Study On Nano-TiO₂ Sensitized With H₂O₂ And Its Visible Light Activity

Nanosize TiO₂ particles are used widely as one of the photocatalysts due to their chemical stability, non-toxicity and high activity, but its high activity can be acquired only under ultraviolet light with a wavelength of 400 nm or less at room temperature due to its broad band gap. Since ultraviolet (UV) light is only 3-5% part of the solar spectrum, the photocatalytic activity of TiO₂ can not be sufficiently activated under solar light irradiation, which strongly limits the use of solar spectra as a source for photodecomposition of pollutant. In addition, the recombination for large numbers of charge carriers would occur in the volume or on the surface of TiO₂. Therefore, the extension of the photoactive wavelength region of TiO₂ into the visible region and improvement of quantum efficiency are desirable for popularizing more TiO₂ photocatalyst, especially under solar light for industrial areas or poor interior lighting illumination in living spaces. For this purpose, pure TiO₂ has been modified by various ways such as impurity doping, inorganic compound and dye sensitization to obtain visible light reactivity. Hydrogen peroxide often was used widely as an electron acceptor in photocatalytic degradation reaction. However, in this paper, H₂O₂ was used as a sensitizer to modify the TiO₂, and the attempt succeeded in extending the optical absorption edge of TiO₂ into the visible region.TiO₂ nanoparticles prepared by hydrolysis of TiCl4 were sensitized with H₂O₂, resulting in absorbing visible light up to 550 nm. Compared with anatase, the stronger visible photoabsorption could be observed for rutile nanopaiticles treated with H₂O₂, but the visible irradiation faded the yellow rutile more easily. The adsorbed water and surface hydroxyl group on TiO₂ nanoparticles treated with H₂O₂ would decrease. The dyes could be degraded selectively. Methylene blue (MB) could be degraded efficiently for all used samples, but a poor activity for decompositions of methyl orange (MO).The mixture phase of anatase and rutile showed the highest photoactivity, and the photoactivity for rutile was most low, due to that more hydroxyl radicals were detected in the suspension of mixture phase of anatase and rutile.The results from XPS, Raman and IR indicated that an amount of SO₄^2- species anchored on the surface of sulfated TiO₂, which resulted in a large number of Br?nsted and Lewis acidic sites on the surface of TiO₂, especially for active sulfate species. More surface chemisorptions centers for some reactants can be facilitated due to surface acidic sites, and these chemisorptions centers also serve hydrogen peroxide. So, the sulfated TiO₂ sensitized with H₂O₂ can adsorb more hydrogen peroxide to form more peroxo-titanium complexes, resulting in more intensive Vis absorption, and the acidic sites could also stabilize the peroxo-titanium complexes. Calcinations at high temperature would lead to the decomposition of a large amount of active sulfate species, which decreased Vis absorption for the used samples treated with H₂O₂.Sensitizing with H₂O₂ to sulfated TiO₂ hardly reduced the adsorbed water and surface hydroxyl group, and more surface hydroxyl groups were observed for sulfated TiO₂ from one-step hydrolysis of boiling TiCl4 solutions due to formation of Ti ions in tetrahedral coordination.All sulfated TiO₂ could degrade efficiently MO, and the sulfated TiO₂ from one-step hydrolysis also could degrade efficiently MB. Photoactivity differed from sulfate species with different coordination to the surface of TiO₂. Higher Vis photoactivity occurred to sulfated TiO₂ with cheating bidentate sulfate species. The active sulfate species were mostly responsible for high Vis photoactivity due to formation of B acidic sites. The photoexcited holes could directly react with B acidic sites on the catalyst surface to produce hydroxyl radicals proven to be powerful oxidants in degrading organics, which would stabilize peroxo-titanium complexes.TiO₂ sol could be prepared via directly sonicating the Ti(OH)4 precipitate. Sol with better crystallization was acquired by hydrothermal treatment. The results from IR and XPS indicated that hydrothermal treatment could increase the absorbed water but reduce surface hydroxyl groups. A large numbers of adsorbed water isolate sol particles from reaction surroundings, so hydrothermal sol sensitized with H₂O₂ could absorb less visible light and adsorb also less MB. The hydrothermal sol had more high photoactivity under ultraviolet radiation; After sensitizing with H₂O₂, UV and Vis photoactivity both was reduced for hydrothermal sol, because the adsorbed water would restrain generation of hydroxyl radicals.Highly dispersive nano TiO₂ particles could obtained by loading TiO₂ particles on the surface of the dispersive nanosize SiO₂ particles by deposition method.TiO₂ loaded on the surface of SiO₂ by Ti-O-Si bond.TiO₂ loaded on the surface of SiO₂ were amorphous, but the crystal TiO₂ were got by calcining and changing the way of loading TiO₂.The catalyst could disperse well if the content of TiO₂ was less 30% in the catalyst; And the content of TiO₂ exceeded 30%, resulting in agglomeration of catalyst. With the increase of TiO₂, the Vis absorption for catalyst treated with H₂O₂ increased.After the content of TiO₂ exceeded 20%, the increase for the Vis absorption become milder. An amount of SiO₂ also could stabilize the peroxo-titanium complexes on the surface of catalyst. The sulfated catalyst could absorb more visible light. The catalyst had highest Vis photoactivity when the content of TiO₂ was about 20-30%. The sulfation and thermal treatment both decreased photoactivity of catalyst. However, sulfated catalyst could degrade MO completely, and other catalyst just discolored MO, due to the generation of small organic molecules. The peroxo-titanium complexes between TiO₂ and SiO₂ particles resulted in high Vis photoactivity for TiO₂/SiO₂ composite catalyst treated with H₂O₂.