxide. 

Fernandez-Garcia, M., Fuerte, A., Hemandez-Alonso, M.D., Soria, J., and Martinez-Arias, A. (2007) Platinization of sunlight active Ti-W mixed oxide photocatalysts. Journal of Catalysis,... 

Ohtani, B., Iwai, K., Kominami, H., Matsuura, T., Kera, Y., and Nishimoto, S. (1995) Titanium(IV) oxide photocatalyst of ultra-high activity for selective N-cyclization of an amino acid in aqueous suspensions. Chemical Physics Letters, 242 (3), 315-319. 

Hitoki, G., Takata, T., Kondo, J.N., Hara, M., Kobayashi, H., and Domen, K., An oxynitride, TaON, as an efficient water oxidation photocatalyst under visible light irradiation (A = <500), Chem. Commun., 16,1698,2002. 

Kim HG, Hwang DW, Lee JS (2004) An undoped, singlephase oxide photocatalyst working under visible light. J Am Chem Soc 126 8912-8913... 

Sato J, Saito N, Yamada Y, Maeda K, Takata T, Kondo JN, Hara M, Kobayashi H, Domen K, Inoue Y (2005) RUO2-loaded beta-Ge3N4 as a non-oxide photocatalyst for overall water splitting. J Am Chem Soc 127 4150-4151... 

However, unlike photosynthesis in green plants, the titanium oxide photocatalyst does not absorb visible light and, therefore, it can make use of only 3-4% of solar photons that reach the Earth. Therefore, to address such enormous tasks, photocatalytic systems which are able to operate effectively and efficiently not only under ultraviolet (UV) but also under sunlight must be established. To this end, it is vital to design and develop unique titanium oxide photocatalysts which can absorb and operate with high efficiency under solar and/or visible-light irradiation [9-16]. 

This chapter deals with the design and development of such unique second-generation titanium oxide photocatalysts which absorb UV-visible light and operate effectively under visible and/or solar irradiation by applying an advanced metal ion-implantation method. 

When titanium oxides are irradiated with UV light that is greater than the band-gap energy of the catalyst (about X < 380 nm), electrons (e ) and holes (h+) are produced in the conduction and valence bands, respectively. These electrons and holes have a high reductive potential and oxidative potential, respectively, which, together, cause catalytic reactions on the surfaces namely photocatalytic reactions are induced. Because of its similarity with the mechanism observed with photosynthesis in green plants, photocatalysis may also be referred to as artificial photosynthesis [1-4]. As will be introduced in a later section, there are no limits to the possibilities and applications of titanium oxide photocatalysts as environmentally harmonious catalysts and/or sustainable green chemical systems. ... 

Figure 1 View of the soundproof-highway walls coated with titanium oxide photocatalysts for the elimination of NO, (the walls were constructed in Osaka, in April 1999) (20 m of the polluted air can be purified at the rate of 1 m" photocatalyst/hr).

Some practical applications of titanium oxide photocatalysts in Japan are as follows ... 

White paper containing titanium oxide photocatalysts... 

Cements containing titanium oxide photocatalyst powders... 

The metal ion-implanted titanium oxide catalysts were calcined in O2 at around 725-823 K for 5 hr. Prior to various spectroscopic measurements such as UV-vis diffuse reflectance, SIMS, XRD, EXAFS, ESR, and ESCA, as well as investigations on the photocatalytic reactions, both the metal ion-implanted and unimplanted original pure titanium oxide photocatalysts were heated in O2 at 750 K and then degassed in cells at 725 K for 2 h, heated in O2 at the same temperature for 2 h, and, finally, outgassed at 473 K to 10 lorr [12-15]. 

Furthermore, as shown in Fig. 5, such red shifts in the absorption band of the metal ion-implanted titanium oxide photocatalysts can be observed for any... 

With unimplanted or chemically doped titanium oxide photocatalysts, the photocatalytic reaction does not proceed under visible-light irradiation (X > 450 nm). However, we have found that visible-light irradiation of metal ion-implanted... 

It is important to emphasize that the photocatalytic reactivity of the metal ion-implanted titanium oxides under UV light (X < 380 nm) retained the same photocatalytic efficiency as the unimplanted original pure titanium oxides under the same UV light irradiation conditions. When metal ions were chemically doped into the titanium oxide photocatalyst, the photocatalytic efficiency decreased dramatically under UV irradiation due to the effective recombination of the photo-formed electrons and holes through the impurity energy levels formed by the doped metal ions within the band gap of the photocatalyst (in the case of Fig. 6)... 

Figure 10 Effect of the depth profile of V ions in the V-ion-implanted titanium oxide photocatalyst on their photocatalytic reactivity for the decomposition of NOx under visible light (X > 450 nm) irradiation at 295 K.

Figure 11 Effect of calcination temperature of the V-ion-implanted titanium oxide sample on the ESR spectra of + species in the V-ion-implanted titanium oxide photocatalyst at 77 K.

Figure 13 UV-vis absorption spectra (diffuse reflectance) of the Cr-ion-implanted titanium oxide photocatalyst (top) and the chemically doped Cr-ion titanium oxide photocatalyst (bottom) and their modified band-gap energy structures.

The advanced metal ion-implantation method has been successfully applied to modify the electronic properties of the titanium oxide photocatalysts, enabling... 

Thus, the advanced metal ion-implantation method has opened the way to many innovative possibilities, and the design and development of such unique titanium oxide photocatalysts can also be considered an important breakthrough in the utilization of solar light energy, which will advance research in sustainable green chemistry for a better environment [4,5,15,16,20-22]. 

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