Preparation Of MOFs And MOF-derived Nanomaterials For Electrocatalytic Ammonia Synthesis

As an important industrial chemical,ammonia is widely used for the production of medicine,fibers,and fertilizers.At present,ammonia is primarily produced by the Harbor-Bosch process,which is energy intensive and has severely relied on the consumption of fossil fuels.In addition,it accounts for>1%of the global CO2 emissions.Therefore,it is imperious to develop a sustainable NH3 synthesis method that can reduce fossil fuels utilization and CO2 emissions.In recent years,electrochemical ammonia synthesis method has attracted wide attention due to its low energy consumption and pollution-free synthesis process.Remarkably,the effect of the catalyst on the reaction rate and selectivity is undoubted.Researchers have been focusing on the designing of efficient,stable and economical new electrocatalysts.Metal organic frameworks(MOFs)as a new class of porous crystalline materials can be used as templates to prepare various metal-based nanomaterials with multifarious shapes and structures.And the unique properties of open pore structure and high active site density are beneficial to achieve rapid mass transfer and improve catalytic performance during the catalysis process.In this paper,MOFs-based catalysts were designed and synthesized for the electrocatalytic nitrogen reduction.The main research contents are as follows:1.In the introduction,several ammonia synthesis methods are introduced first,and then the electrochemical synthesis of ammonia in the aqueous solution is emphasized.Finally,the research significance and contents of this thesis are briefly introduced.2.Fe-ZIF as a sacrifice template reacted with Na2MoO4 to form Fe2(MoO4)3-Fe(OH)3 intermediates in the presence of water.Afterwards,Mo doped iron phosphide(Mo-FeP)nanospheres were synthesized by thermo-assisted phosphorization.For the first time,the performance of electrochemical nitrogen reduction on the Mo-FeP was investigated.The porous nanosphere structure of Mo-FeP provided a high surface area and improved mass transfer enhancing the interaction between the catalyst and N2.The doped Mo species play an important role not only in adsorbing and activating N2 molecules but also in helping the adjacent Fe sites to stabilize the*NNH intermediate.These two effects synergistically facilitate the electrocatalytic N2 reduction activity of Mo-FeP.In 0.1 M HCl electrolyte,this catalyst exhibits excellent catalytic performance with a NH4+yield rate(13.1 ?g h-1 mg-1)and Faradaic efficiency(7.49%)at-0.3 V and-0.2 V vs reversible hydrogen electrode(RHE),respectively.The catalyst also exhibits excellent stability and repeatability.3.CeO2 nanorods(CeO2 NR)synthesized by a simple hydrothermal method used as a sacrificial template to in situ generate a Ce-MOF layer on the surface,and CeO2 encapsulated with Ce-MOF(CeO2@Ce-MOF)was successfully prepared.When this material is used for the electrocatalytic N2 reduction,the MOF layer outside of CeO2 NR plays the role in absorbing and storing N2,which can be concentrated at the active sites of CeO2 NR,increasing the possibility of catalytic reduction of nitrogen.Experiment results show that CeO2 NR@Ce-MOF performance better than the original CeO2 NR for NRR.In 0.1 M Na2SO4 electrolyte,when the applied voltage is-0.8 V,CeO2@Ce-MOF shows the highest ammonia production rate of 7.7 ?g h-1 mg-1,and the highest Faradaic efficiency of 8.4%.