Preparation Of G-C₃N₄ Based Photocatalyst And Its Application In Photocatalytic CO₂ Reduction

Carbon dioxide(CO2)emissions have hit record highs in recent years due to the use of fossil fuels such as coal and oil.It has caused serious damage to the natural ecosystem and threatened the sustainable development of human society.Photocatalytic reduction of CO2 has the advantages of clean,cheap,no secondary pollution and energy consumption,and has attracted wide attention.Photocatalytic reduction of CO2 to energy by light energy is a significant and challenging task to simulate photosynthesis in nature.The research carried out in this field will solve both the energy shortage and the environmental problems caused by the massive accumulation of CO2.Graphite-phase carbon nitride(g-C3N4)is a unique two-dimensional layered material.Because of its low price,easy preparation,good stability and suitable energy band structure,it has become a promising photocatalyst,which is widely used in the fields of photocatalytic water decomposition,CO2 photoreduction,organic pollutant degradation and electrocatalytic hydrogen production.However,due to its small surface area and rapid photoelectron-hole recombination,the photocatalytic activity of g-C3N4 is too low to meet practical application.In this paper,a series of g-C3N4-based photocatalysts were prepared by using heterojunction,supported metal single atom and other methods to promote the adsorption and activation of CO2,further reduce carrier recombination,improve the photocatalytic CO2 reduction activity,and further study the photocatalytic mechanism as follows:1)The low cost g-C3N4/Al2O3 heterojunction was prepared by hydrothermal method before calcination.The sample was determined to be rich in oxygen vacancies by EPR testing.The presence of γ-Al2O3 has a certain regulation effect on the structure of g-C3N4,and the prepared heterojunction has excellent photocatalytic CO2 reduction activity and high CO selectivity.The mechanism of photocatalysis is analyzed.This study provides a promising method for the modification of g-C3N4 and the enhancement of photocatalytic activity.2)CoCo-LDH with oxidizing ability was grown in situ on g-C3N4 with reducing ability to construct redox heterojunction.In situ infrared spectroscopy analysis shows that the addition of CoCo-LDH can improve the adsorption and oxidation capacity of water,ensure the release of proton H species,and promote the protonation of intermediate products.The theoretical calculation(DFT)and experimental results show that the introduction of CoCo-LDH not only reduces the overpotential of water oxidation,but also selectively reduces the energy barrier of*COOH in the photocatalytic CO2 reduction process by adjusting the microelectronic structure.The built in electric field constructed by Fermi level flattening is transferred by the S-mechanism and ensures that the maximum redox potential of g-C3N4/CoCo-LDH is maintained.Without the addition of sacrificial agents,the photocatalytic CO2 process can selectively generate nearly 100%carbon-based products,with CO yield up to 71.39 μmolg-1 h-1.The control of the redox capacity of the oxidation co-catalyst system provides a new way for photocatalytic CO2 reduction to produce carbon products.3)The CN-Co-Ni bimetallic monatomic catalyst with excellent photocatalytic activity and high selectivity was prepared by loading Co and Ni bimetallic monatomic atoms on g-C3N4 by the method of oil bath and then calcination.The HADDF-STEM and XAFS analysis have proved the existence of Co-Ni monatom.Atomic dispersion of active metal substances in monatom catalyst can maximize the utilization of atoms.This study provides reference for the preparation of monatom catalyst.