Photocatalytic Hydrogen Production Performance And Mechanism Of Zirconium-based MOF/g-C₃N₄ Catalysts

Nowadays,as human society develops at a rapid pace,fossil and mineral energy sources are also facing a crisis of near depletion.In addition,the large amount of energy consumed leads to more serious environmental problems such as water pollution,greenhouse gas emissions and soil pollution,which are receiving more attention as people become more aware of environmental issues and pay more attention to their own state of health.Hence,hydrogen energy appears in the public view as a clean energy source,and photocatalytic decomposition of water for hydrogen production is an emerging technology that can use solar energy to convert water into chemical energy by using it as a reaction substrate.However,the current conventional photocatalysts have low hydrogen production effect,high photogenerated electron-hole complex rate,and less wavelength range of absorbed light,so the breakthrough point of this technology should lie in the preparation and improvement of photocatalysts.Graphitic phase carbon nitride（g-C₃N₄）is a class of polymeric semiconductor materials that are promising photocatalysts for hydrogen cracking due to their tunable optical and electrical properties.However,g-C₃N₄ materials exhibit unfavourable photocatalytic properties due to their wide band gap,weak photoadsorption properties and fast photogenerated carrier complexation rates.Metal-organic framework（MOFs）materials are a new type of crystalline porous materials,in which zirconium-based MOFs are characterized by high mesoporous stability,high chemical stability,high specific surface area,functionalized and operable;while porphyrin ligands,which have the advantages of light trapping,catalysis and high activity in nature,are used as ligands and metal ion doping of the ligand centre,which are then linked to Zr₆O₈ nodes to construct 3D metal-organic framework to synthesize MOF materials with bimetallic zirconium-based porphyrins.This bimetallic material was grown in situ on the surface of the conventional catalyst g-C₃N₄（CN）,which can effectively solve the obstacles encountered by g-C₃N₄ in the photocatalytic process,enhance the visible light utilization of the photocatalyst,broaden the light absorption range of the material,improve the separation efficiency of the e^--h⁺pairs,extend the photogenerated carrier lifetime and improve the photocatalytic decomposition of hydrogen from water（HER）.The main research of this paper is as follows:（1）The conventional catalyst graphite-phase carbon nitride（g-C₃N₄）was modified using para-aminobenzoic acid（AA）modification to prepare g-C₃N₄-AA（g-CN-AA）materials.The bimetallic porphyrin zirconium-based MOF:PCN-222（M）（M=H₂,Ni,Mn,Pd）photocatalysts were prepared by assembling H₂TCPP and the porphyrin ligand M-TCPP（M=Ni,Mn,Pd）doped with different central metals with highly stable Zr₆ clusters as nodes for eight-linkage assembly.12 wt%PCN-222（M）（M=H₂,Ni,Mn,Pd）/g-C₃N₄-AA series photocatalysts were prepared by in situ hydrothermal synthesis by loading PCN-222（M）with mass fractions of 1%,3%,and 5%onto the g-C₃N₄-AA,respectively.Among them,the best hydrogen production effect of 1%PCN-222（Pd）/g-C₃N₄-AA can reach 2338.17μmol/g/h.Various characterization methods were used to characterize the composite morphology using scanning electron microscopy（SEM）and transmission electron microscopy（TEM）,Fourier infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）,X-ray diffractometer（XRD）,and Ultraviolet solid-state（UV-vis）to analyze the material structure and elements,absorbance spectral wavelengths,and Tauc Plot calculations were used to calculate the band gap,infer the heterojunction structure formed between the composite interfaces.The PCN-222（M）/g-C₃N₄-AA photocatalyst e^--h⁺on separation efficiency was tested using Photocurrent response tests（PC）as well as Electrochemical impedance（EIS）.The band gap was calculated using the Tauc Plot method,and thus the effect of different metal ions doped into the porphyrin centre on the HOMO and LUMO positions at the interface of the series composites was inferred.Gaussian and first-principles calculations and hydrogen adsorption free energy calculations were also carried out to analyze the effect of the interface between PCN-222（M）and g-C₃N₄-AA on the type of heterostructure building due to the introduction of different metals in the MOF ligands,and to explore in depth the photocatalytic reaction mechanism of the series materials.（2）PCN-224（M）（M=H₂,Ni,Mn,Pd）photocatalysts were prepared by selective doping of transition metal ions(Ni²⁺,Mn²⁺,Pd²⁺)in the porphyrin centres and hexa-linking of metalloporphyrin ligands with Zr₆ cluster nodes,which were compounded with modified graphitic phase carbon nitride g-C₃N₄-AA by in situ hydrothermal synthesis to prepare 12 different mass fractions of PCN-224（M）（M=H₂,Ni,Mn,Pd）/g-C₃N₄-AA series photocatalysts were prepared by in situ hydrothermal synthesis.The results of the photolytic hydrogen precipitation experiments showed that the best catalyst was 5%PCN-224（Pd）/g-C₃N₄-AA with a hydrogen precipitation rate of2456.06μmol/g/h.After a series of characterization,the morphology,physicochemical properties and optical properties of the materials were characterized to investigate the effect of doping different metal ions in the porphyrin ligand centre on the photocatalytic hydrogen production of the PCN-224 series.The effects of the doping of different metal ions in the centre of the porphyrin ligand on the photocatalytic hydrogen production effect,the complexation of photogenerated e^--h⁺pairs and the photogenerated charge transfer rate of the composites were investigated.The changes in the HOMO and LUMO energy levels due to M-TCPP were measured,and the type of heterojunction structure constructed at the interface of the PCN-224（M）/g-C₃N₄-AA composite photocatalysts was inferred.Combining experimental findings with Gaussian and DFT theoretical calculations and hydrogen adsorption free energy calculations to verify the type of construction of heterojunctions and the internal variability of the enhanced hydrogen precipitation performance of photolytic water.