Study Of Polyoxometalates And Metal Oxide Containing Low Cost Transition Metal For Photocatalytic Water Oxidation

In this thesis, polyoxometalates and metal oxide containing low cost transition metal are used as photocatalysts in the photocatalytic O₂ evolution from water splitting. We successfully designed and constructed a high-efficient photosensitized system with polyoxometalates and metal oxide photocatalysts for O₂ evolution by employing [Ru（bpy）₃]Cl₂ as the photosensitizer and Na₂S₂O₈ as the sacrificial electron acceptor. The mechanisms of photocatalytic water oxidation reaction were systematically investigated. The main and new results are obtained and presented as following:1. Study of polyoxometalates containing low cost transition metal for photocatalytic water oxidationAn eleven iron-containing nanoscale inorganic polyanionic oxide cluster Na₂₇[Fe₁₁（H₂O）₁₄（OH）₂(W₃O₁₀)₂(α-SbW₉O₃₃)₆] was firstly reported. The catalytic activity of catalyst for visible light-driven water oxidation was studied in a well-defined system using [Ru（bpy）₃]（ClO₄）₂（bpy =2,2-bipyridine） as the photosensitizer（PS） and Na₂S₂O₈ as the sacrificial oxidant in borate buffer. Na₂₇[Fe₁₁（H₂O）₁₄（OH）₂(W₃O₁₀)₂(α-SbW₉O₃₃)₆] was reported as the first example for exceptional photocatalytic water oxidation. Under optimal conditions, a remarkable turn-over number（TON, 1815）,quantum yield（47 %） and turn-over frequency（TOFinitial, 6.3 s-1） of catalyst were achieved for water oxidation. To our knowledge, these values are also among the highest values reported for homogeneous catalysts for photo-driven water oxidation so far. Collective experiments including FT-IR, dynamic light scattering and capillary electrophoretic measurements substantiate that the title compound is the dominant catalytic species under photocatalytic conditions. The hole-scavenging activity increases with the increase of the concentration of catalyst.2. Study of metal oxide containing low cost transition metal for photocatalytic water oxidationHigh efficient and visible-light-responsive photosensitizer/sacrificial electron acceptor/oxide three-component photocatalytic systems were constructed by using [Ru（bpy）₃]Cl₂ as photosensitizer, Na₂S₂O₈ as a sacrificial electron acceptor and metal oxide as catalyst. The effects of the structure of catalyst, and the interaction between sensitizer and catalyst on the photocatalytic efficiency for O₂ evolution were studied. By combing structural information as well as dynamics and stability investigation on water oxdidation, the optimal configurations of photosensitizer, sacrificial electron acceptor, and O₂ evolution catalyst were achieved. In addition, the photocatalytic mechanism for O₂ evolution was discussed based on the spectral analyses and electrochemical measurements. The main works include:（1） Flower-like 3D Cu O microspheres were synthesized and used to photo-catalyze water oxidation under visible light. The structure of Cu O microspheres was characterized by scanning electron microscopy, transmission electron microscopy, infrared, powder X-ray diffraction, electron dispersive spectroscopy, Raman and X–ray photoelectron spectroscopy. This is the first time that a copper oxide was demonstrated as a photocatalytic water oxidation catalyst under near neutral conditions. The catalytic activity of CuO microspheres in borate buffer shows the best performance with O₂ yield of 11.5 %. No change in the surface properties of Cu O before and after the photocatalytic reaction was seen by XPS, which showed good catalytic stability. A photocatalytic water oxidation reaction mechanism catalyzed by the Cu O microspheres was proposed according to reported publications and experiment proof.（2） Cu Fe₂O₄ nanocrystallines with cubic jacobsite structure have been obtained by heat treatment of the coprecipitation product, which were synthesized by the reaction of Cu²⁺ ions and Fe³⁺ ions under alkaline conditions. This is the first copper-based catalyst for photocatalytic water oxidation using [Ru（bpy）₃]Cl₂ as the photosensitizer and Na₂S₂O₈ as the sacrificial electron acceptor, respectively. An apparent TOF value of 1.2 μmol s-1 m-2 and an oxygen yield of 72.8 % were obtained with CuFe₂O₄. The apparent TOF value with CuFe₂O₄（1.2 μmol s-1 m-2） is the highest value among all heterogeneous photocatalytic water oxidation systems. Cu Fe₂O₄ can be easily separated from reaction solution by magnetic separation while maintaining excellent water oxidation activity in the fourth and fifth runs. The surface conditions of Cu Fe₂O₄ are slightly absent after examination by X-ray photoelectron spectroscopy（XPS） before and after the photocatalytic reaction.（3） Three α-NiO nanocompouds of different morphologies with nanorods, nanowires and nanoplates were synthesized by controlling the ratio of reactants and temperature. The shape and structure of nanocompouds were confirmed by SEM, XRD, FT-IR, Raman, TEM and XPS analysis. These compounds firstly were examined as catalysts in photocatalytic water oxidation using [Ru（bpy）3]2+ and S2O82– as a photosensitizer and a sacrificial oxidant, respectively. All of the samples exhibit high TOFs and perfect stability in slightly alkaline conditions. A characteristic peak around 0.95 V vs. Ag/Ag Cl assigned to Ni³⁺ species was detected by cyclic voltammetry, suggesting that a high-valent nickle species may be responsible for water oxidation. Despite the big difference in crystal morphology among Ni O catalysts, very similarities for catalytic activity have been found experimentally for the three oxides. NiO nancrystals with high surface area have been presented, NiO nancrystals produces superior catalytic activity with increasing of surface area. So, in this study, the photocatalytic activity of Ni O depends mainly on the surface area.（4） Flowerlike noble-metal-free γ-Fe₂O₃@NiO core-shell hierarchical nanostructures have been fabricated. It have been examined as catalyst in photocatalytic water oxidation using [Ru（bpy）₃]（ClO₄）₂ as the photosensitizer and Na₂S₂O₈ as the sacrificial electron acceptor, respectively. An apparent TOF value of 0.29 μmol s-1 m-2 and oxygen yield of 51 % were obtained with γ-Fe₂O₃@Ni O, respectively. The γ-Fe₂O₃@Ni O core-shell hierarchical nanostructures can be easily separated from reaction solution while maintaining excellent water oxidation activity in the fifth runs. The surface conditions of γ-Fe₂O₃@NiO also remained unchanged after examined by XRD, TEM and X-ray photoelectron spectroscopy before and after the photocatalytic reactions.