Photocatalytic reactions, titania

In our experiment, photocatalytic decomposition of ethylene was utilized to probe the surface defect. Photocatalytic properties of all titania samples are shown in table 2. From these results, conversions of ethylene at 5 min and 3 hr were apparently constant (not different in order) due to the equilibrium between the adsorption of gaseous (i.e. ethylene and/or O2) on the titania surface and the consumption of surface species. Moreover it can be concluded that photoactivity of titania increased with increasing of Ti site present in titania surface. It was found that surface area of titania did not control photoactivity of TiOa, but it was the surface defect in titania surface. Although, the lattice oxygen ions are active site of this photocatalytic reaction since it is the site for trapping holes [4], this work showed that the presence of oxygen vacancy site (Ti site) on surface titania can enhance activity of photocatdyst, too. It revealed that oxygen vacancy can increase the life time of separated electron-hole pairs. 

Figure 4.11 Schematic representation of a photocatalytic reaction occurring on a titania particle on which Pt nanoparticles have been deposited. Adapted from [160] with permission from Springer Science and Business Media.

Lorret, O., Franco va, D., Waldner, G., and Stelzer, N. (2009) W-doped titania nanoparticles for UV and visible-light photocatalytic reactions. Applied Catalysis B Environmental, 91 (1-2), 39-46. 

Navio, J.A., G. Colon, M. Trillas, J. Peral, X. Domenech, J.J. Testa, J. Padron, D. Rodriguez and M.I. Litter (1998). Heterogeneous photocatalytic reactions of nitrite oxidation and Cr(VI) reduction on iron-doped titania prepared by the wet impregnation method. Applied Catalysis B-Environmental, 16(2), 187-196. 

Watanabe et al. have reported similar action spectrum analysis of photoin-duced degradation of Rhodamine B with a cadmium sulfide suspension and pointed out a similar dye-sensitization mechanism Watanabe, T. Takizawa, T. Honda, K. J. Phys. Chem. 1977, 81, 1845. Photocatalytic reaction of MB in aerated titania suspensions was reported in 1937 by a Japanese photochemist Horio, M. Nihon Gakujutsu Kyokai Hokoku 1937, 12, 204 (in Japanese). As far as the author knows, this is the first report on titania photocatalysis. 

The fact that irradiation of a dye solution in the absence of a photocatalyst decomposes the dye negligibly has often been described in the papers to support negligible photoinduced reaction by photoexcited dye molecules. However, the photoinduced electron injection requires an acceptor, such as titania, and thereby there are no ideal control experiments to exclude the possibility of photoinduced electron injection, as shown in Fig. 11. When organic dyes themselves are a pollutant to be decomposed, visible light-induced, but not photocatalytic, reaction can be a useful technique, e.g., Chen, X. Zheng, Z. Ke, X. Jaatinen, E. Xie, T. Wang, D. Guo, C. Zhao, J. Zhu, H. Green Chem. 2010, 12, 414. 

The influence of surface hydrophihcity of catalyst on the oxidation of benzylal-cohol to benzaldehyde in aqueous solution has been investigated by Augugliaro et al. [34] using a synthetic titania catalyst with predominant anatase phase and titania P25. More hydrophilic catalyst was prepared by hydrolysis using TiCl4 as precursor. The comparative in situ ATR-FTIR results show that the selectivity to benzaldehyde is increased over the more hydrophilic catalyst. This is attributed to a lower ability of photocatalytic reaction products to adsorb at the surface. 

Another example of visible light-driven reaction through non-band-gap excitation is a photocatalytic reaction that uses surface-plasmon resonance (SPR) absorption of small metal particles loaded on base metal oxides. For example, gold particles of the size of ten to several ten nanometers, presenting purplish red color by the SPR absorption, loaded on titania particles have been used for photocatalytic reactions under visible-light irradiation at the wavelength of ca. 600 nm. Based on the results that titania or a related material is necessary for this visible fight-driven reactimi and that SPR absorption cannot induce electnaiic excitation of electrons, the mechanism of this kind of reaction seems complicated and is now under discussimi. 

Figure 10. Photocatalytic activity of Au/titania nanocomposites containing 0 to 5 % Au in phenol-oxidation and chromium-reduction reactions [57].

---