Synthesis Of Immobilized Single-atom Catalysts Via Ball Milling Method And Study On Their Photocatalytic CO₂ Reduction Mechanism

With the rapid growth of population and industrialization,the global energy supply has increased dramatically.The use of fossil fuels causes large amounts of CO₂ emissions,bringing about a series of ecological and environmental problems such as global warming,glacier melting,and biodiversity loss.Therefore,the conversion and utilization of CO₂have become urgent.Among various CO₂ conversion technologies,photocatalytic CO₂reduction is also considered one of the most promising solutions to energy and environmental problems.However,it faces several challenges:the need to develop a simple synthesis process of catalysts for photocatalytic CO₂ reduction reaction,the need to improve the stability of photocatalysts themselves,the difficulty of CO₂ adsorption activation,the high rate of photo-generated carrier complexation,the lack of in-depth research on the mechanism of photocatalytic CO₂ reduction reaction,and the difficulty of regulating the selectivity of reduction products,etc.These problems become the key to improving the performance of photocatalytic CO₂ reduction reactions and are also the hot spots of research in this field.Compared with conventional photocatalysts as well as loaded photocatalysts,single-atom photocatalysts can have higher catalytic activity and selectivity due to the dispersion of metal particles on the surface up to atomic size.The superior catalytic performance of single-atom photocatalysts gives them great potential for application in photocatalytic CO₂ reduction reactions.To address the above problems,in this dissertation,conventional photocatalysts Ti O₂and g-C₃N₄ were selected as substrates,and metal single-atoms were loaded on the carriers by high-energy ball milling method to construct single-atomic active sites to modulate their catalytic performance and product selectivity.The single-atom active center not only enhances the adsorption and activation of CO₂ but also promotes the separation of photo-generated carriers,thus improving the catalytic performance and product selectivity to solve the problem of difficulty controlling the reduced products of photocatalytic CO₂ reduction reaction.Combining in situ technical means with theoretical calculations,the enhanced mechanisms of catalytic performance and product selectivity of single-atom photocatalysts are systematically investigated.The research work of the dissertation is divided into four main parts as follows:（1）The synthesis of a series of single-atom metals（covering main-group,transition,precious,and rare earth metals）photocatalysts M_SA/Ti O₂ using a simple,efficient,and high-yield mechanochemistry（high-energy ball milling）and evaluate their efficiency towards CO₂ photoreduction is reported.In the synthesized single-atom catalyst（SACs）,the CH₄ yield from CO₂ photoreduction using Pd _SA/Ti O₂ reaches as high as 271.6μmol·g^-1·h^-1 with the selectivity of～98.0%,far surpassing those of conventional Pd clusters and nanoparticles.The experimental results and density functional theory（DFT）calculations reveal that the strong adsorption at single-atom catalytic sites（Pd）leads to significant bending of O=C=O bond angle from 180.0 to 151.0^o and length from 1.16 to 1.20?.The deformation of the CO₂ molecule lowers the energy barrier,resulting in a significant enhancement of the catalytic activity.Meanwhile,combined with in-situ Fourier-transform infrared（FT-IR）,a rational reaction pathway of CO₂ photoreduction over efficient SACs is proposed.（2）The proposed high-energy ball milling method can be extended to different types of substrates for the synthesis of different types of metal SACs.The synthesis process does not require heating,additives,post-etching or other treatments,reducing the complexity and cost of the synthesis process.The loading of Pd onto the g-C₃N₄（CN）surface by high-energy ball milling without significant clusters or nanoparticles confirms the universality of the SAC synthesis method.Pd _SA/CN showed efficient photocatalytic CO₂ reduction performance in the absence of sacrificial agents,indicating a decisive influence of single atoms on the catalytic performance.The distribution of Pd single-atom active sites on the surface of the CN substrate promotes the generation and separation of photogenerated charges.At the same time,it improves the ability of the catalyst to adsorb CO₂.The Pd single-atom active sites lower the overall activation energy barrier of the reaction and improve the photocatalytic CO₂ reduction to CO,exhibiting high yields(～10.2μmol·g^-1·h^-1)as well as good stability,but do not change the reaction path.（3）Atomically dispersed active sites can effectively improve the catalytic activity ofcatalysts,but it is still challenging to construct highly dispersed single-atom active sites.Single-atom Ni was constructed on CN using a high-energy ball milling method for photocatalytic CO₂ reduction reactions.The uniformly loaded single-atomic Ni on the surface of the substrate suggests the effectiveness of the synthesis method in terms of single-atomic dispersion.After optimizing the Ni loading,the photocatalyst containing0.5 at%（0.32 wt.%）single-atomic Ni（Ni/CN-0.5）exhibited the highest CO₂ reduction performance(～19.9μmol·g^-1·h^-1)without any co-catalyst or sacrificial agent.As visualized by aberration-corrected high-angle annular darkfield scanning transmission electron microscopy,the Ni atoms in the Ni/CN-0.5 photocatalyst are most uniformly dispersed for different loadings（0.1,0.3,0.5,0.7,1.0,3.0,and 5.0 at.%）.The homogeneity of the single-atom active sites plays a decisive role in the catalytic performance rather than the loading amount.（4）Due to the diversity of photocatalytic CO₂ reduction products,a major challenge is to achieve single-product selectivity while maintaining high efficiency,and designing single-atom species on the substrate surface is crucial to control the reaction path on the catalyst surface.The stable electron configuration(3d¹⁰)at the center of the single-atom can lower the activation energy barrier of the reaction,and the peripheral empty orbitals can act as electron traps to absorb photo-generated electrons and improve the charge separation efficiency,thus increasing the activity,while changing the reaction pathway to form CO instead of CH₄.After optimization of the single-atomic content,the 0.5 mol%Zn/g-C₃N₄（Zn-CN-0.5）photocatalyst with Zn²⁺single-atoms of stable 3d¹⁰ configuration achieves near 100%selectivity for visible-light-driven CO₂ reduction to CO,with a rate of 21.1μmol·g^-1·h^-1.In contrast,the Cu/g-C₃N₄（Cu-CN-0.5）photocatalyst with an unstable electronic structure can’t achieve high selectivity,although it improves the overall reaction activity.The research content of this dissertation not only provides a convenient high-energy ball milling method to synthesize a series of single-atom catalysts for photocatalytic CO₂reduction,but also finds that the homogeneity of single-atom active sites plays a decisive role in the catalytic performance and explores the mechanism of single-atom electronic structure modulation on the performance and selectivity enhancement of single-atom photocatalysts,and the results are of guiding significance for further development of highly efficient and highly selective metal single-atom photocatalysts.