Construction Of Electrocatalysts With Iron Active Centers And Study On Nitrogen Reduction Reactions Performance For Ammonia Production

Ammonia（NH₃）is not only a basic chemical raw material,but also an important raw material for synthetic fertilizer,which is of great significance to Chinese manufacturing industry and food security.At present,the synthetic ammonia industry uses the Haber-Bosch method to reduce nitrogen under high temperature and high pressure,which suffers a high energy consumption and high carbon emission.As a potential alternative to the Haber-Bosch process,ammonia synthesis by nitrogen reduction under ambient conditions has attracted wide attention of researchers in recent years.Although researchers have done great efforts on nitrogen reduction reaction（NRR）under ambient conditions,the highly stable N≡N covalent triple bond leads to extreme difficult of nitrogen activation and hydrogenation,restricting the further development of NRR under ambient conditions.Finding catalysts with high nitrogen activation ability becomes the key to break through the bottleneck.However,the research at the present stage shows that most catalysts still encounter some obstacles,such as slow reaction kinetics,low Faraday efficiency（FE）and unclear ammonia synthesis mechanism.As the main active metal of nitrogenase,the d orbitals of iron can interact with nitrogen,enhance nitrogen adsorption capacity,and activate nitrogen,which is widely used to develop high-performance nitrogen reduction catalysts.In this thesis,the construction of iron active center is the core starting point.The performance of nitrogen reduction to ammonia is improved by ion replacement,surface doping and metal anchoring strategies to modify the electrocatalyst.The mechanism of promoting nitrogen activation by iron active center is investigated by theoretical simulation,evaluation of NRR performance and electrochemical analysis.A lamella-structured copper iron layered double hydroxides（Fe（Ⅱ）Cu（Ⅱ）Fe（Ⅲ）-LDH）electrocatalyst with adjustable content of Fe²⁺is successfully prepared by solvothermal method with the aid of ethylene glycol as a reducing solvent.Fe（Ⅱ）Cu（Ⅱ）Fe（Ⅲ）-LDH shows favorable electrocatalytic nitrogen reduction reaction（ENRR）performance due to the presence of Fe²⁺active center.Fe（Ⅱ）Cu（Ⅱ）Fe（Ⅲ）-LDH exhibits optimal ENRR performance and acceptable cycle stability at-0.5 V vs.RHE in neutral electrolyte(0.1 mol L^-1 Na₂SO₄),ammonia yield 33.10μg h^-1 mg_cat.^-1and Faraday efficiency 21.70%.The results of chemical toxicity experiments show that Fe²⁺in catalyst is the main active center of NRR.Theoretical calculation is carried out to investigate NRR mechanism of the materials.The results indicate that the existence of Fe²⁺can regulate the d-band center,enhance the nitrogen activation ability,reduce the energy barrier of NRR process,and thus improve the NRR performance of the catalyst.To investigate the promoting effect of iron active center in ENRR,iron active centers are constructed on the surface of non-noble metal catalyst without iron.The feasibility of improving the ammonia synthesis ability for such material and the corresponding promoting mechanism of activity are investigated.Iron doped copper（Fe/Cu）electrocatalysts are prepared by electrodeposition method combined with copper etching and thermal reduction.The results of ENRR experiments show that iron doping can effectively improve the ENRR performance of the catalyst.And the amount of iron doping is controllable.Thanks to iron active center,Fe/Cu shows optimal ENRR performance at a low voltage of-0.1 V vs.RHE in alkaline electrolyte(0.1 mol L^-1 KOH),ammonia yield 22.60μg h^-1 mg_cat.^-1and Faraday efficiency16.04%.The cycle stability of Fe/Cu is acceptable.Compared with the electrocatalytic performance of iron-free doped catalyst,iron in Fe/Cu catalyst is the main nitrogen reduction active center.The results of theoretical calculation exhibit that iron doping can adjust the electronic structure of the catalyst,enhance the nitrogen activation ability of the catalyst and reduce the energy barrier of NRR（0.66e V）,so as to improve the ENRR performance.Iron active centers are constructed on a low-cost graphite phase carbon nitride catalyst.Iron loaded and cyano modified carbon nitride（Fe-CG-CN）electrocatalysts are synthesized by one-step molten salt method with the assistance of melamine and ferric nitrate.The load of iron is controllable.The results of ENRR experiments show that iron load and cyano modification can effectively improve the ENRR performance.Fe-CG-CN exhibits optimal ENRR performance at-0.4 V vs.RHE in acid electrolyte(0.1 mol L^-1 HCl),ammonia yield 20.95μg h^-1 mg_cat.^-1,Faraday efficiency 33.02%.At the same time,the catalyst can be extended to the visible light photocatalytic nitrogen reduction to ammonia.The ammonia yield is 45.21μmol g^-1 h^-1 atλ>400nm.The results of N₂-TPD show that the iron active center plays a major role in enhancing the nitrogen adsorption capacity of the catalyst.The combined action of the iron active center and cyano modification promote the effective improvement of the nitrogen reduction to ammonia performance of the catalyst.