Synthesis Of Metal Composite Catalysts For Electrochemical Nitrate Reduction And In Situ Raman Spectroscopy Study

The increasing concentration of nitrate（NO₃^-）has caused serious harm to human health and environment in water.The method of electrochemical removal of NO₃^-has been widely concerned because of its green environment and simple follow-up treatment.Ammonia（NH₃）,as a high value-added product,is a very important energy carrier.The electrochemical reduction of nitrate to synthesize ammonia not only helps to solve environmental problems,but also reduces the energy consumption of ammonia synthesis.However,this process involves a complex eight-electron transfer process with numerous by-products and intermediates,which is not conducive to improving the selectivity of ammonia production.Therefore,it is an important focus of nitrate reduction to find efficient catalysts and understand the mechanism of the reaction process to improve the yield of synthetic ammonia and the Faradaic efficiency in the future.In this thesis,by synthesizing Ag/Zn O heterostructures,we have studied the mechanism of nitrate reduction in the in situ electrochemical Raman experiment.By using metal-organic frameworks as precursor,we synthesized a kind of electrocatalyst with Cu Ni nanoparticles acting synergistically with nitrogen-doped carbon composite materials.The results showed that the intermediate signals of in situ electrochemical Raman experiments are consistent with the electrochemical reduction process.The synergistic effect between alloy nanoparticles and nitrogen-doped carbon framework is beneficial to the adsorption of NO₃^-at the active site,inhibits the occurrence of hydrogen evolution reaction（HER）,and improves the selectivity of nitrate reduction synthesis of ammonia.The research content of this thesis mainly includes:In the first chapter,in order to investigate the mechanism of the intermediate process of electrochemical nitrate reduction synthesis of ammonia,we rationally designed and constructed a kind of Ag nanocrystal catalyst on the quasi-cavity structure of Zn O nanowalls.This Ag/Zn O heterostructure can not only catalyze nitrate reduction effectively,but also serve as an active substrate for sensitive surface-enhanced Raman signals（SERS）,which makes it possible to explore the mechanism of electrochemical nitrate reduction process of Ag catalyst at the atomic level.The structure of the synthesized Ag catalyst was characterized by XRD,TEM,SEM and HRTEM and the experimental results showed that we have successfully synthesized Ag nanocrystals with a size of about 100 nm,and we have also successfully constructed an Ag/Zn O quasi-cavity structurel with effective coupling between nanocavities and metal nanoparticles,which can induce remarkable SERS performance.The electrochemical test results showed that the Ag nanocatalyst can achieve an ammonia yield of 516 mmol g_cat^-1h^-1and a Faradaic efficiency of 66%at the optimum potential of-0.6 V vs RHE.Further,the SERS signals of N=O,HNH and NH₃were clearly displayed in the in situ electrochemical Raman experiment,which also effectively proved the generation of nitrite and ammonia in the electrochemical reduction process.In the second chapter,noble metal catalysts limit their own application due to their high cost,while MOFs materials can promote the electrocatalytic reaction due to their advantages such as cheap,sustained,large specific surface area,adjustable structure and modifiability of pore surface.As Cu atom has a high Faradaic efficiency in nitrate electrochemical reduction,we synthesized Ni-doped Cu-MOF by solvothermal method as a precursor,and then pyrolyzed at800℃to prepare Cu Ni-NC-800 catalyst for electrochemical nitrate reduction synthesis of ammonia.XRD,XPS,SEM and TEM characterization results showed that we have successfully obtained the MOF-derived materials with octahedral shape,and Cu Ni nanoparticles are limited to the octahedral frame nitrogen-doped carbon composites.The results of electrochemical experiments showed that the Cu Ni-NC-800 catalyst achieved a high Faradaic efficiency of 75%and a yield of 835 mmol g_cat^-1h^-1for the electrochemical synthesis of ammonia in alkaline electrolyte.This is mainly caused by the synergistic effect of Cu Ni nanoparticles and porous nitrogen-doped carbons framework.At the same time,the high porosity of MOF-derived materials is conducive to the adsorption of NO₃^-at the active site,thus promoting the nitrate reduction process.This work provides a reference for the synthesis of bimetallic catalysts with low cost and high selectivity in the future.