Controllable Preparation And Photocatalytic CO₂ Reduction Performances Of M_xO_y@PANACSs（M=Fe,Co,Ni,Cu）

Promoting the development of green and low-carbon economy,enhancing energy utilization,and promoting the implementation of the“double carbon”strategy is a major topic of common concern in China and the world.Photocatalytic CO₂ reduction technology uses green and clean solar energy to convert CO₂ into available chemicals and hydrocarbon fuels,which is a“two birds with one stone”solution to alleviate energy crises and protect the environment.The achievement of excellent photocatalytic CO₂ reduction activity depends on the adsorption and activation capacity of CO₂,the utilization rate of sunlight,and the separation and transfer efficiency of the photo-generated carriers during the reaction.Therefore,the design of the high-efficient photocatalyst system is the core key of the technology.N-doped porous carbon materials are acclaimed in the CO₂ adsorption-reduction field due to their high stability,enriched porosity,and tunable nitrogen-containing base sites.In this paper,acrylonitrile was used as a nitrogen-rich precursor to prepare in-situ millimeter polyacrylonitrile-based activated carbon spheres with high nitrogen content,in order to provide sufficient CO₂ adsorption-activation sites and solve the problem of difficult recovery of powder catalysts;secondly,on this basis,transition metals（Fe,Co,Ni,Cu）active components were introduced in-situ to further improve CO₂ adsorption activation capacity,enhance the separation and migration of photo-generated carriers,and construct M_xO_y@PANACSs（M=Fe,Co,Ni,Cu）N-doped carbon-based metal composite photocatalyst with high-efficiency CO₂ adsorption-reduction activity.The specific research content is as follows:（1）With acrylonitrile as the first monomer,itaconic acid as the comonomer,and the azodiisobutyronitrile as the initiator,PAN small spheres with good sphericity and hardness were prepared by optimizing parameters such as polymerization time and temperature,dimethyl sulfone dilution,stirring rate,dispersant and itaconic acid content in the solution-suspension polymerization process,and PANACSs were obtained after low-temperature carbonization.XPS,SEM,N₂ adsorption/desorption isotherms,CO₂ adsorption isotherms,and CO₂-TPD were used to analyze the atomic configuration and nitrogen content,spherical degree,pore structure characterizations,CO₂ adsorption capacity and types of PANACSs.The results showed that PANACSs is a millimeter-activated carbon sphere with high nitrogen content and excellent CO₂adsorption performance,good spherical degree,and a fan-shaped layered porous structure.PANACSs exhibited good photocatalytic activity and stability for CO₂ reduction under the simulated sunlight,and the reason for its activity was analyzed.（2）M_xO_y@PANACSs（Fe,Co,Ni,Cu）were successfully prepared by adding different types,and different ratios of transition metal（Fe,Co,Ni,Cu）hydroxides in PAN small spheres preparation,after low-temperature carbonization.Under the simulated solar irradiation,5 wt.%Fe（OH）₃-PANACSs and 3 wt.%Ni（OH）₂-PANACSs obtained the best photocatalytic activity of CO₂ reduction to CO and significantly higher than PANACSs.The photoreduction activity of Co（OH）₂-PANACSs,and Cu（OH）₂-PANACSs with different proportions decreased,except that PANACSs with 1 wt.%Co（OH）₂ and Cu（OH）₂ had no significant change.XRD and XPS results indicated that the metal’s crystal structure doesn’t present in a single form,but in the form of mixtures;the morphological structure,pore distribution characteristics,alkaline sites,adsorption CO₂ capabilities,and photoelectric properties of PANACSs before and after the embedding of metal were analyzed by using TG,SEM,N₂ adsorption/desorption isotherms,CO₂ adsorption isotherms,UV-Vis DRS,photocurrent,and electrochemical impedance,etc.The mechanism of the change of CO₂ reduction performance of PANACSs caused by the addition of metal components was proposed:1)the addition of metals provides more basic sites for CO₂ adsorption to improve the adsorption capacity of CO₂,2)The addition of metals promotes the separation and migration of the photo-generated carriers.However,the photocurrent density of PANACSs containing Co and Cu components continues to decrease after the second cycle,which may lead to the inactivation of the active component in the photocatalytic process,thus reducing the photocatalytic reduction of CO₂ performance.