Design And Photocatalytic CO₂ Reduction Performance Study Of Hollow Multi-shelled Heterogeneous Structures

The conversion of solar energy into chemical energy is one of the effective ways to realize the conversion and utilization of solar energy.Among many conversion pathways,photocatalytic reduction of carbon dioxide(CO2)is a process that uses light energy to convert CO2 into valuable carbon based fuels,including carbon monoxide(CO),methane(CH4),methanol(CH3OH)and so on.In the past few decades,the research field of photocatalytic CO2 reduction has made great progress,and researchers have a deeper understanding of the mechanism of CO2 reduction.However,at present,the efficiency of photocatalytic CO2 reduction is low and the selectivity of high value-added products is poor,which makes the conversion process less economical.In order to solve the current problems,the core and difficulty of photocatalytic CO2 reduction research is to design and prepare photocatalysts with excellent catalytic performance,which can not only achieve high efficient CO2 reduction,but also obtain high value-added products.Therefore,three key problems in photocatalytic reaction were needed to solve:light absorption,separation of photo-generated electron/hole pairs,and surface reaction.For this reason,this thesis designed hollow multishelled structures(HoMSs)material with multiple shells and cavities to achieve efficient capture and absorption of sunlight;constructed a shell structure with local heterostructure to achieve effective separation of photo-generated electron/hole pairs;and created oxygen vacancies and lattice distortion on the shell to provide more active sites for catalytic reaction.Based on the above research ideas,a series of HoMSs materials with heterostructure were designed and synthesized in this paper.By optimizing the morphology and component structure,the efficient photocatalytic CO2 reduction performance was achieved,and the reaction mechanism was explored.The main research contents are as follows:(1)SnO2 HoMSs were synthesized by sequential template method(STA),and then SnS2 was grown on the shell of SnO2 HoMSs by solvothermal process using thioacetamide(TAA)as sulfur source.The typical type Ⅱ heterostructure of SnO2 and SnS2 was realized on the nanoscale,which promoted the separation of photo-generated electron/hole pairs and increased the number of photogenerated electrons that could participate in the catalytic reaction.At the same time,lattice distortion structure was successfully introduced into HoMSs,which provided additional active sites for catalytic reaction.In addition,the amount of oxygen vacancies on the surface of the catalyst can be increased.Multiple shells of HoMSs can reflect light many times,which improves the light capture probability of the material.To sum up,they jointly realized the optimization of the photocatalytic system.Under visible light irradiation,the optimal CO production rate was 48.01μmol·g-1 h-1.(2)Compared with the typical type-II structure,Z-scheme structure not only improves the effective separation of photo-generated electron/hole pairs,but also retains the best oxidation and reduction ability of the two materials in the heterogeneous structure.This work designed a photocatalyst with In2S3/FeaO3 Z-scheme structure.Fe2O3 HoMSs were prepared by STA process.Then,the sheet-like of In2S3 was coated on the inner and outer surface of Fe2O3 HoMSs shell by solvothermal method to form the sandwich structure of In2S3/Fe2O3/In2S3.The results of photocatalytic performance show that the Z-scheme structure is beneficial to the separation of photo-generated electron/hole pairs,and the number of photo-generated electrons that can participate in the reduction reaction is increased,and then the photocatalytic reduction process is enhanced.Under visible light irradiation,the optimal CO generation rate of photocatalyst is 27.03 μmol·g-1 h-1.(3)CO is an important intermediate for the formation of CH4 in photocatalytic CO2 reduction.How to enhance the selective adsorption of CO on the catalyst is the key to promote the formation of CH4.In order to obtain CH4,this work designed and synthesized photocatalysts with heterogeneous inner and outer shell components and crystal forms,CeO2@TiO2 HoMSs.The outer shell TiO2 is amorphous,and the inner shell CeO2 is crystalline.Ce-doping can induce the formation of amorphous TiO2,generating impurity energy levels in TiO2,and expanding the range of light response.And,the absorption edge is red shifted from the ultraviolet region to the visible region,which improves the light capture and absorption.The CO produced by CeO2 in the inner layer was re-adsorbed and reduced to CH4 by amorphous TiO2 in the outer layer during the process of outward diffusion.The production rates of CO and CH4 by the optimized photocatalyst under simulated sunlight were 46.02 μmol·g-1 h-1 and 7.59μmol·g-1 h-1 within 8 hours,respectively.