Photocatalysts, titanium oxide

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

Shiraishi, Y. and Hirai, T. (2008) Selective organic transformations on titanium oxide-based photocatalysts. Journal of Photochemistry and Photobiology C Photochemistry Reviews, 9 (4), 157-170. 

Wu, P., Xie, R., Imlay, J.A., and Shang, J.K. (2009) Visible-light-induced photocatalytic inactivation of bacteria by composite photocatalysts of palladium oxide and nitrogen-doped titanium oxide. Applied Catalysis B Environmental,... 

Kitano, M., Tsujimaru, K., and Anpo, M. (2008) Hydrogen production using highly active titanium oxide-based photocatalysts. Topics in Catalysis, 49 (1-2), 4-17. 

Apart from titanium oxide, two other carbon-modified semiconductors were studied in water photoelectrolysis due to their low band gap energy, namely iron (Fe203) and tungsten oxide (W03) [70,90]. Carbon-modified iron oxide demonstrated promising photoconversion efficiency, 4 % and 7 % for modified oxides synthesized in oven and by thermal oxidation respectively [90]. Also, carbon-modified tungsten oxide (C-W03) photocatalysts exhibited a 2 % photoconversion efficiency [70],... 

In this case, titanium oxide is used both as a photocatalyst for the production of electrons and as a sink for holes, preventing recombination. This facilitates the reduction and thus the coloring of tungsten oxide. They also observe the effect of quantum confinement, resulting in band gaps in the sol that are greater than those normally observed in the bulk, as well as a blueshifted absorbance band, both results of the lack of orbital... 

Liu H, Gao L (2004) (Sulfur, Nitrogen)-codoped rutile-titanium oxide as a visible-light-activated photocatalyst. J Am Ceram Soc 87 1582-1584... 

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... 

Lamp covers coated with titanium oxide thin-film photocatalysts... 

Cements containing titanium oxide photocatalyst powders... 

The main characteristics of the various titanium oxide catalysts used in this chapter are summarized in Table 1. Titanium oxide thin film photocatalysts were prepared using an ionized cluster beam (ICB) deposition method [13-16]. In ICB deposition method, the titanium metal target was heated to 2200 K in a crucible and Ti vapor was introduced into the high vacuum chamber to produce Ti clusters. These clusters then reacted with O2 in die chamber and stoichiometric titanium oxide clusters were formed. Tlie ionized titanium oxide clusters formed by electron beam irradiation were accelerated by a high electric field and bombarded onto the glass substrate to form titanium oxide thin films. 

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)... 

It was also found that increasing the number (or amounts) of metal ion implanted into the deep bulk of the titanium oxides caused the photocatalytic efficiency of these photocatalysts to increase under visible-light irradiation, passing through a maximum at around 6 X 10 ions/cm of the catalyst, then decreas-... 

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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