Study On The Controlled Preparation And Photocatalytic CO₂ Reduction Performances Of Bi₂Fe₄O₉/CeO₂@PANACSs

The energy and environmental crisis caused by the excessive burning of fossil fuels and the massive emission of CO₂ has brought a huge impact on human society.Photocatalytic CO₂reduction by solar energy is an effective measure to reduce the CO₂ content in the air.However,starting from the mechanism of photocatalysis,the CO₂ adsorption capacity of a single catalyst is weak,and the photogenerated carriers are easily recombined,which inhibits the catalytic activity.Therefore,it is necessary to seek effective measures to improve the CO₂adsorption and activation ability,and inhibit the recombination of photogenerated carriers,then improve the catalytic activity.Activated carbon spheres have excellent CO₂ adsorption performance,and the existence of nitrogen-containing functional groups can be used as adsorption and activation sites,which can further improve the CO₂ adsorption performance of activated carbon spheres and enhance the photoreduction activity of CO₂.In addition,using activated carbon spheres as carriers can improve the light absorption capacity of the catalyst and inhibit the recombination of photo-generated carriers.Cerium oxide has high temperature resistance and excellent redox ability,which can play a catalytic role in the activation of activated carbon.Bismuth ferrite（Bi₂Fe₄O₉）,a multiferroic Bi-based catalyst,has narrow band gap,good visible light absorption capacity,weak magnetism and easy recovery.Based on this,this thesis takes polyacrylonitrile activated carbon spheres（PANACSs）with high nitrogen content as the main research body,and embeds the highly active metal oxide CeO₂ to construct a CeO₂@PANACSs composite with strong adsorption capacity and high reduction activity.Then,on the basis of binary composite catalyst,Bi₂Fe₄O₉ was loaded by surface impregnation,and Bi₂Fe₄O₉/CeO₂@PANACSs ternary composite photocatalyst was constructed.The rapid transfer of electrons enables the effective separation of electrons and holes,and the reduction performance is further improved.The specific research contents are as follows:（1）Using acrylonitrile as a precursor,polyacrylonitrile activated carbon spheres（PANACSs）with high nitrogen content were prepared by suspension polymerization;Cesalt was added during the preparation of PANACSs,and CeO₂ was embedded in the PANACSs framework by one-pot method.XRD,XPS,SEM analysis confirmed the effective introduction of cerium oxide,the type of its own nitrogen-containing functional groups,the existence of Ceelement,and the change of microscopic morphology before and after doping with Cesalts.The pore structure properties and CO₂ adsorption capacity of CeO₂@PANACSs prepared with different Cesalt doping amount were analyzed by N₂ adsorption/desorption curve and CO₂ adsorption curve.Compared with the photocatalytic CO₂ reduction performance of pure CeO₂,PANACSs,after 10 h,CeO₂@PANACSs showed the highest performance of photocatalytic CO₂ reduction(21μmol·g^-1·h^-1),the best Cesalt doping amount was determined;auxiliary UV-visible diffuse reflection absorption spectrum,impedance and photoluminescence spectrum and other means to determine the enhanced light absorption ability and photogenerated carriers after doping with Cesalt.The separation efficiency was improved;a mechanism for the enhanced activity of CeO₂@PANACSs was proposed.（1）Under the excitation of light,the electrons transition to the conduction band and leave holes in the valence band,which oxidize water into protons.Due to the conductive properties of the activated carbon spheres,the electrons in the conduction band of CeO₂ are rapidly transferred to the surface of PANACSs,and thus,the electron-hole separation is effectively achieved.（2）PANACSs have excellent CO₂ adsorption capacity,and the existence of nitrogen-containing functional groups further promotes the adsorption and activation of CO₂.Among them,pyridinic nitrogen and pyrrolic nitrogen can be used as adsorption and activation sites,and quaternary nitrogen/graphitic nitrogen can be used as electron transport media to transfer electrons from the conduction band of ceria to pyridinic nitrogen and pyrrolic nitrogen.The CO₂ molecules are further activated and reduced.（3）Ce⁴⁺on the surface of CeO₂ acts as an electron capture agent,which captures the electrons that are not transferred in time and reduces itself to Ce³⁺.Ce³⁺reduces the adsorbed CO₂ to CO.（4）Besides,the light absorption range was broadened by using PANACSs as the carrier.Therefore,the CO₂ adsorption capacity of the binary system is improved,the light absorption range is increased,and the photogenerated carriers are effectively separated,thereby improving the photocatalytic activity.（2）Bi₂Fe₄O₉ was prepared by hydrothermal method,and then Bi₂Fe₄O₉ was supported on CeO₂@PANACSs by impregnation method to form a ternary composite photocatalyst.Compared with pure Bi₂Fe₄O₉ or binary system,the photocatalytic CO₂ reduction performance was further improved,reaching 25.61μmol·g^-1·h^-1.XRD,XPS and SEM were used to determine the crystal structure,crystallinity,purity,valence state of main elements,the micromorphology of Bi-based catalyst and the change of morphology before and after loading.The N₂ adsorption/desorption curves and CO₂ adsorption curves compared the effects of loading Bi₂Fe₄O₉ on the specific surface area and CO₂ adsorption capacity of the system.Auxiliary UV-Vis diffuse reflectance absorption spectroscopy,photoluminescence spectroscopy,put forward the mechanism of the enhanced activity of the ternary system.CeO₂@PANACSs have excellent CO₂ adsorption capacity.When Bi₂Fe₄O₉ and CeO₂ are excited by light,electrons transition to the conduction band and leave holes in the valence band.The holes left on the valence band of Bi₂Fe₄O₉ oxidize water into protons and participate in the water oxidation reaction.Due to the metal-like nature of the activated carbon spheres,electrons on the conduction band of Bi₂Fe₄O₉ are rapidly transferred to the surface of CeO₂@PANACSs,so that electron-hole separation can be effectively achieved.The quaternary nitrogen/graphitic nitrogen on the surface of CeO₂@PANACSs acts as an electron transfer medium,which further transfers part of the electrons on the conduction band of Bi₂Fe₄O₉ to pyridinic nitrogen and pyrrolic nitrogen for the subsequent CO₂ reduction reaction.The other part of the electrons transferred from the conduction band of Bi₂Fe₄O₉ is recombined with the holes in the valence band of CeO₂.Ce⁴⁺on the surface of CeO₂@PANACSs acts as an electron capture agent to capture the electrons in the conduction band of CeO₂,and reduces itself to Ce³⁺.Ce³⁺reduces the adsorbed CO₂ to CO.The CO₂adsorption performance is enhanced,the electron holes are effectively separated,and the nitrogen-containing functional groups provide more CO₂ adsorption and activation sites.In addition,the light absorption range is broadened,and therefore,the CO₂ reduction activity is further improved.