Mechanism Research On Hydrogen Peroxide Photosynthesis Based On Efficient Oxygen Utilization Process

Hydrogen peroxide（H₂O₂）is increasingly used as a green and strong oxidant in chemical synthesis,medicine and health,environmental treatment and fuel cells,etc.However,the anthraquinone method currently used in industrial production is an energy-intensive production method,with cumbersome steps,high costs and serious environmental pollution,which can no longer meet the needs of modern low-carbon sustainable development.Photocatalysis,which uses renewable solar energy to convert oxygen and water into H₂O₂under the action of photocatalyst,has become one of the most promising alternative technologies for H₂O₂ synthesis due to its simplicity,non-polluting and safety features,but the efficiency of photocatalytic synthesis of H₂O₂is still low to meet the demand.Based on this,this thesis took the most important oxygen reduction process in H₂O₂ photosynthesis as the object of study,and adopted defect engineering,micro-nano structure modulation and ligand modification to enhance the adsorption,mass transfer and pre-activation process of oxygen on the surface of photocatalyst.By constructing a triphase and fluorocarbon/water system,the photocatalyst could directly utilize the high concentration and ultra-fast mass transfer of gaseous oxygen,and finally realized the efficient synthesis and in situ separation of H₂O₂by promoting the whole process of oxygen utilization.The carrier kinetics and the mechanism of H₂O₂ synthesis during the reaction process were also analyzed in depth.The major research contents and conclusions were summarized as follows:（1）Defective BiVO₄ preparation and its H₂O₂photosynthesis.The oxygen vacancies（OVs）-rich defective BiVO₄photocatalysts（DBVO）were prepared by co-calcination of sodium borohydride with BiVO₄.OVs as typical coordination unsaturated sites（CUS）can effectively adsorb and enrich O₂,and the exuberant chemisorption can spatially promote the transfer of photogenerated electrons to the adsorbed oxygen species.Therefore,compared with pristine BiVO₄,the adsorption capacity of DBVO for O₂ was enhanced by 19 times to4.27 mmol g^-1,the interfacial transfer rate of photogenerated carriers from capture sites to active oxygen species was enhanced by 23 times to 1.1×10⁷ s^-1,and the H₂O₂ photosynthesis efficiency was enhanced by 32 times,the H₂O₂yield of DBVO reached 102μmol g^–1 h^–1under visible light irradiation and without any sacrificial agents.（2）Defective BiVO₄/WO₃/Ti O₂ nanotubes preparation and its H₂O₂photosynthesis.Defective BiVO₄/WO₃/Ti O₂ nanotubes composites were prepared by electrochemical methods,and the OVs introduced by electrochemical reduction were mainly concentrated on BiVO₄.By controlling the reduction potential and time,the precise regulation of the type and concentration of OVs could be achieved,which greatly improved the photo-charge utilization process.In addition,under the influence of the nanoconfined effect,the mass transfer rate of water molecules in the defective BiVO₄/WO₃/Ti O₂ nanotubes was more than 7.6 times that predicted value by the continuous hydrodynamic theory,possessing some superfluidic properties,and the diffusive mobility of dissolved oxygen was also significantly enhanced.The composite photocatalyst achieved an H₂O₂ photosynthesis efficiency of 252μmol g^-1 h^-1in pure water with both improved photovoltaic properties of the catalyst and the mass transfer behavior of the reactants.（3）Catalytic and photoelectric corresponding bifunctional Pd-O_x at the gas-solid-liquid triphase interface for H₂O₂ photosynthesis.The Pd/A/BiVO₄photocatalysts were prepared by modifying ligands on the BiVO₄ surface and then loading Pd nanoparticles.Under the synergistic effect of the ligand and Pd,the photocatalyst exhibited superhydrophobic/superhydrophilic interfacial properties,so it was at the gas-liquid interaction interface during the reaction,which could directly utilize the atmospheric O₂ with high concentration and fast mass transfer.Moreover,this relatively separated products and catalysts system reduced the further decomposition probability of the generated H₂O₂,which contributed to accumulate highly concentrated H₂O₂ solutions.In addition,the charge density transfer from the amino group of the ligand to Pd achieved the enrichment of O₂ and the improvement of carrier dynamics on the catalyst surfaces.Thus,the yield of H₂O₂photosynthesis by hydrophobic Pd/A/BiVO₄ in a triphase system was enhanced by a factor of13 to 805.9μmol g^-1 h^-1 compared to a two-phase system.H₂O₂ was synthesized by a two-channel mechanism of oxygen reduction and water oxidation,in which the one-step two-electron oxygen reduction reaction was the the main pathway.Moreover,Pd-Ox was not only a catalytically active center for the synthesis of H₂O₂,but also had a unique photoelectric response property,which was a bifunctional active site.（4）A fluorocarbon/water system construction and its H₂O₂photosynthesis.The hydrophobic covalent organic framework photocatalyst TTBA with triazine and imine structures,was obtained by monomer polymerization.Based on the superamphiphobicity of perfluoroalkanes,TTBA was at the two-phase interface in the fluorocarbon/water system,and the efficiency of H₂O₂ photosynthesis was as high as 4.9 mmol g^-1 h^-1 under visible light irradiation without any sacrificial agents.This was mainly due to the formation of an ultra-thin dense gas layer between TTBA and perfluoroalkanes under the interaction between hydrophobic surfaces,and the ultra-fast mass transfer and supply of reactant oxygen molecules as superfluid was achieved in this nanoconfined gas layer.Moreover,the H₂O₂generated in this system could be self-separated into the aqueous phase,thus obtaining pure H₂O₂ solutions directly during the synthesis process.H₂O₂ on TTBA was generated through a two-step hydrogen atom transfer process with extremely low energy barriers for the whole process,which confirms the contribution of sufficient O₂ supply to the efficient H₂O₂synthesis.In addition,the two-phase independent hierarchical structure of the fluorocarbon/water system could realize various functionalized applications,such as adding ferrous ions to the aqueous phase to construct a two-phase in situ Fenton system for the efficient degradation of organic pollutants.