Designing Semiconductor Photocatalysts for the Abatement of Aqueous Micropollutants

新型半導體催化劑在處理水中微量污染物方面的研究

Student thesis: Doctoral Thesis

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Award date2 May 2017

Abstract

The aim of the Thesis is to design and evaluate several semiconductor-based photocatalysts for the removal of aqueous micropollutants, including toxic heavy metal ions and endocrine disrupting compounds (EDCs), and to elucidate their underlying mechanisms.

The Thesis begins with the studies on the photoremediation of aqueous hexavalent chromium (Cr(VI)). It was found that the mixture of methanol and Cr(VI) resulted in photosensitive methyl chromate(VI) ester. Homogeneous photodecomposition of chromate(VI) ester occurs spontaneously to form Cr(V) and further to Cr(III) even in the absence of solid photocatalysts. This takes place through the inner sphere ligand-to-metal charge transfer. In fact, the addition of photocatalyst at low concentrations (≤ 0.1 g L-1 TiO2) retarded the rate of Cr(VI) reduction due to the fast electron injection from the methyl chromate ester to the TiO2 conduction band that essentially disrupted the inner sphere charge transfer within the chromate(VI) ester. At higher photocatalyst concentrations, the rate of photoelectron transfer from TiO2 to the chromate(VI) ester becomes dominant, and its rate increases linearly with increasing photocatalyst concentration. Strong electrostatic adsorption is required to negate the slow photoelectron transfer from the photocatalyst surface to the negatively charged chromate(VI) ester. For this reason, the photocatalytic reduction is more efficient on TiO2 than on WO3.

The second part of the work focuses on the development of visible-light active BiVO4 photocatalysts for the oxidation of bisphenol and chlorinated EDCs. Noble metals (Pt, Ag or Pd)-deposited BiVO4 was synthesized, and the loadings of noble metals were optimized accordingly. Because the conduction band edge potential of BiVO4 is incapable of one-electron reduction of O2 to form superoxide radical anion, the net charge separation is limited by the sluggish multi-electron reduction processes. The addition of noble metal deposits was beneficial in enhancing the photocatalytic efficiencies of BiVO4 by catalyzing the surface multi-electron reduction of O2 as well as forming the Schottky barrier for enhanced interfacial charge separation. The trend of photocatalytic activity follows the order Pt > Pd > Ag, coinciding with the work functions of the metals in the same order. The optimum 2% Pt/BiVO4 was highly efficient in the photocatalytic degradation and mineralization of bisphenol A (BPA), 4,4'-thiodiphenol (TDP) and 2,4,6-trichlorophenol (2,4,6-TCP); mildly effective towards bisphenol S (BPS), 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP); and ineffective towards diphenyl sulfoxide (DPSO) and triclosan (TCS). The kinetics of degradation and mineralization of the EDCs are closely related to their Langmuir adsorption capacities on the photocatalyst, suggesting the importance of the direct hole oxidation mechanism in tandem with the oxidation by photocatalytically generated hydroxyl radicals.

The photocatalytic degradation of EDCs was extended to the third part of the Thesis. Glutathione-protected gold nanoclusters supported on titanium dioxide (Au cluster/TiO2) was fabricated and assessed for the visible light degradation of 10 different EDCs. They are BPA, BPS, TDP, DPSO, TCS, 4-CP, 2,4-DCP, 2,4,6-TCP, 4-octylphenol (4-OP) and di-n-octyl phthalate (DNOP). Although unsupported Au clusters are capable of visible light absorption up to 500 nm through the ligand-to-metal charge transfer, the rapid radiative charge recombination prevented any significant surface redox reactions from taking place. The interfacing with TiO2 accepter allows the efficient interfacial charge separation, and made available the photohole on the Au cluster to oxidize the EDCs. As such, oxidation rate is directly proportional to the adsorption capacity of EDCs on the Au cluster/TiO2. Hydroxyl radicals could not be produced under visible light due to the insufficient HOMO level of the gold nanoclusters. Lastly, we showed the superior reusability of the Au cluster/TiO2 under repeated cycles of photocatalytic assessments.