Construction And Performance Study Of Composite Catalysts Photo-splitting Water Based On Covalent Organic Framework

Natural resources are being depleted due to the continuous consumption of fossil fuels and there is an urgent need to develop new energy sources.Among the numerous solar energy conversion technologies,photocatalytic hydrogen production is considered to be one of the most promising pathways.To date,metal-based semiconductor materials have been investigated,however,these semiconductor photocatalysts still have some shortcomings.Covalent organic frameworks（COFs）as a class of porous skeletal materials with permanent porosity,long-range ordering,and high stability,provide an important material platform for solving the global energy and environment crises.The single COFs have serious carrier recombination problem.By combining COFs with other functional materials,the photocarrier migration rate can be improved and the superior composite materials can be prepared.In this thesis,COF-based materials were used as photocatalysts,and the typical Schiff base COF（TpPa-1-COF）was selected.The visible light hydrogen precipitation performance of TpPa-1-COF photocatalyst was improved by two different strategies.The details are as follows:（1）Construction of Ni₁₂P₅（Ni₂P or Co P）/TpPa-1-COF and study of photocatalytic hydrogen production performance:A series of TMP nanoparticles with uniform particle size were prepared by high-temperature mixing and calcination method.Then,the TMP nanoparticles were successfully grown onto the TpPa-1-COF surface by a simple and easy in situ solvothermal strategy,and finally a series of TMP/TpPa-1-COF composite photocatalysts were successfully synthesized.The structure,morphology,bonding and optical properties of Ni₁₂P₅（Ni₂P or Co P）/TpPa-1-COF photocatalysts were characterized by various analytical tests,which showed that the TMP materials did not destroy the structure of TpPa-1-COF and the composites were able to retain the original properties of TpPa-1-COF.The photocatalytic hydrogen production performance of Ni₁₂P₅（Ni₂P or Co P）/TpPa-1-COF were investigated under visible light（λ≥420 nm）irradiation,and the results showed that the Ni₁₂P₅/TpPa-1-COF composite photocatalyst had the highest activity of decomposing water to produce hydrogen,which can reach up to 3.16 mmol g^-1h^-1.It was about 19 times higher than the untreated TpPa-1-COF(0.165 mmol g^-1 h^-1)and comparable to Pt/TpPa-1-COF(3.88 mmol g^-1h^-1).A series of photoelectric and theoretical calculation tests confirmed that TpPa-1-COF compounded with Ni₁₂P₅ has more abundant surface reaction sites and lower H*generation energy barrier.In addition,the conductivity and carrier migration/separation properties of TpPa-1-COF are significantly improved.（2）Fabrication of h-BN/TpPa-1-COF and investigated the performance of photocatalytic hydrogen production:The resulting h-BN samples were added to a two-dimensionalβ-ketoenamine bond-linked COFs（TpPa-1-COF）synthesis system,and the h-BN/TpPa-1-COF composites were directly obtained by a simple solvothermal reaction.Among them,porous h-BN was successfully constructed by high-temperature calcination method and used boric acid and urea as raw materials,which high temperature can induce defects and impart porous structure to h-BN.Benefiting from the introduced defects,h-BN can be endowed with a porous structure,which provides more surface reaction sites.The structure and morphology of h-BN/TpPa-1-COF photocatalysts were tested by using XRD,FT-IR,SEM,TEM and other characterization methods.The bonding of h-BN/TpPa-1-COF was analyzed by XPS and Density Functional Theory.The results confirmed the strong electronic coupling between the two materials and the structural transition of TpPa-1-COF when synthesized with defective porous h-BN.The above result leads to an elevated conduction band position and increase the distance between h-BN and TpPa-1-COF of conduction band,which helps to suppress electron reflux.It facilitates the electron narrow migration to the reactive sites on the surface of porous h-BN to react with H⁺and produce H₂.The results show that the resulting porous h-BN/TpPa-1-COF metal-free VDW heterojunction displays excellent photocatalytic activity without co-catalyst and can reach H₂ precipitation rates as high as 3.15 mmol g^-1h^-1,which is about 67times higher than the pristine TpPa-1-COF.