Photocatalytic hydrogen(H_(2))evolution using covalent organic frameworks(COFs)is an attractive and promising avenue for exploration,but one of its big challenges is low photo-induced charge separation.In this study,w...Photocatalytic hydrogen(H_(2))evolution using covalent organic frameworks(COFs)is an attractive and promising avenue for exploration,but one of its big challenges is low photo-induced charge separation.In this study,we present a straightforward and facile dipole polarization engineering strategy to enhance charge separation efficiency,achieved through atomic modulation(O,S,and Se)of the COF monomer.Our findings demonstrate that incorporating atoms with varying electronegativities into the COF matrix significantly influences the local dipole moment,thereby affecting charge separation efficiency and photostability,which in turn affects the rates of photocatalytic H_(2) evolution.As a result,the newly developed TMT-BO-COF,which contains highly electronegative O atoms,exhibits the lowest exciton binding energy,the highest efficiency in charge separation and transportation,and the longest lifetime of the active charges.This leads to an impressive average H_(2) production rate of 23.7 mmol g^(−1) h^(−1),which is 2.5 and 24.5 times higher than that of TMT-BS-COF(containing S atoms)and TMT-BSe-COF(containing Se atoms),respectively.A novel photocatalytic hydrogen evolution mechanism based on proton-coupled electron transfer on N in the structure of triazine rings in vinylene-linked COFs is proposed by theoretical calculations.Our findings provide new insights into the design of highly photoactive organic framework materials for H_(2) evolution and beyond.展开更多
The kinetics of U(IV)produced by hydrazine reduction of U(VI)with platinum as a catalyst in nitric acid media was studied to reveal the reaction mechanism and optimize the reaction process.Electron spin resonance(ESR)...The kinetics of U(IV)produced by hydrazine reduction of U(VI)with platinum as a catalyst in nitric acid media was studied to reveal the reaction mechanism and optimize the reaction process.Electron spin resonance(ESR)was used to determine the influence of nitric acid oxidation.The effects of nitric acid,hydrazine,U(VI)concentration,catalyst dosage and temperature on the reaction rate were also studied.In addition,the simulation of the reaction process was performed using density functional theory.The results show that the influence of oxidation on the main reaction is limited when the concentration of nitric acid is below 0.5 mol/L.The reaction kinetics equation below the concentration of 0.5 mol/L is found as:−dc(UO_(2)^(2+))/dt=kc^(0.5323)(UO_(2)^(2+))c^(0.2074)(N_(2)H_(5)^(+))c^(−0.2009)(H^(+)).When the temperature is 50℃,and the solid/liquid ratio r is 0.0667 g/mL,the reaction kinetics constant is k=0.00199(mol/L)^(0.4712)/min.Between 20℃ and 80℃,the reaction rate gradually increases with the increase of temperature,and changes from chemically controlled to diffusion-controlled.The simulations of density functional theory give further insight into the influence of various factors on the reaction process,with which the reaction mechanisms are determined according to the reaction kinetics and the simulation results.展开更多
The service water environment of high temperature and high pressure was simulated for the steam generator heat transfer tube of pressurized water reactor.690 TT alloy tube and 405 SS plate were used to form the fricti...The service water environment of high temperature and high pressure was simulated for the steam generator heat transfer tube of pressurized water reactor.690 TT alloy tube and 405 SS plate were used to form the friction pair for impact slip dual-axis fretting corrosion experiments.The microstructure evolution of 690 TT alloy tube during dual-axis impact slip fretting corrosion was investigated.White light interferometer,scanning electron microscope,transmission electron microscope,and Raman spectrum were used to investigate the microstructure and abrasive products of the abraded surface and near-surface.Results indicate that within 105 cycles,the wear mechanism of 690 TT alloy tube is mainly adhesive wear accompanied by material transfer.With the increase in cycles from 5×10^(5) to 2×10^(6),the wear mechanism of 690 TT alloy tube is mainly crack initiation,propagation,and delamination.In terms of microstructure evolution,mixed layer exists under the three body layer in the cross-section microstructure of samples after 105 cycles.The microstructures of samples after 5×10^(5) and 2×10^(6) cycles show slight difference and present a tribological transfer structure layer with thickness of about 500 nm.Additionally,the microstructure evolution enters the stable stage.展开更多
文摘Photocatalytic hydrogen(H_(2))evolution using covalent organic frameworks(COFs)is an attractive and promising avenue for exploration,but one of its big challenges is low photo-induced charge separation.In this study,we present a straightforward and facile dipole polarization engineering strategy to enhance charge separation efficiency,achieved through atomic modulation(O,S,and Se)of the COF monomer.Our findings demonstrate that incorporating atoms with varying electronegativities into the COF matrix significantly influences the local dipole moment,thereby affecting charge separation efficiency and photostability,which in turn affects the rates of photocatalytic H_(2) evolution.As a result,the newly developed TMT-BO-COF,which contains highly electronegative O atoms,exhibits the lowest exciton binding energy,the highest efficiency in charge separation and transportation,and the longest lifetime of the active charges.This leads to an impressive average H_(2) production rate of 23.7 mmol g^(−1) h^(−1),which is 2.5 and 24.5 times higher than that of TMT-BS-COF(containing S atoms)and TMT-BSe-COF(containing Se atoms),respectively.A novel photocatalytic hydrogen evolution mechanism based on proton-coupled electron transfer on N in the structure of triazine rings in vinylene-linked COFs is proposed by theoretical calculations.Our findings provide new insights into the design of highly photoactive organic framework materials for H_(2) evolution and beyond.
基金supported by the National Natural Science Foundation of China(No.11575078)Hunan Provincial Innovation Foundation for Postgraduates(CX20190713)。
文摘The kinetics of U(IV)produced by hydrazine reduction of U(VI)with platinum as a catalyst in nitric acid media was studied to reveal the reaction mechanism and optimize the reaction process.Electron spin resonance(ESR)was used to determine the influence of nitric acid oxidation.The effects of nitric acid,hydrazine,U(VI)concentration,catalyst dosage and temperature on the reaction rate were also studied.In addition,the simulation of the reaction process was performed using density functional theory.The results show that the influence of oxidation on the main reaction is limited when the concentration of nitric acid is below 0.5 mol/L.The reaction kinetics equation below the concentration of 0.5 mol/L is found as:−dc(UO_(2)^(2+))/dt=kc^(0.5323)(UO_(2)^(2+))c^(0.2074)(N_(2)H_(5)^(+))c^(−0.2009)(H^(+)).When the temperature is 50℃,and the solid/liquid ratio r is 0.0667 g/mL,the reaction kinetics constant is k=0.00199(mol/L)^(0.4712)/min.Between 20℃ and 80℃,the reaction rate gradually increases with the increase of temperature,and changes from chemically controlled to diffusion-controlled.The simulations of density functional theory give further insight into the influence of various factors on the reaction process,with which the reaction mechanisms are determined according to the reaction kinetics and the simulation results.
基金National Key Research and Development Program of China(2019YFB1900904)。
文摘The service water environment of high temperature and high pressure was simulated for the steam generator heat transfer tube of pressurized water reactor.690 TT alloy tube and 405 SS plate were used to form the friction pair for impact slip dual-axis fretting corrosion experiments.The microstructure evolution of 690 TT alloy tube during dual-axis impact slip fretting corrosion was investigated.White light interferometer,scanning electron microscope,transmission electron microscope,and Raman spectrum were used to investigate the microstructure and abrasive products of the abraded surface and near-surface.Results indicate that within 105 cycles,the wear mechanism of 690 TT alloy tube is mainly adhesive wear accompanied by material transfer.With the increase in cycles from 5×10^(5) to 2×10^(6),the wear mechanism of 690 TT alloy tube is mainly crack initiation,propagation,and delamination.In terms of microstructure evolution,mixed layer exists under the three body layer in the cross-section microstructure of samples after 105 cycles.The microstructures of samples after 5×10^(5) and 2×10^(6) cycles show slight difference and present a tribological transfer structure layer with thickness of about 500 nm.Additionally,the microstructure evolution enters the stable stage.
