Hydrogen Reduced Rutile Titanium Dioxide Photocatalyst

Abstract

For TiO 2 photocatalysts, recombination of photoexcited electrons and holes would occur in crystalline defects such as oxygen vacancies, Ti 3+ ions, and surface states. Therefore, it is believed that the density of crystalline defects should be decreased to improve the photocatalytic activity of TiO 2 particles. Contrary to this common knowledge, the introduction of crystalline defects by hydrogen reduction treatment is shown to increase the lifetime of photoexcited electrons in rutile TiO 2 photocatalysts with an increase of n-type electrical conductivity. The photocatalytic activities of H 2 -reduced rutile TiO 2 were higher than those of anatase TiO 2 and mixed-phase TiO 2 . This chapter explains the effect of donor doping on the photocatalytic activity of rutile TiO 2 , the relationship between its physicochemical properties and photocatalytic performances, and the mechanism of the enhanced activity of H 2 -reduced TiO 2 . Particle size dependence on the enhanced activities suggests the formation of a space charge layer in large TiO 2 crystallites.

Enhanced Visible Light Response of TiO2 Codoped with Cr and Ta Photocatalysts by Electron Doping

“…This would be because the doped chromium species work as recombination centers. The photocatalytic activity of nondoped TiO 2 under UV irradiation was enhanced by electron doping with H 2 treatment as reported previously, ,, but it did not work under visible light irradiation unlike the “black” TiO 2 prepared by H 2 treatment …”

“…In the case of nondoped rutile TiO 2 photocatalysts, the enhanced photocatalytic activity by H 2 reduction treatment is explained by the increase in electron density in the CB and shallow energy levels, which provide efficient charge separation by built-in potential and fast transport of photogenerated electrons. ,, In this study, we found that the H 2 reduction treatment decreased the midgap states working as deep traps of photoexcited electrons, and the radiative recombination between the deeply trapped electrons and the VB holes. As the H 2 reduction treatment increases the concentration of electrons, the traps at deep levels have already filled with the electrons before photoirradiation .…”

“…H 2 reduction treatment at temperatures higher than 300 °C decreased the intensity of the five lines of the Cr 3+ species substituted in the TiO 2 lattice (Figure ). ESR signal due to the Ti 3+ (d 1 ) species was not formed after H 2 reduction treatment in contrast to the case of nondoped TiO 2 . ,, This indicates that the isolated Cr 3+ species is changed to an ESR-silent species and Ti 4+ is not reduced by H 2 treatment. It is usually difficult to detect the ESR signal of Cr 2+ (d 4 ) and Cr + (d 5 ) even though the species in a distorted environment is ESR active.…”

“…have found that the photocatalytic activity of rutile TiO 2 particles was decreased by high-temperature calcination, but the activity was recovered by H 2 reduction treatment. , The deactivation by calcination at high temperatures is attributed to the decrease of long-lived photoexcited electrons in TiO 2 . Amano et al also found that H 2 reduction treatment at 500–700 °C is an effective method to enhance the photocatalytic activity of rutile TiO 2 with large particle sizes for both H 2 evolution and O 2 evolution from water. − The enhanced photocatalytic activity of H 2 -reduced rutile TiO 2 is explained by the introduction of electrons rather than oxygen vacancies. , The increased electron concentration in large TiO 2 crystallites may enhance the charge separation and charge transport because of the formation of a built-in electric field (upward band bending) at the water interface and the improvement of electrical conductivity. ,, This indicates the importance of the control of electron concentration for rutile TiO 2 photocatalysts by doping treatments.…”

Effect of calcination conditions of P25-TiO2 on photocatalytic selective hydrogenation of 3-nitrostyrene

“…Figure 23(a) shows calcination in air, peak at 529.80eV was reduced because high calcination temperature reduced lattice oxygen due to thermal stress, these effect caused by the rapid change in Ti-O-Ti network, whereas calcination under H 2 flow in the system, which could create more oxygen vacancies then observation area of peak centered at 531.3eV was raised, which is shown in Figure 23(b) Moreover, an electron from hydrogen through surface lattice oxygen then electron was released and transferred to Ti 4+ and then Ti 4+ received electron, so Ti 3+ was formed [28,52,68,69]. However, P25-TiO 2 was calcined in air with different temperature, which found the concentration of oxygen vacancies slightly changed because of additional oxygen in the system, which are shown in Figure 24.…”