Precise Regulation Of Thermal Polymerization Process And Its Effect On Hydrogen Production Performance Of G-C₃N₄

Graphitic carbon nitride（g-C₃N₄）is a promising visible-light responsive polymeric catalyst that has attracted much attention because of its easy synthesis,abundant raw materials,relatively high redox ability,and stable physicochemical properties.Although g-C₃N₄ has various advantages in the field of hydrogen production via photocatalytic water splitting,its photocatalytic hydrogen production activity is still limited by many factors,such as high recombination rate of photogenerated electron-hole pair,low crystallinity,low electron conductivity,small specific surface area,and insufficient absorption of visible light above 460 nm.In order to solve these problems,this thesis investigates the influence of the intermediate products of g-C₃N₄ on its microstructure and hydrogen production performance during the thermal polymerization process by discharging the gases generated at different stages during the heating process;by adding cyanamide and formaldehyde aqueous solutions to silica（Si O₂）sols,hybrid precursors containing cyanamide,formaldehyde and Si O₂ gel was formed after the gelation reaction,which were further calcinated under ventilation and airtight conditions,forming mesoporous g-C₃N₄ with high specific surface area.The effects of formaldehyde on the microstructure and its hydrogen production properties of g-C₃N₄obtained via different thermal polymerization processes.The specific research contents are as follows:（1）The effects of intermediate gaseous products produced at different stages during the thermal polymerization process on the microstructure and hydrogen production properties of the obtained g-C₃N₄.By calcining the dicyandiamide precursor in a sealed tube furnace and venting the gaseous intermediate when the calcination temperature reached certain heating temperatures（20℃,230℃,390℃and 550℃）to investigate the effect of gaseous intermediates self-produced at different stage of thermal polymerization on the microstructure and hydrogen production properties of the obtained g-C₃N₄.The results showed that prepared g-C₃N₄ by venting at 230℃exhibited better crystal quality and hydrogen production properties than the other samples,while the crystal quality and hydrogen production properties gradually decreased when vented at 390℃and 550℃.It is proved that the gaseous intermediates generated before 230℃were unfavorable to the polymerization of g-C₃N₄ during the thermal polymerization,and those generated after 230℃were favorable to the g-C₃N₄polymerization.Releasing the gaseous intermediates that unfavorable for polymerization helps the orderly stacking of 2D planes,which greatly promotes the interlayer charge transfer and enhances the hydrogen production performance.The hydrogen production rate（3500μmol/（h·g））of g-C₃N₄ obtained by exhausting at 230℃（CN-230）was significantly increased,which was 4.7 times higher than that of the original g-C₃N₄.The relationship between the microstructure and photocatalytic activity of g-C₃N₄ and the gaseous intermediates generated at different stages found in this work has an important reference value for the synthesis of g-C₃N₄ with good structure and excellent performance.（2）Preparation of mesoporous g-C₃N₄ materials with large specific surface area and their hydrogen production properties.In order to g-C₃N₄ with large specific surface area and excellent hydrogen production property,the aqueous solution of cyanamide and formaldehyde was dissolved into the Si O₂ sol,forming a hybrid precursor composed of cyanamide,formaldehyde and Si O₂ gel.Mesoporous g-C₃N₄ was obtained by calcination of the hybrid precursor under ventilation condition（named VCN）and under airtight condition（named ACN）,which structure,photoelectric chemical properties and hydrogen production properties were and analyzed comparatively.The effects of formaldehyde aqueous solution and thermal polymerization conditions on the microstructure and hydrogen production properties of g-C₃N₄ was revealed.The results show that the mesoporous g-C₃N₄ prepared under ventilation conditions have large specific surface area（about 312 m²/g）and uniform mesoporous structure,the light absorption range is broadened to 760 nm,improved separation efficiency of photogenerated electron-hole under the ventilation conditions.Among them,VCN-40（VCN by using 40 mg of formaldehyde）demonstrated a hydrogen production rate of2826μmol/（h·g）,which is 1.8 times higher than that of VCN-0（g-C₃N₄ without formaldehyde under ventilated conditions）,indicating that the addition of formaldehyde under aeration conditions was beneficial to the synthesis of g-C₃N₄.Under confined conditions,the hydrogen production rate of ACN gradually decreased with the increase of formaldehyde content,which indicates that the addition of formaldehyde under sealed conditions is detrimental to the synthesis of g-C₃N₄.This study revealed the effects of formaldehyde on microstructure and hydrogen production of g-C₃N₄ under different thermal polymerization conditions,which provides a useful reference for the development of g-C₃N₄-based composite catalysts with high hydrogen production efficiency.