Synthesis Of Metal-confined Nanocomposite Catalysts And Their Application In Nitrate Electrochemical Reduction

As an important energy carrier,ammonia（NH₃）plays a major role in current and future energy systems.The industrial production of ammonia relies mainly on Haber-Bosch which it is energy-intensive and costly.In response to the call of"green chemistry",selective Ammonia synthesis by nitrogen reduction as a new alternative method has emerged.However,the yield and efficiency of ammonia are limited due to the high dissociation energy of N≡N(941 kJ mol^-1)and the low solubility of nitrogen in water.In order to find a suitable nitrogen source,as a nitrogen oxide,nitrate（NO₃^-）has attracted much attention due to its widespread presence in environmental pollutants and the low dissociation energy of the N-O bond(204 kJ mol^-1).From the perspective of solving the energy crisis and environmental pollution,electrochemical reduction of nitrate to ammonia is a promising strategy.In order to improve the Faradaic efficiency and yield of ammonia,it is urgent to find an efficient catalyst for future development of nitrate reduction.We prepared S-doped graphene confined metal In electrocatalysts by spatially confined reduction strategy.We also prepared electrocatalysts with uniform size Co nanoparticles confined in carbon-nitrogen composite materials.The results show that the interaction between the metal and the support.Simultaneously,the special electronic structure formed by the confined metal atoms and the nitrogen-doped carbon supports is beneficial to the adsorption of reactants,promotes the charge transfer,and the hydrogen evolution reaction（HER）is effectively suppressed.This effect greatly improves the performance of electrocatalytic nitrate reduction to ammonia.The research content of this thesis mainly includes:In the first part,to achieve efficient electrochemical nitrate reduction to ammonia,we synthesized an electrocatalyst（In-S-G）that confines metallic In in sulfur doped graphene using a spatially confined reduction strategy.The as-prepared In₂S₃ nanosheets were initially mixed with glucose to form carbon coated status by a hydrothermal method.Then we can obtain In-S-G catalysts by calcination of carbon coated In₂S₃ nanosheets at high temperature.The catalysts were characterized by XRD,TEM and HRTEM.The results show that 5～10 nm metallic In nanoparticles confined in S-doped graphene were prepared successfully.XPS,ICP and other test results show that after high temperature pyrolysis,In₂S₃ were decomposed,the amorphous carbon turned into graphene with heteroatom S doped.At the same time,the remained In³⁺from In₂S₃was reduced to low valence state and inclined to bond with C,forming a unique configuration in which In-S and In-C bonds act simultaneously.The electrochemical test results show that the ammonia yield can reach 220 mmol g^-1_cat h^-1 and the FE can reach 75%at-0.5 V vs RHE.Theoretical calculations show that the synergistic effect of In-S and In-C bonds promotes the adsorption of reactants.As a consequence,the outstanding performance of In-S-G was mainly ascribed to the abundant active sites and special electronic structure.It is beneficial to charge transfer and significantly lowers the reaction barrier for NH₃ generation,effectively suppress the production of hydrogen.In the second part,we take into account that porous materials have the advantages of large specific surface area and high porosity,which are more favorable for the binding of reactants to the active sites.This facilitates the catalytic reaction process.We prepared Co nanoparticles with a size of 5～10 nm by calcining ZIF-67 at high temperature and confined them in nitrogen-doped carbon support composites（Co-NC-600）.The catalyst was used for electrochemical nitrate reduction to ammonia synthesis.The characterization results such as XRD and HRTEM show that we have successfully prepared Co nanoparticles uniformly dispersed in nitrogen-doped carbon support composites.Combined with XPS,EDS and other test results,it is shown that Co nanoparticles and N form Co-N bonds,and each element is uniformly distributed.Electrochemical tests showed that the Co-NC-600 catalyst exhibited nearly 100%Faradaic efficiency for ammonia production,which mainly due to the synergistic effect between Co nanoparticles and nitrogen-doped carbon support.At the same time,the large specific surface area and high porosity of porous materials are favorable for anchoring more active sites.The uniform distribution of a large number of active sites is beneficial for the rapid transfer of electrons,thereby facilitating the catalytic process.This work provides a promising lead for designing efficient and low-cost electrocatalysts.