Research On The Preparation Of Transition-metal-based Catalysts And Their Electrocatalytic Performance For Nitrogen Reduction

Ammonia（NH₃）is a significant chemical raw material.The conventional Haber-Bosch process occupies absolute dominance in the field of ammonia（NH₃）synthesis.However,there exist serious drawbacks that it suffers from heavy CO₂ emission and large energy consumption.Ammonia synthesis via electrocatalytic nitrogen reduction reaction（e NRR）under ambient conditions is deemed as a promising alternative,but stable and efficient electrocatalysts are highly desired.Transition metals can accept the lone pair electrons of N₂ molecules because of their owning empty d orbitals,thus forming M-N bonds（M refers to transition metals）,which prolongs and weakens the N≡N triple bond.Therefore,transition-metal-based catalysts has great potentials to adsorb and activate dinitrogen.In this thesis,the electrocatalytic nitrogen reduction reaction under ambient conditions is the research object,aiming at formulating suitable transition-metal-based catalysts to achieve efficient nitrogen fixation.Therefore,Fe@NC,Fe P-Fe₃O₄and Co₉S₈/NC nanocomposites are prepared,and the structure-performance relationship between catalysts and ammonia-producing activity is studied.The main research contents are as follows:（1）Fe@NC is prepared by a one-step pyrolysis process with Prussian Blue as precursors.The catalyst presents a core-shell structure,and there is a strong interaction between the core and the shell.Due to its unique structural advantages,Fe@NC shows excellent catalytic performance for nitrogen reduction in alkaline environment.The optimal potential of electrolysis is-0.2 V,and the corresponding average ammonia yield rate and Faradaic efficiency are 12.55μg h^-1 mg_cat^-1 and 9.76%,respectively.The by-product hydrazine hydrate is not detected in the experiment and it is verified that the generated ammonia originates from electrocatalysis reaction.Furthermore,the core-shell structure Fe@NC catalyst exhibits excellent electrochemical along with chemical structural stability and durability.（2）FeP-Fe₃O₄ composites are prepared via phosphating and annealing with Fe₂O₃as precursors.Compared with Fe₃O₄ nanomaterials,Fe P-Fe₃O₄ owns more external or internal defects,which makes it own a larger specific surface area.Because of its larger electrochemical active area,Fe P-Fe₃O₄ can expose more reactive sites.Electrons in the Fe P-Fe₃O₄ catalyst tend to aggregate in the Fe P phase.Based on the above factors,Fe P-Fe₃O₄ exhibits excellent ammonia-producing activity in the electrocatalytic nitrogen reduction process.When a potential of-0.1 V is applied,the average ammonia yield rate and Faradaic efficiency of Fe P-Fe₃O₄ are 9.71μg h^-1 mg_cat^-1 and 11.02%,respectively.When a potential of-0.4 V is applied,the maximum average ammonia yield rate is 12.73μg h^-1 mg_cat^-1.No by-product hydrazine hydrate is detected in the experiment and it is verified that the generated ammonia stems from the electrocatalytic reaction.In addition,the Fe P-Fe₃O₄ composite catalyst exhibits excellent electrochemical along with chemical structural stability and durability.Therefore,the proposed phosphating strategy can promote the performance improvement.（3）Co₉S₈/NC nanocomposites are prepared by co-heating ZIF-67 and sulfur powder.Compared with the Co₉S₈ bulk material,macroporous and mesoporous structures coexist in Co₉S₈/NC,while Co₉S₈ presents almost only macroporous structures,which leads to the specific surface area of the former being much larger than that of pure Co₉S₈.Co₉S₈/NC has a larger electrochemical active area and can expose more reactive sites.The above factors together determine the high-yield ammonia activity of Co₉S₈/NC in the electrocatalytic nitrogen reduction process in a neutral environment.When a potential of-0.4 V is applied,the average ammonia yield rate and Faradaic efficiency of Co₉S₈/NC are 9.80μg h^-1 mg_cat^-1and 7.99%,respectively.Under the same conditions,the ammonia yield rate and Faradaic efficiency of Co₉S₈are only4.46μg h^-1 mg_cat^-1 and 5.82%,respectively.No hydrazine hydrate is detected throughout the experiment and it is verified that the generated ammonia comes from the electrocatalytic reaction.Moreover,the Co₉S₈/NC nanocomposite catalyst exhibits excellent electrochemical along with chemical structural stability and durability.