Study On Supramolecular Precursor Modified Polymeric Carbon Nitride And Its Photocatalytic CO₂ Reduction Performance

With the continuous development of science and technology,energy crisis and environmental pollution have become two major problems to be solved urgently in today’s society.Since 1972,when Japanese scientists Fujishima and Honda first proposed photoelectrocatalytic technology to split water,semiconductor photocatalytic technology has been regarded as one of the most promising green and sustainable technologies to alleviate energy crisis and environmental pollution.Nowadays,photocatalytic technology has shown great potential in hydrogen production,CO₂reduction and pollutant degradation.Polymeric carbon nitride（CN）has been widely studied because of its suitable band positions,excellent physicochemical stability and good visible-light response.However,pure CN still exhibits relatively low photocatalytic activity,mainly due to its smaller specific surface area,fewer reactive sites on the surface,and higher recombination rate of photogenerated charge carriers,which limits its real application.Therefore,it is necessary to modify CN photocatalyst to improve its photocatalytic activity.The effective modification strategies mainly include morphology modulation,loading co-catalysts,constructing heterojunction and element doping,etc.In this thesis,CN was modified mainly from the aspects of morphology modulation and loading single-atom co-catalyst.On the one hand,the specific surface area of the CN can be improved and more surface active sites can be provided by morphology modulation.On the other hand,the construction of single-atom active centers facilitates the migration and separation of photogenerated charge carriers,thereby reducing their recombination rate.The main research contents are as follows:Firstly,using melamine and cyanic acid as precursors,nanocages of CN were prepared by low temperature pretreatment of supramolecular precursors and subsequent heat treatment.The low-temperature pretreatment endows supramolecular precursor with stronger hydrogen bonding interaction,which results in a more stretchable and distorted structure and thus well-defined nanocage morphology of CN.The unique nanocage structure,due to its high specific surface area,more extendedπ-delocalization and more exposed lone-pair electrons,promotes the transfer of photogenerated electrons and the adsorption of CO₂molecules,and helps to improve the efficiency of light absorption,thus improving the performance of CO₂ reduction in the photocatalytic system.Secondly,using melamine and cyanuric acid as precursors,the single-atom Cu modified CN was prepared by introducing copper salt and hexamethylenetetramine into the supramolecular precursor solution and combining with subsequent heat treatment.By comparing the photocatalytic CO₂ reduction performance of a series of concentration-gradient single-atom Cu modified CN,the optimal concentration was obtained.At the same time,the synergistic effect of variable-valence catalysis on the transfer and separation of photogenerated charge carriers was analyzed.And a hemocyanin-like Cu-structure which makes our system similar to an artificial enzyme,which can effectively photoreduce CO₂ while liberating dioxygen at the same time.Further experimental results and theoretical calculations show that Cu atoms are uniformly dispersed in CN matrix with strong metal-support interaction.The copper single atoms on C-Cu-N₂ reactive center serve as effective collectors of photogenerated electrons and at the same time,as reactive centers to activate CO₂ molecules,as well as reduce energy barrier towards photocatalytic CO₂ reduction.As a result,the single-atom copper modified carbon nitride enables a much more efficient CO₂ photoreduction.