Photocatalytic Synthesis Of Hydrogen Peroxide From Bipyridyl Covalent Organic Frameworks

In the interest of the sustainable development of human society,there is a growing concern for energy and environmental issues.Since hydrogen peroxide（H₂O₂）has strong chemical activity,and at the same time the products of H₂O₂ are oxygen（O₂）and water（H₂O）after it is used,it does not bring environmental pollution situation.In comparison to the traditional anthraquinone method,photocatalysis is a green and energy efficient production process for H₂O₂.It is desirable that H₂O₂ photosynthesis is carried out in pure water without any sacrificial or buffer reagents in order to improve the stability and practical application of photocatalytic synthesis of H₂O₂.In this regard,organic semiconductor catalysts are suitable semiconductor materials for photocatalytic synthesis of H₂O₂.Because organic semiconductors are mainly composed of light elements such as C,H,N and S,they do not contain metals and can prevent H₂O₂ from being decomposed by metal ions.Covalent organic framework compounds（COFs）are a new type of visible light responsive non-metallic organic semiconductor material developed in recent years.It has great potential in the field of photocatalysis because of its abundant and cheap starting materials,wide visible light absorption,diversity of synthetic strategies and tunable electronic structure.In this paper,we investigate the performance and mechanism of COFs in photocatalytic synthesis of H₂O₂ under water and air conditions.In this paper,covalent organic framework compounds with high crystallinity but different monomer structures were prepared by a solvothermal method to obtain the photocatalysts COF-Tfp Bpy,COF-Tfp Daaq and g-C₃N₄.The elemental composition and crystalline shape of the semiconductor materials were first confirmed by XRD,FTIR,XPS and solid-state NMR,and the morphology of the catalysts was observed by SEM and TEM.It was found that COFs with different monomeric structures did not have the same ability to photocatalytically synthesize H₂O₂,among which bipyridyl COFs（COF-Tfp Bpy）had the best performance in photocatalytic synthesis of H₂O₂.The COF-Tfp Bpy concentration of 1.5 g·L^-1 had the highest photosynthetic rate of 1042μM·h^-1 under one sunlight.To investigate the reaction mechanism of COF-Tfp Bpy photocatalytic precipitation of H₂O₂,electrochemical,in situ Fourier transform infrared and isotope experiments were used,and the photocatalytic activity of COFs in this study was found to be determined by the bipyridine active site,indicating that H₂O₂ was photocatalytically produced by the 2e^-one-step process of COF-Tfp Bpy.To illustrate the difference in selectivity,DFT calculations comparing the adsorption energies of H₂O and O₂ at different N positions of COF-Tfp Bpy illustrate that the presence of bipyridine in the photocatalyst induces a high selectivity for H₂O₂ production by the 2e^-one-step method.To improve the photocatalytic synthesis of H₂O₂,we treated COF-Tfp Bpy with hydrochloric acid acid acidification and obtained the protonated photocatalyst P-COF-Tfp Bpy（referred to as P-COFs）.FTIR,XPS and solid-state NMR illustrate the successful completion of COF-Tfp Bpy protonation.The H₂O₂ production of P-COFs was 1.5 times higher than that of COF-Tfp Bpy,which could reach 702μM,and the H₂O₂ photosynthetic efficiency in one hour was about 1563μM·h^-1.The narrowing of the forbidden band width of P-COFs was observed by DRS and electrochemical characterization,which accelerated the separation and migration of photoexcited carriers and created a good photocatalytic kinetic environment.In this paper,we use free radical quantization experiments and in situ Fourier transform infrared（FTIR）method to further explore the reasons why the performance of H₂O₂ can be improved.It is speculated that the superoxide radical（·OOH）may have been added to the photocatalytic synthesis of H₂O₂ in the form of a 2e^-two-step process based on the production of H₂O₂ by the2e^-one-step method.Meanwhile,H₂O₂ synthesized by prolonged photocatalysis was used to degrade organic pollutants and bacterial inactivation in this paper,and showed good environmental treatment effects.