Photocatalytic Water Oxidation By Cobalt Cubane-Based Polyoxometalate And Transition Metal Oxides

In today’s world, the energy crisis and environmental pollution have been two big problems restricting the development of human society. H₂ is a kind of renewable clean energy, and the product of combustion is water when H₂ combines with O₂. The use of sunlight for artificial photosynthesis water splitting is currently the most ideal energy conversion mode to generate hydrogen. One of the half reaction of water splitting- water oxidation reaction has become the bottleneck of water splitting reaction because it is the complicated process with four electrons and four protons. Therefore, develepment of high efficiency, high selectivity and stability of water oxidation catalysts（WOCs） becomes the key of water splitting. Earlier efforts have focused on oxide particles of iridium and ruthenium and related metal organic complexes, but their cost is prohibitively high for widespread usage. The less-expensive transition metal oxides, such as cobalt oxides, iron oxides, nickel oxides and manganese oxides, are considered to be potential catalysts which can replace precious metals. Main content of this thesis included the research on metal oxides as photocatalysts containing Mn and Co and preparation of Co substituted polyoxometalates as photocatalytic WOCs:1. We reported an all-inorganic, oxidatively and hydrolytically stable polyoxometalate [(A-α-SiW₉O₃₄)₂Co₈（OH）₆（H₂O）₂（CO₃）₃]^16-（Co₈POM）. As a Co（Ⅱ）-based cubane water oxidation catalyst, Co₈ POM embeds double Co^Ⅱ₄O₃ cores. This self-assembly catalyst is similar to the oxygen evolving complex（OEC） of photosystem Ⅱ（PS Ⅱ）. Under light-driven conditions using [Ru（bpy）₃]₂+ as a photosensitizer and persulfate as a sacrificial electron acceptor, Co₈ POM exhibited excellent water oxidation activity. Multiple experimental results（e.g. UV-Vis, effect of pH, FT-IR, density functional theory（DFT） calculations, dynamic light scattering, X-ray photoelectron spectroscopy（XPS）, electrospray ionization mass spectrometry（ESI-MS） and laser flash photolysis） confirmed that Co₈ POM is a stable and efficient catalyst for visible light-driven water oxidation.2. We reported Co₃O₄ porous nanocages derived from simple metal–organic frameworks by a simple self-assemble method and a low temperature anneal process. This method can synthesize MxCo₃-xO₄（M = Co, Mn, Fe） porous nanocages materials and allow precise control of ratio of substituted metal in Co₃O₄ catalysts. These catalysts were investigated for photochemical, chemical-driven（cerium（IV）-driven） and electrochemical water oxidation, and they presented a superior activity for water oxidation. The high turnover frequency（TOF） of （?）3.2 × 10^-4 s^-1 per Co atom was obtained under neutral pH using Co₃O₄ porous nanocages in photocatalytic water oxidation reaction, which was comparable with those of nanostructured Co₃O₄ clusters supported on mesoporous silica. Under cerium（IV）-driven water oxidation condition, a high TOF of （?）3.6 × 10^-3 s^-1 per Co atom was achieved, which was the highest among those of other known reported cobalt oxides. The overpotential of Co₃O₄ porous nanocages for the electrochemical water oxidation(η = 0.42 V at 1 mA cm^-2) was comparable to the reported overpotentials of catalysts based on cobalt. Multiple experimental results（e.g. XRD, TEM, HR-TEM and XPS） confirmed that Co₃O₄ porous nanocages are highly stable. This study illustrates a guideline to the design and synthesis of inexpensive and highly active spinel catalysts for water oxidation.3. The δ-MnO₂/o-MWCNTs were synthesised by simple immersion of the oxidized multi-walled carbon nanotubes（o-MWCNTs） into a KMnO₄ solution. This catalyst possessed high catalytic activity in photocatalytic water oxidation with Na₂S₂O₈ and [Ru（bpy）₃]Cl₂. A series of control experiments confirmed MnO₂ acted as the real active site and o-MWCNTs acted as an electron mediator, respectively. Moreover, our method could give a helpful inspiration to design nanostructure WOC catalysts using a simple method through the CNTs.