Recoverable visible light photocatalytic activity of wide band gap nanotubular titanic acid induced by H2O2-pretreatment

Abstract

Absrtact Nanotubular titanic acid (denoted as NTA) with orthorhombic crystal structure was prepared by combining hydrothermal treatment of TiO2 in alkaline medium with follow-up pickling. Resultant NTA was further treated with the aqueous solution of H2O2 to extend its light absorbance responses to visible light region thereby acquiring visible light photocatalytic activity. The photocatalytic performance of as-prepared NTA modified by H2O2 (denoted as H2O2-NTA) was evaluated by monitoring the oxidation of propylene under visible light irradiation. Moreover, as-fabricated H2O2-NTA was annealed at 300 °C in air to examine its thermal stability, and used H2O2-NTA was retreated with aqueous H2O2 at the end of the third run of propylene oxidation tests to allow regeneration. Results indicate that pretreatment with aqueous H2O2 provides NTA with apparent visible light photocatalytic activity for the oxidation of propylene. H2O2-pretreatment results in a large amount of Ti-peroxide complexes on the surface of NTA; resultant Ti-peroxide complexes serve as visible light absorbing sensitizers and are excited by visible light in association with the state change from the ground state to the excited state. As a result, an electron is injected from the excited state of Ti-peroxide complexes to the conduction band of NTA, thereby generating O2 − radical possessing strong oxidation ability to initiate the degradation of propylene. The visible light photocatalytic activity of H2O2-NTA gradually declines with extending photocatalytic reaction time, which is attributed to the gradual consumption of active Ti-peroxide complexes therewith. However, the lost visible light photocatalytic activity of H2O2-NTA can be regenerated by retreatment with aqueous H2O2. In this way, the visible-light-responses and photocatalytic activities of NTA, an orthorhombic crystalline rather than anatase or rutile phase TiO2, is well regenerated.