Transition Metal Modified G-C₃N₄ And High Photocatalytic Activity Research

With the increasingly serious environmental problems,the traditional catalytic method has been unable to cope with the current form of pollution.Photocatalytic degradation as an extremely green advanced oxidation strategy for pollutants arises at the historic moment.However,traditional photocatalytic materials are difficult to degrade pollutants effectively due to their poor response to visible light,small specific surface area and low yield of active species.Carbon nitride in graphite phase,as a relatively new photocatalyst without metal elements,which has a broad prospect due to its unique physical,chemical properties,and respond ability visible light.However,carbon nitride in graphite phase has low visible light utilization and high recombination rate of photogenerated carriers,which limits its large-scale application.Therefore,improving the charge transfer and transfer rate has become the primary research direction of graphite carbon nitride modification.In this study,two transition metal elements Mo,Ce are mainly used to construct heterojunction structure and element doping modification to improve the efficiency and feasibility of graphite phase nitride carbon-based materials for pollutant degradation.Through basic characterization and part of theoretical calculation,the influence of"structure-activity change"on its properties is explained from the changes of molecular and atomic scale,and its change mechanism is explained and explored reasonably.The research content of this paper has a good reference significance for the reasonable design,efficient application and in-depth mechanism study of transition metal modified graphite carbon nitride photocatalyst.The main research results obtained are as follows:1.Aiming at the highly efficient photocatalytic treatment of dyes（rhodamine B）and heavy metal pollution（hexavalent chromium ions）,the amorphous Mo Ox heterojunction structure supported on g-C₃N₄was constructed in this study It is proved by X-ray diffraction spectrum and X-ray electron spectroscopy that in this structure,because Mo Ox is an amorphous material,which can form a better superposition with layered g-C₃N₄,and a large number of C-O bonds are formed at the interface.The introduction of C-O bond through optoelectronic properties and XPS characterization also leads to the directional movement of charge at the interface,forming a built-in electric field to promote charge separation.The amorphous state of Mo O₃will greatly increase the contact area and heterojunction interface area with graphite carbon nitride,which greatly increases the transfer path of free electrons,which makes oxygen molecules more favorable for adsorption at the heterojunction interface.It is proved by electron spin resonance signals that Mo Ox/g-C₃N₄catalysts form a large number of active oxygen species free radicals after illumination.This work provides a new insight into the performance improvement mechanism of amorphous heterojunctions and emphasizes that the formation of built-in electric field and the introduction of variable valence metal elements are the fundamental reasons for the improvement of catalytic activity.2.Because there are no oxygen vacancies and most superoxide free radicals in the above work,heterostructures with obvious double defects were prepared by using cerium nitrate and melamine as precursors,and the ability to produce hydroxyl radicals was evaluated.The preparation of Ce O_2-x/g-C₃N_4-xdouble defect heterostructure was proved by X-ray diffraction,X-ray electron spectroscopy and electron spin resonance.Through the electron paramagnetic resonance of free radicals,it is proved that the active oxygen species produced by this heterojunction structure is much higher than that of Ce O₂and g-C₃N₄.The preparation of S-scheme heterojunction is proved by UV diffuse reflectance spectrum and Fermi level DFT calculation.The formation of S-scheme heterojunction greatly improves the redox ability of the system.At the same time,the generation of double defects provides more charge transfer channels and speeds up the transformation of excitons.In addition,Ce O₂itself has a very good conductivity,when the oxygen vacancy concentration increases,the charge transfer in Ce O_2-xwill be further improved,resulting in the enhancement of the built-in electric field energy in the interface.This conclusion is further proved by the calculation of work function.This work emphasizes that the double defect caused by the formation of S-scheme heterojunction is the main reason for the improvement of catalytic activity,and the redox semi-reaction mechanism induced by S-scheme heterojunction is explained in more detail.3.There are many studies on the heterojunction types of Ce O₂and g-C₃N₄binding,but there are few detailed studies on Ce doping g-C₃N₄.On the basis of the previous work,Ce was modified by doping g-C3N4 in the form of ions,and its ability to degrade amoxicillin（AMX）and reactive oxygen species in water was evaluated.Cerium ion-doped g-C₃N₄was prepared by one-step synthesis method.The existence of variable valence Ce ions in Ce CN samples was confirmed by XRD and XPS,and g-C₃N₄was modified by ion doping.Combined with the measurement of EPR signal and the DFT calculation of free energy,the most likely site of Ce doped g-C₃N₄was proved.Through the capture of free radicals,it is determined that the superoxide free radical（O₂^-）is the main free radical of Ce CN,and it is further proved by the determination of DOS that the introduction of new orbitals after modification leads to a significant increase in surface charge,and this conclusion is also confirmed by the calculation of electron cloud density.In addition,the introduction of d-orbitals is due to the enhancement of the adsorption and activation ability of oxygen molecules due to the increase of charge density of the conduction band.Through the calculation of adsorption activation energy and the test of TPD-O₂,it is proved that the adsorption ability of this structure to oxygen molecules is much higher than that of g-C₃N₄,so the ability to produce O₂^-is greatly enhanced.In this work,the reason for the improvement of oxidation ability of the catalyst was determined from the point of view of free radical generation ability,and the main mechanism of the improvement of catalytic activity was explained in detail.