Research On Preparation And Electrocatalytic NRR Performance Of Cerium-Based And Iron-Based Catalysts

Ammonia is an important inorganic chemical product,and it plays an important role in industrial and agricultural production in the world.Ammonia synthesis is an important industrial nitrogen fixation method.Traditional industrial nitrogen fixation mainly uses Haber-Bosch process,which requires harsh reaction conditions including high temperature,high pressure and acidic conditions.As a result,this process will lead to a large amount of energy consumption,greenhouse effect and other environmental pollution problems.Therefore,from the perspective of sustainable development,it is necessary to explore a green and economic NH₃ synthesis process.At present,the electrocatalytic synthesis of NH₃ has been widely concerned by researchers due to its low energy consumption,wide and cheap sources of raw materials（nitrogen and water）,low equipment requirements and less pollution.The development of active and selective electrocatalysts is essential because of the high bond energy of nitrogen and its resistance to adsorption and dissociation.Aiming at the above problems,this dissertation has carried out a series of research work on it,and obtained the following innovative research results:1.Firstly,the preparation of porous CeO₂ nanowire arrays and the performance of electrocatalytic nitrogen reduction reaction were studied:Porous CeO₂ nanowire arrays were synthesized by oxalic acid etching and used as electrocatalysts to investigate their NRR performance.In 0.1 M HCl,the catalyst has good ammonia yield(38.6μg h^-1 mg^-1_cat.)with a low Faraday Efficiency（4.7%）at-0.30 V vs.RHE,it maintains good stability in electrochemical tests.The reasons for its higher ammonia yield are that the nanowire array with the porous morphology has a larger specific surface area,which is conducive to exposing more active sites,reducing the resistance of the array,accelerating the diffusion of gas molecules,and thus significantly improving the performance of nitrogen reduction reaction2.Secondly,the preparation of CeP nanoparticle-reduced graphene oxide hybrid and the performance of electrocatalytic nitrogen reduction were studied:CeP nanoparticles-rGO compounds were synthesized by multi-step method.The NRR activity was studied by using it as electrocatalyst.The experimental results show that CeP-rGO achieves a large NH₃ yield of 28.69μg h^-1 mg_cat.^-1and a high Faradaic Efficiency of 9.6%at-0.40V vs.RHE in 0.1 M HCl,resulting from unique structure which further accelerating the electron conduction rate and good electrical properties of rGO.Benefited from rGO which is not easy to collapse and easy to deposit,the hybridization of rGO and CeP nanoparticles is able to improve the NRR performance of the catalyst.Density functional theory（DFT）calculations show that CeP can efficiently catalyze the synthesis of NH₃.3.Since the low oxygen defect concentration of pure CeO₂ show poor NRR performance,the preparation of Ce_1-xZn_xO₂ doped nanoparticles and the performance of nitrogen reduction reaction were also studied.The Ce_1-xZn_xO₂ catalyst was synthesized by hydrothermal method,and the NRR performance of electrocatalysis was studied.The ammonia production capacity and Faraday Efficiency of the catalyst were tested at different voltages.The experimental results show that Ce_1-xZn_xO₂ achieves a large NH₃yield of 29.01μg h^-1 mg_cat.^-1and a high Faradaic Efficiency of 10.3%at-0.2 V vs.RHE in 0.1 M Na₂SO₄.The results also show that the NRR performance can be enhanced by doping metal elements with smaller ionic radius to regulate oxygen defect concentration.The electrocatalytic mechanism of Zn ion doped CeO₂(Ce_1-xZn_xO₂)catalyst was further studied by density functional theory calculation.4.Due to the variable valence property of Mn ion,the preparation of Mn ion doped CeO₂ nanospheres(Ce_1-xMn_xO₂)catalyst and the performance of nitrogen reduction reaction were also studied.Ce_1-xMn_xO₂ catalyst was prepared by hydrothermal synthesis and studied its NRR activity as electrocatalyst.The origin of N in NH₃ was investigated by ¹⁵N₂ isotope labeling method.The results show that the catalyst still has good nitrogen reduction performance.In 0.1 M HCl solution,the ammonia production capacity reaches27.79μg h^-1 mg^-1cat at-0.3 V vs.RHE.Although the Faraday Efficiency is only 9.1%,it is much higher than that of undoped pure CeO₂.The results of isotopic labeling test showed that the N in the product NH₃ is entirely from the labeling N₂.The Ce_1-xMn_xO₂catalyst showed good electrochemical stability during the electrochemical test.Density functional theory further shows that Mn ion doping can effectively regulate the concentration of oxygen vacancy,thus improving the performance of NRR.5.Finally,as iron is an abundant and low-cost transition metal element in the earth’s crust,the preparation of LiFe₅O₈-rGO and the performance of electrocatalytic nitrogen reduction were studied firstly:LiFe₅O₈ was successfully synthesized by sol-gel method.The NRR performance of LiFe₅O₈-rGO which was synthesized by combining LiFe₅O₈with rGO was detaily tested as electrocatalyst.In 0.1 M HCl,The LiFe₅O₈-rGO composite catalyst shows excellent electrocatalytic activity,selectivity and electrochemical stability.It also achieves a large NH₃ yield of 36.025μg h^-1 mg_cat.^-1and a high faradaic efficiency of 13.08%at-0.2 V vs.RHE,the electrocatalytic activity of the catalyst was almost 100%maintained after the 6 cycles of testing.The enhancement of Faradaic Efficiency is mainly attributed to the Li⁺absorbed on the catalyst surface which can effectively retard the H⁺adsorption and evolution,and the high electron conduction rate accelerating by rGO.Additionally,Fe is able to contribute d-orbital electrons to the P*orbital of N₂,thereby weakening the N-N triple bond and enhancing the activation of nitrogen adsorption.Theoretical calculations reveal that LiFe₅O₈-rGO can efficiently catalyze NH₃ synthesis with a low energy barrier,thus showing excellent NRR performance.