Construction Of CeO₂-C₃N₄ Composite Photocatalysts And Study On CO₂ Photoconversion

Energy issue and climate change are two major challenges for human being.Excessive consumption of fossil fuels not only causes air and water pollution,but also leads to an explosive growth of atmospheric CO₂,which has a negative impact on global climate.Recently,photocatalytic CO₂ conversion into energy fuels has been regarded as a promising method to address the energy crisis and global warming.Among numerous semiconductor photocatalysts,CeO₂,as one of the important rare earth oxides,has the advantages of high melting and boiling point,good chemical stability and non-toxicity.In addition,it is easy to form Ce³⁺and Ce⁴⁺pairs in metal oxides,because Ce atom has the unique 4f electron orbit,which is conducive to electron transfer.CeO₂ is a alkaline oxide that facilitates contact with carbon dioxide.However,single CeO₂ has the disadvantages of small particle size,"heavy aggregation",small specific surface area and fast carrier recombination rate,which are not beneficial to the reaction.Therefore,we designed composite material with high specific surface area to improve its catalytic performance.Mesoporous CeO₂with large specific surface area was prepared by hard template method,more defect sites and adsorption sites exposed.g-C₃N₄ quantum dots were introduced to improve the migration of photogenerated carriers in CeO₂.Using Ag as an electron acceptor,we introduced Ag with better conductivity into mesoporous CeO₂ to improve the separation of electron holes,and further introduced g-C₃N₄ nanosheets to inhibit Ag photocorrosion and improve the stability of the photocatalyst.Using the structure-effect betweenthe multi-dimensional structures,wedesigned a one-dimensional CeO₂ nanorod dispersed in the middle of the surface of g-C₃N₄ and rGO to construct a 2D/1D/2D hierarchical structure composite photocatalyst.On the one hand,the dispersion of CeO₂ nanorods were improved and more active sites were exposed.On the other hand,electron mobility and utilization rate were improved largely.Taking advantage of the mass transfer and transmit of high-dimensional multi-channel structure in the photocatalytic process,we designed three-dimensional g-C₃N₄ to improve the dispersion of CeO₂ nanoparticles,and the introduction of Pt nanoparticles reduced the reaction potential of CO₂ conversion,effectively promoting the photoconversion of CO₂ performance of the photocatalysts composite.This paper mainly includes the following four aspects:1.Two-dimensional mesoporous CeO₂ was prepared by‘nano-watering'pathway with SBA-15 as a hard template.Zero-dimensional g-C₃N₄ quantum dots were prepared from urea.Then,mesoporous CeO₂ modified by g-C₃N₄ quantum dots was prepared by hydrothermal method.They were characterized by a series of characterization.The results showed that when the loading of g-C₃N₄ quantum dots was 0.75 wt%,the binary photocatalyst has the best photocatalytic performance.After UV irradiation for 10 h,the yields of CO and CH₄ were 5.34?mol/g and 2.19?mol/g,respectively.2.TheAg/m-CeO₂compositephotocatalystsweresynthesizedby photodeposition using silver nitrate and ascorbic acid as raw materials.Urea and Ag/m-CeO₂ were mixed together,grounded and put into a tube furnace,and calcined in a N₂ atmosphere to obtain g-C₃N₄/Ag/m-CeO₂ composite photocatalyst.The prepared photocatalysts were performed by XRD,SEM,TEM,XPS,UV-vis DRS,etc.,and their properties studied by CO₂ phtotconversion under UV-light irradiation.The results showed that when the loading of Ag was 7 wt%,the prepared g-C₃N₄/Ag/m-CeO₂ composite photocatalyst exhibited the best activity for CO₂photoconversion,and the yields of CO and CH₄ reached 13.94?mol/g and 7.39?mol/g within 10 h.3.Two-dimensional g-C₃N₄ nanosheets were prepared by calcining tricyanic acid at high temperature.One-dimensional CeO₂ nanorods were prepared by controlling the conditions of hydrothermal reaction.Then two-dimensional rGO/CeO₂/g-C₃N₄intercalation composite photocatalyst was prepared by multi-step hydrothermal method.The physicochemical properties and photoelectrochemical tests of the prepared photocatalysts were carried out by a series of characterization techniques,and photocatalytic conversion of CO₂ investigated in this study.The results showed that the introduction of g-C₃N₄ and rGO greatly improved the yield of CO and CH₄,which reached 34.37?mol/g and 21.33?mol/g within 10 h.4.The three-dimensional porous g-C₃N₄ was prepared by twice calcinations with urea and sodium bicarbonate as raw materials.The three-dimensional porous g-C₃N₄was dispersed in a solution of Ce?NO₃?₃ by dipping method,and the adsorption equilibrium was obtained by magnetic stirring at room temperature,filtered and dried,and calcined to obtain a CeO₂/3D-g-C₃N₄ composite photocatalyst.Then,Pt modified CeO₂/g-C₃N₄ composite photocatalyst was prepared by photoreduction technology.The composite photocatalysts were characterized by various measurement techniques,and studied the photocatalytic conversion of CO₂ under UV-irradiation.The results showed that when the loading of CeO₂ was 45 wt%and the loading of Pt was 0.86wt%,the ternary composite photocatalyst exhibited good activity for CO₂phtotorconversion,and the production of CO and CH₄ reached 40.34?mol/g and30.34?mol/g in 10 h,respectively.